The separation of traces of lead as chromate is a fairly good one.
Dissolve 5 grams of the copper in 17 c.c. of nitric acid and an equal
volume of water; boil off nitrous fumes, neutralise with soda, and
afterwards acidulate with acetic acid; and dilute to a litre. Add 20
grams of sodium acetate, warm, and precipitate the lead with a dilute
solution of potassium chromate. Copper chromate (yellow) may be at the
same time thrown down, but it is readily soluble on diluting. Filter off
the precipitate; wash it into a beaker and pass sulphuretted hydrogen;
oxidise the sulphide and weigh as lead sulphate. Treated in this way 5
grams of copper yielded sulphate of lead equal to 2.0 milligrams of
lead. Five grams of the same sample to which 10 milligrams of lead were
added gave 11.4 milligrams.
~Nickel and Cobalt.~--Nickel is always present in larger or smaller
quantities in commercial copper, and, perhaps, has an influence on the
properties of the metal. It is determined as follows:--Dissolve 10 grams
of the copper in 35 c.c. of nitric acid and an equal bulk of water, boil
off nitrous fumes and neutralise with soda, add 2 grams of carbonate of
soda dissolved in water, boil, and filter. Acidify the filtrate with 2
or 3 c.c. of dilute nitric acid and dilute to 1 or 1-1/2 litres. Pass
sulphuretted hydrogen through the cold solution till the copper is all
down and the liquid smells of the gas.
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