Colloid reducing agents

This ion is very easily reduced to gold, and hence alkaline solutions of chloraurates(III) (often wrongly called gold chloride ) are used with a reducing agent to prepare colloidal gold. 

The elements are obtainable in a state of very high purity but some of their physical properties are nonetheless variable because of their dependence on mechanical history. Their colours (Cu reddish, Ag white and Au yellow) and sheen are so characteristic that the names of the metals are used to describe them. Gold can also be obtained in red, blue and violet colloidal forms by the addition of vtirious reducing agents to very dilute aqueous solutions of gold(III) chloride. A remarkably stable example is the Purple of Cassius , obtained by using SnCla as reductant, which not only provides a sensitive test for Au but is also used to colour glass and ceramics. Colloidal silver and copper are also obtainable but are less stable. 

The mechanism of formation of Pt particles by the or-ganometallic reduction route, however, was found to proceed differently, for example in the reductive stabilization of Pt nanoparticles produced by reacting Pt-acetylacetonate with excess trimethylaluminium. Here, derivates of aluminium alkyls act as both reducing agents and colloidal stabilizers. As was shown by a combination... 

Energy-dispersive X-ray analysis showed the absence of nitrogen, indicating that the Pd colloids are free from the metal precursor and reducing agent. Sulfur surface content is three orders of magnitude below the amount in the... 

This mechanism is based on the known importance of hydroxides in other deposition reactions, such as the anomalous codeposition of ferrous metal alloys [38-39], Salvago and Cavallotti claim an analogy with the mechanism of Ni2 + reduction from colloids in support of their proposed mechanism. There is no direct evidence for the hydrolyzed species, however. Furthermore, the mechanism does not explain two experimentally observed facts Ni deposition will proceed if the Ni2 + and the reducing agent are in separate compartments of a cell [36, 37] and P is not deposited in the absence of Ni2 +. The chemical mechanism does not take adequate account of the role of the surface state in catalysis of the reaction. It has no doubt been the extreme oversimplification, by some, of the electrochemical mechanism that has led other investigators to reject it. 

Colloidal metals are usually prepared by reduction of a salt with a reducing agent, such as phosphorus, acetone, tannin, or carbon monoxide. Platinum metals can also be prepared as finely divided very active blacks by reducing the metal salt in an aqueous solution of sodium or potassium borohydride. 

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