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.
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