pH vs. Buffering Capacity

October 30, 2008 | Contact Author | By: Tony O'Lenick
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Industry expert Tony O'Lenick explains the difference between pH and buffering capacity in formulations...

Most formulators have experienced pH drift in formulations. pH is a measure of acid or base strength. Arrhenius, in 1887, was the first person to define an acid and a base--an acid gives rise to excess of H+ in aqueous solution; whereas a base gives rise to excess of OH- in solution.

The theory was refined by Bronsted-Lowry in 1923 such that a proton donor was defined as an acid, and a proton acceptor as a base. They also introduced the familiar concept of the conjugate acid-case pair. Lewis completed the final refinement to acid base theory in 1923 defining an acid as an e-pair acceptor, and a base aa an e-pair donor.

From a practical point of view, pH is a measure of the acidity or alkalinity of a solution. Solutions with a pH level less than seven (7) are considered acidic, while those with a pH greater than 7 are considered basic (alkaline). pH 7 is considered neutral. Since many materials are aqueous-based, pH is a common specification.

Buffering capacity on the other hand is an ability to resist change in pH upon addition of small amounts of acid or base, or upon dilution. Buffer solutions consist of a weak acid and its conjugate base (more commonly) or a weak base and its conjugate acid (less commonly).

pH Drift : Pure water with a pH of 7 has little or no buffering capacity; a drop of acetic acid will lower the pH level to 3, and a drop of base will raise the pH to 13! Small changes in concentration give very large pH changes.

Amphoteric and betaine-based formulations can drop in pH because they can consume small amounts of OH- upon storage. This can result in a significant pH drop. Buffers are added to keep this from happening. Phosphate/citrate buffer is commonly used to keep the pH from drifting.