Photometric Determination of Silica in Aluminous Materials By the Molybdenum Blue Reaction J. A. BRABSON, 1. W. HARVEY, G. E. MAXWELL1,
AND
0. A. SCHAEFFER'
Tennessee Valley Authority, Wilson Dam, Ala.
0.0500 gram of silica and dissolve in 500.0 ml, of water (1 ml. a 0.1 mg. of silica). Prepare this solution on the same day i t is to be used. When making the gravimetric analysis, use a sufficiently large sample so that the error due to solubility of silica in the dehydrating acid will be kept to a minimum. BORICOXIDE. Ignite boric acid in a platinum dish.
A photometric method, bared on the molybdenum blue reaction, is described for the determination of silica in sodium aluminate solutions and calcined alumina, and of silicon in metallic aluminum. Adjustment of p H is made with an indicator, thereby eliminating the necessity for a p H meter. The effect of aluminum salt concentration upon color development is minimized b y the use of two calibration curves. The method may be used for the estimation of silica content from 0.01 to 1 .O% in calcined alumina by varying the size of the sample. A precision of 4% at the optimum amount of silica determinable by this method may be expected when the method is applied to homogeneous samples.
PROCEDURE
The method has a working range of from 0.0 to 1.0 mg. of silica per 250 ml. in the presence of from 0.0 to 0.4 gram of alumina. If a sample containing 0.1 gram of alumina be considered the minimum practicable size, silica in quantities up to 1% may be determined. Alkaline decomposition methods were chosen for attack of solid samples because these methods convert the silica to a soluble form suitable for determination without danger of volatilization or dehydration. Although different methods are used to convert the samples to sodium aluminate solutions, the final step is identical for all materials listed.
THE
rapid and accurate determination of silica in sodium aluminate solutions and calcined alumina, and of silicon in aluminum, is an important factor in the evaluation of alkaline processes for the production of aluminum. The usual gravimetric determinations not only are time-consuming but may yield low results because of the solubility of silica in the acid used for dehydration ( 2 ) . Several procedures have been described for the use of the molybdisilicic acid reaction (1) in the determination of silicon content of aluminum. Pavelka and Morth (6) used the molybdenum blue reaction (3,4,6) for the microanalysis of aluminum for silicon and phosphorus. Possible interference of color-producing impurities in sodium aluminate solutions upon the determination of silica by the molybdisilicic acid method made it desirable to determine silica by the molybdenum blue method. This paper describes a rapid and accurate method based on this reaction.
PREPARATION OF SAMPLES FOR ANALYSIS. Sodium Aluminate Solutions. Transfer a suitable aliquot containing not more than 0.4 gram of alumina to a 250-ml. beaker containing 100 ml. of water and dilute to 170 ml. Calcined Alumina. Transfer a sample of from 0.1 to 0.4 gram of alumina (depending upon the silica content) to a 125ml. platinum dish, cover with 4 grams of anhydrous sodium carbonate and 0.7 gram of boric oxide, and mix intimately by stirring with a platinum wire. Fuse a t 1000" C. with a blast lamp, or preferably in a muffle furnace, until a perfectly clear melt is obtained. A 15-minute fusion time is usually sufficient for complete decomposition of t,he material. Cool, cover the melt with 50 ml. of water, and digest on a steam bath until the melt dissolves. Cool to room temperature and transfer the solution t o a 250-ml. beaker containing 50 ml. of water. Rinse the dish, add the washings to the beaker, and dilute to 170 ml. Aluminum Metal. Transfer