Part 16

LEAD.

Concentrate (1) rapidly by boiling in a 7–inch porcelain dish over a free flame 3 or 4 liters of the sample to be tested, or more if very small amounts of the metals are present, to a volume of about 30 cc. Add 10 or 15 cc. of ammonium chloride solution to assist in the separation of the sulfides, then add a few drops of concentrated ammonium hydroxide, and saturate with hydrogen sulfide. Allow to stand some time, preferably over night, add a little more ammonium hydroxide and hydrogen sulfide, boil the contents of the dish a few minutes, and filter. The precipitate (2) may consist of lead, zinc, copper, and iron sulfides and the suspended organic matter. The soluble coloring matter is in the filtrate (3). Wash the precipitate a few times with hot water, place the precipitate and the filter paper in the original dish and boil with dilute nitric acid, rubbing down the sides of the dish, if necessary, to detach any adhering sulfide precipitate. After again filtering and washing several times with hot water, evaporate the filtrate and washings in the original dish to a bulk of 10 to 15 cc., cool, add 5 cc. of concentrated sulfuric acid, and heat until copious fumes of sulfuric acid are evolved.

Footnote D:

The numbers in parentheses refer to tables 10–12, pages 55–56.

If lead is present dilute the contents of the dish slightly with water, and treat them with 150 cc. of 50 per cent alcohol, in which the lead sulfate is insoluble. Allow to stand some time, preferably over night, filter off the lead sulfate, and wash it with 50 per cent alcohol. Save the filtrate for the determination of zinc.

Dissolve the precipitate of lead sulfate by boiling the filter containing it in ammonium acetate solution in a porcelain dish. (4). Filter into a 50 cc. Nessler tube and wash the filter with boiling water containing a little ammonium acetate. Divide this filtrate in halves and treat one-half with saturated hydrogen sulfide water in order to get an approximation of the amount of lead present. To the other half, or an aliquot portion, if a large amount of lead is present, add a few drops of acetic acid, then an excess of saturated hydrogen sulfide solution, and compare the color with that of standards made by treating known amounts of the standard lead solution with a little acetic acid, ammonium acetate, and hydrogen sulfide.

ZINC.

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.

COPPER.

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 (SnO_{2}).

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. 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. ────────────────────────────────────────────────────────┬────────────── 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. │ ──────────────────────────┬─────────────────────────────┤ 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. │ │ ──────────────────────────┼──────────────┴───────────────────────────── 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. │ ──────────────────────────┴────────────────────────────────────────────

Table 11.—SCHEME FOR DETERMINATION OF COPPER ONLY.

─────────────────────────────────────────────────────────────────────── 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. ───────────────────────────────────────────────────────────────────────

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 H{2}SO{4}, concentrate to less than the capacity of the dish and electrolyze as directed in section 9. ───────────────────────────────────────────────────────────────────────

MINERAL ANALYSIS.

RESIDUE ON EVAPORATION.

See description of method (p. 29). The residue should be dried one hour at 180° C. Turbid waters should be filtered, and the composition of the suspended matter should be determined separately or the amount of it reported as suspended matter.

ALKALINITY AND ACIDITY.

See description of method (pp. 35–41).

CHLORIDE.