EPA-600/4-76-005
March 1976
Environmental Monitoring Series
            DETERMINING TETRAFLUOROBORATES:
                               AN EVALUATION  OF
   FLUOROBORATE ANION  SELECTIVE  ELECTRODE
                                I
                                55
                                \
$32:
\
 LL)
                           Environmental Monitoring and Support Laboratory
                                  Office of Research and Development
                                 U.S. Environmental Protection Agency
                                        Cincinnati, Ohio 45268

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                RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency,  have been  grouped into five series. These five  broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:

     1.    Environmental Health Effects Research
     2.    Environmental Protection Technology
     3.    Ecological Research
     4.    Environmental Monitoring
     5.    Socioeconomic Environmental Studies

This report has been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and  instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion  Service. Springfield, Virginia 22161.

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                                                 EPA-600A-T6-005
                                                 March 1976
DETERMINING TETRAFLUOROBORATES i  AN EVALUATION OP

    THE FLUOROBORATE ANION SELECTIVE ELECTRODE
                 Benjamin T.  Duhart
                  Bennett College
          Greensboro, North Carolina 2?420
              Grant No.   R803006-01-1
                  Project Officer

                Morris E. Gales,  Jr.
  Environmental Monitoring and Support Laboratory
               Cincinnati, Ohio 45268
       U.S. ENVIRONMENTAL PROTECTION AGENCY
        OFFICE OF RESEARCH AND  DEVELOPMENT
 ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
              CINCINNATI, OHIO   1*5268

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                              DISCLAIMER

This report has be^n reviewed by the Environmental Monitoring and Support
Laboratory, U0S. Environmental Protection Agency, and approved for
publication.  Mention of trade names or commercial products does not
constitute endorsement or recommendation for use*
                                     11

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                             FOREWORD

     Environmental measurements are required to determine the quality
of ambient waters and the character of waste effluents.  The Environ-
mental Monitoring and Support Laboratory-Cincinnati conducts research to:
     •  Develop and evaluate techniques to measure the presence and
        concentration of physical, chemical, and radiological pollut-
        ants in water, wastewater, bottom sediments, and solid waste.
     •  Investigate methods for the concentration, recovery, and ident-
        ification of viruses, bacteria and other microbiological org-
        anisms in water.  Conduct studies to determine the responses
        of aquatic organisms to water quality.
     •  Conduct an Agency-wide quality assurance program to assure
        standardization and quality control of systems for monitoring
        water and wastewater.
     There is an ever-increasing interest in the use of electrode methods
to analyze water and waste samples, whether the resulting data are to be
used for research, surveillance, compliance monitoring, or enforcement
purposes.   Accordingly, the Environmental Monitorin-g and Support Laboratory
has an on-going methods research effort in the development,  evaluation,
and modification of electrode procedures.  This particular report pertains
to the evaluation of fluoroborate electrode.  The method has potential
routine application for the analysis of fluoroborates in surface waters
and domestic and industrial wastes.
                                       Dwight G.  Ballinger,  Director
                              Environmental Monitoring and Support Laboratory
                                                 Cincinnati
                                    111

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                               ABSTRACT

Tetrafluoroborates (BF^~) are being used in the plating industry in
place of cyanide.  These compounds are highly desirable because they
eliminate the use of cyanide.  The tetrafluoroborate electrode was
applied to fluoroborate measurements in electroplating bath samples.

The tetrafluoroborate electrode was evaluated for response time, repro-
ducibility, Nerstian behavior, possible potential interferents, and
accuracy.  The electrode technique was compared with the SFADNS colori-
metric technique for fluoride analysis.  The feasibility of using the
fluoroborate electrode and fluoride electrode to observe hydrolysis was
studied.

The tetrafluoroborate electrode was reproducible, with response time
around 6 seconds.  Possible potential Interferents included perchlorate,
iodide, nitrate, and sulfate.  In the case of nitrate and sulfate,
concentrations of 1 and 10 ppm tetrafluoroborate were studied, as was the
effect of pH.  The fluoride electrode technique, tetrafluoroborate elec-
trode technique, and SPADNS colorimetric technique were compared.  With
distillation, the colorimetric analysis gave 9%% recovery in comparsion
with 90# recovery for fluoride electrode analysis.  By subtracting the
fluoride content before distillation from the total fluoride determined
colorimetrically, the amount of fluoride from the tetrafluoroborate may
be determined.  The tetrafluoroborate electrode can be used directly in
electroplating baths to determine some selected fluoroborates.  The
standard addition method was also studied.

This report was submitted in fulfillment of Grant number R803006-01-1
by Bennett College, Greensboro, N. C., under the sponsorship of the U.S.
Environmental Protection Agency.  Work was completed in September 1975*
                                    iv

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                                CONTENTS
                                                         Page

Abstract                                                   iv

List of Figures                                            vi

List of Tables                                            vii

Acknowledgements                                         viii

Sections

  I.     Introduction                                       1

 II.     Conclusions                                        2

III.     Recommendations                                    3

 IV.     Experimental                                       b

  V.     Discussion                                         9

 VI.     References                                        36

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                                FIGURES

No.

1    Calibration Curve for Tetrafluoroborate                13

2    Calibration Curve for Fluoride                         13

3    pH Response Curves for Tetrafluoroborates at
     Various Concentrations                                 19

4    pH Response Curves for Fluoride at Various
     Concentrations                                         19

5    Colorimetric Calibration Curve for Fluoride            2k

6    Calibration Cxurve for Fluoride in TISAB                2k
                               vi

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                                 TABLES

No*                                                               Page

1    Potential Versus Concentration of Tetrafluoroborate           10

2    Potential Versus Concentration of Fluoride                    11

3    Selectivity Constants for Fluoroborate Electrode
     (Method 1)                                                    14

4    Selectivity Constants for Fluoroborate Electrode
     (Method 2)                                                    15

5    Correlation of Sulfate Levels with Tetrafluoroborate
     Concentrations                                                16

6    Correlation of Nitrate Levels with Tetrafluoroborate
     C one entrat ions                                                1 7

7    pH Versus Potentials of Tetrafluoroborate                     18

8    pH Versus Potentials of Fluoride                              20

0    Data for Calibration Curves                                   23

10   Comparison of Methods for Sodium Fluoride                     25

11   Comparison of Methods for Sodium Tetrafluoroborate            26

12   Comparison of Methods for Copper Tetrafluoroborate            28

13   Comparison of Methods for Tin Tetrafluoroborate               29

1^   Comparison of Methods for Lead Tetrafluoroborate              30
15   Comparison of Methods for 60# Tin - W Lead
     Tetrafluoroborate                                              31

16   Correlation of Tetrafluoroborate Electrode,
     Subtracted Background Fluoride Content and
     Standard Addition                                              32

1?   Standard Addition Method                                       33

.18   Calibration Curves Data at **0°C                                34

19   Potential - Time Data for Tetrafluoroborate  in water
     at 40°C                                                        35
                                 VI1

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                           ACKNOWLEDGEMENTS

The author would, like to thank the personnel of the Environmental
Monitoring and Support Laboratory, especially Morris E. Gales, for their
assistance to the project*  Personal thanks are due to Bertie Mitchell,
Jacqueline Face, and Judy Smith for their research assistance, and thanks
to Mr. W. G. Nixon of Harshaw Chemical Company for his assistance.
                                   viii

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                              SECTION I

                             INTRODUCTION

In the plating industry, tetrafluoroborate has been used in place of
cyanide in electroplating baths.  These tetrafluoroborate baths have
many advantages over cyanide bathst  no severe waste disposal problems
and no handling of dangerous, highly toxic cyanide.  However, hydrolysis
occurs giving rise to fluoride as waste in tetrafluoroborate solutions.

