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The study of chemical equilibrium in reactions requires the concentrations of both reactants and the products. However, the pH value is a measure that depicts how acidic or basic is the given solution. Henderson and Hasselbalch, in 1916, came forward with a combined equation that contains the value of concentrations of the weak acid and their conjugate base, the acid dissociation constant pka, to calculate the pH value of buffer solutions. The derivation of this equation is essential to understand how chemical equilibrium works and, therefore, can be understood in simple steps. The equation has a few shortcomings, and thus the Henderson-Hasselbalch Equation limitations are also described in the article.
Henderson Hasselbalch Equation
The Henderson-Hasselbalch equation can be written in terms of the dissociation constant pka, the concentration of conjugate acid and weak acid as follows :-
pH= pka + log10[conjugate base][weak acid]
The Henderson-Hasselbalch equation helps calculate the pH of any buffer solution provided that we have the value of the conjugate base concentration and concentration of weak acid. By putting these values in the equation, one can calculate the desired results. Although this equation is thorough, there are certain shortcomings of the Henderson-Hasselbalch equation since it is an approximate equation for calculating pH. However, Henderson-Hasselbalch Equation limitations are given in subsequent sections.
Henderson Hasselbalch Equation History
The historical aspects of this equation come from 1908, when a chemist, biologist, physiologist Lawrence Joseph Anderson gave out an equation to find out the concentration of hydrogen ion in a buffer solution (bicarbonate), which was in the form :-
[H+] [HCO3-] = K [CO2] [H2O]
This equation was termed the Henderson equation in 1908. Furthermore, in 1909, a Danish chemist named S.P.L Sorenson introduced the concept of pH. With the concentration of conjugate base, weak acid, and pH, Karl Albert Hasselbalch, in 1916, combined the three aspects in logarithmic terms and finally derived an equation famously known today as the Henderson-Hasselbalch equation for a buffer solution.
Henderson Hasselbalch Equation Derivation
The derivation is essential concerning Henderson hasselbalch equation limitations jee notes. To derive the Henderson-Hasselbalch equation, one has to have some preliminary information that a buffer solution consists of an aqueous solution of an acid and a conjugate base salt of an acid. The Henderson-Hasselbalch equation successfully incorporates the concentrations of such solutions along with the dissociation constant Ka with their pH.
The derivation is as follows, consider the following reaction,
HA ⇌ H+ + A-
Where HA is a weak acid. Now the Ka (acid dissociation constant) can be written in terms of concentrations of reactants and products as :-
Ka = [H+] [A-][HA]
Now by taking negative of logarithm of base ten on both left and ride sides of the above equation, we can write,
-log10 Ka = -log10 [H+] [A-][HA]
Using the mathematical identities of logarithmic, we can simplify the above equation as,
-log10 Ka = – log10 H+- log10 [A-][HA]
Rewriting the above equation in terms of pH as,
pka = pH – log10 [A-][HA]
rearranging the terms in the above equation, we finally write the famous Henderson-Hasselbalch equation as,
pH= pka + log10[conjugate base][weak acid]
The Henderson-Hasselbalch equation helps calculate the equilibrium pH in many reactions, such as acid and base reactions.
Henderson Hasselbalch Equation Limitations
- The Henderson-Hasselbalch Equation’s limitations do not account for precise strong acids or bases values. The Henderson-Hasselbalch equation presumes that the concentrations of acids and their conjugate bases at chemical equilibrium shall remain the same.
- Also, since it relates pH to the concentrations and dissociation constants, it has been noticed that the equation does not give accurate values of pH when it comes to highly diluting buffer solutions .
- It also does not take into consideration the hydrolysis of water and the overall effect on the value of pH. Besides such Henderson-Hasselbalch Equation limitations, it is still helpful in many situations for calculating pH value.
Conclusion
The Henderson-Hasselbalch equation gives the pH value for buffer solutions when the concentrations of weak acids and their conjugate base are given. However, it can be derived by considering the reversible reaction between an acid and its corresponding ions. The limitations of this equation are also addressed in the article: it does not account for strong acids and bases and the correct pH value for very dilute solutions. The report provides detailed knowledge about the equation, its derivation, and Henderson Hasselbalch Equation Limitations.
