Chloride in water and sewage has its origin in common salt, from mineral
deposits in the earth, from ocean vapors carried inland by the wind, or
from polluting materials like sewage and trade wastes, which contain the
salt used in the household and in manufacturing. Comparison of the
chloride content of a water with that of other waters in the vicinity
known to be unpolluted frequently affords useful information as to its
sanitary quality. If, however, the chloride normally exceeds 20 parts
per million because of chloride-bearing mineral deposits the chloride
content of a water has little sanitary significance.
_Reagents._—1. Standard sodium chloride solution. Dissolve 16.48 grams
of pure fused sodium chloride in 1 liter of distilled water. Dilute 100
cc. of this stock solution to 1 liter in order to obtain a standard
solution each cubic centimeter of which contains 0.001 gram of chloride.
2. Standard silver nitrate solution. Dissolve about 2.40 grams of silver
nitrate crystals in 1 liter of distilled water. Standardize this with
the standard salt solution, and adjust, correcting for volume (see p.
43), so that 1 cc. will be exactly equivalent to 0.0005 gram of
chloride.
3. Potassium chromate indicator. Dissolve 50 grams of neutral potassium
chromate in a little distilled water. Add enough silver nitrate to
produce a slight red precipitate. Filter and dilute the filtrate to 1
liter with distilled water.
4. Aluminium hydroxide. Electrolyze ammonia-free water, using aluminium
electrodes. Wash the precipitate until it is free from chloride,
ammonia, and nitrite. Or dissolve 125 grams of potassium or ammonium
alum in 1 liter of distilled water. Precipitate the aluminium by adding
cautiously ammonium hydroxide. Wash the precipitate in a large jar by
successive additions and decantations of distilled water until free from
chloride, nitrite, and ammonia.
_Procedure._—Add 1 cc. of potassium chromate indicator to 50 cc. of the
sample in a 6–inch white porcelain evaporating dish or a 150 cc.
Erlenmeyer flask over a white surface. Titrate with the silver nitrate
solution under similar conditions of volume, light, and temperature as
were used in standardizing the silver nitrate until a faint reddish
coloration is perceptible. The detection of the end-point is facilitated
by comparison of the contents of the porcelain dish with those of
another dish containing the same quantity of potassium chromate
indicator in 50 cc. of distilled water. Some analysts prefer to make the
titration in a dark-room provided with a yellow light. The end-point is
very sharp by electric light and also by daylight with photographic
yellow glass. The titration may be made in Nessler tubes[68a] if the
solutions are standardized under similar conditions.
If the amount of chloride is very high use 25 cc., or even a smaller
quantity, dilute the volume taken to 50 cc. with distilled water. If the
amount of chloride is very low concentrate 250 cc. of the sample to 50
cc. by evaporation. Rotate the liquid to make sure that no residue
remains undissolved on the walls of the dish, and, if necessary, use a
rubber-tipped glass rod to assist in this operation.
Chloride is determined by some observers by extracting with hot
distilled water the residue in the platinum dish in the determination of
the residue on evaporation and proceeding as just described. This is
permissible if a little sodium carbonate is added before evaporation to
prevent loss of chloride through decomposition of magnesium chloride in
the residue.
If the sample has a color greater than 30 it should be decolorized by
shaking it thoroughly with washed aluminium hydroxide (3 cc. to 500 cc.
of the sample) and allowing the precipitate to settle. Make the
determination on a portion of the clarified sample, filtered if
necessary. If the sample is acid, neutralize it with sodium carbonate;
if hydroxide is present, add dilute sulfuric acid until the cold liquid
will just discharge the color of phenolphthalein. If the presence of
sulfide and sulfocyanate renders it necessary, make proper
corrections[24c][100b] or modifications in treatment.
Make correction for the error due to variations in the volume of the
liquid and precipitate by means of the formula[39] X = 0.003V + 0.02, in
which X = the correction in cubic centimeters of silver nitrate solution
and V = cubic centimeters of liquid at the end of the titration. If 50
cc. of the sample is titrated chloride (Cl) in parts per million is
equal to the number of cubic centimeters of silver nitrate solution
multiplied by 10. The correction to be applied is 0.2 cc. unless unusual
accuracy is required.