Chemical characteristics of water

Chemical tests are performed to determine chemical characteristics of water. These chemical tests are listed below.

1.  Total solids and suspended solids.

2.  pH value of water

3.  Hardness of water

4.  Chloride content

5.  Nitrogen content

6.  Harmful metals and chemical substances

7.  Dissolved gases

8.  Biochemical oxygen demand

Total solids and suspended solids: The total amount of solids present in water can determined by evaporating sample of water and weighing dry residue left. The suspended solids can be found by filtering the water and weighing the residue left on filter paper. 

The difference between the total solids and suspended solids will represent dissolved solids. The permissible limits solids range from 500 to 1000ppm.

pH value of water: The pH value of water indicates the logarithm of reciprocal of hydrogen ion concentration present in water. It is a indicator of acidity or alkalinity of water.

The pH value of water is measured using two methods namely

1.  Electric potential using Potentiometer.

2.  Colour indicators

pH of water more than 7 indicates alkaline and less than 7 shows acidic. The permissible pH values of drinking water range from 6.6 to 8.5.

Hardness of water: Hardness in water is that characteristic which prevents the formation of sufficient foam, when such hard waters are mixed with soap.

Hardness is caused by calcium and magnesium salts present in water. Hard water lead to greater soap consumption, scaling of boilers, causes corrosion, incrustation of pipes and makes food tasteless.

Bicarbonates and carbonates of calcium and magnesium are responsible for temporary hardness. Whereas, sulphates, chlorides and nitrates of calcium or magnesium responsible for permanent hardness. Hardness limit for drinking purposes ranges from 75 to 115ppm.

Chloride content: The presence of chloride content in river or stream indicates pollution due to sewage or industrial wastes. Therefore chloride content in water should be regularly tested. The chloride content of treated water should not exceed 250ppm.

Nitrogen content: The presence of Nitrogen in water is am indication of the presence of the organic matter. It occurs due to 4 forms of nitrogen.

• Free ammonia: The presence of free ammonia indicates first stage of decomposition of organic matters. Free ammonia for drinking water should not exceed 0.15ppm
• Albuminoid: The presence of albuminoid indicates the pollution in water. It should not exceed 0.3ppm.  
• Nitrites: Nitrites are highly dangerous. It should not be present in water.
• Nitrates: High quantity of nitrates causes mathemoglobinemia (Blue baby diseases). The nitrate concentration in drinking water should be limited to 45ppm.

Dissolved gases: The various gases dissolves into water due to contact with atmosphere or ground surface. High quantity of Methane gas has a explosive behaviour.  Hydrogen sulphide (H₂S) in small quantity will give bad taste and odour. The presence of carbon dioxide indicates polluted water. The presence of less oxygen quantity also polluted water.

Harmful metals and chemical substances: Tests are carried out to determine the amounts various metals and substances such as iron, manganese, copper, lead, barium, cadmium, arsenic, selenium, fluorine present in water.

Iron and manganese concentrations should not be greater than 0.3ppm and 0.05ppm. Lead and barium should not present in water due to high toxicity. The fluoride concentration limit should not exceed 1.5ppm.

Biochemical oxygen demand (BOD): The amount of oxygen required in the process until oxidation get completed is known as BOD. If sufficient oxygen is present in water, the aerobic bacteria will flourish and cause the biological decomposition of waste and organic matter.

The BOD is calculated for 5 days at 20^oC is taken as oxygen demand. It is measured using dilution method.BOD of drinking water must be zero, which indicates no organic matters are present.