As the strength of a standard solution may change upon standing, the molarity factor should be redetermined frequently. Volumetric solutions should not differ from the prescribed strength by more than 10% and the molarity should be determined with a precision of 0.2%. When solutions of a reagent are used in several molarities, the details of the preparation and standardization are usually given for the most commonly used strength. Stronger or weaker solutions are prepared and standardized using proportionate amounts of the reagent or by making an exact dilution of a stronger solution. Volumetric solutions prepared by dilution should be re-standardized either as directed for the stronger solution or by comparison with another volumetric solution having a known ratio to the stronger solution.
The water used in preparing volumetric solutions complies with the requirements of the monograph on Purified Water, unless otherwise specified. When used for the preparation of unstable solutions such as potassium permanganate or sodium thiosulphate, it should be freshly boiled and cooled. When a solution is to be used in an assay in which the endpoint is determined by an electrochemical process (e g. potentiometrically), the solution must be standardized in the same way.
Where it is directed that "any necessary correction" be made by a blank determination, the determination should be done using the same quantities of the same reagents treated in the same manner as the solution or mixture containing the portion of the substance under examination but omitting the substance under examination.
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