ATOMIC ABSORPTION  ANALYSIS OF  PHOSPHATES  IN  WATER

Calvin  O.  Huber

University of  Wisconsin
Milwaukee,  Wisconsin

October  1973
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                                                  BPA-670/2-73-079
                                                  October, 1973
ATOMIC ABSORPTION ANALYSIS OF PHOSPHATES IN WATER

                        by

                  Calvin 0. Huber

             Department of Chemistry

        University of Wisconsin-Milwaukee

           Milwaukee,  Wisconsin  53201


                Project 16020 DHD

 Program Element  »  310401  (1972),  IB1027 (1973)


                Project Officer:

               Mr.  Robert  L.  Booth

      Analytical Quality Control  Laboratory

     National Environmental Research Center

             Cincinnati, Ohio  45268
                  Prepared For:

       OFFICE  OF  RESEARCH AND MONITORING

     U.S. ENVIRONMENTAL PROTECTION AGENCY

            WASHINGTON,  D.C.   20460
                                      -670/2-73-079, October, 1973.
StLECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
    ue
      Atomic Absorption Analysis of Phosphates in Water
7. Aatbot(l)
     Huber, C.O.
                Department of Chemistry.
                               -—
                                                                10.  Pro/tctlfo.
                                                                   /403LO
                                                               II. Coatrtct/GraatMo.
is. AtMrict       The project investigatwf»utilization of phosphate,  silicate,

           and sulfate chemical inhibition effects in magnesium  atonic absorption

           spectrometry for the purpose of determination of  these anions.   The

           variables found to be of greatest significance in the inhibition pro-

           cesses were flame temperature and solution stoichiometry.   The  procedure

           termed 'atomic absorption inhibition titration1  (MIT) was  developed.  It

           provides useful, new determination methods for single anions and also for

           simultaneous determination of phosphate, silicate,  and sulfate  in  a

           single sample.  The methods have been  evaluated  and applied to  water and

           waste water samples.
                                                                                17*.Descriptors       *AluBili. Absorption,  'Inhibition Titfation.,  *Phosphates, *Water
                                                                                                     i  X        -*          */) fln  ~l-   i   <•
                                                                                              Analyses, Sulfates,  Silicates.
                                                                                nb.idtatiSa,   *Pheii|jlinl8t,  'Inhibition Titration,  Wa«o> S  Wactac
                                                                                                          ^
Uc. CO WRR Field A Group   £) 5 /\
                                                                                     vailability
                                                                                     ? Trnm flic Naf)n
                                                                                                        19.
                                                                                                           (Rtpcrt) -•••
                                                                               paper cop/, |1.45 in mlgmfl.k.
                                                                                                                          t JTO.O/  ,  Send To;
                                                                                                                         33. Price
                                                                                                                                    WATER RESOURCES SCIENTIFIC INFORMATION CENTER
                                                                                                                                    US DEPARTMENT OF THE INTERIOR
                                                                                                                                   ; , WASHINGTON. D. C 2O24O
                                                                                          Robert  L. Booth
                                              AQCL/NERC, USEPA
                                                                               wRsrc 102 (nev

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                         ABSTRACT

     The project investigated utilization of phosphate,
silicate and sulfate  chemical inhibition effects in atomic
absorption spectrometry for the purpose of determination of
these anions.  The magnesium atomic absorption signal was
used.  The variables  found to be of greatest significance
in the inhibition processes were flame temperature and
solution stoichiometry.  As a result of the latter, the
procedure termed atomic absorption inhibition titration,
AAIT, was developed.   It involves constant rate addition
of magnesium to the stirred sample solution while monitoring
the latter solution for magnesium atomic absorption.  Rather
remarkable titration  curve shapes are obtained, especially
for solutions  containing all three anions.  Based on such
titration curves, useful new determination methods were
developed for  single  anions and also for simultaneous
determination  of the  three anions present in a single
sample.
     The methods developed as applied to water and waste
water have been investigated and found to be suitable in
most respects  and superior to present methods in some.
     This report was  submitted in fulfillment of Research
Grant * 16020  DHD by  the University of Wisconsin-Milwaukee
                             ii
                                                                                  under the partial  sponsorship of the Office of Research and Monitoring,
                                                                                  U.S. Environmental Protection Agency.  Work was completed as of July,  1973.
                                                                                                                 iii

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                       TABLE OF CONTENTS


