pH

What is pH?

pH is a measure of the acidity, or hydrogen ion concentration, of an aqueous (watery liquid). Every water solution contains H+ ions. pH is the negative common logarithm of the hydrogen ion concentration expressed in mol/L.

Most of us know that water is H2O. When we draw it more correctly as H-O-H we can see that water can dissociate into H+ and OH-. Neutral water has a pH of 7. If more acid (H+ ions) is added the pH drops. The lower the pH the more acid the solution, however, because it is a logarithmic scale, a pH of 5 is 10 times more acid than a pH of 6, and a pH of 4 is 100 times more acid than a pH of 6. A pH of say 1 is very acid indeed.

Conversely, as solutions become less acid, or more alkaline, the pH rises above 7. A solution of pH 12 is very alkaline.

Some typical pH values are:

Most substances have a pH in the range 0 to 14, although pH values outside this range do occur

pH may be measured by use of a pH meter or by using special indicator papers such as litmus which change colour at known pH values. pH may also be measured by titration.

pH effects can be very marked in the environment. More than 70% of Australian soils have either a very high or low pH. Australian soils are acidic because they are geologically old and have been leached of most of their minerals except sands and metal oxides, and in many cases they are residual sand deserts from earlier periods of aridity. Naturally acidic soils occupy about one third of Australia, but many agricultural soils in the Intensive Land-use Zone become more acidic as the result of removal of calcium in harvested product, leaching of nitrate and calcium from nitrogen-producing pastures, and use of acidifying fertilisers. The process is particularly rapid in high-rainfall pasture-based systems. Acid sulfate soils, generally found in low lying coastal areas, are not acid until they dry out and oxidise, releasing much sulfuric acid.

When rock surfaces are exposed to air and rain, a reaction can occur with the elements in the rock resulting in a change in the characteristics of the water that drains off. If the rock contains sulphides, a natural oxidation process can acidify the water. This is known as acid drainage (also acid rock drainage or acid mine drainage). As the water becomes more acidic, its capacity to leach out other elements from the rock, such as metals, increases. The resulting drainage can become very acidic and contain a number of harmful constituents. Although this is a natural process, mining activities can trigger this phenomenon by exposing large surface areas of rock to water and oxygen. The most significant newly exposed rock surfaces are in the fragmented pieces of waste rock that are removed from the ground and placed in dumps.

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