Buffer Calculations

Calculating the pH of Buffers - Real Chemistry

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Key Questions

• A buffer solution will maintain a constant pH with the addition of small amounts of acid and alkali.

There is a limit to the amount of acid or alkali that the buffer can handle.

The buffer capacity is given by the expression:

$\frac{\mathrm{dn}}{d \left(p H\right)}$

$\mathrm{dn}$ is a tiny amount of added base which results in tiny change in pH shown as $d \left(p H\right)$

If you have a weak acid whose dissociation constant is ${K}_{a}$ then the buffer capacity is given by :

(From Wiki)

${C}_{A}$ is the concentration of the acid.

• Buffer solutions are used to calibrate pH meters because they resist changes in pH.

When you use a pH meter to measure pH, you want to be sure that if the meter says pH = 7.00, the pH really is 7.00.

So you use solutions of known pH and adjust the meter to display those values.

Buffers are ideal for this purpose because:

• They are easily prepared for a given pH.
• They are stable for long periods of time.
• They resist changes in pH if you accidentally add a little acid or base or even water.

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