This standard specifies a general method, known as the Karl Fischer method [1], for the determination of free water, or water of crystallization, in most solid or liquid chemicals, both organic and inorganic. In some cases, it is necessary to take certain precautions. These are specified, for each product, in the corresponding UNE standards. Two valuation methods are specified. In one of them, the end point is determined by observing a turn, or color change (visual method). In the other method, the end point is determined electrometrically (electrometric method). The visual method can only be used with colorless solutions, when an electrometric device is not available. A direct assessment must always be carried out. The electrometric method can be applied both by direct titration and by using a back titration. In all cases, this method is more precise than the visual one. For this reason it is the one that is recommended to use.