Acidic Buffers: Definition, Composition, and How They Work

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What is an Acidic Buffer?

Firstly, we need to define what is a pH buffer.

A pH buffer is a solution which resists pH change when acid or base are added to the solution. That doesn’t mean that the pH doesn’t change at all, but that the pH change is much smaller than a comparable solution without buffering function.

An acidic buffer is a buffer that has an initial pH less than 7. It is possible to formulate buffers to be a specific pH by selecting the acid, the salt and their concentrations.

What are pH Buffer Solutions Used For?

Buffer systems occur in natural as well as man-made systems. There are two types of pH buffer – acidic buffers and alkaline buffers.

There are many natural buffer systems within living organisms (i.e. animal and plant cells). Buffering systems are also used in industrial applications such as a huge variety of fermentations.

Processed food and drink is often formulated to include a buffer system.

Skincare and haircare products usually have a buffer system too.

Buffer solutions are used for calibrating apparatus used to measure pH, and you may have seen pH buffer solutions in your college lab if the lab has a pH meter.

There are many, many more examples of application of pH buffers too.

What are Acidic Buffers Made From?

An acidic buffer is a solution of a weak acid and a salt. The acid and the salt must have the same anion; for example, if the acid used is ethanoic acid, then an ethanoate salt such as sodium ethanoate must be included in the buffer.

How Does an Acidic Buffer Work?

The key is the reversible reaction that is the dissociation of the weak acid, which we have discussed in detail in a previous article here.

We can illustrate the dissociation of a generic weak acid HA in this equation:

The presence of a salt with the same anion A^– has an influence over the position of equilibrium of this dissociation, and that equilibrium is established in the freshly made buffer solution.

Adding Base to the Buffer

When base is added it reacts with the H⁺ ions in the buffer, and this (temporarily) reduces the concentration of the H⁺ ion. This causes a disturbance to the equilibrium, and causes a response predicted by the Le Chatelier Principle – more of the acid molecules dissociate to increase the concentration of H⁺ ions back to near the original concentration.

As the concentration of H⁺ ions is maintained close to its original concentration, the pH is ‘buffered‘ to remain close to its original value.

Adding Acid to the Buffer

When acid is added the concentration of H⁺ ions is (temporarily) increased, disturbing the equilibrium that existed in the buffer. The change in the position of equilibrium can again be predicted using the Le Chatelier Principle – some of the H⁺ ions combine with anions (A^– in our earlier illustrative equation) to form acid molecules (HA), until the H+ concentration is very close to its original value.

Therefore, the pH concentration remains close to its original value and the pH is said to be buffered.

Calculations For Acidic Buffers

You can be certain that you will face a question in your exam related to acidic buffers, and that you will need to do calculation to answer the question fully. That may sound scary but actually it’s not if you already understand how to do calculations for weak acids.

That’s because we use the same equation, the same logic, and the same acid dissociation constant as we use with weak acid calculations – no need to learn two ways!

There are some cunning bits in some questions, and the best approach is not to jump to conclusions about the question but to read it carefully. That’s important in every question, of course, and never more so than with the buffer question.

The difficulty is that there are several ways to make or prepare the buffer solution, and the question may ask you about one of several possible ways. We’ll look more closely at these in a moment.

The foolproof way of dealing with this is to always make sure you know :

• the number of moles of the acid
• the number of moles of the salt
• the volume of the buffer when the individual substances have been combined

So, why is this sometimes tricky? It’s because there are several ways to prepare the buffer, to make the solution. It’s super important to pay attention to the narrative in the question that describes the “ingredients” used. There will always be a description of how the buffer is prepared and it is key to focus on it and realise what considerations are needed when calculating the concentrations in the buffer.

The Four Ways of Making an Acid Buffer

There are four methods of preparing an acid buffer solution that you may see in your exam question. If you look carefully at questions in past papers you will likely find examples of each.

Actually, there are many more ways, but these are the four you will potentially see in an exam question.

The four methods are:

• Mix a solution of weak acid with a solution of the salt.
• Dissolve solid salt into a solution of weak acid.
• Mix a solution of weak acid with a solution of base (such as KOH). The base and some of the acid react to produce the salt.
• Dissolve solid base into a solution of weak acid. The base and some of the acid react to produce the salt.

Let’s look at each in more details so we can be aware of the various considerations.

Mixing Solutions of Weak Acid and Salt

In this method the question will describe the mixing of known volume of weak acid solution, and a known volume of salt solution.

You will be given the concentration of each, so it is possible to calculate the number of moles of acid and of salt present.

It may seem obvious to realise that calculating the concentration of each in the final buffer must consider the total volume of the combined solutions, but this is a common error. The concentrations of the acid and the salt are lower in the buffer than they were in the individual solutions.

Dissolving Salt into Weak Acid Solution

In this method the question will describe a known mass of salt is dissolved into a known volume of acid solution.

You will have the concentration of the acid solution. We don’t consider there to be a volume change when we dissolve the salt into the solution, so the concentration of weak acid does not change.

You will need to calculate the number of moles of salt that was added. As you know the volume of acid solution used you can easily calculate the salt concentration in the buffer solution.

