Rapid Gravimetric Determination of Silicon in Aluminum Alloys PHILIP LISA" AND HENRY L. KATZ Industrial Test Laboratory, Philadelphia Naval Shipyard, Philadelphia 12, Pa. A rapid gravimetric method is described for the determination of silicon in aluminum alloys for amounts exceeding 1.5%. An acid attack is employed using phosphoric, nitric, and sulfuric acids. The silicon is oxidized by evaporation of the solution, an excess of perchloric acid is added, and the silicic acid is dehydrated by moderate boiling of the perchloric acid. The usual procedure of handling dehydrated silicic acid is then followed. The precision and accuracy of the new method are determined by a statistical study of the data.
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N INVESTIGATION was recently undertaken a t this laboratory to obtain a rapid and accurate gravimetric method for the determination of silicon in aluminum alloys. Although many colorimetric procedures which have the necessary speed and accuracy have appeared in the literature (7, 8,9,I C ) , the control of pH, temperature, and time was so critical as to render these methods unsuccessful in the hands of routine analysts running Large numbers of determinations. This laboratory usually determines lower amounts of silicon (under 5%) spectrographically, but the need still existed for a rapid chemical procedure for determining silicon contents of 5y0 and above. Two gravimetric procedures have been used in this laboratory for the determination of silicon in aluminum alloys. One is a modified A.S.T.M. method (2), employing an alkali attack of the aluminum alloy followed by a single evaporation with hydrogen peroxide and a subsequent single dehydration with perchloric acid; the other is the Alcoa procedure ( I ) , similar to the A.S.T.M. method but using a triple evaporation with hydrogen peroxide followed by a double dehydration with a perchloric-sulfuric acid mixture. Since the alkali attack common to both procedures is tedious and time-consuming, efforts were directed toward evolving an accurate and rapid gravimetric method employing an acid attack of the aluminum alloy. Many papers have been published on the use of acid mixtures (3,IS,l4,16,18,19)for attacking aluminum alloys in an attempt to speed up the analysis. In general these procedures tend to give low results which have been attributed to two factors-the incomplete oxidation of silicon in the metal to silica and loss of volatile silicon compounds, such as silicon hydride, incurred during acid decomposition (4,6, IT), indicating that the proper combination of acids had not been attained. Fuchshuber, however, proposed a method (6) using a mixture of phosphoric, nitric, and sulfuric acids which, besides effectively dissolving the alloy without any apparent loss of silicon by volatilization, oxidized all the silicon present directly to silicic acid upon evaporation of the solution. Hydrochloric acid was then added and the solution evaporated again until the silicic acid was dehydrated. The main objection to this procedure is that continuance of the dehydration for as little as one minute longer than is necessary tends to cause dissolution of silica in phosphoric acid. A later modification of this method ( 2 1 ) introduced cobalt nitrate as a means of indicating exactly when the silicic acid was completely dehydrated. The authors, however, have found that the change in color of the cobalt nitrate is not so sharp as suggested and have experienced difficulty in obtaining precise and accurate results.
addition of perchloric acid to the solution after the conversion of silicon to silicic acid, followed by a moderate boiling of the perchloric acid, appeared to solve the problem. Subsequent experiments showed that when sufficient perchloric acid was present, the boiling time was not critical and could be extended from 5 t o 15 minutes without causing a significant change in the silicon results. In the course of the investigation the authors confirmed the lower limit on the percentage of silicon (1.5%) that could be determined by means of a phosphoric, nitric, and sulfuric acid mixture (11). Further observations on alloys containing up to 1% magnesium and more than 1.5% silicon showed no apparent loss of silicon due to the possible presence of magnesium silicide. To establish the fact that the acid mixture used in the proposed method effected a complete oxidation of all the silicon present in the alloy, four samples in duplicate (types 43, 355, 356, and 32) were dissolved and the silicon was oxidized in the manner described below. The solutions were then diluted and filtered, and the amount of residue was determined. I n practically every CBW the residue wm negligible and the greatest amount found represented less than 0.05% silicon. The procedure finally evolved employs an acid attack of the alloy by a phosphoric, nitric, and sulfuric acid mixture in a ratio of 3 4 1 , respectively. The solution is evaporated until the silicon is completely oxidized, as evidenced by a clearing of the solution. Perchloric acid is then added and the solution boiled moderately. Gelatin is added to facilitate the filtration of the silica (10) and the usual procedure for handling dehydrated silicic acid is followed. REAGENTS