Preliminary investigations have shown that tetrafluoroborates break down
to fluoride ions in the Bellack fluoride distillation.  An ion-selective
electrode technique that is capable of determining tetrafluoroborate
ions at low concentrations requires less preparation, eliminates the
distillation step and would find use in the water pollution control
field*  The tetrafluoroborate electrode capability would also save time,
require less sample volume and fewer chemical reagents.

This project was divided into three phases.  Phase one involvedi
(a) evaluation of reproducibility and response time, (b) preparation
of calibration curves, and (c) evaluation of interferences for the
fluoroborate electrode.  In phase two, a colorimetric method was com-
pared with the fluoride and fluoroborate electrode methods.  In phase
three, the feasibility of using fluoride and fluoroborate electrodes
to observe the hydrolysis of fluoroborate was investigated.

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                              SECTION II

                             CONCLUSIONS

The tetrafluoroborate electrode was evaluated for response time,
reproducibility, and Nernstian-like behavior.  The time required for the
electrode to reach a relatively stable potential, as indicated by a
recorder curve, was around 6 seconds for decade changes in fluoroborate
concentrations.  The reproducibility of the electrode depended on the
concentration.  As the concentration of tetrafluoroborate decreased, the
standard deviation became higher with regard to a calibration curve.
The Nernstian-like behavior for the tetrafluoroborate gave a slope
around 56 millivolts down to about 0.6 ppm tetrafluoroborate.

The greatest interferents appeared to be perchlorate, nitrate, and
iodide.  Sulfate also interfered.  Lower results would be obtained for
samples containing more than 10 ppm sulfate in 1 ppm tetrafluoroborate.
In 1 ppm tetrafluoroborate, nitrate should be absent for accurate results.
The pH showed little effect on concentrations of 1 and 10 ppm of tetra-
fluoroborate.  But, it may affect the sample if the pH is greater than
6 for 100 ppm tetrafluoroborate.  Due to the interference of nitrate, the
tetrafluoroborate electrode may not be useful in river water.

The colorimetric method appeared to give the best recovery of total
fluoride from tetrafluoroborate.  Ninety percent recovery was obtained
with the fluoride electrode method whereas 98# was recovered with the
SPADNS colorimetric method.  The fluoride electrode method and the
tetrafluoroborate electrode can be used before distillation in pure
sodium tetrafluoroborate to determine the amount of fluoride coming from
the tetrafluoroborate0  The amount of fluoride measured with the elec-
trode before distillation was subtracted from the total fluoride
obtained.  The use of the colorimetric method for fluoride content before
distillation was not practical because of the speed of hydrolysis.
Copper and lead fluoroborate compared favorably with all methods whereas
tin and 60# tin - 40J6 lead mixture did not.  The results from the fluoro-
borate electrode were low for tin fluoroborate, and this would be ex-
pected if there were some strong complexation.  However, the tetrafluoro-
borate electrode should not be used directly in the presence of inter-
ferents or complexed fluoroborate ions.  It might be possible to use the
standard addition method.

The use of the fluoride and fluoroborate electrodes to measure the rate
of hydrolysis may be feasible with an appropriate buffer solution.  A
buffer solution should be found that would not interfere with the hydrol-
ysis and measuring devices.  Without a buffer system, the hydrolysis
determinations would be cumbersome with the two electrodes.

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                         SECTION III

                       RECOMMENDATIONS

The tetrafluoroborate electrode can be used to determine tetrafluoro-
borate In electroplating baths.  However, it is advisable that a
colorimetric analysis be performed initially for comparison purposes.
In using the colorimetric analysis for total- fluoride content, the
fluoride content before the analysis should be subtracted before
calculating the amount of tetrafluoroborate.

Further studies should also be made on the hydrolysis reaction and
rates of tetrafluoroborates.

It is recommended that a study be made on the possible automation of
the distillation and analysis of tetrafluoroborates.

It is recommended that further investigations be made on the
complexation of tetrafluoroborate.

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                              SECTION IV

                             EXPERIMENTAL

PHASE I

A.  Preparation of Standard Solutions
    Stock solutions of lO""  M sodium tetraflurooborate (Alpha
    Inorganic Inc.) were made by dissolving 10.9790 grams in approxi-
    mately 100 ml of de-ionized water and filtered.  The filtrate was
    transferred to one liter volumetric flask and diluted to the mark
    with de-ionized water.  The stock solution was kept about five days
    due to hydrolysis.

    A stock solution of 10"1 M sodium fluoride was prepared by dissolv-
    ing 4.1990 grams in one liter of de-ionized water.

    Stock solutions of 1000 ppm sodium tetraf luoroborate were made
    essentially the same as above with the exception that 1,264822 grams
    were dissolved in approximately 100 ml of distilled water.  A stock
    solution of 100 ppm sodium fluoride was prepared by dissolving
    0,221023 grains in one liter of distilled water.

    All reagents were used without further purification.  Sodium fluoride
    and sodium tetraf luoroborate were dried in an oven at 110°G for 24
    hours.  The stock solutions were stored in polyethylene bottles in
    a water bath at 25°C.  From the stock solutions, standard solutions
    were made by serial dilution each day.

B.  Measurements

    Electrode potentials measurements were made on an Orion model 801A
    ionalyzer.  The solutions were magnetically stirred at a constant,
    slow rate using a Teflon-coated stirring bar, and EMP readings were
    taken after six minutes as indicated by the Orion model 751 printer.
    The Orion model 92-05 fluoroborate and model 94-09 fluoride specific
    ion electrodes were used with a single junction reference electrode.

    The Orion model 90-91 single junction reference electrode was filled
    with 0.1 M potassium chloride saturated with silver chloride.

PHASE II

A.  Preparation of Tetraf luoroborates

    Stock solutions of 1000 ppm sodium tetrafluoroborate (Alpha
    Inorganic Inc.) were made by dissolving 1.264832 grams in approxi-
    mately 100 ml of distilled water and filtered.  The filtrate was

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    transferred to one liter volumetric flask and diluted to the mark
    with distilled water.  From the stock solutions, 4 ppm tetrafluoro-
    borate were made each day.

    From the pint sample bottles, 2 ml were diluted to one liter.  Then,
    subsequential dilutions of 3 ml of the 2 cc/1 were diluted to 500 ml
    with distilled water each day.  These methods were used to make the
    cupric, stannous and lead tetrafluoroborate solutions.  The sixty-
    forty mixtures were made by taking 200 ml of the 3 cc/500 ml lead
    tetrafluoroborate and diluting to the mark of 500 ml with 3 cc/500 ml
    of tin tetrafluoroborate.  This gave a 60% tin and 40% lead tetra-
    fluoroborate.

B.  Preliminary Distillation

    1.   Apparatus

          Corning No. 3360

    2.   Procedure

        a.   Place 400 ml distilled water in the distilling flask and
            carefully add 200 ml concentrated sulfuric acid,  H2S04.
            Swirl until the flask contents are homogeneous.   Add 25-35
            glass beads.  Begin heating slowly at first, then as rapidly
            as the efficiency of the condenser will permit (the distil-
            late must be cool) until the temperature of the flask con-
            tents reaches exactly 180°C.   Discard the distillate.   This
            process served to remove fluoride contamination and to adjust
            the acid-water ratio for subsequent distillation.

        b.   After cooling the acid mixture remaining from steps outlined
            in (a), or previous distillations, to 120°C or below,  add 300
            ml of sample, mix thoroughly, and distill as before until the
            temperature reaches 180°C.   To prevent sulfate carry-over, do
            not permit the temperature to exceed 180°C.

        c.   After distillation of fluoride, flush the still with 300 ml
            distilled water.

