BLEACH ANALYSIS
1. A student followed the procedure of this experiment to
determine the percent NaOCl in a commercial bleaching solution that was found in
the basement of an abandoned house. The
student diluted 50.00 mL of bleach to 250.0 mL in a volumetric flask and
titrated a 20.00mL sample of the diluted bleaching solution. The titration required 35.46 mL of 0.1052 M Na2S2O3
solution. A faded price label on the gallon bottle read $0.79. The density of the bleaching solution was
1.10 g/mL. Note: 1.00gal = 3.78L
a) Calculate
the number of moles of S2O32- ion required for
the titration.
b) Calculate
the number of moles of I2 produced in the titration mixture.
c) Calculate
the number of moles of OCl- ion present in the diluted bleaching
solution
titrated.
d)
Calculate
the mass of NaOCl present in the diluted bleaching solution titrated (the
20.00mL sample).
e)
Calculate
the mass of NaOCl present in the original 50.00mL sample of undiluted bleach.
f) Calculate
the mass of the original 50.00mL sample of undiluted bleach.
g) Determine
the mass percent NaOCl in original 50.00mL sample of undiluted bleach.
h) Calculate
the mass of undiluted bleaching solution required to give 100.0 g of NaOCl.
i) Calculate
the volume of undiluted bleaching solution required to give 100.0 g of
NaOCl.
j) Determine the cost of the amount of undiluted bleaching
solution required to give
100.0 g of NaOCl.
BLEACH ANALYSIS
Objectives:
·
To
carry out a redox reaction between acidified iodide ions and hypochlorite ions.
·
To
titrate the triiodide ion (I3-) formed by the
iodine combining with the excess iodide ion to determine the concentration of
commercial bleach solution.
·
To
use a starch complex indicator to determine the end point in the titration of
bleach with acidified iodide potassium iodide solution.
Theory:
Many commercial products are
effective because they contain oxidizing agents. Some products, which contain
oxidizing agents, are bleaches, hair coloring agents, scouring powders, and toilet
bowl cleaners. The most common oxidizing agent in bleach is sodium
hypochlorite, NaOCl. Commercial bleaches are produced by bubbling chlorine gas
into a sodium hydroxide solution. Some of the chlorine is oxidized to the
hypochlorite ion, OCl- and some is reduced to the chloride ion, Cl-. The solution remains strongly basic. The
chemical equation for the process is:
Cl2 (g) + 2 OH- (aq) ® ClO- (aq) + Cl- (aq) + H2O (l)
The amount of hypochlorite
ion present in a solution can be determined by an oxidation-reduction
titration. One of the best methods is the iodine-thiosulfate titration
procedure. Iodide ion, I-, is easily oxidized by many oxidizing
agents. In acid solution, hypochlorite
ions oxidize iodide ions to form iodine, I2. The iodine that forms is then titrated with
a standard (known molarity) solution of sodium thiosulfate.
The analysis takes place in
a series of steps:
(1) Acidified iodide ion is added to the hypochlorite ion
solution and the iodide is oxidized
to iodine.
2 H+ (aq) +
OCl- (aq)
+ 2 I- (aq) ® Cl- (aq) + I2 (aq) +
H2O (l) (reaction 1)
(2) Iodine
is only slightly soluble in water. It
dissolves very well in an aqueous solution of
iodide ion, in which it forms a complex ion called the triiodide ion, I3-.
Triiodide ion is a
combination of a neutral I2 molecule and an I-
ion. The triiodide ion is yellow in
dilute
solution, and dark red-brown
when concentrated.
I2 (aq) +
I- (aq)
® I3- (aq) (reaction 2)
(3) The
triiodide ion is titrated with a standard solution of thiosulfate ions, which
reduces the
iodine back to iodide ions.
I3-
(aq) + 2 S2O3-2
(aq) ® 3
I- (aq) + S4O6-2
(aq) (reaction 3)
During the last reaction the
red-brown color of the solution of the triiodide ion fades to yellow and then
to the colorless solution of the iodide ion. It is possible to use the
disappearance of the color of the I3- ion as a method of
determining the end point in the titration, but this is not a very sensitive
procedure. Addition of starch to a solution that contains iodine or triiodide
ion forms a reversible deep blue complex. The disappearance of this blue
colored complex is a much more sensitive method of determining the end point.