                                                       Page

 ABSTRACT	ii

 LIST OF FIGURES  ....... 	  V

 LIST OF TABLES	 vi

 ACKNOWLEDGMENTS	 vii

 CONCLUSIONS	viii

 RECOMMENDATIONS  .............  	  .  x

 INTRODUCTION  ..........  	  1

EXPERIMENTAL	4

RESULTS   .....  	  .......  	  6

DISCUSSION 	  ......... 32

REFERENCES	 34

LIST OF PUBLICATIONS	 35
                                                                                                       LIST OF FIGURES
                                                      Pages
Figure
   1   Inhibition and Gas Flow Hates	9-10

   2   Titration of Blank and 8 ppm Orthophosphate  .  .12-13

   3   Effect of Air:Hydrogen Flow Ratio, R,
       on Titration Curve	25-26

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                          LIST OF  TABLES
Table

  I


 II

III


 IV


  V

 VI

VII
                                                     Page
Titration PO
                   Mole  Ratios for
J. .L l^, VI..I.WA1 ^u./lly rawXC  rUil-4.UEl  1.UX
Solutions Containing Phosphate  and Sulfate ...  16

River Hater  Analysis	18

Determination of Phosphate in Detergent
Products by  AMT	19

Determination of Silica in Artificial
Drinking Hater 	  21

Determination of Sulfate by AAIT	23

Data  Comparing AAIT and Gravimetric Methods  .  .28

Simultaneous Determination of SiO,, PO.,
and S04 in Haste Hater	31
                             ACKNOWLEDGMENTS




     Mr. Walter Crawford, Dr.  Robert Looyenga, and Dr.  Chuang-I Lin were

the co-investigators in this work.  Their contributions were absolutely

indispensible to its success.



     The help and liaison provided by project officers,  Mr. Robert L. Booth

of the Analytical Quality Control Laboratory, and Dr. George W. Bailey of the

Southeast Water Research Laboratory, is gratefully acknowledged.



     The project was supported by the Office of Research and Monitoring,

U.S.  Environmental Protection  Agency.
                                                                                            The cooperation and assistance of The Center for Great Lakes Studies of

                                                                                        this campus was helpful.

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                         CONCLUSIONS





     1.  New and  improved  analytical methods  for  analysis



of common anions  in water  are  achieved  to meet  the  challenges



of speed, accuracy, convenience,  sample deterioration,



precision, and cost.






     2.  Flame spectroscopic chemical inhibition  effects can



be a basis for such improved methods.






     3.  Flame temperature and solution ion ratios  are the



most significant  parameters for utilizing the chemical



inhibition methods.  Solution  ion ratios are  most readily



regulated by use  of a titration technique.






     4.  Phosphate, silicate,  and sulfate can be  determined



in real water and waste water  samples,  separated  or in



combination.  A procedure requiring less than ten minutes



allows the determination of these three anions  in a single



fifty  milliliter  sample.






      5.   Chemical inhibition processes  in flames  are shown



 to be rate  controlled in the depleting  droplet  and  resulting



 particle rather than controlled by solution equilibria.
                           viii
     6.   The success in achieving these improved analytical



methods indicates that further research should be pursued to



exploit the above advances for water analysis problems of




various types.
                                                                                                              ix

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                       RECOMMENDATIONS


     An advantageous new technique has been made  available

for water analysis.  As a result, the following specific

courses of action regarding it  are recommended.


     1.  The method should be applied by  a number of

analytical chemistry laboratories for increased efficiency

and for in-the-field evaluation of its capability.


     2.  The method should be completely  automated by means

of automatic sample handling and data acquisition and

treatment computer equipment.


     3.  Simple minimal adequate design flame  photometric

apparatus should be developed or applied  to exploit the cost

and dependability advantages of the  inherent simplicity of

this new method.


     4.  The method should be extended to other anions,

flame emission and fluorescence signals,  and analysis of

cations.


     5.  Finally, the technique provides  a new tool for

investigation of flame chemical inhibition reactions and

very high temperature rate processes and  stoichiometry.

These capabilities should be applied to fundamental studies

in these areas of chemistry.

                              x
                        INTRODUCTION
     The importance for phosphate and other anion analysis

is well-stated in an American Chemical Society report  on

pollution abatment:

        Progress in analytical chemistry and instrumentation
     is vital to both water pollution research and survellance
     and research on eutrophication is a notable example of
     the need.  The identification and measurement of
     limiting nutrients would be greatly eased by improved
     analytical methods for low levels of the forms of
     phosphorous compounds, the various forms of nitrogen
     compounds, trace metals, and trace organic growth
     factors in water.

