KWAME NKRUMAH UNIVERSITY OF SCIENCE AND
TECHNOLOGY
DEPARTMENT OF CHEMISTRY
YEAR TWO (CHEM 270)
TITLE: IODOMETRY
NAME: OPOKU ERNEST
EMAIL: ernest.opoku@gasp.knust.edu.gh
EXPERIMENT: I.2.2.3
DATE: 28TH JANUARY, 2014
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TITLE: IODOMETRY
AIMS AND OBJECTIVES:
1. To determine the amount of the active ingredient, hypochlorite in two household
bleaches, by titration of iodine released by reaction of hypochlorite with iodide.
2. To determine which bleach is the best to buy.
INTRODUCTION
Iodometry is the quantitative analysis of a solution of an oxidizing agent by adding an iodide
which reacts to form iodine, which is then titrated.
Iodometric titration, is a method of volumetric analysis, a redox titration where the
appearance or disappearance of elementary iodine indicates the end point. Iodometry involves
indirect titration of iodine liberated by reaction with the analyte, whereas iodimetry involves
direct titration using iodine as the titrant.
Iodine can be used as an oxidizing agent in many oxidation-reduction titrations and iodide
can be used as a reducing agent in other oxidation-reduction titrations:
I2
+ 2 e–
= 2 I–
(1)
If a standard iodine solution is used as a titrant for an oxidizable analyte, the technique is
iodimetry. If an excess of iodide is used to quantitatively reduce a chemical species while
simultaneously forming iodine, and if the iodine is subsequently titrated with thiosulfate, the
technique is iodometry. Iodometry is an example of an indirect determination since a product
of a preliminary reaction is titrated.
The use of iodine as a titrant suffers from two major disadvantages. First, iodine is not
particularly soluble in water, and second, iodine is somewhat volatile. Consequently, there
is an escape of significant amounts of dissolved iodine from the solution. Both of these
disadvantages are overcome by adding iodide (I–) to iodine (I ) solutions.
In the
2
presence of iodide, iodine reacts to form triiodide (I3) which is highly soluble and not
volatile.
I2 + I– = I3
(2)
The major chemical species present in these solutions is triiodide. The reduction of
triiodide to iodide is analogous to the reduction of iodine.
(3)
I3
+ 2 e = 3 ITriiodide reacts with thiosulfate to yield iodide and tetrathionate.
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I2 + 2S2O2-3
2I- + S4O2-6
(4)
Dilute triiodide solutions are yellow, more concentrated solutions are brown, and even
more concentrated solutions are violet. Iodide solutions are colorless. If all of the other
solution
components are colorless, it is possible to detect the endpoint of titrations involving triiodide
without the use of an indicator. Endpoint detection is considerably easier, however, with
an indicator. The indicator that is usually chosen for titrations involving iodine (triiodide) is
starch. Starch forms a dark blue complex with iodine. The end point in iodimetry
corresponds to a sudden color change to blue. Likewise the end point in iodometry
corresponds to a sudden loss of blue color due to the complex. Potato starch, rather than
corn starch, is preferred for making the indicator solution since the color change due to the
starch complex at the end point is sharper. In iodometry the starch is added only after the
color due to triiodide has begun to fade, i.e., near the endpoint, because starch can be
destroyed in the presence of excess triiodide.
In the first portion of this experiment a sodium thiosulfate solution is prepared and
standardized with the primary standard potassium iodate. Iodate (IO3) reacts with an excess
of iodide in acid solution to yield triiodide, which is subsequently titrated with the
standardized thiosulfate solution.
Iodometric Determination of Hypochlorite in Commercial Bleach Product
In most liquid laundry bleaches and in some solid bleaches, the active ingredient is
hypochlorite (OCl–). Liquid bleaches usually contain sodium hypochlorite and the
bleaching action is caused by the strong oxidizing properties of OCl–, which also are
exploited in this iodometric determination of hypochlorite. In the experiment, iodide is used
to reduce the hypochlorite in bleach. The reaction in acid solution yields chloride and
triiodide.
OCl– + 3 I– + 2 H+ = Cl– + I3
+ H2O
(5)
The triiodide formed in this reaction is titrated with the standard thiosulfate solution that
was prepared earlier.
(6)
+ 3 I2 S2O32– + I3- = S4O6
Thiosulfate ↑
↑ Tetrathionate
The endpoint of the titration is located with the starch indicator solution that was also
prepared earlier.
