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Anal. Chem. 1985, 57,258-262
Ion Chromatographic Determination of Silicic Acid in Natural Water Tetsuo Okada a n d Tooru Kuwamoto* Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan
A nonsuppressed Ion chromatography uslng potassium hydroxide solution as an eluent was applied to the determinatlon of sillclc acid in natural water (lake water, river water, and tap water). Some anlons eluted rapldly, such as fluoride Ion and low molecular organic acids, were separated from sillclc acld at the eluent concentratlon of 7 could not be determined by a suppressed IC because the conductance was measured in an acidic solution (18). The authors previously reported that a nonsuppressed IC using potassium hydroxide solution as eluent was more sensitive than a conventional nonsuppressed IC (19, 20) and could be applied to the determination of anions in environmental water (16,21). This method was effective for the determination of weak acids such as phenol or its derivatives, because their separation and detection were carried out in a basic solution (16). As monomer silicic acid is dissolved in the form of H3Si04-in potassium hydroxide solution used as an eluent, it can be determined by this chromatographic method (22). In this paper, the effects of the coexistent ions and the application of the analysis of silicic acid in natural water were discussed in detail. EXPERIMENTAL SECTION Apparatus. A Toyo Soda Model nonsuppressed ion chromatograph HLC-601equipped with an anion exchange column (50 mm X 4.6 mm id.) packed with TSKgel IC-Anion-PW(particle size 10 f l pm, capacity 0.03 f 0.005 mequiv/g) was used. The flow rate was maintained at 1.2 mL/min under a pressure of 20-25 kg/cm2. The separator column and a conductivity detector were set in the oven regulated at 30 “C. The volume of the sample loop was 100 pL. A Shimadzu spectrophotometer, Model UV200S, was used in order to determine “reactive silica” by the molybdenum blue method. Reagents. The eluent was daily prepared by dissolving the guaranteed reagent of potassium hydroxide in distilled deionized water and by deaerating. A silicate standard solution was prepared by diluting 1000 ppm (as Si) solution of sodium silicate (Nakarai ChemicalsCo., Ltd.) stocked in 0.6 M sodium carbonate solution. A fluoride solution (1000 ppm) was prepared by dissolving potassium fluoride, dried under vacuum at 110 “C overnight, in distilled deionized water. Standard solutionsof organic acids (0.05 M) were prepared by dissolving organic acids or their potassium salts. Solutions of magnesium and calcium ions were prepared by dissolving magnesium metal and calcium carbonate in a small excess of HC1. Working standard solutions were prepared by diluting stock solutions in distilled deionized water. A solution of boric acid (0.1 M) was prepared by dissolving the guaranteed reagent of boric acid in distilled deionized water. All solutions were stocked in plastic bottles. The other reagents were used without purifying the guaranteed reagents. Dowex 50W-X2, strong acidic cation exchange resin (100-200 mesh), was used in order to remove the interfering cations in sample water. Lake water and river water were supplied to the experiment aftm they were filtered through a Millipore filter (pore size 0.45 pm, type HA). RESULTS AND DISCUSSION Ion Chromatogram of Silicic Acid. As a potassium hydroxide solution is a weak eluent, it is difficult to determine quantitatively the polyvalent ions retained by an ion exchange resin. Monomer silicic acid is dissolved as H4Si04 or its dissociated forms. It was expected that monomer silicic acid
0003-2700/85/0357-0258$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
259
5-
e
10
0
30
20
Concentration of
F-
, ppm
Figure 3. Peak depression of silicic acid by fluoride ion: a, peak area; b, peak height. Sample solution contained 1 ppm (as Si) of silicic acid. Eluent was 0.5 mM KOH. Other conditions are given in the text.
Concentration of KOH, m M
Flgure 1. Ion chromatograms of silicic acid and the relationship between the peak height and eluent concentration. Sample solution was 5 ppm (as Si) of silicic acid. Other conditions are given in the text.