Accordingly, the project objective was stated as follows:

        The initial aim is to determine phosphate in natural
     and waste waters by atomic absorption with high
     sensitivity, speed and specificity.   The linear
     absorbance decrease for metals due to flame compound
     formation will be the observed quantity.   The
     investigation will necessarily include studies of flame
     reaction stoichiometries and rates and should provide
     improved understanding of compound formation in flames.
     This will be used to establish a specific determination
     for the various phosphates as well as for other nonmetallic
     anions.  The extension of the advantages of atomic
     absorption to analysis of many anions at concentrations
     present in natural waters is the final objective.

     Present analytical methods for measurement of anions as

outlined in Public Health Services' "Standard Methods of

Water and Waste Water Analysis" are usually laborious and

sometimes empirical.  Interferences by silicate, arsenate,

and other anions are troublesome.  Because the methods are

relatively slow errors sometimes enter during storage due to

                              1

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micro organism action and adsorption effects.   The above
reference points  out the desirability of fast  field methods
whenever possible.
     A vast  literature exists aimed at removing or minimizing
chemical interference effects in flame spectroscopy.  These
effects of inhibiting anions  due to formation  of refractory
species in the  flame remain as probably the most important
limitation on accuracy for most practical analysis of  metals.
     In contrast, little has  been reported on  the use  of
these inhibitions as  a means  of analyzing the  corresponding
anions.   The most common inhibiting anions are coincidentally
also among the  anions  of most interest in natural and  waste
water chemistry and  pollution studies,  i.e., phosphate,
sulfate, and silicate.
     Studies in our  laboratory have been aimed at obtaining
specific inhibitions for  each of  these  anions.   Among  the
parameters investigated were:   pH,  beam height,  fuel:oxidant
ratio, burner type,  total  gas  flov,  rate,  and metal     ;
cation:inhibiting anion  ratio.  Among  these parameters,
 fuel:oxidant ratio  (i..e. ,  flame temperature) and metal
 cation:anion ratio  U.e.,  solution  stoichiometry)  were found
 to be most useful.
     To provide for  convenient manipulation of  solution
stoichiometry  as well  as  for  an accurate  analysis method
                               2
a titration procedure was adopted.   We have termed the
technique AAIT—atomic absorption inhibition titration.   In
this procedure the titrant, typically 100 yg/ml MgCl2 is
added by an infusion pump to 100 ml of the sample solution
which contains 0.1 to 25 yg/ml of anion while the sample
solution is being sampled by aspiration into the flame  and
the magnesium atomic absorption is monitored.  Magnesium is
selected as the monitor metal because of its sensitivity
and its high extent of inhibition by refractory forming anions.
     Atomic absorption inhibition titration  (AAIT) has  been
successfully employed to determine phosphate, silicate, and
sulfate as well as simultaneous determination of any
combination of these anions in drinking and waste water.

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                        EXPERIMENTAL





     Methods based on inhibition effects require the use of



relatively cool flames in which the compounds formed in the



evaporating droplets are stable.  The hydrogen-air  flame



provides conditions at which  inhibition of these anions



 (Si02, PO4, and SOj) is realized without significant



interference from less stable compounds.  A number  of



experimental adjustments influence inhibition by anions.



Inhibition ordinarily increases with excess of either fuel



or oxidant gas flow, but decreases as beam height or total



gas flow rate increases.  These variables establish flame



temperature in the light path.  The optimized conditions for



silicate determination has the combination of hydrogen flow



rate of 10 ft3/hr (30 psi), air, 10 ft3/hr (40 psi) and for



sulfate, 30 ft3/hr  (30 psi) for hydrogen and 10 ft3/hr



 (30 psi) for air.  The previous statement indicates a



possibility to determine both silicate and sulfate  anions



at certain combinations of instrumental parameters  with



minimum interference from each other.



     To obtain well characterized titration curves  it is



necessary to exchange cations with a hydrogen form  ion



exchange resin for standard as well as sample solutions,



i.e.,  sodium and potassium ions impair sharpness of



                              4
titration signals.  Such separations are accomplished in



jost a few minutes via a batch process because the ion



concentrations involved are so low.

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                           RESULTS





     Phosphate Determination by AAIT:  The anions selected



in this study are ortho-, pyro-, tri-, tetra-, and hexa-



phosphates.  Observations were made with both total



consumption and pre-mix slot burners.  A hydrogen-air flame



was used in order to have the relatively low flame



temperatures and the flame temperature control which is



necessary.  The total consumption burner was a Jarrell-Ash



"HETCO" model.  The pre-mix burner had a 0.5 x 100 mm slot.



For manual titration an ordinary glass buret was used with



0.027 in. I.D. plastic tubing leading into the titration



solution in a 250-ml beaker stirred with a magnetic stirrer.



     All solutions were prepared from reagent grade material



using deionized distilled water (unless specified otherwise).



