| "Ferrocyanide" required. | ||||
| With | nothing added. | 20.3 c.c. | ||
| " | 0.050 | gram | lead (as chloride) | 20.9 " |
| " | 0.050 | " | manganese (as sulphate) | 25.5 " |
| " | 0.050 | " | cadmium (as sulphate) | 23.5 " |
| " | 0.050 | " | nickel (as sulphate) | 26.2 " |
Effect of Varying Zinc.—These were titrated under the usual conditions, and gave the following results:—
| Zinc added | 1.0 c.c. | 10.0 c.c. | 20.0 c.c. | 50.0 c.c. | 100.0 c.c. |
| "Ferrocyanide" required | 1.1 " | 10.2 " | 20.3 " | 50.6 " | 101.0 " |
Determination of Zinc in a Sample of Brass.—Take the solution from which the copper has been separated by electrolysis and pass sulphuretted hydrogen until the remaining traces of copper and the lead are precipitated, filter, boil the solution free from sulphuretted hydrogen, put in a piece of litmus paper, and add sodic hydrate solution in slight excess; add 10 c.c. of dilute hydrochloric acid (which should render the solution acid and clear); warm, and titrate.
A sample of 0.5 gram of brass treated in this manner required 16.4 c.c. of "ferrocyanide" (standard 100 c.c. = 0.9909 zinc), which equals 0.1625 gram of zinc or 32.5 per cent.
Determination of Zinc in Blende.—Dissolve 1 gram of the dried and powdered sample in 25 c.c. of nitric acid with the help of two or three grams of potassium chlorate dissolved in the acid. Evaporate to complete dryness, taking care to avoid spirting. Add 7 grams of powdered ammonium chloride, 15 c.c. of strong ammonia and 25 c.c. of boiling water; boil for one minute and see that the residue is all softened. Filter through a small filter, and wash thoroughly with small quantities of a hot one per cent. solution of ammonium chloride. Add 25 c.c. of hydrochloric acid to the filtrate. Place in the solution some clean lead foil, say 10 or 20 square inches. Boil gently until the solution has been colourless for three or four minutes. Filter, wash with a little hot water; and titrate with standard ferrocyanide.
Determination of Zinc in Silver Precipitate.—This precipitate contains lead sulphate, silver, copper, iron, zinc, lime, &c. Weigh up 5 grams of the sample, and extract with 30 c.c. of dilute sulphuric acid with the aid of heat. Separate the copper with sulphuretted hydrogen, peroxidise the iron with a drop or two of nitric acid, and separate as acetate. Render the filtrate ammoniacal, pass sulphuretted hydrogen; warm, and filter. Dissolve the precipitated zinc sulphide in dilute hydrochloric acid, evaporate, dilute, and titrate. Silver precipitates carry about 2.5 per cent. of zinc.
GASOMETRIC METHOD.
Metallic zinc is readily soluble in dilute hydrochloric or sulphuric acid, hydrogen being at the same time evolved.[74] The volume of the hydrogen evolved is obviously a measure of the amount of zinc present in the metallic state. The speed with which the reaction goes on (even in the cold) and the insolubility of hydrogen renders this method of assay a convenient one. It is especially applicable to the determination of the proportion of zinc in zinc dust. The apparatus described in the chapter on gasometric method is used. The method of working is as follows: Fill the two burettes with cold water to a little above the zero mark, place in the bottle about 0.25 gram of the substance to be determined, and in the inner phial or test tube 5 c.c. of dilute sulphuric acid; cork the apparatus tightly and allow to stand for a few minutes; then bring the water to the same level in the two burettes by running out through the clip at the bottom. Read off the level of the liquid in the graduated burette. Turn the bottle over sufficiently to spill the acid over the zinc, and then run water out of the apparatus so as to keep the liquid in the two burettes at the same level, taking care not to run it out more quickly than the hydrogen is being generated. When the volume of gas ceases to increase, read off the level of the liquid, deduct the reading which was started with; the difference gives the volume of hydrogen evolved. At the same time read off the volume of air in the "volume corrector," which must be fixed alongside the gas burettes. Make the correction. For example: A piece of zinc weighing 0.2835 gram was found to give 99.9 c.c. of gas at a time when the corrector read 104 c.c.[75] Then the corrected volume is
104 : 100 :: 99.9 : x.
x = 96.0 c.c.