(Sulphuric acid of specific gravity 1·84.)
NOTE.--The directions must be followed carefully, and glass measuring vessels should be used if procurable.
(_a_) Measure out 1 pint of strong acid, and pour it carefully and slowly _down the inner surface_ of a jar containing 20 gallons of water. Do not pour it directly into the water.
The heavy acid will sink to the bottom of the jar, and a good mixture must be obtained by stirring well.
(_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 1-1/2 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.