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1.2 VOLUMETRIC SOLUTIONS

          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).

          Volumetric solutions do not differ from the prescribed strength by more than 10 per cent. The molarity of the volumetric solutions is determined by an appropriate number of titrations. The repeatability does not exceed 0.2 per cent (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.

Disodium Edetate, Twentieth-Molar (0.05 M)

C₁₀H₁₄N₂Na₂O₈.2H₂O = 372.24

Dissolve 18.6 g of disodium edetate in sufficient water to produce 1000 ml.

Standardization: Weigh accurately about 200 mg of chelometric standard calcium carbonate, previously dried at 110º for 2 hours and cooled in a desiccator, transfer to a 400-ml beaker, add 10 ml of water, and swirl to form a slurry. Cover the beaker with a watch glass, and introduce 2 ml of dilute hydrochloric acid from a pipette inserted between the lip of the beaker and the edge of the watch glass. Swirl the beaker to dissolve the calcium carbonate. Wash down the sides of the beaker, the outer surface of the pipette, and the watch glass with water, and dilute with water to about 100 ml. While stirring the solution, preferably with a magnetic stirrer, add about 30 ml of the disodium edetate solution from a 50-ml burette. Add 15 ml of sodium hydroxide TS and 300 mg of hydroxy naphthol blue mixture; 100 mg of calcon mixture or calconcarboxylic acid mixture may be used in place of hydroxy naphthol blue mixture. Continue the titration with the disodium edetate solution to a blue end-point. Each ml of 0.05 M disodium edetate is equivalent to 5.004 mg of CaCO₃.

Hydrochloric Acid, Molar (1 M)

HCl = 36.46

Dilute 85 ml of hydrochloric acid in sufficient water to produce 1000 ml.

Standardization: Weigh accurately about 1.5 g of anhydrous sodium carbonate that previously has been heated at about 270º for 1 hour. Dissolve it in 100 ml of water and add 0.1 ml of methyl red TS. Add the acid slowly from a burette, with constant stirring 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.

Iodine, Twentieth-Molar (0.05 M)

I₂ = 253.81

Dissolve 14 g of iodine in a solution of 36 g of potassium iodide in 100 ml of water, and add 0.15 ml of hydrochloric acid. Dilute slowly, with continuous stirring, to 1000 ml with water.

Standardization: Weigh accurately about 150 mg of arsenic trioxide, previously dried at 105º for 1 hour, and dissolve in 20 ml of 1 M sodium hydroxide by warming if necessary. Dilute with 40 ml of water, add 0.1 ml of methyl orange TS, and follow with dilute hydrochloric acid until the yellow colour is changed to pink. Then add 2 g of sodium hydrogencarbonate, dilute with 50 ml of water, and add 3 ml of starch TS. Slowly titrate with the iodine solution until a permanent blue colour is produced. Each ml of 0.05 M iodine is equivalent to 4.946 mg of As₂O₃.

Preserve in amber-coloured, glass-stoppered bottles.

Lead Nitrate, Twentieth-Molar (0.05 M)

Pb(NO₃)₂ = 331.21

Dissolve 16.6 g of lead(II) nitrate in sufficient water to produce 1000 ml.

Standardization: Dilute 25.0 ml of the lead nitrate solution with 200 ml of water, add 10 ml of ammonia buffer pH 10.9 and about 20 mg of mordant black 11 mixture. Titrate with 0.05 M disodium edetate VS. Calculate the molarity.

Perchloric Acid, Tenth-Molar (0.1 M)

HClO₄ = 100.46

Caution The perchloric acid must be well diluted with acetic acid before adding the acetic anhydride. Failure to observe this precaution leads to formation of the explosive acetylperchlorate.

           Mix thoroughly 8.5 ml of perchloric acid with 500 ml of anhydrous glacial acetic and then add 21 ml of acetic anhydride; cool, and add anhydrous glacial acetic acid to make 1000 ml. Allow to stand for 24 hours.

          Determine the water content by the Karl Fischer Method, Appendix 4.12. If necessary, add sufficient water or acetic anhydride to adjust the water content to between 0.02 per cent and 0.05 per cent, and allow to stand for a further 24 hours.

