Materials Included In Kit

Additional Materials Required

Prelab Preparation

Stock sodium hypochlorite solution, ≅ 2000 ppm: Add 100 mL of 5% sodium hypochlorite solution to a 1-L beaker containing 500 mL of distilled or deionized water. Stir to dissolve and dilute to 1 liter with water. Place in a 1-L bottle and cap. Label as “Stock NaOCl Solution.”

Simulated pool water solution: Add 50 mL of “Stock NaOCl Solution” in a 1-L beaker containing 500 mL of distilled or deionized water. Stir to dissolve and dilute to 1 liter with water. Place in a 1-L bottle and cap. Label Simulated Pool Water.

Sodium bicarbonate solution (Disposal solution): Add approximately 5 g of sodium bicarbonate to an empty 400-mL beaker. Have students add 250 mL of water to the beaker before beginning the lab. One beaker is needed for each student group.

Safety Precautions

Sodium hypochlorite solution is a corrosive liquid and mildly toxic by ingestion and inhalation. Avoid skin contact. Avoid contact or mixing of sodium hypochlorite with acids, which can release toxic chlorine gas. Hydrochloric acid solution is mildly toxic by ingestion and inhalation. Avoid skin contact. Methyl orange solution is mildly toxic by ingestion. Place reagent bottles in a fume hood. Because of the bleach odor, it is best to work in a well-ventilated room. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the lab. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All titrated solutions should be neutralized with sodium bicarbonate, then disposed of according to Flinn Suggested Disposal Method #26b. The methyl orange solution may be disposed of according to Flinn Suggested Disposal Method #26b. The stock sodium hypochlorite solutions, along with the simulated pool samples, may be disposed of according to Flinn Suggested Disposal Method #6.

Lab Hints

• The laboratory work for this experiment can be completed within two typical 50-minute lab periods. Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students.
• Chlorine gas is generated when HOCl is placed in strongly acidified solution. Always add the pool water to the acidified methyl orange so that the hypochlorite reacts with dye before Cl₂ gas is formed. Remind students to follow the procedure and add the methyl orange and the HCl, then the pool water. Using the micro-techniques described in the kit, there is a minimum amount of chlorinated water in the reaction vessel.
• The methyl orange solution is acidified to ensure the conversion of all hypochlorite to the acid form. Once all of the dye has reacted, any extra pool water added to the acidic solution will generate chlorine gas. Although the amount of gas generated from such dilute solutions is very small, the classroom air could become polluted if large numbers of tests overshoot the endpoint. Each group should have a large beaker of sodium bicarbonate solution at the workstation. After each titration, the titrated solution should be emptied into this container. This will return the pH to about 7 and prevent release of chlorine gas from that sample.
• Have students practice the drop technique with water to make sure consistent sized drops are produced.
• In Part B, the student data can be captured on a chalkboard or dry erase board for students to use in producing their graphs.
• Since microchemistry techniques involve small volumes of reagents, students tend to look closely at the titration samples. Student use of goggles is especially important in this stage of the experiments.
• Have students label their pipets so the pipets are not mixed up. Clean test tubes at the end of day one.
• In Part B, the reduced levels of chlorine will occur in the samples with higher levels of added urea solution. If the volume of pool sample added to the well exceeds 50 drops, have the students repeat the titration with 5 drops of methyl orange solution and 5 drops of distilled water. This reduces the molarity of the methyl orange solution and the drops of pool water needed to reach the endpoint both by two.
• Calculations are simplified if the density of all solutions is assumed to be 1.0 g/mL. The worksheets make this assumption. Recorded drops of solutions are interpreted as volumes.
• Students’ initial samples may be significantly lower in chlorine concentration than calculated due to the natural decomposition of hypochlorite ion.
• Place reagent bottles in a fume hood.
• Be sure to assign to each student group a different number of drops of the urea solution to add to their simulated pool water.

Teacher Tips

• A supplemental experiment idea is included that looks at the effect of UV light on chlorine levels.
• The kit has been designed to keep exposure of students far below what would be encountered by using an average household cleanser. Nevertheless, chlorine and chloramines can affect the respiratory system at low levels so care must be taken to minimize the chance that either will be generated in the classroom. The teacher is urged to do each experiment first and note where confusion might occur.

