_Procedure._—Mix 1 cc. of the tolidin reagent with 100 cc. of the sample in a Nessler tube and allow the solution to stand at least 5 minutes. Small amounts of free chlorine give a yellow and larger amounts an orange color.

For quantitative determination compare the color with that of standards in similar tubes prepared from the solutions of copper sulfate and potassium bichromate. The amounts of solution for various standards are indicated in Table 13.

Standard methods for the examination of water and sewage — Part 54

Table 13.—PREPARATION OF PERMANENT STANDARDS FOR CONTENT OF CHLORINE.

Standard methods for the examination of water and sewage — Part 54

───────────────────────┬───────────────────────┬─────────────────────── Chlorine. │ Solution of copper │ Solution of potassium │ sulfate. │ bichromate. ───────────────────────┼───────────────────────┼─────────────────────── _Parts per million._ │ _cc._ │ _cc._ │ │ 0.01│ 0.0│ 0.8 .02│ .0│ 2.1 .03│ .0│ 3.2 .04│ .0│ 4.3 .05│ .4│ 5.5 .06│ .8│ 6.6 .07│ 1.2│ 7.5 .08│ 1.5│ 8.7 .09│ 1.7│ 9.0 .10│ 1.8│ 10.0 .20│ 1.9│ 20.0 .30│ 1.9│ 30.0 .40│ 2.0│ 38.0 .50│ 2.0│ 45.0 ───────────────────────┴───────────────────────┴───────────────────────

Standard methods for the examination of water and sewage — Part 54

DISSOLVED OXYGEN.[16][65][68][71b][99][100c][120]

_Reagents._—1. Sulfuric acid, concentrated. (Sp. gr. 1.83–1.84.)

2. Potassium permanganate. Dissolve 6.32 grams of the salt in water and dilute the solution to 1 liter.

3. Potassium oxalate. A 2 per cent solution.

4. Manganous sulfate. Dissolve 480 grams of the salt in water and dilute the solution to 1 liter.

5. Alkaline potassium iodide. Dissolve 700 grams of potassium hydroxide and 150 grams of potassium iodide in water and dilute the solution to 1 liter.

6. Hydrochloric acid. Concentrated (Sp. gr. 1.18–1.19).

7. Sodium thiosulfate. A N/40 solution. Dissolve 6.2 grams of chemically pure recrystallized sodium thiosulfate in water and dilute the solution to 1 liter with freshly boiled distilled water. Each cc. is equivalent to 0.2 mg. of oxygen or to 0.1395 cc. of oxygen at 0°C. and 760 mm. pressure. Inasmuch as this solution is not permanent it should be standardized occasionally against a N/40 solution of potassium bichromate. The keeping qualities of the thiosulfate solution are improved by adding to each liter 5 cc. of chloroform and 1.5 grams of ammonium carbonate before diluting to the prescribed volume.