0
Concentration of KOH , mM 1 2 3 4 I
c
I
I
I
3l ’I,j:::
rr2
cc
I
Figure 2. Variation of the retention times of fluoride and silicic acid with the concentration of KOH. A chromatogram shows separation of siliclc acld from fluoride ion when using 1 mM KOH: A, fluorlde; B, silicic acid. Conditions are given in the text.
was dissolved as a monovalent ion (H3Si04-)and was eluted rapidly with a potassium hydroxide eluent, because its dissociation constants were pKl = 9.8 and pK2 = 12.16 ( I ) . Figure 1shows the typical ion chromatogramsof monomer silicic acid using eluents of 0.5-4 mM. The elution peaks of sample solutes were observed as the decrease in the conductance because of the large ion equivalent conductance of hydroxide ion (AOH= 198.3 cm2molm1at 26 “C). The first large peak in chromatogram is a “dip peak”, which is due to the elution of sample water, an eluent anion replaced by sample anions, and sample cations excluded by an ion-exchange resin (23). As seen in Figure 1, “dip peak” affected the peak of silicic acid when using concentrated eluents. The retention time decreased and the peak height of silicic acid increased with the increase in the eluent concentration. Its retention time corresponds to those of fluoride ion and some low molecular organic acids such as acetic, formic, and glycolic acids (16). The authors previously reported that the determined value of fluoride ion in environmentalwater obtained by this method (2 mM of eluent was used) was larger than that obtained by spectrophotometric or potentiometric method (21). This positive error in the fluoride ion concentration was caused by the simultaneous elution of monosilicic acid. Effect of the Coexistent Ions and Separation of Silicic Acid from Interfering Substances. Since monomer silicic acid and fluoride ion were simultaneously eluted with the eluent of 2 mM, their separation was investigated by varying the eluent concentration. Figure 2 shows the relationship
L
TI me
Flgure 4. Elimination of Interference of fluoride ion by adding boric acid: a, 1 ppm (as Si) of silicic acid; b, 1 ppm (as Si) of silicic acid and 2 ppm of fluoride ion; c, 1 ppm (as Si) of silicic acid, 2 ppm of fluoride M of boric acid. Eluent was 0.5 mM of KOH. Other ion and 1 X conditions are given in the text.
Table I. Peak Depression Fluoride Ion by Silicic Acida peak area,
concn of silicic acid (ppm as Si) 0 1
5 10
peak height, pS
arbitrary units
20.2 17.0 12.0 6.8
122 112 88 50
asample, 1 ppm of fluoride ion. between the eluent concentration and the retention times of silicic acid and fluoride ion. These two ions could be separated a t the eluent concentration of 1
KOH
0.85
>1 97.8 93.0 95.7 82.6 87.0 86.1 98.0
99.6
>1 >1 >1 >1 0.97
>1 >1 >1
>1
100.0
>1
Sample contained 1 ppm (as Si) of silicic acid. Table 111. Cation Interference Study” recovery, %
cation
molar ratio (M2’/Si)
without pretreatment
0.5 1 10 0.5
99.2 89.2 70.4 97.4 97.8 91.3
Mg2+
Ca2+
1 10 a
pretreatment with cation exchanger
97.1 97.9
Sample contained 1 ppm (as Si) of silicic acid.
I
IO
0 Time
~
min
Figure 5. Separation of slllcic acid from organic acids. Sample was 1 ppm of slllcic acid (as SI), glycolate, and formate. Eluent was 0.5 mM of KOH. Other conditions are given In the text.
ion is not a serious problem in the analysis of silicic acid in such cases. Table I1 shows the interference of anions when using 0.5 mM and 1mM potassium hydroxide eluent. Organic acids, except glycolic acid, eluted rapidly and were completely separated from silicic acid as shown in Figure 5. Acetic and lactic acids were not shown in the figure but were eluted at the almost same position with glycolic acid. These organic acids did not interfere with the determination of silicic acid in cases studied. Molybdic acid did not interfere because it did not form silicomolybdic acid in a basic solution. Subsequently,the interferences of cations were investigated, as magnesium and calcium ions, which existed with silica in minerals such as forsterite or xonotlite (I),seriously interfered as shown in Table 111. These cations do not interact with silicic acid in an acidic or neutral solution, but form complexes, ion pairs or precipitates, in a basic solution. These interfering cations had to be removed for the accurate determination of silicic acid. At the first time, a Na+-form cation exchange resin was used in order to remove these interfering ions, but the recovery of silicic acid decreased with the concentration of magnesium ion because of the adsorption or the coagulation of silicic acid. Consequently, the use of a H+-form cation exchange resin (Dowex 50W X2) gave the satisfactory results as seen in Table 111. The adsorption of silicic acid with resin matrix was not observed. An ion-exchange column (40 mm X 15 mm i.d.) was frequently regenerated by 2 M HC1 so that interfering cations ion-exchanged by a resin did not adsorb silicic acid. The recovery of silicic acid during ion chromatography was nearly 100% regardless of sample pH, though the effluent from the cation exchange column was acidic. This treatment was not necessary when a sample solution contained only equimolar calcium ion or half molar magnesium ion of silicic acid; they did not interfere as seen in Table 111.
