It has often happened that shippers or local sellers, by inadvertence, have supplied the one chemical in place of the other—to the detriment of the rubber and the discomfiture of managers of estates. The error, as a rule, has not been detected for some time, and then perhaps only as a result of complaints or enquiries reaching the laboratories. To the layman, and certainly to the native who usually has charge of these substances, it is not a simple matter to distinguish between them without special knowledge. There are certain elementary tests, however, which can be applied on all estates serving to make the distinction, but affording no information regarding the actual quality of the chemicals. They are given in a comparative form on [page 116]. Samples of doubtful specimens may be sent to the laboratories for analysis, but the bulk of the chemical should not be used.

During the War some very poor shipments were received, and much trouble was caused. Under normal conditions there can be no question that it is to the interests of chemical manufacturers to supply the best article; and it is anticipated that in future there should be no difficulty in procuring shipments of a high degree of purity.

Sodium Bisulphite. Sodium Sulphite.
1.If in good condition it has a powerful odour of sulphur dioxide.[9]1.It has no perceptible odour.
2.In solution it should turn a blue litmus-paper red.2.In solution it should turn a red litmus-paper blue.
3.It exhibits a marked tendency to “cake” if the drum is allowed to remain open.3.The tendency to “cake” is less marked than in the case of the bisulphite.

[9] High-grade sodium bisulphite has very little odour, but by the time it reaches the estate, and as a result of short exposure to the moist atmosphere of the tropics, a little decomposition sets in and a strong odour of sulphur dioxide gas is noticeable.

It will be evident that, as sodium bisulphite under normal conditions gives off a gas when exposed to the atmosphere, it deteriorates in quality continuously. It is the potential presence of this gas which makes the powder effective as an anti-oxidant and disinfectant. It is within the experience of all accustomed to the handling of this chemical, that in addition to the loss of gas, the powder cakes into a hard mass on exposure. If only the top layer is caked, the remainder may be in fair condition; but no caked portions should be used, as they cannot be of good quality. They may, however, be used for the treatment of scrap rubber, to be discussed later.

Care of Sodium Bisulphite.—The ready tendency of sodium bisulphite to deteriorate on exposure should give sufficient indication regarding its treatment in storage. It should be bought only in drums (or other air-tight containers), and should be stored in a dry place. No drum should be opened until required, and the common practice of keeping an open drum on the floor of the factory should be avoided.

Drums are of two sizes, generally containing 1⁄4 or 1⁄2 cwt. respectively. It will be obvious that, although the prime cost may be cheaper with the larger quantity, it would always be preferable to secure the smaller drums, as the loss on exposure will be less.

Most commonly the 56 lb. drum is purchased. It should not be difficult to calculate the period during which the contents will be consumed, on the basis of a maximum of 1 lb. per 100 gallons of latex. A 56 lb. drum, assuming no loss or waste, should be sufficient to treat at least 5,600 gallons of latex (say, 8,500 lbs. of rubber)—if the bisulphite is of first-class quality, and the use is applied only to the preparation of fine pale crepe.

Where the quantity used per diem is small, it is advised that precautions should be taken to preserve the quality of the chemical when a drum is opened. It might be of advantage to place the contents of the drum in smaller sealed tins, or to have made a special container (with a closely fitting lid) into which the powder can be placed as soon as the drum has been opened.

Mixing Solution with Latex.—Emphasis has been laid, in [Chapter VIII.], upon the necessity for care in the preparation of the solution. Equal regard must be given to the mixture of the solution with the latex.