Acidity and alkalinity are important for many biological processes. Azaleas will only grow well in acid soils. Saliva is alkaline but your stomach contents are acid. Healthy urine is acidic but infected urine can be alkaline.
The previous paragraph raises several questions like – how acid does soil need to be to grow azaleas?
We can answer questions like this by defining a scale of acidity or alkalinity – this scale is shown below and is called the pH scale.
A neutral solution is defined to have a pH value of 7. Why? Because in every mole of water there are 10-7 moles of H+ ions and 10-7 moles of OH– (post 17.49); because there are equal numbers of these ions, water is neutral (see posts 17.49 and 17.50). (See post 16.7 to find out what 10-7 means and post 17.48 to find out about moles.)
A solution that is 10 times more acidic than water has a pH value of 6 because 10-7 × 10 = 10-6 (see post 16.7); a solution that is 100 times more acidic than water has a pH value of 5 because 10-7 × 100 = 10-5, and so on.
A solution that is 10 times less acidic than water (so 10 times more alkaline) has a pH value of 8 because 10-7/10 = 10-8 (see post 16.7); a solution that is 100 times less acidic than water (so 100 times more alkaline) has a pH value of 9 because 10-7/100 = 10-9, and so on.
How do we measure pH? There are two different ways. The most reliable is to use a pH meter. This meter measures the potential difference (post 17.44) for moving H+ ions between the solution and a reference solution. The H+ ions in an acidic solution will tend to repel H+ ions (because they both have a positive charge – see post 16.25) and so has a higher potential than the reference solution. The OH– ions in an alkaline solution will tend to attract H+ ions (because they have unlike charges – see post 16.25) and so has a lower potential than the reference solution. A simpler method for measuring pH is to use universal indicator paper. This paper contains several dyes that change colour when the pH changes. As each dye changes colour there is an overall change in the colour of the paper – so the colour of universal indicator paper tells us the pH of a solution that we dip it into. Many dyes change colour in this way – litmus changes colour from red to blue when the pH value rises above between 4.5 and 8.3. If you grow hydrangeas in soil whose pH value is less than 5.5, its flowers will be blue; above 6.5, the flowers are pink.
The concept of pH is defined for solutions and pH measurements are made on liquids. So, how can we measure the pH of soil? The answer is that soils are wet and contain liquid trapped between their tiny lumps of solid.
The pH of a solution depends on the strengths (posts 17.49 and 17.50 of any acid or alkali present in the solution and on their concentrations, because both determine the numbers of H+ and OH– ions. In order to calculate the pH of a solution we need to know some mathematics covered in the next two posts.