C.  Electrode Method

    1.   Apparatus

        a.   Orion Model 801A ionalyzer

        b.   Single junction reference electrode

        c.   Fluoride electrode, Orion Model 94-09

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   2.  Reagents

       a.  Stock fluoride solution!   Dissolve 221.0 rag anhydrous sodium
           fluoride,  NaF, in distilled water and dilute to  one  liter
           (100  ppm fluoride).

       b.  Standard fluoride solutiont   Dilute  100 ml stock fluoride
           solution to one liter (10 ppm fluoride).

       c.  Total ionic strength adjustment buffer (TISAB).   Place
           approximately 500 ml distilled water in a one  liter  beaker.
           Add 57  ml  concentrated (glacial) acetic acid,  58 g sodium
           chloride,  NaCl, and 12 g sodium citrate dihydrate,
           Na^CgHcO,,  . 2H20.  Stir to dissolve.  Place the  beaker  in  a
           water bath (for cooling), insert a calibrated  pH electrode
           and reference electrode into the solution and  slowly add
           approximately 6 N sodium hydroxide (about 125  ml) until the
           pH is between 5*0 and 5«5«  Cool to  room temperature.   Put
           into  a one liter volumetric flask and add distilled  water  to
           the mark.

    3.  Procedure

       a.  Preparation of fluoride standards and standard curve:
           Measure 0, 1.0, 2.0, 3.0, 4.0, 5.0,  6.0, 8,0,  and 10,0, ml
           standard fluoride solution into a series of 100 ml volumetric
           flasks to produce fluoride concentrations of 0,  0.1, 0.2,  0,3,
           0.4,  0.5,  0.6, 0.8, and 1.0 rag/1. To each flask, add by
           pipet 50 ml TISAB solution and dilute to 100  ml with distilled
           water.  Mix well.

       b.  Preparation of Standard Curve.
           The solutions above were measured in duplicates and  the
           average calculated.  A plot of EMF versus part per million
           fluoride was made.

D.  Colorimetric  Method

    1.   Apparatus

          Coleman Model 124- Double Beam Spectrophotometer

    2.   Reagents

        a.  Standard fluoride solution*  Prepared essentially as directed
            in the electrode method,
                                     6

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        b.  SPADNS solutioni  Dissolve 958 mg SPADNS, sodium 2 (para
            sulfophenylazo)-!, 8 - dihydroxy - 3» 6 - naphthalene
            disulfonate, also called 4,5-dihydroxy -3-(para
            sulfophenylazo)-2,7-naphthalene disulfonic acid trisodium
            salt, in distilled water and dilute to 500 ml.  This solution
            is stable indefinitely if protected from direct sunlight.
            It was stored in a brown bottle.

        c.  Zirconyl-acid reagent*  Dissolve 133 rag Zirconyl chloride
            octahydrate, ZrOCl2 . 8HoO, in about 25 ml  distilled water.
            Add 350 ml concentrated HCL and dilute to 500 ml with
            distilled water.

        d.  Acid Zirconyl - SPADNS reagents  Mix equal volumes of SPADNS
            solution and zirconyl - acid reagent to produce a single
            reagent, which is stable for at least 2 yrs.

    3.  Procedure

        a.  Preparation of Standard Curvet
            Prepare fluoride standards in the range of 0 to 1.40 rag/1 by
            diluting appropriate quantities of the standard fluoride
            solution to 50 ml with distilled water.  Pipet 10.00 ml of
            the mixed acid-zirconyl-SPADNS reagent to each standard and
            mix well, exercising care to avoid contamination during the
            process.  Set the spectrophotometer to zero absorbance with
            distilled water in both curvettes.  Use distilled water as
            the reference solution.  Plot a curve of the fluoride con-
            centration -absorbance relationship at 570 nm.

E.  Sampling

    For the electrode method, the distillate or distillant (50 ml) was
    diluted to the mark in a 100 ml volumetric flask with TISAB.   For
    the colorimetric method, the distillate (5 ml) was diluted to 100 ml
    with distilled water.  In the case of sodium fluoride, the distillate
    or distillant (100 ml) was used.  The samples were measured in
    duplicates and averaged.  This process was carried out in triplicates,

F.  Standard Addition Method

    To 100 ml of sample, one millillter of 100 ppm sodium tetrafluoro-
    borate was added.  The electrode potentials were measured before
    and after the addition.  Only in the case of sodium tetrafluoroborate
    was two milliliters of 1000 ppm sodium tetrafluoroborate added.

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PHASE III

Calibration

A calibration curve was made for tetrafluoroborate at 0.5, 1.0, 5.0,
10.0, and 100 ppm in a polyethylene thermostatic water jacketed cell at
40°C.  A calibration curve was also made for fluoride at 0.01, 0.1, 1.0,
and 10.0 ppm fluoride at 40°C.  The pH meter was calibrated at 40°C with
a buffer solution.

Hydrolysis

Water (100 ml) at 40°C was placed in the cell and an aliquot equal to the
aliquot to be added was removed.  The electrodes were immersed, and the
aliquot added.  The electrodes were placed in the 40°C solution inter-
mittently, and the measurements were taken intermittently as measured by
the printer.  Standards were measured before intermittent readings.
Four ppm tetrafluoroborate were used.  Later^ 8 ppm were used in unbuffered
solutions.

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                              SECTION V

                              DISCUSSION

PHASE I

Reproducibility and Response Time

Tetrafluoroborate solutions (10~^M) were freshly prepared five times and
read in duplicates.  The standard deviation obtained for a concentration
of lO^^M was 0.27.

The dynamic response time was obtained by adding with a plastic syringe
one ml of a more concentrated solution to a rapid stirred solution to
give the desired decade change.  In the case of the decade change in
concentration of 10"4 to lO-^M, one cc of 9.1 x 10~2M sodium tetrafluoro-
borate (NaBFjj.) was added.  The time required to reach a relatively stable
potential as indicated by a recorder curve was around six seconds for
decade changes in fluoroborate concentrations.  For the fluoride elec-
trode, the response time for a decade change in concentration was some-
what slower than fluoroborate electrode.  The response time for a decade
change of 1(H* to 10"3n was about 1?,0 seconds,

Calibration Curve

A plot of potential against the logarithm of the concentration of fluoride
and fluoroborate was made from data in Tables 1 and 2.  Potentials were
measured in replicates with averages and standard deviations calculated,
According to the equations below, the slope of the calibration curves
(1) should be 59. 16 millivolts per decade.


          E  -  E1   - 2.3  RT  log   a                (1)
                             F
          E  -  E1   - 2.3  RT  log   a                (2)
                             F         F"
Where
          E  *  the measured total potential of the system.

          2' =•  the portion of the total potential due to choice of
                external reference electrodes and internal solutions.

