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Two methods of titration depending on whether the end point is detected visually or electrometrically are specified. The visual method is only applicable to colourless solutions but can be used when no electrometric apparatus is available. The electrometric method, whether by direct titration or back-titration, is the more accurate one and for this reason recommended. The principle of the test consists in reaction of any water present in a test portion with a solution of iodine and sulphur dioxide in a pyridine/methanol mixture (Karl Fischer reagent), previously standardized by titration with an exactly known mass of water.