Unit 5: Case Studies

This unit involves applying the analytical techniques from previous units to assess the quality of a real-world water or soil sample.

Case Study 1: Water Sample Analysis

This involves collecting a water sample (e.g., from a river, lake, or tap) and determining its key quality parameters. This is crucial for checking its potability (safety for drinking) or its level of pollution.

1. Determination of pH

Method: Measured using a calibrated pH meter.
Importance: Most aquatic life thrives in a neutral range (6.5-8.5). Highly acidic or basic water is indicative of industrial pollution.

2. Determination of Acidity

Principle: Measures the water's capacity to neutralize a strong base. It's often due to dissolved CO₂ (carbonic acid).
Method: A simple acid-base titration. The water sample is titrated with a standard NaOH solution using phenolphthalein indicator.
Endpoint: Colorless to faint pink.

3. Determination of Alkalinity

Principle: Measures the water's capacity to neutralize a strong acid (its "buffering capacity"). It is mainly due to bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.
Method: An acid-base titration. The water sample is titrated with a standard H₂SO₄ or HCl solution.

4. Determination of Dissolved Oxygen (DO)

Principle (Winkler's Titration): DO is the amount of O₂ dissolved in water, which is essential for aquatic life. Low DO indicates pollution. This is a complex redox titration.

  1. The O₂ in the water is "fixed" by adding MnSO₄ and an alkali-iodide solution. This forms a precipitate of Mn(OH)₂.
  2. The dissolved O₂ oxidizes Mn(II) to Mn(III) or Mn(IV).
  3. When acid is added, the manganese oxidizes the iodide (I⁻) from the reagent to form free Iodine (I₂).
  4. The amount of I₂ formed is *equivalent* to the amount of O₂ originally in the water.
  5. This I₂ is then titrated with a standard sodium thiosulphate (Na₂S₂O₃) solution using starch indicator.
  6. Endpoint: Blue to colorless.

5. Determination of Biochemical Oxygen Demand (BOD)

Principle: BOD measures the amount of dissolved oxygen consumed by microorganisms (bacteria) while they decompose organic waste in the water.
It is the most important indicator of organic pollution (e.g., sewage).

Method:

  1. Take two bottles of the water sample.
  2. Measure the Dissolved Oxygen in the first bottle immediately (this is DO_initial).
  3. Incubate the second bottle in complete darkness at 20°C for 5 days.
  4. After 5 days, measure the Dissolved Oxygen in the second bottle (this is DO_final).
Calculation:
BOD (in mg/L) = DO_initial - DO_final

A high BOD (e.g., > 5 mg/L) means the water is heavily polluted with organic waste.

6. Determination of Chemical Oxygen Demand (COD)

Principle: COD measures *all* chemically oxidizable substances in water (both biodegradable and non-biodegradable). It is always higher than BOD.
Method: A powerful chemical oxidizing agent, potassium dichromate (K₂Cr₂O₇), is used to chemically "digest" (oxidize) the organic matter in the sample.
The amount of dichromate *consumed* is measured by titration, which is proportional to the amount of organic matter.

Case Study 2: Soil Sample Analysis

This involves collecting a soil sample and analyzing it for key macro-nutrients to assess its fertility, building on the techniques from Unit 2.

1. Determination of pH

Method: A soil-water suspension (e.g., 1:2 ratio) is prepared, and the pH is measured with a pH meter.
Importance: Most plants prefer a pH between 6.0 and 7.5. pH affects nutrient availability; for example, in highly acidic soil, aluminum can become toxic, and nutrients like phosphorus become unavailable.

2. Determination of Macro Nutrients (K, Ca, Mg)

These are "secondary" macro-nutrients, essential for plant growth.