BIO101: Introduction to Molecular and Cellular Biology

pH

The hydrogen ion concentration, [H⁺] of a solution is an important property because biological systems contains functional groups whose properties are changed by changes in the hydrogen ion concentration.

Since the hydrogen ion concentrations are usually much less than one, and can vary over many orders of magnitude, a different scale is used for the hydrogen ion concentration, the pH scale. The pH is the -log of the proton concentration:

$\textrm{pH}=-\log[\textrm{H}^{+}]$

The log conversion reduces a 10 fold change in hydrogen ion concentration to a one unit change in pH. The minus sign changes the negative numbers that would be obtained from log[H⁺] to positive ones. Since the pH scale is an inverse scale the concentration of protons is high at low pH and low at high pH. A solution is said to be acidic if the pH is less than seven, and basic if the pH is above 7. A solution is neutral if its pH is equal to 7.0.

The image below shows the pH of a number of common fluids.

pH of Various Compounds.

On the left are biological compounds and on the right are some foods and cleaning products.

Acid Dissociation and pH

For our studies, the Bronsted definition of an acid will be used, we will define an acid as a proton donor and a base as a proton acceptor. Hydrochloric acid, like sodium chloride, is a strong electrolyte because it completely dissociates in aqueous solution into charged ions. Hydrochloric acid is also a strong acid because when it completely dissociates it also completely donates all of its protons.

Many molecules are weak electrolytes and exist in an equilibrium (indicated by $\Leftrightarrow$ in the general equation below) between the starting molecule and its dissociated parts. Thus dissociation can be seen as an acid (HA) in equilibrium with a proton (H⁺) and the corresponding conjugate base (A^-).

In general:

$\textrm{HA}\ \Leftrightarrow\ \textrm{A}^{-} +\ \textrm{H}^{+}$

Specifically for Acetic acid:

$\textrm{CH}_{3}\textrm{COOH}\ \Leftrightarrow \textrm{CH}_{3}\textrm{COO}^{-} + \textrm{H}^{+}$

Source: Carnegie Mellon University, https://oli.cmu.edu/jcourse/workbook/activity/page?context=df459e6d0a0001dc64e18bdb857f2432
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