ELSEVIER FoodChemistry50 (1994) 423-428 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0308-8146/94/$7.00 Analytical MethodsSection Flow injection titration of chloride in food products with a silver tubular electrode based on an homogeneous crystalline membrane Isabel M. P. L. V. O. Ferreira, Jos~ L. F. C. Lima* Departamento de Qulmica-Fisica, Faculdade de Farmdcia, Universidade do Porto, Rua Anlbal Cunha 164, 4000 Porto, Portugal & Ant6nio O. S. S. Rangel Escola Superior de Biotecnologia, Universidade Cat61ica Portuguesa, Rua Dr. Ant6nio Bernardino de AImeida, 4200 Porto, Portugal (Received 9 August 1993; accepted 8 November 1993) A flow injection system for the pseudo-titration of chlorides in food products with potentiometric detection has been developed. For this purpose, a silver ion selective tubular electrode based on a homogeneous crystalline membrane, pre- pared by pressing silver sulphide at high pressure, was constructed. Its opera- tional characteristics were evaluated and compared with the corresponding con- ventionally shaped electrodes obtained with the same sensor. The results obtained for chloride determination in wine, milk, beer and vine- gar by using the FIA methodology were in good agreement with those provided by reference procedures. The sampling rate achieved varied between 120 and 360 samples per hour. INTRODUCTION The determination of chloride in food products such as wine, milk, beer or vinegar is usually accomplished by potentiometric titration with silver (I) cations. The automation of these determinations by developing a flow injection analysis (FIA) pseudo-titration manifold is described in this paper. As a detection system for the flow titration, a silver ion selective electrode based on a homogeneous crystalline membrane with a tubular configuration was constructed. In the so-called pseudo-titration FIA systems (Stew- art, 1986), the sample is introduced into the titrant carrier stream, mixed in an exponential gradient device (typically a well-stirred mixing chamber), and the con- centration of one of the reactants, or the product moni- tored with a suitable detector. Under these conditions, there is a linear relationship between the time interval (peak width at a preset signal level) and the logarithm of analyte concentration. In the manifold described here for chloride deter- mination in food products, the sample is injected into a carrier stream containing silver cations, and the decrease in the silver concentration is monitored * To whom correspondence should be addressed. 423 by a silver tubular detector constructed in our laboratory. The utilization of tubular electrodes as detectors for the chloride titration in some matrices in a flow injec- tion system has already been described by some authors. Alegret et al. (1988) constructed a silver ion selective electrode based on a silver sulphide hetero- geneous crystalline membrane for the titration of chlo- ride in serum, and Lima and Rangel (1989) constructed a second kind of electrode of AgES/Ag for the determi- nation of chloride in wines. In this paper we describe the construction of a tubular silver (I) electrode based on a homogeneous crystalline membrane obtained by pressing silver sulphide powder at high pressure. As the quality of the tubular electrodes is strongly dependent upon the quality of the conventionally shaped elec- trodes, conventional units were also constructed and the operating characteristics compared. This type of electrode presents less dependence upon the matrix composition and a faster response over those using heterogeneous membranes (Alegret et al., 1987a; Alegret et al., 1989). Regarding the second kind of electrode, its potential is strongly affected by the sample matrix, so its use is rather limited, unless some previous matrix separation such as dialysis is per- formed before measurement.