Gold (Au) Colloids

Chemistry Level 3

Au Colloid Preparation:

20 mL of an aqueous 0.1 mM hydrogen tetrachloroaurate(III) hydrate solution was added to a 50 mL beaker. A magnetic stirring bar was added to the beaker, and the solution was heated on a hotplate (with magnetic stirring) until it starts to boil. Upon boiling, 2 mL of an aqueous 38.8 mM sodium citrate dihydrate solution was added to the boiling hydrogen tetrachloroaurate(III) hydrate solution. Boiling was allowed to continue for a further 10 minutes, after which, the resulting solution was removed from the heat, and allowed to cool to room temperature.

Upon the addition of the 2 mL of aqueous 38.8 sodium citrate dihydrate solution to the boiling 0.1 mM hydrogen tetrachloroaurate(III) hydrate solution, a deep red colour developed. This resulted from the citrate anions reducing the Au(III) cations to Au(0) nanoparticles, while most of the citrate is oxidised to citric acid. However, there is an excess of citrate anions in the solution. The excess (non-oxidised) citrate anions physisorb to the surface of the Au(0) nanoparticles, stabilising the Au(0) nanoparticles, forming a Au colloid.

QUESTION:

Considering that an aqueous solution of hydrogen tetrachloroaurate(III) hydrate is yellow/orange, and an aqueous solution of sodium citrate dihydrate is colourless, something has obviously happened to the resulting solution, for it to be turn a deep red colour. At a specific wavelength (frequency) of light, collective oscillation of electrons on the Au nanoparticle surface cause a phenomenon called surface plasmon resonance, resulting in strong extinction of light (absorption and scattering). The particular wavelength, or frequency, of light where this occurs is strongly dependent on what factors?

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