DSC-152: Section-C (Physics and Chemistry of Environment)
Table of Contents
Practical 11: Determination of pH of water samples
pH is a measure of the hydrogen ion (H⁺) concentration in a solution. It indicates whether the water is acidic, neutral, or alkaline. The formula is pH = -log[H⁺]. Most aquatic life thrives in a narrow pH range (6.5-8.5).
Objective
To measure the pH of different water samples using a pH meter.
Materials
- Digital pH meter
- Beakers
- Standard buffer solutions (pH 4.0, 7.0, and 9.2)
- Distilled water (for rinsing)
- Water samples (e.g., tap water, pond water, distilled water, rain water)
Procedure
- Calibration:
- Turn on the pH meter and let it warm up.
- Rinse the glass electrode with distilled water and gently blot dry.
- Place the electrode in the pH 7.0 buffer solution. Calibrate the meter to read "7.00".
- Rinse the electrode again. Place it in the pH 4.0 buffer (for acidic samples) or pH 9.2 buffer (for alkaline samples). Calibrate to the respective value.
- Measurement:
- Rinse the electrode with distilled water.
- Take a water sample in a clean beaker.
- Immerse the electrode in the sample. Swirl gently.
- Wait for the reading on the display to stabilize.
- Record the final pH value.
- Repeat step 2 for all other water samples, making sure to rinse the electrode with distilled water between each sample.
Observation Table
| Sample No. | Source of Water Sample | pH Value | Nature (Acidic/Neutral/Alkaline) |
|---|---|---|---|
| 1 | Distilled Water | ||
| 2 | Tap Water | ||
| 3 | Pond Water |
Practical 12: Determination of pH of soil samples
Soil pH is a "master variable" that controls the chemical and biological processes in the soil. It strongly affects the availability of plant nutrients and the toxicity of metals.
Objective
To measure the pH of different soil samples.
Materials
- Digital pH meter (calibrated as in Practical 11)
- Beakers (100 mL)
- Soil samples (e.g., garden soil, roadside soil, forest soil)
- Distilled water
- Weighing balance, stirring rod
Procedure
- Prepare Soil Slurry:
- Weigh 20g of the air-dried soil sample.
- Place it in a 100 mL beaker.
- Add 40 mL of distilled water (This creates a 1:2 soil:water ratio).
- Stir the mixture (slurry) thoroughly with a glass rod for 5 minutes.
- Let the slurry stand for 30 minutes to allow the soil to settle and the ions to dissolve.
- Measurement:
- Stir the slurry again just before measuring.
- Immerse the calibrated pH electrode into the supernatant liquid (the liquid above the settled soil).
- Wait for the reading to stabilize and record the pH.
- Repeat for all soil samples.
Observation Table
| Sample No. | Source of Soil Sample | pH Value | Nature (e.g., Highly Acidic, Neutral, Alkaline) |
|---|---|---|---|
| 1 | Garden Soil | ||
| 2 | Roadside Soil |
Practical 13: To measure soil temperature of different sites
Soil temperature is a key abiotic factor that influences seed germination, root growth, and the metabolic activity of soil microbes (which drive nutrient cycling).
Objective
To measure and compare the soil temperature at different sites with varying conditions (e.g., sun vs. shade).
Materials
- Soil thermometer (a long, metal-probe thermometer)
- Notebook and pen
Procedure
- Select Sites: Choose at least 3-4 sites with different characteristics. For example:
- Site A: Open, bare soil in direct sunlight.
- Site B: Shaded soil (e.g., under a large tree).
- Site C: Moist soil (e.g., near a water body).
- Site D: Soil covered with grass.
- Measurement:
- At each site, gently insert the soil thermometer into the ground to a standard depth (e.g., 10 cm).
- Leave the thermometer in place for 3-5 minutes, or until the reading stabilizes.
- Read the temperature without removing the thermometer from the soil, if possible, to avoid rapid changes.
- Record the temperature and a description of the site.
Observation Table
| Site No. | Site Description (Location, Cover, Sun/Shade) | Soil Temperature (°C) |
|---|---|---|
| 1 | Open, bare soil, full sun | |
| 2 | Shaded soil, under tree canopy | |
| 3 | Moist soil, near pond edge |
Practical 14: To measure water holding capacity and moisture percentage of soil
Soil Moisture Percentage is the amount of water *currently* present in a soil sample. Water Holding Capacity (WHC) is the *maximum* amount of water a soil can hold against the force of gravity (also called field capacity).
