030 – Chemistry of river water

This laboratory guides students through the chemical analysis of river water using simple colorimetric and volumetric techniques. They measure the pH, determine water hardness with EDTA titration, and quantify nitrate and phosphate concentrations using reagent-based tests. The activity emphasizes the environmental importance of water quality monitoring. Students develop skills in sample handling, precision, and data interpretation. By the end, they will relate experimental findings to environmental health.

Educational Goals

The primary educational objective of this laboratory is to foster an understanding of how chemistry contributes to environmental science. Students will appreciate that water quality assessment is central to ecosystem health, human consumption, and pollution monitoring. On a conceptual level, students will:

  • Recognize the meaning of pH, hardness, nitrates, and phosphates in natural waters.
  • Learn the principles behind titration (EDTA–metal ion complexation) and colorimetric assays (indicator dyes, reagent-based reactions).
  • Link numerical results (ppm, mg/L, pH) with environmental implications such as eutrophication, acidification, and water potability.

On a technical level, students will:

  • Safely manipulate laboratory glassware, reagents, and electronic instruments (pH meter).
  • Gain fluency in measuring small liquid volumes with droppers, pipettes, and graduated cylinders.
  • Record, interpret, and compare results with standard reference scales.

Overall, this lab combines environmental chemistry concepts with essential laboratory competencies, helping students connect chemical analysis to real-world sustainability concerns.

Protocol

Preparation

To collect river water, you have two choices :

  • Choice 1 : Using an empty 100 mL beaker, collect a sample of the river water at your feet
  • Choice 2 : Use directly the water already collected this morning in the beaker “river water”

PART A : pH measurement

  1. Using the graduated cylinder, measure 20 mL of river water.
  2. Transfer the 20 mL of river water into test tube 1.
  3. Using the dropper, add 2 drops of universal pH indicator (pH i) into test tube 1.
  4. Empty the excess from the dropper into the recovery tray.
  5. Immerse the pH meter electrode in test tube 1.
  6. Using a glass rod, mix the contents of test tube 1.
  7. Compare the color obtained to the pH chart.
  8. Rinse the pH meter electrode with distilled water.
  9. Dry the pH meter electrode with a paper towel.

PART B : Water hardness measurement

  1. Using the graduated cylinder, measure 20 mL of river water.
  2. Transfer the 20 mL of river water into test tube 2.
  3. Using the dropper, add 10 drops of buffer solution into test tube 2.
  4. Empty the excess from the dropper into the recovery tray.
  5. Check the pH of the solution using the pH meter. Make sure it is greater than 10.
  6. Using the pipette, add 10 mL of 0.5% Eriochrome Black (EB) into test tube 2.
  7. Mix with the glass rod and note the color of the solution.
  8. Using the pipette, add 1 mL of EDTA into test tube 2.
  9. Stir the solution using the glass rod.

Note: Make sure the specimen is placed in front of a black cardboard background in order to clearly distinguish color changes.

  1. Repeat steps 8 and 9 until the solution changes color again. Note the number of mL added.
  2. The volume of EDTA added is found in the results table.
  3. According to the quantity added, determine the hardness of the river water.
  • Low hardness : 5 mL or less of EDTA added
  • Medium hardness : Between 5 and 10 mL of EDTA added
  • High hardness : Between 10 and 15 mL of EDTA added
  • Very high hardness : More than 15 mL of EDTA added.

Note: The colorimetric scale is approximate, additional quantitative tests are required to determine the exact result.

  1. Make sure the pipette is completely empty (0 mL).

PART C : Nitrate concentration measurement

  1. Using the graduated cylinder, measure 20 mL of river water.
  2. Transfer the 20 mL of river water into test tube 3.
  3. Using the dropper, add 10 drops of nitrate reducing agent solution (NO3 (R)).
  4. Empty the excess from the dropper into the recovery tray.
  5. Mix with the glass rod.
  6. Using the dropper, add 10 drops of Griess reagent.
  7. Empty the excess from the dropper into the recovery tray.
  8. Mix with the glass rod.
  9. Consult the colorimetric scale (ppm NO3) to determine the nitrate concentration of the river water.

