Find out the %age purity of FeSO4. The given solution contains 1g/50ml. You are provided with 0.0166 K2Cr2O7 solution.

 

Theory

Chemicals

K₂Cr₂O₇ soln.(50ml), FeSO₄ soln (50ml), Diphenylamine

Apparatus

Burette, pipette, funnel, conical flask, burette stand, beaker, weight balance

Principle:

Redox titration

Indicator

Diphenylamine

End point

Violet Blue Colour

Chemical Equation

K₂Cr₂O₇  + 7H₂SO₄  +   6 FeSO₄     ---------->    K₂SO₄ + 3Fe₂(SO₄)₃ + Cr₂(SO₄)₃ + 7H₂O   

Mole ratio

K₂Cr₂O₇      :      FeSO₄

1        :           6

Procedure

Prepare 50ml solution of FeSO₄ by taking 1g of FeSO₄ and dissolving it to 50 ml of distilled water

Prepare 50ml solution of K₂Cr₂O₇ by taking 2g of K₂Cr₂O₇ and dilute up to 50ml with distilled water

Prepare indicator by adding 0.5g of diphenylamine in 90% H₂SO₄

Prepare the sample solution by taking 10ml of FeSO₄ solution in titration flask and add 2ml H₃PO₄ and 2 drops of indicator.

Take K₂Cr₂O₇ in burette and titrate against 10ml FeSO₄ solution in the flask till we get violet blue colouration

Result

The %age purity of FeSO₄ is ……….

Standardization of Sodium Thiosulphate Solution (Na2S2O3) by Iodometry

 

Theory

Sodium thiosulphate solution (Na₂S₂O₃) is titrated against potassium dichromate (K₂Cr₂O₇) in presence of HCl and KI. K₂Cr₂O₇ oxidizes the iodide ion in acidic medium to equivalent amount of iodine. The iodine formed in the reaction oxidizes Na₂S₂O₃ giving sodium tetrathionate ion and the end point is detected by starch solution

Apparatus

Burette, Flask, weight balance,

Chemicals

Sodium Thiosulphate, Potassium dichromate, Potassium iodide, HCl, Starch solution

Principle

Iodometric Titration (A type of redox reaction)

Reaction Equation

K₂Cr₂O₇     +    6KI  +   14HCl        -------->    3I₂   +   2CrCl₃

2Na₂S₂O₃    +    I₂         ----------->      Na₂S₄O₆     +  2NaI                 

Indicator

Starch solution

Endpoint

Colour change from dark blue to bottle green

Procedure

Preparation of sodium thiosulphate solution:

Dissolve 12.5g of sodium thiosulphate in water and make the volume up to 500ml. Keep the solution aside and filter to remove any cloudiness if appears.

Preparation of starch solution:

Add one gram of starch to few ml of water, prepare slurry and add gradually to 100ml of boiling water till a translucent solution is obtained

Standardization:

Dissolve 0.125g of accurately weighed potassium dichromate in 25ml of water present in a 250 ml flask. Add 10 ml of HCl and 2g of KI, close with stopper, shake well and keep in dark for 15 minutes. Add 100ml of water to the above mixture and titrate with sodium thiosulphate using starch as the indicator.

Calculations:

Result

 

Discussion

From the above experiment it was evident that sodium thiosulphate can be effectively standardize by using potassium dichromate and iodide with HCl and starch as the visual indicator, after performing the calculations, strength of the prepared sodium thiosulphate solution was found to be ……….. N.

 

Why did we used normality instead of molarity in this experiment?

How is normality different from molarity? 

Determination of amount /dm3 of Cu+2 in CuSO4.5H2O using Na2S2O3 and KI by iodimetry

 

Image by master1305 on Freepik

Theory

In this experiment the amount of Cu⁺² in CuSO₄.5H₂O was determined with the help of iodometric titration. 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 main purpose is to determine the amount of Cu⁺² ions in copper sulphate (CuSO₄.5H₂O ) for this we first determined the molarity (moles/dm3) of CuSO₄.5H₂O and then performed the mathematical calculations to obtain the end result.   

Apparatus

Burette, Burette stand, beaker, stirrer, weight balance, measuring cylinder

Chemicals

1g KI, 0.1 M Na₂S₂O₃, Sample solution (CuSO₄.5H₂O)

Principle

Redox reaction

Reaction equation

2 CuSO₄ + 4KI   →  Cu₂I₂  +  2K₂SO₄  +  I₂

I₂  +  I^-   →  I₃^-

Starch  + I₃^-  →  Starch-I₃ (dark blue)

2S₂O₃^-2  +   I₂   →    S₄O₆^2-  +  2I^-

Indicator

Starch solution

End point

Colourless

Procedure

1.    Make of 0.1 M Na₂S₂O₃ solution by dissolving 1.24g of Na₂S₂O₃ in distilled water in a 50ml beaker and then make the volume of the solution up to the mark.

