V O L U M E 25, NO. 1, J A N U A R Y 1 9 5 3 composition of potassium acid phthalate up to the same temperature consists of a mixture of the acid salt and the normal salt. The presence of admixed potassium chloride has no significant effect on the initial rate of decomposition of potassium acid phthalate, and mixtures of the two saks used as unknowns may be dried in the same way as potassium acid phthalate itself. LITERATURE CITED
( 1 ) Hendrixson, W. S., J . Am. Chem. Soc., 42, 726 (1920).
145 (2) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis.” p. 140, New York, John Wiley & Sons, 1929. (3) Kolthoff, I. M., and Stenger, V. A., “Volumetric Analysis,” 5’01. 11,p. 94, New York, Interscience Publishers, 1947. (4) Willard, H. H., and Furman, N. H., “Elementary Quantitative Analysis,” p. 136, New York, D. Van Nostrand Co., 1940. RECEIVED for review November 2 , 1951. Accepted October 30, 1952. Constructed from a thesis presented to T h e Graduate School of T h e Ohio S t a t e University in partial fulfillment of the requirements for t h e h1.S. degree, June 1961.
Photometric Determination of Silica in Alkalies 0. A. KENYON
AND
H. A. BEWICK
Solvay Process Division, Allied Chemical & Dye Corp., Syracuse,
N. Y .
Because the usual gravimetric determination of silica in alkali products is lengthy and inadequate for very small amounts, an investigation was undertaken. A photometric method employing the molybdenum blue complex was developed, which is rapid and has precision and accuracy suitable for quality control. A standard deviation of f 0 . 9 microgram in 100 micrograms of silica has been realized. No treatment of the sample other than solution, pH adjustment, and proper salt concentration is required. No serious interference is encountered. and the method has a wide range.
N
O PREVIOUS critical study of the photometric mol?-bdate method for silica in thp high salt concentrations encountered in the analysis of sodium hydroxide and other alkalies is known. A recent review (3) ha. indicated the need for a study of the effect of salinity on color development of molybdenum complexes in the determination of silica in sea n-ater. The usual gravimetric determination of silica in sodium hydroxide or other alkali products is lengthy and inadequate for very small amounts. The photometric determination of silica using the molybdate reaction is the recognized procedure for small amounts of silica, and this paper reports the development of the proper conditions for its application t o alkalies. Early investigations of this method Tvere made by Jolles and Xeurath ( 7 ) and Dieneit and Wandenbulcke ( 6 ) . The method with various modifications has been used for the determination of silica in fresh water ( 9 ) , in sea tvatcr (10, 15, 1 6 ) , and in boiler feed water ( 4 , 8,11, I S ) . The optimum conditions for the color reactions were determined with special emphasis upon the effects of p H and varying concentrations of sodium or potassium chlorides. Other important factors such as concentration of reagents, time required for color development, color stability, method of neutralizing samples, and reproducibility of the method were studied. I n this work the authors xere unable to obtain reproducible results with Bunting’s molybdate reagent ( 4 ) . Stoloff’s reagent (12)was found to be satisfactory. The volume of the sample a t the time the molybdate reagent was introduced was found to be important. Great variations in the intensity of the color formed were noted for all volumes below i 5 ml. Published data vary concerning p H recommended for maximum color formation. I n this study color formation was found to be jointly dependent upon p H and chloride concentration. The findings of this work may also be applied to the photometric determination of the much larger amounts of silica present in alkali silicates according to Hiskey’s technique (e),in which the spectrophotometer is adjusted to zero absorbancy using a high silica standard and a lyider slit width. APPARATUS
The absorbancy measurements were made with a Beckman RIodel DU spectrophotometer and 1.00-cm. cells. The red-sensi-
tive phototube was used a t 660 and 825 mp to measure the molybdenum blue complex and the blue-sensitive phototube was used a t 410 mp to measure the yellow molybdisilicic acid. The reference cell contained distilled water. Slit widths were 0.09, 0.10, and 0.15 mm., respectively. Other apparatus required included a Beckman pH meter, Model H-2, 100-ml. Kohlrausch flasks, and 100-ml. mixing cylinders. REAGEhTS
