Question

There are a lot of textbook examples showing what happens to the pH of a buffered solution when you add strong acids and strong bases. Searching for an example calculation where you add weak acids or bases instead, I did not find any information.

The purpose of this question is to establish a model answer for this type of problem. To give a specific example, we start with a phosphate buffer (total phosphate concentration of 100 mmol/L) with a pH adjusted to 8. What will happen when you add weak acid to a concentration of 10 mmol/L? Because the strength of the weak acid might matter, give the answer for a pKa
of 6.0, 8.0 and 10.0. Does it matter if the weak acid is one of the substances already present in the buffer?

This question is related, but not a duplicate of questions about titrating weak acids.

Answer 1

Adding a weak acid to a phosphate buffer will result in a slight change in pH. The extent of the change depends on the pKa of the added acid and its relation to the buffer's pH. The buffer system helps maintain pH stability and is particularly effective if the added weak acid is part of the buffer's constituents.

Step-by-step explanation:

Effect of Adding a Weak Acid to a Phosphate Buffer

When a weak acid is added to a phosphate buffer with a total phosphate concentration of 100 mmol/L and a pH of 8, the buffer's pH will slightly change depending on the pKa of the added weak acid.

A weak acid with a pKa of 6.0 would dissociate less in the buffer, since the pH of the buffer is higher than the pKa, leading to a smaller change in pH.

A weak acid with a pKa of 8.0 will have more influence, since its pKa matches the initial pH of the buffer, thus potentially causing a modest decrease in the pH.

However, the buffer system will still resist large changes in pH. Lastly, for a weak acid with a pKa of 10.0, because the buffer pH is below the pKa, the acid would remain largely undissociated, leading to an even smaller effect on pH.

It generally matters if the weak acid is one of the substances present in the buffer, as it would then participate directly in the buffer's acid-base equilibrium.

Calculating the specifics of the pH change would involve using the Henderson-Hasselbalch equation and considering the weak acid's dissociation equilibrium.

The buffer’s capacity to neutralize added acids or bases demonstrates its essential role in maintaining pH stability, unlike the drastic pH changes that would occur in unbuffered solutions.

Answer 2

Adding a weak acid to a phosphate buffer will result in a slight change in pH. The extent of the change depends on the pKa of the added acid and its relation to the buffer's pH. The buffer system helps maintain pH stability and is particularly effective if the added weak acid is part of the buffer's constituents.

Step-by-step explanation:

Effect of Adding a Weak Acid to a Phosphate Buffer

When a weak acid is added to a phosphate buffer with a total phosphate concentration of 100 mmol/L and a pH of 8, the buffer's pH will slightly change depending on the pKa of the added weak acid.

A weak acid with a pKa of 6.0 would dissociate less in the buffer, since the pH of the buffer is higher than the pKa, leading to a smaller change in pH.

A weak acid with a pKa of 8.0 will have more influence, since its pKa matches the initial pH of the buffer, thus potentially causing a modest decrease in the pH.

However, the buffer system will still resist large changes in pH. Lastly, for a weak acid with a pKa of 10.0, because the buffer pH is below the pKa, the acid would remain largely undissociated, leading to an even smaller effect on pH.

It generally matters if the weak acid is one of the substances present in the buffer, as it would then participate directly in the buffer's acid-base equilibrium.

Calculating the specifics of the pH change would involve using the Henderson-Hasselbalch equation and considering the weak acid's dissociation equilibrium.

The buffer’s capacity to neutralize added acids or bases demonstrates its essential role in maintaining pH stability, unlike the drastic pH changes that would occur in unbuffered solutions.