The Meaning of the pH of Water. Another Installment in Pure Water Annie’s Water
Pure Water Annie’s technical articles appear regularly in the Pure Water Occasional.
Treatment 101 Series.
The term pH is used to describe the activity of the hydrogen ion in a solution. It measures the relative acidity and alkalinity of the solution. It is not a measure of quantity but of the relationship between acidity and alkalinity.
The pH scale ranges from 0 to 14, with a value of 7 considered neutral. Pure water is considered to be neutral, pH 7.0, although most water ranges between 6.0 and 8.0 on the scale. Values above 7.0 are considered alkaline and below 7.0 acidic.
The pH value of water decreases as the amount of carbon dioxide, CO2, increases, and pH increases as the amount of bicarbonate alkalinity increases. The ratio of carbon dioxide and bicarbonate alkalinity within the ranges of 3.6 to 8.4 is an indication of the pH value of the water.
The pH of water figures into almost all aspects of water treatment. The reduction of such contaminants as iron, manganese, arsenic, fluoride, and hydrogen sulfide is highly dependent on the pH of water. Water that is low in pH is corrosive and must be corrected. Even disinfection of water with chlorine is highly dependent on pH.
As far as health is concerned, there is no evidence to support claims by sellers of pH amending machines that only highly alkaline water is suitable for drinking.
pH Adjustment in Water
The pH can be raisedby feeding soda ash (sodium carbonate), caustic soda (sodium hydroxide), sodium
bicarbonate, or potassium hydroxide into the water. Calcite (calcium carbonate, CaCO3) and/or Corosex (MgO—magnesium oxide) are used as filters to increase pH from the 5 and 6 range. The peak flow of neutralizing filters is limited to about 6 gpm per square foot of medium. Downflow filters must be backwashed frequently to prevent “cementing” of the bed. Upflow filters do not experience cementing of the bed, but they do not work if iron is present in the water.
Lowering pH is less frequently done, but can be accomplished by feeding a variety of acids with a standard metering pump. Both sulfuric and hydrochloric acids will work, but weaker acids are usually preferred. The most commonly used are phosphoric acid (H3PO4), acetic acid (CH3COOH), and citric acid (C6H8O7). Vinegar can also be injected to lower pH.