Hydrochloric acid, 10.0 Y. Citric acid solution, 10% w./v. Tartaric acid solution, 10% w./v. Sodium chloride solution, 2 M, 116.9 grams per liter. Adjusted to p H 1.3. Potassium chloride solution, 2 X , 149.1 grams per liter. Adjusted to p H 1.3. Ammonium molybdate solution, 5%. Five grams of ammonium molybdate Tvere dissolved in 80 ml. of warm distilled water. The solution mas cooled, 2.8 ml. of concentrated sulfuric acid were added, and the solution was made up to 100 ml. Ammonium hydroxide solution, silica-free. Air was passed for a t least 1 hour through 250 ml. of concentrated ammonium hydroxide in a 500-ml. gas washing bottle and conducted into 250 ml. of distilled water in a polyethylene bottle. As this ammonia solution is used only for p H adjustments, the strength is unimportant. Reductant Solution. Sinety grams of sodium bisulfite, NaHSOS, were dissolved in 800 ml. of distilled water. Seven grams of anhydrous sodium sulfite and 1.5 grams of l-amino-2naphthol-4-sulfonic acid (Eastman No. 360) were dissolved in 100 ml. of distilled water. The two solutions were mixed, diluted to 1 liter, and stored in an amber glass bottle under refrigeration. Silica Solution. A stock solution was prepared by dissolving 5.1 grams of sodium metasilicate nonahydrate, Ka2Si03.9Hz0 in 1 liter of distilled water and standardized gravimetrically ( 2 ) . Appropriate dilutions were used for the working standard silica solution in the preparation of calibration curves. EXPERIMENTAL WORK
In this investigation the concentration of reagents, stability of color complexes, method of sample neutralization, effect of pH, effect of varying concentration of sodium and potassium chlorides, and precisionof the method were carefully studied. The procedure of Bunting ( 4 ) was used as a starting basis in this method of investigation. Stability of Silica Solutions. Investigators have stated ( 4 , 9) t h a t silica solutions stored in a hard-rubber container will not deteriorate. During the present work it was found that a standard
146
ANALYTICAL CHEMISTRY
silica solution (1 ml. = 0.004 gram of 9iO2) a t p H 12.0 stored in a hard-rubber bottle had not deteriorated over a period of 29 months. However, a working standard silica solution ( 1 ml. = 0.2 mg. of SiO,) a t p H 8.5 stored in a hard-rubber bottle had deteriorated 4% over the same period of time. Neutralization of Sample. .%dams ( 1 ) found that silica may undergo polymerization and various colloidal changes in solutions which are about neutral. This fact raised the question whether alkali solutions should be neutralized by pouring into acid solutions or vice versa. A scimpie of sodium hydroxide solution was prepared and the test performed both waja. Pouring the acid solution into the a l k d i solution produced 38% lower values for silica. The method herein dewibed can be used only for the determination of soluble or crystalloidal silica and not colloidal silica.
0
160 240 320 MICROGRAMS SILICA
80
400
F i g u r e 2. Calibration Curves Molybdenum blue, 10-mm. cells, 660 mp A . No salts present B . 0.58 gram of sodium chloride C. 5.8 grams OF sodium chloride
I /
p H 1.5
amount's of iron v-hich may be present. Thechoice of acid is dependent upon the color comples to be measured and the salt concentration of the sample. Citric acid is preferred to titrtaric acid in the low salt concentrations present when the yellow complex is measured photometrically. This is due to the lesser effect of the citric acid on the stability of the color. After the addition of citric acid the absorbancy must be read within 2 t o 10 minutes. For the high salt concentrations likely to occur when the blue complex is used, tartaric. acid is preferred because here the tartaric acid results in greater stahilit>-of the yellow color complex prior to the reduction to the blue complex. When the salt concentrat,ion is greater than 2 grams pcr 100 ml., 2 ml. of 10% t,artaric acid are used and for lesser concentrations 4 ml. of 1070 tartaricarepreferrcd. Thevolumesof tart,aric acid used are governed by the l o x iron content in the high salt-low silica solutions rcsulting xhen mercury cell sodium hydroxide is neutralized. h series of papers by Strickland ( I $ ) , published after the prescnt work was completed, serves to explain some irregularities encountered in the stabilitj- of the >-elloa-complex. The forniation of either the alpha- or ~)ct,a-silicomolyhdic acids was shown by Strickland to be dependent on p H and extraneous ion (chloride in this caw) concentration. The convrrpion of the beta acid to the more stable alpha form is accclcrated by presence of citrate or tartrate, account,ing for the rapid fading of the yellow complex which the authors experienced. Stability of Molybdenum Blue Color. The molybdenum blue comples was found t o be completely developed in the presence of sodium chloride 5 minutes after the addition of the reductant solution and the absorbancy remained constant for the 65-hour period tested. The molybdenum blue complex used in the present work is considered to be essentially the royal blue complex described by Strickland (14) as a reduction product of the beta acid. The rontiitions for itn formation outlined b!- Strickland are fulfilled. How-
