This document specifies representative materials suitable for the determination of the performance characteristics, including uncertainty, during the initial assessment of a quantitative method, used in a laboratory, for physico-chemical water analysis. This document focuses on five main types of water: — waters intended for consumption (5.2); — natural waters (5.3); — waste waters (5.4); — marine waters (5.5); — recreational waters (5.6). NOTE – Other more specific or less common types of water can be incorporated in any of the above types provided appropriate justifications. The characteristics of the standard matrix are compatible with the characteristics of the samples handled.