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                         Table  1.  POTENTIALS VERSUS CONCENTRATION OF TETRAFLUOROBORATE
Concentration,
ppm
0.1
1.0
Calculated ppm
10.0
Calculated ppm
100.0
Calculated ppm
1000.0
Calculated ppm
mvj
198.9
176.8
1.08
123.1
9.69
65.7
101.50
10.0
991.70
mvg
202.7
176.3
1.10
122.9
9.77
65.2
103.60
10.2
983.60
mv~
221.2
180.2
0.94
124.4
9.63
65.4
102.76
9.6
1008.06
mv^
209.8
176.7
1.08
123.8
9.42
64.9
104.98
9.6
1008.06
mv,.
213.2
180.0
0.94
124.3
9.23
65.4
102.76
9.8
999.85
mv6
208.6
177.9
1.03
123.3
9.61
65.1
104.03
10.0
991.70
Average
209.0
178.0
1.03
123.6
9.58
65.3
103.20
9.9
997.00
Standard
Deviation
7.9
1.7
± 0.07
0.6
± 0.2
0.3
t 1.2
. 0.2
-± 9.7
H
O

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Table 2.  POTENTIALS VERSUS CONCENTRATION OF FLUORIDE
Concentration!
ppm
0.01
0.10
1.00
10.00
100.00
rav.
136.0
84.9
25.6
-32.4
-90.1
mv2
129.5
83.7
24.3
-33.2
-90.5
mv,j
135.7
84.4
24.9
-32.7
-89.6
"%
134.1
84.0
25.8
-32.7
-90.3
nv
137.9
85.7
27.9
-39.9
-88.7
mv6
135.5
84.7
26.6
-31.8
-89.3
Average
134.8
84.6
25.9
-32.3
-89.8
Standard
Deviation
2.9
0.7
1.3
0.8
0.7

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2.3  SI    -    nernst factor  (59.16 m at 25°C),
      F         R and F are constants and T is the temperature In
                degrees Kelvin

a   -      •=>    the fluoroborate  ion activy in the sample solution.
 BFjj,

a .        *    the fluoride ion  activity in the sample solution.
 F

The Nernstian-like behavior, illustrated in Figures 1 and 2, gave a
slope around  58 millivolts for fluoride electrode and  56 millivolts
for fluoroborate electrode.  The  lower curvature for fluoroborate
electrode was probably caused  by  the water solubility of the ion
exchanger.

Interferences

The electrode responds to certain other anions as well as fluoroborate
ions.  The selectivity constants  (If2,3f^t5) were calculated by two
methods.  Measurements were made  in duplicates and averaged.  The
selectivity for one ion over another is expressed in equation number  3«


               E  -  Ec   - 2.3   RT log (a  +  K   a )            (3)
                                  ~p      A     A,B B
Where             a   «  activity of ion BF^""


                   a   «  activity of interferent ion B
                    B

                   Kt n »  selectivity constant
                    A,B

When measurements are made in two solutions each containing the sodium
cation only at a concentration of 10"*-^Mf then


      E(MB)   -   E(MA)  -  - 2.3 RT log   K     (aB / a )          (4)
                                   F        A,B         A

hence the selectivity constant can be determined as (ag /  a )
                                                            A
approximately equal to one0  Results are shown in Table 3.


For the univalent anions, perchlorate, nitrate, and iodide appear to be
                                   12

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H
00
         1000
          100
        E
        Q.
        a
        it  10
        O
        O
           1.0
          0.1
                j	I
I	I
             0   24  48  72  96  120  144  168 192  226


                       POTENTIAL, mv


                       FIGURE 1.


     CALIBRATION-CURVE FOR  TETRAFLUOROBORATE
                                                              100
                                                               10
                                E
                                a
                                a
                                _  1.0
                                O
                                O
                                                               0.1
                                                              0.01
                                                                         I
                                    -96 -72  -48 -24  0   24  48  72   96  120  144


                                                  POTENTIAL, mv


                                                   FIGURE 2.


                                       CALIBRATION-CURVE  FOR FLUORIDE

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      Table 3.   SELECTIVITY CONSTANTS FOR FLUOROBORATE ELECTRODE

                              (Method 1)
Interferent
Ion
OV
Ac (acetate )
F"
Cl"
Br
l"
NO "
Difference
-42.65
139.60
154.40
146.55
118.95
55.75
96.75
K
5.25
4.37 x 10"3
2.46 x 10"3
3.33 x 10"3
9.76 x lo"3
1.14 x 10"1
2.32 x l(f-
|
the greatest interferents for the fluoroborate electrode.

The second method for calculating the selectivity constants involves a
constant concentration of 10~^ interferent ion and a decade change of
fluoroborate (10"-^ and lO'^M) ion.  In other words, two different
solutions with a decade change in fluoroborate ion concentrations and
same level of interferent ion concentration were measured.  The
selectivity constant was expressed in equation number 5.
          K
                                                        (5)
where
a.
                       activity  of fluoroborate in solution one.

-------
                     *=  activity of fluoroborate in solution two.
             a  l/zi  a  activity of interferent ion with anionic charge


                                 F/RT
V
Activity coefficients were calculated from the Debye Huckel Limiting Law
equation (6).  Results of method 2 are shown below in Table 4.
      Table 4.  SELECTIVITY CONSTANTS FOR FLUOROBORATE ELECTRODE

                              (Method 2)
Interferent
Ion
Metaborate (Bo0/,*5)
Borate (B/,07=)
Citrate (C^HcCy-)
Tartrate (CJi^Ox")
Phosphate (P0j5)
Sulfate (SO^")
Difference
57.4-5
58.60
58.65
58.85
58.20
60.30
K
3.53 x 10"3
3.53 x 10"3
1.26 x 10"3
3.52 x 10~3
1.2? x 10"3
3.53 x 10"3
A correlation of sulfate levels with tetrafluoroborate concentrations
was made.  (See Table 5).  The greatest potential difference between the
standard solution and the solutions containing both the 10 ppm tetra-
fluoroborate and sulfate concentrations was 1.8 mv.  From the calibration
curve» a difference of 3»9 mv gave 9*0 ppm tetrafluoroborate.  This
indicated an error around ten percent for the 10 ppm level of tetrafluoro-
borate.  For the one part per million tetrafluoroborate , the error was
somewhat larger as would be expected from the standard deviation given in
Table 1.  From the data, it is indicated that lower results will be
obtained for samples that contain more than 50 ppm sulfate in 10 ppm
tetrafluoroborate.  Also, lower results will be obtained for samples that
                                    15

-------
     Table 5.   CORRELATION OF SULFATE LEVELS WITH TETRAFLUOROBORATE
                              CONCENTRATIONS
                                                            Calculated
                10 ppm of Tetrafluoroborate    124.0 mv       9.34 ppm
                 1.0 ppra of Tetrafluoroborate  178.0 mv       1.025 PP»
Sulfate Concentration
(ppm)
250
Calculated ppm of BF^~
150
Calculated ppm of BF^
50
Calculated ppm of BF^
10
Calculated ppm of BF^~
1
f a!! culated ppm of BF^"
0.1
Calculated ppm of BF^
In 10 ppm BF/"
mv
125.8
8.6?
125.6
8.70
125.0
8.92
123.7
9.40
123.2
9.60
123.2
9.60
Difference
1.8
- 1.33
1.6
- 1.30
1.0
- 1.08
0.3
- 0.6
0.8
- 0.4
0.8
- 0.4
In 1.0 ppm BF^~
mv
183.6
0.81
183.3
0.82
183.0
O.R3
181.9
0.87
1°9.4
0.96
178.9
0.98
Difference
5.6
- 0.19
5.3
- 0.18,
5.0
- 0.17
3.9
- 0.13
1.4
- 0.04
0.9
- 0.02
contain more than 10 ppm sulfate in 1 ppm tetrafluoroborate.

