(b) Of this solution (which is approximately 1 per cent. by weight), use 1 gallon to 20 gallons of latex.
Readers are doubtless now well aware of the corrosive action of strong sulphuric acid, and we scarcely need point out that even the dilute acid should not be kept in contact with the usual iron vessels found in factories. The mixing of solutions should be done in one of the glazed earthenware jars commonly in use.
The formula given above works out at approximately 1 part strong acid to 2,000 parts of latex (of dry rubber content 11⁄2 lbs. per gallon). The formula for using acetic acid with the same latex works out at about 1: 1,200. It will be apparent, therefore, that relatively sulphuric acid is a more powerful coagulant than acetic acid. In terms of dry rubber obtained from latex of the consistency indicated above—
1 lb. sulphuric acid will produce 300 lbs. dry rubber. 1 lb. acetic acid will produce 180 lbs. dry rubber.
With both acids selling at the same rate, sulphuric acid would be more economical in use; when its cost is less than that of acetic acid, which is the normal condition, the economic advantage in favour of sulphuric acid is augmented still further.
It may be found that the standard formula for sulphuric acid will not always give a perfectly clear remaining serum, even though an attempt is made daily to work to a uniform consistency for all latices. It is inevitable that the manipulation of the latices should be slightly in error on occasions, or that a small mistake might occur in preparing the solution of acid. Hence a clear remaining serum after coagulation may be secured less often than a slightly turbid serum. This is as it should be. The minimum quantity of acid may be adjusted so closely as to give such results. If a clear serum is obtained always, that should be an indication of continual excess of coagulant. Naturally, if a milky serum is always obtained, the reverse is the case.
As a last word on the subject, it may again be emphasised that the use of sulphuric acid is not advised, except in an emergency; and that the greatest possible care must be exercised in the observance of the strict formula for use.
Hydrochloric Acid, Nitric Acid.—These mineral acids would prove more expensive than sulphuric acid. In addition they are much more uncertain in action. For example, the use of a certain excess of hydrochloric acid would not hasten coagulation, but would prevent it. Above all their effect, in excess, is deleterious to the rubber.
Hydrofluoric Acid.—This has a strong corrosive action on porcelain or glass. Hence it has to be contained in bottles of gutta-percha or lead. It is mentioned here merely because some years ago it found a use as a coagulant, chiefly in Ceylon. It was sold in the form of a 10 per cent. solution under the name of “Purub,” and was the subject of a patent.
It is effective as a coagulant, and has also an anti-oxidant action, which was its chief recommendation when cheap and harmless anti-oxidants were not commonly known. It is comparatively expensive, and, as indicated above, difficult to handle and store. In short, it has nothing to commend it, in comparison with acetic or formic acids.