gL e
pH 1.7
0
20
1
2
3
4
5
GMS.NACL PER 100 ML. F i g u r e 1. Effect of pH and S o d i u m Chloride C o n c e n t r a t i o n on Silica Re-
covery 200 micrograms of silica added
Solution Volume. The volume of the sample solution, a t the time the molybdate reagent was introduced to produce the color, was found to be of great importance, With a solution volume of 25 ml. or less an intensely blue color \vas obtained, very different from the normal molibdenum blue. Duplicate samples did not give check results. K h e n the solution volume was inrreased to 75 ml. before the addition of the molybdate reagent, excellent reproducibility was obtaintd. 1-ariations of f 5 nil. a t this levcl had no effect on reproducibility. Concentration of Reagents. The use of 2 ml. of 5% ammonium molybdate was found t o be sufficient for maximum color development. Five milliliters of reductant solution were required t o produce a maximum color and give good reproducibility. Stability of Yellow Complex. The yellow complex, molybdisilicic acid, was completely developed within 5 minutes in a silicate solution in the absence of t h e citric and tartaric acids, and remained stable for the GO minutes tested. However, citric or tartaric acid is required t o complex small
147
V O L U M E 25, N O . 1, J A N U A R Y 1 9 5 3 ever, the stability of the bluecomplex as formed in the present procedure exceeds that claimed by Strickland. Effect of pH and Salt Concentration. I n agreement with the findings of Knudson, Juday, and Meloche (9) it was confirmed that the p H of the final 100-nil. volume must be in the range 1.6 to 2.0 in the absence of sodium chloride for maximum development of the yellow complex. However, the presence of sodium chloride, or other chlorides, had considerable effect upon the color development of the yellow complex. As the blue romplex is the reduction product of the yellow complex, the blue color is affected in like degree. T o investigate both factors, the effect of p H and sodium chloride concentration was studied. Hydrochloric acid was used for sample n~utralixation and acidity control, in line with its use in this laboratory as a general reagent in the preparation of master alkali solutions for spectrographic and photometric determiniition of all trace eleinents of interest. I n preliniinarg work, it was demonstrated that the effect of sodiuni chloride on the color development of the yellow complex was marked. As the concentration \vas increased from 0 to 6 grams of sodium chloride per 100 nil., there was a definite decrease in the amount of yellow complex formed. Slight variations in pH became increasingly important as the sodium chloride con( entration increased. These tu-o variables were then studied in more detail in the determination of silica as the blue complex. Synthetic samples each containing 200 micrograms of silica and 0.0, 0.5, 1.0, 2 0. 3.0, 4.0, 5.0, and 6.0 grams of sodium chloride, respectively. \I ere analyzed for silica according to the recommended procedure, with the cweption of the p H adjustment at the i5-ml. volume. The rerovery of silica was studied a t p H 1.1, 1.3, 1.4, 1.5, and 1 T u.inp 3 it:tnclard analytical curve prepared a t pH 1.3 from standard. c ontdining no salt. Figure 1 shows that the effect of the sodiuni c,liloIide is less and more nearly linear when the p H a t the i’5-mI volume is adjusted to 1.4. A slight deviation in the st:ind:trd~zingbuffer or pH instrument on the low side of p H 1.3 could produce erratic results. The interdependent effect of p H and sodium chloride conLentration as shown i n Figure 1 is a most important factor in the determination of siliea In alkalies. In order to shou the relative effect of different salts on th(, formation of the niolvbdenuni blue complex, the following experiment was carried out. Fifty-milliliter solutions containing, respectively, sodium chloride (5.85 grams), calcium chloride dihydrate (7 35. grams), ammonium chloride ( 5 35 grams), and potassium chloride (7.45 grams) were taken and 200 milligrams of silica were added to each. The molybdenum blue complex was developed according to the recommended procedure and 0.24 the absorbancy readings were found to be 0.210, 0.196, 0.190, and 0.163, & r e s p e c t i v e l y . In thc presence of no salt the absorbancy was 0.268.
