Determination of Small Amounts of Silicon in Cathode Nickel G. VICTOR POTTER Sylvania Electric Products, Znc., Towanda, Pa. A method is described for the separation of small amounts of silicon from the major component and interfering elements in cathode nickel by using perchloric acid. The final determination is made by the heteropolp blue procedure recently discussed by Boltz and Mellon.
B
ECAUSE the determination of small amounts of silicon in
cathode nickel by gravimetric procedures seems to require very large samples, a spectrophotometric procedure in which a much smaller sample is used has been desired. A number of colorimetric procedures had been investigated but did not seem to give consistently reliable results in the hands of different analysts. The “heteropoly blue” procedure of Boltz and Mellon ( I ) has given consistently reliable results in this laboratory, when used under carefully controlled conditions. DISCUSSION
Colorimetric methods were first tried, in which no separation of other elements was made. However, because it was noted that as the sample size was increased the percentage of silicon decreased, it was felt that some of the silicon was present in a form that was not acid-soluble. I t was found by experiment that some of the elements present in cathode nickel tend to increase and others decrease the sensitivity of the color reaction. Therefore, the determination of silicon in the presence of the major component and other elements was abandoned. Scparation of the silicon from the major component and interfering elements was then tried by dehydration with hydrochloric, sulfuric, and perchloric acids, respectively. But in dealing with such small amounts of silicon, the loss entailed due to necessary washing made the error too high. The method that finally gave consistently reliable results was to dehydrate the silica with perchloric acid in a platinum dish, filter through a small hydrofluoric acid-treated filter paper, and wash just once with a small amount of 1 to 99 hydrochloric acid. The small amount of other elements left is so small as t o be entirely without effect. The filter paper containing the silica is then put back in the platinum dish, oxidized with a small amount of nitric and perchloric acid, evaporated to dryness, and gently ignited. The residue is fused with a small amount of anhydrous sodium bicarbonate, the fusion is taken up in water, the pH is properly adjusted, the solution is diluted to volume, and suitable aliquots are taken for the final determination. In working with such small amounts of silica, a rather large error was sometimes caused by contamination from glassware. Therefore, all operations were carried out in platinum ware and polyethylene beakers, with exception of the final color development, which was of such short duration that no error was encountered.
1000 nil. with water, add a small amount of 6 .V sulfuric acid to make the solution faintly acid, about pH 6.0, then dilute to 1000 ml., and store in wax-lined or polyethvlene bottles. Ammonium Molybdate. Dissolve’ 18.8 grams of ammonium molybdate [ (SH4)&1o&.4H20] in water containing 23 ml. of concentrated sulfuric acid and dilute to 250 ml. Reductant. Solution A. Dissolve 2 grams of anhydrous sodium sulfite in 25 ml. of water and add 0.4 gram of l-amino-2naphthol-4-sulfonic acid. Solution €3. Dissolve 25 grams of sodium bisulfite in 200 ml. of water. Add solution A to solution B and dilute to 250 ml. Specially Prepared Filter Paper. Immerse Whatman Yo. 42, 7.0-cm. filter papers or paper of similar quality in 1 to 1 hydrofluoric acid for 15 to 20 minutes and then wash thoroughly. Polyethylene Beakers. The polyethylene bottles used for hydrofluoric acid have been found very satisfactory after the tops have been removed. Other Reagents. All other reagents were of reagent grade. Sodium bicarbonate is used for the fusion because it can generally be obtained with higher purity than sodium carbonate and hence results in a lower blank. PROCEDURE
Calibration Curve. Transfer suitable aliquots of the standard solution containing from 0.01 to 0.06 mg. of silicon to 100-ml. volumetric borosilicate glass flasks, dilute to about 95 ml., and add 1 ml. of 7.5% ammonium molybdate solution. ilfter 5 minutes add 4 ml. of 10% tartaric acid and mix ( 2 ) . Add 1 ml. of reductant and dilute to the mark. After 20 minutes make transmittancy measurements a t 820 mp, using a blank of the same quantities of reagents in the reference cell. The color is stable for a t least 12 hours ( 1 ) . Plot the transmittancy value. obtained against milligrams of silicon per 100 ml. of solution.
Table I. Sample N o . E-39-45 H-1400 63 66 71 72
73 74
R
N.B.S. AI alloy 80 81
Weight of Sample, Gram 1 .o 1.0
0.5 0.5 0.5 0.5 0.5
0.J 1 .o
0.4 1.0 1.0
Accuracy of %lethod % Si 0.029,o. 029
0.017,0.017 0 , 0 2 2 , o .020 0 . 0 2 0 , o ,022 0.026,0.026 0.022,O.023 0.023,0.025 0.089.0.087 0.22.0.22 0.108 0 , 0 2 0 , o. 0 1 9 . 0 . 0 1 ~ 0,021,o .019,0.019
% Si b y Other Laboratories 0 026-0.040 0 012-0 020 0 03 0 03 0 03 0 03 0 024 0 091 0 22 0 108-0.12 0 01.5 0 020
Analysis of Unknown. To 1 gram of sample in a platinum dish add about 5 ml. of concentrated nitric acid, cover with a horosilicate cover glass, and heat gently to dissolve. When dissolution is complete, carefully wash down the sides of the dish with :I small amount of water, then add about 4 I d . of perchloric acid, replace cover on dish, and evaporate to copious fumes and to as small a volume as possible without allowing the contents of the dish to become solid; if this occurs the separation of silica is always incomplete (3). Cool slightly, until crystallization starts, then quickly dilute with 1 to 99 hydrochloric acid, bringing the volume to a total of 25 to 30 ml. Add a small pirich of the
APPARATUS AND REAGENTS
Instruments. Beckman spectrophotometer Model DU and pH meter. Standard. Dissolve 2.5 grams of sodium metasilicate (NazSiOs.9Ht0) in water in a sqainless steel or polyethylene beaker, dilute to 500 ml., and store in a polyeth lene or wax-lined bottle. This stock solution must be standar&ed by gravimetric determination. Calculate the re uired amount to make 1000 ml. of silicon per ml., dilute to nearly solution containing 0.01 mg.
