Gravimetric Determination of Zinc By the Mercuric Thiocyanate Method a. C.
VOSBURGH, GERALD COOPER, WILLIAM J. CLAYTON, Duke University, Durham, N. C.
IK
AND
HARRY PFANN
crystallization and then the mixture was transferred quantitatively to the main body of the solution. If a visible turbidity did not result, the process was repeated. The reagent was then added in small drops a t a rate of about 2.5 ml. per minute with mechanical stirring.
PLANKING some solubility measurements requiring the precise determination of zinc, a test was made of the gravimetric mercuric thiocyanate method studied by Cohn (S), Lundell and Bee ( 6 ) ,and Jamieson (4). The method has been used by Metler and Vosburgh (7) and Clayton and Vosburgh (2) and is one of the methods given for the determination of zinc in aluminum by Churchill and Bridges (1). The experiments indicated that, while the method had some definite advantages, a further study of some of the sources of error was desirable.
Procedure The procedure used in the study of other conditions was as follows : A solution was prepared by dissolving a weighed portion, about
1 gram, of spectroscopically pure zinc, obtained through the courtesy of the Research Division of the New Jersey Zinc Company, by means of 8 or 9 ml. of 8 molar nitric acid solution, making up to about 200 ml., and weighing the solution. For analysis, weighed portions, usually about 10 grams each, were taken. The sample was diluted to about 100 ml. Enough nitric
Washing the Precipitate Because of the appreciable solubility of zinc mercuric thiocyanate ( 5 ) , Lundell and Bee (6) and Jamieson (4) washed the precipitate with a 0.002 molar solution of potassium mercuric thiocyanate and Metler and Vosburgh (7) washed twice with a 0.01 molar solution and twice with a 0.001 molar solution. Churchill and Bridges recommend a 0.001 molar solution. Estimation based on the water remaining in a Gooch crucible after a n ordinary washing with 0.001 molar wash solution gives 0.2 to 0.25 mg. for the amount of nonvolatile material remaining in the crucible. This estimate was confirmed by experience. Experiments on the change in weight of a zinc mercuric thiocyanate precipitate on washing showed that washing with a 0.001 molar wash solution at a temperature of 5" to 10" C. caused negligible change. Preliminary washing with a more concentrated solution was undesirable, as i t often resulted in the retention of significant quantities of the reagent. A 0.001 molar solution at 20" to 30" C. caused a n appreciable loss of precipitate.
acid was present with the sample to make the acid concentration after dilution about 0.1 molar. In a few cases sulfuric acid was used, and in some of the experiments additional sulfuric or nitric acid mas added. The solution was next seeded and the precipitation carried out as described above. A 0.1 molar solution of the reagent, the preparation of which is discussed below, was used. The solution was 0.01 to 0.02 molar with respect to excess reagent after precipitation; for 0.03 gram of zinc, either 15 or 30 ml. of 0.1 molar solution of the reagent were added, and45ml. wereshown to benot too much. Theprecipitate was allowed to digest under the mother liquor at room temperature for a t least 1 hour. A Gooch crucible was rinsed with water (or wash solution) and the mother liquor decanted through it. The precipitate was washed a t least twice by decantation with cold 0.001 molar wash solution, transferred to the crucible, washed twice in the crucible, and finally dried at 105' to 110" C. and weighed,
Composition of the Reagent It was found best to prepare the reagent with excess of thiocyanate. When 10 per cent excess of mercuric chloride was present, the results of two determinations were 8 parts in 1000 too high. When the reagent was made of equivalent quantities of the two compounds, five determinations gave an average of 2.4 parts in 1000 too high. When thiocyanate was present in excess, however, the results were practically correct. The reagent was usually made with a 10 per cent excess of thiocyanate, but 17 per cent excess gave equally good results. The reagent may also be made from mercuric thiocyanate. One sample of commercial mercuric thiocyanate was found
WASHING PROCEDURE. I n the preparation of the Gooch
crucibles the final washing before drying and weighing was made with 0.001 molar wash solution. Before a filtration was begun, the crucible was well rinsed with water or the wash solution. The mother liquor was then decanted through the crucible and the precipitate washed at least twice by decantation with cold (5' to 10" C.) 0.001 molar wash solution. Then the precipitate was transferred to the crucible and washed at least twice. Erratic results could often be explained as the result of inefficient washing. It is believed that the procedure just given will lead to satisfactory washing if followed exactly.