200 mg. of ferric oxide as ferric chloride, 100 mg. of phosphorus pentoxide as disodium hydrogen phosphate, 100 mg. of sodium nitrate, 200 mg. of sodium sulfate, and 2 mg. of sodium chlorate cause no interference when present separately.
0.6-
0.5-
>
0
2 0.4-
m
a
8
0.30.20.1 -
0
50 100 150 200 MICROGRAMS SILICA
Figure 4. Calibration Curve Molybdenum blue, 10-mm. cells, 825 A . No salts present B . 5.8 grama of sodium chloride
mp
Precision of Method. On twenty synthetic samples containing 100 micrograms of silica and 2.9 grams of sodium chloride, the standard deviation was &0.9 microgram and the maximum deviation 2 micrograms. The absorbancy of the molybdenum blue was measured a t 660 mp. DISCUS SlON
Careful control of pII, proprr method of sample preparation, and concentration of alkali chlorides are important factors Rhich affect the formation of molybdisilicic acid and the molybdenum blue complex. The 10.0 -V hydrochloric acid was used to neutralize the alkali samples and the p H n-as adjusted with silica-free ammonia. This permitted a constant chloride content during color development not otherwise conveniently posPible because of the slight variation in strength of caustic liquors. The silica can be easily determined in four sodium hydroxide samplrs in 1 hour.
0.30m 0
80
160
240
MICROGRAMS SILICA Figure 3. Calibration Curves Molybdenum blue, IO-mm. cells, 660
A. B.
mp
0.75 g r a m of potassium chloride 1 . 5 grams of potassium chloride
Effect of Diverse Ions. Citric or tartaric acid is used t o destroy a n y molybdiphosphoric acid formed, as it, too. results in the formation of m o l y b d e n u m blue. Phosphates may be cncountered in analyzing alliali detergents and in sodium hydroxide produced from milk of lime anti soda ash. Up to
REC04IRIENDED PROCEDURES
Molybdenum Blue Method. were used in the analysis.
Sodium carbonate Sodium bicarbonate Sodium sesquicarbonate Caustic soda. 98 t o 100% Caustic soda, 73% Caustic soda, 00% Caustic potash 90 to 92% Potassium carbonate, 9 9 5
The following sample weights 10 grams
1 3 grams
14 grams 7 . 5 grams
10 grams 15 grams 12 grams 13 5 grams
The indicated weight of sample was diluted with distilled water in a polyethylene beaker to a volume of approximately 50 ml. and . hydrochloric acid in anpoured with stirring into 20 ml. of 10.0 V other polyethylene beaker. When cool, the solution was tramferred to a 100-ml.volumetric flask and made up to the mark. This solution contains a small excess of acid. -15-ml. (0.585 gram of sodium chloride or 0.746 gram of potassium chloride) or 50-151. (5.85 grams of sodium chloride or 7.46 grams of potassium chloride) aliquot was transferred to a 100-ml. beaker and diluted to 70 ml. and the acidity was adjusted to pH 1.4 n i t h a pH meter using the silica-free ammonia solution, The aliquot taken was governed
148
ANALYTICAL CHEMISTRY
by the silica content. If the silica in the original weighed samples was in the range 0 to 1.0 mg. of Sios, a 50-ml. aliquot of the neutralized solution was taken and for the range 1.0 to 20 mg. of SiOz a 5-ml. aliquot was used. The adjusted solution was transferred to a 100-ml. mixing cylinder and made u p to 75 ml. and 2 ml. of 5% ammonium molybdate added. After 5 minutes, 10% tartaric acid was added, dependent on the salt concentration.
0,201 0.15
A blank for all reagents used was prepared and measured in the same manner. Molybdenum Yellow Method. When samples containing more than 400 micrograms of silica per sample aliquot were regularly encountered, the absorbancy measurements were made on the yellow molybdisilicic acid complex a t 410 m p . To the p H adjusted solution a t 75 ml., 7.5 ml. of 5% ammonium molybdate were added and mixed. After 5 minutes, 2 ml. of 10% citric acid were added, the solution was made up to 100 ml. and mixed, and the absorbancy measurements were made 2 to 10 minutes following the addition of the citric acid. Four calibration curves for standard solutions were usually made a t the concentrations 0.585 and 5.85 grams of sodium chloride and 0.746 and 7.46 grams of potassium chloride per 100 ml. These curves are illustrated in Figures 2 t o 5 . ACKNOWLEDGMENT
0
2 0.10 U
0.051
0
The authors express their appreciation to G. L. Murphy and George Oplinger for helpful advice.
