Experiment No. ___________________
Date ___________________
IODOMETRY
AIM
To determine the amount of copper by a Redox Titration.
INTRODUCTION
Iodide ion (I-) is a moderately effective reducing agent that has been used for the analysis of
oxidants.
The lack of a good method of end point detection makes direct titration of oxidizing agents by
solutions of iodide salts impractical. Thus, indirect procedure is always employed. This involves
reduction with a moderate unmeasured excess of potassium iodide. Liberated iodine (I2) is then
titrated with a standard solution of a reducing agent. The quantity of the iodine is equivalent in
quantity to the oxidant being determined. Sodium thiosulfate, Na2S2O3, is mostly used for this
purpose.
The reaction between iodine and thiosulfate ion is as follows:
2S2O32- + I2 ↔ S4O62- + 2IThe end point in the titration is readily established by means of starch solution. It should be
emphasized that starch is partially decomposed in the presence of a large excess of iodine. For
this reason the indicator is never added to an iodine solution until the bulk of that substance has
been reduced. The change in color of the iodine from a red-brown to a faint yellow signals the
proper time for the addition of the indicator.
REAGENTS AND APPARATUS
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CuSO4.5H2O (unknown solution, already prepared)
Sodium thiosulfate, Na2S2O3.5H2O (0.5 L of 0.1 M for 3 students)
Sodium carbonate, Na2CO3 (in the balance room)
Potassium iodate, KIO3 (primary standart)
Potassium iodide, KI (in the balance room)
Potassium thiocyanate, KSCN (in the balance room)
Hydrochloric acid, HCl (25 mL of 6.0 M for 2 students)
Sulfuric acid, H2SO4 (25 mL of 3.0 M for 2 students)
Starch indicator (already prepared)
250mL conical flasks
100 mL graduated cylinder
buret
PROCEDURE
A) Preparation of Na2S2O3 Solution
1)
2)
Heat 0.5 L of distilled water to boiling in a beaker covered with a watch glass. Boil for at
least 10 minutes.
Cool and add necessary amount of Na2S2O3.5H2O to prepare 0.5 L of 0.1 M Na2S2O3 and
add 0.05 g of Na2CO3.
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3)
Thiosulfate solution is readily effected by microorganisms and its concentration is
changed. This effect can be minimized by addition of several substances like Na2CO3,
NaOH, borax (NaB2O4) and chloroform (CHCl3).
Stir until Na2S2O3.5H2O and Na2CO3 are dissolved.
B) Standardization of Na2S2O3 Solution
1)
2)
3)
4)
Weigh 0.12 to 0.15 g samples of KIO3 into 250 mL erlenmeyer flask.
Dissolve in 50 mL of water and add 2.0 g of KI
After KI salt has dissolved, add 2.0 mL of 6.0 M HCl and titrate immediately with Na2S2O3
solution until the color of the solution becomes pale yellow.
Add 5.0 mL of starch indicator and titrate to the disapperance of the blue color. The
reactions involved can be expressed as follow:
IO3  5I   6H   3I2  3H2O
2  6I   S O2
3I2  6S2O3
4 6
2  3H O
IO3  6H   6S2O2  I   3S4 O6
2
3
5)
Two replicates will be performed for each student. Calculate the molarity of the thiosulfate
solution for the results of 4 students.
C) Determination of Copper
1)
2)
3)
Take your unknown sample into a 250 mL erlenmeyer flask and add 50 mL distilled water.
Add 1.0 mL of 3.0 M H2SO4 and then 4.0 g of KI.
Titrate immediately with standardized Na2S2O3 solution until the solution becomes pale
yellow and then, add 2.0 g of potassium thiocyanate, KSCN and 5.0 mL of starch indicator.
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It has been found experimentally that the titration of iodine by thiosulfate in the
presence of CuI tends to yield slightly low results because appreciable quantities of
iodine are adsorbed on the solid. This difficulty is largely overcome by the addition of
thiocyanate (SCN-) ion which also forms a sparingly soluble copper (I) salt. Part of the
copper (I) iodide is converted to the corresponding thiocyanate at the surface of the
solid:
2Cu2   4I   2CuI(s)  I2
CuI(s)  SCN   CuSCN(s)  I 
Accompanying this reaction is the release of the adsorbed iodine, I2, thus making it
available for titration. Early addition of SCN- must be avoided, however, because of the
tendency for that ion to reduce iodine slowly.
4)
5)
Swirly vigorously for several seconds.
Continue the titration with vigorous mixing until the blue starch/iodine color is decolorized
and does not return for several minutes. The reactions involved can be expressed as follow:
2Cu 2  4I   2CuI(s)  I2
2  2I   S O2
I2  2S2O3
4 6
2  2CuI(s)  S O2
2I -  2Cu 2  2S2O3
4 6
6)
Report the result in terms of mg Cu in your unknown sample.
PRE-LAB STUDIES
Read pages 512-514 (20B-2) from the textbook (9th Ed)
1) Why KMnO4 is preferred as an oxidizing agent in red-ox titrations?
2) Why do we apply an indirect method in iodometry?
3) What is the function of starch in this experiment and why do not we add it at the beginning of
the titration?
4) What is the name of primary standard used in the standardization of thiosulfate solution?
5) What is the reason of rarely using iodine as primary standard for the standardization of
thiosulfate solution?
POST-LAB STUDIES
1)
2)
3)
4)
During the preparation of sodium thiosulfate solution,
i) why do we add Na2CO3?
ii) why should we store Na2S2O3 solution in dark and closed bottles?
Answer the following questions for the standardization of sodium thiosulfate:
i) Explain what happens during the titration and write the related reactions.
ii) Why should the medium be acidic?
iii) What kind of error (positive/negative) is expected to arise if the titration is not started right
after the addition of acid?
What is the function of KSCN in the Cu2+ determination? Write the related reaction(s).
Why do not we add KSCN at the beginning of the experiment? What kind of error is obtained
if it is not added during the titration? Explain.
Name surname:
Section:
Date:
REPORT SHEET FOR IODOMETRY
B. Standardization of Na2S2O3 Solution
Student
1
Student
2
Student
3
Mass of
Volume of
Concentration of
KIO3, g
Na2S2O3, mL
Na2S2O3, mol/L
Concentration of
Na2S2O3, mol/L
̅  𝐬)
(𝐗
Replicate 1
Replicate 2
Replicate 1
Replicate 2
Replicate 1
Replicate 2
C. Determination of Copper
Volume of Na2S2O3,
Mass of
Mean mass,
True mass of
% Relative
mL
copper, mg
̅  𝐬)
mg (𝐗
copper, mg
Error
Replicate 1
Replicate 2
TA`s Name and Signature: