Sodium Chloride (Common Salt).
This should be examined for the following:—
Actual Chloride, either titrated with N/10 silver nitrate solution, using neutral potassium chromate solution as indicator, or, preferably, estimated gravimetrically as silver chloride by precipitation with silver nitrate solution, the precipitate transferred to a tared filter paper, washed, dried and weighed.
Insoluble matter, remaining on dissolving 5 grammes in water, and filtering. This is washed, dried, ignited and weighed.
Moisture.—5 grammes are weighed into a platinum crucible, and heat gently applied. The temperature is gradually increased to a dull red heat, which is maintained for a few minutes, the dish cooled in a desicator, and weighed.
Sulphates are estimated by precipitation as barium sulphate and calculated to Na2SO4.
Sodium.—This may be determined by converting the salt into sodium sulphate by the action of concentrated sulphuric acid, igniting to drive off hydrochloric and sulphuric acids, and fusing the mass until constant in weight, weighing finally as Na2SO4.
Potassium Chloride.
This should be examined, in the same way as sodium chloride, for chloride, insoluble matter, moisture, and sulphate. The potassium may be determined as potassium platino-chloride, as described under carbonate of potash.