An easy uncertainty evaluation of the COD titrimetric analysis in correlation with quality control and validation data. Method applicability region

Abstract

Chemical Oxygen Demand (COD) estimates the amount of organic oxidizable matter in wastewaters. It is a critical parameter for assessing the quality of effluents prior to discharge, in the design and operation of wastewater treatment plants, and in environmental modelling and monitoring. In this study COD was determined by a closed reflux titrimetric method according to 5220C, APHA, 21^st edn, 2005. This method is applicable to COD values between 40 and 400 mg L⁻¹ and chloride concentrations up to 2000 mg L⁻¹. Chloride interference is a serious problem in COD measurement for wastewaters containing low organic matter concentrations. The reliability of the results depends on a proper validation of the measurement procedure that includes the calculation and evaluation of the measurement uncertainty. This study develops a practical, understandable and simple way for calculating and assessing the Combined Standard Uncertainty (CSU) of COD measurement, mainly based on already existing quality control and validation data. The CSU assessment is based on the existing literature trueness and precision target limits, without any additional assumptions. Diagrams correlating the trueness of the COD measurement mean values with different chloride concentrations and the laboratory control charts' limits offer a tool for improving the performance of the determination and define the method application area. The validation data that were obtained in our laboratory (accredited according to ISO 17025) and the proposed methodology for CSU calculation were verified in the frame of intra and inter-laboratory measurements.