An acid mixture of 150 ml. of phosphoric acid (sp. gr. 1.71), 200 ml. of nitric acid (sp. gr. 1.42), and 50 ml. of sulfuric acid (sp. gr. 1.84). Ammonium nitrate, C.P. Perchloric acid, 70%. Gelatin solution, 1%. Sulfuric acid (1 to 99). Sulfuric acid (1 to 1). Hydrofluoric acid, 480j0. PROCEDURE
Dissolve 2 grams of the sample (1.5 to 4.5% silicon) or 1 gram of the sample (4.5t o 10.O~o silicon) in an 800-ml. beaker in 80 ml. of acid mixture. For samples containing more than 10% silicon, dissolve 0.5 gram of the sample in 60 ml. of acid mixture. After the initial reaction has subsided and most of the alloy has dissolved, cover the beaker and warm until the sample is completely in solution. Remove the cover glass and evaporate until the solution clears. Immediately add a pinch of solid ammonium nitrate to oxidize any sulfur, swirl, and remove the beaker from the hot plate. Allow to cool slightly, add 60 ml. of perchloric acid (707,), and evaporate to fumes of perchloric acid. Cover the beaker and moderately boil the solution for 8 or 9 minutes. Cool somewhat, cautiously add 300 ml. of hot distilled water and 40 ml. of gelatin solution (1%), and stir well. Filter immediately on an 11-cm. KO.41 Whatman filter paper containing some paper pulp. Swab the beaker and wash the paper 12 times with hot dilute sulfuric acid (1 to 99), followed by a few washes of hot dis-
EXPERIMENTAL
Investigations were undertaken by the authors to eliminat? the critical time factor inherent in the above procedures. The , I Preaent address, Franklin Institute, Laboratories of Research and Development, Philadelphia 3. Pa.
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V O L U M E 19, NO. 4, A P R I L 1 9 4 7
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tilled water. Transfer the Paper and Precipitate to a platinum crucible, char off the paper completely, and ignite the residue a t 11000 C. to constant weight. Add 3 drops of dilute sulfuric acid (1 to 1) and 5 of hydrofluoric acid (4870), and evaporatecarefully to dryness. Ignite a t 1100” C. to constant weight. The difference in the two weights represents the weight of silica which contains 46.72% silicon. PRECISION AND ACCURACY
To test the reliability of the proposed method, six samples were chosen as representative of the types of aluminum alloys usually run for silicon in this laboratory ranging from 2.0 to 12.5YGof
Table I.
not significant-Le., all methods give substantially the same averOn the Other are age* The proposed method nificantly different from those obtained with the umpire method in only one case (chemist 1, sample 32). In other words, the Alcoa method and the proposed method gave substantially the same results, while the modified A.S.T.M. procedure tended t o give low results on the high silicon-bearing samples. In view of the demonstrated precision and accuracy, together with the large saving in operational time effected, the p r o p o d method, in the opinion of the authors, constitutes an excellent procedure for routine laboratory use.
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Comparison of Methods for Determining Silicon in Aluminum Alloys
Chemist 2 StandardC Standard‘ 70Silicon Av. deviation 70Silicon Av. de\,iation Sample* hfethodb 195A I 2.17 2.00 2.00 2.01 0.01 2.09 2.03 2.10 0.07 2.02 I1 1.99 2 . 0 8 2.03 0.05 2.01 1.99 2.04 2.01 0.02 2.01 3.56 3.56 108 I 3.58 0.03 3.61 3.66 3.59 3.61 3.62 0.04 3.64 I1 3.64 0.03 3.58 0.10 3.65 3.61 3.67 3 . 6 3 3.47 5 . 4 2 5.37 5.18 355 I 5 . 3 1 5.37 5.28 5.26 5.24 0.05 0.05 5.40 I1 5.42 5.43 5.29 0.03 5.42 0.01 5.26 5.30 5.31 5.48 5.38 43 I 5.42 5.42 5.41 0.02 5.46 0.03 5.47 5.43 5.66 I1 5.60 5.54 5.60 0.06 5.57 5.52 5.50 5.53 0.04 5.52 5.45 111 5 . 5 5 5.54 5.53 5.49 5.49 0.04 5 . 5 4 .0.02 6.69 6.82 6.68 6.73 6.63 356 I 0.08 6.63 6.61 6.62 0.01 6.76 6.75 6.82 6.78 6 . 7 8 6.74 6.88 6.80 I1 0.04 0.07 6.86 6.83 6.84 6.79 I11 6.84 0.01 6.78 6.81 6.79 0.01 0.16 32 I 11.94 11.95 11.98 11.96 0.02 1 1 . 6 0 1 1 . 9 0 1 1 . 8 2 11.77 0.10 12.36 12.28 12.32 12.32 0 . 0 4 12.35 12.29 12.15 12.26 I1 111 12.14 12.16 12.26 12.19 12.20 12.06 12.04 12.10 0 . 0 9 0.06 All samples I 0.07d 0.07d All samples I1 0.06d 0.05d a Sample 3 3 0.5% magnesium. Sample.43, 0.13% titanium. Sample 356, 0.107, titanium and 0.3% magnesium. Sample 32 1.0% magnesium. b I, modified A.S.T.M. metdod. 11, acid attp=ethod. 111, Alooa method.
