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Water PH in South Africa

Written by Janice M on . Posted in Water & Environmental

What is pH?

The term pH derives from the French word, “pouvoir hydrogene” and means “hydrogen power”. pH is a measurement of the concentration of hydrogen ions in a solution.

Because of its mathematical formulation, low pH values are associated with solutions with high concentrations of Hydrogen ions, whilst high pH values occur for solutions with low concentrations of Hydrogen ions.

Pure water has a pH of 7.0, and other solutions are usually described with reference to this value.

Acids are defined as those solutions that have a pH of less then 7 (ie more Hydrogen ions than water); while bases are defined as those solutions that have a pH greater than 7 (ie less Hydrogen ions than water).

pH value of pure water = -log {H+} = – log 10 -7 = 7

pH is one of the most important parameters measured in a wide variety of industries such as water and wastewater treatment, agricultural research and production, environmental monitoring, etc. For example, the pH levels for drinking water should be pH 6.5 – 8.5.

Measurement of pH

Litmus paper is one of the most common methods of pH measurement. However, it only provides a rough indication that is insufficient in many applications.

The more accurate method involves the use of a measurement system that consists of a pH meter and a pH electrode that has a Hydrogen ion sensitive glass bulb. The most common sensing element used in the electrode is the glass membrane as it is selective for H+ ions, ie H+ ions can permeate through the hydrated layer of glass membrane. However, the electrode body may not necessarily be glass.

The movement of ions into the hydrated membrane changes the electrochemical effect inside the glass which is measured in mV and then converted via the pH meter to be reflected as a pH value.

Therefore depending on the concentration of ions in the solution, the mV and hence pH varies.

Offset in pH electrode

Theoretically, when placed in pH7 buffer at 25°C, a pH electrode produces 0 mV which the pH meter reads as 7.00 pH. The difference between 0 mV and the electrode’s actual reading is called the offset error which can be as high as 25 mV.

In other words, when the electrode is not in measurement or in pH7 buffer solution, the output (or reading) is known as the offset.

While in theory, the mV value should be zero, in practice this is rarely the case because of the following reasons:

– Liquid difference

– Bulb composition

– Wire geometry difference and other factors.

In practice, it is unrealistic to achieve zero offset in electrodes.

Slope in pH electrode

A pH electrode produces mV in different solutions.Therefore, the slope of the electrode can be defined as follows:

Slope = mV / pH unit

The slope of a new pH electrode should fall between 92% and 102% of 59.16 mV. If the slope falls below 92%, cleaning of the electrode may be required.

Temperature Compensation

In a perfect electrode – one that measures zero at exactly pH7 – there is no temperature effect on the electrode sensitivity at pH7, regardless of temperature change. Most pH electrodes are not perfect, but the errors from changes in temperature are still very minute when near pH7, plus or minus one-tenths of a pH, and can be disregarded. However, the further from pH7 the solution is, the greater the expected measurement error due to changes in the electrode sensitivity. For most electrodes, the error is approximately 0.003 pH / C / pH away from pH 7.

For example, if a pH meter is calibrated at room temperature (25°C) and is measuring a sample around pH4 at around 5°C,

Temperature difference: 25°C – 5°C = 20°C

pH away from neutral: 7 pH – 4 pH = 3 pH

Total error: 0.003 x 20 x 3 = 0.18 pH

To overcome this error, pH meters require some form of temperature compensation to ensure standardised pH values. Meters and controllers with Automatic Temperature Compensation (ATC) receive a continuous signal from a temperature sensing element and automatically correct the pH value based on the temperature of the solution. Manual Temperature Compensation requires the user to enter the temperature of the solution in order to correct pH readings for temperature. ATC is considered to be more practical for most pH applications.

