Experiment 39: Analysis of Commercial Bleach

PurposeIntroduction: Many commercial products are effective because they contain oxidizing agents.

Some products that contain oxidizing agents are bleaches, hair coloring agents, scouring powders, and toilet bowl cleaners. The most common oxidizing agent in bleaches is sodium hypochlorite, NaClO (sometimes written NaOCl).

Commercial bleaches are created by bubbling chlorine gas into a sodium hydroxide solution (remember this from your “funky redox rxns”?). Some of the chlorine is oxidized to the hypochlorite ion, ClO- and some is reduced to the chloride ion, Cl- (a disproportionation reaction).

The chemical equation for the process is:

• Cl2(g) + 2OH- (aq) ---> ClO- (aq) + Cl- (aq) + H2O

Terms

Oxidizing agent – transfers oxygen atoms or gains electronsOxidized – to give electronsReduced- compound gains electrons

• The amount of hypochlorite ion (ClO- ) present in a solution of bleach can be determined by oxidation-reduction titration.

• One of the best methods is the iodine-thiosulfate titration procedure. Iodide ion, I-, is easily oxidized by almost any oxidizing agent

• (It has many electrons to lose!). In an acid solution, hypochlorite ions oxidize iodide ions to form iodine, I2.

• The iodine that forms is then titrated with a standard solution of sodium thiosulfate.

The analysis takes place in a series of steps:

• 1.Acidified iodide ion is added to hypochlorite ion solution, and the iodide is oxidized to iodine.

2H+ (aq) + HOCl(aq) + 2I-(aq) ---> Cl-(aq) + I2(aq) + H2O (l)

Oxidizing agent

reduced

oxidized

• 3.The Iodine solution is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide ions:

• I2+ (2S2O3) 2----> 2I-+ (S4O6 )2-

Part A Dilute 1M Na2S2O3 to 0.05M Na2S2O3

• Pour 5 ml of 1M Na2S2O3 into a 250 ml beaker• Dilute with DI water until it reaches 100 ml• This will create a 0.05M Na2S2O3

Part A standardization of Na2S2O3

• Rinse and fill a buret with Na2S2O3

• Rinse and fill a second buret with 3 M KIO3

• Dispense 15 ml of KIO3 solution into a 250 ml Erlenmeyer flask– Add to flask 25 ml of water– 3ml of 3M KI– 2ml of 3M H2SO4

Part A begin titration• Slowly add Na2S2O3

• When the solution turns a light yellow add 0.5ml of starch indicator, the solution will turn black

• Continue to add Na2S2O3 until the solution reaches an endpoint (turns colorless)

• Calculate Molarity of Na2S2O3 using the formula below as seen on page 481

lO3-2 + 6S2O3

-2 + 6H+ ---> l- + 3S4O6- 2+ 3H2O

lO3-2 + 5I-1 + 6H+ ---> l- + 3I2+ 3H2O

Calculations for standarization

• Moles of lO3-2 = (Molarity lO3

-2 x volume of lO3-2)

• 6 Moles of lO3-2 = 1 moles of Na2S2O3

Part B Determination of the oxidizing agent of bleach

• Fill buret with 0.05M Na2S2O3

• Add 0.5g of bleach into a 250ml flask • Record the mass of the unknown bleach

Part B Determination of the oxidizing agent of bleach

– To the flask add• 25 ml of di water • 3ml of 3M KI• 2ml of 3 M H2SO4

• 5 drops of 3 % ammonium molybdate

It should turn a dark brown, if it does not call me overIf it does immediately begin to titrate

2H+ (aq) + HOCl(aq) + 2I-(aq) ---> Cl-(aq) + I2(aq) + H2O (l)

Part B begin titration

• Slowly add Na2S2O3 to flask• When the solution turns a light yellow add

0.5ml of starch, the solution will turn black\• Continue to add Na2S2O3 until it turns clear

I2+ (2S2O3) 2----> 2I-+ (S4O6 )2-

Part B begin titration

• Calculate Moles of NaOCl using the formula below as seen on page 480

NaOCl + 2S2O3-2 + H+ ---> Cl- + S4O6

+ H2O

Calculations for % oxidizing agent (OCL)

• Moles of Na2S2O3 = (Molarity Na2S2O3 x volume of

Na2S2O3)

• 1 Moles of NaOCL = 2 moles of Na2S2O3