AP Chemistry Lab
Analysis of Commercial Bleach
Introduction
Many commercial products are successful because they use
oxidizing agents. These produces include bleaches, toilet cleaners, and
scouring powders. Bleaches contain the hypochlorite ion or: OCl 1 as in
the case of common household Clorox ( NaOCl ). Bleaches are made from
bubbling chlorine gas into a sodium hydroxide solution. The reaction is
strongly basic and is seen as follows:
Cl2 ( g )  2OH 1 (aq)  ClO1 (aq)  H 2 0(l )
The amount of hypochlorite ion present can be determined be an
oxidation-reduction titration. The method that will be used in this
experiment will be an iodine-thiosulfate titration. Any oxidizing agent easily
oxidizes the iodide ion. In this case hypochlorite ions oxidize iodine ions
to form the element iodine. The iodine that forms is then titrated with a
standard solution of sodium thiosulfate.
(1)
Acidic iodide ion is added to hypochlorite ion solution, and the
iodide is oxidized to iodine.
2H 2 (aq)  ClO1 (aq)  2 I 1  Cl 1 (aq)  I 2 (aq)  H 2O
(2)
Iodine is only slightly soluble in water. It dissolves very well in
aqueous solution of iodide ion in which it forms the complex
triiodide ion ( I 31 ) consider the following reaction:
I 2 (aq)  I 1 (aq)  I 31 (aq)
The triiodide ion is yellow when in dilute solution and dark red
brown when concentrated.
(3)
The triiodide ion is titrated with a standard solution of thiosulfate
ions, which reduces the iodine back to the iodide ion.
I 31 (aq)  2S 2 032 (aq)  3I 1 (aq)  S 4 062 (aq)
During this last reaction the red brown color of the triiodide ion
fades to yellow then to clear color of the iodide ion. It is possible to use
the disappearance of the color of the triiodide ion as the method of
determining the end point. Addition of starch to the iodide ion or triiodide
ion forms a reversible blue complex. Starch must be added slowly or a
non-reversible reaction may form. Add the starch only near the end point.
1
Pre-Lab Questions
Answer the following questions on another sheet of paper. Be sure to show all
work. BE neat and concise.
1.
A student uses 24.35 ml of sodium thiosulfate solution to reach the
end-point in a titration. What is the concentration of the bleach?
2.
Remember the original bleach was diluted. What is the molarity of
the original solution?
3.
What is the percent sodium hypochlorite in the bleach? Calculate
the mass of one liter of bleach, using 1.08 gm/ml as the density of
the bleach. Also, calculate the number of grams of sodium
hypochlorite in 1 liter of bleach.
4.
A student calculates the percentages in each trial as follows:
Trial #
1
2
3
4
% Sodium hypochlorite
6.08
6.15
6.15
7.08
Calculate the standard deviation for this data and determine if any
data can be statistically considered invalid.
Chemicals Needed
Commercial Bleach Sodium Hypochlorite
Starch solution 2%
Hydrochloric Acid solution 3.0 M
Sodium Thiosulfate Solution .10 M
Potassium Iodide (solid)
Baking Soda (solid)
HCl
Na2 S2O3
KI
NaHCO3
Equipment Needed
1.
2.
3.
4.
5.
6.
5 ml and 25 ml pipets
Buret
Ring stand Assembly
Pipet Bulb
Flasks to hole chemicals (500 ml size) 4 required
Buret Clamp
2
Safety Considerations
Bleach is harmful to all clothing. Be careful not to get this chemical on
your clothes. Fumes from the iodine and the Hydrochloric Acid are also
dangerous. Be careful.
Procedure
1.
Use a pipet bulb and measure 5 ml of commercial bleach and add
to a 100 ml flask. Dilute with 25 ml of distilled water.
2.
Weigh out approximately 2 grams of Potassium Iodide. Hold for
later use.
3.
Add the solid Potassium Iodide to the bleach solution.
4.
Swirl the entire solution several times to ensure mixing. Add 2 ml
of 3.0 M HCl to the bleach solution to complete the ion ionization.
The solution should be dark yellow to a red brown. The I 31 ion is
now present.
5.
Record the initial buret reading. Titrate with .10 M of sodium
Thiosulfate until the iodine color become light yellow. Add one
dropper of starch solution. The blue color of the starch –iodine
solution should appear. Continue titrating until the blue color
disappears. Record the final buret reading.
6.
Repeat step 5 two more times.
3
Data Sheet
Trial 1
Trial 2
Trial 3
Initial Buret Reading
_________ ________
_________
Final Buret reading
_________
________
_________
Volume of Thiosulfate added
_________ ________
_________
Moles of Thiosulfate added
_________ ________
_________
Moles of Hypochlorite in dilute
_________ ________
_________
Concentration of Dilute
_________ ________
_________
Concentration of Undilute
_________ ________
_________
Mass of 1 L of Bleach
_________ ________
_________
Mass of NaOCl
_________ ________
_________
% NaOCl
_________ ________
_________
Standard Deviation
_________ ________
_________
Data Thrown out
_________ ________
_________
Density = 1.08 g/ml
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Write a paragraph describing a conclusion for this lab --
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