Sodium hexametaphosphate and tetraphosphoric acid were



commercial materials (Benlo Chemical Co.).  To remove cations,



the solution was treated with cation exchange resin (Dowex



50 x 8, 20-25 mesh)  using a separatory funnel by a batch



method.  The cation exchange 'resin was exposed to dilute




acid solution between uses.



     Several detergent product brands were selected randomly




from among those sold locally.
     Before beginning a titration, gas flow rates were



adjusted for maximum inhibition using a magnesium plus



phosphate solution.  The atomic absorption signal was



observed at intervals during the titration by dipping the



aspirator tube into the titration vessel.  Steps were taken



to insure that the titration solution level with reference



to the burner did not change and that solution uptake rates



were not sufficient to introduce significant errors.  A



complete manual titration curve was obtained in less than



one-half hour.  Semi-automatic titration requires less than



five minutes.




     Results for the total consumption burner inhibitions by



the three anions, orthophosphate, silicate, and sulfate, were



similar.  The extent of inhibition was proportional to the



magnesium absorption signal and generally followed the



temperature profile of the flame.   Rather wide variation of



hydrogen to air flow ratios (0.5 to 4.0) and total gas flow



rates (15 to 50 ft /hr) showed little effect on the relative



amounts of inhibition among the three anions.  The degree of



inhibition is similar for phosphate and silicate and is



somewhat less for sulfate.



     The inhibition effect is relatively linear with anion



concentration for mole ratios of inhibitor  (anion) to

-------
 absorber (Mg)  less than unity and anion concentrations down
 to 0.5 ug/ml.
      This linear response suggests the possibility of
 relatively precise and sensitive indirect measurement of these
 anions by their inhibiting effect.  The specific analysis of
 one anion in the presence of the others is unlikely, however,
 because the relative inhibitions of sulfate, silicate, and
 phosphate are independent of vertical position, fuel to
 oxidant ratio, total gas flow rate, and anion concentration.
      Data taken using a pre-mix burner are significantly
 different from those with the total consumption burner due
 to the smaller range of droplet sizes.  Although with the
 latter, phosphate and silicate inhibited similarly. Figure 1
 shows that with the pre-mix flame a progression in inhibition
 is observed.  Inhibition for various fuel to oxidant flow
 rates is also shown in Figure 1.  Several features of these
 plots are significant.  Absorption by magnesium is at a
 maximum value and inhibitions are at or very near minimum
 values at hydrogen to air flow ratios near 2:5.  This
 ratio corresponds to a stoichiometric mixture for combustion
 of hydrogen with air and thus a maximum in flame temperature.
. At lower or higher ratios the cooler flame results in more
 inhibition for all three anions, but also less absorption by
 magnesium.  Observations for many more combinations of gas
                               8
Figure 1.  Inhibition and Gas Flow Rates

           Pre-mix burner
           Burner height, 10 mm; X = 285.2 nm; pH = 7

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                           OT


                      %   ABSN
                                           OB
                                           O
                                             8
o
o
3
O
«•

2
          ro
          O
         • ot
          o
IN5

O
•o
•O

3
O  o
      m
         (J>
         o
                               r—r—i
          ro
          O
          w
          o
                                                     o
                                                     c
flow rates all yielded a similar pattern.   When total gas


flow rate is increased while maintaining constant hydrogen


to air flow ratio the inhibition decreases due to the


increased thermal input to the flame.


     Inhibition for all three anions increases moderately


with pH and with decreasing beam height in the flame.  These


trends were not distinctive for any one of the anions and


were not considered suitable for application to selective


determination of anions.


     Data obtained here, in agreement with other workers,


show that when the mole ratio of inhibiting anion to metal


exceeds approximately unity the extent of inhibition no


longer increases on increasing the concentration of anion.


At anion to magnesium mole ratios less than unity the


occurrence of discontinuities in solution stoichiometry


vs_. absorption was observed for phosphate and silicate


solutions.  Titration permits convenient variation of the


solution stoichiometry.  Titration also allows easy


maintenance of other experimental conditions during


measurements.  An example of  a  curve obtained when titrating


phosphate with magnesium is  shown in Figure 2.   The  remarkable


shape of this curve  forms the basis  of  the new measurement


technique for low concentrations of  phosphate.   It represents


a unique type of titration  chemistry in that high temperature


                               11

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Figure 2.   Titration of Blank and 8 ppm Orthophosphate



           Titrant, MgCl2 solution

           Gas flow rates:  Hydrogen, 25 ft /hr  (60 psi)


                           Air, 5 ft3/hr (80  psi)
                             12
                                                                                                      FIGURE 2
                                                                                     100
                                                                                     90
                                                                                     80
                                                                                     70
to
CD
       0    LO   2,0   3.0   4.0   5.0   60   7.0   ftO   9.0  10.0


