MANGANESE.

If the sample contains less than 10 parts per million of manganese, use a colorimetric method in which the manganous salt is oxidized to permanganate and the color produced thereby is compared with that of a standard solution similarly treated. The persulfate method and the bismuthate method are suitable. If the sample contains more than 10 parts per million of manganese it is sometimes preferable to use a volumetric or gravimetric method.

PERSULFATE METHOD.

Reagents.—1. Nitric acid. Dilute concentrated nitric acid with an equal volume of distilled water. Free the diluted acid from brown oxides of nitrogen by aeration.

2. Silver nitrate. Dissolve 20 grams of silver nitrate in 1 liter of distilled water.

3. Standard manganous sulfate. Dissolve 0.288 gram of purest potassium permanganate in about 100 cc. of distilled water. Acidify the solution with sulfuric acid and heat to boiling. Add slowly a sufficient quantity of dilute solution of oxalic acid to discharge the color. Cool and dilute to 1 liter. One cc. of this solution contains 0.1 mg. of manganese.

4. Ammonium persulfate. Crystals, free from chloride.

Procedure.—Use an amount of the sample that contains not more than 0.2 mg. of manganese. Add 2 cc. of nitric acid and boil down to about 50 cc. Precipitate the chloride with silver nitrate solution, adding at least 1 cc. in excess. Shake and heat to coagulate the precipitate, and filter. A sample that contains much chloride should be evaporated with a few drops of sulfuric acid until white fumes appear and then diluted before the nitric acid and silver nitrate are added as directed above. If the sample is highly colored by organic matter it should be evaporated with sulfuric acid, and the residue ignited and dissolved in dilute nitric acid. Add about 0.5 gram of ammonium persulfate crystals and warm the solution until the maximum permanganate color is developed. This usually takes about ten minutes. At the same time prepare standards by diluting portions of 0.2, 0.4, 0.6 cc., etc. of the standard manganous sulfate solution to about 50 cc. and treating them exactly as the sample was treated. Transfer the sample and the standards to 50 cc. Nessler tubes, and compare the colors immediately. Manganese in parts per million is equal to the number of cubic centimeters of standard manganous sulfate solution in the tube that the sample matches multiplied by 100, divided by the number of cubic centimeters of the sample used.

BISMUTHATE METHOD.[^[2a]][^[113]]

Reagents.—1. Nitric acid. Dilute 1 part of concentrated nitric acid with 4 parts of distilled water. Free the dilute acid from brown oxides of nitrogen by aeration.

2. Sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3 parts of distilled water.

3. Dilute sulfuric acid. Dilute 25 cc. of concentrated acid to 1 liter with distilled water. Add enough permanganate solution to color faintly the dilute acid.

4. Standard manganous sulfate. The standard solution of manganous sulfate prepared as described under persulfate method (p. [48]) should be used and the standards should be prepared by following the same procedure as is used for the sample. This solution is more permanent than a solution of potassium permanganate, which may, however, be used. To prepare it dissolve 0.288 gram of potassium permanganate in distilled water and dilute the solution to 1 liter.

5. Sodium bismuthate. Purest dry salt.

Procedure.—Use an amount of the sample that contains not more than 0.2 mg. of manganese. Add 0.5 cc. of sulfuric acid and evaporate to dryness. Heat until the sulfuric acid is volatilized and ignite the residue. Dissolve in 40 cc. of nitric acid, add about 0.5 gram of sodium bismuthate, and heat until the permanganate color disappears. Add a few drops of a solution of ammonium or sodium bisulfate to clear the solution and again boil to expel oxides of nitrogen. Remove from the source of heat, cool to 20° C., again add 0.5 gram of sodium bismuthate, and stir. When the maximum permanganate color has developed, filter through an alundum or Gooch crucible containing an asbestos mat ignited and washed with potassium permanganate. Wash the precipitate with dilute sulfuric acid until the washings are colorless. Transfer the filtrate to a 50 cc. Nessler tube and compare the color of it with that of standards prepared from the potassium permanganate solution. To prepare the standards, dilute portions of 0.2, 0.4, 0.6 cc., etc. of the permanganate solution to 50 cc. with dilute sulfuric acid. The content of manganese is calculated as described under persulfate method (p. [49]).

LEAD, ZINC, COPPER, AND TIN.[^[7]][^[60]]

Determinations of lead, zinc, copper, and tin are important in certain mining regions and in places where the water has a solvent action on pipes and other containers. The use of certain “germicides” also makes it necessary to test for some of these metals.

Lead, zinc, and copper may be determined colorimetrically or electrolytically. The colorimetric methods are not so accurate as a combination of both, and are chiefly of value as qualitative tests.

It is possible to make a rough estimation of the amount of lead in clear waters by acidifying with acetic acid, saturating with hydrogen sulfide, and comparing the color produced with that produced by standard lead solutions in Nessler tubes, treated in similar manner. This method, however, is not applicable if the water is colored or contains iron.

Reagents.—1. Standard lead solution. Dissolve 1.60 grams of lead nitrate (Pb(NO₃)₂) in 1 liter of distilled water. One cc. of this solution contains 1 mg. of lead (Pb). As a check it is desirable to determine lead as sulfate in a measured portion of this solution.

2. Standard copper solution. Dissolve about 0.8 gram of copper sulfate crystals (CuSO₄.5H₂O) in water and, after the addition of 1 cc. of concentrated sulfuric acid, dilute the solution to 1 liter. Determine the copper in 100 cc. of this solution in the usual way by electrolytic deposition. Dilute the solution so that 1 cc. contains 0.2 milligram copper (Cu). This solution is permanent.

3. Ammonium chloride. Twenty-five per cent solution.

4. Ammonium acetate. Fifty per cent solution.

5. Ammonium hydroxide. (Sp. gr. 0.96.)

6. Hydrogen sulfide. Saturated solution.

7. Potassium sulfide. An alkaline solution of potassium sulfide made by mixing equal volumes of 10 per cent potassium hydroxide and a saturated aqueous solution of hydrogen sulfide.

8. Potassium oxalate. Crystals.

9. Potassium sulfate. Crystals.

10. Alcohol. Ninety-five per cent.

11. Alcohol. Fifty per cent.

12. Acetic acid. Fifty per cent.

13. Nitric acid. Concentrated acid (Sp. gr. 1.42).

14. Nitric acid. Dilute 1 part of the concentrated acid to 10 parts with distilled water.

15. Hydrochloric acid. (Sp. gr. 1.20.)

16. Sulfuric acid. Concentrated acid (Sp. gr. 1.84).

17. Sulfuric acid. Dilute the concentrated acid with an equal volume of distilled water.

18. Urea. Crystals.

LEAD.

Concentrate (1)[^[D]] 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.

[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.[^[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.		3. Reject the filtrate which contains the coloring matter.
	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.