The pH Scale

The pH scale is used to measure how acidic or basic an aqueous solution is. The scale runs from 0 to 14:—the lower the pH, the more acidic the solution is; the higher the pH, the more basic (or alkaline) it is.

• A liquid like water, with a pH of 7, is said to be neutral
• Soda, with a pH of 2-4, is acidic
• Stomach tablet antacid medicines which have a pH of 10-11, are basic (alkaline)

Chemistry of Acids

Acids are chemicals that will dissolve in water to form positively charged hydrogen ions (H+). A strong acid forms more of these hydrogen ions than a weak acid. The reactivity of an acid—the acid’s corrosiveness—is dictated by the concentration of these hydrogen ions. When we measure pH, we are really measuring the concentration of these reactive hydrogen ions that form from the acid, and the pH value is a guide to the corrosiveness of the acid. Hydrochloric acid, for example, which can have a pH as low as zero, is extremely corrosive.

How is pH Measured?

There are multiple methods for measuring pH. One simple and commonly used method is litmus/pH paper where strips of paper change color to provide an indicative measure of pH. In a laboratory environment, precise measurements of pH have for many years been determined indirectly using titration, the same technique that is used to measured TBN and TAN. However, modern equipment has superseded titrations and pH is now measured directly using an electrode probe. The hydrogen ions that form from the acid increase the voltage across the solution, and this is measured by the electrode in the pH meter.

The fourth article in this series will cover the measurement of pH for lubricant samples using the industry standard ASTM D7946 method.

For more information, please contact your Lubrizol representative.