What is pH?

by Winston Kao

The term pH comes from the French p(ouvoir) H(ydrogene), which literally means hydrogen power.

So let’s start looking at chemistry from the simplest point of view.

What is pH? It is the concentration of H+ (hydrogen) in a solution.

Unlike most scales which are linear scales [for example, difference between 1.0, 2.0 and 3.0 all have equal distances or quantities of 1 division for each number], the pH Scale is a logarithmic scale, in other words, the difference between pH 1 & pH 2 is 10; and the difference between pH 1 & pH 3 is 100. Example: There is ten trillion times more hydrogen in pH 1 than there is in pH 14.

The chart below shows the difference in Hydrogen concentration between between pH numbers:

14 pH and 1 pH is 10,000,000,000,000 H+ – – – – – – – – – – – – – – strong acid

14 pH and 2 pH is 1,000,000,000,000

14 pH and 3 pH is 100,000,000,000

14 pH and 4 pH is 10,000,000,000

14 pH and 5 pH is 1,000,000,000

14 pH and 6 pH is 100,000,000 – – – – – – – – – – – – – weak acid

14 pH and 7 pH is 10,000,000 – – – – – – – – an arbitrary mid-point, called “neutral”

14 pH and 8 pH is 1,000,000 – – – – – – – – – – – – – weak alkaline

14 pH and 9 pH is 100,000

14 pH and 10 pH is 10,000

14 pH and 11 pH is 1,000

14 pH and 12 pH is 100

14 pH and 13 pH is 10

14 pH and 14 pH is 0- – – strong alkaline

In other words pH 1 [the stongest acid] has ten trillion times more hydrogen than pH 14 [the strongest alkaline]. Looking at it from another point of view, pH 14 has (1x10 to negative 14 power) (1x10–14) LESS hydrogen than pH 1.

So for a soil pH to change from a high pH of 8 down to a pH of 5 it is not a factor of 3; because the pH Scale is a logarithmic scale, it is a factor of 1000. Therefore one has to add 1000 times more acid to bring down the pH. In other words, one has to add 1000 times more H+.

Difference between 5 pH and 6 pH is 10

Difference between 5 pH and 7 pH is 100

Difference between 5 pH and 8 pH is 1000

Winston W. J. Kao