Acidity and Alkalinity
The pH scale is a measure of the acidity or alkalinity of a liquid. The scale ranges from 0 to 14 with a pH of 7 being neutral.
- A pH greater than 7 indicates alkalinity – the higher the pH, the greater the alkalinity.
- A pH less than 7 indicates acidity – the lower the pH, the greater the acidity.
It is not generally realised that pH is a logarithmic scale. Each unit of pH change represents a ten times change in acidity or alkalinity.
Acid rain is a good example of this point. Unpolluted rain has a pH of 5.6 whilst highly polluted rain can have a pH of below 3.0. There are nearly 3 pH units difference between unpolluted and acid rain indicating a 1000 fold increase in acidity. It is little wonder that our ancient cathedrals are being rapidly damaged by acid rain. Examples of the pH of everyday items are given as illustration:
- Apples 2.9 – 3.3
- Limes 1.8 – 2.0
- Rhubarb 3.10 – 3.20
- Vinegar 2.40 – 3.40
- Bananas 4.50 - 5.20
- Honey 3.70 - 4.20
- Potatoes 5.40 - 5.90
The pH of tap water should be in the range of 6.5 to 8.5 which is around neutral. Water Authorities will often control the pH of water to within this range because some water sources tend to be acidic. Lime and sodium carbonate are added to make the water less acidic.
Water which is too acidic will be corrosive to metals, especially the lead in lead piping. Controlling the pH reduces the corrosive nature of the water and, therefore, the concentration of lead in the water. Too high a pH reduces the effectiveness of chlorination and leads to the risk of bacterial proliferation.
From the foregoing it can be seen that the pH can give much information about the nature of the water. A low pH, for instance, will indicate aggressive water which can give high lead levels from lead piping. A high pH could suggest insufficient available chlorine with the possibility of the presence of bacteria. A high pH may also be a result of the presence of alkaline hardness which will suggest that the water will have scale-forming tendencies.
One other use of pH is as an indicator to the contamination of water. A low pH was recorded by the South West Water Authority when aluminium sulphate was accidentally added to the water supply at Camelford. Unfortunately, it was 24 hours before the cause of the low pH was found. By then the contamination had spread throughout the town water supply.