618
INDUSTRIAL AND ENGINEERING CHEMISTRY
measurements, along with deviations from the mean and the
average deviation from the mean value, are shown in Table I. The value obtained at the 5-ml. point for a single observation is, therefore, subject to an average error of 0.05, and this error would increase somewhat for smaller amounts of water. For a given set of observations the average error of measurement involved in the extrapolation would be of the order of 0.1 pH unit. Greater accuracy may be obtained by averaging several measurementa on similar samples. Some inaccuracy in extrapolation is encountered when the curvature of the plot at higher concentrations is fairly pronounced. As was pointed out in connection with Figure 1, however, the extreme value of pH attainable by extrapolation would be that of a saturated solution. Time is not a critical factor in measurements of the pH, since equilibrium between the water and the fabric is usually attained rather rapidly. All extractions were made at room temperature because a hot extraction may result in decomposition of materials in the cloth and thus lead to abnormal values.
Discussion Using the method described, a large number of pH measure'ments have been made in this laboratory on cloth samples under prevailing atmospheric conditions. It is apparent from the results of this investigation that the pH of a fabric depends upon ita moisture content, which, in turn, is a function of the relative humidity and temperature of the surrounding atmosphere. I n setting up specifications and testa, therefore, it would seem essential that a precise definition including the cloth conditions be
VoL 15, No. 10
adhered to. In order that a definite value may be asaigned to it, the pH of a fabric or textile material might be defined m the p H of the water present under A. S. T. M. standard conditions of 21 C. and 65 per cent relative humidity. This value is the one which would be obtained by the proposed extrapolation method using cloth samples with a moisture content brought to equilibrium in a standard conditioning room.
Literature Cited A. S. T. M. Standards, Part 111, D232-36, p. 448, and D25939T, p. 998 (1939).
A. S. T. M. Standards, Suppl. 111,D548-41, p. 193 (1941). A. S. T. M. Standards on Textile Materials, D629-41T, p. 26 (1941).
Crist, J. L., Am. DyeatufReptr., 31, 133 (1942). Goode, E. A., and Cox, A. B., Proc. SOC.Chem. Ind. Vidork, 37. 1215 (1937).
Goodings,A. C., Am. Dysstuf Reptr., 24,109 (1935). Kolsky, S. I., and Jones, B. M., Ibid.. 20,133 (1931). Launer, H. F., J. Research Natl. Bur. Standards, 22, 653 (1939). U. 5. Army Quartermaster Corps, Tentative Specification for Barracks Bags, 0. D . , P. Q. D. No. 142, par. F-3a, Feb. 27, 1942, as amended by the elimination of the use of buffer solution, Amendment No. 1, Aug. 26, 1942 (Mimeographed). Urquhart, A. R., J. Tezt. Inst., 18, T55 (1927). Urquhart, A. R., Bostock, W., and Eckersall, N., Ibid., 23, T136 (1932).
Urquhart, A. R., and Eckersall, N., Ibid., 21, T499 (1930); 23, T163 (1932).
von Bergen, W., and Crowley, T. N., private communication, 1943.
Wallace, E. L., J . Am. Leather Chem. Aesoc., 30,370 (1935). Wehmer, P. F., Paper Trade J . , 111, No. 12,33 (1940).
Photometric Estimation of Silicon in Magnesium and Magnesium Alloys A. J. BOYLE AND V. V. HUGHEY, Basic Magnesium, Incorporated, Las Vegas, Nevada In a rapid and accurate method for the determination of small amounts of silicon in magnesium metal a n d magnesium alloys, use is m a d e of the molybdenum blue color developed f r o m sodium sulfite reduction of the silicomolybdic acid complex at a pH range of 7.0 to 7.3. Interfering weak bases are complexed by the use of a m m o n i u m tartrate after color development.
T
H E necessity for the determination of small amounts of silicon in magnesium metal and its alloys has become increasingly urgent. According to Mellor (8), silicon and silica are converted to silicides in molten magnesium and appear as a eutectic mixture. Occasionally, with larger amounts of silicon, crystals of MgZSi form, according to Beck ( d ) , which may be detected metallographically. Examination of the hydrochloric acid residue from 100 grams of incendiary bomb alloy revealed no free silicon. I n view of these findings, it is assumed that all silicon in magnesium metal is present as silicide which may be readily converted to silica by treatment with nitric acid. This work is intended to describe a rapid and accurate photometric method for the determination of silicon in magnesium and its alloys. It is based on the reduction of the silicomolybdic acid complex H&Si(MoO,)s].HzO (3,6) to molybdenum blue, Mo,Os.H:O (9).