1
I
0
5
Concentration d Sillcic Acid
I
ppm
Figure 6. Calibration curve for sllicic acid and the elution peaks of standard solutions. Eluent was 1 mM KOH. Other conditions are given in the text.
The elution peak of silicic acid was affected by the dip peak which became large with the increase in the amount of electrolytes in a sample solution. Although their separation was adequate in ordinary samples, it was difficult to determine quantitatively silicic acid when a sample solution contained large amounts of electrolytes over the 10-fold concentration of an eluent. Detection Limit and Calibration Curve for Silicic Acid. The peak height of silicic acid increased with the increase in the concentration of eluent as described above. However, the noise of the base line also increased because of the increase in the background conductance and some coexistent ions interfered with the determination of silicic acid when using high concentrated eluent. Therefore, the eluent of 0.5 mM was used in order to investigate the precision of this method.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
261
Table IV. Determination of Silicic Acid"
lake water
river water
amt of Mg2+,ppm
amt of Ca2+,ppm
1.9 1.9 1.9 1.9 1.9 1.9 1.9 0.9 0.4 4.0
9.5 9.5 9.5 9.5 9.5 9.5 9.5 1.8 0.6 8.2
1
2 3 4 5 6 7 1 2
tap water
concn (pprn) of silicic acid (as Si) IC spectrophotometry 0.40 0.39 0.35 0.41 0.42 0.59 0.39 3.53 3.44 10.85
molar ratio Mg2+/Si Ca2+/Si 16.6 17.0 19.0 16.2 15.8 11.3 17.0 0.36 0.12 0.53
5.5 5.6 6.3 5.4 5.2 3.7 5.6 0.29 0.13 0.43
0.41 0.34 0.37 0.45 0.39 0.57 0.41 3.59 3.52 10.19
Lake water was treated by cation exchange resin column in order to remove magnesium and calcium ions. Magnesium and calcium ions were measured by atomic absorption spectrometry. 203040-
.-
.. .. . A
\
70n
>r
8 80s3
\ \
D
L L 0
I
1
10 20 T i m e , rnln.
I
30
\
; 904
a 4
Flgure 7. Ion chromatogram of five anions in lake water. Lake water was treated by H+-form cation-exchange resin. Eluent was 0.8 mM KOH. Other conditions are given in the text.
u
VI
0
The calibration curve was linear over the range of 0.1-2.5 ppm (as Si) in the peak height method (Figure 6). The peak height became small compared with the expected value because of the broadening of the peak and the decrease in the degree of the dissociation of silicic acid, when high concentrated standard solutions (>5 ppm) were injected as seen in Figure 6. Especially, a remarkable broadening of the peak was observed upon injecting 10 ppm of silicic acid. The percent standard deviation was 2.1% for 1 ppm of silicic acid level and the detection limit was 22 ppb as Si or 47 ppb as Si02 (detection limit was defined as the concentration corresponding to twice the value of the base line noise). This value is satisfactory compared with the other methods such as the molybdenum blue method (5-6 ppb as Si), electrochemical methods (6.4-100 ppb as Si), ICP-AES (8-9 ppb as Si), and flame AAS (600 ppb as Si). The detection limit will be improved by the elimination of pulses from the pump and the temperature deviation in the oven (conductance is the function of temperature). Application to the Determination of Silicic Acid in Natural Water. This method was applied to the determination of silicic acid in lake, river, and tap water and compared with the molybdenum blue method. Figure 7 shows a ion chromatogram of lake water. Five peaks (silicic acid, fluoride, chloride, nitrate, and sulfate ions) were observed and silicic acid was completely separated from fluoride ion. As fluoride ion did not interfer because of its low concentration (about 100 ppb), it was not necessary to add boric acid which reduced the interference of fluoride ion. However, boric acid must be added to sample solutions, when they contain a large amount of fluoride, the concentration of which is more tKan that of silicic acid. Table IV shows the concentrations of calcium and magnesium ions determined by atomic absorption spectrometry
0.5 Reaction
1
Time
, rnin
Flgure 8. Reaction of silica In standard and natural water wlth molybdic acld: (0)standard solution of monomer sllicic acid (500 ppb); (0) natural water sample. A and B represent the reaction curves of monomer and dimer silicic aclds reported by Alexander (ref 25).
