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ANALYSIS OF METAL FINISHING BATHS
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2.1
Turbidimetry * or **
Also known as nephelometry (from the Greek, nephelos, cloudy). This technique uses similar optical equipment (see Figure) to that required for colorimetry. The underlying principle is based on the fact that rather than having a clear, homogeneous solution, it is used for the study of liquids which have a second phase in suspension. This second phase, which is substantially insoluble in the liquid (aqueous) phase, scatters the light. In most configurations, it is the intensity of this scattered light which is measured.
Turbidimetry is now a very widely-used technique in medicine, where it forms the basis of many important, though routine, tests. Metal Finishers are beneficiaries of this, in that equipment has become more widely available and at lower cost.
Among the systems studied by turbidimetry are: concentrations of gas or air bubbles in solution, oily or greasy emulsions, and suspended solid particles. The method can be used not only to determine the concentrations of these, but also particle size distribution. Outside of medicine, the most important analysis – and one also of use in Metal Finishing – is determination of sulphate concentration. The method has also been used for chloride analysis in plating baths.
Turbidity is measured in one of two ways. Either the same optical configuration is used as in colorimetry (Figure), where as before, the intensity of the emerging light beam is attenuated, but now by a "blocking" mechanism rather than adsorption. Or, another geometry is employed (Figure) in which the detector (II) is mounted at 90º to the incident light beam, in order to capture the scattered radiation. Strictly speaking, nephelometry is the term used to describe the former geometry, turbidimetry describes the latter. However the two terms are often used interchangeably.
Turbidimetry is thus seen to have much in common with colorimetry. However it suffers one drawback which, though it need not be serious, should be constantly borne in mind. Two-phase solutions, whether the second phase is a gas bubble, a second liquid insoluble in the first or a solid in suspension - all are metastable, which is to say that sooner or later, solids will sediment, gas bubbles rise to the surface, and liquids or emulsions will segregate. In contrast to a colorimetric sample, which will scarcely change over the course of a week, turbid solutions have a limited lifetime, that is to say that measurements and results are time-sensitive.
For further details of this technique, details of instrumentation including low-cost home-build instruments and specific methods for Metal Finishing, including analysis of sulphates or chlorides in process solutions, our Special Report is now published.