Further Extensions

Supplementary Information

Experiment Overview
In this lab, a simulated sample of swimming pool water is analyzed by microscale techniques. Students determine the effect ultraviolet light has on the chlorine level of the pool sample. A sample of pool water will be exposed to ultraviolet light. The sample of the pool water will be taken and analyzed for chlorine content at regular intervals. The chlorine concentration will then be graphed versus time.

Procedure
Modify the Part B procedure to fit this experiment. Have students expose their simulated pool water to ultraviolet light, then analyze the simulated pool water for free residual chloride concentrations at 10- to 15-minute intervals. The resultant graph of their data should be similar to Figure 4.

Answers to Prelab Questions

A solution containing 10 drops of 0.0015 M methyl orange solution and 5 drops 0.5 M HCl solution is titrated to a pale yellow endpoint with 7 drops of the simulated pool water.

1. Calculate the molarity of free chlorine residual (M_Chlorine) in the pool sample.

   M_Chlorine • V_Chlorine = M_{Methyl Orange} • V_{Methyl Orange}

   M_Cl = (V_{Methyl Orange}/V_Chlorine) • M_{Methyl Orange}

   {13935_PreLabAnswers_Equation_1}
2. Convert this concentration to parts per million of chlorine (ppm Cl) in solution.

   ppm Cl(mg/L) = (1000 mg/g) • (35.45 g Cl/mole) • (0.0021 moles Cl/L) = 74 ppm Cl

Sample Data

Data Table Part A
Molarity, Methyl Orange Solution ___0.015___ M

Data Table Part B
Molarity, Methyl Orange Solution ___0.015___ M

Answers to Questions

1. For each titration, calculated the concentration of chlorine in the sample, in units of molarity, M, and parts per million chlorine, ppm. Enter values in the data tables.
   1. Part B—Well 1
      {13935_Answers_Equation_9}
      {13935_Answers_Equation_10}
   2. Part C—Well 1
      {13935_Answers_Equation_11}
      {13935_Answers_Equation_12}
2. Average the concentration values for each sample. Enter these averages in the data rables.
   1. Part B—Well 1
      M_AVG = (0.0019 + 0.0017 + 0.0019 + 0.0019)/4 = 0.0019 M
      ppm_AVG = (67 + 60 + 67 + 67)/4 = 65 ppm
   2. Part C—Well 1
      M_AVG = [(4.1 x 10^–4) + (3.8 x 10^–4) + (4.2 x 10^–4) + (3.9 x 10^–4)]/4 = (4.0 x 10^–4) M
      ppm_AVG = (15 + 13 + 15 + 14)/4 = 14 ppm
3. Submit your Part C data to the instructor. When all student data are turned in, take the class data and plot, on graph paper, the chlorine concentration, in ppm, versus the drops of urea solution added. Place concentration on the y-axis and drops of urea solution added on the x-axis.

   Sample Data: Swimming Pool Chlorine Levels 24 Hours after Addition of Urea

   {13935_Answers_Table_3}
   {13935_Answers_Figure_5}

References

Special thanks to Doug De La Matter, retired chemistry teacher, Madawaska Valley D.H.S., Barry’s Bay, ON, for providing Flinn with the instructions and background information for this demonstration.

Introduction

Swimming pool chemistry involves a rich variety of chemical concepts and practical applications. In this lab, students will perform microscale analysis of a simulated swimming pool sample. The results will then be used to understand the chlorination process.

Concepts

• Chlorination process
• Equilibrium
• Le Chatelier’s principle

Background

Water used for drinking and in swimming pools is disinfected to remove bacteria and destroy other comtaminants that may cause color, odor, and turbidity in the water. The most common disinfection process used in pools is chlorination. Chlorination uses a chlorine species to oxidize the bacteria or unwanted chemicals, thus killing them or making them inactive. The chemical species used in this process is not, in fact, chlorine, Cl₂, but hypochlorous acid, HOCl.

