chemistry 04

7/25/2019 Chemistry 04

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Experiment Number: 04

Name of the experiment:

Standardization of Sodium Thiosulphate solution withstandard Potassium Dichromate solution.

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Name:# $tanu %umar Saha

&oll:# 0'0(")*

+roup:# c"

Partner,s &oll:#0'0(")4

Department:# E.E.E.

Date of Performance:# "(!0-!)00-

Date of Submission:# )!0-!)00-


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Objective:-

The objective of this experiment is to determine the strength of Sodium

Thiosulphate (Na2S2O3) with a standard Potassium Dichromate (K2Cr2O7)

solution. This experiment is based on oxidation and reduction reaction, as well as

iodometry. Sodium Thiosulphate has very important and huge industrial usage.

It is widely used in the photographic processing and to remove excess chlorine in

the bleaching industry. This examination is very useful to find out the percentage

of purity of Sodium Thiosulphate.

Theory:- Standardization is the process by which the strength of a solution is

determined with the help of a standard solution. A solution of known

concentration is called a standard solution. This experiment is done by means of

titration. In presence of a suitable indicator, a chemical substance that detects the

end point of reaction by changing its color, the volumetric analysis in which a

standard solution is added in another solution (whose strength is unknown) to

reach its end point to determine the strength of that solution is called titration.Titration involving iodine or dealing with liberated iodine in chemical

reaction is called iodometry and iodometry respectively. This reaction is

iodometry because iodine is obtained 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

(S2O32-+ e-= S4O6

2-+ I-)

Here K2Cr2O7is an oxidizing agent and I-is a reducing agent. Again in thesecond reaction I2

is an oxidizing agent and S2O3--is a reducing agent.

In the 2ndStep of the reaction a specific indicator is used that is Starch-

which has a significant effect on iodine.

Starch+I2=Starch-Iodine (blue color)

Starch-Iodine+6S2O3--=Starch +6I-+ 3S4O6


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If to a solution containing a little iodine, some starch solution is added and

Na2S2O3is 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. Conical flask, 2. Burette , 3. Pipette ,

4. Volumetric flask , 5. Stand, 6. Funnel

Chemicals:-1. Na2S2O3, 2. K2Cr2O7, 3. KI , 4. NaHCO35. HCl (Concentrated) , 6. Starch (Indicator)

Data:-Standardization of Na2S2O3solution with standard K2Cr2O7solution

No. of

obs.

Volume

of

K2Cr2O7(ml)

Burette reading Volume of

Na2S2O3

(ml)

Average

(Volume of

Na2S2O3)

(ml)

Initial

(ml)

Final

(ml)

1. 10 0 10.3 10.3

10.42. 10 10.3 20.7 10.4

3. 10 20.7 31.1 10.4

Calculation:-

We know, VredX Sred= VoxX Sox

Here,

VK2Cr2O7= 10 ml , SK2Cr2O7= 0.1 N

VNa2S2O4= 10.4 ml SNa2S2O4=?

SNa2S2O4= (10 X 0.1)/10.4

=0.097 N

Result:- Determined strength of Na2S2O3solution is = 0.097 N


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Error Analysis:-Percentage of error = (known value ~observed value) X 100 known value

= (0.098 0.097) X 100 / 0.098

= 1%

Discussion:- As the color change of the titration of Na2S2O3with K2Cr2O7is very

confusing, the end point of the titration may not have been properly

determined. This may be the cause of error.

chemistry 04

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