EXPERIMENT 5 REDOX TITRATION: TITRATION USING SODIUM THIOSULPHATE Objectives 1. 2. To prepare a standard solution of potassium iodate for use to determine the concentration of sodium thiosulphate solution accurately. To acquire the proper techniques of carrying out a titration.
Introduction Redox titrations using sodium thiosulphate as a reducing agent is known as iodometric titration since it is used specifically to titrate iodine. The reaction involved is: I2 + 2Na2S2O3 I2 + 2S2O322NaI + Na2S4O6 2I- + S4O62-
In this equation I2 has been reduced to I- :2S2O32I2 + 2e S4O62- + 2e 2I-
The iodine/thiosulphate titration is a general method for determining the concentration of an oxidising agent solution. A known volume of an oxidising agent is added into an excess solution of acidified potassium iodide. The reaction will release iodine:Example: (a) With KMnO4 2MnO4- + 16H+ + 10I(b) With KIO3 IO3- + 5I+ 6H+ 3I2 + 3H2O 2Mn2+ + 5I2 + 8H2O
The iodine that is released is titrated against a standard thiosulphate solution. From the stoichiometry of the reaction, the amount of iodine can be determined and from this, the concentration of the oxidising agent which released the iodine, can be calculated. In an iodometric titration, a starch solution is used as an indicator as it can absorb 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 standardised thiosulphate solution, iodine will react with the thiosulphate solution. When all the iodine has reacted with the thiosulphate solution, the dark blue colour will disappear. So, the end point of the titration is when the dark blue colour disappears. Iodine is usually dissolved in water by adding an excess of KI so that KI3 which has similar properties to iodine is formed. I2 + KI I3- + 2e KI3 3I-
Oxidising agents used other than thiosulphate are iron (II) salts, arsenic III oxide, sulphur dioxide and stibium (III) oxide. The following are reactions of sulphur dioxide and stibium (III) oxide with iodine: I2 2I2 + + SO2 + 2H2O 2H2O 2HI 4HI + H2SO4 + Sb2O5
Experimental aspects to be considered: 1. The indicator (starch) in the iodometric titration is not added in the early stage of the experiment as in acid-base titrations. Starch is only added after titration has begun, i.e. when the colour of the reaction mixture has changed from brown to a light yellow colour. Starch is a colloid that can absorb iodine and form a complex. When this happens, it would be difficult to release the iodine when titrating with the thiosulphate. This will influence the determination of the end point. Hence, the addition of the starch should only be done when the solution is light yellow in colour. At this point, there is only a small amount of released iodine left and the complex formed would also be small in quantity and
can easily react with the thiosulphate. 2. As soon as the solution is mixed with KI, titrate immediately with the thiosulphate. This will prevent the iodine from evaporating. Chemical Reagents Potassium iodate crystals Potassium iodide solids H2SO4 1.0M Starch solution Sodium thiosulphate solution 0.1 M Distilled water
Apparatus weighing bottle/50 mL beaker glass rod analytical balance pipette filler filter funnel 250 mL volumetric flask burette 25 mL pipette 250 mL conical flask 5 mL and 25 ml measuring cylinder 100 mL beaker Method A. 1. 2. 3.
Preparation of Potassium Iodate Solution (To be shared by two students) Weigh approximately 0.75g of potassium iodate crystals in a 50 mL beaker. Add 25 mL of distilled water into the beaker and stir with a glass rod to dissolve all the potassium iodate. Pour the potassium iodate solution through a filter funnel into a 250 mL volumetric flask. Rinse the beaker with distilled water and pour this water into the volumetric flask. Add in more distilled water to the volumetric flask up to the mark on the neck of the flask....
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