Water Quality - Chemical Characteristics

1) pH value of Water/Hydrogen Ion Concentration

The acidity or alkalinity of water is measured in terms of its pH value or H-ion concentration. pH value is the logarithm of reciprocal of hydrogen ion activity in moles per litre. Depending upon the nature of dissolved salts and minerals, water may be acidic or alkaline. When acids or alkalis are dissolved in water, they dissociate into electrically charged hydrogen and hydroxyl radicals, respectively. pH of natural water is generally in the range of 6 - 8. Industrial wastes may be strongly acidic or basic and their effect on pH value of receiving water depends on the buffering capacity of receiving water. It is desirable to maintain pH value of water very close to 7. The acidic water causes tuberculation and the alkaline water causes incrustation. For potable water, the pH value should be between 7 and 8.50. Following are the two methods which are employed to measure the pH value of water.

a) Electrometric Method

In this method, potentiometer is used to measure the electrical pressure exerted by positively charged H-ions. The pH value is then correspondingly expressed.

b) Colourimetric Method

In this method, chemical reagents are added to water and the colour produced is compared with standard colours of known pH values. A set of sealed tubes containing coloured waters of known pH values is kept in the laboratory for ready reference. This test is simple and hence, it is commonly carried out in public health laboratories. The usual indicators are Benzol yellow, Methyl red, Bromphenol blue, etc., for acidic range and Thymol blue, Phenol red, Tolyl red, etc. for alkaline range.

2) Chlorides

The chloride contents, especially of sodium chloride or salt, are worked out for a sample of water. The excess presence of sodium chloride indicates pollution of water due to sewage, minerals, etc. and is dangerous and unfit for use. The water has lower contents of salt than sewage due to the fact that salt consumed in food is excreted by body. For potable water, the highest desirable level of chloride content is 200 mg/litre and its maximum permissible level is 600 mg per litre.

The natural waters near the mines and sea dissolve sodium chloride and also presence of chlorides may be due to mixing of saline water and sewage in the water. Sodium chloride is the main substance in chloride water. The natural water near the mines and sea has dissolved sodium chloride. Similarly, the presence of chlorides may be due to the mixing of saline water and sewage in the water. Excess of chlorides is considered as dangerous and makes the water unfit for many uses. Chloride content is determined by titrating the wastewater with silver nitrate and potassium chromate. Appearance of reddish colour confirms presence of chlorides in water. The measurement of chloride contents is carried out as follows.

1. 50 cc of sample of water is taken by pipette in a porcelain dish.
2. Two or three drops of potassium chromate solution are added to the sample of water.
3. The chloride content is then determined by titrating with standard solution of silver nitrate.
4. The silver reacts first with all chlorides and silver chloride thus formed then reacts with potassium chromate.
5. The silver chromate appears as reddish precipitate and the amount of silver nitrate required to produce such reddish precipitate determines the amount of chlorides present in water.

3) Dissolved Gases

The water contains various gases from its contact with the atmosphere and ground surfaces. The usual gases are nitrogen, methane, carbon dioxide and oxygen. In addition, water may contain some amount of hydrogen sulphide and ammonia depending upon the pH and anaerobic/aerobic condition of water. The contents of these dissolved gases in a sample of water are suitably worked out. The methane concentration is to be studied for its explosive property. The hydrogen sulphide gives disagreeable odour to water even if its amount is very small. The carbon dioxide content indicates biological activities, causes corrosion, increases solubility of many minerals in water and gives taste to water. Dissolved oxygen is necessary for sustenance of aquatic life in water and to keep it fresh. Oxygen in the dissolved state is obtained from atmosphere and pure natural surface water is usually saturated with it. The simple test to determine the amount of dissolved oxygen present in a sample of water is to expose water for 4 hours at a temperature of 27°C with 10% acid solution of potassium permanganate. The quantity of oxygen absorbed can then be calculated. This amount, for potable water, should be about 5 to 10 ppm.

4) Hardness

The hardness or soap-destroying power of water is of two types – temporary hardness and permanent hardness. The temporary hardness is also known as carbonate hardness and it is mainly due to the presence of bicarbonates of calcium and magnesium. It can be removed by boiling or by adding lime to the water. The permanent hardness is also known as non-carbonate hardness and it is due to the presence of sulphates, chlorides and nitrates of calcium and magnesium. It cannot be removed by simply boiling the water. It requires special treatment of water softening.

Total hardness = Carbonate hardness or alkalinity + Non carbonate hardness

The excess hardness of water is undesirable because of various reasons such as it causes more consumption of soap, affects the working of dyeing system, provides scales on boilers, causes corrosion and incrustation of pipes, makes food tasteless, etc. Presence of hardness in water prevents the lathering of the soap during cleaning of clothes, etc.

Hardness is usually expressed in mg of calcium carbonate per litre of water. Hardness is generally determined by Versenate Method. In this method, the water is titrated against EDTA salt solution using Eriochrome Black T as indicator solution. While titrating, colour changes from wine red to blue. In general, under a normal range of pH values, water with hardness up to 75 mg/L are considered as soft and those with 200 mg/L and above are considered as hard. In between, the water is considered as moderately hard. Underground water is generally harder than the surface water, as they have more opportunity to come in contact with minerals. For boiler feed water and for efficient cloth washing, etc., the water must be soft. However, for drinking purposes, water with hardness below 75 mg/L is generally tasteless and hence, the prescribed hardness limit for drinking ranges between 75 to 150 mg/L.

5) Alkalinity

The alkalinity is the capacity of a given sample to neutralize a standard solution of acid. The alkalinity is due to the presence of bicarbonate (HCO₃), carbonate (CO₃) or hydroxide (OH). The determination of alkalinity is very useful in waters and wastes because it provides buffering to resist changes in pH value. The alkalinity is usually divided into the following two parts. Total alkalinity i.e. above pH 4.5 and Caustic alkalinity i.e. above pH 8.2. The alkalinity is measured by volumetric analysis. The commonly adopted two indicators are given below.

