To determine chemical parameters such as hardness, alkalinity, and chemical oxygen demand COD) of water samples.
It is needless to emphasize the importance of water in our life. Without water, there is no life on our planet. We need water for different purposes. We need water for drinking, for industries, for irrigation, for swimming and fishing, etc.
Water for different purposes has its own requirements for composition and purity. Each body of water needs to be analysed on a regular basis to confirm to suitability. The types of analysis could vary from simple field testing for a single analyte to laboratory based multi-component instrumental analysis. The measurement of water quality is a very exacting and time consuming process, and a large number of quantitative analytical methods are used for this purpose.
Hardness in water is that characteristic, which “prevents the lathering of soap”. This is due to presence in water of certain salts of calcium, magnesium and other heavy metals dissolved in it. A sample of hard water, when treated with soap does not produce lather, but on other hand forms a white scum or precipitate. This precipitate is formed, due to the formation of insoluble soaps of calcium and magnesium.
Thus, water which does not produce lather with soap solution readily, but forms a white curd, is called hard water. On the other hand, water which lathers easily on shaking with soap solution, is called soft water. Such water consequently does not contain dissolved calcium and magnesium salts in it.
Temporary or carbonate hardness: It is caused by the presence of dissolved bicarbonates of calcium, magnesium and other heavy metals and the carbonate of iron. Temporary hardness is mostly destroyed by mere boiling of water, when bicarbonates are decomposed, will produce insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel.
Permanent or non-carbonate hardness: It is due to the presence of chlorides and sulphates of calcium, magnesium, iron, and other heavy metals. Unlike temporary hardness, permanent hardness is not destroyed on boiling.
The degree of hardness of drinking water has been classified in terms of the equivalent CaCO3 concentration as follows:
In a hard water sample, the total hardness can be determined by titrating the Ca2+ and Mg2+ present in an aliquot of the sample with Na2EDTA solution, using NH4Cl-NH4OH buffer solution of pH 10 and Eriochrome Black-T as the metal indicator.
Na2H2Y (Disodium EDTA solution) → 2Na+ + H2Y-
Mg2+ + HD2- (blue) → MgD (wine red) + H+
D (metal-indicator complex, wine red colour) + H2Y- →Y- (metal EDTA complex colourless) + HD- (blue colour) + H+
Ethylenediamine tetra-acetic acid (EDTA) and its sodium salts form a chelated soluble complex when added to a solution of certain metal cations. If a small amount of a dye such as Eriochrome black T is added to an aqueous solution containing calcium and magnesium ions at a pH of 10 ± 0.1, the solution will become wine red. If EDTA is then added as a titrant, the calcium and magnesium will be complexed. After sufficient EDTA has been added to complex all the magnesium and calcium, the solution will turn from wine red to blue. This is the end point of the titration.
Units of Hardness:
1. Parts per million (ppm): Is the parts of calcium carbonate equivalent hardness per 106 parts of water, i.e, 1 ppm = 1 part of CaCO3 eq hardness in 106 parts of water.
2. Milligram per litre (mg/L): Is the number of milligrams of CaCO3 equivalent hardness present per litre of water. Thus:
1 mg/L = 1 mg of CaCO3 eq hardness per L of water.
3. Clarke’s degree (oCl): Is number of grains (1/7000 lb) of CaCO3 equivalent hardness per gallon (10 lb) of water. Or it is parts of CaCO3equivalent hardness per 70,000 parts of water. Thus,
1oClarke = 1 grain of CaCO3 eq hardness per gallon of water.
4. Degree French (oFr): Is the parts of CaCO3 equivalent hardness per 105 parts of water. Thus,
1o Fr = 1 part of CaCO3 hardness eq per 105 parts of water.
Relationship Between Various Units of Hardness:
1ppm =1 mg/L=0.1oFr =0.07oCl
1mg/L=1 ppm=0.1oFr =0.07oCl
1oFr=10 ppm=10 mg/L=0.7oCl
Alkalinity is an aggregate property of the water sample which measures the acid-neutralizing capacity of a water sample. It can be interpreted in terms specific substances only when a complete chemical composition of the sample is also performed. The alkalinity of surface water is due to the carbonate, bicarbonate and hydroxide content and is often interpreted in terms of the concentrations of these constituents. Higher the alkalinity, greater is the capacity of water to neutralize acids. Conversely, the lower the alkalinity, the lesser will be the neutralizing capacity.
Alkalinity of sample can be estimated by titration with standard H2SO4 or HCI solution. Titration to pH 8.3 or decolourisation of phenolphthalein indicator will indicate complete neutralization of OH- and 1/2 of CO32-, while to pH 4.5 or sharp change from yellow to orange of methyl orange indicator will indicate total alkalinity.
To detect the different types of alkalinity, the water is tested for phenolphthalein and total alkalinity, using Equations:
A = titrant (mL) used to titrate to pH 8.3
B = titrant (mL) used to titrate to pH 4.5
N = normality of the acid (0.02N H2SO4 for this alkalinity test)
50,000 = a conversion factor to change the normality into units of CaCO3
Once PA and TA are determined, then three types of alkalinities, i.e, hydroxides, carbonates and bicarbonates can be easily calculated from the table:
Result of Titration
OH alkalinity as CaCO3
CO3 alkalinity as CaCO3
HCO3 alkalinity as CaCO3
PA = 0
PA < 1/2TA
TA - 2PA
PA = 1/2TA
PA > 1/2TA
2PA - TA
2(TA - PA)
PA = TA
Chemical Oxygen Demand (COD):
COD is used as a measure of oxygen equivalent to organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. For samples from a specific source, COD can be related empirically to BOD. COD determination has advantage over BOD determination in that the result can be obtained in about 5 hours as compared to 5 days required for BOD test.
The organic matter gets oxidized completely by K2Cr2O7 in the presence of H2SO4 to produce CO2 and H2O. The excess of K2Cr2O7 remained after the reaction is titrated with ferrous ammonium sulphate. The dichromate consumed gives the O2 required for oxidation of organic matter.
DRINKING WATER QUALITY STANDARDS:
BIS- Beaurau of Indian Standards.
ICMR- Indian Council for Medical Research.
WHO- World Health Organization.
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