/
LITERATURE CITED
400 600 800 MICROGRAMS S I L I C A
200
1000
Figure 5. Calibration Curve Yellow complex, IO-mm. cells, 410 mo 0.58 g r a m of sodium chloride
When the salt concentration was greater than 2 grams of sodium chloride per 100 ml. or 3 grams of potassium chloride per 100 ml., 2 ml. of 10% tartaric acid were used; otherwise 4 ml. of 10% tartaric acid. After the addition of tartaric acid the solution was allowed to stand 5 minutes, 5 ml. of reductant solution were added, and the solution was u p to 100 ml. The solution was mixed after the addition of each reagent. After 10 minutes the absorbancy was measured in a 1-cm. cell a t 660 mp. Very small amounts of silica may be measured more accurately a t 825 mp.
(1) Adams, M. F., IND.ENG.CHEM.,ANAL.ED.,17, 542 (1945). (2) Am. SOC. Testing Materials, ASTM Designation 859-50T, Referee Method B. (3) Barnes, H., Clayton, H. R., Irving, H., and Leyton, L., Ann. Repts. Progress Chem. (Chem. SOC.London), 21, 341 (1949). (4) Bunting, W.E., IND.ESG. CHEM.,ANAL.ED., 16, 612 (1944). (5) Dienert, F., and Wandenbulcke, F., Cornpl. rend., 176, 1478 (1923). (6) Hiskey, C. F., ANAL.CHEM.., 21, 1440 (1949). (7) Jolles, A,, and Keurath, F., 2. angew. Chem., 11, 315 (1898). (8) Kahler, H. L., IND.ERG.CHEM.,ANAL.ED., 13, 536 (1941). (9) Knudson, H. W.,Juday, C.. and bleloche, V. W., Zbid., 12, 270 (1940). (10) Robinson, J., and Spoor, H. R., Ibid., 8, 455 (1936). (11) Schwartz, M. C., Ibid., 14, 893 (1942). (12) Stoloff, L. S., Ibid., 14, 636 (1942). Ibid., 16, 574 (1944). (13) Straub, F. G., and Grabowski, H. -4,, (14) Strickland, J. D. H., J . Am. Chem. Soc., 74, 862-76 (1952). (15) Thayer, L., IND.ENG.CHEM.,ANAL.ED., 2, 276 (1930). (16) Thompson, T. G., and Houlton, H., Ibid., 5 , 417 (1933). RECEIVED for review July 7, 1952. l c c e p t e d October 3, 1952.
Photometric Determination of Silicon in Ferrous, Ferromagnetic, Nickel, and Copper Alloys A Molybdenum Blue Method C. L. LUKE Bell Telephone Laboratories, Inc., Murray Hill,N . J.
T
HE purpose of the present investigation was the development
of a simple photometric method for the determination of silicon in all types of ferrous, ferromagnetic, nickel, and copper alloys. A few qualitative experiments and a study of the recent literature on photometric silicon analysis ( 2 , 4,7 ) indicated that it would be virtually impossible to develop such a widely applicable method without resorting to separations before the photometric determination was made. From the work of Guenther and Gale (3) it appeared probable that the interfering constituents could be separated by one of the new precipitation-extraction techniques. Serfass and his associates (8)used a sodium diethyldithiocarbamate-chloroform extraction to remove nickel from nickel plating baths prior to the determination of aluminum. This extraction method proved to be satisfactory for the removal of the metals which interfere in the silicon determination.
As a result, a method has been developed which is rapid, convenient, and widely applicable. The method is not applicable to alloys where solution of the silicon is incomplete-e.g., National Bureau of Standards aluminum base alloy, No. 86-e-but almost all ferrous, ferromagnetic, nickel, or copper alloys can be successfully analyzed. The method consists of solution of the sample in a mixture of hydrochloric and nitric acids, destruction of the nitrio acid by heating with formic acid, removal of all interfering metals by a carbamate-chloroform extraction, and finally, determination of silicon by the usual photometric molybdenum blue method using, essentially, the technique described by Minster ( 5 , 6 ) . The time of analysis is approximately 2 hours. The method is applicable in the range of 0.01 to 2% of silicon if the rather poor precision and accuracy in the upper ranges, which result from the limitations of photometric technique, can be
