The apparatus required is shown in fig. 82. It consists of a small retort or evaporating vessel made out of a pipette of 200 c.c. capacity. This is heated by means of a paraffin-bath at 130° or 140° C. It is connected with an upright condenser, at the lower end of which is a small flask which serves as a receiver.
The quantity of the borate taken should contain not more than 0.2 gram of the trioxide. Insoluble compounds are "dissolved in nitric acid at once, or, if necessary, first fused with sodium carbonate." With soluble and alkaline borates sufficient nitric acid is added to render it faintly acid. The solution is then introduced into the retort.
"The lime, to retain the boric acid in the distillate, is ignited in the crucible in which the evaporation of the distillate is to be made subsequently." It is then cooled in the desiccator for ten minutes, and weighed. The lime is transferred to the receiving flask and slaked with a little water. The retort is lowered into the bath so that "only the rear dips below the surface." The evaporation is carried to dryness, the retort being lowered further into the bath as the evaporation proceeds. Ten c.c. of methyl alcohol are introduced upon the residue, and the evaporation again started. Six such portions of alcohol are thus distilled and 2 c.c. of water are introduced and evaporated between the second and third, as also between the fourth and fifth distillations. If acetic acid is used instead of nitric in the first instance this addition of water is unnecessary.
The distillate is evaporated in the crucible ignited over the blowpipe, cooled in the desiccator for ten minutes and weighed. The increase in weight gives the boron trioxide. The results tend to be from 1 to 2 milligrams too high.
VOLUMETRIC METHOD.
This method is applicable to the indirect determination of boric acid in borax and similar compounds. It is based on the measurement of the quantity of normal solution of acid required to replace the boric acid, and, consequently, is rather a measure of the soda present. The process is an alkalimetric one, and is carried out as follows:—Weigh up 3 grams of the sample and dissolve in water. Tint with methyl orange, and run in from an ordinary burette normal solution of sulphuric acid until a pink tint is got. 100 c.c. of the normal solution of acid are equal to 7.0 grams of boron trioxide (B2O3), or 10.1 grams of anhydrous borax (Na2B4O7).
Examination of Borax.—In addition to the determination just given, the following determinations are also required:—
Water.—Take about 2 grams and heat to tranquil fusion in a platinum crucible. Count the loss in weight as water.
Sulphuric Oxide.—Take 2 grams, dissolve in water, acidify with hydrochloric acid, filter, and precipitate with barium chloride. Wash the precipitate, ignite, and weigh as barium sulphate (see Sulphur).