§ 798. Solubility of Copper in Water and Various Fluids.—The solubility of copper in water and saline solutions has been very carefully studied by Carnelley.[856] Distilled water exerts some solvent action, the amount varying, as might be expected, according to the time of exposure, the amount of surface exposed, the quantity of water acting upon the copper, &c. It would appear that, under favourable circumstances, 100 c.c. of distilled water may dissolve ·3 mgrm. of copper (·2 grain per gallon).
[856] Journ. Chem. Soc., 1876, vol. ii. p. 4.
With regard to salts, those of ammonium exert a solvent action on copper more decided than that of any others known. With the others, however, the nature of the base exerts little influence, the action of the salt depending chiefly on the nature of its acid radical. Thus, beginning with the least effective, the following is the order of dissolving strength:—Nitrates, sulphates, carbonates, and chlorides. It will then at once be evident that a water, contaminated by sewage, and therefore containing plenty of ammonia and chlorides, might exert a very considerable solvent action on copper.
Almost all the oils and fats, as well as syrups, dissolve small quantities of copper; hence its frequent presence in articles of food cooked or prepared in copper vessels. In the very elaborate and careful experiments of Mr. W. Thompson,[857] the only oils which took up no copper, when digested on copper foil, were English neats’-foot oil, tallow oil, one sample of olive oil, palm-nut oil, common tallow oil, and white oil, which was protected from the air by a thick coating of oxidised oil on its surface.
[857] “Action of Fatty Oils on Metallic Copper,” Chem. News, vol. xxxiv. pp. 176, 200, 313.
The formation of copper compounds with the fatty acids takes place so readily that Jeannel[858] has proposed the green colouring of fats by copper as a test for the presence of copper; and Bottger[859] recommends a copper holding brandy to be shaken up with olive oil to free it from copper.