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5.13 ACID-NEUTRALIZING CAPACITY

(Note All tests shall be conducted at a temperature of 37º±3º.)

Standardization of pH Meter

 ​           Standardize a pH meter using Potassium Hydrogenphthalate Standard Buffer Solution and Potassium Tetraoxalate Standard Buffer Solution as described under the “Determination of pH” (Appendix 4.11).

Magnetic Stirrer

 ​           Transfer 100 ml of water to a 250-ml beaker containing a 40- × 10-mm (or other suitable size) magnetic stirring bar that is coated with solid perfluorocarbon and has a spin ring at its centre. Adjust the power setting of the magnetic stirrer to produce a stirring rate of 300±30 rpm when the stirring bar is centred in the beaker, as determined by a suitable optical tachometer. 

Test Preparation

 ​           Powders Transfer the accurately weighed portion of the substance specified in the individual monograph to a 250-ml beaker, add 70 ml of water and mix on the magnetic stirrer for 1 minute.

 ​           Effervescent solids Transfer an accurately weighed quantity, equivalent to the minimum labelled dosage, to a 250-ml beaker, add 10 ml of water, and swirl the beaker gently while allowing the reaction to subside. Add another 10 ml of water, and swirl gently. Wash the walls of the beaker with 50 ml of water, and mix on the magnetic stirrer for 1 minute.

 ​           Suspensions and other liquids Shake the container until the contents are uniform, and determine the weight per millilitre. Transfer an accurately weighed quantity of the uniform mixture, equivalent to the minimum labelled dosage, to a 250-ml beaker, add water to make a total volume of about 70 ml, and mix on the magnetic stirrer for 1 minute.

 ​           Non-chewable tablets Weigh not less than 2 tablets, and determine the average tablet weight. Grind the tablets to a fine powder, mix to obtain a uniform mixture, and transfer an accurately weighed quantity of it, equivalent to the minimum labelled dosage, to a 250-ml beaker. If wetting is desired, add not more than 5 ml of ethanol (neutralized to an apparent pH of 3.5), and mix to wet the sample thoroughly. Add 70 ml of water, and mix on the magnetic stirrer for 1 minute.

 ​           Chewable tablets Prepare as directed for Nonchewable tablets.

 ​           Tablets that are required to be chewed Transfer 1 tablet to a 250-ml beaker, add 50 ml of water, and mix on the magnetic stirrer for 1 minute.

 ​           Capsules Weigh accurately not less than 20 capsules. Remove the capsule contents completely, with the aid of a cotton swab if necessary. Accurately weigh the empty capsules, and determine the average weight of the contents per capsule. Mix the combined capsule contents to obtain a uniform mixture, and proceed as directed for Non-chewable tablets, beginning with “transfer an accurately weighed quantity of it, ...”.

Procedure

 ​           Powders, effervescent solids, suspensions and other liquids, non-chewable tablets, chewable tablets, and capsules Pipette 30 ml of 1 M hydrochloric acid VS into the test preparation while continuing to stir with the magnetic stirrer. (Note Where the acid-neutralizing capacity of the sample is greater than 25 mEq, use 60.0 ml of 1 M hydrochloric acid VS, and make the appropriate modifications in the calculation.) Stir for 15 minutes, accurately timed, after the addition of the acid, begin to titrate immediately, and in a period not to exceed an additional 5 minutes, titrate the excess hydrochloric acid with 0.5 M sodium hydroxide VS to attain a stable (for 10 to 15 seconds) pH of 3.5. Calculate the number of mEq of acid consumed by the sample taken by the formula:

(V_HCl × M_HCl) – (V_NaOH × M_NaOH),

in which M_HCl and M_NaOH are the molarities of the hydrochloric acid VS and the sodium hydroxide VS, respectively; and V_HCl is the volume of the hydrochloric acid VS added into the test preparation and V_NaOH is the volume of the sodium hydroxide VS used for titration. Express the result in terms of mEq of acid consumed.

 ​           Tablets that are required to be chewed Pipette 30 ml of 1 M hydrochloric acid VS into the test preparation while continuing to stir with the magnetic stirrer for 10 minutes, accurately timed, after the addition of the acid. Discontinue stirring briefly, and without delay remove any gum base from the beaker using a long needle. Promptly rinse the needle with 20 ml of water, collecting the washing in the beaker, and resume stirring for 5 minutes, accurately timed, then begin to titrate immediately, and in a period not to exceed an additional 5 minutes, titrate the excess hydrochloric acid with 0.5 M sodium hydroxide VS to attain a stable (for 10 to 15 seconds) pH of 3.5. Calculate the number of mEq of acid consumed by the Tablet taken by the formula:

(30 × M_HCl) – (V_NaOH × M_NaOH),

in which the terms are as defined above.