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Chapter 15 Volumetric Analysis
Oxidation- Reduction
Primary standard for redox titration’s: Ammonium Iron (II) Sulphate.
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Stable-does not react with the gases in the air and has good solubility.
High relative molecular mass of Ammonium Iron (II) Sulphate(392) ensures a
high degree of accuracy when weighing.
When dissolving in water, sulphuric acid must be added to prevent it from
reacting with the water (HYDROLYSIS) and oxygen present. Fe2+ oxidised
to Fe3+
Solutions should be made up with Distilled water as deionised water may
contain chlorine.
Potassium Permanganate (KMnO4) is a good Oxidising Agent. It itself is
reduced(gains 5 electrons).
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Because KMnO4 is not very soluble in water a dilute solution (0.02 mol L-1) is
used
The MnO4- ion is purple and Mn2+ is colourless, so no indicator is needed.
The endpoint is a permanent pale pink colour.
The meniscus is read from the top.
The H+ ions are provided by H2SO4.
o HCl would be oxidised
o HNO3 is an oxidising agent.
If sufficient H2SO4 is not added the MnO4- is reduced to MnO2 which is a
brown solid. More H2SO4 should be added.
Experiments.
1. To prepare a standard solution of Ammonium Iron (II) Sulphate. And to use
this solution to standardise a solution of KMnO4 by titration.(Page 193 of
Book)
2. Using the standardised KMnO4 to determine the amount of Iron in an Iron
tablet. (Page 197 of Book).
The reaction of Sodium Thiosulfate (Na2S2O3)
Sodium Thiosulphate (Na2S2O3) is an important reducing agent used in chemistry. It
reacts with iodine molecules and reduces them to Iodine ions (I-) and tetraionate ion.
I2 + 2S2O32-  S4O62- + 2IThe colour change is from the brown/red colour of iodine, I2, to a straw yellow
colour to a colourless iodide ion, I- .
Starch is used as an indicator to detect the end point. It goes from blue to colourless.
The starch is added near the end point (when the solution is a straw yellow colour)
because the solution changes colour so abruptly. If the starch is added at an early
stage, the iodine present may become strongly adsorbed on to the starch and make the
titration less accurate.
Sodium thiosulphate is not a primary standard. It is standardised by titration it against
acidified potassium permanganate with an excess of acidified potassium iodide. When
potassium permanganate reacts with potassium iodide, the potassium iodide is
oxidised to free iodine (I2). The free iodine can then be titrated against the unknown
thiosulphate solution using starch as an indicator.
The overall reactions are:
2 mol KMnO4 = 5mol I2 = 10 mol Na2S2O3
The iodine cannot be used as a primary standard because it does not dissolve readily
in water and iodine sublimes.
Experiments.
1. To prepare a solution of sodium thiosulfate and standardise it by titration against a
solution of iodine. (See page 202 book)
2. To determine the percentage (w/v) of sodium hypo chlorite in bleach. (Page 204
Book)