1. Phenolphthalein : Pink above pH 8.5 and colourless below pH 8.2
2. Methyl Orange : Red below pH 4.5 and yellow orange above pH 4.5

6) Acidity

Acidity is a measure of the capacity of water to neutralise bases. Acidity is the sum of all titrable acid present in the water sample. Strong mineral acids, weak acids such as carbonic acid, acetic acid present in the water sample contributes to acidity of the water. Usually dissolved carbon dioxide (CO₂) is the major acidic component present in the unpolluted surface waters. The volume of standard alkali required to titrate a specific volume of the sample to pH 8.3 is called phenolphthalein acidity (Total Acidity). The volume of standard alkali required to titrate a specific volume of the water sample (wastewater and highly polluted water) to pH 3.7 is called methyl orange acidity (Mineral Acidity).

Acidity interferes in the treatment of water. Carbon dioxide is of important considerations in determining whether removal by aeration or simple neutralisation with lime /lime soda ash or NaOH will be chosen as the water treatment method. The size of the equipment, chemical requirements, storage spaces and cost of the treatment all depends on the carbon dioxide present. Aquatic life is affected by high water acidity. The organisms present are prone to death with low pH of water. High acidity water is not used for construction purposes. Especially in reinforced concrete construction due to the corrosive nature of high acidity water. Water containing mineral acidity is not fit for drinking purposes. Industrial wastewaters containing high mineral acidity is must be neutralized before they are subjected to biological treatment or direct discharge to water sources.

7) Total Solids

Total solids include suspended and dissolved solids. In this test, the amounts of dissolved and suspended matter present in water are determined separately and then added together to get the total amount of solids present in water. The highest desirable level of total solids is 500 mg/litre and its maximum permissible level is 1500 mg/litre. For measuring suspended solids, water is filtered through a fine filter and dry material retained on the filter is weighed. The filtered water is evaporated and weight of residue that remains on evaporation represents the amount of dissolved solids in water. Total solids can also be considered as the sum of organic and inorganic solids. Amount of inorganic solids can be determined by fusing the residue of total solids in a muffle-furnace and weighing the fused residue. Amount of organic solids is the difference between the amount of inorganic and total solid.

8) Nitrogen and its Compounds

The nitrogen is present in water in the following four forms. The presence of nitrogen in the water indicates the presence of organic maters in the water. 

• Free ammonia 
• Albuminoidal ammonia 
• Nitrites 
• Nitrates

The amount of free ammonia in potable water should not exceed 0.15 ppm and that of albuminoidal ammonia should not exceed 0.3 ppm. The term albuminoidal ammonia is used to represent the quantity of nitrogen present in water before decomposition of organic matter has started. The presence of nitrites indicates that the organic matter present in water is not fully oxidized or in other words, it indicates an intermediate oxidation stage. The amount of nitrites in potable water should be nil.

The presence of the nitrites in the water, due to partly oxidized organic matters, is very dangerous. Therefore, in no case nitrites should be allowed in the water. For potable water, the highest desirable level of nitrates is 45 mg per litre. The free ammonia is measured by simply boiling the water. The ammonia gas is then liberated. The albuminoidal ammonia is measured by adding strong alkaline solution of potassium permanganate to water and then boiling it. The ammonia gas is then liberated. The nitrites and nitrates are converted chemically into ammonia and then measured by comparison with standard colours. The nitrites are rapidly and easily converted to nitrates by the full oxidation of the organic matters. The presence of nitrates is not so harmful. But nitrates > 45 mg/L can cause “mathemoglobinemia” disease to the children.

9) Chlorine

Dissolved free chlorine is never found in natural waters. It is present in the treated water resulting from disinfection with chlorine. The chlorine remains as residual in treated water for the sake of safety against pathogenic bacteria. Residual chlorine is determined by the starch-iodide test. In starch-iodide test, potassium iodide and starch solutions are added to the sample of water due to which blue colour is formed. This blue colour is then removed by titrating with sodium thiosuplhate solution and the quantity of chloride is calculated. On the addition of ortho-iodine solution if yellow colour is formed, it indicates the presence of residual chlorine in the water. The intensity of this yellow colour is compared with standard colours to determine the quantity of residual chlorine. The residual chlorine should remain between 0.5 to 0.2 mg/L in the water so that it remains safe against pathogenic bacteria.

10) Iron and Manganese

These are generally found in ground water. The presence of iron and manganese in water makes it brownish red in colour. Presence of these elements leads to the growth of micro-organism and corrodes the water pipes. Iron and manganese also causes taste and odour in the water. The quantity of iron and manganese is determined by colorimetric methods.

11) Lead and Arsenic

These are not usually found in natural waters. But sometimes lead is mixed up in water from lead pipes or from tanks lined with lead paint when water moves through them. These are poisonous and dangerous to the health of public. The presence of lead and arsenic is detected by means of chemical tests.

Water contains various types of minerals and metals such as, copper, barium, cadmium, selenium, fluoride etc. Arsenic and selenium are poisonous; therefore, they must be removed totally. Human lungs are affected by the presence of high quantity of copper in the water. Fewer cavities in the teeth will be formed due to excessive presence of fluoride in water. The quantity of the metals and other substances can be done indirectly by colorimetric methods using UV-visible spectrophotometer or directly by the use of sophisticated instruments such as Atomic Absorption Spectrophotometer (AAS), Atomic Emission Spectrophotometer (AES), Inductively Coupled Mass Spectrophotometer (ICP-MS) etc.