Buffer Solutions

Buffers regulate pH around a specific value

What are buffers?

The buffers, also called buffers, pH regulators or buffers, are aqueous solutions formed by a weak acid and its conjugate base or a weak base and its conjugate acid.

They are called buffers, since they have the ability to buffer the pH change after adding acids or bases in small amounts, even if it is strong acids or bases.

Buffer solutions are very useful, since they allow the pH to be kept constant. There are many chemical reactions that need a certain and constant pH to work well.

Properties of buffer solutions

They are aqueous solutions

Both the concept of pH and the pH scale only make sense in aqueous solutions, so all buffers or buffers are prepared in water.

Its operation is based on the acid-base balance and the principles of Le Chatelier

Buffers are based on reversible acid / base cleavage reactions that are in equilibrium. By adding strong acids or bases to the medium, this balance is disturbed, so the system reacts to counteract the disturbance, following the principles of Le Chatelier. This is how buffers manage to buffer large changes in pH.

Its pH is easily calculated with the Henderson-Hasselbalch equation

Regardless of the type of buffer, its pH, both before and after adding small amounts of strong acids or bases, can be calculated using the Henderson-Hasselbalch equation:

Where [base] refers to the molar concentration of the base (or the conjugate base salt) and [acid] refers to the molar concentration of the acid (or the conjugate acid salt).

They are capable of partially neutralizing both acids and bases

Buffers are made up of a basic substance and an acidic substance, which can neutralize bases and acids, respectively. For this reason, they can buffer the pH after the addition of both strong acids and strong bases.

Its operation depends on the temperature

The pH of a buffer depends on the acidity or basicity constant of the weak acid or base it contains. This constant depends on temperature, so the operation of these solutions also depends on temperature.

The more concentrated, the greater their ability to buffer or regulate the pH

The buffering capacity depends on the total concentration of the solution being greater than the final concentration of the added strong acid or base. For this reason, the higher the total concentration of the buffer, the greater its ability to buffer strong acids or bases.

Components of a buffer solution

Chemically, buffers are made up of only two components. These components can be a weak acid mixed with a salt of its conjugate base or a weak base mixed with a salt of its conjugated acid.

There are two different ways to obtain these components in the same solution, as we will see below:

1. Preparation of buffers by mixing solutions of their components

This is the most direct way to prepare a buffer solution. For this, separate solutions of the weak acid (eg acetic acid) and a salt of the conjugated base (eg sodium acetate) are prepared. Then both solutions are mixed little by little until the desired pH is obtained.

    The solutions are mixed little by little until the desired pH is obtained.

2. Preparation of buffers by partial neutralization of a weak acid solution

In this case, a weak acid solution is prepared with the desired total concentration, and then sodium or potassium hydroxide is added little by little until the desired pH is reached.

Types of buffer solutions

Buffers can be classified according to the types of components they have or according to their final pH.

According to the types of components:

  • Weak acid and conjugate base buffer: In these cases the pH depends on the pK a of the weak acid.
  • Weak base buffer and conjugated acid: In these cases, the pH depends on the pK b of the weak base.
  • Polyprotic acid salts buffer: In some cases, both the one that plays the role of the weak acid and its conjugate base are both salts from the partial neutralization of an acid that has several protons, such as sulfuric or phosphoric acid.

According to its final pH:

Depending on the acidity or basicity constant, a buffer solution can regulate the pH around different pH ranges, giving rise to three types of buffers:

  • Acid buffers: They are those that regulate the pH around values ​​less than 7. They are prepared with weak acids whose pK a is less than 7 or with weak bases whose pK b is greater than 7.
  • Neutral buffers: These are those that regulate the pH around 7. They are usually made up of weak acids or bases that have a pK a or a pK b close to 7.
  • Alkaline buffers: These are those that regulate the pH around values ​​greater than 7. They are prepared with weak acids whose pK a is greater than 7 or with weak bases whose pK b is less than 7.

Examples of buffers or buffers

Acetic acid / sodium acetate buffer

This is a buffer of a weak acid (acetic acid) and a salt of its conjugate base (sodium acetate). The equilibrium involved and its equilibrium constant are:

This buffer regulates the pH around 4.74 .

Ammonia / Ammonium Chloride Buffer

This is a buffer of a weak base (ammonia or ammonium hydroxide) and a salt of its conjugated acid (ammonium chloride). The equilibrium involved and its equilibrium constant are:

This is an alkaline buffer that regulates the pH around 9.26.

Bisulfate / sulfate buffer

In this case, the bisulfate ion plays the role of a weak acid whose conjugate base is the sulfate ion. The balance involved is:

This is an acidic buffer that regulates the pH around 3.05 .

Carbonic acid / bicarbonate buffer

This buffer regulates the pH around 6.38.

Dihydrogen phosphate / hydrogen phosphate buffer

This is one of the most widely used pH regulation systems in biology and biochemistry, since it allows regulating the pH very close to the physiological pH at which most chemical reactions occur within cells. The reaction is:

This buffer regulates the pH around 7.20.

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