Substoichiometry in Isotope Dilution Analysis:Automation of the Process with Reference to Mercury

Abstract

Substoichionetry is a radiochemical technique which greatly improves the sensitivity of trace metal analysis by solvent extraction procedures. The basis of the technique is the use of a smaller amount of reagent than the stoichiometric amount required by the metal. In this way the same amount of metal is always extracted. This is convenient in isotope dilution analysis because the activity of the organic phase is then proportional to the specific activity. There is no need to measure the amount of metal actually extracted and a reagent can be used at a low concentration which cannot be measured by conventional techniques. A method of determining mercury by substoichiometric solvent extraction, which has previously been described by Ruzicka and Stary (RL), has been automated and its limitations investigated and compared with those of the manual method (B5). During the course of this investigation chloride was found to interfere by forming a ternary complex, mercuric chloride dithizonate. The reactions and properties of this substance have been studied and its stability constant has been determined (B6). Several methods of overcoming this chloride interference have been examined. Whilst exploring one of these methods, the addition of ethylene-diamine-tetra-acetic acid, another ternary complex, mercuric chloride ethylene-diamine-tetra-acetate, was recognised and its stability constants have also been measured. Two papers have been published reporting this work (B5, B6) and a third is being prepared.