Analysis of Water Quality | Plant Ecology

In this article we will analyse various properties of water which affect plant growth:- 1. Hardness 2. Chloride 3. Dissolved Oxygen (DO) 4. Iron.

1. Hardness (EDTA Titrimetric Method):

Water hardness is the traditional measure of the capacity of water to react with soap, hard water requiring a considerable amount of soap to produce lather. Hardness of water is not a specific constituent but a variable and complex mixture of cations and anions. The principal hardness-causing ions are calcium and magnesium. They can be removed by boiling.

The degree of hardness of drinking water is classified in terms of the equivalent CaCO₃concentra­tion:

Soft 0-60 mg/L

Medium 60-120 mg/L

Hard 120-180 mg/L

Very hard 180 mg/L

Principle:

In alkaline condition EDTA reacts with Ca and Mg to form a soluble chelated complex. Ca and Mg ions develop wine red colour with Erichrome Black-T under alkaline condition. When EDTA is added as a titrant the Ca and Mg divalent ions get complex resulting in sharp change from wine red to blue which indicates the end-point of the titration.

Reagents:

(a) EDTA solution (0.01 M): 3.732 gm. EDTA dilutes in 1 lit. distilled water.

(b) Erichrome Black-T indicator: Mix 0.5 gm. of dye with 100 gm. of NaCl.

(c) Buffer solution:

Solution A: Dissolving 16.9 gm. NH₄C1 in 143 ml conc. NH₄OH soln.

Solution B: Dissolving 1.179 gm. of disodium EDTA and 0.780 gm. of MgSO₄, 7H₂O in 50 ml distilled water.

Mixed solution A & B and dilute to 250 ml with distilled water.

(d) Sodium sulphide solution:

Dissolving of 5 gm. of Sodium sulphide or 3.7 gm. of di-Sodium sulphide in 100 ml of distilled water.

Procedure:

1. Take 50 ml of sample in a conical flask (in case of sample having higher calcium, a smaller volume should be taken and make up the volume to 50 ml).

2. Add 1 ml of the buffer solution, followed by addition of 1 ml of Na₂S solution if necessary (for samples having higher amount of heavy metals).

3. Add a pinch of Eriochrome Black T indicator so as to develop a wine-red colour.

4. Titrate the solution against EDTA solution. The end-point is reached when the colour changes from wine-red to blue.

Calculation:

Total hardness as mg/L CaCO₃ = ml of EDTA used x 1,000/ml of sample

2. Chloride:

The presence of chloride in natural waters is attributed to the dissolution of salt deposits, discharge of effluents from chemical industries, irrigation drainage, and contamination from refuge leachates and seawater intrusion in coastal areas. The salty taste produced by chloride depends on the chemical compo­sition of the water.

Principle:

Chloride is determined in a neutral or slightly alkaline solution by titration with standard silver nitrate solution, using potassium chromate as an indicator. Silver chloride is quantitatively precipitated before red silver chromate is formed.

Reagents:

(a) Silver nitrate, 0.02 N: Dissolve 3.4 gm. of dried AgNO₃ in distilled water to make 1 L of solution and store in a dark bottle.

(b) Potassium chromate, 5%: Dissolve 5 gm. of K₂CrO₄ in 100 ml distilled water.

Procedure:

1. Take 50 ml of filtered sample in a conical flask and add 2 ml of K₂CrO₄ indicator.

2. Titrate the contents against 0.02 N AgNO₃ until a persistent red tinge appears.

Calculation:

Chloride (mg/L) = (ml × N) of AgNO₃ × 1,000 × 35.5/ml of sample

3. Dissolved Oxygen (DO):

Dissolved oxygen (DO) is one of the most important parameters in water quality assessment and reflects the physical and biological processes prevailing in the waters. Non-polluted surface waters are usually saturated with DO.

Oxygen can be rapidly removed from the waters by discharge of oxygen- demanding wastes. Other inorganic reducing agents such as H₂S, NH₃, and Fe²⁺ as well as some oxidizable substances tends to decrease the DO content. Low DOs are indicative of heavy contamination with organic matter. It is determined by the Winkler’s method (Azide modification method).

Principle:

The manganous sulphate reacts with the alkali to form a white precipitate of manganous hydroxide, which — in the presence of oxygen — gets precipitated into a brown coloured compound.

In a strong acidic medium, manganic ions are reduced by iodide ions which get converted into iodine equivalent to the original concentration of oxygen in the sample. The iodine can be titrated against thiosulphate using starch as an indicator.

