AS Bleach Redox Titration
- Both bleach and the thiosulfate must be measured as accurately as possible. Use pipets with a narrow tips. Carefully squeeze individual drops. Count the drops of both bleach and thiosulfate carefully.
- Place 20 drops of (diluted) bleach solution in one well of a 24-well plate.
- Add 20 drops of 3 M acetic acid.
- Add 12 drops of KI to the bleach. Stir with a toothpick. The solution should have an orange-brown color.
While stirring, titrate the solution dropwise with the Na2S2O3 until the solution turns yellow. Be sure to keep track of the number of drops used.
When the solution is a light yellow which indicates that most of the I2 has reacted, add one drop of the starch solution; continue stirring. The solution should turn a deep blue black color. Continue titrating dropwise with Na2S2O3until the solution turns colorless. Continue counting drops.
- Record the total drops of Na2S2O3 used.
- Wash hands.
Determine the concentration of bleach in the original solution before dilution.
- Write a balanced equation for the reaction of bleach with potassium iodide. Write a balanced equation for the reaction of thiosulfate ion with triiodide ion.
- Describe the effect on bleach of standing in an opened bottle.
- Solutions of bleach are very basic. Explain.
- Identify the role of starch in the experiment.
Name ___________________________ Class _______
SmallScale 052 Bleach Analysis
Watch the movies.
Describe the color changes observed.
Determine the concentration of bleach in the original solution before dilution. Use the following data:
Solution of Na2S2O3 is 0.15 M Na2S2O3.
Answer the questions.
This experiment fits in with discussions of oxidation and reduction. Also, it works with solution stoichiometry and volumetric assays. It can be performed in an applied chemistry class.
Although a common household item, bleach is a dangerous material that causes burns and is toxic. Reacting bleach with acid produces chlorine. Reacting bleach with ammonia produces toxic chloramine. Potassium iodide and iodine are toxic. Wear goggles and apron. Avoid contact with chemicals. Wash spills immediately with large amounts of water. Wash hands after the experiment.
Teacher Preparation: 20 minutes to prepare the solutions. The Na2S2O3 is not standardized because the accuracy required is low. Making a Na2S2O3 solution of known concentration by careful weighing is satisfactory.
Class Time: 30 minutes + time for calculations
- 0.5 mL of 0.1 % of starch– Spray starch into a beaker of boiling water while stirring. This will serve as an indicator solution for the whole class. Test the indicator before using.
- 5 mL of fresh bleach diluted (1 mL liquid bleach in 10 ml solution). It may be desirable to compare a new bottle of bleach with an old one
- 5 mL of 3 M Acetic Acid (use stock solution)
- 3 mL of 1 M KI (Dissolve 8.300 g potassium iodide (KI) in enough water to make 50 mL solution.)
- 5 mL of 0.15 M Na2S2O3 (Dissolve 3.72 g sodium thiosulfate pentahydrate (Na2S2O3•5H2O) in enough water to make 100 mL solution.)
- 24-well plate
- 10-mL cylinder
The materials used in this experiment may be disposed of safely using running water at the sink.
- In two trials, one required 17 drops Na2S2O3 and the other 18. An average of 17.5 drops was used in calculations.
- The reactions involved are:
- ClO- + 3 I- + 2 H+ –> Cl- + I3- + H2O
- 2 S2O32- + I3- –> S4O62- + 3 I-
- Adding these equations together gives:
- 2 S2O32- + ClO- + 2 H+ –> S4O62- + Cl- + H2O
- (M S2O32-) x (V S2O32-) x (1 mol ClO-/2 mol S2O32-)
- = (M ClO-) x (V ClO-)
- (M ClO-) = (M S2O32-) x (V S2O32-) x
- (1 mol ClO-/2 mol S2O32-) x (1/(V ClO-)
- = (0.15 ) x (17.5 drops S2O32-) x
- (1/2) x (1/20 drops bleach)
- = 0.066 M (dilute) = (M ClO-)
- M conc x V conc = M dil x V dil
- M conc = M dil x ( V dil/V conc )= 0.066 x 10 = 0.66
- g bleach/ L = (0.66 mol NaClO /1 L) x
- (74 g NaClO/1 mol NaClO)
- = 49 g NaClO/L
- [In an experiment performed by the instructor, a 10-mL cylinder was tared on a top-loading balance. A 9.63 mL sample had a mass of 10.3 g. From this, the density was determined to be 10.3 g /9.63 mL = 1.07 g/mL.]
- % bleach = 100% x (49 g NaClO/L) x
- (1 L/1000 mL) x (1 mL/1.07 g)
- = 4.6 % NaClO in bleach
- Q1. Write a balanced equation for the reaction of bleach with potassium iodide. Write a balanced equation for the reaction of thiosulfate ion with triiodide ion.
- A1. ClO- + 3 I- + 2 H+ –> Cl- + I3- + H2O
- 2 S2O32- + I3- –> S4O62- + 3 I-
- Q2. Describe the effect on bleach of standing in an opened bottle.
- A2. The bleach slowly loses chlorine to the air:
- ClO- + Cl- + H2O –> Cl2 + 2 OH-
- Q3. Solutions of bleach are very basic. Explain.
- A3. If the bleach is not very basic, chlorine loses by the reaction shown above are significant. Adding base shifts the equilibrium to the left.
- Q4. Identify the role of starch in the experiment.
- A4. Starch is an indicator. It makes very low concentrations of I3- detectable.
oxidation, reduction, redox, applied chemistry, bleach, titration, volumetric,
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