Experiment 5: Determination of Residual Chlorine in Water by Iodometric Titration

• Volumetric Flask – for preparing and storing the water sample

• Conical Flask (Erlenmeyer Flask) – for titration

• Burette – to deliver standard sodium thiosulphate solution

• Pipette – to measure exact volume of water sample

• Measuring Cylinder – for measuring acid and KI

• Glass Rod – for mixing

• Beaker – for transferring solutions

• Funnel – for transferring liquids safely

• White Tile – to observe color change during titration

• Wash Bottle with Distilled Water – for cleaning and rinsing

• Water Sample – chlorinated water to be tested

• Acetic Acid (CH₃COOH) – to acidify the solution (pH 3–4)

• Potassium Iodide (KI) – reacts with chlorine to liberate iodine

• Starch Solution – indicator that forms a blue-black complex with iodine

• Sodium Thiosulphate (Na₂S₂O₃) – standard titrant that reduces iodine to iodide

• Distilled Water – used for dilution and rinsing

3.1 Background

Chlorination is a primary method for disinfecting water supplies. When chlorine is added to water, it reacts with organic and inorganic materials. This initial consumption of chlorine is known as the chlorine demand. The amount of chlorine remaining in the water after the demand has been satisfied is called residual chlorine.

3.2 Definitions and Principles

Residual Chlorine: The concentration of chlorine that remains in the water after a specific contact period. It acts as a safeguard against recontamination as the water travels through the distribution system. It exists in two forms:

• Free Chlorine: The most effective disinfectant, consisting of hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻).

• Combined Chlorine: Formed from the reaction of chlorine with ammonia or organic nitrogen compounds (chloramines). It is a less potent but more persistent disinfectant.

Breakpoint Chlorination: A process where chlorine is added until the chlorine demand is fully met, and all ammonia is oxidized. Beyond this “breakpoint,” any additional chlorine results in a proportional increase in free residual chlorine.

Principle of Iodometric Titration: This experiment is based on a redox titration. In an acidic medium (created by adding acetic acid), residual chlorine oxidizes iodide ions (from Potassium Iodide, KI) to free iodine (I₂). The liberated iodine imparts a yellow-brown color to the solution. This liberated iodine is then titrated against a standard solution of sodium thiosulphate (Na₂S₂O₃) using starch as an indicator. The endpoint is reached when the blue color of the starch-iodine complex disappears.

Chemical Reactions:

1. Liberation of Iodine:

2. Titration with Sodium Thiosulphate:

Types of Chlorination

1. Pre-chlorination: Chlorine is added before filtration to control biological growth and remove odors.

2. Post-chlorination: Chlorine is added after filtration or treatment, usually just before distribution, to maintain residual chlorine for disinfection.

3. Breakpoint Chlorination: Chlorine is added in sufficient quantity to oxidize all ammonia and organic matter, ensuring only free chlorine remains.

4. Super-chlorination: A high dose of chlorine is added to treat heavily polluted water. Dechlorination is usually done afterward.

5. Re-chlorination: Additional chlorine is added at intermediate points in the distribution system to maintain disinfectant levels.

6. Shock Chlorination: Used for disinfection of wells or small water systems; involves very high chlorine doses to eliminate microbial contamination.

Methods of Disinfection

1. Chlorination: Most common method using chlorine or its compounds.

2. Ozonation: Uses ozone gas (O₃); a powerful oxidizing agent that kills bacteria and viruses. No residual effect.

3. UV Radiation: Ultraviolet light damages the DNA of microorganisms. Effective but leaves no residual protection.

4. Boiling: Physical method to kill pathogens. Suitable for small-scale or emergency use.

5. Iodine/Bromine Disinfection: Often used in portable water systems (like camping). Less commonly used in large-scale treatment.

6. Membrane Filtration (Micro/Ultrafiltration): Physically removes microorganisms through very fine membranes.

3.3 Significance of the Experiment

Measuring residual chlorine is crucial for ensuring both effective disinfection and acceptable water quality. Insufficient residual chlorine (< 0.2 mg/L) may lead to microbial contamination and waterborne diseases. Conversely, excessive levels (> 0.5 mg/L) can cause unpleasant taste and odor, and potentially lead to the formation of harmful disinfection by-products (DBPs). This test helps water treatment operators maintain the optimal chlorine concentration in the public water supply.

1. A 200 ml sample of the chlorinated water was collected in a volumetric flask.

2. 50 ml of the water sample was measured and transferred into a clean conical flask using a pipette.

3. Approximately 5 ml of acetic acid was added to the flask to lower the pH to between 3 and 4.

4. About 1 gram of potassium iodide (KI) crystals was added to the flask, and the solution was mixed, which turned a yellow-brown color due to the liberation of iodine.

5. The liberated iodine was titrated against a standard N/40 sodium thiosulphate solution taken in a burette until the solution turned a pale straw-yellow color.

6. About 1-2 ml of starch solution was added as an indicator. The solution immediately turned a deep blue-black color.

7. The titration was carefully continued, adding the sodium thiosulphate drop by drop, until the blue color completely disappeared and the solution became colorless. This marked the endpoint.

8. The final burette reading was recorded.

9. The procedure was repeated multiple times to obtain a set of concordant readings.

Given Data:

• Concordant volume of Na₂S₂O₃ (hypo) used: 5.1 ml

• Volume of water sample taken for titration: 50 ml

• Normality of standard Na₂S₂O₃ solution (N): 1/40 N (or 0.025 N)

• Normality Factor (f): 1.009

• Equivalent weight of Chlorine: 35.5

Calculation:

The residual chlorine concentration is calculated using the formula:

Substituting the values:

Breaking it down step by step:

Final Result: 91.39 mg/L (after rounding)

Result: The concentration of residual chlorine in the given water sample was found to be 91.39 mg/L.

Discussion: The experimental result of 91.39 mg/L is exceptionally high. According to the World Health Organization (WHO), the recommended level of residual chlorine in drinking water is between 0.2 and 0.5 mg/L. The Nepal Drinking Water Quality Standards (NDWQS, 2005) recommend a range of 0.1 to 0.2 mg/L.

Our result is more than 180 times the maximum recommended WHO guideline. Such a high concentration is completely unsuitable for drinking purposes and poses significant health risks, including severe irritation to the eyes, skin, and respiratory system, as well as creating a highly unpalatable taste and odor. This extreme value could be due to a gross error in the chlorination process (over-chlorination) or a significant error during the laboratory procedure, such as using an incorrect concentration of the sodium thiosulphate titrant.

The experiment successfully determined the residual chlorine concentration in the water sample using iodometric titration. The resulting concentration of 91.39 mg/L is drastically above the permissible limits set by both WHO and national standards. Therefore, the water is not safe for human consumption and requires immediate corrective action.

If a water supply is found to have excessively high residual chlorine, the following control strategies can be implemented:

• Dechlorination: The water can be treated with dechlorinating agents like sodium thiosulphate or activated carbon filters to reduce the chlorine level.

• Process Optimization: The chlorination process at the treatment plant must be reviewed and adjusted to ensure the correct dosage is applied.

• Aeration: Allowing the water to stand in an open tank can help some of the chlorine dissipate into the atmosphere.

• All glassware must be thoroughly cleaned and rinsed with distilled water.

• Potassium Iodide (KI) is sensitive to light and air; it should be added just before titration.

• The starch indicator solution should be freshly prepared, as it deteriorates over time.

• The endpoint of the titration should be approached carefully, drop by drop, to avoid overshooting.

• Burette readings must be taken accurately from the bottom of the meniscus.