- 1). Measure the pH of your buffer solution using the pH meter or pH paper. If you're using pH paper, you may only be able to obtain a rough estimate of the pH, but this may be sufficient depending on how you plan to use your buffer solution.
- 2). Calculate the ratio of weak acid to conjugate base in the buffer solution using the Henderson-Hasselbalch equation, where pH = pKa + log (concentration of conjugate base / concentration of weak acid) . The pKa is the negative log of the acid dissociation constant. You can find the pKa of most weak acids under the table at the University of Waterloo website. The pKa of acetic acid, for example, is 4.75. Since you know both the pH and the pKa, you can easily plug both values into the equation to find the ratio of weak acid to conjugate base.
- 3). Put on the chemical splash goggles, long-sleeved coat and gloves.
- 4). Measure out the amount of acid and conjugate base you need to increase buffer capacity to the desired level while maintaining the same pH. Given the Henderson-Hasselbalch equation from step 2, pH will not change as long as the ratio of conjugate base to weak acid remains the same. In other words, a solution that is .02 molar in sodium acetate and .04 molar in acetic acid will have roughly the same pH after you double the concentration of both compounds. The more acid and conjugate base you have in the solution, the greater the pH capacity will be.
- 5). Add the acid and conjugate base to your solution and stir to mix them in.
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