,mination of Si lica in Sea Water Using Solvent Extraction blank was run a t an H2S04concentration of 2.25M. From Figure 1 one may note that the absorbance of the aqueous phase minus the blank in the aqueous phase ( A - B ) is 0.077, whereas the final absorbance in the extractant less the blank in the extractant (C) is 0.093. This color enhancement of about 20'34 in the organic phase was consistently observed. After a successful single step extraction subsequent re-extractions with fresh ethyl acetate produced essentially blank values as indicated in Table I.
SIR: The analysis of soluble silica in surface waters of the ocean requires a method of high sensitivity, free from phosphate interference. Because large numbers of these analyses are done at sea, the method must be relatively simple to perform. Of the two methods in common use, the spectrophotometric measurement of silicomolybdic acid is simple but insensitive because of blank interference in the region of the absorbance peak; the spectrophotometric analysis of silicomolybdous acid as described by Mullin and Riley (4) is more sensitive and free from blank interference, but more laborious and somewhat tricky. Silicomolybdic acid may be partially extracted into various oxygen-containing solvents ( 2 , 6 )from the solution in which it is formed. If, however, the aqueous solution is strongly acidified, the efficiency of extraction rises markedly as the acid drives the silicomolybdic acid into the organic phase. Figure 1 shows the relative absorbance of silicomolybdate a t 3600 A. in the ester and aqueous phases for various concentrations of sulfuric acid. Silicomolybdate was formed by adding 2 ml. of a mixture of 2%% ammonium molybdate solution and 1M sulfuric acid (5:2) to 40 ml. of 18.5 pLV Na2SiF6 solution. a f t e r 20 minutes, 10 ml. was removed for absorbance measurement, and the remaining silicomolybdate solution b a s acidified by 10 ml. of HzS04of appropriate strength, cooled to room temperature, then extracted by shaking for 1 minute with 10 ml. of ethyl acetate. The phases were separated and volumes measured. For purposes of comparison, absorbances were adjusted as if dilution to 40 ml. had been carried out in each case. The
Table I. Absorbances at 3350 A. of Ethyl Acetate from Successive Extractions of Same Aqueous Phase
Extraction 1st 2nd 3rd Distilled water
Absorbance 0,879, 0.870, 0.874
0.022, 0.022, 0.032 0.004, 0.003, 0.005 blank 0.011
Other esters were tested for a more favorable combination of extraction and insolubility. The results of an experiment similar to that in Figure 1 are shown in Figure 2, although in this case the absorbances were not measured for blanks or for the aqueous phase. More acid is required to drive silicomolybdate into these other esters. The efficiency of extraction correlates with the aqueous solubility. Ethyl acetate is the most soluble ester tested, isopropyl acetate second, isobutyl and isoamyl acetate about the same, and butyl butyrate the least soluble. If solubility and extraction efficiency are related then there is little chance of finding an insoluble ex-
:!I
tractant, but the ester solubility is no disadvantage. That fraction of the ester which is dissolved in the aqueous phase does not hold any silicomolybdate; therefore, the loss of ester by solution merely concentrates the absorbing species in the organic phase. Ethyl acetate is cheap, readily available, separates quickly and cleanly from the acidified solution, and extracts completely at reasonable acidities. It was therefore chosen as the extractant. A sulfuric acid concentration of 3.W was chosen as the extraction condition. Strickland (5) has shown that the color intensity of silicomolybdic acid is affected by the conversion from the initial beta to the more stable alpha form. However, the two forms have identical absorbance a t 3350 A. By working a t this wavelength any errors due to this spontaneous conversion may be avoided. The acidified molybdate ion (reagent) present in the aqueous phase is too strongly absorbing to permit use of this wavelength, but by solvent extraction the reagent interference is essentially eliminated. Morrison and Wilson (3) have shown that silicomolybdic acid fades in the presence of strong a ~ i d s - O . 4 ~ ~per minute for the beta form and 0.1% per minute for the alpha form in 3.V HC1 a t 4300 A. I n this work fading of 0.570 per minute was observed in 3M H2S04 a t 3350 A, indicating that the time of contact should be minimized. To test for interference by phosphate the analysis was run in triplicate on a solution 33 p M S a H 2 P 0 4and in triplicate on 3.3 p M NaH2P04. I n the former case the absorbance was equivalent to 0.3 p M of silica and in the latter case to slightly less than that amount. I n addition, phosphomolybdic acid (16 p M ) was prepared from NaH2P04 solution
.IO0
2m .os0
-cc
6
,
1
I
$0
I.bO MOLARITY
PI70 H2SO4
1
I
3.60
,060
2 a
.040
.020
n
Figure 1 . Silicomolybdate absorbance (adjusted to equal volumes) in ethyl acetate and aqueous phases after extraction from various concentrations of H2S04 solutions A. 6.
