Determination of pH and Chlorine Level in MIS Swimming Pool
by Vivian & Ethan, DP2 student, Class of 2025
Recently, my classmate and I were in class studying redox titrations. Reduction-oxidation titrations are carried out in a similar way to acid-base titrations and can be used to determine unknown concentrations in redox reactions.
We were then curious about the pH and chlorine concentration in the school’s swimming pool. Our teacher was also curious so our teacher set up a lab for us to investigate. Chlorine is a chemical added to pools to help destroy bacteria and organic matter. However, the chlorine level should stay between 1.0 and 3.0 parts per million (ppm). Anything higher might cause swimmers to have red eyes and itchy skin. The pH level of the pool water is also important because if the pH level is too high or too low, it would not be safe to swim in. Moreover, chlorine loses its effectiveness in acidic water. The recommended healthy pH level for pools ranges from around 7.2 to 7.6.
Therefore, the objective was to determine whether the chlorine concentration is within 1.0 and 3.0 ppm and whether the pH of our school swimming pool was between the recommended range of 7.2 and 7.6.
We can measure chlorine levels using titration. Titration is an important technique in volumetric analysis. It is used to determine the concentration of a solution when it reacts exactly with another solution of known concentration. Titrations typically involve reactions between acids and bases or between oxidizing and reducing agents. Precise glassware, including burettes and pipettes, is used to obtain precision. A pipette is used to measure a known volume but unknown concentration of one solution in a conical flask. The other solution of known concentration is put into a burette. The point at which the two solutions have reacted completely is called the equivalence point. An indicator will help us identify when the reaction has completed, as it changes colour at the endpoint (slightly after the equivalence). This can be observed by the human eye. Titration involves multiple trials to gain a more accurate result of the volume required to reach an equivalence point; this volume is known as the titre.
During the lab activity, we collected the MIS swimming pool water into a water bottle and set our titration systems. We then filled the system with Na2S2O3 (sodium thiosulfate) in order to titrate the pool water. This sodium thiosulfate is called the “titrant” while the pool water is called the “titrate.” During the titration, we added sodium thiosulfate into the conical flask containing the pool water and swirled it. We noticed colour changes from black to transparent solution with the help of indicators, which we had added to the pool water beforehand. This tells us that we have reached the end point. We then record the volume of titrant used and use this to calculate the concentration of chlorine.
However, we found more successful results for the pH as we used a pH sensor. We placed the pH sensor into a beaker containing the pool water, and it created a graph on our computer. The pH of the small and big pools is around 7.2, which is within the recommended range. The pools are therefore of a safe pH to swim in. It is a bit over 7, which is normal because the type of chlorine commonly used in pools, calcium hypochlorite, is a bit alkaline.
Unfortunately, our results were not very accurate as Vivian calculated 815 ppm and Ethan calculated 47.95 ppm of chlorine in the pool water. If there were really 815 ppm or 47.95 ppm of chlorine then it would be very dangerous to swim in the pool. This may be due to various factors, such as the burette leaking, or dropping the sodium thiosulfate too fast into the conical flask. Difficulties we faced during this process were when our system had air bubbles, which could really affect our result when we titrated the Na2S2O3 with the swimming pool water. The experiment is also on a school lab scale, so we don’t have the accuracy of larger-scale experiments.
Ways for further improvement in future titrations is to ensure that we slow down the drops of the titrant when the colour is changing, and making sure the system has no air bubbles. It would be better if we could use burettes that do not leak.