Standardization: Weigh accurately about 700 mg of potassium hydrogenphthalate previously crushed lightly and dried at 120º for 2 hours. Dissolve in 50 ml of anhydrous glacial acetic acid in a 250-ml flask. Add 0.1 ml of crystal violet TS, and titrate with the perchloric acid solution until the violet colour changes to blue-green. Deduct the volume of the perchloric acid consumed by 50 ml of the anhydrous glacial acetic acid. Each ml of 0.1 M perchloric acid is equivalent to 20.42 mg of C₈H₅KO₄.

Potassium Hydroxide, Molar (1 M)

KOH = 56.11

          Dissolve 68 g of potassium hydroxide in sufficient carbon dioxide-free water to produce 1000 ml.

Standardization: Titrate 20.0 ml of the solution with 1 M hydrochloric acid VS using 0.5 ml of phenolphthalein TS as indicator. Perform a blank determination, and make any necessary correction. Calculate the molarity.

Potassium Hydroxide, Tenth-Molar (0.1 M)

          Dissolve 6.8 g of potassium hydroxide in sufficient carbon dioxide-free water to produce 1000 ml.

Standardization: Titrate 20.0 ml of the solution with 0.1 M hydrochloric acid VS using 0.5 ml of phenolphthalein TS as indicator. Perform a blank determination, and make any necessary correction. Calculate the molarity.

Potassium Hydroxide, Ethanolic, Half-Molar (0.5 M)

          Dissolve 34 g of potassium hydroxide in 20 ml of water, and dilute with aldehyde-free ethanol to produce 1000 ml. Allow the solution to stand in a tightly stoppered bottle for 24 hours, and then quickly decant the clear supernatant liquid into a suitable, tight container.

Standardization: Dilute 25.0 ml of 0.5 M hydrochloric acid VS with 50 ml of water, add 0.1 ml of phenolphthalein TS and titrate with the ethanolic potassium hydroxide solution until a permanent, pale pink colour is produced. Perform a blank determination, and make any necessary correction. Calculate the molarity.

Store protected from light.

Potassium Permanganate, Fiftieth-Molar (0.02 M)

KMnO₄ = 158.03

          Dissolve about 3.3 g of potassium permanganate in 1000 ml of water in a flask, and boil the solution for about 15 minutes. Insert the stopper in the flask, allow it to stand for at least 2 days, and filter through a fineporosity, sintered-glass crucible. If necessary, the bottom of the sintered-glass crucible may be lined with a pledget of glass wool. Standardize the solution as follows.

Standardization: Weigh accurately about 200 mg of sodium oxalate, previously dried at 110º to constant weight, and dissolve it in 250 ml of water. Add 7 ml of sulfuric acid, heat to about 70º, and then slowly add the permanganate solution from a burette, with constant stirring, until a pale pink colour, which persists for 15 seconds, is produced. The temperature at the conclusion of the titration should be not less than 60º. Perform a blank determination, and make any necessary correction. Each ml of 0.02 M potassium permanganate is equivalent to 6.700 mg of C₂Na₂O₄.

          Note Since potassium permanganate is reduced on contact with organic substances such as rubber, the solution must be handled in apparatus entirely of glass or other suitably inert material. It should be frequently restandardized.

Store in glass-stoppered, amber-coloured bottles.

Silver Nitrate, Tenth-Molar (0.1 M)

AgNO₃ = 169.87

          Dissolve 17.5 g of silver nitrate in sufficient water to produce 1000 ml.

Standardization: Transfer about 100 mg, accurately weighed, of sodium chloride, previously dried at 110º for 2 hours, to a 150-ml beaker. Dissolve in 5 ml of water and add 5 ml of acetic acid, 50 ml of methanol and 0.15 ml of eosin Y TS. Stir, preferably with a magnetic stirrer, and titrate with the silver nitrate solution until a purplish red colour appears. Each ml of 0.1 M silver nitrate is equivalent to 5.844 mg of NaCl.

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 120º 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.

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

Restandardize the solution frequently.

Sodium Hydroxide, Ethanolic, Tenth-Molar (0.1 M)

          Dissolve 4.2 g of sodium hydroxide in 10 ml of water, and dilute with aldehyde-free ethanol to produce 1000 ml. Allow the solution to stand in a tightly stoppered bottle for 24 hours, and then quickly decant the clear supernatant liquid into a suitable, tight container.

