Unit 2: Analysis of Soil and Water

Part A: Analysis of Soil

Soil analysis determines the composition and properties of soil, which is crucial for crop management and fertility.

1. Collection and Preparation of Soil Sample

A composite sample is collected by taking small samples from multiple locations in a field (e.g., in a 'W' pattern), mixing them, drying them in the air, grinding them, and sieving them to get a uniform, representative sample.

2. Determination of Soil Moisture

Principle (Gravimetric): The moisture is determined by finding the mass lost from the soil upon heating.

Method:

1. Weigh a clean, dry crucible (W1).
2. Add a sample of fresh soil and weigh again (W2). Mass of soil = W2 - W1.
3. Heat the crucible in an oven at 105-110°C for several hours (or until constant weight).
4. Cool in a desiccator and weigh again (W3). Mass of dry soil = W3 - W1.
5. Mass of moisture lost = W2 - W3.

Calculation:
% Moisture = ( (W2 - W3) / (W2 - W1) ) × 100

3. Determination of Soil pH

Principle: Soil pH measures the acidity or alkalinity of the soil, which affects nutrient availability for plants. A soil suspension is made, and the pH is measured using a pH meter or universal indicator solution.

4. Estimation of Calcium (Ca) and Magnesium (Mg)

Principle (Complexometric Titration): The Ca²⁺ and Mg²⁺ ions are extracted from the soil into a solution. They are then titrated with a standard solution of EDTA (Ethylenediaminetetraacetic acid), which is a chelating agent. It forms a stable, 1:1 complex with metal ions.

• Total Ca²⁺ + Mg²⁺: A buffer (pH 10) is added, and Eriochrome Black-T (EBT) indicator is used. The endpoint is a color change from wine red to blue.
• Ca²⁺ only: The titration is repeated at a high pH (12-13), where Mg²⁺ precipitates as Mg(OH)₂. A different indicator (e.g., Murexide) is used.
• Mg²⁺ content is then found by subtraction.

5. Estimation of Potassium (K)

Principle (Flame Photometry): This technique is used for alkali and alkaline earth metals.

1. The K⁺ ions are extracted from the soil into a solution.
2. The solution is sprayed into a hot, non-luminous flame.
3. The heat of the flame excites the potassium atoms, which then emit light at a characteristic wavelength (lilac/violet for K).
4. The *intensity* of the emitted light is measured by the photometer and is directly proportional to the *concentration* of potassium in the sample.

Part B: Analysis of Water

1. Determination of Water pH

Principle: Measures the hydrogen ion (H⁺) concentration. It is a critical indicator of water quality. Measured using a calibrated pH meter or pH paper.

2. Estimation of Water Hardness (Ca²⁺ and Mg²⁺)

Hardness is the property of water that prevents lathering with soap, caused by dissolved Ca²⁺ and Mg²⁺ ions.

• Temporary Hardness: Due to bicarbonates (HCO₃⁻). Can be removed by boiling.
• Permanent Hardness: Due to chlorides (Cl⁻) and sulfates (SO₄²⁻). Cannot be removed by boiling.

Principle (Complexometric Titration): Exactly the same as for soil. The total hardness (Ca²⁺ + Mg²⁺) is determined by titrating the water sample with standard EDTA solution at pH 10 using Eriochrome Black-T (EBT) indicator. The endpoint is a wine red to blue color change.

3. Estimation of Chloride (Cl⁻)

Principle (Argentometry - Mohr's Method): The water sample is titrated with a standard solution of silver nitrate (AgNO₃).
Reaction: Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s) (White precipitate)
The indicator used is potassium chromate (K₂CrO₄). After all the chloride has precipitated, the very next drop of Ag⁺ reacts with the chromate indicator to form a red-brown precipitate of silver chromate (Ag₂CrO₄), which signals the endpoint.
Endpoint Reaction: 2Ag⁺ (aq) + CrO₄²⁻ (aq) → Ag₂CrO₄ (s) (Red-brown)

4. Estimation of Sulphate (SO₄²⁻)

Principle (Gravimetric Analysis): This method involves precipitating the target ion (sulphate) from solution as a highly insoluble, stable compound that can be weighed.

1. A known, large volume of the water sample is taken.
2. It is acidified, and a hot solution of barium chloride (BaCl₂) is added in excess.
3. Sulphate precipitates as highly insoluble barium sulphate (BaSO₄).
   Reaction: Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s) (White precipitate)
4. The precipitate is filtered, washed, dried, and ignited in a furnace.
5. The final, stable BaSO₄ is cooled in a desiccator and weighed.
6. From the mass of BaSO₄, the original mass of sulphate can be calculated.