The proportion of carbonate is calculated by multiplying the amount of N/10 acid required in the second titration by 2, and then by either 0.0031 or 0.0047 to give the amount of carbonate present, expressed as Na2O or K2O respectively.
An alternative method is to determine the alkalinity before and after the elimination of carbonate by chloride of barium.
About 7-8 grammes of the sample are dissolved in water, and made up to 100 c.c., and the total alkalinity determined by titrating 20 c.c. with N/1 acid, using methyl orange as indicator. To another 20 c.c. is added barium chloride solution (10 per cent.) until it ceases to give a precipitate, the precipitate allowed to settle, and the clear supernatant liquid decanted off, the precipitate transferred to a filter paper and well washed, and the filtrate titrated with N/1 acid, using phenol-phthalein as indicator. The second titration gives the amount of caustic alkali present, and the difference between the two the proportion of carbonate.
When methyl orange solution is used as indicator, titrations must be carried out cold.
Reference has already been made (p. 39) to the manner in which the alkali percentage is expressed in English degrees in the case of caustic soda.
Chlorides are estimated by titrating the neutral solution with N/10 silver nitrate solution, potassium chromate being used as indicator. One c.c. N/10 AgNO3 solution = 0.00585 gramme sodium chloride.
The amount of acid necessary for exact neutralisation having already been ascertained, it is recommended to use the equivalent quantity of N/10 nitric acid to produce the neutral solution.
Sulphides may be tested for, qualitatively, with lead acetate solution.
Aluminates are determined gravimetrically in the usual manner; 2 grammes are dissolved in water, rendered acid with HCl, excess of ammonia added, and the gelatinous precipitate of aluminium hydrate collected on a filter paper, washed, burnt, and weighed.