Calculate Buffer Capacity

A titration exam is something you probably did in high school chemistry or in a college course. It’s like the liquid percolate in the bottom of the glass flask, carefully waiting for the solution to turn a tinge of pink or magenta.

 Why, though, does this solution only change color when a specific amount of chemicals is added? To obtain that answer, we must first comprehend the solution’s fundamental features.

Definition of buffer capacity

To understand what a buffer capacity is, we must first grasp what a buffer is. A buffer is a substance that, when exposed to a small amount of acid or base, resists pH changes. A weak acid or weak base, along with its conjugate salt, usually makes up the chemical makeup of a buffer solution.

Buffer Capacity is now defined as the measure of a buffer’s efficiency in resisting a pH change. The question of “what is the major change?” is raised by this definition. A change of one unit does not always result in a considerable change. In other situations, a 0.1-unit difference can make a big difference. To give a more precise definition, buffer capacity can be defined as the amount of a strong acid or strong base that must be added to one liter of a solution in order to modify its pH by one unit. The following is the buffer capacity equation:

 n is several equivalents of additional strong bases in the buffer equation (per 1 L of the solution). It’s worth noting that adding n moles of acid to the solution will modify the pH by the same amount, but in the opposite direction. We’ll create a formula that relates buffer capacity to pH, pKa, and buffer concentration.

Buffer capacity calculation

Let’s try to deduce the buffer equation now that we’ve seen how it might be expressed to gain a better grasp of how we got at the above equation. We’ll make the base monoprotic to make this derivation a little easier (a base that will accept only one proton). 

We’ll also assume the volume is one, which will allow us to consider concentration and mole count interchangeably. 

The basic elements of [HA] in the following equation can be broken down further. The dissociation constant is the rate at which a larger, more complicated compound is broken down into smaller fundamental constituents. The dissociation constant facilitates the derivation’s simplicity. The acid dissociation constant, Ka, is found in the equation below. It has to do with how easy a molecule can become an acid.

[HA]=([H+][A–])/Ka

Before we proceed, we must first grasp a crucial definition known as the water ionization constant or self-ionization of water, which will serve as a prerequisite for neatly wrapping up this derivation. Water self-ionization is an ionization reaction in which H2O loses the nucleus of one of its hydrogen atoms to form the hydroxide ion, OH, in pure water or an aqueous solution.

The graph above depicts the changes in buffer capacity in an acetic buffer at 0.1 M. The buffer, as expected, resists acid and base addition in order to maintain an equimolar solution (pH=pKa). The buffer capacity of the solution has relatively high values only for pH close to the pKa value, as shown in the graph: the further away from the ideal value, the lower the buffer capacity of the solution.

A conjugate base solution with a pH of 8-10 has little buffer capacity, however at higher pHs, the presence of the strong base becomes more relevant. A pure acetic acid solution with a pH below 3 already has a low enough pH to withstand changes caused by the large concentration of H+ cations.

Conclusion

The buffer capacity of a solution is a measure of how resistant it is to a pH shift when an acid or base is introduced to it.

To understand what a buffer capacity is, we must first grasp what a buffer is. A buffer is a substance that, when exposed to a small amount of acid or base, resists pH changes.