a sample of from 0.05 to 0.20, ram (depending upon the silicon content) to a 125-mi. platinum jish, add 50 ml. of water and five pellets (about 0.6 gram) of sodium hydroxide. Let stand until the sample disintegrates, then add a few drops of 3% hydrogen peroxide, and digest for 15 minutes on a steam bath. Cool to room temperature and transfer the solution to a 250-ml. beaker containing 50 ml. of water. Rinse the dish, add the washings to the beaker, and dilute to 170 ml. DETERMINATION OF SILICA. Immediately following the transfer of the alkaline solution to the beaker, add 8 drops of thymol blue indicator and add concentrated hydrochloric acid, dropwise, until the aluminum hydroxide, which first precipitates, is nearly dissolved and the solution has a yellow color. Do not brin the color of the indicator to red. Add 1 9 hydrochloric acid iropwise, stirring constantly, until the aluminum hydroxide completely dissolves and the indicator assumes a permanent p h k color. This operation is critical in the adjustment of pH and may require as much as 5 minutes' time with samples of higher aluminum salt concentration. Add 5.0 nil. of 1 9 hydrochloric 2 acetic acid, and 5.0 ml. of ammonium acid, 5.0 ml. of 1 molybdate solution in the succession named. Stir the solution between additions of reagents and stir vigorously for 1 minute after the addition of the molybdate reagent. Wait 5 minutes for the molybdisilicic acid to develop, then transfer to a 250-ml. volumetric flask. Reduce the molybdisilicic acid by adding sloxly from it pipet, with vigorous shaking, 20 ml. of 17% sodium sulfite solution. Eight minutes after the addition of sulfite, add 5.0 2 acetic acid, dilute to the mark, and mix thorml. more of 1 oughly. Thirty minutes after the addition of the sulfite, determine the color intensit,y of the solution with a photometer fitted with a red filter. Use distilled water in the reference cell. (When applying the method to highly colored liquors from the Bayer process, use an equivalent portion of this solution in the reference cell.) Read the amount of silica present from the proper cali-
APPARATUS
A filter photometer (Fisher AC Electrophotometer) fitted with a 650-mp red filter and 2-cm. absorption cells waa used for color measurements. A glass electrode ap aratus (Leeds & Northrup Universal pH potentiometer) was useifor the pH determinations. REAGENTS
+
HYDROCHLORIC ACID (1 9). Dilute 50 ml. of the concentrated acid to 500 ml. ACETICACIDBUFFER. Mix one volume of glacial acetic acid with two volumes of water. THYMOL BLUE INDICATOR SOLUTION. Dissolve 0.4 gram of thymol blue in 10 ml. of freshly prepared 5% sodium hydroxide solution contained in a 300-ml. platinum dish. Dilute to 250 ml. and neutralize with dilute hydrochloric acid to an orange color; avoid an excess, which would precipitate the indicator. Transfer the solution to a 500-ml. flask and dilute to the mark. AMMONIUM MOLYBDATE SOLUTION.Dissolve 25 grams of the salt, (NH4)J40,01r 4H20, in 250 ml. of water. Allow to stand 24 hours and filter. Do not allow solution to stand over a week before usin . SODIUM SOLUTION.Dissolve 170 grams of the anhydrous salt in about 900 ml. of water. Filter and dilute to 1000 ml. ALUMINUMCHLORIDESOLUTION.Dissolve in water either 52.3 grams of the anhydrous salt or 94.8 rams of the hexahydrate, acidify with a few drops of hydrochoric acid, filter, and 0.4 gram of alumina). dilute to 500 ml. (10 ml. STANDARD SILICASOLUTION.Weigh out an amount of sodium metasilicate, Ka2SiOj,5H20, the silica content of which has been carefully determined gravimetrically, which mjll be equivalent to
+
+
BULFITE
1 2
+
Preaent address, U. 8.Navy. Present address, Harvard University, Cambridge, Mass.
705
+
PO6
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 1.