A correlation of nitrate levels with tetrafluoroborate concentrations is
shown in Table 6.  From the table, it was noted that interference was
greater for lower concentrations of tetrafluoroborate than for higher
concentrations.  It can also be seen that as the concentration of the
interfering nitrate increased, the error increased.  For 1 ppm tetra-
fluoroborate, nitrate should be absent for accurate results.
                                    16

-------
   Table 6.  CORRELATION OF NITRATE LEVELS WITH TETRAFLUOROBORATE
                           CONCENTRATIONS

              10 ppm of Tetrafluoroborate    124,0 mv
               1.0 ppm of Tetrafluoroborate  178.0 mv
Nitrate Concentration
(ppm)
250
Calculated ppm of BF^~
150
Calculated ppm of BF^
50
Calculated ppm of BF^
10
Calculated ppm of BF^~
1
Calculated ppm of BF^~
,0.1
Calculated ppm of BF^~
In 10 ppm BF ~
mv
119.0
11.40
120.2
10.90
121.9
10.10
122.5
9.90
122.3
9.96
122.7
9.8
Difference
5.0
+ 1.4
3.8
+ 0.9
2.1
+ 0.1
1=5
- 0.1
1.7
0.1
1.3
0.2
In 1.0 ppm BF^"
mv
140.0
4.85
142.6
4.36
157.1
2.57
173.0
1.26
174.5
1.18
176.9
1.07
Difference
38.0
+ 3.85
35.4
+ 3.36
20.9
+ 2.41
5.0
+ 0.25
3.5
+ 0.18
1.1
+ 0.07
Potential measurements were also made as the pH was changed.  The pH was
varied by adding increments of 10~-*M sodium hydroxide and 10~-^M
hydrochloric acid to 100 ml of solution.  Volume corrections and cali-
brations were made.  From the data in Tables 7 and 8, Figures 3 and 4
were plotted for 1.0, 10, and 100 ppm tetrafluoroborate and fluoride,
respectively.  Little potential change was noted between pH 3 and pH 10
for the tetrafluoroborate concentrations.  Some results may be affected
if the sample is greater than pH 6 for 100 ppm tetrafluoroborate.  Lower
results are obtained below pH 3.  For the fluoride electrode, the pH
effect on the electrode response was small between pH 4 and pH ?,
depending on the concentration.
                                    17

-------
                               Table ?.  pH VERSUS POTENTIALS OF TBTRAFLUOROBORATE
                                               Concentrations (ppm)

PH
2.00
2,15
3.19
4.16
5. 20
6.20
6.9?
6.02
3.97
9.99
11.06
12001
1
mv
162.9
165.6
177.1*
178.6
179.0
177.1
179.1
177.1
1?4.5
176.5
17**. 0
177.4
.0
Calculated ppm
1.89
1.69
1.04
0.99
0.98
1.06
0.97
1.06
1.18
1.08
1.20
1.04
10
pH
1.99
2.15
2.98
3.48
4.84
6.34
7.51
8.84
10.11
10.99
12.02

mv
117.4
119.2
121.7
122.1
123.0
122.3
123.5
124.0
126.9
128.5
133.0

Calculated ppm
12.20
11.30
10.20
10.00
9.67
9.96
9.48
9.29
8.25
7.73
6.70

100
PH
2.00
3.01
4.13
5.06
6.07
7.23
8.95
9.78
10.72
10.81
11.08
12.01
mv
60.9
65.2
66.0
66.4
66.8
67.7
68.1
68.3
69.6
70.0
71.0
77.6
Calculated ppm
122.90
103.60
99.80
98.20
96.60
93.00
91.00
90.80
86.50
85.00
81.70
62.40
CO

-------
  180
  160
  140
  120
 £  100
5  80


O
Q.
   60
   40
   20
                    lOppm
                    lOOppm
                 I	I
                      6



                      PH
                                 10    12
                  FIGURE  3.

          pH RESPONSE CURVES  FOR

TETRAFLUOROBORATE AT VARIOUS CONCENTRATIONS
                                                         80
                                                         60
                                                         40
                                                         20
                                                      E  0
2 -20

O
a.
                                                        -40
                                                        -60
                                                        -80
                                                       -100
                       6



                     PH
                                                                                       10     12
                  FIGURE 4.

      pH RESPONSE CURVES FOR FLUORIDE

        AT VARIOUS CONCENTRATIONS

-------
Table 8.  pH VERSUS POTENTIALS OF FLUORIDE
          Concentration (Part-Per-Million)
1.0 ppm
PH
2.00
2.95
3.57
4.18
5.78
5.90
6.56
8.25
8.90
9.49
9.99
10.51

mv
89.9
45.2
30.8
25.8
25.9
25.9
27.2
17.7
5.6
-30.9
-48.7
-65.5

10 ppm
PH
2.00
2.67
2.94
3.85
4.39
5.86
5.95
7.07
8.53
8.92
9.52
10.01
11.12
mv
35.7
0.3
-10.5
-29.9
-31.4
-32.3
-32.3
-31.9
-34.1
-37.9
-47.7
-57.0
-71.6
100 ppm
PH
2.16
3.00
3.96
5.00
5.96
5.98
6.8?
9.01
9.91
10.51



mv
-43.9
-73.8
-88.3
-89.8
-89.8
-89.8
-89.9
-89.8
-90.7
-91.6



                      20

-------
SUMMARY

The response time, reproducibility, calibration curve, and interferences
of the tetrafluoroborate electrode were determined.  The time required
to reach a relatively stable potential was around 6 seconds for decade
changes in fluoroborate concentrations.  The calibration curve was
Nernstian-like down to about 0.6 ppm of tetrafluoroborate.  The slope
of the curve was around 56 millivolts for the tetrafluoroborate elec-
trode.

The univalent anions, perchlorate, iodide, and nitrate gave the greatest
interference as indicated by the calculated selectivity constants.  Of
the divalent anions tested, metaborate (I^O^), borate (B^P^=),
tartrate (C^H^0?*"), and sulfate (SO^) gave approximately the same
selectivity constants.  Further studies of the nitrate and sulfate anions
in 1 and 10 ppm tetrafluoroborate gave an indication of the levels of
interference.  For instance, lower results than expected were obtained
for samples containing more than 50 PP"i sulfate in 10 ppm tetrafluoro-
borate.  Lower results were also obtained for samples containing more
than 10 ppm sulfate in 1 ppm tetrafluoroborate.  In the case of the
nitrates in 1 ppm tetrafluoroborate, accuracy would be obtained if
nitrates were absent.

In the study of pH, higher results were obtained for samples below pH 3»
Depending on the tetrafluoroborate concentration, little change in the
tetrafluoroborate electrode potentials was noted between pH 3 and pH 10.
However, hydrolysis was occurring rapidly between the two pH limits.
This was indicated by the need to obtain the potentials of the tetra-
fluoroborate electrode at a specific time interval after addition of the
acid or base.  This also gave tetrafluoroborate the appearance of being
amphoteric.
                                   21

-------
PHASE II

Calibration curves for the colorimetric method (SPADNS) and fluoride
electrode methods are shown in Figures 5 and 6.  The calibration curve
for the fluoride was made in a solution containing the total ionic
strength buffer.  The curvn was linear down to 0.2 ppm fluoride in the
TISAB solution.  The colorimetric calibration curve was linear down to
0.6 ppm fluoride or 1.3 ppm sodium fluoride.  At zero fluoride concen-
tration, this measurement was made on distilled water and the mixed
colored reagent.  (See Table 9.)