Hypochlorous acid, a weak acid in solution, is created in solution by adding its conjugate base, hypochlorite ion, OCl^–, in the salt form; as sodium hypochlorite, NaOCl, potassium hypochlorite, KOCl, or lithium hypochlorite, LiOCl. Once the hypochlorite salt is dissolved in water, equilibrium is established between the weak acid hypochlorous acid, HOCl, and its conjugate base hypochlorite ion, OCl^– (Equation 1).

While hypochlorous acid is a weak acid in solution, it acts as a strong disinfection agent in oxidizing bacteria and other contaminants. The hypochlorite ion also acts as an oxidizing agent, but is much less effective than hypochlorous acid.

The equilibrium for Equation 1 depends on pH and is very sensitive in the pH range of 7 to 8.

At a pH value of 7.0, about 75% of the dissolved chlorine is present as hypochlorous acid and 25% as hypochlorite ion. At a pH value of 8.0, these values are reversed (Figure 1). pH values lower than 7.0 will give a higher percentage of hypochlorous acid, and thus more effective disinfection. However, these acidic conditions also produce eye irritation, corrosion of metal piping and fittings, leaching of calcium ions, Ca²⁺(aq), from tile grout, and other undesirable side effects.

At pH values higher than 8.0, eye irritation also occurs, and the Equation 1 equilibrium is shifted to the left, lowering the concentration of hypochlorous acid and making disinfection ineffective. The water may also become cloudy and calcium and iron precipitates may form as scale or discoloration on the pool surfaces.

The ideal pH range for effective disinfection and to minimize these undesirable effects is 7.2–7.8. For effective disinfection, HOCl concentrations are usually maintained between 1.0 and 3.0 parts per million (1 ppm = 1 milligram/1 liter of solution).

Ammonia and its derivatives in pool water react with HOCl to form compounds called chloramines (Equations 2 and 3). The primary source of chloramines (or ammonia and its derivatives) is humans, usually from skin oils, perspiration and other byproducts. Chloramine production is responsible for the chlorine odor in swimming pools. The effect of these compounds is to reduce the amount of HOCl in solution, possibly below the amount needed for effective disinfection. The amount of HOCl left in solution after reaction with ammonia compounds is called the free chlorine residual, the amount of chlorine that is free to react with bacteria.

If too much HOCl is available, an undesirable competing reaction takes place. Nitrogen trichloride is very irritating to eyes and mucous membranes. It off-gases into the air creating a distinctive “swimming pool” smell that most people identify as the smell of chlorine. Ironically, if a swimming pool has the correct level of chlorine, there will be little if any odor. The odor problem is especially troublesome in indoor pools, where recent efforts at energy conservation have reduced airflows. When the degradation products are not efficiently vented, too much nitrogen trichloride will build up and off-gases intact.

A process called superchlorination is used to rid pools of ammonia compounds (Equations 5 and 6). In superchlorination, the hypochlorous acid concentration is increased to the point where all ammonia and chloramine compounds in the water are completely oxidized. This process is illustrated in the graph below (Figure 2). Region A shows a slow increase in the measured chlorine level as some of the added HOCl is used up in reactions that produce chloramines.

Region B shows a rapid decrease in the free chlorine residual as the added HOCl is used in the conversion of chloramines to N₂ and HCl.

Region C shows a steep increase in free chlorine residual concentration, as added chlorine no longer combines with organic compounds.

Disinfection of pool water is accomplished by maintaining adequate free chlorine residual levels and optimum pH.

Free chlorine residual levels are rapidly reduced by the ultraviolet rays (UV) in sunlight [λ = 290 to 350 nm]. The sunlight breaks down the hypochlorite ions to chloride ions (Equation 7). As the concentration of hypochlorite ion is reduced, the reaction in Equation 1 shifts to the left to reestablish equilibrium and the concentration of hypochlorous acid is reduced.

Experiment Overview

In this lab, a simulated sample of swimming pool water will be analyzed by microscale techniques. Students will determine the free chlorine concentration of their sample by titration with an acidified solution of the weak acid methyl orange. The endpoint is reached when the solution color changes from red to pale yellow. Urea, an ammonia compound, is added to a new sample and students will determine the effect of urea on the chlorine level of the pool water.

