Experiment Number: 04
Name of the experiment:
Standardization of Sodium Thiosulphate
solution with standard Potassium
Dichromate solution.
Course: Chem-114
Name:—
Roll:—
Group:—
Partner’s Roll:—
Department:— C.S.E.
Date of Performance:—
Date of Submission:—
Objective:—
‘Sodium Thiosulphate’ is a very important industrial raw
material. It is widely used in photographic studio. The
objective of this experiment is to determine the strength of
this Sodium Thiosulphate.
This experiment is based on oxidation and reduction
reaction, as well as “Iodometric reaction”. The main purpose
of the experiment is to standardize Na2S2O3 solution with
standard K2Cr2O7 solution.
Theory:—
In this experiment the strength of Sodium Thiosulphate
solution is determined with the help of a standard Potassium
Dichromate solution. This is an oxidation and reduction
reaction, as well as “Iodometric reaction”.
Titration involving with iodine or dealing with iodine liberated
in chemical reaction is called Iodimetric and Iodometric
titration respectively. This reaction is iodometric because
iodine is got from KI.
The reactions of this experiment are:
6KI+14HCl+ K2Cr2O7 =2CrCl3+3I2 +7H2O+8KCl
(6 I- + 14 H+ + Cr2O7-2 + 6e- = 2 Cr+3 + 3 I2+ 7 H2O)
2Na2S2O3 + I2 = Na2S4O6 + 2NaI
(S2O3 2- + e- = S4O6 2- + I-)
Here K2Cr2O7 is an oxidizing agent and I- is a reducing agent.
Again in the second reaction I2 is an oxidizing agent and S2O3-is a reducing agent.
In the 2nd Step of the reaction a specific indicator is used that
is “Starch”- which has a significant characteristic on iodine.
Starch+I2=Starch-Iodine (blue colour)
Starch-Iodine+6S2O3--=Starch +6I- + 3S4 O6—
If to a solution containing a little iodine, some starch solution
is added and Na2S2O3 is run in from the burette, the blue color
of the starch-iodine complex will disappear from the solution
as soon as all the iodine has been reduced to iodide ion.
Apparatus:—
1.
2.
3.
4.
5.
6.
Conical flask
Burette
Pipette
Volumetric flask
Stand
Funnel
Name of the chemicals used:—
1. Na2S2O3
2. K2Cr2O7
3. KI
4. NaHCO3
5. HCl (concentrated)
6. Starch (Indicator)
Data:—
(Standardization of Sodium Thiosulphate solution with
standard potassium Dichromate Solution)
Burette
Number of
Obser-vation
Volume
of
K2Cr2O7
Initial
Reading
reading
Final
Reading
Volume of
Na2S2O3
Average
Volume of
NaS2O3
ml
01
02
03
04
Ml
10
10
10
10
ml
4.9
14.6
24.3
34.1
ml
14.6
24.3
34.1
43.8
Calculation:—
from Vred X Sred = Vox X Sox
Here,
VK2Cr2O7 = 10 ml
SK2Cr2O7 = 0.1065 N
VNa2S2O4 = 9.725 ml (average)
SNa2S2O4 = 0.1095 N
Result:—
Determined strength of Na2S2O3 solution is:
SNa2S2O3 = 0.1095 N
Discussion:—
This experiment is very important because it gives the
idea of oxidation and reduction reaction and at the same time,
it views a clear idea about iodometric reaction.
ml
9.7
9.7
9.8
9.7
9.725