Ion chromatography provides a simple, fast, small sample volume demanding, and fit-for-purpose methodology for the concurrent determination of 10 ions (F- Cl-, Br-, NO3-, PO43-, SO42-, Na+, K+, Ca2+ and Mg2+) in surface, ground and potable water samples. Current law legislations, as well as the need to constantly provide accurate and reliable results, enforce public sector laboratories to practice quality management system, according to the EN ISO 17025 standard. In this paper, the work that was undertaken for the accreditation of the above determinations at the chemical laboratory of EYDAP SA, is presented. The laboratory has developed two in-house methods, which are carried out simultaneously by a dual chromatographic system. The method for the analysis of anions is based on the 4110B APHA standard method and the one for the cations based on the ISO14911. Deviations from the standard procedure made a full method validation imperative, therefore, for all 10 ions, method characteristics were determined and assessed towards the fitness-for-purpose scope (range of measurement, calibration, method detection level, level of quantification, repeatability, reproducibility, precision, accuracy, peak resolution). Additionally, measurement uncertainty was determined and procedures for daily measurement quality control, assessment of individual analyst capability, measurement, sample and reagent traceability, and regular participation in proficiency testing schemes were implemented. Methodology and results for method validation, uncertainty quantification and quality control is presented in detail in the full paper.