Page:The British pharmacopœia.djvu/524

490

VOLUMETRIC SOLUTION OF HYPOSULPHITE OF SODIUM.

(Hyposulphite of Sodium crystallised, Na2S2O3,5H2O = 248.)

Take of

Dissolve the hyposulphite of sodium in 10,000 grain-measures of water. Fill a burette with this solution, and drop it cautiously into 1,000 grain-measures of the volumetric solution of iodine, until the brown colour is just discharged. Note the number of grain-measures (n) required to produce this effect; then put 8,000 grain-measures of the same solution into a graduated jar, and augment this quantity by the addition of distilled water until it amounts to $$\frac{8000\times 1000}{n}$$ gram-measures. If, for example, n=950, the 8,000 grain-measures of solution should be diluted to the bulk of $$\frac{8000 \times 1000}{n}$$=8,421 grain-measures. 1,000 grain-measures of this solution contain 24.8 grains of the hyposulphite ($1⁄10$th of Na2S2O3,5H2O, in grains), and therefore correspond to 12.7 grains of iodine ($1⁄10$ th of an atomic weight in grains).

Grammes and cubic centimetres may be employed instead of grains and grain-measures, but for convenience $1⁄10$ th of the numbers should be taken. 100 cubic centimetres of this solution contain 2.48 grammes of the hyposulphite ($1⁄100$ th of Na2S2O3,5H2O, in grammes), and therefore correspond to 1.27 gramme of iodine ($1⁄100$th of an atomic weight in grammes).

This solution is used for testing the following substances.