and of silicic acid determined by this method and the molybdenum blue method. Silicic acid in tap water and river water was directly determined by this method, because both Mg2+/Siand Ca2+/Siratios were less than 0.5 (Table IV). As mentioned above, equimolar magnesium and half molar calcium ions of silicic acid did not interfere with the determination of silicic acid. However, silicic acid in lake water had to be determined after removing calcium and magnesium ions with the cation exchange column. There was a good agreement between these two methods as seen in Table IV. Alexander (24,25)reported that dimer silicic acid reacted with molybdic acid more slowly than monomer silicic acid. The broken lines (A and B) in Figure 8 show the reaction of monomer and dimer silicic acid with molybdic acid investigated by Alexander (24). The reaction rate of silicic acids in natural water samples with molybdic acid was investigated and the difference in the reaction rate between silicic acids in natural water samples and standard monomer silicic acid was not observed (solid line in Figure 8). Therefore, only monomer silicic acid existed in natural water studied in this work. If polymer silicic acids exist in sample water, they may decompose in the hydoxide eluent during the ion chromatographic analysis. However, the evidence of the existence of dimer silicic acid has been found in only a saturated silica solution or a solution equilibrated with an amorphous silica (4-6,24,25). In the authors' experiment,polymer silicic acids, prepared by leaving a concentrated silica solution, decomposed by dilution. This phenomenon was confirmed by both mo-
262
Anal. Chem. 1985, 57, 262-265
lybdenum blue and IC methods. Thus, “reactive silica” in these water samples is not dimer silicic acid, but monomer silicic acid. Consequently, both ion chromatographic and molybdenum blue methods measured only monomer silicic acid in this caee, and the ion chromatographic method measures “reactive silica” in diluted silica because dimer silicic acid is not contained. In conclusion, this method is effective for the simultaneous determination of anions including silicic acid in natural water.
ACKNOWLEDGMENT The authors thank Shiga Prefectural Institute of Public Health and Environmental Science for the supply of river and lake water samples. LITERATURE CITED (1) Kolthoff, I. M.; Elving, J. E. “Treatise on Analytical Chemistry”; Wiiey and Interscience: New York, 1962; Vol. 2, Part 11, pp 108-120. Tarutanl, T. J. Chromatogr. 1970, 50, 523-526. Shlmada, K.; Tarutani, T. J . Chromafogr. 1979, 168, 401-406. Lentz, C. W. Inorg. Chem. 1984, 3, 574-579. Wu, F. F. H.; Wtz. J.; Jamelson, W. D.; Masson, C. R. J . Chromatogr. 1970, 48, 515-520. (6) Cary, L. W.; de Jong, 6. H. W. S.; Dibble, W. E., Jr. Geochlm. Cosmochim. Acta 1982, 46, 1317-1320. (7) Iwasaki, I. Bunsekl Kagaku 1980, 9 , 184-198. (2) (3) (4) (5)
(8) Chow, D. T. W.; Robinson, R. J. Anal. Chem. 1953, 25, 646-648. (9) Fogg, A. 0.; Osakwe, 4. A. Talanta 1978, 25, 226-228. (10) Iyer, C. S. P.; Valenta, P.; Nurnberg, H. W. Anal. Lett. 1981, 1 4 , 921-931. (11) Horl, T.; Itoh, T.; Okazakl, S.; Fujinaga, T. BunseklKagaku 1981, 30, 582-587. (12) Garbarlnl, J. R.; Taylor, H. E. Appl. Spectrosc. 1979, 33, 220-226. (13) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801- 1809. (14) Zolotov, Yu. A.; Shplgun, 0. A.; Bubchikova, L. T. Fresenlus’ 2.Anal. Chem. 1983, 316, 8-12. (15) Ficklln, W. H. Anal. Lett. 1982, 15, 865-871. (16) Okada, T.; Kuwamoto, T. Anal. Chem. 1983, 55, 1001-1004. (17) Rokushlka, S.; Qui, 2. Y.; Hatano, H. J . Chromafogr. 1983, 260, 81-87. (18) Pohl, C. A.; Johnson, E. L. J . Chromatogr. Scl. 1980, 18, 442-452. (19) Gjerde, D.T.; Frltz, J. S.; Schmuckler, G. J . Chromafogr. 1979, 186, 509-519. (20) Gjerde, D. T.; Schmuckler, G.;Frltz, J. S. J . Chromafogr. 1980, 767, 35-45. (21) Okada, T.; Kuwamoto, T. Bunsekl Kagaku 1983, 32, 595-599. (22) Okada, T.; Kuwamoto, T. Anal. Lett ., in press. (23) Okada, T.;Kuwamoto, T. Anal. Chem. 1984, 56, 2073-2078. (24) Alexander, G. B. J . Am. Chem. SOC.1953, 75, 5655-5657. (25) Alexander, G. B. J . Am. Chem. SOC.1954, 76, 2094-2096.