The first attempt in this laboratory to determine the amount of silicon in magnesium metal and its alloys made use of the yellow complex (10, 1 1 ) . Iron, which offered the principal interference in this procedure, was removed as bask formate ( I d ) . Following this practice, the silicon found was consistently low. A procedure for the determination of silica has been reported utilizing the blue color from a sodium sulfite reduction of the silicomolybdic acid. complex (4, 6, 7). In order to prevent the hydrolysis of salts of aluminum, zinc, and iron present in the solution of the alloys, the molybdenum blue was first developed by sulfite a t a pH of 3, using formic acid to attain this acidity. As in the case of acetic acid ( 7 ) continued reduction of the ammonium molybdate reagent yielded utterly unreliable results. At thie pH, microscopic bubbles of sulfur dioxide very definitely interfere with transmission readings. To make possible the comparison of the molybdenum blue on the neutral or alkaline side, ammonium tartrate was employed to complex the aluminum, zinc, and iron present in the alloys. This procedure gave consistent results. The cloudiness due to the initial hydrolysis of the salts of weak bases during the develop ment of molybdenum blue disappeared completely a t room temperature after 50 minutes. Longer periods of standing resulted in extremely small changes in transmission. In the higher concentrations of silicon the development time had greater significance. This was not s d c i e n t , however, to warrant a develop ment period longer than one hour. Reagents and samples were prepared in paraffin-lined f l h and quartz beakers, respectively. Vycor glassware was also found
ANALYTICAL EDITION
October 15, 1943
TABLE I. ADDITIONOF SILICONAS SODIUMSILICATE Weight of Samde QiNd
2.0000 2.0000 2.0000 2.0000 2.0000
Present by analvsia
-_----
Added Found Ma. MU. To Weighed Amounts of Magnesium Metal
Mi.
0.062 0.072 0.074 0,080 0.099
0.051 0.051 0.051 0.051 0.051
0.126 0.121 0.124 0.128 0.143
Silicon % 0.0031 0,0036 0.0037 0.0040 0.0050
To Weighed Amounta of Magnesium Alloys Containing Zinc, Aluminum, and Manganese 1.2756 0.402 0.100 0.502 0.0315 0.405 0,0267 0.294 0.100 1.0884 0.100 0.513 0.0375 1.0584 0.397 0.100 0.438 0.0283 1.5240 0.431
TABLE 11. INFLUENCE OF COPPERON STANDARD SODIUM SILICATESOLUTION
Silicon Present
Copper Present
Mo.
MU.
Mo.
0.625 0.625 0.625 0.625 0.625
1 2 3 4
0.640 0.610 0.578 0.575 0.625
None
Silicon Found
to be suitable. The only contact with less resistant glassware came at the time of making the sample up to volume and in comparison in the colorimeter cell. Color measurements were made on a Lumetron colorimeter, Model 402 E, using a narrow-band filter havin maximum transmission a t 610 millimicrons, which effectivefy eliminated the emor due to iron. If this element is present in amounts no greater than 0.02 er cent, any visual or photometric instrument is adequate for tge estimation of silicon. The 50-mm. depth cell was employed. A Beckman pH meter was used for all pH adjustments. Aluminum, iron, nickel, zinc, lead, tin, manganese, and titanium occurring in amounts characteristic of magnesium alloys offered no interference. Copper exceeding 0.2 per cent resulted in slightly lower transmission values. This error did not increase markedly with larger amounts of copper. Alloys containing 0.1 to 0.2 per cent copper were dissolved in cold 6 N sulfuric acid. The copper was removed by filtration and the filtrate boiled with nitric acid to complete the oxidation of the silicon compounds. Small losses of silicon hydride resulted from following this procedure. Chloride, sulfate, perchlorate, and nitrate were without influence. Phosphate very definitely interfered, but only in concentration greater than was found in a large number of samples of magnesium and magnesium alloys.
619
of ammonium tartrate reagent and dilute to 100 ml. Read the transmission at about 610 milfimicrons in the colorimeter after a 50-minute period of standing. If co per is present in amounts greater than 0.2 per cent, diesolve tge sample in a uartz or Vycor beaker by the dropwise addition of cold 6 N sdiuric acid. Filter off the copper residue, add 10 ml. of concentrated nitric acid, and boil, proceeding ao above. If fluoride is present, error due to this element may be avoided by the addition of 5 ml. of saturated boric acid prior to solution of the sample in nitric acid (1). Aliquots containing no more than 0.05 mg. of silicon were used to establish the results in Tables I to IV.
