In this work, a procedure for total hardness determination in drinking and natural waters, expressed by the equivalent mass concentration of CaCO3, , using an in-house complexometric titration with ethylenediaminetetraacetic acid based on standardized procedure was detailed. Compliance of the procedures requirements with legislation is discussed. Validation and quality control actions were implemented, and a limit of detection of 0.6 mg L-1 was determined fulfilling the target limit of detection of 15 mg L-1 imposed by specific legislation. Measurement precision was estimated under repeatability conditions, where relative standard deviations close to 1 % were estimated, and in within-laboratory reproducibility, where relative standard uncertainties of 4 % and 0.5 % were obtained for lower and higher than 100 mg L-1, respectively. The results were considered unbiased whenever recoveries were within the range 0.90-1.10, fulfilling the target trueness value stated as 10 % of parametric value. The performance of the measurement procedure was also assessed by proficiency testing and z scores less than 2 were obtained. Measurement uncertainty was estimated by a modeling approach, using the law of propagation of uncertainty for variables not independent, and by empirical ones, such as single-laboratory validation and quality control data and proficiency testing. Measurement uncertainty computed from each model was assessed by xi score performance indicator showing a good estimated uncertainty (|xi| < 2). Finally, a way is proposed to assess the suitability of the combined uncertainty by the different models, comparing them to a target expanded measurement uncertainty within the analytical range, calculated from maximum decreed precision and trueness criteria since no target uncertainties were stated in the legislation.
