Chemical parameters of water quality/ Chemical characteristics of water
- These are some chemical aspects of water quality that helps to determine whether water is polluted or not.
1. Chloride ion:
- Normally all types of water contains chloride ion but its concentration is very low in natural water system.
- Chloride ion concentration increases in case of urine and sewage contaminated water.
- High concentration of chloride ion give salty taste and also corrodes pipelines of water.
- Normally 150mg/ltr of chloride ion is harmless.
- Maximum permissible limit of chloride ion in drinking water is 200mg/ ltr.
2. Ammonia:
- In water ammonia come from decomposition of organic matter like protein, amino acids etc. Its concentration also increases during water disinfection process using chloramine.
- In water Ammonia (NH3) is first oxidized into nitrite and then into nitrate. Therefore by measuring the concentration of NH3, nitrite and nitrate, we can predict the time of contamination of organic matter in water.
- In recently contamination, concentration of NH3 is very high than nitrite and nitrate.
- Concentration of NH3 in ground water system is usually 3mg/ltr. If its concentration is greater than 50mg/ltr, it gives characteristic taste and odor.
3. Nitrite:
- It is very unstable intermediate formed during conversionof NH2 into nitrate.
- In aerobic condition nitrite is oxidized into nitrate whereas in anaerobic condition, nitrite is reduced to ammonia.
- If concentration of nitrite is greater in drinking water, it brings serious health hazard to the consumers.
- Disease caused by high concentration of nitrite in infants is called Blue baby syndrome, which is characterized by blue coloration of skin
- Level of nitrite in drinking water should not exceed 3mg/ltr.
- **Note; blue baby syndrome: hemoglobin has greater affinity for nitrite than oxygen. Therefore, if level of nitrite is high in blood due to consumption of contaminated water then it inhibit formation of oxyhemoglobin, instead it form nitrosomyoglobin. This prevent transport of Oxygen to tissue giving characteristic blue color. In infants nitrite poisoning occurs even due to high concentration of nitrate in drinking water because stomach of infant is less acidic. In this situation nitrate is reduced by intestinal bacteria into nitrite causing blue baby syndrome.
4. Nitrate:
- It is most stable oxidized form of nitrogen. In water nitrate comes from organic matter decomposition and from atmospheric nitrogen fixation.
- Like nitrite Nitrate should not exceed 3mg/ltr in drinking water. It is because nitrate can be reduced into nitrite in gut of infants and causes nitrite poisoning.
- Nitrate is very important in natural water system like lake and pond because high concentration of nitrate facilitates heavy growth of aquatic plants causing eutrophication.
5. Phosphate:
- In water phosphate is present in the form of H2PO4-, polyphosphate and as organic phosphate.
- Phosphate in water sources comes from agricultural wastes, sewage and from industrial effluent.
- Phosphate is not toxic to human being but it is important chemical in natural water system like pond because its high concentration facilitates eutrophication.
6. Hardness:
- Hardness of water is merely due to salt of calcium and magnesium.
- Temporary hardness is due to carbonate and bicarbonate of calcium and magnesium while permanent hardness is due to chloride and sulfate of calcium and magnesium.
- Water is classified as 9on the basis of concentration of calcium carbonate)
- Soft water:<5omg/ltr
- Moderately hard water: 50-150 mg/ltr
- Hard water: 150-300 mg/ltr
- Very hard water:>300 mg/dl
- Hard water is not suitable for industrial use. But hard water is usually beneficial for drinking purposes. However hardness caused by MgSO4 give some serious health effects. So, concentration of Mg++ should not exceeds 50 mg/lt in drinking water.
7. Biological oxygen demand (BOD):
- Biological oxygen demand represents amount of oxygen required by living organism (microorganism) for oxidation of biodegradable organic matters present in water under aerobic condition.
- BOD is a direct measurement of quality of oxygen needed for biodegradation of organic matter and indirect measurement of quantity of biodegradable organic matter in water.
- When biodegradable organic matter is added in water. Microorganism utilizes dissolved oxygen to oxidize organic matter. If the rate of consumption of oxygen during biodegradation is greater than solubilization of atmospheric Oxygen into water, level of dissolved oxygen gradually decreases.