Standardization: Weigh accurately about 200 mg of benzoic acid, previously dried, and dissolve in a mixture of 10 ml of ethanol and 2 ml of water and titrate with the ethanolic sodium hydroxide solution, using phenolphthalein TS as indicator. Perform a blank determination, and make any necessary correction. Each ml of 0.1 M ethanolic sodium hydroxide is equivalent to 12.21 mg of C₇H₆O₂.

Sodium Thiosulfate, Tenth-Molar (0.1 M)

Na₂S₂O₃.5H₂O = 248.17

          Dissolve 25 g of sodium thiosulfate and 200 mg of sodium carbonate in sufficient recently boiled and cooled water to produce 1000 ml.

Standardization: Weigh accurately about 210 mg of potassium dichromate, previously pulverized and dried at 120º for 4 hours, and dissolve in 100 ml of water in a 500-ml iodine flask. Swirl to dissolve the solid, remove the stopper, and quickly add 3 g of potassium iodide, 2 g of sodium hydrogencarbonate and 5 ml of hydrochloric acid. Insert the stopper in the flask, swirl to mix, and allow to stand in the dark for 10 minutes. Rinse the stopper and the inner walls of the flask with water. Titrate the liberated iodine with the sodium thiosulfate solution until the solution is yellowish green in colour. Add 3 ml of starch TS, and continue the titration to the discharge of the blue colour. Each ml of 0.1 M sodium thiosulfate is equivalent to 4.903 mg of K₂Cr₂O₇.

Restandardize the solution frequently.

Sulfuric Acid, Half-Molar (0.5 M) 

           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).

Sulfuric Acid, Twentieth-Molar (0.05 M)

           Dilute 100 ml of 0.5 M sulfuric acid VS to 1000 ml with water.

Standardization: Standardize the solution as described under Hydrochloric Acid, Molar (1 M) using about 100 mg of anhydrous sodium carbonate, accurately weighed and dissolved in 20 ml of water.

Tetrabutylammonium Hydroxide, Tenth-Molar (0.1 M)

(C₄H₉)4NOH = 259.48

          Dissolve 40 g of tetrabutylammonium iodide in 90 ml of anhydrous methanol in a glass-stoppered flask. Place in an ice-bath, add 20 g of powdered silver oxide, insert the stopper in the flask, and shake vigorously for 60 minutes. Centrifuge a few ml, and test the supernatant liquid for iodide (Appendix 5.1). If the test is positive, add an additional 2 g of silver oxide, and continue to allow to stand for 30 minutes within termittent shaking. When all of the iodide has reacted, filter through a fine-porosity, sintered-glass filter. Rinse the flask and the filter with three 50-ml portions of anhydrous toluene, adding the rinsings to the filtrate. Dilute with a mixture of 3 volumes of anhydrous toluene and 1 volume of anhydrous methanol to 1000 ml, and flush the solution for 10 minutes with dry carbon dioxide-free nitrogen.

Store in a reservoir protected from carbon dioxide and moisture and discard after 60 days.

          Alternatively, the solution may be prepared by diluting a suitable volume of commercially available tetrabutylammonium hydroxide solution in methanol with a mixture of 4 volumes of anhydrous toluene and 1 volume of anhydrous methanol.

Standardization: Weigh accurately about 400 mg of benzoic acid, previously dried, and dissolve in 80 ml of dimethylformamide. Add 0.15 ml of a 1 per cent w/v solution of thymol blue in dimethylformamide, and titrate to a blue end-point with the tetrabutylammonium hydroxide solution, delivering the titrant from a burette equipped with a carbon dioxide absorption trap. Perform a blank determination, and make any necessary correction. Each ml of 0.1 M tetrabutylammonium hydroxide is equivalent to 12.21 mg of C₇H₆O₂.

Note 1. If necessary to obtain a clear solution, further small quantities of anhydrous methanol may be added. 

         2. Tenth-Molar Tetrabutylammonium Hydroxide VS must be standardized immediately before use.

Zinc Sulfate, Twentieth-Molar (0.05 M)

ZnSO₄.7H₂O = 287.54

          Dissolve 14.4 g of zinc sulfate in sufficient water to produce 1000 ml.

Standardization: To 10.0 ml of 0.05 M disodium edetate VS, add, in the order given, 10 ml of acetic acid-ammonium acetate buffer TS, 50 ml of ethanol, and 2 ml of dithizone TS. Titrate with the zinc sulfate solution to a clear, rose-pink colour. Calculate the molarity.