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C
Chemist 1
Calculated from Standard deviation
of 3 results from their average and n d Calculated fromI$’$-
where
292
-
-ds,
where Zd* = sum of squares of deviations of each
3.
is the sum of squares of standard deviations and N is the number
of standard deviations = 6 .
silicon. The series of six samples was analyzed for silicon by two operators, using the proposed method and the modified A.S.T.M. procedure, both operators running each series three separate times. The results obtained are shown in Table I. In studying precision, a distinction should be made between the ability of a chemist to reproduce his own results with a given method and the ability of different chemists, perhaps in different laboratories, to check each other. Since only two chemists were used in this study, both from one laboratory, only the first can be estimated with any validity. In this sense, the results show the two methods to be substantially the same, with an average standard deviation of about 0.06% silicon. Using this figure, the precision may be expressed as follows: In the long run, a single determination on a given sample by a given chemist by either method would not be expected to differ from the average of a large number of determinations under the same conditions, by more than 0.12% silicon, 95% of the time. Examination of the averages in Table I shows that in the case of the high silicon-bearing alloys, the modified A.S.T.M. method tends to give lower results than the proposed method (note especially sample 32 and chemist 2 results on samples 43 and 356). The testing of these differences by the t test ( 1 2 ) indicates that they are statistically significant on the 95% level. To determine whether the higher or lower results are the more reliable, three of the samples (43, 356, and 32) were reanalyzed by the Alcoa method which, because of the additional peroxide evaporations and the double dehydration employed, may be considered as an umpire method. The values obtained by the latter procedure are higher than those obtained by the modified A.S.T.M. method, as can be seen from Table I. In all cases except one, these differences are statistically significant on the 9570 level; in the one case, chemist 1 on sample 356, the difference is
ACKNOWLEDGMENT
The authors wish to express their gratitude to M. Kaplan for the statistical analysis of the data, to G. Norwitz for aasistance in the experimental work, and to K. L. Proctor for his continued interest in the investigation. LITERATURE CITED
(1) Aluminum Co. of America, “Chemical Analysis of Aluminum”, 2nd ed., pp. 28-30, 1941. (2) Am. Soc. Testing Materials, “Methods of Chemical Analysin of Metals”, p. 131, 1943. (3) Callender, L. H., Anal&, 58, 81 (1933). (4) Churchill, H. V., Bridges, R. W., and Lee, M. F., IND. EBB. CHEM.,ANAL.ED., 9, 201, (1937). (5) Zbid, 9, 533-4 (1937). (6) Fuchshuber. H.. 2 . anal. Chem.. 116. 421 (1939). (7j Hadley, B. W., Analyst, 66, 486 (1941). ( 8 ) Ibid., 67, 5 (1942). (9) Ibid., 70, 43 (1945). (10) Nikolaev, N. S., Zavodskaya Lab., 10, 536-8 (1941). (11) Osborn, G. H., and Clark, J., Metallurgia, 31, No. 186, 230 (1945). (12) Rider, P. R., “Modern Statistical Methods”, New York, John Wiley & Sons, 1939. (13) Salzer, Erhard, and Theissig, F., Chem.-Ztg., 64, 468 (1940). (14) Shishkin, B. &I.,Zavodskaya Lab. 7, 96 (1938). (15) Shkotova, S.N., Ibid., 8, 213 (1939). (16) Stross, W., Analyst, 69, 44 (1944). (17) Terni, A., and Rloravia, G., Atti X o congr. intern. chim., 3,470 (1939). (18) Urech, P., 2. anorg. allgem. Chem., 214, 111 (1933). (19) Vasil’ev, K. A , , and Vegrin, M. L., Zavodskaya Lab., 7, 263 (1938). ~I
THEopinions expressed are chose of the authors and are not to be construed as reflecting the o5cial views of the Navy Department, through whow permission this article is published.
