Constant-current coulometry and ion chromatography bromide determination to characterize the purity of the potassium chloride

Abstract

A coulometria é considerada o método de maior qualidade metrológica para medição de quantidade de substância de padrões primários. Entretanto, a contaminação de sais de elevada pureza está presente em níveis traços sendo necessária a sua identificação e quantificação. Este trabalho apresenta os estudos desenvolvidos pelo Inmetro (Instituto Nacional de Metrologia, Normalização e Qualidade Industrial, Brasil) para determinar a pureza do cloreto de potássio (KCl) por titulação coulométrica a corrente constante, além da determinação do brometo como seu principal contaminante por cromatografia iônica (IC). Esses estudos foram necessários para desenvolver material de referência certificado (MRC) de KCl para garantir a qualidade dos resultados das medições químicas principalmente as de titulometria. A caracterização do candidato a MRC de KCl apresentou pureza de 99,9662%, a qual foi obtida após a correção do brometo, em fração mássica, obtido por IC, de 182,47 ± 3,03 mg kg -1 (incerteza expandida, k = 2, para um nível de confiança de aproximadamente de 95%).Coulometry is considered the highest metrological method for measuring the amount of substance of high-purity compounds. However, the contamination of the high purity salts is present in trace levels and its identification and quantification are necessary. This work aims to present the studies performed by Inmetro (National Institute of Metrology, Standardization and Industrial Quality, Brazil) to determine the purity of potassium chloride (KCl) by constant-current coulometric titration, as well as the determination of bromide as the main impurity in the KCl through ion chromatography (IC). These studies were needed to develop certified reference material (CRM) of KCl to guarantee the quality of the chemical measurements, mainly those performed by titrimetric analysis. The characterization of the candidate CRM of KCl by coulometric titration presented the purity of 99.9662%. This value was obtained after bromide correction of 182.47 ± 3.03 mg kg -1 (expanded uncertainty for k = 2, for a confidence level of approximately 95%) for the mass fraction bromide assayed by IC.

Metrological evaluation of the certification of primary reference materials characterized by high-precision constant-current coulometry for the reliability of the titration analyses

“…The procedures for characterizing the candidate CRM of KCl were previously described [6]. The electrochemical reactions for the coulometric titration of KCl were similar to the reactions (2) and (3) presented before; however, it is worth emphasizing that in this case, the AgCl was precipitated on a high-purity silver electrode without the presence of light since the AgCl could undergo degradation, interfering in the coulometric results [25,33].…”

“…As a consequence, chemical laboratories require suitable certified reference materials (CRMs) for calibration [2,3], method development [4], validation [5], and quality control [6,7]. In volumetric analyses, which are among the most widely used analytical techniques for determining the concentration of solutions [8][9][10][11], primary standards or CRMs are needed to standardize the reagent solution-the titrant-for stoichiometrically reacting with the analyte in an unknown solution-the solution being titrated-to accurately determine the analyte's concentration.…”

“…The basic requirement is 100 % current efficiency of the titrant generation at the working electrode caused by a well-defined electrode reaction. In the National Metrology Institutes (NMIs), the main type of coulometry is high-precision, constant-current coulometry that was first developed by Taylor and Smith [27] as an analytical technique for certifying primary reference materials [6,[24][25][26][27][28][29][30][31].…”

A Simple and Accurate Measurement Procedure for Serum Chloride by Ion Chromatography and Bias Evaluation of Six Ion Selective Electrode Measurements in China

“…For coulometric titration, a silver rod of high purity (99.999%) was required to en-sure high current efficiency [3] and the exposure of electrolytic product (AgCl) would cause errors because of photodecomposition [4]. In another study of purity valuation of KCl by constant-current coulometric titration, bromide was titrated the same as chloride, and the mass fraction of chloride was over evaluated [5]. In Germany, inductively coupled plasma (isotope dilution) sector field mass spectrometry, ICP-(ID) SFMS, was established as a candidate reference measurement procedure for serum chloride [6].…”