Chemistry (Redox)

Topics: Iodine, Titration, Redox Pages: 7 (1485 words) Published: November 25, 2012
Experiment 1 : Redox Titration Using Sodium Thiosulphate
This experiment is to determine the concentration of oxidizing solution using the iodine/ thiosulphate titration where the reducing solution is potassium iodate solution and the oxidizing solution is sodium thiosulphate solution. Potassium iodate solution which is an oxidizing agent is added into an excess solution of acidified potassium iodide. This reaction will release iodine. Potassium iodide is acidified with sulphuric acid and the iodine released quickly titrated with sodium thiosulphate until it become light yellow. The iodine then detected with starch solution and it turn into dark blue solution and titrated again with sodium thiosulphate until colourless. From the reaction occur, the amount of the iodine can be determined and based on this amount, the concentration of oxidizing agent which released iodine can be determined.

Redox titration using sodium thiosulphate as a reducing agent is also known as iodometric titration. The reaction is:-
I2(aq) + 2NA2S2O3(aq) 2Nal(aq) + 2Na2S4O6(aq)
I2(aq) + 2S2O32-(aq)2I-(aq) + S4O62-(aq)
In this equation, I2 has been reduced to I-:
2S2O32-(aq)S4O62-(aq) + 2e-
I2(aq) + 2e- 2I-(aq)
The iodine/ thiosulphate titration is a general method for determining the concentration of oxidizing solution. A known volume of an oxidizing agent is added into an excess solution of acidified potassium iodide. The reaction will release iodine:-

a. With KMnO4
2MnO4-(aq) + 16H+ (aq)+ 10I-(aq) 2Mn2+(aq) + 5I2(aq) + 8H2O(I)

b. With KIO3
O3-(aq) + 5I-(aq) + 6H (aq) 3I2(aq) + 3H2O(aq)

The iodine that is released is titrated with a standard thiosulphate solution. From the stoichiometry of the reaction, the amount of iodine can be determined and from this the concentration of the oxidizing agent, which released the iodine, can be calculated.

a. To prepare a standard solution of potassium iodate use in determining the concentration of a sodium thiosulphate solution accurately. b. To learn the proper technique for titration.

In an iodometric titration, starch solution is used as an indicator as it absorbs the iodine that is released. This absorption will cause the solution to change to a dark blue colour. When this dark blue solution is titrated with the standard thiosulphate, iodine will react with the thiosulphate solution, the dark blue will disappear. So the end point of the titration is when the dark blue solour disappear.

It is difficult for iodine to dissolve in water. Iodine is usually dissolves in water by adding an excess of KI so that KI3, which has similar properties to iodine, is formed.

I2(aq) + KI(aq)KI3

.I3-(aq) + 2e-3I-(aq)


50-mL beaker250-mL volumetric flask
Glass rodBurette
25-mL pipettePipette filler
3 × 250-mL conical flaskMagnetic flea
Potassium iodate crystals (KIO3)Potassium iodide (KI)
1M H2SO4Starch solution
ca. 0.1M sodium thiosulphate solution

Level of miniscus

Pull the stopcock in against the taper each time you turn it.

A sheet of white paper or towel below the flask will help in recognizing the colour change at the end point. Swirl the flask continuosly until one drop of titrant causes a colour change throughout the entire solution.

Swirl the flask continuously until the drop of titrant causes a colour change throughout the entire solution.

A. Preparation of potassium iodate solution

1. 0.75 g of potassium iodate crystals is weighed accurately in 50-mL beaker. 2. 25 mL distilled water are added into beaker and stirred with glass rod to dissolve all the potassium iodate. 3. The potassium iodate solution is poured into a 250-mL volumetric flask. The beaker is rinsed with...
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