Mixing Solutions of Weak Acid and Base

In this method the question will describe the mixing of known volume of weak acid solution, and a known volume of base solution. The base is typically potassium hydroxide or sodium hydroxide.

Note that there is no salt added to the buffer. The salt is produced by a reaction between the base and the acid. All the base will be neutralised in this reaction, and there will be some weak acid remaining. (This is always the case as there must be weak acid in the final buffer solution).

You will be given the concentration of the acid and base solutions, so it is possible to calculate the number of moles of acid and of base that were combined.

It is vital to take into account that some of the acid will be used in the reaction with the base, and you will need to calculate the number of moles of acid that remain. How do you do this?

Firstly, write a balanced equation for the reaction between the acid and base. This will define the ratio (or stoichiometry) of the reaction between the two. The below shows an example for the reaction between our generic weak acid HA and sodium hydroxide.

Notice that the equation does not include a reversible symbol. This is because the reaction is not reversible – it ‘goes to completion’ meaning that the reaction will continue until all the acid or all the base are used up in the reaction.

Also consider that the number of moles of the acid must be greater than the number of moles of the base, X > Y. This is because our acidic buffer must include weak acid otherwise it wouldn’t be an acidic buffer and wouldn’t have any pH buffering function. All the base will be neutralised in the reaction leaving some of the weak acid in the final buffer solution.

The number of moles of weak acid in the final buffer will be X – Y for our example. The number of moles of salt produced will be Y.

It is possible to calculate the acid concentration and the salt concentration if you know the final volume of the buffer solution. This is usually the total of the volumes of the acid and base solutions that were mixed together.

Dissolving Base into Weak Acid

In this method the exam question will describe a known mass of base is dissolved into a known volume of acid solution.

Note that there is no salt added to the buffer. The salt is produced by a reaction between the base and the acid. All the base will be neutralised in this reaction, and there will be some weak acid remaining. (This is always the case as there must be weak acid in the final buffer solution).

You will have the concentration of the acid solution. We don’t consider there to be a volume change when we dissolve the base into the solution.

The concentration of the weak acid does change because some of the acid reacts with the base. The reaction is identical to that described in the equation in the section above.

Again, the number of moles of acid (X) is greater than the number of moles of base (Y).

The number of moles of acid remaining after reaction with the base is X – Y, and the number of moles of salt produced is Y.

The concentrations of weak acid and salt can be calculated. The volume to use is the original volume of weak acid solution.

Assumptions When Using K_a For Acidic Buffer Calculations

You may recall from our previous article about using K_a for weak acid calculations that it was necessary to make some assumptions when doing the calculations.

We use a different set of assumptions when using K_a for acidic buffer calculations.

The first assumption is that the concentration of anions [A^–] at equilibrium is equal to the salt concentration.

This is a reasonable assumption that makes the calculation simpler. The salt does dissociate completely. The assumption ignores the relatively small concentration of anions present as a result of dissociation of the weak acid.

The second assumption we make is about the concentration of undissociated acid, HA, at equilibrium. We make the assumption that the acid concentration [HA] is unchanged from the initial concentration.

But we know that some of that acid has dissociated, so we know that this isn’t the true concentration. So why can we make this assumption?

It’s because the proportion of molecules that dissociate in aqueous solution is small, typically less than 1%. That means that using the original acid concentration is a reasonable approximation, so our assumption is a fair one.

Calculating [H⁺] and pH for Acidic Buffers

Typically you will be asked to find the pH for an acidic buffer solution, and you will be given the acid concentration, salt concentration and the K_a value. (Read more about the K_a value, including the equation for calculating K_a).

To calculate the hydrogen ion concentration, we need to rearrange the equation to isolate the [H⁺] term:

Once the hydrogen ion concentration is known, the pH can be calculated using the usual equation:

pH = -log [H⁺]

Calculating [HA] for Acidic Buffers

You may also be asked to find the concentration of the acid.

To make the calculation you need to make a simple rearrangement of the acid dissociation constant again, this time to make [HA] the subject. When you have done this you will have:

To solve the equation, it is necessary to know the hydrogen concentration, salt concentration (which we assume to be the same as [A^–], and the value of the acid dissociation constant K_a. You may need to calculate the hydrogen ion concentration if you are given the pH instead – use the usual equation:

[H⁺] = 10^-pH

Calculating [A-] for Acidic Buffers

You may be asked to calculate the concentration of the salt present in the buffer. We mentioned earlier in this article that we make the assumption that the salt concentration is the same as [A^–]. That means we need to rearrange the equation to isolate the [A^–] value:

[A^–] / [HA]: The Ratio of Acid and Salt in an Acidic Buffer

Occasionally you may be asked a question that relies on using the ratio of salt and acid in the buffer, rather than knowing their concentrations.

A rearrangement of the equation for the dissociation constant gives us this expression, which isolates the ratio of salt : acid.

This can be useful where the values of the acid dissociation constant and hydrogen ion concentration (or the pH) are known.

If there seems to be “not enough information” in an exam question, consider whether you could use the ratio to bridge that gap.