                        ml 200 ppm  Mg
                                                                                                           13

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 processes on small bits of solution is the observed quantity.
 Reverse titrations of metals with inhibiting anions might
 also be advantageous in certain cases.
     On the titration curve shown in Figure 2, points
 suitable for application to phosphate determinations are
 designated as A,  B,  and C.   The shape of the curve combined
with the titration data indicates that any of these points
may be  used for convenient  endpoint designation.   The three
titration endpoints  suggest refractory compound formations
with approximately two,  three,  and four magnesium atoms  per
phosphorus  atom.   Such  stoichiometries do not correspond to
magnesium phosphate  compounds or  complexes of ordinary
solution  chemistry,  but  rather  reflect high temperature
processes occurring  in the  evaporating droplets.
     The sample droplet  is  subject to drastic reactions
 including formation, dehydration,  and volatilization of  dry
 aerosol particles.  All  these processes occur in  about one
millisecond.   Stoichiometries quite  different from those
 of ordinary solution chemistry are  to be  expected.   The
 decreasing signal before point C can  be  assigned  to a  rate
 process  involving formation of a more  stable  refractory
 species  from  a less stable initial  species.   The  effect  is
 even more prominent for solutions containing  sulfate.  This
 titration method allows investigation  into the  inhibition
mechanisms.
                              14
     Table I summarizes results for titrations of solutions
containing sulfate as well as phosphate.   The data show
general agreement with titrations where no sulfate is
present.  The effects of sulfate on the titration curve
were to decrease the absorption throughout the titration
and to enhance the minimum at point C.

     Polyphosphate Titrations:  Polyphosphate titration curves
had the same general shapes as those for orthophosphate.
Titration stoichiometry was examined under the same
instrumental settings as for phosphate titration.  The
endpoints used corresponded to point A of Figure 2.  The
magnesium to phosphorus atom ratio appears to approach a
constant atom ratio of unity with increasing size of the
polyphosphate, whereas for orthophosphate the ratio is near
two.  These results suggest a kinetically determined
inhibiting process in the droplets.
     Silicate can be expected to  interfere when  present in
samples.  It must either be first separated  or it must be
                  4
determined by AAIT  and subtracted from total silicate  and
phosphate results.
     Analysis of Milwaukee River  water was performed in
order to examine the utility  of the  analysis system.   The
slightly acidified  sample was filtered  through paper and
                               15

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                  TABLE I
Titration PO4/Mg Mole Ratios  for  Solutions
     Containing Phosphate  and Sulfate
ppm P04
5
5
10
10
10
10
15
ppm S04
0
10
0
5
10
15
10
A
0.551
0.613
0.556
0.527
0.551
0.538
0.602
B
0.319
0.376

0.297
0.321
0.366
0.334
C
0.261
0.285

0.244
0.263
0.289
0.248
                 0.562
                             0.336
                                          0.262
                     16
 then a 0.4  micron membrane  filter.   It was then passed
 through a cation exchange column in the acid form to replace
 naturally occurring  magnesium,  calcium, and other cations.
 The sample  was  diluted ten-fold for convenience.   Titrationa
 were performed  on river water,  river water plus standard
 addition of phosphate,  and  standardized phosphate solutions.
 The results are shown  in  Table  II.   Accuracy and precision
 are good considering the  speed  and  sensitivity of the method.
 The absence of  measurable silicates in the samples titrated
 was confirmed by observing  a negligible endpoint  for a
 titration at conditions under which only silicate would  be
 detected.   Thus,  either the river water contained negligible
 dissolved silicate or  silicate  was  quantitatively held in
 the cation  exchanger.  For  best results,  a standard addition
 must be titrated with  the same  flame adjustment and
 instrument  settings  used  for the samples.
      Phosphate  determination was also performed on four
 brands  of commercial detergent  products (Table III).   The
 detergent solutions  were  either boiled for one hour or aged
 for several  days before titration.
      Determination of  sample No.  1  of the series  of water
 analysis  standard  solutions offered by the Analytical  Qua-
lity Control Laboratory yielded  0.050 (s = 0.001)  ppm
 phosphorus  compared  with  the stated value  by AQCL  of 0.05 ppm.
                              17

