Small quantities of tin are occasionally found in waters that have
passed through tin or tin-lined pipes. This metal, if present, is
precipitated with the iron by ammonia in the lead, zinc, and copper
separations. In the method for copper alone, it is removed in the same
way and may be further avoided by dissolving the sulfides in
concentrated nitric acid. Any tin present will then separate as an
insoluble compound, which may be ignited and weighed as the oxide
(SnO_{2}).
The following schematic tables illustrate the procedures given.
Table 10.—SCHEME FOR THE SEPARATION OF LEAD, ZINC, AND COPPER.
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1. Concentrate sample. Add 10 cc. NH_{4}Cl, a few drops NH_{4}OH and
saturate with H_{2}S. Allow to stand, add more NH_{4}OH and H_{2}S.
Boil, filter, and wash.
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2. Dissolve the precipitate in dilute HNO_{3}. Filter │3. Reject the
and wash. Evaporate to 10 or 15 cc. Cool. Add 5 cc. │filtrate which
concentrated H_{2}SO_{4}, and heat until white fumes are│contains the
given off. Dilute slightly and treat with 150 cc. of 50 │coloring
per cent alcohol. Allow to stand; filter, and wash with │matter.
50 per cent alcohol. │
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4. The precipitate │5. The filtrate contains the │
contains the Pb. Dissolve │Zn and Cu. Concentrate to │
in NH_{4}C_{2}H_{3}O_{2} │expel alcohol. Add excess of │
solution. Filter into a 50│NH_{4}OH, filter and wash │
cc. Nessler tube and wash │precipitate. │
with water containing ├──────────────┬──────────────┴──────────────
NH_{4}C_{2}H_{3}O_{2}. │6. Reject the │7. The filtrate contains the
Divide filtrate in halves.│precipitate │Zn and Cu. Neutralize with
Saturate one-half with │which contains│H_{2}SO_{4}. Add 10 cc.
H_{2}S. Determine the Pb │the Fe. │concentrated H_{2}SO_{4} and
in the other half by │ │1 g. urea. Electrolyze for
adding HC_{2}H_{3}O_{2} │ │two hours with a current of
and H_{2}S and comparing │ │0.5 ampere. Break circuit,
with standards containing │ │empty dish and wash.
known amounts of Pb. │ │
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8. The deposit is Cu. │9. The solution contains the Zn. Nearly
Immerse the cathode in a │neutralize with NH_{4}OH. Concentrate to
small amount of hot, │less than the capacity of the dish. Add 2 g.
dilute HNO_{3}; wash off │K_{2}C_{2}O_{4} and 1.5 g. K_{2}SO_{4}.
and evaporate to dryness. │Electrolyze for 3 hours with a current of
Take up in water and wash │0.3 ampere. Siphon off solution, break
into a Nessler tube. Make │circuit, wash with water, then alcohol, dry
up to mark, and add 10 cc.│at 70° C., cool and weigh.
of potassium sulfide ├────────────────────────────────────────────
solution. Compare with │10. The weighed residue is metallic Zn.
standard. If large amount │
is present, dry and weigh │
as Cu. │
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Table 11.—SCHEME FOR DETERMINATION OF COPPER ONLY.
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11. Concentrate sample to 75 cc. Add 2 cc. conc. H_{2}SO_{4} for clear,
soft waters and 5 cc. for alkaline or turbid waters. Electrolyze
following procedure in 7 and 8.
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Table 12.—SCHEME FOR DETERMINATION OF ZINC ONLY.
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13. Follow scheme for all three metals as given in Table 10 through
section 5. Nearly neutralize the filtrate with H_{2}SO_{4}, concentrate
to less than the capacity of the dish and electrolyze as directed in
section 9.
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