Thioureas reaction with copper

The copper complex is very stable at neutral pH, but it fades very rapidly in the presence of hydrogen ions. Other complex formers such as tartaric acid or citric acid and thiourea interfere with the reaction and, therefore, should not be included in mobile phases used for the separation of amino acids [3]. 

The major source of interference and inaccuracy is iron contamination. It is thus essential to ensure that all the reagents, especially the ascorbate and the hydrochloric acid, are iron-free. Copper ions may also interfere in this assay (Duffy and Gandin, 1977). This can be eliminated by incorporating thiourea which, at a low pH, forms a stable colourless complex with copper without affecting the reaction of ferrozine with iron, thereby reducing the colour formation of ferrozine with copper. The affinity of thiourea for copper is maximal in the low pH range used here. 

The positional isomer of the heterocyclic carbonate used here is also known. Instead of using benzoquinone as a starting material with thiourea as the sulfur source (giving the 1,4- oxygen orientation), one can start with resorcinol in reaction with ammonium thiocyanate as the sulfur source (in the presence of copper sulfate) and get the positional isomer with a 1,3- oxygen orientation. This material (also known as thioxolone, or tioxolone, or... 

Free radicals derived from thiourea have been proposed as intermediates in several oxidations of thiourea. However, the reactions have not yielded much information regarding the identity or thermochemistry of the species implicated. For example, oxidation by IrCl62- occurs with a second-order dependence on [thiourea], a complex pH dependence, and hints of copper catalysis (244). Oxidation by Cu-(me2-phen)2+ is suggested to be an inner-sphere mechanism (92). At this time it is difficult even to guess at the redox potential of the thiourea radical. 

As a result of increasingly stringent environmental regulations, much attention has been devoted in recent years to the possibility that gold and silver can be leached by ligands other than cyanide. Other considerations, for example the presence of excessive quantities of soluble copper in the ore or the occurrence of gold in refractory (to cyanidation) minerals such as arsenopyrite or stibnite, have also prompted the search for alternatives to cyanide. Of the various possibilities, the one most studied and applied is the use of thiourea in acidic solutions with iron(III) as the oxidant. The principal leaching reaction is... 

Copper, which interferes by forming a coloured extractable complex with a-furildioxime, is effectively masked with thiourea or thiosulphate [16]. Addition of Na-K tartrate prevents the hydrolysable metals precipitating under the conditions of this reaction. 

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