In this experiment, the concentration of free residual chlorine in a simulated swimming pool will be determined by microscale titration. The chlorine concentration is expressed both as moles per liter (M) and as parts per million chlorine (ppm or mg/L).

A fixed volume of standard methyl orange solution will be titrated with the swimming pool water. The acidified methyl orange is oxidized by the hypochorous acid in the pool water. A proposed reaction is listed to the right.

Once all the methyl orange is oxidized, the solution color changes from red to yellow. The volumes are delivered drop by drop from the capillary-tip pipets. At the endpoint,

where

M_MeOr = molarity of the methyl orange solution
V_MeOr = drops of methyl orange solution
M_chlorine = molarity of chlorine in the pool sample
V_chlorine = drops of pool sample

Equation 10 can be rearranged to solve the unknown molarity of chlorine in the pool water:

M_chlorine = [V_MeOr/V_chlorine] • M_MeOr

The parts per million of chlorine (ppm Cl) in solution is related in turn to M_chlorine:

ppm Cl (mg/L) = (1000 mg/1g) • (molar mass of chlorine) • M_chlorine
ppm Cl (mg/L) = (1000 mg/1g) • (35.45 g Cl/mole) • M_chlorine

Materials

Prelab Questions

A solution containing 10 drops of 0.0015 M methyl orange solution and 5 drops 0.5 M HCl solution is titrated to a pale yellow endpoint with 7 drops of the simulated pool water.

1. Calculate the molarity of free chlorine residual (M_chlorine) in the pool sample.
2. Convert this concentration to parts per million of chlorine (ppm Cl) in solution.

Safety Precautions

Sodium hypochlorite solution is mildly toxic by ingestion and inhalation. Avoid skin contact. Hydrochloric acid solution is mildly toxic by ingestion and inhalation. Avoid skin contact. Methyl orange solution is mildly toxic by ingestion. Work in a well-ventilated room. Wear chemical splash goggles, chemical-resistant gloves and a chemical resistant apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

Part A. Measuring Chlorine Levels

Caution: Wear goggles at all times during the procedure.

1. Obtain 25 mL of simulated pool water and add it to the plastic container. Cap the container.
2. Place four test tubes in a test tube rack and label the test tubes 1–4.
3. Place the test tube rack on top of a sheet of white paper.
4. Using a clean Beral-type microtip pipet, obtain 2–3 mL of methyl orange indicator solution.
5. Transfer 10 drops of methyl orange solution to reaction test tube 1. Be certain to hold the pipet vertically. Avoid having air bubbles in the stem of the pipet (see Figure 3). Record the number of drops in Part A of the data tables.
   {13935_Procedure_Figure_3}
6. Using a clean Beral-type microtip pipet, obtain 1–2 mL of 0.5 M HCl solution.
7. Add 5 drops of the HCl solution to test tube 1. Record the number of drops in Part A of the data tables.
8. Mix the solution by swirling the test tube.
9. Uncap the plastic container and, using a clean Beral-type micro-pipet, obtain 2–3 mL of the simulated pool water.
10. Carefully add the simulated pool water drop by drop to test tube 1, mixing the solution between drops by swirling the test tube, until the red color has faded to a pale yellow. Record the number of drops in Part A of the data tables.
11. Repeat steps 1–10 for three more pool samples using test tubes 2 through 4. Record all data in Part A of the data tables.
12. Dispose of the final solutions in the test tubes in the waste beaker as directed by the instructor.

Part B. Effect of Urea on Chlorine Level

13. Using a clean Beral-type microtip pipet, obtain 1–2 mL of the 0.1 M urea solution.
14. Add the instructor-assigned number of drops (0 to 15 drops) of urea solution to the simulated pool water in the plastic container. Record this number in Part B of the data tables.
15. Cap the plastic container and label the container cap with the number of drops of urea solution added. Place the container in the hood.
16. Allow the pool sample to react for about 24 hours.
17. After 24 hours, test the chlorine level in your pool using steps 1–12.
18. Enter all data in Part B of the data tables.

Student Worksheet PDF

13935_Student1.pdf