A Text-book of Assaying: For the Use of Those Connected with Mines Part 41

_x_ = 0.2795 gram of zinc.

This being contained in 0.2835 gram of metal is equivalent to 98.5 per cent.

As an example of a determination in which reducing the volume of liberated hydrogen to 0 C. and 760 mm. is avoided, the following may be taken:--

0.2315 gram of pure zinc gave 82.1 c.c. of gas; and the volume of air in the corrector was 103.6 c.c.

0.2835 gram of the a.s.say gave 99.9 c.c. of gas; and the volume of air in the corrector was 104.0 c.c.;

104 : 103.6 :: 99.9 : _x_.

_x_ = 99.5 c.c.

This is the volume of gas got in the a.s.say if measured under the same conditions as the standard,

82.1 : 99.5 :: 0.2315 : _x_.

_x_ = 0.2806.

Then 0.2835 : 0.2806 :: 100: _x_.

_x_ = 98.9 per cent.

As these a.s.says can be made quickly, it is well for the sake of greater accuracy to make them in duplicate, and to take the mean of the readings. One set of standardisings will do for any number of a.s.says.

The student must carefully avoid unnecessary handling of the bottle in which the zinc is dissolved.

~Colorimetric Method.~--Zinc salts being colourless, there is no colorimetric determination.

EXAMINATION OF COMMERCIAL ZINC.

Take 20 grams of zinc, and dissolve them in dilute nitric acid; boil, allow to settle; filter; wash, dry; ignite the precipitate, if any, and weigh as oxide of tin. Examine this for a.r.s.enic.

~Lead.~--Add ammonia and carbonate of ammonia to the liquid, and boil, filter off the precipitate, wash with hot water. Digest the precipitate with dilute sulphuric acid; filter, wash, and weigh the sulphate of lead.

~Iron.~--To the filtrate from the sulphate of lead add ammonia, and pa.s.s sulphuretted hydrogen; digest, and filter. (Save the filtrate.) Dissolve the precipitate in hydrochloric acid, oxidise with nitric acid, and precipitate with ammonia. Wash, ignite, and weigh as ferric oxide.

Calculate to iron.

~a.r.s.enic.~--To the filtrate from the sulphide of iron add hydrochloric acid in slight excess; filter off, and wash the precipitate. Rinse it back into the beaker, dissolve in nitric acid, filter from the sulphur, and add ammonia, in excess, and magnesia mixture. Filter off the ammonic-magnesic a.r.s.enate, and wash with dilute ammonia. Dry, ignite with nitric acid, and weigh as magnesic pyra.r.s.enate. Calculate to a.r.s.enic, and add to that found with the tin.

~Copper.~--To the filtrate from the ammonia and ammonic carbonate add sulphuric acid in small excess, and pa.s.s sulphuretted hydrogen. Allow to settle, filter, and wash. Rinse the precipitate into a beaker, boil with dilute sulphuric acid, and filter. (Save the filtrate.) Dry, burn the paper with the precipitate, treat with a drop or two of nitric acid, ignite, and weigh as copper oxide. Calculate to copper.

~Cadmium.~--To the filtrate from the sulphide of copper add ammonia, so as to nearly neutralise the excess of acid, and pa.s.s sulphuretted hydrogen. Collect and weigh the precipitate as cadmium sulphide, as described under _Cadmium_.

PRACTICAL EXERCISES.

1. What weight of hydrogen will be evolved in dissolving 1 gram of zinc in dilute sulphuric acid?

2. How many c.c. would this quant.i.ty of hydrogen measure at 0 C. and 760 m.m.? (1 litre weighs 0.0896 gram).

3. 0.23 gram of zinc are found to give 77.9 c.c. of hydrogen. In another experiment under the same conditions 80.2 c.c. are got. What weight of zinc was used for the second experiment?

4. A sample of blende is found to contain 55 per cent. of zinc. What percentage of zinc sulphide did the sample contain?

5. How much metallic lead would be precipitated from a solution of lead acetate by 1 gram of zinc?

CADMIUM.

Cadmium occurs in nature as cadmium sulphide in greenockite, CdS, which is very rare. It is widely diffused in calamine, blende, and other zinc ores, forming, in some cases, as much as 2 or 3 per cent. of the ore.

Oxide of cadmium forms the "brown blaze" of the zinc smelters.

Sulphide of cadmium is used as a pigment (cadmium yellow); and the metal and some of its salts are useful reagents.

The salts of cadmium closely resemble those of zinc. The hydrate, however, is insoluble in excess of potash, and the sulphide is insoluble in dilute acids. It forms only one series of salts.

Cadmium is detected by giving with sulphuretted hydrogen in solutions, not too strongly acid, a yellow precipitate, which is insoluble in solutions of the alkalies, alkaline sulphides, or cyanide of pota.s.sium.

~Solution and Separation.~--Substances containing cadmium are soluble in acids. The solution is evaporated to dryness (to render any silica that may be present insoluble) and taken up with 10 c.c. of dilute hydrochloric acid. Dilute to 100 c.c., and pa.s.s sulphuretted hydrogen.

Filter, digest the precipitate with soda, wash, and boil with dilute sulphuric acid. Filter; the filtrate contains the cadmium and, possibly, a small quant.i.ty of zinc, from which it is best separated by reprecipitating with sulphuretted hydrogen.

GRAVIMETRIC DETERMINATION.

The solution containing the cadmium freed from the other metals is precipitated with sulphuretted hydrogen in a moderately-acid solution.

The precipitate is collected on a weighed filter, and washed, first with an acid solution of sulphuretted hydrogen, and afterwards with water. It is dried at 100 C. and weighed. If free sulphur is suspected to be present, extract with bisulphide of carbon, and again weigh. The residue is cadmium sulphide, which contains 77.78 per cent. of cadmium. It is a yellow powder insoluble in solutions of the alkalies, alkaline sulphides, or cyanide of pota.s.sium. It dissolves readily in acid. It cannot be ignited in a current of hydrogen without loss.

VOLUMETRIC METHOD.

The solution containing the cadmium is concentrated by evaporation, and mixed with an excess of oxalic acid and alcohol. The precipitate is filtered, washed with alcohol, dissolved in hot hydrochloric acid, and t.i.trated with permanganate of pota.s.sium.

FOOTNOTES:

[64] When chromium is present some of the iron may escape precipitation but it can be recovered from the solution by means of ammonic sulphide.

[65]

(1) 10FeSO_{4} + 2KMnO_{4} + 8H_{2}SO_{4} = 5Fe_{2}(SO_{4})_{3} + 2MnSO_{4} + K_{2}SO_{4} + 8H_{2}O.

(2) 6FeCl_{2} + K_{2}Cr_{2}O_{7} + 14HCl = 3Fe_{2}Cl_{6} + Cr_{2}Cl_{6} + 2KCl + 7H_{2}O.

[66] (1) Fe_{2}Cl_{6} + SnCl_{2} = 2FeCl_{2} + SnCl_{4}.

(2) Fe_{2}Cl_{6} + SH_{2} = 2FeCl_{2} + 2HCl + S.

(3) Fe_{2}Cl_{6} + Na_{2}SO_{3} + H_{2}O = 2FeCl_{2} + Na_{2}SO_{4} + 2HCl.

(4) Fe_{2}Cl_{6} + Zn = 2FeCl_{2} + ZnCl_{2}.

[67] 20 grams of stannous chloride and 20 c.c. of dilute hydrochloric acid are diluted to one litre.

[68] The maximum reducing effect of zinc is obtained by exposing as large a surface as possible of the metal in a hot concentrated solution containing but little free acid (Thorpe).