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          In this Pharmacopoeia, all concentrations of volumetric solutions are expressed in terms of molarity. The molarity of a solution is stated as the number of moles of substance contained in 1000 mL of the solution. A solution which contains x moles of substance per 1000 mL is designated as x molar (x M).

          Volume solution do not differ from the prescribed strength by more than 10 percent. The molarity of the volumetric solutions is determined by an appropriate number of titrations. The repeatability does not exceed 0.2 percent (relative standard deviation).

          Throughout the Pharmacopoeia, molar solutions to be standardized before use in assays and other quantitative tests are designated by appending the letters VS to the name of the reagent.

Preparation and Standardization

          For each solution the preparation and standardization of the most commonly used strength is described. Stronger or weaker solutions than those described are prepared and standardized using proportionate amounts of the reagents or by making an exact dilution of a stronger solution with carbon dioxide-free water. Solutions of molarity below 0.01 M are freshly prepared using carbon dioxide-free water.

          The water used in preparing volumetric solutions complies with the requirements of the monograph on Purified Water. When used for the preparation of unstable solutions such as potassium permanganate and sodium thiosulfate, it should be freshly boiled and cooled. When a solution is to be used in an assay in which the end-point is determined by an electrochemical process, the solution must be standardized in the same way. All volumetric solutions, if practicable, are to be prepared, standardized and used at the same temperature.

Blank Determinations

          Where it is directed that “any necessary correction” be made by a blank determination, the determination is to be conducted with the use of the same quantities of the same reagents treated in the same manner as the solution or mixture containing the portion of the substance under assay or test, but with the substance itself omitted. Appropriate blank corrections are to be made for all Pharmacopoeial titrimetric assays.

          All Pharmacopoeial assays that are volumetric in nature indicate the weight of the substance being assayed to which each ml of the primary volumetric solution is equivalent. In general, these equivalents may be derived by simple calculation from the respective molecular formulae and weights.

Hydrochloric Acid, Molar (1 M)

HCl = 36.46

          Dilute 85 mL of hydrochloric acid with water to produce 1000 mL.

Standardization: Weigh accurately about 1.5 g of anhydrous sodium carbonate that previously has been heated at about 270° for 1 hour. Dissolve it in 100 mL of water, and add 0.1 mL of methyl red TS. Titrate with the hydrochloric acid, until the solution becomes faintly pink. Heat the solution to boiling, cool, and continue the titration. Heat again to boiling, and titrate further as necessary until the faint pink colour is no longer affected by continued boiling. Each mL of 1 M hydrochloric acid is equivalent to 52.99 mg of Na₂CO₃.

Sodium Hydroxide, Molar (1 M)

NaOH = 40.00

          Dissolve 162 g of sodium hydroxide in 150 mL of carbon dioxide-free water, cool the solution to room temperature and filter through hardened filter paper. Dilute 54.5 mL of the clear filtrate with carbon dioxide-free water to 1000 mL.

Standardization: Weigh accurately about 5 g of potassium hydrogenphthalate, previously crushed lightly and dried at 120° for 2 hours, and dissolve in 75 mL of carbon dioxide-free water. Add 0.1 mL of phenolphthalein TS, and titrate with the sodium hydroxide solution to the production of a permanent pink colour. Each mL of 1 M sodium hydroxide is equivalent to 204.2 mg of C₈H₅KO₄.

Note  1. Solutions of alkali hydroxides absorb carbon dioxide when exposed to air. They should therefore be stored in bottles with suitable non-glass, well-fitting stoppers, provided with a tube filled with soda lime.

           2. Prepare solutions of lower concentration (e.g., 0.1 M, 0.01 M) by quantitatively diluting accurately measured volumesof the 1 M solution with sufficient carbon dioxide-free water to yield the desired concentration.

          Restandardize the solution frequently.

Sulfuric Acid, Half-Molar (0.5 M)

H₂SO₄ = 98.07

          Carefully add 28 mL of sulfuric acid to water, and dilute to 1000 mL with the same solvent.

Standardization: Standardize the solution as described under Hydrochloric Acid, Molar (1 M).