Objective
To determine the moisture content and the maximum water holding capacity of a given soil sample.
Materials
- Soil sample
- Crucible or moisture tin (with lid)
- Hot air oven
- Weighing balance
- Funnel, filter paper (or cotton wool plug)
Procedure A: Moisture Percentage
- Weigh a clean, dry crucible (W1).
- Add a small amount of fresh (moist) soil sample to the crucible. Weigh it (W2).
- Place the crucible (with soil, lid off) in a hot air oven set at 105°C for 24 hours.
- Remove the crucible, let it cool in a desiccator, and weigh it again (W3).
Calculation:Weight of moist soil = W2 - W1
Weight of water lost = W2 - W3
Moisture % = [(Weight of water lost) / (Weight of moist soil)] × 100
Moisture % = [(W2 - W3) / (W2 - W1)] × 100
Procedure B: Water Holding Capacity (WHC)
- Take a plastic funnel and plug its stem with cotton wool or a piece of filter paper.
- Weigh the funnel + filter paper (W_a).
- Take a known weight of the oven-dried soil (from Procedure A) and place it in the funnel (e.g., 50g). Let this be W_soil.
- Place the funnel over a beaker.
- Gently and slowly add water to the soil until it is fully saturated and water starts to drip from the funnel.
- Cover the funnel to prevent evaporation and allow it to drip until no more water comes out (this may take an hour). This represents the field capacity.
- Wipe any drops from the outside of the funnel and weigh the funnel + wet soil (W_b).
Calculation:Weight of wet soil + funnel = W_b
Weight of dry soil + funnel = W_a + W_soil
Weight of water held by soil = W_b - (W_a + W_soil)
WHC % = [(Weight of water held) / (Weight of dry soil)] × 100
Practical 15: Determination of total hardness of water
Water Hardness is caused by dissolved divalent cations, primarily Calcium (Ca²⁺) and Magnesium (Mg²⁺). It is determined by a complexometric titration using EDTA (Ethylenediaminetetraacetic acid) as the titrant. Hardness is expressed as mg/L of CaCO₃.
Objective
To determine the total hardness of a given water sample by EDTA titration.
Materials
- Burette, pipette (50 mL), conical flask (250 mL)
- Standard Hard Water (for standardizing EDTA)
- EDTA solution (0.01 M)
- Ammonia buffer solution (to maintain pH ≈ 10)
- Eriochrome Black T (EBT) indicator
- Water sample
Procedure
- Rinse and fill the burette with the 0.01 M EDTA solution. Note the initial reading.
- Pipette 50 mL of the water sample into a clean conical flask.
- Add 1-2 mL of ammonia buffer solution (to bring the pH to 10).
- Add 2-3 drops of EBT indicator. If hardness is present, the solution will turn wine-red.
- Titration: Add the EDTA solution from the burette drop by drop, while constantly swirling the flask.
- Endpoint: Continue titrating until the color changes sharply from wine-red, through a purplish-blue, to a clear sky-blue.
- Record the final burette reading. The volume of EDTA used is (Final Reading - Initial Reading).
Key things to remember:
- Titrant: EDTA
- Indicator: Eriochrome Black T (EBT)
- pH: 10 (maintained by buffer)
- Color Change: Wine-Red to Sky-Blue
Calculation
The calculation uses the Molarity equation (M₁V₁ = M₂V₂). For this specific titration:
Hardness (in mg/L as CaCO₃) = (V_EDTA × M_EDTA × 100.1 × 1000) / V_sample
- V_EDTA = Volume of EDTA used in titration (in mL)
- M_EDTA = Molarity of the EDTA solution (e.g., 0.01 M)
- 100.1 = Molar mass of CaCO₃ (g/mol)
- 1000 = Conversion factor (mg to g, and mL to L)
- V_sample = Volume of water sample taken (e.g., 50 mL)
Simplified Formula for 0.01 M EDTA and 50 mL sample:
Hardness = (V_EDTA × 0.01 × 100.1 × 1000) / 50
Hardness = V_EDTA × 20.02