Notes

In this module, you will be introduced to pH and buffering. pH is the measurement of the activity of hydrogen ions (H+) in a solution. We express pH as a number, but without units. In general, pH values are governed by a numeric scale that goes from 0 to 14. This is known as the pH scale. On the pH scale, values that are in between 0 and 6 are acidic. Those values that are in between 8 and 14 are basic or alkaline. Values that are at 7 are considered neutral. A buffer is an agent that adjusts the pH of a given solution. A buffer will drive a basic or acidic solution to a certain pH and then prevent the pH of this solution from changing.

pH and pOH

As mentioned previously, pH is a measure of the activity of hydrogen ions in a solution. When we refer to the activity of hydrogen ions that are being measured, we express the ions as a concentration denoted, or symbolized [H+]. From the pH scale, we can determine the extent, or amount, of acidity and alkalinity of a given solution. The range on the pH scale lies in between 0 to 14. Zero is highly acidic, and 14 is highly basic, or alkaline. A pH value of 7 is considered to be neutral. On the pH scale, an increase in the pH value by one number means we actually have a ten-fold decrease in the concentration of hydrogen ions [H+]. For example, if we have an increase in pH from 1 to 2, we will have a decrease (by an order of 10 times less) in the H+ concentration. If we increase the pH from 2 to 4, we have a decrease in the H+ concentration of 100 times (10 X 10).

pOH is the measurement of the activity of hydroxide ions OH- in a given solution. Similar to the pH, we are measuring the activity of the concentration of OH-, expressed as [OH-]. We classify the value pOH similar to pH. The exception is that for a pOH value in between 0 and 6, we have an alkaline solution. A pOH value between 8 and 14 yields an acidic solution, and a pOH value of 7 is neutral. The illustration below shows a pH scale with the relationship between pH and pOH and acidity and alkalinity