Method of Precipitation The size of the crystals varies considerably with the method of precipitation. When the reagent is added rapidly, some of the crystals are too small. A slow rate of addition results in supersaturation followed by a sudden precipitation of small crystals. However, if crystallization is induced by rubbing the sides of the beaker or by seeding, the crystals are large enough to be handled conveniently and do not creep or stick to the beaker or stirring rod. The following was found to be a convenient method of obtaining crystals of the right size and was used in all the experiments described below: Three small drops of the solution containing the sample were transferred on the end of a stirring rod to a small test tube or vial and a drop or two of a 0.1 molar solution of the reagent was added. The solution was stirred and the glass rubbed to induce
TABLE I. DETERMINATION OF ZINC IN KNOWN SAMPLES (The quantity of zinc present was from 0.037 to 0.046 gram the weight of the precipitate varying from 0.28 to 0.38 gram) Error (Observed - Calculated) A
No. of Detns.
Additions Grams
393
Maximum
Algebraic Arithmetic mean mean Parts in 1000
INDUSTRIAL A S D ENGINEERING CHEMISTRY
394
unsatisfactory, impurities which were present probably being responsible for the erratic results obtained.
Results Table I gives the results of a nuniber of analyses made as described. To some of the samples various additions were made as shown. While the results of analyses that failed because of improper control of conditions are not included, it is believed that the data in the table are representative. The method is capable of giving very good results when no interfering substances are present. The algebraic mean of the errors is 0.3 part in a thousand, indicating slightly high results. The average error without regard to sign was 1.2 parts in a thousand Sulfuric acid up to 5 grams and nitric acid up to 2.5 grams may be present, but larger quantities lead to high results. The quantities of salts permissible are less.
VOL. 10, NO. 7
Results obtained with samples containing 0.02 and 0.09 gram of zinc were as satisfactory as the remits with 0.04 to 0.05 gram.
Literature Cited (1) Churchill, H. \'., and Bridges, R. W., "Chemical Analysis of Aluminum," p . 36, New Kensington, Pa., Aluminum Research Laboratories, 1935. (2) Clayton, \V. J., and Vosburgh, W. C., J . Am. Chern. SOC.,59, 2415 (1937). (3) Cohn, Ber., 34, 3507 (1902). (4) Jamieson, G. S.,J . 4 m . Chem. SOC.,40, 1036 (1918). (5) Kolthoff, I. N., "Volumetric Analysis," 1-01. 11, p. 267, New York, John Wiley & Sons, 1929. (6) Lundell, G. E. F., and Bee, M. K . , Trans. Am. Inst. J l i n i n g M e t . Engrs., 8, 146 (1914). (7) Metler, V., and Vosburgh, W. C., J . - i m Clrem. Soc., 55, 2625 (1933). RECEIVED February 9 i i , 1938.
Moisture Determination CHARLES W. GRIFFITHS, R. M. Hollingshead Corporation, Camdcn, S . J
0
F INTEREST to those who use the Dean-Stark method
of moisture determination will be this simple supplementary apparatus. It assists in reading the volume of aqueous distillate and entirely eliminates the error due to parallax which is so prevalent in this determination. The experienced operator will appreciate the difficulty of obtaining an accurate reading by attempting to hold the trap level and taking a reading with the naked eye. I n instances where the material to be tested is of high moisture content and only a 10-gram sample can be used, a n error of 0.1 cc. in reading the meniscus will mean an error of 1.0 per cent in the ultimate result. The error in larger samples will of course be proportionately less, but i t is difficult if not impossible t o eliminate such a n error without some kind of a n aid to hold the trap in its intended position while reading, and something to aid the eye and keep the eye and meniscus in the same plane. The author has found the supplementary apparatus particularly valuable where the Dean-Stark method of
moisture determination has been adapted to high-moisture compounds. The base of the apparatus stands upon three legs. Two of these are of the adjusting-screw type, and are used in conjunction with the bubble level to facilitate the leveling of the base. From the base arises a support for holding the trap perpendicular. A small spring clamp at the bottom holds the trap rigid. The eyepiece is an ordinary meniscus reader (Arthur H. Thomas Co. 2501) to which is added a wooden spool, one end of which is beveled to rest snugly in the eyepiece and is tightly held by a short piece of Gooch rubber tubing. Through the exact center of the spool, which is about 35 mm. long and 30 to 35 mm. in diameter, a 3-mm. hole is bored.
It is advisable to add to the top layer of distillate a few cubic centimeters of a solution of a n oil-soluble dye. This will color only the top layer, further distinguishing the two layers of liquid. In practice i t is best to place the eyepiece below the junction of the liquid and raise it till the entire upper layer appears colored (by the oil-soluble dye). If the junction of the liquids should come a t the tapered part of the trap, the jaws of the eyepiece will obviously not hold it in a horizontal position. This difficulty is overcome by using a block of wood 90 X .55 X 24 mm., holding the 55-mm. end flat against the base with the 90-mm. end a g a i n s t t h e graduated portion of the trap. The part of the clamp outside the V-shaped jaws may be clamped against the block and the eyepiece raised or lowered until t h e p r o p e r l e v e l is reached. RECEIVEDM a r c h 16, 1938.
