Marine Chemistry, 4 (1976) 29--42 © Elsevier Scientific Publishing Company, Amsterdam--Printed in The Netherlands OPTIMISATION OF A CATALYTIC PROCEDURE FOR THE DETER- MINATION OF TOTAL IODINE IN SEAWATER VICTOR W. TRUESDALE and PIERS CHAPMAN Institute of Hydrology, Maclean Building, Wallingford (Great Britain) School of Environmental Sciences, University of East A nglia, Norwich (Great Britain) (Received February 17, 1975; revision accepted June 24, 1975) ABSTRACT Truesdale, V. W. and Chapman, P., 1976. Optimisation of a catalytic procedure for the determination of total iodine in seawater. Mar. Chem., 4: 29--42. A precise method for the determination of total iodine in seawater, which uses a Technicon Auto-Analyser II system and is based on Truesdale and Spencer's (1974) and Truesdale and Smith's (1975) earlier works with a catalytic procedure, is described. The procedure operates over a range of 30--80 pgl -l of iodine. As requested by Mark (1973), the way in which the procedure can be varied to suit other applications, and therefore ranges, is explained in detail. The standard deviation at the 50 #g1-1 level was found to be 0.12 pg1-1 showing that the method's precision exceeds most, if not all, previously published procedures for this variable. This high degree of resolution makes the procedure suitable for oceanic investigations. The effects of varying the reaction time, the temperature at which the reaction proceeds and the salinity of samples and standards are described. The advantages of using a Technicon Auto-Analyser II system rather than an Auto- Analyser I system are discussed. With slight modification the procedure can also be used in the measurement of river flow where sodium iodide is used as tracer in the dilution method. INTRODUCTION The total concentration of iodine in most oceanic waters seems to fall within the 40--60/~g1-1 range (Sugawara and Terada, 1957; Barkley and Thompson, 1960a; Tsunogai, 1971a; Liss et al., 1973). Further, at most of the hydrographic stations studied, the total concentration of iodine in the water column below 200 m does not appear to vary by more than a few/~g1-1. Existing methods for total-iodine determination are too imprecise to resolve these differences and it is likely, therefore, that these small differences merely reflect imprecision in the measurements made so far. While much is known about the distribution of total iodine in the oceans, little is known about the variations in the concentration of the constituent species. Although Sill~n (1961) calculated that a very large proportion of the iodine in seawater should exist as iodate, all the workers cited above have reported varying levels of iodide, especially in surface waters. While several methods for the precise