Note: Make sure the specimen is placed in front of a black cardboard background in order to clearly distinguish color changes.

Note: The colorimetric scale is approximate, additional quantitative tests are required to determine the exact result.

PART D : Phosphate concentration measurement

  1. Using the graduated cylinder, measure 20 mL of river water.
  2. Transfer the 20 mL of river water into test tube 4.
  3. Using the dropper, add 10 drops of ammonium molybdate solution (MoO42-).
  4. Empty the excess from the dropper into the recovery tray.
  5. Mix with the glass rod.
  6. Using the dropper, add 10 drops of ascorbic acid solution (Vit C).
  7. Empty the excess from the dropper into the recovery tray.
  8. Mix with the glass rod.
  9. Consult the colorimetric scale (ppm PO4) to determine the phosphate concentration of the river water.

Note: Make sure the specimen is placed in front of a black cardboard background in order to clearly distinguish color changes.

Note: The colorimetric scale is approximate, additional quantitative tests are required to determine the exact result.

  1. Empty the test tubes into the recovery tray.

Anticipated Outcomes

At the conclusion of the experiment, students should be able to:

  • Define the role of each parameter measured: pH, hardness, nitrates, phosphates.
  • Interpret how individual measurements reflect the chemical status of a river ecosystem.
  • Recognize that each test relies on specific chemical reactions.

Charts

pH indicator

Nitrates

Phosphates

Technical Outcomes

  • Prepare samples consistently and perform measurements with attention to reproducibility.
  • Apply both instrumental methods (pH meter) and classical methods (color indicators, EDTA titration).
  • Accurately document results in a logbook and compare them to reference charts.

Analytical Outcomes

  • Translate raw experimental results into meaningful water quality assessments:
  • pH: 5.9 (slightly acidic).
  • Hardness: 60 mg/L Mg²⁺, based on 6 mL EDTA (average hardness).
  • Nitrates: 60 ppm, above recommended drinking water limits.
  • Phosphates: 40 ppm, high risk of eutrophication.
  • Critically evaluate whether results indicate safe, borderline, or problematic water quality.
  • Discuss potential sources of error.

Summary of Assignment by Grade Range

Grade 9–10 (Introductory Level)

  • Focus: Observations and simple interpretations.
  • Tasks: Add reagents, observe color changes, use the pH scale, and record basic results.
  • Learning Outcome: Students identify whether water is acidic, hard, or nutrient-rich, without detailed calculations.

Grade 11 (Intermediate Level)

  • Focus: Quantitative reasoning and stoichiometry introduction.
  • Tasks: Carry out EDTA titration, measure reagent volumes, and compare data to hardness classification.
  • Learning Outcome: Students connect reagent volumes (mL of EDTA) to water hardness values and interpret ppm concentrations.

Grade 12 (Advanced Level – Pre-university)

  • Focus: Environmental impact analysis and error consideration.
  • Tasks: Compute hardness in mg/L CaCO₃ equivalents, compare nitrate/phosphate values with standards, propose pollution sources.
  • Learning Outcome: Students interpret chemistry data in environmental terms and evaluate water quality against standards.

Enrichment Level

  • Focus: Holistic water quality evaluation and broader context.
  • Tasks: Critically assess method sensitivity, discuss improvements (e.g., spectrophotometry), propose management strategies.
  • Learning Outcome: Students bridge lab-scale measurements with professional approaches and environmental monitoring frameworks.

Laboratory essentials

Instruments

  • 100mL beaker
  • 25mL graduated cylinder
  • Dropper
  • 10mL pipette
  • Glass rod
  • pH meter
  • 50mL test tubes x4

Products

  • River water
  • Buffer
  • Universal pH indicator
  • Eriochrome black
  • EDTA
  • Nitrate reducing agent
  • Griess reagent
  • Ammonium molybdate solution
  • Ascorbic acid solution
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