Experiment 39: Analysis of Commercial
Bleach
Purpose
Introduction: 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, ClOand 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
electrons
Oxidized – to give electrons
Reduced- 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.
reduced
2H+ (aq) + HOCl(aq) + 2I-(aq) ---> Cl-(aq) + I2(aq) + H2O (l)
Oxidizing agent
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 + 5I-1 + 6H+ ---> l- + 3I2+ 3H2O
lO3-2 + 6S2O3-2 + 6H+ ---> l- + 3S4O6- 2+ 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 over
If 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