Candidate primary reference measurement procedure for the amount-of-substance content of a complexing agent expressed as EDTA based on the coulometric titration with electrogenerated Hg ions

Abstract

Abstract The primary reference measurement procedure is a method to realize the definition of the quantity without relation to a reference standard of the same quantity. This paper presents a candidate primary reference measurement procedure for the measurement of the amount-of-substance of a complexing agent expressed as ethylenediaminetetraacetic acid (EDTA) based on coulometric titration. Herein, mercury (Hg) was used as the anode because it does not undergo spontaneous oxidation. Furthermore, a 1:1 complex is formed between the electrogenerated Hg2+ ions and EDTA (in the form of Y4−). Although the electrochemical oxidation of Hg may generate Hg2 2+ ion, Hg2 2+ undergoes disproportionation in the presence of EDTA, thereby forming the desired Hg2+ species. The equivalence point occurs at the maximum of the derivative of the theoretical titration curve. As such, the measurement results were obtained by assuming that the maximum of the derivative of the experimental titration curve was the end point. The developed procedure was employed for the measurement of the amount-of-substance content of the complexing agent in EDTA reference material certified at the Slovak Institute of Metrology. The experimental measurements agreed well with the certified value for the reference material within the measurement uncertainty. The candidate procedure will be useful in certifying EDTA reference material and establishing international equivalence of amount content of the complexing agent.