Components of Henderson Equation

The Henderson Hasselbalch formula formalises this concept, with A denoting the percentage of base ions (OH^–) and HA denoting the concentration of acid ions (H⁺).

The Henderson-Hasselbach equation is a formula that connects the pH of an acid’s aqueous solution to the acid’s acid dissociation constant.

pH = pK_A + log₁₀[conjugate base]/[weak acid]

Whenever the percentages of the acid and conjugate base are equal, i.e. whenever the acidity is 50% dissociated, the pH of the solution equals the pKa of the acid, according to the Henderson-Hasselbach equation.

That is, when  [HA] = [A^–]

The Henderson Hasselbalch formula approximates the link between a solution’s pH or pOH, its pKa or pKb, and the ratio of dissociated chemical species concentrations. The acid dissociation factor must be known before the equation could be used.

Equation

There are many ways to write the equation. Two of the most common ones are:

pH = pK_a + log ([conjugate base]/[weak acid])

pOH = pK_b + log ([conjugate acid]/[weak base])

In 1908, Lawrence Joseph Henderson devised a formula for calculating overall pH of a buffer solution. This formula was rewritten into logarithmic terms around 1917 by Karl Albert Hasselbalch.

Limitations

• The most important assumption within that equation is that during the equilibrium, the concentrations of acid as well as its conjugate base would stay unchanged.

• The importance of water hydrolysis and its impact on the total pH of the solution is often overlooked.

• Hydrolysis of a base and acid dissociation are also ignored.

• When working with strong acids and bases, the assertion made within the equation may fail.

Understanding K_a and pK_a

When determining whether a species would donate or take protons at a given pH value, K_a, pK_a, pK_b are by far the most useful parameters. While addition of water to a solution does not modify the equilibrium constant, they are accurate measures of acid or basic strength. Acids are represented by K_a and  pK_a, whereas bases are represented by K_b and pK_b. These values, like pH and pOH, take into account the concentration of hydrogen ions or proton (for K_a and  pK_a,), as well as the concentration of hydroxide ions (for K_b and pK_b).

K_a and K_b are related to each other by the ion constant for water, Kw:

K_w = K_a × K_b

The acid dissociation constant, K_a, is a number that represents how long it takes for an acid to separate from This constant’s pK_a is simply its -log. K_b is a base dissociation constant, and pK_b is the -log of it. The moles per litre (mol/L) unit of measurement for acid or base dissociation constants. Dissociation of acids and bases is governed by the following equations:

H_A + H₂O ⇆ A^– + H₃O⁺

H_B + H₂O ⇆ B⁺ + OH^–

In the formula, A stands for acid and B for base.

Ka = [H⁺][A^–]/ [H_A]

pK_a = – log K_a

half the equivalence point is, pH = pK_a = -log K_a

A high K_a value denotes a strong acid since the acid is fully dissolved into its ions. When the K_a value is large, it’s also preferable to generate products within the reaction. A low K_a value means that just a small amount of an acid dissociates, implying that the acid is weak. The K_a value of most weak acids ranges from 10² to 10¹⁴ for the most part. When the pK_a value is low, the acid is more powerful. Weak acids have a pK_a of two to fourteen.

Conclusion 

When a little amount of acid or base is introduced or when the solution is diluted with pure solvent, a buffer is typically characterised as a solution that resists changes in pH. This characteristic is very beneficial for keeping the pH of a chemical system at an optimal level so that reaction kinetics and equilibrium processes can be influenced appropriately. A buffer solution is made up of a weak acid and its conjugate base, or a weak acid and its conjugate base. “Salts” is a term that refers to the conjugate forms. The Henderson-Hasselbalch equation is a particular form of the ionisation or dissociation constant expression. The pH of a solution that contains a weak acid as well as its conjugate base can be calculated using this equation (or salt). By partially neutralising a weak acid with a strong base, you can make a solution of the weak acid and its salt (conjugate base). To make the weak acid component, combine a surplus of salt with either a strong acid. The most common method for preparing a weak acid-conjugate base buffer would be to mix the weak acid with its salt directly. The concentrations of both weak acid and the strong acid may be precisely controlled with this third method.