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      TABLE II
River Water Analysis
2 ppm PO4
(ml)
0.73
0.59
0.50
0.51
0.57
:un-i .ruim
River
(ml)
1.50
1.23
1.08
1.05
1.29
River +
2 ppm P04
(ml)
2.24
1.80
1.60
1.61
1.84
	 uoncen
P04 -in
River Water
(ppm)
4.14
4.10
4.40
4.32
4.46
trataon 	
Added PO4
Determined
(ppm)
1.97
1.93
2.08
2.16
2.12
              x = 4.28      x = 2.05
              s = 0.16      s = 0.10
                      TABLE III
           Determination of Phosphate in
             Detergent Products by AAIT
Brand
Number
1
2
3
4
POa Found—
(ABIT),
36.2 ± 0.5%
36.1 ± 0.2%
36.1 ± 0.5%
28.9 ± 1.0%
PO. Found
(Std Method)
35.8%
36.5%
36.6%
24.6%
^Standard deviations based on five replicate
titrations.  Percent PO. in commercial material
as received.
         18
                                                                                       19

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     In conclusion, the results of this  investigation  indicate
that atonic absorption inhibition titration  (AAIT)  is
applicable to phosphate determination  in surface  and waste
waters.  A pre-nix burner  is  necessary for the AAIT method.
Hhen the phosphate solution is titrated  with magnesium,
three break points were found. At these points the magnesium
to phosphate ratio remain  essentially  constant, and therefore
can serve as the endpoints for titration, even in the  presence
of sulfate.
     These details for AAIT determinations of phosphates  are
cited in reference S.

     Silicate Determination by AAIT:   AAIT titration curves
for silicate are of  the same  shape as  those  for phosphate.
With use of a flame  of  sufficiently  high temperature,  silicate
inhibits magnesium atomic  absorption,  but phosphate and sulfate
do not.  Accordingly,  silicate can be  determined  in the presence
of these ions in water  samples.   Typical data for such determina-
tion is given in Table IV.   A report  on silicate determination
by this new method developed  here is already in print*.
                  TABLE IV
         Determination of Silica in
          Artificial Drinking Water
   SiO, added
      Tppm)
SiO, det'nd
   tpp»>
Number of
   runs
0.50
1.00
2.00
3.00
4.00
0.54
1.00
2.06
3.09
3.77
2
2
1
1
1
X (mean yg SiO2 found/yg SiO2 present)  = 1.02
                  s = 5.1%
                               20
                                                                                                              21

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      Sulfate Determination by AAIT;  Of the three anions
 considered here, sulfate has least tendency to undergo
 inhibiting reactions with magnesium in the droplets.
 Therefore, the AAIT determination of this anion requires
 use of a. relatively cool flame.   This was accomplished by
 using a fuel rich flame with hydrogen flow rate, 30 ft /hr,
 and air flow rate,  10 ft /hr.  Inlet pressure to the  flow
 meters was 30 psi.   Under these  conditions a simple titration
 curve is  obtained in which a linearly increasing absorbance
 is  observed beyond  the titration endpoint.   A linear
 extrapolation of this segment to the base line conveniently
 defines the endpoint.   Typical data is indicated in Table V.
      Determinations  over a pH range of 1-9  resulted in no
 significant change  in results obtained.   Precision
measurements  on  a 5  ppm  SO,  indicate a standard deviation
 of 0.2% for  six  determinations.   Such titrations using
 100 ppa Mg  as titrant  required less  than  1.5 min to perform.
 Chloride, acetate, nitrate,  and  fluoride  do not interfere;
 however, phosphate and silicate  do  interfere and must  be
 removed.  The latter two anions  are much more  basic than
 sulfate and so previous separation of  them  (e.g_. , ion
 exchange) is readily accomplished.
     A report describing sulfate determinations  by  AAIT as
developed in this project is already  in print   and  can be
                              22
             TABLE V
Determination of Sulfate by AAIT

ppm S04 added      ppm SO^ found
    1.00
    2.00
    4.00
    8.00
    16.0
    20.0
0.86
1.94
4.00
8.00
15.9
20.1
                                                                                                               23

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consulted..
     Simultaneous Determination of Anions by AAIT:  Studies



of AAIT curve shapes with more than one of  the anions



present indicated that with proper choice of flame  conditions



remarkably useful signals were obtained.  Figure  3  shows



titration curves obtained for solutions containing  one yg/ml



silicate and 25 vg/ml sulfate.  Points A and B serve as



endpoints for silicate and  C minus B serves as the  endpoint



for sulfate.  Figure 3 also shows data obtained for



optimizing hydrogen to air  flow ratios.  Up to R  =  0.5,



this small amount of silicate does not show a titration



signal, whereas at ratios greater than 1.5  the silicate



signal predominates.