CHEMICALS AND EQUIPMENT
1. 0.1N Sodium thiosulphate
2. Potassium iodide
3. 2M sulphuric acid
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4. 0.1M KMnO4
5. Starch indicator
6. White magic bleaching solution ( GH¢3.50)
7. Janola bleaching solution ( GH¢ 5.0)
8. 10ml pipette
9. 3 Conical flask
10. Electronic balance
11. Burette
12. Funnel
PROCEDURE
1. Saturation of sodium thiosulphate
Experiment
2g of KI + 10ml of KMnO4
2g of KI + 10ml of KMnO4 + 5ml of 2M
H2SO4, follow by titration with the sodium
thiosulphate.
Observation
Purple colour decolourised
The decolouration persist,
Colour changes were observed until pale
yellow colouration occurs, the colour change
was left for a while.
1ml of freshly prepared iodine was added to
A blue black solution was formed indicating
the pale yellow solution
the presence of iodine.
The blue solution was titrated against a drop
The blue black colour disappeared to form a
of sodium thiosulphate.
colourless solution.
The procedure was repeated for another two experiments.
2. Analysis of bleaching solution.
JANOLA
Experiment
Observation
1g of KI was measured into a conical flask
and then 20ml of distilled water added to it.
1ml of the bleach (janola) + 10ml of 2M
Orange solution was observed
H2SO4 was added to the prepared solution.
The liberated iodine was then titrated against There was a colour change from orange to
the thiosulphate solution.
pale yellow
1ml of starch solution was added to the pale
A blue black solution form indicating the
yellow solution
presence of iodine.
The blue solution was titrated against a drop
The blue black colour disappeared to form a
of sodium thiosulphate.
colourless solution
The procedure was repeated for two more experiment.
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WHITE MAGIC
Experiment
Observation
1g of KI was measured into a conical flask
and then 20ml of distilled water added to it.
1ml of the bleach solution (white magic) was
added to the prepared solution.
The liberated iodine was titrated against the
The colour change from orange to pale
thiosulphate solution.
yellow.
1ml of starch solution was added to the pale
A blue black solution form indicating the
yellow solution
presence of iodine.
The blue solution was titrated against a drop
The blue black colour disappeared to form a
of sodium thiosulphate
colourless solution
The procedure was repeated for two more experiment.
TABLE OF RESULTS
STANDARDIZATION OF SODIUM THIOSULPHATE
Titration
Final volume/ml
Initial volume/ml
Titre value/ml
1
10.80
0.00
10.80
2
10.85
0.00
10.85
3
10.80
0.00
10.80
Average titre = (10.80 + 10.85 +10.80) = 10.82ml
3
JANOLA
Titration
1
Final volume/ml
1.50
Initial volume/ml
0.00
Titre value/ml
1.50
Average titre= (1.30 + 1.30)= 1.30ml
2
2
2.80
1.50
1.30
3
4.10
2.80
1.30
2
6.65
6.45
0.20
3
6.89
6.65
0.24
WHITE MAGIC
Titration
Final volume/ml
Initial volume/ml
Titre value/ml
1
6.45
6.10
0.35
Average titre= (0.20 + 0.24) = 0.22ml
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2
CALCULATIONS AND EVALUATION OF DATA
From the reaction equation
2MnO-4 + 16H+ + 10I-
2Mn2+ + 5I2 + 8H2O
n(MnO-4) = 2
n(I2)
5
n(I2)= 5 × n(MnO2
n(MnO-4) = concentration x volume
n(MnO-4)= 0.1mol/L × 0.01L = 0.001mol
n(I2) = 5 × 0.001mol= 2.5×10-3mol
2
from the equation
I2 + 2S2O42-
2I- + S4O-26
n(S2O-23) = 2 × 2.5×10-3mol = 0.005mol
from the titration average titre = 10.82ml = 0.01082L
concentration = mole
volume
[S2O-23] = 0.005mol
0.01082L
= 0.462M
JANOLA
From
I2 + 2S2O2-3
2I- + S4O-26
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n(S2O2-3) that reacted with the liberated I2 = conc. x average titre volume
n(S2O2-3) that reacted with the liberated I2 = 0.462 M × 0.00130L = 6.006 × 10-4mol
but n(I2) = ½ × n(S2O-23) = ½ × 6.006 × 10-4= 3.003 ×10-4mol
hence n(I2) liberated= 3.003 ×10-4mol
from the equation
ClO- + 2I- + 2H+
Cl- + I2 + H2O
n(I2) liberated = n(ClO-)
hence n(ClO-)= 3.003 ×10-4mol
since the volume of bleach taken = 1ml =0.001L
then [ClO-]= (3.003×10-4) = 0.3003M
0.001L
Mass(ClO-) = mole (ClO-) x molar mass of (ClO-)
Molar mass(ClO-) = (35.5 + 16) = 51.5g/mol
m (ClO-) in Janola= 3.003 ×10-4 × 51.5 = 0.0155g
WHITE MAGIC
n(S2O2-3) that reacted with the liberated I2 = 0.462×0.22×10-3 = 1.0164 ×10-4mol
but n(I2) = ½ × n(S2O2-3)= ½ ×1.10164×10-4 = 5.082 × 10-5mol
since n(I2)= n(ClO-),
n(ClO-)= 5.082 ×10-5mol
since the volume of bleach used = 1ml = 0.001M
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thus [ClO-]= (5.082 × 10-5= 0.05082M
0.001L
Mass(ClO-) = mole (ClO-) x molar mass of (ClO-)
Molar mass(ClO-) = (35.5 + 16) = 51.5g/mol
m (ClO-) in white magic = 5.082×10-5 × 51.5= 2.61723 × 10-3g
1. Janola
If the cost is 5.00 Ghana cedis
A unit in cedis per gram = GH¢ 5.00 = GH¢ 322.58p/g
0.0155g
= GH¢ 322.58p/g
ii. white magic
If the cost is GH¢ 3.50p
A unit in cedis per gram = GH¢ 3.50p
= GH¢ 1337.29p/g
-3
2.61723 × 10 g
= GH¢ 1337.29p/g
Based on my results, Janola is the best to buy amongst the two.
