In chemistry, the Henderson-Hasselbalch (often misspelled as Henderson-Hasselbach) equation describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. Chemistry (from Egyptian kÄ“me (chem meaning "earth") is the Science concerned with the composition structure and properties pH is the measure of the acidity or alkalinity of a Solution. The equation is also useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions (it is widely used to calculate isoelectric point of the proteins). For an individual weak acid or weak base component see Buffering agent. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change The isoelectric point (pI is the PH at which a particular Molecule or surface carries no net electrical charge.

Two equivalent forms of the equation are

$\textrm{pH} = \textrm{pK}_{a}+ \log \frac{[\textrm{A}^-]}{[\textrm{HA}]}$

and

$\textrm{pH} = \textrm{pK}_{a}+\log \left ( \frac{[\mathrm{base}]}{[\mathrm{acid}]} \right ).$

Here, pKa is âˆ’ log(Ka) where Ka is the acid dissociation constant, that is:

$\textrm{pK}_{a} = - \log(K_{a}) = - \log \left ( \frac{[\mbox{H}_{3}\mbox{O}^+][\mbox{A}^-]}{[\mbox{HA}]} \right )$ for the non-specific BrÃ¸nsted acid-base reaction: $\mbox{HA} + \mbox{H}_{2}\mbox{O} \rightleftharpoons \mbox{A}^- + \mbox{H}_{3}\mbox{O}^+$

In these equations, A âˆ’ denotes the ionic form of the relevant acid. Bracketed quantities such as [base] and [acid] denote the molar concentration of the quantity enclosed.

In analogy to the above equations, the following equation is valid:

$\textrm{pOH} = \textrm{pK}_{b}+ \log ( \frac{[\textrm{BH}^+]}{[\textrm{B}]} )$

Where B + denotes the salt of the corresponding base B.

## History

Lawrence Joseph Henderson wrote an equation, in 1908, describing the use of carbonic acid as a buffer solution. Lawrence Joseph Henderson (b June 3 1878 Lynn Massachusetts &ndash d Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3 For an individual weak acid or weak base component see Buffering agent. Karl Albert Hasselbalch later re-expressed that formula in logarithmic terms, resulting in the Henderson-Hasselbalch equation [1]. Karl Albert Hasselbalch ( 1 November 1874, Aastrup Denmark - 19 September 1962) was a physician and chemist In Mathematics, the logarithm of a number to a given base is the power or Exponent to which the base must be raised in order to produce Hasselbalch was using the formula to study metabolic acidosis, which results from carbonic acid in the blood. In Medicine, metabolic acidosis is a process which if unchecked leads to acidemia (i Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells €”such as nutrients and oxygenâ€”and transports Waste products

## Limitations

There are some significant approximations implicit in the Henderson-Hasselbalch equation. The most significant is the assumption that the concentration of the acid and its conjugate base at equilibrium will remain the same as the formal concentration. This neglects the dissociation of the acid and the hydrolysis of the base. The dissociation of water itself is neglected as well. These approximations will fail when dealing with relatively strong acids or bases (pKa more than a couple units away from 7), dilute or very concentrated solutions (less than 1 mM or greater than 1M), or heavily skewed acid/base ratios (more than 100 to 1).