If the starch is added to a solution, which contains a great deal of iodine,
the starch-iodine complex, which forms, may not be reversible. Therefore, the starch is not added until
shortly before the end point is reached. The moles of thiosulfate used in the
titration and the stoichiometry in reactions 1-3 are used to determine the
amount of hypochlorite initially present in the sample.
Procedure:
1. Rinse a 50.00mL buret with a small amount of the standardized thiosulfate solution. Fill the buret with the thiosulfate solution. Be sure the tip of the buret is filled.
2. Record the initial buret reading and the
molarity of the thiosulfate solution.
3. Use a 5.00mL pipet and bulb to transfer a 5.00mL
portion of commercial bleach into a 100.00 mL volumetric flask. Dilute to the line with distilled water,
stopper and mix well.
4. Mass 2.00 g of solid KI in to plastic
weighing boat. The KI is in excess so a centigram balance can be used and the mass
of KI does not need to be known accurately.
5. Pipet 25.00mL of the dilute bleach into a
125mL Erlenmeyer flask.
6. Add the KI and approximately 25mL of
distilled water, swirl to dissolve the KI.
7. Add approximately 2.0mL of 3 M HCl to the solution. It will be dark
yellow to red-brown from the presence of the triiodide complex ion.
8. Titrate the iodine with the standardized
sodium thiosulfate solution until the iodine coloris light yellow. Add 1-2
droppers of starch solution and observe the blue starch-iodine complex.
9. Slowly continue titration dropwise
with good mixing until the blue color disappears.
10. Record the final buret reading and
calculate the volume of thiosulfate solution used.
11. Rinse the flask with hot water being
careful not to lose the magnetic stir bar,
12. Repeat the titration twice more.
NAME:__________________________________________
COURSE:___________
LAB
PARTNER:___________________________________ PERIOD:___________
DATA TABLE
Trial number |
1 |
2 |
3 |
|
Initial
buret reading (mL) |
|
|
|
|
Final buret reading (mL) |
|
|
|
|
Volume of thiosulfate used
(mL) |
|
|
|
|
Molarity of thiosulfate
(M) |
|
|
|
|
Moles S2O3-2 |
|
|
|
|
Moles I2 |
|
|
|
|
Moles OCl-1 in the
25.00mL of dilute bleach |
|
|
|
|
Moles OCl-1 in
the 5.00mL of undiluted bleach |
|
|
|
|
Density of the undiluted
bleach |
1.08 g/mL |
|
|
|
Mass of 5.00mL of
undiluted bleach (g) |
|
|
|
|
Molar mass of NaOCl |
|
|
|
|
Mass NaOCl in the 5.00mL
of undiluted bleach |
|
|
|
|
Mass % NaOCl in the
undiluted bleach |
|
|
|
|
Relative % Error* |
|
|
|
*Use 5.25% as the actual value or the percent
listed on the label of the bleach bottle.
Show a sample calculation for one trial
below:
Discussion: Comment on precision,
errors and difficulties, and whether or not the commercial bleach meets,
exceeds, or falls short of the advertised NaOCl content (5.25%).
BLEACH ANALYSIS
Equipment:
10 Burets
labeled: Na2S2O3
10 Buret
reading cards
10 25.0
mL graduated cylinders
10 250
mL Erlenmeyer flasks
10 10
mL graduated cylinders
10 Magnetic
stirrers
10 Medium
magnetic stir bars
3 Stir
bar retrievers
4 Wide
mouth screw top bottles labeled: KI
3 250ml
Plastic reagent bottles labeled Clorox
3 Dropper
bottles of starch indicator solution
10 Wash
bottles with distilled water
10 Boxes
Kimwipes
3 1
Liter plastic reagent bottles labeled Na2S2O3
10 Medium
plastic weigh boats
10 Scoopulas
labeled KI
10 100mL
volumetric flasks with stoppers
10 Pipet
racks
10 Rubber
pipet bulbs
10 5.00mL
volumetric pipets labeled undiluted
10 25.00mL
volumetric pipets labeled undiluted
10 liters of 0.100 M Na2S2O3 (24.8 g /l.or 248g /10 l)
Standardize the solution.
3 liters of Chlorox (100 mL..1 liter x 3)
Solution
Preparation:
2% starch solution: prepare 200 mL of starch solution by first dissolving 4 grams of soluble starch into ~10 mL of distilled water. Pour into 80 mL of boiling distilled water while stirring. Dilute to 200mL.
3 M HCl:
500 mL of HCl solution by adding 125 mL of 12 M HCl to 200 mL of
distilled water and diluting to a final volume of 500 mL.