AliOa Present, Gram 0.00 0.08 0.20 0.40 0.60 0.80
Effect of Aluminum Salt Concentration upon pH Adjustment and Color development^ pH before pH after Molybdate Transmittance Transmittance, Addition Measurement % 1.38 * 0.03 4.24 0.00 61.5 0.0
1.41 * 0.04 1.38 0.03 1.34 0.05 1.26 0.04 1.09 * 0.01 f
f
f
4.18 0.00 4.00 * 0.01 3.72 * 0.02 3.40 f 0.02 3.12 0.00 f
f
61.0 A O . 1 61.9 h 0 . 1 62.9 * 0.1 61.6 fO.1 61.0 t0.1
All values are averages for three determinations.
bration curve. Determine a blank on the reagents and subtract this value from the total silica to obtain the net silica in the sample. PREPARATION OF CALIBRATION CURVE. To counteract the effect of aluminum salt concentration on color development, prepare two calibration curves based on 0.2 and 0.4 gram of alumina. Use suitable aliquots of the standard silica solution to determine 11 points on each curve (including the blank) in steps of 0.1 mg. of silica. Dissolve four sodium hydroxide pellets (0.5 gram) in 50 ml. of water contained in a latinum dish and add the desired quantities of standard silica antaluminum chloride solutions. Transfer the alkaline solution to a 250-ml. beaker containing 50 ml. of water. Rinse the dish, add the washings to the beaker, and dilute to 170 ml. Continue the analysis as outlined under the determination of silica. Construct a calibration curve, plotting the silica concentration against the color intensity. Draw a curve beginning a t 100% transmittance, or zero extinction, parallel to the ori inal curve which includes the silica derived from the sodium hyfroxide and aluminum chloride, as well as the other reagents. This represents the true calibration curve and, of course, necessitates the determination and subtraction of a blank for each set of reagents used. The curves are slightly concave and their slopes vary with different amounts of alumina.
Vol. 16, No. 11
procedure slightly to incorporate the addition of 5 ml. more of 1 2 acetic acid, 8 minutes after the addition of sulfite. EFFECT OF ALUMINUM SALTS.Using the conditions outlined above, a series of tests was made to see what effect variations in aluminum salt concentration have upon color development. During these tests, 0.5 mg. of silica was added to each sample, and the addition of alumina was varied from 0.0 through 0.8 gram. These tests also included pH determinations before the addition of ammonium molybdate and after the measurement of transmittance. The results are given in Table I. Apparently, increases in concentrations of aluminum salts, up to 0.4 gram of alumina, cause a corresponding retardation of color development. These variations, obviously too significant to be ignored, were minimized by the preparation of two calibration curves covering the range 0.1 to 0.4 gram of alumina. At least three factors may exert an influence upon the lower transmittance values found with the 0.6- and O.&gram alumina concentrations. These are lower initial pH values before addition of molybdate, lower pH during reduction with sulfite, and traces of silica from the aluminum salts. Difficulty in redissolving these larger amounts of alumina made continued study of these points inadvisable.
+
EXPERIMENTAL
ADJUSTMENT OF pH. The determination of silica by the molybdenum blue reaction is dependent upon the amount of molybdisilicic acid formed, which in turn is affected by variations in pH. I t was established experimentally that for maximum development of molybdisilicic acid the pH must be adjusted to between 1.0 and 1.4 before the addition of molybdate. Of the several techniques tried for the adjustment of pH the one described above proved most successful. Measurements of the pH of prepared solutions of varying alumina content used in the preparation of calibration curves and of actual samples indicated that the pH could be adjusted by the method described to 1.35 * 0.1 pH before the addition of molybdate. Typical data on pH obtained by this procedure are given in Table I. PROGRESSION OF COLOR. Kahler (4) pointed out that the use of acetic acid at a relatively high pH caused considerable color progression. This effect could not be disregarded if acetic acid were. to be used to prevent the hydrolysis of aluminum salts. To check the color progression of molybdenum blue, with and without the presence of aluminum salts, solutions were prepared in the same manner as for the calibration curves. The molybdisilicic acid formed by the addition of ammonium molybdate was reduced with sodium sulfite as described in the procedure, the solutions were immediately diluted to 250 ml., and transmittance measurements were made a t several time intervals. Results of typical tests are shown in Figure 1. I t was found that the rate of color development levels off after 10 minutes, although the maximum intensity of color is not reached for ssveral hours. The presence of aluminum salts also appeared to retard maximum color development. Although the results shown in Figure 1 were obtained on clear solutions, interference from hydrolysis of aluminum salts was encountered on some of the samples. This difficulty was met by varying the
15 20 25 TIME, MINUTES Figure 1.
