Theory

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 such as drinking, industries, irrigation, swimming, 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 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.

Total Hardness

Theory

Hardness in water is that characteristic which “prevents the lathering of soap”. This is due to the presence 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 instead forms a white scum or precipitate. This precipitate results from 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 does not contain dissolved calcium and magnesium salts.

Types of Hardness

Temporary or Carbonate Hardness

Caused by dissolved bicarbonates of calcium, magnesium, and other heavy metals and carbonates of iron. Temporary hardness is mostly removed by boiling, which decomposes bicarbonates into insoluble carbonates or hydroxides that settle as a crust.

Permanent or Non-Carbonate Hardness

Caused by chlorides and sulphates of calcium, magnesium, iron, and other heavy metals. This type of hardness cannot be removed by boiling.

Classification of Hardness

Water Type	CaCO3 Concentration (mg/L)
Soft	0–60 mg/L
Medium	60–120 mg/L
Hard	120–180 mg/L
Very Hard	>180 mg/L

Determination of Hardness

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 turns wine red. Upon adding EDTA as a titrant, the calcium and magnesium ions are complexed, and the solution changes from wine red to blue at the end point of the titration.

Units of Hardness

1. Parts per million (ppm): Parts of calcium carbonate equivalent hardness per 10⁶ parts of water.
   1 ppm = 1 part of CaCO₃ eq hardness in 10⁶ parts of water.
2. Milligram per litre (mg/L): mg of CaCO₃ equivalent hardness per litre of water.
   1 mg/L = 1 mg of CaCO₃ eq hardness per L of water.
3. Clarke’s degree (°Cl): Number of grains (1/7000 lb) of CaCO₃ eq hardness per gallon (10 lb) of water.
   1 °Cl = 1 grain of CaCO₃ eq hardness per gallon of water.
4. Degree French (°Fr): Parts of CaCO₃ eq hardness per 10⁵ parts of water.
   1 °Fr = 1 part of CaCO₃ hardness eq per 10⁵ parts of water.

Relationship Between Various Units of Hardness

• 1 ppm = 1 mg/L = 0.1 °Fr = 0.07 °Cl
• 1 mg/L = 1 ppm = 0.1 °Fr = 0.07 °Cl
• 1 °Cl = 1.43 °Fr = 14.3 ppm = 0.7 mg/L
• 1 °Fr = 10 ppm = 10 mg/L = 0.7 °Cl

$$\text{Total hardness as } \mathrm{CaCO_3} \, (\text{ppm}) = \frac{\text{Vol. of EDTA (mL)} \times 0.1 \times \text{molarity of EDTA} \times 10^6}{\text{Vol. of the sample (mL)}}$$

Alkalinity

Theory:

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 of 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 a sample can be estimated by titration with standard H₂SO₄ or HCl solution. Titration to pH 8.3 or decolourisation of phenolphthalein indicator will indicate complete neutralization of OH⁻ and 1/2 of CO₃²⁻, while titration to pH 4.5 or a sharp change from yellow to orange using 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:

$$\text{Phenolphthalein alkalinity} \left( \frac{\text{mg}}{\text{L}} \text{ as } \mathrm{CaCO_3} \right) = \frac{A \times \text{Normality of acid} \times 50,000}{\text{mL of sample}}$$

$$\text{Total alkalinity} \left( \frac{\text{mg}}{\text{L}} \text{ as } \mathrm{CaCO_3} \right) = \frac{B \times \text{Normality of acid} \times 50{,}000}{\text{mL of sample}}$$

Where

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	0	0	TA
PA < 1/2 TA	0	2PA	TA - 2PA
PA = 1/2 TA	0	2PA	0
PA > 1/2 TA	2PA - TA	2(TA - PA)	0
PA = TA	TA	0	0

Chemical Oxygen Demand (COD)

Theory

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 remaining after the reaction is titrated with ferrous ammonium sulphate. The dichromate consumed gives the O₂ required for oxidation of organic matter.

$$COD \left( \frac{mg}{L} \right) = \frac{\text{Vol. of FAS for sample} \times \text{Normality of FAS} \times 8000}{\text{Vol. of sample}}$$

DRINKING WATER QUALITY STANDARDS:

Sl. No.	Characteristic/Parameter	BIS	ICMR	WHO
1	Colour	5	2.5	-
2	Odour	Agreeable	Unobjectionable	Unobjectionable
3	Turbidity	10 NTU	5 NTU	2.5 NTU
4	pH	6.5-8.5	7.0-85	7.0-8.5
5	TDS	500 mg/l	500 mg/l	500 mg/l
6	Hardness	300 mg/l	300 mg/l	200 mg/l
7	Ca	75 mg/l	75 mg/l	75 mg/l
8	Mg	30 mg/l	50 mg/l	30 mg/l
9	Cl	250 mg/l	200 mg/l	200 mg/l
10	Sulphate	200 mg/l	200 mg/l	200 mg/l
11	Fe	0.3 mg/l	0.1 mg/l	0.1 mg/l
12	Nitrate	45 mg/l	20 mg/l	45 mg/l
13	Phenolic compounds	0.001 mg/l	0.001 mg/l	0.001 mg/l
14	Cd, Sc	0.01 mg/l	-	0.01 mg/l
15	Cu, As	0.05 mg/l	0.05 mg/l	0.01 mg/l
16	Cyanides	0.05 mg/l	-	0.01 mg/l
17	Pb	0.1 mg/l	-	0.01 mg/l
18	Anionic detergents	0.2 mg/l	-	-
19	PAH	-	-	-
20	Residual Chlorine	0.2 mg/l	-	-
21	Pesticides	Absent	-	-

BIS – Bureau of Indian Standards.

ICMR – Indian Council for Medical Research.

WHO – World Health Organization.