DISCUSSION
From the experiment performed above, the reaction between the measured gram of potassium
iodide and 10ml of potassium permanganate, the purple colour of the permanganate was
decolourised by iodide ion. 5ml of sulphuric acid was added to the permanganate- iodide
solution to prevent the decomposition of the ions in the solution and also to create medium
for the solution to proceed.
2MnO-4 + 16H+ + 10I-
2Mn2+ + 5I2 + 8H2O
The solution was then titrated against a standard sodium thiosulphate, the titration proceeds
with colour changes till pale yellow colour was observe, showing the presence of iodine in
solution. The solution was allow for two minutes, this was done to see whether the pale
yellow colour will stand, 1ml of starch solution was added to the pale yellow solution and
the colour changes to blue black, confirming the presence of the iodine. A few drop of the
thiosulphate turn the solution colourless. The volume of the titration was recorded and
addition two experiments were conducted. Average titre value was determined for the three
experiments.
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The iodide content of two different bleaches (Janola and White magic) were given, 1ml of
each sample was taken into different conical flask, a measured amount of potassium iodide
was dissolves into a certain volume of water. The measured samples of the bleach was
transferred into the iodide solution which gives yellow solution. This was also titrated
against sodium thiosulphate till a pale yellow solution was obtain, 1ml of starch solution was
added to the pale yellow solution which turns blue black. A drop or two of thiosulphate
changes the solution to colourless. It was observed that the Janola has higher titre values
whilst white magic has less titre values.
I2 + 2S2O32-
2I- + S4O62-
In the calculation, the mole ratio of permanganate and iodine was used together with their
stochiometric ratio was used to find mole of the iodine, by so doing concentration and
volume of the permanganate was used. Knowing the mole of iodine, which is in ratio one is
to two (1:2) with the thiosulphate, mole of thiosulphate is equal two times the mole of iodine.
So knowing the mole of thiosulphate, the concentration of the thiosulphate was calculated
using the volume and the mole.
With the mole of iodine, which is in ratio of (1:1) with perchlorate? The mole and
concentration of hypochlorate was calculated, masses of the hypochlorates in both janola and
white magic are calculated. These masses are used to determine the price of each of the
bleach.
ClO- + 2I- + 2H+
Cl- + I2 + H2O.
CONCLUSION
From the experiment and the calculation above, we can conclude that the active ingredients
of the two bleaches are the hypochlorite. The respective masses of hypochlorite present in
Janola and White magic were found to be 15.5mg and 2.86172mg. Their price tags were
found to be right because of the quantity difference. However it is recommended that Janola
is the best to buy.
PRECAUTIONS
1. The gases produced during the dissolution of the brass sample are toxic. Therefore all
reactions that involved the evolution of a gas were done in fumehood.
2. Goggles were worn to prevent any of the solution from getting into contact with the eye
during boiling.
3. Gloves were worn to prevent any of the solution from getting into contact with the body.
REFERENCES
1. Modern Inorganic Chemistry, Second Edition by William L. Jolly, 2010, pages 46 to 49.
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2. Journal of Solid State Chemistry, 2012, pages 4 to 7, retrieved from
http://www.sciencedirect.com/science.
3. Concise Inorganic Chemistry, Fifth Edition by J.D Lee, 2007, pages 202-204 and 951.
4. KNUST Chemistry laboratory manual for second year, CHEM 269 & 270, page 32- 34.
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