Summary Silicon in magnesium alloys,may be determined colorimetrically by the molybdenum blue method with an average error of 5 per cent on amounts ranging from 0.002 to 0.05 per cent. Probably higher concentrations might be determined with the same degree of accuracy, provided su5cient care were maintained in making dilutions. The maximum amount of silicon that may be present in the final aliquot should not exceed 0.05 mg. Use of ammonium tartrate prevented the interference of aluminum, iron, and zinc, making possible the measurement of a stable molybdenum blue at a pH of approximately 7.2. Copper concentrations up to 0.2 per cent gave no interference. Error due to larger amounts of copper was eliminated by solution of the alloy in cold 6 N sulfuric acid with subsequent filtration and oxidation of the soluble silicon hydride with nitric acid. Lead, manganese, tin, titanium, and nickel, in concentrations normally encountered in magnesium alloys, did not interfere.
TABLE 111. INFLUENCE OF PHOSPHORUS ON STANDARD SODIUM SILICATESOLUTION PO4
Silicon Present
MU.
MO.
Silicon Found
Mu.
0.0 0.05 0.10 0.15 0.20 0.30 1.35
0.125 0.125 0.125 0.125 0.125 0.125 0.125
0.127 0.131 0.127 0.129 0.134 0.126 0.116
TABLEIv. INFLUENCE ON SULFURIC ACID SOLUTION OF MhQNESIUM ALLOYCONTAINING ZINC, MANGANESE, AND ALUMINUM Weight of Alloy Qram
1.0000 1.0000
Silicon Found, Solution in Sulfuric Acid
Silicon Present, Solution in Nitric Acid
MU. 0.265 0.258
0.283 0.283
Mu.
Reagents
Acknowledgment
Hydrochloric acid, 1 N . Ammonium carbonate, dilute solution. Ammonium molybdate, 100 grams (81.0 PR cent Moot) Sodium sulfite, 170 grams of anhydrous sodium sulSodium silicate, 0.025 mg. of silicon per ml., by dehydration with perchloric acid followed by sulfuric acid and hydrofluoric acid treatment. Ammonium tartrate, 400 rams per liter. Nitric acid, equal volumes of concentrated acifand water.
The authors wish to express appreciation to H. H. Willard of the University of Michigan for helpful suggestions and cooperation in the development of this procedure.
Procedure for Determining Silicon i n Magnesium Metal and Magnesium Alloys Dissolve 2 grams of the metal in a quartz or Vycor beaker by the dropwise addition of 50 ml. of 1 to 1 nitric acid. Boil for 15 minutes to remove all oxides of nitrogen. Cool, transfer to a 100-ml. volumetric flask, and make up to the mark. Pipet a 25-ml. aliquot into a quartz or Vycor beaker, adjust to a pH of approximately 4 with ammonium carbonate, add 5 ml. of ammonium molybdate reagent, and adjust with the hydrochloric acid reagent to a pH range of 2.4 to 2.7. Allow the silicomolybdic acid complex to develop for 2 minutes. Add 25 ml. of sodium sulfite reagent. After letting stand for 10 minutes, add 10 ml.
Literature Cited (1) Am. SOC.Testing Materials, BUZZ.23, p. 26,1942, Preprint. (2) Beck, Adolph, “Technology of Magnesium and Its Alloys”, London, F . A. Hughes & Co., 1940. (3) Dienert, F., and Wandenbulcke, F., Compt. rend., 176, 1478 (1923). (4) Foulger, J. H., J. Am. Chem. Soc., 49,431 (1927). (5) Isaacs, L.,BUZZ.soc. chim. bioZ., 6 , 157-68 (1924). (6) Jolles and Neurath, 2. angew. Chem., 11, 315 (1898). (7) Kahler, H.L., IND.ENQ.CHEM.,ANAL.ED., 13, 536 (1941). (8) Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. IV, p. 271, New York, Longmans, Green & Co., 1940. (9) Munro, Proc. Trans.Nova Scotian Inst. Sci., 16, 9 (1928). (10) Schwartr, M. C., IND.ENQ.CHEM.,ANAL.ED., 14, 893 (1942). (11) Schwartr, M. C., La. State Univ., BUZZ.30, No. 14 (1938). (12) Sheldon, J. L., thesis, University of Michigan, 1940.
