Page:The British pharmacopœia.djvu/527

VOLUMETRIC SOLUTION OF OXALIC ACID.
(Crystalised Oxalic Acid, ${\ce {H2C2O4,2H2O = 126}}$.)

Take of
Oxalic Acid, in crystals	660 grains
Distilled Water	a sufficiency

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, ${\ce {Na2CO3,10H2O}}$ crystalised carbonate of sodium (${\ce {Na2CO3,10H2O}}$), 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 1/10th of a molecular weight in grammes of an alkali.

The following substances are tested with this solution:—