Chemistry Laboratory Presidency
University
CHEM 102
Summer 2004
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Experiment 4: Standardization of sodium thiosulphate solution with a
standard potassium dichromate solution.
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Type of reaction: Oxidation – reduction titration
Theory: a known weight of pure dichromate crystal is treated with excess potassium
iodide in hydrochloric acid solution and liberated iodine is treated with sodium
thiosulphate solution which is to be standardized.
Reaction:
6KI = 3I2 (Oxidation)
3I2 + 3I- = 3I3-
K2Cr2O7 + 6e = 2K+ + 2Cr3+ + 7O2- (reduction)
6 Na2S2O3 – 6e = 3 Na2S4O6 + 6 Na+ (oxidation)
3I3- + 6e = 9I- (reduction)
Therefore,
1 K2Cr2O7 = 3I2= 6 Na2S2O3
Apparatus:
1. Conical flask
2. Pipette
3. burette
4. Beaker
5. Funnel
6. Volumetric Flask (250 ml)
7. Electric balance
Reagents and Chemicals:
1. Potassium dichromate(K2Cr2O7)
2. Sodium thiosulphate
3. Concentrated Hydrochloric acid
4. Sodium bicarbonate
5. Potassium Iodide
Indicator: Starch solution
Procedure:
a. Preparation of .05 (M) K2Cr2O7 solution
1) Make a 250 ml of this solution by dissolving the calculated amount of potassium
dichromate in 250ml of distilled water in a 250 ml volumetric flask. Use electronic
balance very carefully to measure the amount of potassium dichromate.
b. Standardization of sodium thiosulphate using standard potassium
dichromate
1) Take out 50 ml distilled water in a conical flask.
2) Add 0.75 gm NaHCO3 and 2.0 gm KI and dissolved them.
3) Add 4.0ml concentrated HCl.
4) Take 10 ml of the 0.05(M) potassium dichromate in the same conical flask and
leave it in a dark place covered with watch glass for 5 – 10 min. The solution should
be deep brown.
5) Now add 50 ml distilled water and titrate with sodium thiosulphate solution
following the sequence:
i) Titrate until the brown color becomes fade and add 2-3 ml starch
solution.
ii) Continue titration until the blue color is changed to light blue or light
green.
iii) Carry out two more titrations and calculate the volume of the
thiosulphate and note them in your lab book. Take the average of these
three volumes.
iv) Now write the balanced oxidation – reduction reaction and find the
equation to calculate the strength of sodium thiosulphate solution.
Experiment # 4: Standardization of sodium thiosulphate solution with a standard
potassium dichromate solution.
Name: Presidency University
Student ID: Chemistry Laboratory
Date: CHEM102
Prelab
Answer the following question:
1. How does a redox indicator works?
2. What is the difference between acid base and a redox indicator?
3. What type of standard is K2Cr2O7?
4. Can you use any other chemicals instead of NaHCO3?
5. Why you should cover the conical flask after adding NaHCO3?
6. What type of titration you are performing?
7. Can you use KMnO4 instead of K2Cr2O7 ? Which is stronger oxidizing acid?
8. What is the function of starch?
9. Why is it necessary to add starch at the end point of titration?
Experiment # 4: Standardization of sodium thiosulphate solution with a standard
potassium dichromate solution.
Name: Presidency University
Student ID: Chemistry Laboratory
Date: CHEM102
Calculations & Results
Observation No Burette reading (ml) Amount of Average
Initial reading Final Na2S2O3 amount of
(ml) reading(ml) consumed(ml) Na2S2O3
consumed (ml)
Reaction:
Calculation:
Result:
Discussion:
