Qualitative analysis / by William Briggs and R.W. Stewart.

  • William Briggs
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: Qualitative analysis / by William Briggs and R.W. Stewart. Source: Wellcome Collection.

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    copper salts; it is best seen when the reacticn takes place in a watch-glass placed on a sheet of white paper. K4Fe(CN)6 affords one of the most delicate reactions for copper salts. 4. Potassium cyanide, ICON, gives a greenish-yellou precipitate of cupric cyanide, Cu(CN)2. 2 KCN + CuS04 =-- Cu(CN)2 + K2S04. The precipitate rapidly loses cyanogen, forming cuprous cyanide, Cu2(CN)2. 2 Cu(CN)2 = Cu2(CN)2 + C2N2. If excess of potassium cyanide is added the precipitate redissolves, forming a double cyanide of potassium and copper, KCu(CN)2. 2 KCJST + Cu2(CN)2 = 2 KCu(CN)2. As stated above, this compound is not decomposed by sulphuretted hydrogen. The explanation of this is that the ions of the double cyanide are K and [Cu(CN)2], not K, Cu and (CN), i.e.y there are no free copper ions (or more correctly only a very few). Now it is on the presence of free copper ions that the precipitation of copper by H2S depends, the sulphur ions of H2S combining with them to form insoluble CuS. 5. Caustic potash, KOH, or caustic soda, NaOH, gives a bulky light blue precipitate of cupric hydrate, Cu(OH)2. 2 KOH + CuS04 = Cu(OH)2 + K2S04. The precipitate is insoluble in excess. If the mixture is boiled the precipitate becomes black, the cupric hydroxide losing part of its water. 3 Cu(OH)2 = Cu(OH)2, 2 CuO + 2 H20. 6. If a piece of bright iron be dipped into a cupric solution acidulated with HC1, it becomes coated with copper; if clean iron filings be boiled in the solution, it