Overview
pH describes the acidity or basicity of a solution. A low pH indicates acidic conditions, while a high pH indicates basic conditions. pH is actually a measure of the amount of hydrogen ions in a solution which can be thought of as the power of hydrogen. A low pH has many free hydrogen ions in the water, whereas a high pH has few. Technically, pH is the negative logarithm of the free hydrogen activity in a solution.
Influencing Factors
Alkalinity is important in protecting water against acid pollution. Alkaline waters are more resistant to pH change than lower alkaline waters. For instance, when acid enters water with a high alkalinity, the pH of the water does not change as much as it would if there was no alkalinity. Most alkaline materials come from the surrounding rocks and the local geology of the area. The most common alkaline substance is limestone.
Photosynthesis, respiration, and decomposition have a large effect on pH in aquatic systems because they affect carbon dioxide levels in the water. Carbon dioxide combines with water to produce a mild acid called carbonic acid, thus lowering pH levels. While respiration and decomposition lower pH because they add carbon dioxide, photosynthesis increases pH because it removes carbon dioxide from the water. The combination of these processes leads to daily fluctuations in pH, especially during the summer when the rate of photosynthesis is high during the day, and the rate of respiration is high at night.
Although air pollution can originate from many sources, the burning of fossil fuels is particularly significant. Burning coal, gas, and oil emits nitrous oxide and sulfur dioxide gases which combine with atmospheric water to produce acid precipitation. Acid precipitation falls and lowers the pH of surface water.
Mining, industry, and factory discharges can have drastic effects on pH. Acid mine drainage is a common form of water pollution in mining areas. Factory and industry can cause drastic rises or falls in the pH of surrounding waters. Industrial strength cleaners and chemicals often have extreme pH values, which can devastate aquatic life if released untreated.
Importance
pH is very important and is used to monitor safe water conditions. A lot of aquatic life cannot live below a pH of 5 or above a pH of 9. Once the normal pH range for an aquatic environment has been established, a rise or fall in pH can indicate chemical pollution or acid rain.
The pH of natural waters typically ranges from 6 to 9. The pH of marine waters is close to 8.2, whereas most natural freshwaters have pH values in the range from 6.5 to 8.0. Pure rainwater and snow have a pH around 5.6. The pH of rain and snow is naturally acidic due to carbon dioxide in the air, which dissolves in the water to produce a mild acid. Rain is not considered to be acid rain until the pH drops below 5.5.
Measurement
pH is measured on a scale of 0 to 14. A pH of 7 is neutral, a pH below 7 is acidic, and a pH greater than 7 is basic. A lower number represents a greater number of hydrogen ions, and each pH unit represents a 10-fold change in the hydrogen ion concentration. For instance, a pH of 6 has ten times more free hydrogen ions than a pH of 7, and a hundred times more free hydrogen ions than a pH of 8.
The most accurate way to measure pH is with an electric pH probe. When the probe is immersed in water, it measures the pH of the water relative to the pH of a solution within the probe. The pH probe electrode has a glass bulb filled with a pH 7 solution. The difference between the pH in the water and the pH 7 solution within the glass bulb generates an electric potential, which is measured relative to a stable electric potential generated by a reference electrode within the pH probe. The pH of the solution is then determined by the difference between these two electric potentials.