ATOMIC ABSORPTION ANALYSIS OF PHOSPHATES IN WATER
Calvin O. Huber
University of Wisconsin
Milwaukee, Wisconsin
October 1973
DISTRIBUTED BY:
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This document has been approved for public release and sale.
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
OT
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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
-------
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
-------
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
-------
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
-------
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.
-------
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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