2.    Prepare sample solution by taking small amount of CuSO₄ and dissolve in distilled water make solution up to 20 ml

3.    In titration flask take 10ml of sample solution and add 1g of KI in it, then dilute it with 10ml of distilled water

4.    Titrate this solution with Na₂S₂O₃ taken in burette until the colour of the solution became pale yellow

5.    Add 2ml of 5% starch solution in the above solution the colour will change into blue

6.    Again, titrate until colourless solution is obtained

Starch solution:

Make a paste of 5g of soluble starch with a little water and pour the paste with constant stirring into 100 ml of boiling water and boil for 1 minute. Allow the solution to cool and add 2-3g of KI. Keep the solution in a stoppered bottle.

Calculations:

Molarity of CuSO₄.5H₂O = M₂ = ………M  (suppose its ‘A’ moles)

 

Cu⁺²  :  CuSO₄.5H₂O

1         :         1

According to the above mole ratio 1 mole of CuSO₄.5H₂O contains 1 mole of Cu⁺² ions which means

A moles of CuSO₄.5H₂O = A mole of Cu+2

Result

Amount of Cu⁺² in CuSO₄.5H₂O is ………moles/dm3

Discussion:

In the titration of iodine, starch must not be added until just before the end point is reached. Apart from the fact that the fading of iodine colour is a good indication of the approach at the end point. If the starch solution is added when the iodine concentration is high, some iodine may remain adsorbed even at the end point. 

Standardization of Sodium thiosulphate (Na2S2O3) solution by Iodometry

 

Theory

In this experiment the strength of Sodium Thiosulphate (Na₂S₂O₃) 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 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

Chemicals

Sodium bicarbonate, distilled water, KI solution, conc. HCl, Potassium dichromate (K₂Cr₂O₇) solution, Na₂S₂O₃ solution, Starch solution 

Apparatus

Burette, burette stand, pipette, funnel, conical flask, beaker, stirrer, weight balance, watch glass

Principle

Redox titration

Reaction Equation

K₂Cr₂O₇ + 6KI +14HCl → 2CrCl₃ + 3I₂ + 7H₂O + 8KCl

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI

Indicator

Starch solution

End point

Disappearance of blue colour

Preparation of 100ml of 0.01M K₂Cr₂O₇ solution

Molecular weight of K₂Cr₂O₇ = 294g

It means 1000ml of 1M K₂Cr₂O₇ solution contains = 294 g of K₂Cr₂O₇

We want to calculate “Given mass” for 0.01M solution

Given mass = Molarity × molar weight

0.01  × 294

= 2.94 g

For 1000ml of 0.01M solution we require 2.94g of K₂Cr₂O₇

Take 0.294g of K₂Cr₂O₇ in 100ml volumetric flask and dissolve in distilled water, ten fill the flask up to the mark

Procedure

1.    Dissolve about 1gm of sodium bicarbonate (NaHCO₃) in about 50ml of distilled water in a conical flask

2.    Add 4ml of 12% solution of iodate-free potassium iodide (KI) and shake well

3.    Now add about 4ml of concentrated HCl acid slowly by while rotating the flask in order to mix the liquids well

4.    Pipette out 10ml of standard dichromate (K₂Cr₂O₇) solution and pour into the same flask. Shake gently for thorough mixing. Cover the flask with a watch glass and allow standing in the dark for 5 minutes for completion of reaction. The solution will be deep brown. In the meantime, fill the burette with the supplied thiosulphate solution in the appropriate manner.

5.    Titrate the liberated iodine (brown colour solution of flask) with the Na₂S₂O₃ solution while shaking the flask until the brown colour fades (light yellow)

6.    Add 2ml of starch solution. At the end point the deep blue colour of the starch-iodine complex disappears leaving the light green colour of the chromate ion 

7.    Calculate the strength of the supplied thiosulphate solution

Table: Standardization of Na₂S₂O₃ by K₂Cr₂O₇ solution

Result

The strength of supplied thiosulphate (Na₂S₂O₃) solution is ………… M.

Discussion

The solution was made acidic enough to make sure the redox reactions get completed. In order to avoid oxidation of HCl by air a reducing environment was provided by adding a small amount of NaHCO₃ into solution. As iodine can be easily lost from solution due to its volatility, flask was kept closed. Iodine in water solutions is usually coloured strong enough so that its presence can be detected visually. However, close to the end point the concentration of iodine is very low, its yellow colour is very pale and can be easily overlooked. If we add starch, iodine gets adsorbed on the surface of starch and the product gives a deep blue colour. In the presence of small amounts of iodine adsorption and desorption are fast and reversible which makes the detection of end point difficult. This is why starch is used. However, when the concentration of iodine is high, its bonding with starch is relatively strong which makes the desorption slow which makes the detection of end point relatively difficult. Thus, at the end point some absorbed iodine may remain untitrated giving erroneous endpoint. This is why starch solution is added at the end point when the concentration of iodine is very small. [Discussion should be written in past-passive form]