30
35
Progression of Molybdenum Blue
EFFECTOF SODIUM AND BORON SALTS. Before applying the method t o the analysis of calcined alumina, the effects of larger amounts of sodium salts and of boron salts were investigated. Solutions similar to those used for the preparation of calibration curves and containing 2.6 grams of sodium hydroxide and 0.7 gram of boric oxide were analyzed for silica. The results, after subtraction of reagent blanks, indicated no effect from these fluxing reagents. EFFECTOF IRON SALTS.Solutions identical with those used for checking the effects of boron and sodium, to which had been added 0.5 and 1.5 mg. of FelOI as ferric chloride, were analyzed for silica. N o significant difference in results was observed. EFFECT OF PHOSPHORUS. Solutions similar to those used for preparation of calibration curves and containing 0.2 gram of
Table
11.
Effect of Phosphorus on Photometric Determination of Silica SiOr Added PlOl Added SiO, Found Mo. Mo. M47. 0.20 0.20 0.20 0.60 0.60 0.60 1.00 1.00 1.00
0.2 0.5
1.0 0.2 0.5 1.0 0.2 0.6 1.0
0.21 0.22 0.24 0.61 0.65 0.70 1.02 1.07 1.11
ANALYTICAL EDITION
November, 1944
alumina in each instance and varying quantities of silica and phosphorus pentoxide as phosphoric acid were analyzed in the usual manner. Results are shown in Table 11. The interference from phosphorus was considerable. However, the lowest amount tested was larger than the amount ordinarily encountered in 0.4 gram of alumina. Approximate corrections for phosphorus are probably feasible in the few instances where the phosphorus pentoxide content of the sample exceeds 0.2 mg. APPLICATION TO SODIUM ALUMINATE SOLUTIONS
Two samples of highly colored sodium aluminate liquors from the Bayer process were analyzed gravimetrically by double dehydration with sulfuric acid and by the photometric method. Results are shown in Table 111.
111.
Comparative Analyses for Sitica in Sodium Aluminate Solutions Sample SiO:, Grams per Liter No. Gravimetric Photometric
Table
0.287 * 0.005 0.319 0.014 Average of three values.
;:
a
f
0.287 0.002 0.326 A 0.001 f
APPLICATION TO CALCINED ALUMINA
Three samples of calcined alumina previously analyzed by the standard gravimetric method, which involves decomposition of the sample with potassium pyrosulfate and double dehydration of the silica with sulfuric acid, were analyzed by the photometric method (Table IV). APPLICATION TO ALUMINUM METAL
Three samples of aluminum metal were analyzed gravimetrically by double dehydration with sulfuric acid after decomposition with sodium hydroxide and hydrogen peroxide and by the photometric method. The results are shown in Table V. For comparative purposes, the data in Tables 111, IV, and V were reported to one significant figure more than is justified by the accuracy of the methods used.
