The American Carbon Manual/The Chromic Salts

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Either chromic acid alone, or various of its salts, may be used to render certain soluble organic bodies insoluble after exposure to the action of light. Practically, for a variety of reasons, the bichromate of potash, or the bichromate of ammonia, is found to answer best. Bichromate of potash, as being the cheapest, is more generally employed, otherwise bichromate of ammonia has certain advantages. It is a little more sensitive, and has been said to yield a tissue less liable to spontaneous insolubility. The latter quality, however, seems doubtful, and certainly requires verifying; as, for more than one reason, it seems probable that it will, on the contrary, produce a tissue more prone to spontaneous decomposition. In the first place, this tendency will necessarily accompany extreme sensitiveness. Mr. Swan finds that dampness in the sensitive tissue is a chief cause of the spontaneous change which produces insolubility, and as bichromate of ammonia is more greedy of water than bichromate of potash, the tissue will more readily absorb moisture from the atmosphere, and thus secure the condition favorable to spontaneous change. The difference in the solubility of the two salts is the chief element of advantage in favor of the latter. Bichromate of potash is soluble in about ten times its weight of water, at 60 degrees; bichromate of ammonia is soluble in about four times its weight of water, at the same temperature.

The double chromate of potash and ammonia has been recommended as possessing certain advantages over the bichromates. M. Emile Kopp, who first suggested its use, prefers it as more convenient. Mr. Carey Lea points out that its especial advantage is found in the fact that it is less liable to spontaneous decomposition, and not seriously less sensitive to the action of light. It is not necessary to prepare the crystallized double salt, but simply to neutralize a saturated solution of bichromate of potash with liquid ammonia. The advantage of the bichromate in point of sensitiveness is however very great. It will be found advantageous, therefore, wherever the greatest sensitiveness is required, to use a bichromate in preference to a neutral chromate.

The salts of uranium have, in some instances, been proposed in place of those of chromium, but the reactions do not correspond, and practically this substitution is inadmissible.