Page:The British pharmacopœia.djvu/523

Volumetric solutions, before being used, should be shaken, in order that they may be throughout of uniform strength. They should also be preserved in stoppered bottles. All measurements should be made at 60° F. (15°.5 C.).

VOLUMETRIC SOLUTION OF BICHROMATE OF POTASSIUM.

(Bichromate of Potassium, ${\ce {K2Cr2O7}}$ = 295.)

Take of

Bichromate of Potassium	147.5 grains
Distilled Water	a sufficiency

Put the bichromate of potassium into the 10,000 grain flask, and, having half filled the flask with water, allow the salt to dissolve; then dilute the solution with more water, until it has the exact bulk of 10,000 grain-measures. 1,000 grain-measures of this solution contain 14-75 grains of the bichromate (1/20th of ${\ce {K2Cr2O7}}$, in grains), and, when added to a solution of a ferrous salt acidulated with hydrochloric acid, are capable of converting 16.8 grains of iron (1/20th of 6Fe, in grains) from the ferrous to the ferric state.

Grammes and cubic centimetres may be employed instead of grains and grain-measures, but for convenience 1/10th of the numbers should be taken. Thus 14.75 grammes of bichromate of potassium should be made to form 1,000 cubic centimetres of solution. 100 cubic centimetres of this solution contain 1.475 grammes of the bichromate (1/200 of ${\ce {K2Cr2O7}}$, in grammes), and, when added to a solution of a ferrous salt acidulated with hydrochloric acid, are capable of converting 1.68 gramme of iron (1/200th of 6Fe, in grammes) from the ferrous to the ferric state.

This solution is used for determining the proportion of ferrous salt in the following preparations. It is known that the whole of the ferrous salt has been converted into a ferric salt when a minute drop of the liquid, placed in contact with a drop of a very dilute solution of ferricyanide of potassium on a white plate, ceases to strike with it a blue colour.