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.
COPPER.[[77]]
Use 1 liter of a sample containing 0.1 to 1.0 part per million of copper, and proportionate amounts for other concentrations. Evaporate to about 75 cc., and wash into a 100 cc. platinum dish. Add 2 cc. of dilute sulfuric acid for clear and soft waters; add more acid to very alkaline waters to offset the alkalinity; add 5 cc. of acid to waters carrying much organic matter or clay to insure the formation of a soluble copper salt. Then place the dish as the anode in a direct current circuit, suspend a spiral wire cathode in the solution so that it is parallel to and about half an inch from the bottom of the dish, and close the circuit.
Electrolyze for about four hours with occasional stirring, or over night, if convenient. The current may be supplied by two gravity cells in series, yielding a current through the solution of about 0.02 ampere. Lift out the cathode without previously opening the circuit, and immerse the spiral in a small amount of dilute nitric acid previously heated to boiling. Wash off the wire and evaporate the nitric acid solution to dryness on the water bath. If the presence of silver is suspected add a few drops of hydrochloric acid before evaporation. Dissolve the residue in water and wash it into a 50 cc. Nessler tube. Dilute to 50 cc. and add 10 cc. of the potassium sulfide solution. The color of the copper sulfide develops at once and is fairly permanent, lasting at least several hours. Add 10 cc. of the potassium sulfide solution to a similar tube containing 50 cc. of distilled water, and then add to it standard copper solution in 0.2 cc. portions until the colors of the two tubes match. If 1 liter of the sample is used copper in parts per million is equal to the number of cubic centimeters of standard copper solution required to match the color of the sample multiplied by 0.2.
TIN.
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 (SnO2).
The following schematic tables illustrate the procedures given.
| Table 10.—Scheme for the separation of lead, zinc, and copper. | |||
|---|---|---|---|
| 1. Concentrate sample. Add 10 cc. NH4Cl, a few drops NH4OH and saturate with H2S. Allow to stand, add more NH4OH and H2S. Boil, filter, and wash. | |||
| 2. Dissolve the precipitate in dilute HNO3. Filter and wash. Evaporate to 10 or 15 cc. Cool. Add 5 cc. concentrated H2SO4, and heat until white fumes are given off. Dilute slightly and treat with 150 cc. of 50 per cent alcohol. Allow to stand; filter, and wash with 50 per cent alcohol. | 3. Reject the filtrate which contains the coloring matter. | ||
| 4. The precipitate contains the Pb. Dissolve in NH4C2H3O2 solution. Filter into a 50 cc. Nessler tube and wash with water containing NH4C2H3O2. Divide filtrate in halves. Saturate one-half with H2S. Determine the Pb in the other half by adding HC2H3O2 and H2S and comparing with standards containing known amounts of Pb. | 5. The filtrate contains the Zn and Cu. Concentrate to expel alcohol. Add excess of NH4OH, filter and wash precipitate. | ||
| 6. Reject the precipitate which contains the Fe. | 7. The filtrate contains the Zn and Cu. Neutralize with H2SO4. Add 10 cc. concentrated H2SO4 and 1 g. urea. Electrolyze for two hours with a current of 0.5 ampere. Break circuit, empty dish and wash. | ||
| 8. The deposit is Cu. Immerse the cathode in a small amount of hot, dilute HNO3; wash off and evaporate to dryness. Take up in water and wash into a Nessler tube. Make up to mark, and add 10 cc. of potassium sulfide solution. Compare with standard. If large amount is present, dry and weigh as Cu. | 9. The solution contains the Zn. Nearly neutralize with NH4OH. Concentrate to less than the capacity of the dish. Add 2 g. K2C2O4 and 1.5 g. K2SO4. Electrolyze for 3 hours with a current of 0.3 ampere. Siphon off solution, break circuit, wash with water, then alcohol, dry at 70° C., cool and weigh. | ||
| 10. The weighed residue is metallic Zn. | |||
| Table 11.—Scheme for determination of copper only. | |||
| 11. Concentrate sample to 75 cc. Add 2 cc. conc. H2SO4 for clear, soft waters and 5 cc. for alkaline or turbid waters. Electrolyze following procedure in 7 and 8. | |||
| Table 12.—Scheme for determination of zinc only. | |||
| 13. Follow scheme for all three metals as given in Table 10 through section 5. Nearly neutralize the filtrate with H2SO4, concentrate to less than the capacity of the dish and electrolyze as directed in section 9. | |||