C.
Absorbance of aqueous phase before extraction Absorbance of aqueous blank after extraction Absorbance of blank in ethyl acetate after extraction. Concentration of silicomolybdate 14 wM. Absorbance measured at 3600 A.
764
ANALYTICAL CHEMISTRY
' 0
2 4 M 0 LAR IT Y
6
8
H 2 S 0,
Figure 2. Silicomolybdate absorbance (adjusted to equal volumes) in various esters after extraction from various concentrations of H2S04 solutions Concentration of silicomolybdate is 14 wM. at 3600 A.
Absorbance measured
in 1 M HC1, then acidified with various amounts of sulfuric acid and extracted into ethyl acetate. Phosphate-free solutions were similarly treated. The differences in absorbance are given in Table I1* It may be seen from Table I1 that phosphomolybdic acid is extractable by ethyl acetate, but that the strong acid solutions cause the destruction of the absorbing species. Solvent extraction will normally eliminate the necessity for filtering turbid estuarine waters before analysis. A sample of water collected from Narragansett Marine Laboratory pier in Sarraganaett Bay was analyzed with and without filtration of the original water sample. The results were essentially identical (4.44 i 0.18 and 4.55 f 0.12 p M , respectively). EXPERIMENTAL
Apparatus. dbsorbance measurements were made in I-cm. borosilicate glass cells using a Beckman Model DU spectrophotometer with Beckman No. 73600 power supply. Labware. Glass should be avoided in handling and storing solutions when low concentrations of silica are to be measured. Solutions. a. Molybdate. Dissolve 21.5 grams of ammonium molybdate, (NH4)sMo7OZ4 4Hz0, in 500 ml. of silica-free water. Filter. 6 . Acid. Make 1.OM HzS04 using silica-free water. c. Mixed reagent. Mix two parts of (a) to one part of ( b ) . Make fresh before each set of analyses. d. Make 1:l H2S04 by adding concentrated acid to an equal volume of silica-free water. Store in polyethylene after cooling. e . Standards. Prepare standards by weighing out and diluting NazSiFs in silica-free water. All solutions except the mixed reagent are stable for months. Procedure. T o 30 ml. of sample, add 1 ml. of mixed reagent. Mix well a n d wait 20 minutes for silicomolybd a t e t o form completely. Add 15 ml. of 1 : 1 sulfuric acid, mix and transfer to a separatory funnel containing exactly 10 ml. of ethyl acet>ate. Shake for 1 minute and allow to settle briefly; then discard the aqueous and collect the ester phase in a test tube. Transfer to 1-cm. spectrophotometer cell and read absorbance a t 3350 A. A calibration curve should be prepared by pipetting various amounts of sodium silicofluoride solution into lowsilica sea water. Blank Determination. Accurate blank determination depends on the preparation of silica-free sea water. Merrill ( I ) has described an insoluble ferric hydroxide dispersed cation exchange resin-known as the Honda column-which will extract silica from about 20 column-volumes of sea or fresh water. The water should be passed through the column at the rate of 1 to 2 column-volumes per hour.