     In order for simultaneous determination of silicate and



sulfate together, the background intensity  after  the silica



peak must be as low as possible without sacrificing the



clarity of the silicate  endpoint.  The best condition for



our work is hydrogen, 10 ft /hr (30 psi) and air, 7.5 ft /hr



(30 psi).  In the course of optimization we have  used both



the "Tri-Flame" pre-mix  laminar flow burner head  and the



newer, improved teflon-lined burner head  (Jarrell-Ash



82-374).  It is found that  the newer burner is more



efficient in nebulizing  solution.



                              24
Figure 3.  Effect of Air:Hydrogen Flow Ratio, R, on



           Titration Curve





           Hydrogen flow rate, 10 ft3/hr (30 psi)
                              25

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                      FIGtTKE 3
9Or
80-
                                                      R=0.8
                                                      = 0.75
     Simultaneous determination of silicate-sulfate system
requires calibration  standards for both components.   These
standards are least square fitted and show a linear
correlation coefficient of 0.998 for 0.50 to 4.00 pg/ml  SiO2
in the presence of 20 pg/ml SO, and 0.984 for 5.00 to 30.0
ug/ml SO. in the presence of 1.0 iig/ml SiO2.  Thus,  linear
calibration for simultaneous determination of silicate and
sulfate is possible.
     This method has  been applied to determine silicate  and
sulfate in raw water  from Lake Michigan and drinking water
from the Milwaukee Water Purification Plant.  The results
are summarized in Table VI.  Data obtained by their laboratory
using standard gravimetric methods are included for
comparison.  It is important to point out that the recovery
of added one ug/ml silica in raw and effluent waters is
almost 100% in the AAIT method even though the calibration
curve was obtained in deionized distilled water.   The small
discrepancy between the standard gravimetric and AAIT
methods was further investigated by synthesizing drinking
water according to the data published by the American
Chemical Society.
     The high accuracy and precision shown in Table  IV lead
to the conclusion that the AAIT method is valid for  Si02 at
these levels.  The gravimetric data apparently contains
                             27
                           TIME,  min.

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                          TABLE VI
         Data Comparing AAIT  and  Gravimetric Methods
  Sample
Raw H2O
Raw H2O
               -Concentration  of  SO.	Concentration  of  SiO.—
                        (ppm)                    (ppm)
                AAIT      Gravimetric  AAIT
               Method-      Methods   Method-
               21.5±0.1

               21.7±0.5
20.7
1.0 ppm SiO2
               25.3±0.2
         1.39*0.04

         2.41+0.05
1.0 ppm SiO2
Drinking HjO   24.8±0.1      26.0     0.95±0.01
Drinking H.
                                       2.03±0.03
                                                   Gravimetric
                                                      Method^.
1.2
                                                        1.0
^Deviations shown indicate range  for two or three
determinations.  -As reported by  Milwaukee Water Purification
Plant.
                               28
negative error.  Table  IV  indicates that the common anions
present in drinking water  do  not interfere in the silica
determination.  A series of simultaneous titrations of
samples' containing various relative amounts of silicates
and sulfate shows that  the method is valid whenever the ratio
of sulfate to silicate  is  larger than 2.  A survey of data
for municipal water supplies  indicates that many  fall into
this category.  Addition of known amounts of sulfate provides
easy accomodation of those which do not.
     Further investigations were aimed at even more general
application to waste waters and  at including phosphate in
the analysis.
     Data obtained for  solutions containing silicate, sulfate,
and phosphate showed that  relatively linear relationships
between points A, B, and C on the titration curve  of Figure 3
were obtained as follows:
     A = k1[Si02) + b                           (1)
     B - k2(Si02] + k3[P04l + b                 (2)
     C = k4[Si02l + k5[P04] + kg[S04]  +  b       (3)
where bracketed terms indicate concentration.
     A computer-assisted multiple linear regression analysis
treatment of an array of experimental data resulted in
evaluation of the coefficients in the set  of  equations:
                              29

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      [S04]
KgC - K?B + KgA + Kg
(4)
(5)
(6)
      Thus a single titration for analysis of all  three  anions
of  interest is  possible.   Such titrations ordinarily require
about five minutes to perform.  Actual titration  time is
usually less than three minutes.
      Data obtained for artificial waste water show  that
when  total yg/ml  of SiO2  and PO,  exceed one-half  of ug/ml
SO. the.error for SiCX, and PO. determination becomes
  4                   24
excessive.   This  is attributed to merging of points B and C
of the titration  curve (Figure 3).   This limitation, however,
does  allow determination  of most  surface waters and many
waste waters  (See  Table VII).
     Again,  a full report of the  new method  developed for
simultaneous determination of  the three anions in a single
sample has been published ,
                              30
                                                                                                       TABLE VII
                                                                                            Simultaneous Determination of
                                                                                           SiO2, PO4, and SO4 in Waste Water
	 Aaaea
Si02
1.
1.
4.
10.
1.
50
50
00
00
00
I
PO
3.
2.
2.
5.
4.
Mg/m
4
00
00
00
00
00
-U 	
so4
20.
20.
20.
20.
20.