- If organic matter content is very high, complete loss of dissolved oxygen occurs. This creates anaerobic environment in water. In this case aerobic aquatic organism cannot survive.
- Furthermore, if dissolved oxygen is absent then organic matter starts to decompose anaerobically that creates taste and odor problem.
- High value of BOD is an indicator of water pollution.
Determination of BOD
- To determine BOD, water sample is placed in a 300ml BOD bottle, seal it and incubated at 20C for 5 days in dark room.
- Light must be excluded to prevent the growth of algae that may produce O2 inside bottle.
- BOD = (DO1-DO5) * dilution factor * temperature factor
Limitation of BOD measurement
- For complete oxidation of organic matter, it will take about 20-30 days which is practically difficult. Therefore BOD is calculated by 5 days incubation method.
- One of the major limitation pf BOD calculation is that it gives quantity of only biodegradable organic matte but not non-biodegradable organic matters.
- BOD cannot determine if water contains toxic chemicals or antimicrobial substances. It is because microorganism are killed by these toxic chemicals.
8. Chemical oxygen demand (COD):
- COD is amount of oxygen needed for oxidation of organic matter present in water by strong chemical oxidizing agents such as K2Cr2O7.
- COD measurement is particularly valuable to determine amount of organic matter in water which contains toxic or antimicrobial chemicals.
- K2Cr2O7 is strong oxidizing agents and it oxidizes both biodegradable and non-biodegradable organic matters. Therefore the value of COD is always greater than BOD for particular water sample.
- COD gives amount of total organic (biodegradable + non-biodegradable) matter present in water.
Measurement of COD:
- To determine COD, water sample is mixed with excess of K2Cr2O7 in sulfuric acid solution. At the same time add AgSO4 (as catalyst) and HgSO4 to eliminate interference by chloride ion in water.
- K2Cr2O7 oxidizes organic matter into water, CO2 and NH3. Therefore, level of K2Cr2O7 decreases, then remaining K2Cr2O7 is measured by titration with ferrous ammonium sulfate.
- COD = (V1-V2)*N*8*1000/X
- Where, V1= Initial volume of ferrous ammonium sulfate
- V2= final volume of ferrous ammonium sulfate remaining
- N= normality of ferrous ammonium sulfate
- X= volume of water sample used during titration
9. Dissolved Oxygen (DO):
- Oxygen is highly soluble and get dissolved in water in the form of dissolved oxygen (DO).
- DO is a source of oxygen for aquatic aerobic organism like fishes, aerobic bacteria etc. therefore, DO is an important characteristics of natural water system that determines its quality.
- If DO is absent in water, aquatic organism cannot survive. Total absent of Dissolved oxygen in water creates anaerobic condition which facilitates anaerobic decomposition of organic compounds.
- Dissolved oxygen in water is affected by many factors like temperature, organic matter content etc.
- Temperature affects DO by two ways- first, solubility of oxygen in water decreases with increase in temperature of water. Second, high temperature promotes growth and activity of aquatic microorganisms so that they consume oxygen faster.
- If organic content is high, microorganisms utilizes DO for its oxidation causing depletion of dissolved oxygen level.
Measurement of Dissolved oxygen (DO) by modified Winkler’s method:
- DO content of water is determined iodometrically.
- At first water sample is taken and alkaline Potassium iodide (KI) is added
- When MnSO4 is added, dissolved oxygen in water oxidize MnSO4 into basic manganese oxide {Mn(OH)2}.
- Adding H2SO4 in the solution liberates iodine as gas. Amount of iodine gas is estimated by titration with sodium thiosulfate containing starch indicator.
- The amount of iodine liberated is equivalent to amount of dissolved oxygen present in water.
10. pH:
- Water become alkaline due to presence of Nh3, OH-, Ca++, Mg++ etc. in alkaline water certain minerals are deficient. Alkaline pH also decreases efficiency of disinfection process (chlorination).
- Water become acidic due to presence of dissolved acids and dissolved carbon-dioxide. Acidic pH corrodes pipeline. Similarly certain minerals are more than excess in acidic water.
- pH of drinking water should be around 6.5-8.5