Sea water thus treated gave absorbances as low as 0.016 by the solvent extraction method, or 0.020 by the conventional silicomolybdic acid method measured at 3600 A. These blanks are equivalent to 0.3 and 4 p M silica, respectively. Reagent blanks suggest that very little of the acidified molybdate ion is driven into the organic phase. However, the spectrum of the blank in ethyl acetate is not the same as the spectrum of silicomolybdate, implying that the blank is not entirely due to silica contamination.
Table 111.
Table It. Absorbances at 3600 A. after Acidification of 16 ph4 Phosphomolybdate Solution and Extraction with Ethyl Acetate
Absorbance (above blank) Ester Aqueous phase phase
Molarity acid 1.0 1.6 2.5 3.3 4.6
0,372 0.061 0.008 0.007 0.004
0.039 0.008 0.019 0.031 0.018
Determination of Silica in Surface Sea Water before and after Standard Additions. Micrograms of Si02 in 30 MI.
Originally Dresent, 0.5 2.3 1.8“
Added,
Total,
Found,
3.2 3.2 10.8
3.7 5.5 12.6
3.4 5.4 12.5a
Per cent of total 92 98 99
Average of five analyses, all others are average of three.
Comments on Procedure. Once properly set up, twelve analyses may be r u n in slightly more t h a n a n hour. Six separatory funnels should be used, shaken in pairs. T h e phase separation is quick and clean when acid has been added. Any water carried with t h e ester phase will settle out in the test tubes a n d be left behind when the ester is decanted into the spectrophotometer cells. Occasionally water gets into the spectrophotometer cells: if this does not settle out, the cell must be drained and refilled. Water clinging to the sides of the cell can be removed by an acetone rinse.
Table IV. Comparison of Silica Concentration in Sea Water Concentrations by Two Methods
Samples from R/V Trident Cruise 21 Concns. in rmoles per Liter Ethyl Sample acetate Conventional no. method method 7-8 7-9 9-4 .~ 10-19 11-5 11-6 12-5 12-6
24.2p M 29.3
24 p M 29
18 3
18
5.3
19.0 23.8 18.1 21.4
~~
19 19 24 18
21
RESULTS
Sensitivity and Precision. Separate Beer’s law curves were r u n at sea by three different analysts, two of them inexperienced in the method. T h e molar absorptivities obtained were 50,500, 49,500, and 52,600 liters per mole-cm. on sea water. Using a different stock solution for t h e dilution series a value of 49,800 liters per mole-cm. was obtained in the laboratory on sea water a n d 51,300 liters per mole-cm. on distilled water. Analysis in quintuplicate on 1 p M sea water gave absorbances of 0.079 f 0.002; on 7 p M sea water 0.370 f 0.009; sea water blanks were 0.030 i. 0.001 (standard deviations indicated). Accuracy. Replicate samples of surface sea water were analyzed, then sDiked with a known amount of silica and again analyzed. T h e results are shown in Table 111. Several Atlantic Ocean samples were analyzed by two analytical methodsethyl acetate extraction and conventional silicomolybdic acid analysis at
3600 A.
IV .
Results are shown in Table ACKNOWLEDGMENT
The author thanks Benjamin Buglio, Kenneth Wunschel, and Kent Fanning for assistance in the development of this method. LITERATURE CITED
(1) Merrill, J. R., Honda, M., Arnold, J. R., ANAL.CHEM.32,1420 (1960). ( 2 ) Morrison, G. H., Freiser, H., “Solvent
Extraction in Analytical Chemistry,” Wiley, ,New York, 1957. (3) Morrison, I. R., Wilson, A. L.,
Analyst 88, 88 (1963). ( 4 ) Mullin, J. B., Riley, J. P., Anal. Chim. Acta 12, 162 (1955). (5) Strickland, J. D. H., J . Am. Chem. SOC.74, 862 (1952). (6) Wadelin, C., Mellon, M. G., Anal. Chem. 25, 1668 (1953).
DAVIDR. S C H I N K Narragansett llarine Laboratory University of Rhode Island Kingston, R. I. woRKsupported by ofice of Naval Research Contract Nonr-396(08). VOL. 37, NO. 6 , M A Y 1965
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