Colloidal gold colour

Turkevich, J. (1985) Colloidal gold. Partll. Colour, coagulation, adhesion, alloying and catalytic properties. Gold Bulletin, 18, 125-131. 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

The spot-test technique is as follows. Mix a drop of the neutral test solution with a drop of 0-33m. gold chloride solution in a micro crucible and warm. After a short time, the solution is coloured red or blue with colloidal gold. 

Tin(II) chloride solution purple precipitate, purple of Cassius , consisting of an adsorption compound of tin(II) hydroxide, Sn(OH)2, and colloidal gold, in neutral or weakly acid solution. In extremely dilute solutions only a purple colouration is produced. If the solution is strongly acid with hydrochloric acid, a dark-brown precipitate of pure gold is formed. 

A summary of investigations on colloidal gold has been given by Ostwald,2 and also by Cornejo.3 Its optical properties have been examined by Steubing,4 and the various colours of the product have been studied by Bancroft.5... 

Colloidal gold has a very long history. Colourful aqueous solutions of gold colloids date back to Roman times and were known to the medieval alchemists... 

Shortly after this a mineral found in lead mines at Strontian in Argyll was mistaken for witherite. In 1790 Crawford suggested that it contained a new earth which he called strontia. His views were confirmed by numerous other investigators. The mineral was named strontianite it is the carbonate, SrCOa. The sulphate was first found by Clayfield near Bristol, where it is still incorrectly called strontia in the trade. The beautiful blue colour of some specimens led to the name celestine it is probably caused by traces of colloidal gold. 

Apart from particle size and shape, the scattering field is considerably influenced by the optical particle properties, i.e. by the dielectric function s(f) in optical domain or equivalently by the complex refractive index m. Indeed, Mie s paper (1908) on sphere scattering aimed at an explanation for the optical phenomena in colloidal gold sols (e.g. different colour of transmitted and reflected light), which can not be properly understood without adequate consideration of the wavelength dependency of the real and imaginary part of the refractive index. 

Gold compounds are all easily reduced in alkaline solution to metallic gold which may occur in colloidal form and so be red, blue or intermediate colours. Reduction to gold, followed by weighing of ihe metal precipitated, may be used in quantiiaiive analysis. 

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