If zinc is present and copper is absent concentrate the filtrate from
the lead sulfate to expel the alcohol, and remove the iron by adding an
excess of ammonium hydroxide. Filter, wash, and acidify the filtrate
with sulfuric acid. Concentrate the filtrate to about 150 cc. and
transfer to a weighed platinum dish. Add 2 grams of potassium oxalate
and 1.5 grams of potassium sulfate. Deposit the zinc electrolytically by
means of a current of about 0.3 ampere for three hours. After deposition
is complete and while the current is on, siphon off the solution and at
the same time run into the dish a stream of distilled water in order to
expel the free sulfuric acid, which might dissolve some of the zinc if
the circuit were broken. After the acid has been removed break the
circuit, wash the dish with water, then with 95 per cent alcohol, dry at
70° C., cool, and weigh it. The difference between this weight (10) and
the weight of the platinum dish equals the amount of metallic zinc. Some
difficulty has been experienced in this determination in obtaining pure
reagents. It is therefore advisable to make blank determinations with
each new lot of reagents and to correct the results if necessary.
If copper also is present (5) concentrate the filtrate from the lead
sulfate until the alcohol is expelled, and add an excess of ammonium
hydroxide. (6) Remove any iron precipitate by filtration. Neutralize the
filtrate (7) with sulfuric acid, and add 2 cc. of concentrated sulfuric
acid and 1 gram of urea. Electrolyze the solution and determine copper
colorimetrically as described in the procedure for copper (p. 54). After
the copper has been deposited add ammonium hydroxide to the solution
containing the zinc until nearly all the sulfuric acid has been
neutralized, concentrate to slightly less than the capacity of the
platinum dish, add 1.5 grams of potassium sulfate and 2 grams of
potassium oxalate, and electrolyze for zinc. As this solution is usually
saturated with ammonium salts due to neutralizing the large quantity of
sulfuric acid, it is frequently impossible to get the zinc deposited
firmly on the dish before the salts interfere by crystallization. To
avoid this difficulty, dilute half the solution and electrolyze it for
zinc; or, if the amount of zinc is very small, precipitate the zinc as
sulfide in acetic acid solution, wash, ignite to oxide, and weigh the
precipitate. This difficulty will not be encountered if copper is absent
as there will then be no excess of ammonium salts.
If lead and copper are known to be absent and zinc alone is to be
determined (13), after treating with sulfuric acid for separation of
lead, slightly dilute the contents of the dish. Add an excess of
ammonium hydroxide to precipitate iron and filter. Make the filtrate
slightly acid with sulfuric acid, concentrate to about 150 cc., transfer
to a weighed platinum dish, add potassium oxalate and sulfate, and
electrolyze the solution as described for deposition of zinc.