707
metric method is applied to aluminum. This discrepancy cannot be explained by incomplete oxidation of silicon, since clear solutions resulted when the sodium hydroxide-hydrogen peroxide solutions of the samples were acidified with sulfuric acid. An explanation of the discrepancy between the results obtained in the analysis of alumina and those obtained in the analysis of aluminum may be derived from consideration of (a) the loss of silica during a double dehydration with sulfuric acid, and ( b ) the silica content of the analytical reagents used. Hillebrand and Lundell (8) reported consistently low results in the gravimetric determination of silica, and attribute the losses to the solubility of silica in the dehydrating acid (hydrochloric or sulfuric). The error inherent in the gravimetric method was verified experimentally by analyzing solutions of aluminum sulfate to which known amounts of silica had been added. Low results were obtained in every analysis. The principal reagents entering into the analyses were those used to effect dissolution of the samples-namely, potassium pyrosulfate for the alumina and sodium hydroxide for the aluminum. Both reagents contained traces of silica detectable by spectrographic means. Since the quantity of reagent used with the alumina was about 12 times that used with the aluminum, the silica loss during dehydration of the alumina solution may have been compensated by reagent impurity, whereas no such compensation occurred in dehydration of the solution of aluminum. I n view of this probability, together with the fact that the discrepancies between the gravimetric and photometric analyses of aluminum represent differences of the order of only 1 mg. of silica per 5 grams of alumina, it was concluded that the results obtained photometrically probably represent more nearly the absolute silica content of aluminous materials. ~~
~~
Table V. Comparative Analyses for Silicon in Aluminum Sample Si, Per Cent NO. Gravimetric Photometric 15 0.094 0.008 0.117 0.001 f
2" 0.259 3s 0.278 O Average of six valuea. b Average of three values.
f f
0.005 0.008
0.293 * 0.013 0.297 * 0.004
PRECISION AND ACCURACY
DISCUSSION
The precision of the method is illustrated best by the analysis of the sodium aluminate solutions. On the basis of the results in Table I11 and similar results from the analysis of less highly colored liquors, the method is believed to have a precision within 0.02 mg. of silica. On the basis of the optimum amount of silica determinable, 0.5 mg., this represents a precision of 4%.
The method described makes use of the molybdenum blue reaction for determining silica under carefully controlled conditions. Variables are compensated for by means of calibration curves for combinations of reagents that simulate actual samples. Some of the conditions were chosen arbitrarily; others were determined experimentally. A number of calibration curves for solutions of varying concentrations of aluminum salts were prepared. Although the curves for 0.1 and 0.2 gram of alumina were sufficiently concordant to be considered identical, an appreciable difference was noted in the curves based on 0.08 and 0.40 gram of alumina. This made advisable the use of more than one calibration curve. When curves are constructed for 0.2 and 0.4 gram of alumina per 250 ml., samples containing from 0.0 to 0.4 gram of alumina may be analyzed with reasonable accuracy. For more precise work it is necessary to prepare calibration curves covering the range 0.0 to 0.1 gram of alumina per 250 ml. No attempt has been made to apply this method to aluminum alloys.
Table IV. ComDarative Analvser for Silica in Calcined Alumina Sample SiO,, Per Cent No. Gravimetric" Photometric b 0.041 * 0.001 1 0.041 0.011
0.076 0.010 2 0.077 * 0.006 0.266 0.008 0.294 * 0.009 36 Average of three values. b Average of six values. O,Sample3 contained 0.67% of PaOs. Difference in results is attributed to positive error caused by presence of PaOe. f f
This degree of precision was not attained in the analysis of alumina and aluminum. A comparison of the results in Tables IV and V, obtained by both gravimetric and photometric methods, indicates that the precision of the two methods is essentially the same. Possible heterogeneity of the samples may have affected results obtained when using both the gravimetric and photometric procedures. The data in Tables IV and V indicate that, although there is good agreement between the two methods when applied to alumina, consistently high results are obtained when the photo-
LITERATURE CITED (1) Dienert, F., and Wandenbulcke, F., Compt. rend., 176, 1478 (1923). (2) Hillebrand, W. F., and Ldndell, G. E. F., "Applied Inorganio Analysis", p. 540, New York, John Wiley & Sons, 1929. (3) Isaacs, L., Bull. SOC. china. biol., 6, 157 (1924). (4) Kahler, H. L., IND.ENO.CHPM., ANAL.ED., 13, 536 (1941). (5) Pavelks, F., and Morth, H., Mikrochemie, 16, 239 (1934). (6) Woods, J. T., and Mellon, M. G., IND. ENG.CHEM.,ANAL.ED., 13, 760 (1941).