In Table 10t a comparison of the colorimetric method before and after
distillation was made on sodium fluoride.  There was essentially no
difference between the average part per million fluoride before and after
distillation.  In relation to the fluoride electrode, there was about 406
difference.  It was also noted that the fluoride electrode measurements
after distillation were less than the colorimetric measurements.

Comparisons of methods for sodium tetrafluoroborate are shown in Table 11*
Starting with four parts per million (ppm) tetrafluoroborate (TFB),
distillation should give around 3«50 ppm fluoride.  Comparing the 3«50
ppm fluoride with 3«^2 ppm and 3»15 ppm» the fluoride recovered from the
TFB is about 9&%> for colorimetric method and 90# for fluoride electrode
method.  The amount of fluoride was also determined before distillation
(distillant).  Before distillation, the fluoride content was around an
average of 0.41 ppm.  Calculating the fluoride content using the TFB
electrode, a fluoride content of 2.98 ppm was obtained.  A total fluoride
content of 3,^2. and 3«15 PPm was obtained by the colorimetric and
fluoride electrode methods, respectively.  If the amount of fluoride
found before distillation was subtracted from the total fluoride content
(3.42 - 0.41 - 3.01), the fluoride content from the TFB was 3.01 ppm.
This gave rise to an error of one percent between the calculated amount
of fluoride from the TFB electrode and the colorimetric total fluoride
content minus the fluoride content measured before distillation by the
fluoride electrode.  On the other hand, the fluoride electrode gave 2,?4
ppm fluoride from the TFB.  However, this gave an error around 8# for the
fluoride electrode using the same difference technique.  Colorimetric
analysis of the fluoride content in TFB before distillation was not
possible.  It was noted that within approximately two hours after the
addition of the mixed reagents, the absorbance was almost identical to
the TFB solution after distillation.  This may be caused by the zirconium
and hydrochloric acid mixture which may speed up the hydrolysis process.
However, it may be possible to make colorimetric measurements on TFB
solutions at quick specific time intervals.  The amount of fluoride before
distillation became larger as the TFB solution ages.  However, a fresh
stock solution and subsequential dilutions gave around 0.18 ppm fluoride
before distillation.
                                    22

-------
Table 9.  DATA FOR CALIBRATION CURVES
Fluoride
Concentration
(ppm)
Blank
0.1
0.2
0.3
0.4
0.5
0.6
0.8
1.0
5.0
10.0
Absorbance
0.751
0.720
0.68?
0.651
0.621
0.590
0.562
0.521



Potentials
(MV)

101.0
84.9
74.8
67.3
62.0
57.2
49.5
43.6
1.9
-15.4
                   23

-------
       0.8
ro
       0.4
       0.3
       0.2
        0.1
                            I
                  0.2
                           0.4


                       FLUORIDE, ppm
                                    0.6
                                             0.8
                       FIGURE 5.

          COLORIMETRIC CALIBRATIONS CURVE

                      FOR FLUORIDE
                                                                10.0 —
                                                                 5.0 —
                                                              E
                                                              Q.
                                                              a
                                                              O
                                                              O
         I   I    I    I    I    I    I   I  VI
                15   30  45  60  75  90  105
                POTENTIAL, mv



                FIGURE 6.


CALIBRATION CURVE FOR  FLUORIDE IN TISAB

-------
Table 10.  COMPARISON OF METHODS FOR SODIUM FLUORIDE



                Before Distillation
Absorbance
0.665
0.66?
0.665

Absorbancc
0.664
0.665
0.670
From
Curve
0.265
0.260
0.265
After
From
Curve
0.265
0.265
0.250
Distillant
Concentration
0.53
0.52
0.53
Distillation
Distillate
Concentration
0.53
0.53
0.50
Average
Standard Deviation
0.53 ± 0.01



Average
Standard Deviation
0.52 I 0.02


Fluoride Electrode After Distillation
MV
77.7
81.5
79.8
From
Curve
0.270
0.232
0.250
Distillate
Concentration
0.5^
0.46
0.50
Average
Standard Deviation
0.50 1 0.04


                          25

-------
    Table 11.  COMPARISON OF METHODS FOR SODIUM TBTRAFLUOROBORATE
MV

149.1
148.8
149.6
From
Curve

 3.4
 3.5
 3.3
                     Tetrafluoroborate Electrode
                       Average
                       Standard Deviation

                          3.40 i  0.10
                         Colorimetric Method

Absorbance
0.640
0.638
0.640
From
Curve
0.340
0.345
0.340
Distillate (F**)
Concentration
3.40
3.45
3.40
Average
Standard Deviation
3.42 - 0.03


                          Fluoride Electrode

MV
32.1
31.6
32.6
From
Curve
1.58
1.60
1.55
Distillate
Concentration
3.15
3.20
3.10
Average
Standard Deviation
3.15 - 0.05


               Fluoride Electrode Before Distillation
MV

82.5
74.9
105.0
From
Curve

0.22
0.31
0.09
Distillant (F**)
Concentration

    0.44
    0.62
    0.18
Average
Standard Deviation

  0.41 t  0.22
                                    26

-------
Copper, tin, lead and 60$ tin - 40$ lead TFB results are shown in Tables
12, 13» 1^» and 15, respectively.  Using the difference technique, tin
and the 60-40 mixture gave the largest errors in fluoride content.  These
errors were greater than 10#.  Any metal in the electromotive series will
displace a metal below it (a less active metal) from its salt in water
solution.  Considering the metals tin, lead, and copper, tin is the most
active and probably will displace lead in solution.  In the case of the
metallic tetrafluoroborates, the measurements on the tetrafluoroborate
electrode would be expected to be low if there were some strong com-
plexing occurring.  This may be the cause of the tin tetrafluoroborate
potentials, as measured with the fluoroborate electrode, to be low.

A summary of the correlation of the fluoride contents is given in Table
16,  It appeared as if the tetrafluoroborate electrode and the fluoride
electrode could be used in some instances to determine fluoroborate and
fluoride concentration in distilled water.  For the case of four parts
per million tetrafluoroborate, it is expected that 3*50 PPo of fluoride
would be formed from four parts per million tetrafluoroborate.  The
tetrafluoroborate electrode gave J,^Q ppm fluoroborate which corresponds
to about 2.98 ppm fluoride.  The amount of fluoride present as measured
with the fluoride electrode was an average of 0,41 ppm.  This gave a total
of 3.39 ppm fluoride from both the fluoride present and that calculated
from the 3»^ PPm tetrafluoroborate.  The percent difference between the
expected fluoride from four ppm tetrafluoroborate and the total fluoride
obtained from measurement was 3«1^«  However, tetrafluoroborate electrode
cannot be used directly in the presence of interferents or complexed
fluoroborate ions.  It might be possible to use the standard addition
method.

