Buffer Solutions

 Buffer solutions are an important, but potentially confusing idea involving weak acids and bases. We’ll start with a simple definition, then look at what buffers can do that makes them so interesting. 

The Definition 

A buffer is a solution that contains a weak conjugate pair.

Let’s pull that apart. First, a buffer solution must contain BOTH an acid and a base. Secondly, both must be weak, since if one or both was strong, they would immediately react ~100%. Lastly, the acid and base need to be conjugates.

A simple example of a buffer is a solution that contains some acetic acid (\(HC_2H_3O_2\)) and some acetate (\(C_2H_3O_2^{-1}\)). Another example would be a solution that contains both ammonia (\(NH_3\)) and ammonium (\(NH_4^{+1}\)).

What makes a buffer so interesting? 

A buffer solution is one that resists changes in pH when an acid or base is added to the solution.

To understand that, let’s compare what happens when you add HCl (a strong acid) to water and when you add it to a buffer solution (we’ll use the \(HC_2H_3O_2 ~and~ C_2H_3O_2^{-1}\) for our example.

When you add HCl to water, the following reaction occurs:

\(HCl + H_2O \rightleftharpoons H_3O^{+1} + Cl^{-1}\)

Because this reaction produces hydronium (\(H_3O^{+1}\))the pH will go down.

However, if you add HCl to the buffer solution, the HCl will react with the conjugate base (\(C_2H_3O_2^{-1}\)) in the following reaction:

\(HCl + C_2H_3O_2^{-1} \rightleftharpoons HC_2H_3O_2 + Cl^{-1}\)

Because this reaction does NOT produce hydronium, the pH is unchanged.

We see the same sort of thing when we add a base, like NaOH.

When we add NaOH to water, it dissolves, creating a solution of hydroxide:

\(NaOH_{(s)} \rightleftharpoons Na^{+1}_{(aq)} + OH^{-1}_{(aq)}\)

The addition of hydroxide ions to the solution means that the pH will rise.

However, if you add NaOH to the buffer solution, the NaOH will react with the acid present (\(HC_2H_3O_2\)). You can think of this as a double displacement reaction:

\(NaOH + HC_2H_3O_2 \rightleftharpoons NaC_2H_3O_2 + H_2O\)

Although it is more accurate to think about the net-ionic equation that occurs with the hydroxide ions:

\(OH^{-1}_{(aq)} + HC_2H_3O_{2 (aq)} \rightleftharpoons H_2O_{(l)} + C_2H_3O_{2 (aq)}^{-1}\)

No matter which version of the reaction you look at, no hydroxide is added to the solution, so the pH is unchanged.

Buffer Math 

It is, of course, not quite correct to suggest that the pH doesn’t change at all. What we really mean is that the pH does not change dramatically.

How it changes, and by how much, can be worked out using the Henderson/Hasselbalch equation. This math is beyond the scope of most first year chemistry classes, but you can find some information here.

Buffers in Your Life 

It turns out that buffer solutions are present in lots of situations. Here are just a few:

Swimming Pools

As we all know, children make regular acidic “donations” to swimming pools. In addition, because an in-ground pool is walled in cement which contains lime, the walls themselves can make the pool  more basic. In addition, rain not only adds water to the pool, but also washes in debris. Insects, and perhaps frogs, fall in the pool and die. All in all, there are multiple factors that can change the pH of the water in your pool. Since you cannot control, or predict these in any meaningful way, the pH of your pool is controlled with a buffer.

The Ocean

The ocean is a vast solution containing an incredible number of different compounds and ions. Two of those ions are carbonate (\(CO_3^{-2}\)) and bicarbonate (\(HCO_3^{-1}\)). These two ions, along with the carbonic acid formed from the dissolution of carbon dioxide, create a complex buffer solution that holds the ocean’s pH steady.

The Bloodstream

As your body burns sugars for energy, carbon dioxide is produced. As we know, \(CO_2\) forms carbonic acid when it reacts with water. If that process was unmodulated, the pH of your bloodstream would drop dramatically when you exercised (burning lots of sugars) and would rise as you exhaled. Such an unstable situation would be unworkable for the complex chemistry that keeps us alive. Fortunately, the same complex buffer solution found in the ocean (\(H_2CO_3, ~HCO_3^{-1}, ~and~ CO_3^{-2}\)) is also present in your body. In fact, the tiny variations in pH (generally less than 0.02 points on the pH scale) are how your body regulates breathing. The urge to breathe when you are holding your breath is determined, NOT by the lack of oxygen, but instead by the slight drop in blood pH as the concentration of \(CO_2\) increases.

Country Time Lemonade\(^®\)

Country Time Lemonade mix is advertised as being “not too tart, and not too sweet”. Of course making sure that your lemonade mix is not too sweet is simple. Just add too much sugar. However “tart” is a little different. Tart is just a nice word for sour. That means that Country Time is claiming that their mix will not be too acidic. However, they have no control over the pH of the water in my sink that I use to make their lemonade. To control the pH of my drink mix, they have used a buffer. The first ingredient here is citric acid. The second and third ingredients are potassium and sodium citrate. Citrate ions are, of course, the conjugate base of citric acid.