The Use of a Custom-Built Coulometric Karl Fischer Instrument for the Determination of Water Content in Chocolate
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The Use of a Custom-Built Coulometric Karl Fischer Instrument for the Determination of Water Content in Chocolate Carrie M. Clippard & Joseph C. Nichisti & Rodney A. Kreuter & Balwant S. Chohan & Danny G. Sykes
Received: 14 March 2014 / Accepted: 28 July 2014 # Springer Science+Business Media New York 2014
Abstract A Karl Fischer (KF) coulometric titration method was designed and optimized for the quantitative determination of water in chocolate. A number of essential KF experimental parameters were investigated and assessed, including a study of polar extraction solvents, extraction times, interfering side reactions, and a comparison of pyridine-free KF coulometric reagents. The optimized protocol was found to be applicable to a wide variety of chocolate samples. Typical coulometric titration times for the chocolate samples were between 1 and 5 min, translating to water contents from 0.18 to 0.38 %w/w. With this contribution, the quantification of water in chocolate using KF titration methods is now a simple, safe, and reliable process, comparable in terms of precision and accuracy to any commercially available KF coulometric apparatus. Keywords Food analysis . Moisture content . Water content . Electrochemistry . Karl Fischer . Coulometry . Coulometric titration
Introduction Karl Fischer (KF) titration is a quantitative method for determining water (Fisher 1935; Scholz 2011) in a large variety of substances over a wide concentration range, including Electronic supplementary material The online version of this article (doi:10.1007/s12161-014-9949-9) contains supplementary material, which is available to authorized users. C. M. Clippard : J. C. Nichisti : R. A. Kreuter : D. G. Sykes (*) Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA e-mail: [email protected] B. S. Chohan Math & Natural Sciences Department, Centenary College, Hackettstown, NJ 07840, USA
laboratory solvents (Dantan et al. 2000), pharmaceuticals (Cachet and Hoogmartens 1988), lubricating oils, cosmetics, and food products (Basantia and Rai 1999; Gallina et al. 2010; Isengard 2006; Lanz et al. 2006; McClure and Steffen 2005; Sanchez et al. 2010; Sloman et al. 1954; Supartono et al. 1998). Unlike gravimetric measurements, that assume all volatiles removed by the process as being water, the KF titration is a direct method that is specific for liquid water. The total moisture present in foodstuffs, particularly dry foodstuffs and powders consists of free moisture as well as crystal bounded water, and thus the accuracy of the method is very much dependent on an effective extraction procedure. The KF method is especially useful for low moisture levels (5 %) of water (Grünke 2001; Grünke and Wünsch 2000; Verhoef and Barendrecht 1976; Verhoef and Barendrecht 1977). In alcoholic media, the alcohol and SO2 initially form an alkylsulfite species, which is then oxidized by iodine (or I3−) to an alkylsulfate in the presence of water (Cedergren 1996; Grünke 2001; Scholz 2011; Wunsch and Seubert 1989). Under these conditions,
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