The concentration of the TFB solution used for standard addition (see
Table 1?) were calculated using the equation (2).
                                        E/S                   i
               Cx - Cs (Vs/Vx + Vs)[ 10     -  (Vx + Vs/Vx)l
Where Cs is the standard solution concentration, Cx is the unknown con-
centration, Vs is the volume of the standard addition aliquot, Vx is the
test solution,  E is the difference in potential and S is the slope.  In
the above equation, the slope used was 56 millivolts.  The concentration
calculated compared favorably with the background subtracted fluoride
contents for the fluoride electrode.  In Table 16, the standard addition
results for sodium tetrafluoroborate cannot be compared favorably to the
background subtracted fluoride concentration because the measurements were
made in different standard solutions.
                                    27

-------
Table 12.  COMPARISON OF METHODS FOR COPPER TETRAFLUOROBORATE
Tetrafluoroborate Electrode
MV
142.1
142.6
144.5

Absorbance
0.590
0.665
0.664

MV
20.0
19.9
20.1
From
Curve
4.6
4.4
4.1

From
Curve
0.495
0.265
0.265

From
Curve
2.50
2.50
2.50
Average
Standard Deviation
4.37 t 0.25
Colorimetric Method
Distillate
Concentration
4.95
5.30
5.30
Fluoride Electrode
Distillate
Concentration
5.0
5.0
5.0
Percentage
Tetrafluoroborate
36.42*

Average
Standard Deviation
5.18 - 0.2

Average
Standard Deviation
5.0 - 0.0
Fluoride Electrode Before Distillation
MV
58.4
53.8
51.6
From
Curve
0.57
0.69
0.75
Distillant
Concentration
1.14
1.38
1.50
Average
Standard Deviation
1.34 - 0.18
                               28

-------
      Table 13.  COMPARISON OF METHODS FOR TIN TETRAFLUOROBORATE
                     Tetrafluoroborate Electrode
MV

150.4
152.2
153.3
From
Curve

 3.3
 3.0
 2.9
Average
Standard Deviation

   3.0? ± 0.21
                                         Percentage
                                         T etrafluoroborate

                                              25.58^
                         Colorimetric Method
Absorbance
0.664
0.673
0.665
From
Curve
0.265
0.245
0.265
Distillate
Concentration
5.30
4.90
5.30
Average
Standard Deviation
5.1? t 0.23
MV

18.9
18.9
20.1
From
Curve

2.60
2.60
2.50
              Fluoride Electrode

               Distillate
               Concentration

                   5.20
                   5.20
                   5.00
                          Average
                          Standard Deviation

                             5.13 i 0.12
                Fluoride Electrode Before Distillation
MV

54.6
40.4
36.7
From
Curve

0.67
1.14
1.31
Distillant
Concentration

    1.33
    2.27
    2.62
                                         Average
                                         Standard Deviation

                                            2.0? - 0.6?
                                   29

-------
     Table 14.  COMPARISON OF METHODS FOR LEAD TETRAFLUOROBORATE
MV

143.1
143.^
142.9
From
Curve

4.4
4.3
4.4
Tetrafluoroborate Electrode

      Average
      Standard Deviation

         4.37 i 0.06
Percentage
Tetrafluoroborate
                         Colorimetric Method

Absorbancn
0.680
0.6??
0.6?4
From
Curve
MMMMMB^B
0.220
0.225
0.235
Distillate
Concentration
4.40
4.50
4.70
Average
Standard
4.53


Deviation
i 0.15

MV

24.9
24.1
24.6
From
Curve

2.05
2.15
2.10
     Fluoride Electrode

      Distillate
      Concentration

          4.10
          4.30
          4.20   '
Average
Standard Deviation

   4.20 i 0.10
                Fluoride Electrode Before Distillation
MV

75.8
67.7
64.6
From
Curve

0.29
0.40
0.45
      Distillant
      Concentration

          0.58
          0.80
          0.90
Average
Standard Deviation

   0.76 t 0.16
                                    30

-------
Table 15.  COMPARISON OF METHODS FOR 60% TIN-40# LEAD TETRAFLUOROBORATE
MV

150.2
150.4
151.5
From
Curve

3.3
3.3
3.1
Tetrafluoroborate Electrode

      Average
      Standard Deviation

         3.23 i 0.12
            From
Absorbance  Curve
             Colorimetric Method

               Distillate
               Concentration
                             Average
                             Standard Deviation
0.678
0^674

MV
22.9
21.8
22.0

MV
39.2
38.0
38.6
0.225
0.235
0.235

From
Curve
MBi*»^Bm^^
2.25
2.35
2.33
Fluoride
From
Curve
1.20
1.25
1.23
4.50
4.70
4.70
Fluoride Electrode
Distillate
Concentration
4.50
4.70
4.66
4.63

- 0.12

Average
Standard Deviation
4.62
1 0.11
Electrode Before Distillation
Distillant
Concentration
2.40
2.50
2.46
Average
Standard
2.45
Deviation
£ 0.05
                                   31

-------
                           Table 16.  CORRELATION OF TETRAPLUOROBORATE ELECTRODE,
                        SUBTRACTED BACKGROUND FLUORIDE CONTENT AND STANDARD ADDITION
Solution
NaBF^

Cu(BF^)2

Sn(BF4)2

Pb(BF4)2

60% Tin -
40)6 Lead

Electrode
3.40.

4.3?

3.0?

4.3?

3.23

Calculated
Fluoride
Expected
2.98**

3.83**

2.69**

3.83**

2.83**

Total
Fluoride
3.42
3.15*
5.18
5.00*
5.17
5.13*
4.53
4.20*
4.63
4.62*
' I
Background
Subtracted
3.01
2.74*
3.84
3.66*
3.10
3.06*
3.77
3.44*
2.18
2.17*
Standard
Addition

3.27**

3.46**

2.85**

3.48**

2.54**
Percent
Difference
1.0
8.0*
0.3
4.4*
15.2
13.8*
1.6
10.2*
23.0
23.3*
ro
       *    Fluoride electrode method

       **   Fluoride concentration calculated from TFB electrode measurements

-------
SUMMARY

The colorimetric method (SPADNS) was compared with the fluoride and
tetrafluoroborate electrode methods.  The calibration curve for the
fluoride electrode in total ionic strength buffer was linear down to
0.? ppm fluoride.  The colorimetric calibration curve was linear down to
0.6 ppm fluoride.

In comparing fluoride concentrations before and after distillation in
standard sodium fluoride solutions, there were essentially no difference
in the average ppm of fluoride.  However, the fluoride electrode gave
slightly lower results than the colorimetric measurements.  After distil-
lation of 4 ppm tetrafluoroborate solution, the total fluoride recovered
was 98$ for the colorimetric method and 90# for the fluoride electrode
method.  In sodium tetrafluoroborate, the fluoride concentration (as
calculated from the measurements of the tetrafluoroborate electrode) and
the fluoride content present before distillation (as measured with the
fluoride electrode) were summed to be approximately equal to the total
colorimetric fluoride content.  Among copper, tin, lead, and 6($ tin -
40/6 lead tetrafluoroborates, the tin and the 60-40 mixture gave the
largest difference in fluoride content when comparing the tetrafluoro-
borate electrode method with the colorimetric fluoride content minus
fluoride electrode content before distillation.  The standard addition
method also compared favorably with the background subtracted fluoride
content from the fluoride electrode.
                    Table 17.  STANDARD ADDITION METHOD

                                 Average                 Concentrations,
Solutions                 Potential Differences               ppm BF4


NaBF4                             44.60                         3.74

Sn(BF4)2                           6.63                         3.26

Cu(BF4)2                           5.63                         3.95

Pb(BF4)2                           5.60                         3.98

60% Sn(BF4)2 - 40% Pb(BF4)2        7.33                         2.90
                                   33

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PHASE III

This phase involved the feasibility of using both the fluoride and fluoro-
borate electrode to observe the hydrolysis of fluoroborate in aqueous
solution.  Since fluoroborate is expected to give rise to fluoride upon
hydrolysis (8,9,10,11) the fluoride electrode would measure the amounts
of fluoride formed.  Because the hydrolysis appeared to be very slow at
25°C and at high concentrations (1000 ppm), a concentration of 4 ppm
was observed at 40°C.  Temperatures above 50°C were not recommended for
the tetrafluoroborate electrode.

