tandardisation of sodium thiosulphate solution with potassium iodate
Sodium thiosulphate is not of ultra-high purity, and hence does not qualify to be used as a primary standard in titrations. It should therefore be standardised
against a primary standard, potassium iodate (KIO3), in order to determine its accurate concentration. The standardisation involves two redox equations
described below:
2S2O32â+ I2 â S4O62- + 2I-
IO3- + 5I- + 6H+ â 3I2 + 3H2O
In the second equation IO3- plays an oxidative role, where I- ions are oxidisedto I2 under acidic conditions, causing the solution to turn a brownish colour.
The iodine produced is then titrated with Na2S2O3 solution, using starch as an indicator. The starch indicator is not added until the brownish colour, due to
iodine, has changed to pale yellow. When the starch is added to the solution, the yellow colour changes to blue-black. The end point in the titration is reached
when a drop of the thiosulphate causes the solution to become colourless.
The aim of this experiment is to determine the actual concentration of the thiosulfate solution (in mol/L) using potassium iodate as a standard. The molecular mass of KIO3 is 214 g mol-1.
Reagents: KIO3 (A.R.)
20 g Kl (iodate free) (for use in Experiments 3 and 5)
1 M HCI solution
0.1 M Na2S2O3 stock solution
Starch indicator solution
(a) Calculate the mass of A.R. KIO3 needed to prepare a 0.016 M solution in a 250 mL volumetric flask. Ask a demonstrator to check your calculations.
(b) Following the procedure given in Experiment 1, weigh by difference the KIO3, as supplied in the glass vial, accurately to the nearest 0.1 mg. (Use
a 4-place balance.)
(c) Transfer the weighed KIO3 into a 250 mL volumetric flask, dissolve it, and then make to the mark with deionized water. Shake to mix well.
(d) Fill the burette with 0.1 M Na2S2O3 stock solution, ensuring that there is no air pocket in the burette tip.
(e) Pipette a 25 mL aliquot of the KIO3 solution into each of three 250 mL conical flasks.
(f) Treat each aliquot individually from this point. Weigh 2 g Kl into a weighing beaker and add it to the conical flask, followed by 10 mL of 1 M HCI.
(g) Immediately titrate the liberated iodine with the 0.1 M Na2S2O3 stock solution. When the colour of the solution becomes pale yellow, dilute to 100 mL with deionized water and add 3 to 5 drops of starch indicator solution. Continue the titration until the colour changes from blue to colourless.
QUESTIONS:
Calculate the molarity of sodium thiosulfate.
tandardisation of sodium thiosulphate solution with potassium iodate
Sodium thiosulphate is not of ultra-high purity, and hence does not qualify to be used as a primary standard in titrations. It should therefore be standardised
against a primary standard, potassium iodate (KIO3), in order to determine its accurate concentration. The standardisation involves two redox equations
described below:
2S2O32â+ I2 â S4O62- + 2I-
IO3- + 5I- + 6H+ â 3I2 + 3H2O
In the second equation IO3- plays an oxidative role, where I- ions are oxidisedto I2 under acidic conditions, causing the solution to turn a brownish colour.
The iodine produced is then titrated with Na2S2O3 solution, using starch as an indicator. The starch indicator is not added until the brownish colour, due to
iodine, has changed to pale yellow. When the starch is added to the solution, the yellow colour changes to blue-black. The end point in the titration is reached
when a drop of the thiosulphate causes the solution to become colourless.
The aim of this experiment is to determine the actual concentration of the thiosulfate solution (in mol/L) using potassium iodate as a standard. The molecular mass of KIO3 is 214 g mol-1.
Reagents: KIO3 (A.R.)
20 g Kl (iodate free) (for use in Experiments 3 and 5)
1 M HCI solution
0.1 M Na2S2O3 stock solution
Starch indicator solution
(a) Calculate the mass of A.R. KIO3 needed to prepare a 0.016 M solution in a 250 mL volumetric flask. Ask a demonstrator to check your calculations.
(b) Following the procedure given in Experiment 1, weigh by difference the KIO3, as supplied in the glass vial, accurately to the nearest 0.1 mg. (Use
a 4-place balance.)
(c) Transfer the weighed KIO3 into a 250 mL volumetric flask, dissolve it, and then make to the mark with deionized water. Shake to mix well.
(d) Fill the burette with 0.1 M Na2S2O3 stock solution, ensuring that there is no air pocket in the burette tip.
(e) Pipette a 25 mL aliquot of the KIO3 solution into each of three 250 mL conical flasks.
(f) Treat each aliquot individually from this point. Weigh 2 g Kl into a weighing beaker and add it to the conical flask, followed by 10 mL of 1 M HCI.
(g) Immediately titrate the liberated iodine with the 0.1 M Na2S2O3 stock solution. When the colour of the solution becomes pale yellow, dilute to 100 mL with deionized water and add 3 to 5 drops of starch indicator solution. Continue the titration until the colour changes from blue to colourless.
QUESTIONS:
Calculate the molarity of sodium thiosulfate.
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