The Water Course

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Dissolved Solids

TWC 1.01 – Potential of Hydrogen (pH)

What is pH?



In the natural environment, water is always gaining extra ions from the many substances that it dissolves. If water dissolves an acid, it causes the atoms that make up the acid to dissociate (break apart) and in doing so, the water acquires extra hydrogen ions (H+). Because water is a polar molecule, it can attract the positively charged H+ cations and bond with them. This introduces H3O+ molecules, known as the hydronium cations, to the water. This video explains exactly how this cation is formed. When there is an abundance of H3O+ cations, water becomes more acidic.

The reverse occurs when water dissolves an alkali (the opposite of an acid). When alkalis dissolve in water, their atoms dissociate to release a hydroxide anion, an OH–. When water contains exactly the same amount of H3O+ and OH– ions, then water is considered to be neutral or balanced. Too much H3O+ and it is acidic. Too much OH– and it becomes alkaline.

Acidity is measured on the pH scale. In this scale, a neutral solution (such as pure water) has a value of 7. Smaller numbers indicate more acidity, and larger numbers indicate more alkalinity. The scale is logarithmic: thus, one number on the scale represents a factor of ten. In other words, a solution with a pH of 5 is ten times as acidic as a solution with a pH of 6.

How to Measure pH


There are two main ways to measure pH. Farmers who need to monitor the acidity of their soil usually use test strips, known as litmus paper, that contain universal indicator. When the test strips turn pink, they indicate an acidic environment. When they turn blue, they indicate an alkaline one. The range of colours between pink and blue can be used to estimate where, on the pH scale, a substance sits. This is not a precise system of measurement.