3
927
ANALYTICAL CHEMISTRY
928 specially prepared filter paper pulp to help collect the silica, filter immediately through one of the specially prepared filter papers, and wash once with 1 to 99 hydrochloric acid. Put the paper containing the silica back in the platinum dish, add 5 ml. of concentrated nitric acid and 3 ml. of perchloric acid, cover with the cover glass, and heat gently to oxidize the paper. After oxidation is complete, wash down the cover and sides of the dish carefully with a small amount of water, continue the evaporation to dryness, and then ignite gently. Fuse the residue with 2 grams of anhydrous sodium bicarbonate, allow to cool, take up fusion with water, and then neutralize and adjust to pH 4.5 to 5.0 with a pH meter, using 6 N sulfuric acid. Keep dish covered while neutralizing, in order to avoid loss from spattering. I t was found by experiment that this pH will result in the optimum pH of 1.6 a t the point of formation of molybdisilicic acid (1). It will be necessary to have some silica-free ammonium hydroxide on hand to help in adjusting the pH a t 4.5 to 5.0. Heat the solution t o remove carbon dioxide, cool, and then dilute to 100 ml. The oxides of nickel that will be present soon settle and a clear solution can be pipetted off the top, or if desired, the solution can be filtered through one of the specially prepared filter papers and then diluted to volume. Suitable aliquots are treated as in the preparation of the calibration curve, and transmittancy measurements are made a t 820 mp after 20 minutes. Values are obtajned from the calibration curve.
Data. Table I indicates the accuracy of the method; the results are compared to those reported by other laboratories.
SUMMARY
Although to date this method has been used only for determining silicon in cathode nickel and a Iiational Bureau of Standards sample of aluminum alloy, it should be applicable to the determination of silicon in almost any metal or material that is acid-soluble, unless the major component is an element that interferes seriously with the color development. In that case, it would be necessary to wash the filter more thoroughly, determine the loss of silica for the necessary number of washings, and apply a correction in subsequent similar analyses. ACKNOW LEDGhIENT
The author wishes to express appreciation to Wendell L. Hummer, a former member of the analytical staff, for his iuggestions and generous assistance in preparation of the manuscript. LITERATURE CITED
(1)
Bolts, D. F., and Mellon, 31. G., IND.ENG.CHEM., A N ~ LED,, . 19,
873 (1947). (2) Bunting, W. E., Ibid., 16, 612 (1944). (3) Smith, G. F., “Perchloric Acid,” 4th ed., p. 18, Columbus, Ohio, G. Frederick Smith Chemical Co., 1940.
RECEIVED February 20, 1950.
Simultaneous Determination of Ethylene and Propylene Chlorohydrins W. A. CANNON Wyandotte Chemicals Corporation, Wyandotte, Mich. Ethylene and propylene chlorohydrins are determined simultaneously with an average error of 1.9% for ethylene chlorohydrin and 2.0% for propylene chlorohydrin. The mixed chlorohydrins are hydrolyzed to glycols by heating with sodium bicarbonate in sealed bottles. The glycols are oxidized with periodic acid and the acetaldehyde and formaldehyde are determined polarographically after a simple distillation. The method is applicable to mixtures
A
LTHOUGH several methods have been described for the quantitative determination of chlorohvdrins, none is applicable to a quantitative determination of ethylene and propylene chlorohydrins in the presence of each other. Trafelet (4) determined total water-soluble chlorohydrins by selective hydrolysis with sodium bicarbonate and titration of the chloride ions liberated. Other methods mentioned in the literature, such as Uhrig’s (6) method for determination of ethylene chlorohydrin, cannot be used to distinguish between ethylene and propylene chlorohydrins in mixtures. The method described in this paper consists essentially of hydrolyzing the chlorohydrin mixture with sodium bicarbonate to the corresponding glycols.
+ NaHC08 --+C H ~ O H . C H ~ 0+H KaC1 + CO, CHa.CHOH.CH&I + NaHC03-+ CHs.CHOH.CHzOH + NaCl + COS
of ethylene and propylene chlorohydrins in water solution or water-soluble solvents. The principal limitation to the method is that there must not be present or formed any volatile substances polarographically reducible at the same potential range as the aldehydes. The presence of monohydric alcohols or moderate concentrations of hydrochloric acid, aliphatic dichlorides, or dichloro ethers does not interfere with the determination.
+ HI04 +2HCHO + HI03 + Hz0 CHa.CHOH.CH20H + HI04 HCHO + CHaCHO + HIOs + HsO CHzOH.CH20H
.----f
The aldehydes produced are removed by distillation and determined polarographically according to the procedure described by Warshowsky and Elving (6) and Whitnack and Moshier (7‘). The acetaldehyde produced by periodic acid oxidation is a measure of the propylene chlorohydrin concentration of the mixture before hydrolysis. The ethylene chlorohydrin can be estimated by deducting the formaldehyde produced by the oxidation of propylene glycol from the total formaldehyde produced.
CHzOII.CH&l
After neutralization, the glycol mixture is oxidized with periodic acid to give formaldehyde and acetaldehyde according to the following equations:
MATERIALS AND APPARATUS
Periodic Acid, 0.5 N. A solution is pre ared by dissolving 11 grams of eriodic acid (G. Frederick Smitg Chemical Compan ) in distillexwater and diluting to 100 ml. This solution should
