“Polymer” Inclusions in Cobalt‐Hardened Electroplated Gold

When cobalt‐hardened electroplated gold is dissolved in either aqua regia or mercury, carbon‐containing inclusions are extracted. The chemical identity of the extracts is established here by a variety of chemical and physical analyses. The indiscriminate use of the word “polymer” to refer to both the original inclusions and the extracts is inaccurate. The “polymer” separated upon dissolution of the gold in aqua regia is not an organic material but a known cobalt cyanide compound with the formula Co 3 II [ Co III ( CN ) 6 ] 2 · x H 2 O . On the other hand, the extract obtained after dissolution in mercury was shown to contain K 3 Co III ( CN ) 6 . Based on the observation of the two different extracts and the chemistry of the plating solution, the form of the original inclusions is proposed to be either a Co(II)‐ or Co(III)‐cyanide complex. Additional analyses on the bulk gold and on the aqua regia filtrate led to the following conclusions: ( i ) There is an additional source in the gold deposit for both cobalt and carbon apart from the cobalt cyanide complex. ( ii ) For cobalt, this additional source is the substitutional metallic form. ( iii ) For carbon, the additional source is an as yet unidentified compound probably derived from cyanide ions but containing a C:N ratio greater than one. This compound may be the source of the carbon found in the aqua regia filtrate, and it may account for the extraordinary amounts of hydrogen found in hard gold deposits. ( iv ) Some cautions are necessary in the aqua regia analyses for Co and C because of the limited solubility of “polymer.”