Optimised determinations of water in ethanol by encoded photometric near-infrared spectroscopy: a special case of sequential standard addition calibration.

A special limiting case of sequential standard addition calibration (S-SAC) has been applied to measurement of the water content of ethanol using encoded photometric near infrared spectroscopy. The method has shown good comparability with certified reference materials and to measurements made by Karl Fischer titration. The technique is quick and easy to use and should have application in high throughput and process measurement, for instance in biofuels analysis at port-of-entry or in bio-refineries. The characteristics of this limiting case of S-SAC have been fully described, and the corrections required to the value obtained by extrapolation to avoid bias have been calculated. The precision of the S-SAC procedure has been studied, and proposals have been made to optimise this with respect to the analytical precision. The technique should be applicable for the measurement of water in ethanol mass fractions of up to 0.1g g(-1) with an expanded uncertainty of less than 2% (relative).