BUFFER SOLUTIONSThe successful completion of many Pharmacopeial tests and assaysrequires adjustment to or maintenance of a specified pH by the additionof buffer solutions. In pH measurements, standard buffer solutions arerequired for reference purposes. For convenience, the preparation ofthese solutions is in some instances described in the sections in whichtheir use is specified; i.e., five separate phosphate buffers aredescribed under Antibiotics -- Microbial Assays <81>, and severalmiscellaneous single-purpose solutions are described in the individualmonographs.A solution is said to be buffered if it resists changes in theactivity of an ion on the addition of substances that are expected tochange the activity of that ion. Buffers are substances or combinationsof substances that impart this resistance to a solution. Bufferedsolutions are systems in which the ion is in equilibrium with substancescapable of removing or releasing the ion.Buffer capacity refers to the amount of material that may be added toa solution without causing a significant change in ion activity. It isdefined as the ratio of acid or base added (in gram-equivalents perliter) to the change in pH (in pH units). The capacity of a bufferedsolution is adjusted to the conditions of use, usually by adjustment ofthe concentrations of buffer substances.Buffers are used to establish and maintain an ion activity withinnarrow limits. The most common systems are used
to establishhydrogen-ion activity for the calibration of pH
meters, in thepreparation of dosage forms that approach isotonicity, in
analyticalprocedures, and to maintain stability of various dosage forms.Buffers
used in physiological systems are carefully chosen so as not tointerfere with pharmacological activity of the medicament or normalfunction of the organism. It is essential that buffers used in chemicalanalysis be compatible with the substance
determined and the reagentsused.STANDARD BUFFER SOLUTIONS -- Standard solutions of definite pH arereadily available in buffer solutions prepared from the appropriatereagents. In addition, buffer solutions, buffer tablets, and buffersolids may be obtained from commercial sources in convenient prepackagedform. Such preparations are available for the entire working range inpharmaceutical analysis, but are not recommended for pH meterstandardization (see pH <791>.The required reagents are described in the section, Reagents.Previously dry the crystalline reagents, except the boric acid, at 110degrees to 120 degrees for 1 hour.NOTE -- Where water is specified for solution or dilution of testsubstances in pH determinations, use carbon dioxide-free water.Store the prepared solutions in chemically resistant, tight containerssuch as Type I glass bottles. Use the solutions within 3 months.Standard buffer solutions for various ranges between pH 1.2 and 10.0may be prepared by appropriate combinations of 0.2 M solutions describedherein, used in the proportions shown in the accompanying table. Thevolumes shown in the table are for 200 mL of buffer solution.1. Hydrochloric Acid, 0.2 M, and Sodium Hydroxide, 0.2 M -- Prepareand standardize as directed under Volumetric Solutions.2. Potassium Biphthalate, 0.2 M -- Dissolve 40.85 g of potassiumbiphthalate [KHC[6]H[4](COO)[2]] in water, and dilute with water to 1000mL.3. Potassium Phosphate, Monobasic 0.2 M -- Dissolve 27.22 g ofmonobasic potassium phosphate (KH[2]PO[4]) in water, and dilute withwater to 1000 mL.4. Boric Acid and Potassium Chloride, 0.2 M -- Dissolve 12.37 g ofboric acid (H[3]BO[3]) and 14.91 g of potassium chloride (KCl) in water,and dilute with water to 1000 mL.5. Potassium Chloride, 0.2 M -- Dissolve 14.91 g