0
0
0
0
0
Si02
1.44
1.60
3.48
7.30
1.06+0.15-
Pi
3
1
IMg/aij.;-
.04
.56
2.38
9
.56
4.04+0.10-
SO,
20.
20.
20.
20.
21.3±0

5
3
3
0
„





S3-
                                                                ^Mean and estimated standard deviation for five replicate
                                                                titrations.
                                                                                                            31

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                         DISCUSSION

     The analytical technique developed  in this  project  is
characterized by speed and convenience,  especially when
compared to alternative methods  for  determining  the  three
anions  (POj, SiOj, and SOj)  in a single  sample.   The lower
determination limits  are comparable  to present standard
methods.  The apparatus required,  even for the semi-automatic
titration, is not extremely  costly,  i.e., an atomic  absorption
spectrometer and an infusion pump.   The  first of these items
is already part of the equipment of  many water and waste
water laboratories.   Several of  the  advantages of atomic
absorption are thus made available for these common  important
anions.  An additional advantage is  that since procedurally
the method is a titration, the demands on the spectrometer
are not high, i^.e., only changes in  signal are observed.
Hopefully instrument  suppliers will  exploit  these features
to provide a relatively low  cost unit  which  will allow for
determination of magnesium and other common  metals by atomic
absorption or emission and these anions  by AAIT.
     The apparatus used in the method  is suitable for
automation.  Although a beginning  in the procurement of
equipment for this task was  possible under the budgetary
limitations of the project,  this effort  is not yet complete.
                              32
     At least two benefits of the program  to  chemistry have
occured.  One is that AAIT provides a new  tool  for observing
inhibition processes in flame spectrometry.   This provides
elucidation of inhibition mechanisms.   In general these
studies have shown that the inhibition processes are rate
determined processes as opposed to solution equilibrium
regulated processes.  Finally, it should be noted that,
although the method is based on inhibition effects, it is
not an indirect method but rather employs  the procedural
advantages of direct titrations and atomic absorption
measurements.
                              33

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                          REFERENCES
1.  Anon. Cleaning Our Environment.  The Chemical Basis for
    Action.  American Chemical Society, Washington, D.C.,
    1969, pp 109* 152.
2.  Alkemade, C. Th. J., Flame Emission and Atomic Absorption
    Spectroaetry, J.A. Dean and T.C. Rains, Eds., New York,
    Marcel Dekker, 1969.  Vol. I, Chpt. 4.
3.  Smith, R., Stafford, C.M., and Winefordner, J.D.  Anal.
    Chem. 41:946, 1969.
4.  Looyenga, R.W. and Huber, C.O.  Determination of Silicate
    in Wastewater by Atomic Absorption Inhibition Titration.
    Anal. Chem. 43:498-501, 1971.
5.  Crawford, W.E., Lin, C.I. ,  and Huber, C.O.  Atomic Absor-
    ption Inhibition Titration of Orthophosphate and Polyphos-
    phates.   Anal. Chim. Acta.   64:387-395, 1973.
6.  Looyenga, R.W. and Huber, C.O.  The Determination of Sulfate
    by Atomic Absorption Inhibition Titration.  Anal. Chim.
    Acta. 55:179-183,  1971.
7.  Lin,  C.I. and Huber, C.O.  Determination of Phosphate, Sili-
    cate, and Sulfate  in Natural and Wastewater by Atomic
    Absorption Inhibition Titration.  Anal. Chem. 44:2200-2204,
    1972.
                    LIST OF PUBLICATIONS

"Determination of Silicate in Waste Water by Atomic
Absorption Inhibition Titration," R. W. Looyenga and C. O.
Huber, Analytical Chemistry, 43, 498-501 (1971).

"The Determination of Sulfate by Atomic Absorption Inhibition
Titration," R. W. Looyenga and C. O. Huber, Analytica Chimica
Acta, 55, 179-183 (1971).

"Determination of Phosphate, Silicate, and Sulfate in
Natural and Waste Water by Atomic Absorption Inhibition
Titration," C. I. Lin and C. O. Huber, Analytical Chemistry,
44, 2200-2204 (1972).

"Atomic Absorption Inhibition Titration of Orthophosphate
and Polyphosphates,"  W. E. Crawford, C. I.  Lin, and C. 0.
Huber, Analytica Chimica Acta, 64, 387-395 (1973).
                                                                                                               35

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