Reagents:

(a) Sodium thiosulphate, 0.025 N:

Dissolve 24.82 g of Na₂S₂O₃. 5H₂O in boiled distilled water and make up the volume to 1 L. Add 0.4 g of borax or a pellet of NaOH as stabilizer. This is 0.1 N stock solution. Dilute it four times to get 0.025 N and store in an amber-coloured bottle.

(b) Alkali iodide azide solution:

Dissolve 500 g of NaOH or 700 g of KOH and 150 g of KI in distilled water to make 1 L of solution. To it add a solution containing 10 g of NaN₃ in 40 ml of distilled water. Mix the contents properly.

(c) Manganous sulphate solution:

Dissolve 100 g of MnSO₄.4H₂O in 200 ml of boiled distilled water.

(d) Starch solution:

Dissolve 1 g of starch in 100 ml of warm distilled water (80°C-90°C) and add a few drops of formaldehyde solution.

(e) Sulphuric acid:

H₂SO₄ conc. (SG: 1.84).

Procedure:

1. Fill the sample in a BOD bottle of known volume carefully, avoiding any kind of bubbling and trapping of air bubbles in the bottle after placing the stopper.

2. Pour 2 ml each of MnSO₄ and alkali (in case the volume of the sample is about 300 ml, add 2 ml each of the reagents), well below the surface from the walls. A precipitate will appear.

3. Place the stopper and shake the contents well by inverting the bottle repeatedly. Allow the precipitate to settle.

4. Add 1-2 ml of conc. H₂SO₄ and shake well to dissolve the precipitate.

5. Remove either the whole content or a part in a conical flask for titration.

6. Titrate with contents with sodium thiosulphate using starch as an indicator. At the end-point, initial dark blue colour changes to colourless.

Calculation:

When whole content has been titrated,

DO, mg/L = (ml × N) of titrant × 8 × 1000/ (V₁ – v)

When part of the content has been titrated,

DO, mg/L = (ml × N) of titrant × 8 × 1000/ V₂ (V₁ – V/V₁)

where, V₁ = volume of sample bottle after placing the stopper

V₂ = volume of part of the contents titrated

V = volume of MnSO₄ and alkali added

4. Iron:

It is one of the most abundant elements of the rocks and soils. All kinds of water including ground water have appreciable quantities of iron. Iron has more solubility at acidic pH, therefore large quantities of iron are leached out from the soil by acidic water.

In addition to natural source of iron, corrosion of pipes, pumps etc. can also increase its concentration in distribution systems. Although iron has little concern as a health hazard, it is still considered as a nuisance in excessive quantities. The limits on iron in waters are based on aesthetic and taste consideration rather than physiological effect.

Principle:

All the iron is converted to ferrous state by boiling with HCl and hydroxylamine. The reduced iron chelates with 1, 10-phenon-throline at pH 3.2-3.3 to form a complex of orange-red colour. The intensity of this colour is proportional to the concentration of iron and it is determined colorimetrically.

Reagent:

(a) Conc.HCI

(b) Hydroxylamine hydrochloride:

Dissolve 10 g of NH₂OH.HCl in distilled water to prepare 100 ml of solution.

(c) Ammonium acetate buffer solution:

Dissolve 250 g of NH₄CH₃COOH in 150 ml distilled water and add 700 ml of glacial acetic acid.

(d) Phenonthroline solution:

Dissolve 100 g of 1, 10-phenonthroline monohydrate in 100 ml distilled water by thorough stirring and heating up to 80°C (should not boil).

(e) Stock iron solution (200 mg/L iron):

Add 20 ml of concentrated H₂SO₄ to 50 ml of distilled water and dissolve 1.404 g ferrous ammonium sulphate-hex-a-hydrate. Add 0.1 N KMnO₄ slowly till a persistent faint pink colour appears. Dilute to 1 L.

(f) Standard solution (10 mg/L):

Dilute this stock iron solution twenty times with distilled water.

Procedure:

1. Take 50 ml sample or an aliquot containing not more than 4 mg/L iron in a 150 ml conical flask.

2. Add 2 ml conc. HCl and 1 ml of Hydroxylamine hydrochloride solution.

3. Boil the content to half of the volume for dissolution of the iron.

4. Cool and add 10 ml of buffer and 2 mi of phenanthroline solution.

5. Make up the volume to 100 ml and after 10 min take the reading at 510 nm.

6. Prepare a standard curve in the range of 1-4 mg/L of iron using various standard iron solu­tions.

7. Calculate the concentration of iron in the sample from the standard curve.