BUFFER SOLUTIONS
The successful completion of many Pharmacopeial tests and assays requires adjustment to or maintenance of a specified pHby the addition of buffer solutions.In pHmeasurements,standard buffer solutions are required for reference purposes.For convenience,the preparation of these solutions is in some instances described in the sections in which their use is specified;i.e.,five separate phosphate buffers are described under Antibiotics—Microbial Assays á81ñ,and several miscellaneous single-purpose solutions are described in the individual monographs.
Asolution is said to be buffered if it resists changes in the activity of an ion on the addition of substances that are expected to change the activity of that ion.Buffers are substances or combinations of substances that impart this resistance to a solution.Buffered solutions are systems in which the ion is in equilibrium with substances capable of removing or releasing the ion.
Buffer capacity refers to the amount of material that may be added to a solution without causing a significant change in ion activity.It is defined as the ratio of acid or base added (in gram-equivalents per liter)to the change in pH(in pHunits).The capacity of a buffered solution is adjusted to the conditions of use,usually by adjustment of the concentrations of buffer substances.
Buffers are used to establish and maintain an ion activity within narrow limits.The most common systems are used (a)to establish hydrogen-ion activity for the calibration of pHmeters,(b)in the preparation of dosage forms that approach isotonicity,(c)in analytical procedures,and (d)to maintain stability of various dosage forms.Buffers used in physiological systems are carefully chosen so as not to interfere with pharmacological activity of the medicament or normal function of the organism.It is essential that buffers used in chemical analysis be compatible with the substance determined and the reagents used.
Standard Buffer Solutions
—Standard solutions of definite pHare readily available in buffer solutions prepared from the appropriate reagents.In addition,buffer solutions,buffer tablets,and buffer solids may be obtained from commercial sources in convenient prepackaged form.Such preparations are available for the entire working range in pharmaceutical analysis,but are not recommended for pHmeter standardization (see pHá791ñ).
The required reagents are described in the section,Reagents.Previously dry the crystalline reagents,except the boric acid,at 110
to 120for 1hour.
to 120for 1hour.
[NOTE—Where water is specified for solution or dilution of test substances in pHdeterminations,use carbon dioxide-free water.]
Store the prepared solutions in chemically resistant,tight containers such as Type Iglass bottles.Use the solutions within 3months.
Standard Buffer Solutionsfor various ranges between pH1.2and 10.0may be prepared by appropriate combinations of the solutions described herein,used in the proportions shown in the accompanying table.The volumes shown in the table are for 200mLof buffer solution,except that the volumes shown for Acetate Bufferare used to prepare 1000mLof buffer solution.
- Hydrochloric Acid,0.2M,and Sodium Hydroxide,0.2M—Prepare and standardize as directed under Volumetric Solutions.
- Potassium Biphthalate,0.2M—Dissolve 40.85g of potassium biphthalate [KHC6H4(COO)2]in water,and dilute with water to 1000mL.
- Potassium Phosphate,Monobasic 0.2M—Dissolve 27.22g of monobasic potassium phosphate (KH2PO4)in water,and dilute with water to 1000mL.
- Boric Acid and Potassium Chloride,0.2M—Dissolve 12.37g of boric acid (H3BO3)and 14.91g of potassium chloride (KCl)in water,and dilute with water to 1000mL.
- Potassium Chloride,0.2M—Dissolve 14.91g of potassium chloride (KCl)in water,and dilute with water to 1000mL.
- Acetic Acid,2N—Prepare and standardize as directed under Volumetric Solutions.
| Composition of Standard Buffer Solutions | |||||||||||
| Hydrochloric Acid Buffer | |||||||||||
| Place 50mLof the potassium chloride solution in a 200-mLvolumetric flask,add the specified volume of the hydrochloric acid solution,then add water to volume. | |||||||||||
| pH | 1.2 | 1.3 | 1.4 | 1.5 | 1.6 | 1.7 | 1.8 | 1.9 | 2.0 | 2.1 | 2.2 |
| 0.2M HCl,mL | 85.0 | 67.2 | 53.2 | 41.4 | 32.4 | 26.0 | 20.4 | 16.2 | 13.0 | 10.2 | 7.8 |
| Acid Phthalate Buffer | ||||||||||
| Place 50mLof the potassium biphthalate solution in a 200-mLvolumetric flask,add the specified volume of the hydrochloric acid solution,then add water to volume. | ||||||||||
| pH | 2.2 | 2.4 | 2.6 | 2.8 | 3.0 | 3.2 | 3.4 | 3.6 | 3.8 | 4.0 |
| 0.2M HCl,mL | 49.5 | 42.2 | 35.4 | 28.9 | 22.3 | 15.7 | 10.4 | 6.3 | 2.9 | 0.1 |
| Neutralized Phthalate Buffer | |||||||||
| Place 50mLof the potassium biphthalate solution in a 200-mLvolumetric flask,add the specified volume of the sodium hydroxide solution,then add water to volume. | |||||||||
| pH | 4.2 | 4.4 | 4.6 | 4.8 | 5.0 | 5.2 | 5.4 | 5.6 | 5.8 |
| 0.2M NaOH,mL | 3.0 | 6.6 | 11.1 | 16.5 | 22.6 | 28.8 | 34.1 | 38.8 | 42.3 |
| Phosphate Buffer | ||||||||||||
| Place 50mLof the monobasic potassium phosphate solution in a 200-mLvolumetric flask,add the specified volume of the sodium hydroxide solution,then add water to volume. | ||||||||||||
| pH | 5.8 | 6.0 | 6.2 | 6.4 | 6.6 | 6.8 | 7.0 | 7.2 | 7.4 | 7.6 | 7.8 | 8.0 |
| 0.2M NaOH,mL | 3.6 | 5.6 | 8.1 | 11.6 | 16.4 | 22.4 | 29.1 | 34.7 | 39.1 | 42.4 | 44.5 | 46.1 |
| Alkaline Borate Buffer | |||||||||||
| Place 50mLof the boric acid and potassium chloride solution in a 200-mLvolumetric flask,add the specified volume of the sodium hydroxide solution,then add water to volume. | |||||||||||
| pH | 8.0 | 8.2 | 8.4 | 8.6 | 8.8 | 9.0 | 9.2 | 9.4 | 9.6 | 9.8 | 10.0 |
| 0.2M NaOH,mL | 3.9 | 6.0 | 8.6 | 11.8 | 15.8 | 20.8 | 26.4 | 32.1 | 36.9 | 40.6 | 43.7 |
| Acetate Buffer | ||||||||||
| Place the specified amount of sodium acetate NaC2H3O2·3H2Oin a 1000-mLvolumetric flask,add the specified volume of the acetic acid solution,then add water to volume,and mix. | ||||||||||
| pH | 4.1 | 4.3 | 4.5 | 4.7 | 4.9 | 5.1 | 5.2 | 5.3 | 5.4 | 5.5 |
| pH(measured) | 4.10 | 4.29 | 4.51 | 4.70 | 4.90 | 5.11 | 5.18 | 5.30 | 5.40 | 5.48 |
| NaC2H3O2·3H2O,g | 1.5 | 1.99 | 2.99 | 3.59 | 4.34 | 5.08 | 5.23 | 5.61 | 5.76 | 5.98 |
| 2N CH3COOH,mL | 19.5 | 17.7 | 14.0 | 11.8 | 9.1 | 6.3 | 5.8 | 4.4 | 3.8 | 3.0 |