0.1000M Na2S2O3:
Dissolve 24.8 g of Na2S2O3
· 5 H2O in 150 mL of distilled water in a
one liter volumetric flask and dilute with distilled water to a final volume.
Mix thoroughly.
Alternate.Starch
prep.(Shakasheri).
1% starch:1 liter of
solution (heat 500 mL of distilled water to boiling in a 1-liter beaker.
Prepare a slurry in a 50mL beaker of 10 g starch in 20mL of distilled water.
Pour the slurry into the boiling water and boil for 5 minutes. Put 450 mL of
cold DI water in a 2 liter beaker and pour the hot starch solution into it.
Dilute to 1 liter after the solution reaches room temperature. Rinse the
storage bottle with boiling DI water before storing the solution to inhibit
bacterial growth.
BLEACH ANALYSIS
1. Put a white surface under the titration
flask to see the loss of the iodine color and titrate to a very pale yellow, If
the solution does not turn blue when the starch is added, the sample has been
over-titrated (the endpoint overrun) and the sample needs to be repeated.
Adding the starch too early results in too much iodine adhering to the starch and
an endpoint that is not sharp.
2. Chlorine bleach is very basic and should
be handled with care. Safety glasses must be worn. The students should be
reminded to wear old clothes during this lab. The bleach will affect many dyed
fabrics.
3. Answers to the prelab questions.
a. Calculate the number of moles of S2O32-
ion required for the titration.
Mole S2O3-2
= (3.546x10-2L)(0.1052M) = 3.730x10-3mole S2O3-2
b. Calculate the number of moles of I2
produced in the titration mixture.
3.730x10-3mole S2O3-2
(1 mol I2/ 2 mol S2O3-2) = 1.865x10-3
mol I2
c. Calculate the number of moles of OCl-
ion present in the diluted bleaching solution titrated.
1.865x10-3 mol I2
(1 mol OCl-1/ 1 mol I2) = 1.865x10-3 mol OCl-1
d. Calculate the mass of NaOCl present in the diluted
bleaching solution titrated (the 20.00mL sample).
1.865x10-3 mol
NaOCl (74.45g/mol) = 0.1388g NaOCl
e. Calculate the mass of NaOCl present in the
original 50.00mL sample of undiluted bleach.
0.1388g NaOCl (250mL total
sample/ 20mL sample titrated) = 1.736g
f. Calculate the mass of the original 50.00mL
sample of undiluted bleach.
(50.00mL)(1.10g.mL) = 55.0g
bleach
g. Determine the mass percent NaOCl in
original 50.00mL sample of undiluted bleach.
(1.736g NaOCl)/(55.0g
bleach) x 100 = 3.16%
h. Calculate the mass of undiluted bleaching
solution required to give 100.0 g of NaOCl.
100.0g
NaOCl = (0.0316)(X g bleach) X g bleach =
3.16x103g
i. Calculate the volume of undiluted
bleaching solution required to give 100.0 g of NaOCl.
V=mass/volume (3.16x103g)/ (1.10g/mL) =
2.88x103mL 2.88L
j. Determine the cost of the
amount of undiluted bleaching solution required to give 100.0 g of NaOCl.
2.88L(1.00gal/L)(0.79/gal) =
0.60 $0.60
4. Sample Data
Trial number |
1 |
2 |
3 |
|
Initial
buret reading (mL) |
0.00 |
|
|
|
Final buret reading (mL) |
19.30 |
|
|
|
Volume of thiosulfate used
(mL) |
19.30 |
|
|
|
Molarity of thiosulfate
(M) |
0.1000 |
|
|
|
Moles S2O3-2 |
1.930x10-3 |
|
|
|
Moles I2 |
9.650x10-4 |
|
|
|
Moles OCl-1 in
the 25.00mL of dilute bleach |
9.650x10-4 |
|
|
|
Moles OCl-1 in
the 5.00mL of undiluted bleach |
3.860x10-3 |
|
|
|
Density of the undiluted
bleach |
1.08 g/mL |
|
|
|
Mass of 5.00mL of
undiluted bleach (g) |
5.40 |
|
|
|
Molar mass of NaOCl
(g/mol) |
74.45 |
|
|
|
Mass NaOCl in the 5.00mL
of undiluted bleach (g) |
0.2874 |
|
|
|
Mass % NaOCl in the
undiluted bleach (%) |
5.32 |
|
|
|
Relative % Error* (%) |
1 |
|
|