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Chemicals Preparation of Solutions 268

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CHEMICALS ; PREPARATION OF SOLUTIONS 268. Choice of Chemicals. Substances which are entirely free from chemical impurities cannot be obtained as ordinary commercial products, and are only necessary for certain scientific investigations, for which purposes they are specially prepared by elaborate processes of purification. Intermediate between the purest chemical products and the raw materials, there are a large number of grades, which differ from one another in the amount of active constituent they contain, as well as in the nature and pro portion of the various impurities. Certain im purities, according to the use for which the sub stance is needed, are not injurious. For instance, impurities which merely lower the amount of active substance present may be permitted, but others, which would retard or interfere with the intended reaction, should be excluded.

For this reason, chemical substances to be used in pharmacy must comply with the stan dards of purity laid down in the British Pharnza copoeia. Similarly, many firms issue certain specifications to their purveyors of chemicals, which state the limits of admissible impurities in each product, and fix the price according to the actual content of active substance. It may be gathered from these remarks that care should be exercised in buying photographic chemicals, and that a comparison of prices is not the only consideration to be taken into account.

In his own interests, therefore, the photog rapher should purchase his chemicals from reputable specialized firms, who, from their knowledge of the ultimate uses of the materials, can supply the required qualities.

269. Anhydrous, Crystalline, Efflorescent, and Deliquescent Substances. The amount of active substance contained in a product varies con siderably with the chemical form in which it is obtained, and also with the extent to which it may have been changed by the action of the air.

Many salts exist in two forms, anhydrous and hydrated, the latter appearing most usually in the form of crystals. Anhydrous sulphite of soda, 1 for example, is equivalent to exactly twice its own weight of the crystalline sulphite, the difference (assuming both substances to be pure) representing the water of crystallization contained in the crystalline salt. The fact that

this numerical relationship is fortuitous and applies only to the case of sodium sulphite, is often overlooked. Thus, anhydrous carbonate of soda is equivalent to 2-7 times its own weight of the crystalline carbonate, and anhydrous hyposulphite of soda to approximately 1-5 times its own weight of the crystalline salt.

The water, which constitutes an integral part of hydrated salts, is not always firmly retained, and, in a very dry atmosphere, certain of these salts effloresce, the outer coating of the crystal being converted to the powdery anhydrous compound.

Other salts, both anhydrous and hydrated, readily absorb moisture from the dissolving progressively in the water they have absorbed. These deliquescent or hygroscopic salts, as they are called, are very difficult to keep in good condition, and their weighing-out becomes so uncertain that it is often advisable to prepare from these substances, immediately they are received, a stock solution of known concentra tion, from which, at any future time, the various mixtures may be prepared.

270. Unstable Substances. Many other stances change very rapidly, either spontane ously or due to the influence of atmospheric oxygen. These changes are usually accelerated by the presence of moisture. In this manner sulphite of soda, particularly as the crystalline salt or in solution (in which case the weaker the solution the more rapid the change), is gradually converted into a mixture of sulphate and di thionate by absorption of oxygen. Similarly, developing agents turn brown or black, in time, by oxidation, especially if they have been trans ferred to a damp vessel cyanides are gradu ally converted into carbonates by the carbon dioxide in the air. In a manner similar to the loss of gas from seltzer water, ammonium hy drate constantly loses ammonia gas, which, dissolved in water, forms the active constituent. Solutions of formaline, or formaldehyde, deposit a white insoluble mass (trioxymethylene) pro duced, without external interference, by a transformation of the original gaseous product.

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