Chlorine Decay in Tap Water Under Sunlight

ISEF Category: Chemistry

Subcategory: Analytical Chemistry  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Tap water does not stay the same after it leaves the pipe. Sunlight can break down disinfectants, which changes how well water stays safe. You can measure that change with a color test, then turn the numbers into a prediction model. That gives you a real chemistry project with a public health angle.

What Is It?

Water treatment plants add chlorine or chloramine to kill germs. These disinfectants do not last forever. Light, heat, and time all push them to break apart or react with other compounds in the water.

Think of the disinfectant like a battery charge. Fresh water starts with a full charge, then the charge drops as the water sits. DPD colorimetry uses a dye called DPD, which turns pink when it reacts with chlorine. The darker the pink, the more disinfectant is left. You can compare colors with a smartphone camera or a simple colorimeter.

Arrhenius modeling adds the temperature piece. This model links reaction speed to temperature, so you can test whether warmer sunlight speeds up chlorine loss. If you collect clean, repeatable data, you can estimate how fast disinfectant decays under different light conditions and build a small prediction tool.

Why This Is a Good Topic

This topic works well because you can change one main variable, sunlight exposure, and measure a clear outcome, disinfectant concentration. You can also compare chlorine and chloramine, which connects your work to real water treatment choices. The project teaches colorimetry, controls, calibration, and basic modeling, which are strong research skills for a first project.

Research Questions

• How does sunlight exposure change free chlorine concentration in tap water over time?
• How does sunlight exposure change total chlorine concentration in tap water over time?
• What is the effect of container material on chlorine decay under the same light exposure?
• To what extent does water temperature change the rate of chlorine loss in sunlight?
• Which lighting condition causes the fastest drop in DPD color intensity, direct sun, window light, or darkness?
• Does chloramine decay more slowly than free chlorine under the same exposure conditions?

Basic Materials

• Tap water samples from one source.
• DPD chlorine reagent kit or DPD test tablets.
• Clear glass jars or bottles with matching lids.
• Opaque container or aluminum foil for dark controls.
• Smartphone with a fixed camera.
• White background or printed color reference card.
• Digital timer.
• Notebook or spreadsheet for data logging.
• Thermometer.
• Measuring cups or graduated cylinder.
• Safety glasses.
• Disposable gloves.

Advanced Materials

• Free chlorine and total chlorine test standards.
• Spectrophotometer or school colorimeter.
• DPD reagent set for free and total chlorine.
• UV-transparent and UV-blocking sample containers.
• Light meter or UV meter.
• Temperature-controlled water bath or incubator.
• pH meter.
• Chloramine standard solution, if available through a school lab.
• Analytical balance.
• Image calibration target for smartphone imaging.
• Lab notebook and data sheet templates.

Software & Tools

• Google Sheets: Organizes measurements, builds calibration curves, and graphs decay trends.
• ImageJ: Measures color intensity from photos and helps turn images into numerical data.
• Python: Fits decay models and compares sunlight groups with simple statistics.
• PubChem: Helps you look up chemical properties and safety details for DPD and related reagents.
• NOAA Solar Calculator: Estimates sunlight angle and day length for your exposure schedule.

Experiment Steps

1. Define whether you will track free chlorine, total chlorine, or both, so your measurements match one clear outcome.
2. Choose the exposure setup, then lock down the container type, light source, and control group before you collect data.
3. Build a calibration plan so each color reading can become a concentration estimate instead of a raw photo.
4. Decide how you will separate sunlight effects from temperature effects, since both can change decay speed.
5. Plan the model you will test, such as a simple exponential decay model and an Arrhenius temperature comparison.
6. Set your data table and graph format first, so every trial gets recorded the same way.

Common Pitfalls

• Using different jars or bottle colors between trials, which changes how much light reaches the water.
• Taking photos under shifting room light, which makes DPD color readings drift from one sample to the next.
• Mixing up free chlorine and total chlorine tests, which gives you numbers that do not answer the same question.
• Ignoring temperature changes during sun exposure, which makes it hard to tell whether light or heat caused the decay.
• Starting with tap water from different sources, which adds extra chemistry and hides the effect of sunlight.

What Makes This Competitive

A strong version of this project does more than report that chlorine drops in sunlight. It compares free chlorine and chloramine, controls temperature, and uses calibration data to turn photos into real concentrations. You can also test whether an exponential model fits better than a straight-line trend. If your smartphone app predicts decay under new conditions and you check that prediction against fresh samples, your project feels much more like real analytical chemistry.

Project Variations

• Test chlorine decay in bottled water, filtered water, and tap water to compare how water source changes the trend.
• Compare direct sunlight, window light, and indoor LED light to see how different spectra affect disinfectant loss.
• Add pH as a second variable to see whether acidity changes how fast chlorine or chloramine disappears.

Learn More

• US EPA Drinking Water Basics: Learn how chlorine and chloramine are used in water treatment, and find the topic through the EPA drinking water pages.
• NIH PubMed: Search review articles on chlorine decay, disinfection byproducts, and DPD colorimetry.
• NOAA Solar Calculator: Estimate sunlight timing and angle for your exposure schedule by searching the NOAA solar calculator.
• MIT OpenCourseWare, Principles of Chemical Science: Review reaction rates, equilibrium, and Arrhenius ideas through the free course materials.
• USGS Water Science School: Read plain-language explanations of water chemistry and find it by searching the USGS Water Science School.