Factors affecting pH

The concentration of carbon dioxide in the water

Carbon dioxide (CO2) enters a water body from a variety of sources, including the atmosphere, runoff from land, release from bacteria in the water and respiration by aquatic organisms. This dissolved CO2 forms a weak acid. Natural, unpolluted rainwater can be as acidic as pH 5.6, because it absorbs CO2 as it falls through the air. Because plants take in CO2 during the day and release it during the night, pH levels in water can change from daytime to night.

Geology and soils of the watershed

Acidic and alkaline compounds can be released into water from different types of rock and soil. When calcite (CaCO3) is present, carbonates can be released, increasing the alkalinity of the water, which raises the pH. When sulfide minerals, such as pyrite are present, water and oxygen interact with the minerals to form Sulfuric acid. This can significantly drop the pH of water. Drainage from forests and marshes is often slightly acidic, due to the presence of organic acids produced by decaying vegetation.

Drainage from mine sites

Mining for gold, silver, and other metals often involves the removal of sulfide minerals buried in the ground. When water flows over or through sulfidic waste rock or tailings exposed at a mine site, this water can become acidic from the formation of sulfuric acid. In the absence of buffering material, such as calcareous rocks, streams that receive drainage from mine sites have low pH levels.

Air Pollution

Air pollution from car exhaust and power plant emissions increases the concentrations of nitrogen oxides and sulfur dioxide in the air. These pollutants can travel far from their place of origin, and react in the atmosphere to form nitric acid and sulfuric acid. These acids can affect the pH of streams by combining with moisture in the air and falling to earth as acid rain or snow.

pH values in South Africa

According to the National Water Resource Quality Status Report – First Edition June 2002, pH values vary across the country. pH was low in the Klip Spruit (of the Olifants WMA) and would likely result in irritation of the mucous membranes of water users in this area. It is likely that the source of the low pH is the acid mine drainage from the coal mines and mine dumps in the area. A notable effect of the low pH would be “burning eyes” with the use of the water for recreational purposes.

From an irrigated agriculture use perspective, there were high pH levels in the Luvuvhu and Letaba, Crocodile West and Marico, Olifants, Usuntu to Mhlatuze, Mzimvubu to Keiskamma, Upper Orange and lower Orage WMAs.

The Fish to Tsitsikamma, Gouritz and South Western Cape (Breede and Berg) WMAs had low pH values, making irrigated agriculture in these areas more challenging.

Water Quality Standards for pH

According to the DWA Requirements for the Purification of Waste Water or Effluent Water, the pH should be 5.5 – 7.5.

Very high (greater than 9.5) or very low (less than 4.5) pH values are unsuitable for most aquatic organisms.

Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5 and may die at these low pH values. High pH levels (9 – 14) can harm fish by denaturing cell membranes.

Changes in pH can also affect aquatic life indirectly by altering other aspects of water chemistry. Low pH levels accelerate the release of metals from rocks or sediments in the stream. These metals can affect a fish’s metabolism and the fish’s ability to take in water through the gills and can kill fish fry.

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Historical comments (4)

  • Anonymous

    |

    Hello,

    A few months ago, a group and I went to a stream to test test the affect of pH levels on Macro inveretebrate populations. We tested throughout the stream in five places, three trials each time and found the pH level always came to about 6.0 as the average. This had no visible affect on the invertebrates but I’m sure if we had varied results of pH levels, we could have seen some real results. If we would have received more varied results, would the acidity of the stream water have affected the Macro invertebrate populations and caused them to decrease?

    • Janice

      |

      Aquatic Macro Invertebrates can tolerate a wide range in pH. Acidity is not equivalent to pollution. High levels of oxygen will for instance tend to make water alkaline whereas CO2 will make it acidic. The pH can for instance vary between day and night due to the presence of aquatic plants. Calcium rich streams will have a pH of 8. The composition of the aquatic invertebrate community will change, depending on the extent of pollution as some have a wider tolerance than others.

  • proximate analyzer

    |

    Great delivery. Solid arguments. Keep up the great effort.

    • Janice M

      |

      Thank you for taking the time to post a comment.

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