ACIDS AND BASES: OXIDATION AND REDUCTION 1071501000700 50 100 150
Added Ce (IV)as% Fe (ED
Figure 4.7. Potentiometric titration of Fe(II) with Ce(lV)number of redox reactions; quantitatively they have several ad-
vantages over ordinary indicator methods.
Thus, for example, an analysis using coloured solutions can be
carried out, where an indicator cannot be used. Moreover, it is not
easy to find a redox indicator which will change colour at the right
point. Potentiometric methods can fairly readily be made automatic.TESTS FOR REDUCING AND OXIDISING AGENTS
The redox properties of all reagents are relative and a given reagent
may be both a reducing and an oxidising agent depending upon the
reaction in which it is involved. Thus, for example, sulphur dioxide
in aqueous solution is an oxidising agent with respect to hydrogen
sulphide, but a reducing agent with respect to acidified potassium
dichromate(VI) solution. Similarly hydrogen peroxide in acidic
solution is an oxidising agent relative to iron(II) ions but a reducing
agent relative to manganate(VII) ions in aqueous solution. How-
ever, it is convenient to establish approximate 'reference points' for
laboratory reagents, which can then be loosely classified as follows:Reagents are reducing if they:
- Decolorise a solution of potassium manganate(VII) acidified
with dilute sulphuric acid. - Turn a solution of potassium dichromate(VI) acidified with dilute
sulphuric acid from orange to green.