Measurement uncertainty assessment for potentiometric determination of trace anionic impurities in graphite post ultrasonic extraction: A bottom-up approach

An evaluation of measurement uncertainty in the analysis of trace level fluoride (F−) and chloride (Cl−) impurities in the refractory matrix (graphite) using a bottom-up approach based on the EURACHEM CITAC guide is presented herein. The major uncertainty contribution in the measurement was from calibration slope in case of F− and calibration intercept in case of Cl−. The expanded uncertainties calculated by considering the coverage factor (k= 2) was found to be 7.1 % for F− and 3.9 % for Cl−. The higher uncertainty in case of F− in comparison with Cl− could be attributed to the presence of F− in the limiting concentration range. The impact of calibration intercept in the measurement uncertainty for Cl− analysis can be attributed to the significant contribution of the blank towards the electrode potential. The studies demonstrated the effect of analyte concentration on the measurement uncertainty. The content of analytes along with the expanded uncertainty in the graphite sample were reported as 17.2 ± 1.2 mg/kg of  F− and 3300 ± 130 mg /kg of Cl− (calculated within 95% confidence level). The statistical concurrence of the results obtained from Ion Selective Electrode (ISE) and Ion chromatography (IC) analyses was confirmed by student’s t-test values.