Page:The British pharmacopœia.djvu/527

Rh VOLUMETRIC SOLUTION OF OXALIC ACID.

(Crystalised Oxalic Acid, H2C2O4,2H2O = 126.)

Put the oxalic acid into the 10,000 grain flask, fill the flask to about two-thirds of its bulk with water, allow the acid to dissolve, and then dilute the solution with more water until it has the exact volume of 10,000 grain-measures. Fill a burette with the fluid, and add it gradually to a solution of 10.6 grams of pure carbonate of sodium (which may be obtained by heating the ordinary pure bicarbonate of sodium to redness in a platinum crucible for a quarter of an hour), containing a few drops of solution of litmus, until the red colour produced ceases to change to blue on boiling. Note the number of grain-measures used {n), then put 9,000 grain-measures of the solution of oxalic acid into a graduated jar, and augment this quantity by the addition of distilled water until it amounts $$\frac{9000\times 300}{n}$$ to grain-measures. 1,000 grain-measures of this solution contain half a molecular weight in grains (63 grams) of oxalic acid, and are therefore capable of neutralising one molecular weight in grains of such alkalies as potash, KHO, or soda, NaHO; or half the molecular weight in grains of such salts as anhydrous carbonate of sodium, Na2CO3,10H2O crystalised carbonate of sodium (Na2CO3,10H2O), etc.

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 contain $1⁄20$ th. of a molecular weight in grammes (6.3 grammes) of oxalic acid, and will neutralise $1⁄10$th of a molecular weight in grammes of an alkali.

The following substances are tested with this solution:—