RECEIVED for review July 30,1984. Accepted October 3,1984. This work has been supported by Grant-in-Aid for Scientific Research (Grant No. 58470029) from the Ministry of Education, Science and Culture, Japan.
Thermometric Ion Exchange: An Approach for Measurement of Acid or Base Concentration above 1 N Theodore E. Miller, Jr.* Central Research, Building 1712, The Dow Chemical Co., Midland, Michigan 48640
Hamish Small
4176 Oxford Drive, Leland, Michigan 49654
Thermometric ion exchange comblnes an Ion exchange resln wlth a mlcrothermlstor In a mlnlature cartridge, affordlng measurement of acld or base content of aqueous samples outside the pH range of 0-14. Advantages over conventional automated tltratlon are compactness, fast analysis, and slgniflcantly reduced electromechanlcalcomplexity. The method is to measure the slzable (- 10 “C) thermal translent eluted from a 100 pL acld or base form Ion-exchange bed In response to base or acld sample injection, respectively. The magnitude of thls conflned “heat of neutrallratlon” thermal peak is dlrectly related to sample acldlty or baslclty from 1 N upward. Salt interference Is Insignificant over a wide range, owlng to Its smaller heat of dilution and only partlai mixing In contrast to the complete neutrallzatlon of acld or base In the cartridge. The measurement process Is rapld, Is hlghly speclflc to acids or bases, and produces a sizable signal. Relative preclslon Is found to be f2% relatlve at the 95% confidence level.
A number of industrial processes require effectively measured and maintained amounts of acid or base in aqueous solutions well above concentrations appropriate for conventional pH metering. This range includes acids or bases above
1N, about 4% by weight. Such solutions are found in alkaline scrubber systems for corrosive acid vent gas removal, process feed reagents, wastewater neutralization, and chloralkali electrolytic cell effluents. Important uses of concentrated acid or base streams also include electroplating bath solvents, paper pulp cooking liquors, and metal finishing processes. On-stream titrators are frequently employed in industry for determining these higher concentrations of acid or base but require relatively excessive maintenance due to their complex electromechanical composition, electrodes, and reagent requirement. Another existing approach is the in situ conductivity probe with a sufficiently high cell constant to deal with elevated concentrations of acid or base. However, solution conductivity exhibits a plateau at intermediate levels of acid or base and, in fact, declines as concentrations approach saturation, limiting direct conductivity systems to monitoring acid or base well below saturation. For example, the specific conductance (conductivity) of aqueous sulfuric acid peaks at 30% and declines at higher concentrations (I). Conductivity and bulk density measurements for acid or base assay typically also suffer from salt interference. The above shortcomings have led to efforts to find new alternatives to the above approaches,particularly for alkalinity determination. Among these are in situ electrode and flowinjection/colorimetry methods along with a flow injection
0003-2700/85/0357-0262$01.50/0 0 1984 American Chemical Society