Calibration curves were constructed for both electrodes at 40°C.  The
data are presented in Table 18.  In Table 19, potential - time data are
presented for tetrafluoroborate in water.  The pH was also recorded
because pH affects both electrodes to a certain extent.  However, the
fluoride electrode was most affected.  From Table 19» the pH and the
tetrafluoroborate concentration were decreasing, and the fluoride con-
centration was somewhat increasing.  After 12 days, it appeared as if
only 40# of tetrafluoroborate was loss at 4 ppm and about 2Q# was loss
at 8 ppm.  However, this can not be taken too seriously because of errors
involving pH and limited samples.  After approximately 24 hours, a white
deposit was noted in the polyethylene cell.  This was not further investi-
gated.

Clark and Jones (12) used sodium citrate to quench the reaction after
aliquots were taken in his study of catalyzed hydrolysis of tetrafluoro-
borates.  Then, the solutions were buffered.  A buffer probably should be
used with care.  Care should be taken to make sure the buffer does not
interfere with the reaction or electrode measurements.  A buffer would
also raise the detection limit for the liquid ion exchange tetrafluoro-
borate electrode.

BF4
Concentration,
ppm
0.5
1.0
5.0
10.0
100.0
Table 18. CALIBRATION
Average
Potential
MV
210.3
203.1
162.4
145.1
83.5
CURVES DATA AT 40°C
F
Concentrations ,
ppm
0.01
0.1
1.0
10.0


Average
Potential,
MV
188.4
137.3
66.7
8.2


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SUMMARY

In the hydrolysis of tetrafluoroborate, the tetrafluoroborate concen-
tration and the pH were decreasing, and the fluoride concentration was
increasing.  The use of the tetrafluoroborate electrode and fluoride
electrode together to determine the hydrolysis concentrations became a
problem as the pH decreased.  The use of a buffer in some manner may
eliminate one aspect of the problem.  But, a buffer probably would cause
the tetrafluoroborate electrode not to respond below 2 ppm.
Table 19.  POTENTIAL - TIME DATA FOR TETRAFLUOROBORATE IN WATER AT 40°C
Time,
hours
0
24
48
72
96
120
144
168
192
240
264
288
312
BF4
MV
171.5
168.2
169.2
168.6
171.9
171.5
172.2
175.3
175.2
179.2
178.8
180.1
182.3
Concentration ,
ppm
3.4
3.9
3.9
3.9
3.4
3.4
3.4
2.9
2.9
2.5
2.5
2.4
2.3
F
MV
132.2
126.5
128. 4a
87.9
89.4
84.9
76.6
62.4
74.0
67.9
26. 4a
43.6
50.1
Concentration,
ppm
0.12
0.14
0.13
0.54
0.52
0.60
0.82
1.40
0.90
1.10
5.00
2.60
2.10
pH
5.483
5.028
4.783
4.483a
4.618
4.506
4.461
4.355
4.299
4.204
4.178
4.103
4.096
  appear to be artifacts
                                   35

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                              SECTION VI

                              REFERENCES

 1.   Durst,  R. A.   Ion-Selective Electrodes.  National Bureau of
     Standards, Washington, D.C.  Publication Number 3140   1969.

 2.   Rechnitz, G.  A.c M. R. Kresz, and S. B. Zamochnick.   "Analytical
     Study of an Iodide-Sensitive Membrane Electrode." Anal, Chem.
     38i973.  1966.

 3»   Rechnitz, G.  A0  "Ion-Selective Electrodes."  Chen. Eng. News.
     45j146  1957.

 4.   Srinivasan, K. and G0 A. Rechnitz„  "Selectivity Studies on Liquid
     Membrane, Ion Selective Electrodes."  Anal, Chem. 41il203.  1969.

 5.   Krull, I. H., C. Ac Mask, and R. E. Cosgrove.  "A Solid Potassium
     Ion Selective Electrode."  Anal. Letters.  3i43.  1970.

 6.   Meites, L.  Handbook of Analytical Chemistry0  1st edition.  New
     York, McGraw-Hill, Inc., 1963»

 ?.   Standard Methods for Examination of Water and Wastevrater.  13th
     edition.  Washington* D. C., American Public Health Association,
     1971.

 8.   Wamser, Christian A.  "Hydrolysis of Fluoboric Acid in Aoueous
     Solution."  Journal of the American Chemical Society.  70»1209.
     1948.

 9.   Wamser, Christian A.  "Equilibria in the System Boron Trifluoride-
     Water at 25°C.  Journal of the American Chemical Society.  73«409.
     1951.

10.   Anbar, M. and S. Guttmann.  "The Isotopic Exchange of Fluoroboric
     Acid with Hydrofluoric Acid."  J. Physical Chem.  64*1897.   I960.

11,   Ryss, I. G.  "The Chemistry of Fluorine and its Inorganic Compounds."
     United States Atomic Energy Commission. AEC-tr-3927  (pt.2).  1956.

12.   Clark, Howell R. and M. M. Jones.  "Ligand Substitution Catalysis
     via Hard Acid-Hard Base Interaction."  Journal of the American
     Chemical Society.  92»8l6.  1970.
                                    36

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                                    TECHNICAL REPORT DATA
                             (Please read Instructions on the reverse before completing)
 1. REPORT NO.

 EPA-600/4-76-005
                              2.
                                                             3. RECIPIENT'S ACCESSION-NO.
4. TITL>= AND SUBTITLE
    Determining Tetrafluoroborates:  An Evaluation of

    Fluoroborate Anion  Selective Electrode
               5. REPORT DATE
                 March 1976  (Issuing Date)
               6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)

    Benjamin T. Duhart
                                                             8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS

    Bennett  College
    Greensboro, North Carolina  27420
                Q. PROGRAM ELEMENT NO.
                P.E.     TtOAP      Task
                IPAQ??	09ABZ	18
                1JCQ$fO3)W£>C/
                         I/GRANT NO.
                                                                 R-803006-01-1
 12. SPONSORING AGENCY NAME AND ADDRESS
                                                             13. TYPE OF REPORT AND PERIOD COVERED
    Environmental Monitoring  and Support Laboratory
    Office  of Research and  Development
    U.S.  Environmental Protection Agency
    Cincinnati,  Ohio  45268
                    Final Report
               14. SPONSORING AGENCY CODE

                    EPA-ORD
 15. SUPPLEMENTARY NOTES
 18. ABSTRACT

          The Orion fluoroborate electrode  was evaluated  to determine its
    applicability to water and waste.  The calibration curve was Nernstian
    down  to 0.6 mg/1 and  the slope of the  curve was 56 millivolts per decade
    change.  Interference of nitrate and sulfate was studied.  Low results
    were  obtained for samples that contained 50 mg/1 of  sulfate and 10 mg/1
    tetrafluoroborate or  10 mg/1 sulfate and 1 mg/1 tetrafluoroborate.  To
    determine 1 mg/1 of tetrafluoroborate,  nitrate should be absent.  The
    fluoroborate electrode can be used directly to determine some selected
    fluoroborates.
17.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
 b.IDENTIFIERS/OPEN ENDED TERMS  C. COSATI Field/Group
    fluorides, electrodes
    water analysis, monitors
  fluoroborate
  tetrafluoroborate
  copper  tetrafluoroborate
  tin tetrafluoroborate
  lead  tetrafluoroborate
                                                                                13B
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