Turbidity metal content

A piece of pure metallic sodium about half the size of a pea is dropped into 0-5 c.c. of the oil in a dry test tube, and heated until all chemical action has ceased. The test tube and contents are immersed whilst still hot in 10 c.c. of distilled water in a porcelain dish. The solution is. filtered, acidulated with nitric acid, and silver nitrate solution added. Any turbidity or opalescence indicates the presence of chlorine compounds. 

This is the typical Italian grape-based pomace distillate. Produced and kept with a higher alcohol content, before sale it is diluted with water to a content of between 38% and 60% alcohol by volume (usually 42-43%) and filtered. As a consequence of water dilution, haze and turbidity of the product may occur. The elemental composition of grappa is related in particular to the stability of the product -mainly caused by Cu, Fe and related colour changes, and Ca and Mg precipitations with sulphates - and the presence of heavy metals eventually leached by distillation apparatus, tanks and pumps, wooden casks, filter pads and glass bottles. A survey of limpid and apparently stabile 58 grappa samples produced in northern Italy, some processed by very small distilleries, is shown in Table 8.10. The data show that the maximum values of several elements are clearly due to the cited leaching phenomena. 

The optical spectroscopy is commonly applied to such colloidal suspensions that obey the Rayleigh limit or the Rayleigh-Debye-Gans limit of scattering (cf. Appendix B.2). In this case the spectra usually have a smooth and monotone shape, from which only a few details of the size distribution can be deduced. Yet, for metals with a surface plasmon resonance in the optical domain (e.g. Ag or An), one observes a distinct, size dependent maximum in the turbidity spectra of nanoparticles (Fig. 2.17 cf. Njoki et al. 2007). The presence of such a maximum can clearly enhance the information content of the spectrum. 

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