Hydrogen peroxide is usually encountered in the form of an aqueous solution containing about 6 per cent, 12 per cent or 30 per cent hydrogen peroxide, and frequently referred to as '20-volume', '40-volume', and '100-volume' hydrogen peroxide respectively; this terminology is based upon the volume of oxygen liberated when the solution is decomposed by boiling. Thus 1 mL of ' 100-volume' hydrogen peroxide will yield 100 mL of oxygen measured at standard temperature and pressure.
The following reaction occurs when potassium permanganate solution is added to hydrogen peroxide solution acidified with dilute sulphuric acid:
2MnO4- +5H2O2
+ 6H+ = 2Mn2++ 5O2 + 8H2O
This forms the basis of the method of analysis given below.
It is good practice to use a
fairly high concentration of acid and a reasonably low rate of addition in
order to reduce the danger of forming manganese dioxide, which is an active
catalyst for the decomposition of hydrogen peroxide. For slightly coloured
solutions or for titrations with dilute permanganate, the use of ferroin as
indicator is recommended. Organic substances may interfere. A fading end point
indicates the presence of organic matter or other reducing agents, in which
case the iodometric method is better.
Potassium permanganate solutions
may be standardised using arsenic(III) oxide or sodium oxalate as primary
standards: secondary standards include metallic iron, and iron(II)
ethylenediammonium sulphate (or ethylenediamine iron(II) sulphate), FeSO4,C2H4(NH3)2SO4,4H2O.
Weigh out about 3.2-3.25 g
potassium permanganate, transfer it to a 1000mL beaker, add 1 L water, cover
the beaker with a clockglass, heat the solution to boiling, boil gently for
15-30 minutes and allow the solution to cool to the laboratory temperature.
Filter the solution through a funnel containing a plug of purified glass wool,
or, more simply, through a sintered-glass or porcelain filtering crucible or
funnel. Collect the filtrate in a vessel which has been cleaned with chromic
acid mixture and then thoroughly washed with distilled water. The filtered
solution should be stored in a clean, glass-stoppered bottle, and kept in the
dark or in diffuse light except when in use: alternatively, it may be kept in a
dark brown glass bottle.
Sodium oxalate is readily obtained pure and anhydrous, and
the ordinary material has a purity of at least 99.9 per cent. The best way is
to make a more rapid addition of
90-95 per cent of the permanganate solution (about 25-35 mL.min -1) to a solution of sodium
oxalate in 1 M sulphuric acid at 25-30 °C, the solution is then warmed to 55-60
°C and the titration completed, the last 0.5- 1 mL portion being added
dropwise. The method is accurate to 0.06 per cent. Experimental details are
given below.
2MnO4- +5H2C2O4
+ 6H+ = 2Mn2+ + 10CO2 + 8H2O
It should be mentioned that if oxalate is to be determined it is often not convenient to use the room temperature technique for unknown amounts of oxalate. The permanganate solution may then be standardised against sodium oxalate at about 80 °C using the same procedure in the standardisation as in the analysis.
Dry some sodium oxalate (p.a.) at 105-110 °C for 2 hours, and allow it to cool in a covered vessel in a desiccator. Weigh out accurately about 0.3 g of the dry sodium oxalate, put it into a beaker, add 240 mL of recently prepared distilled water, and 250 mL of 1M sulphuric acid. Cool to 25-30 °C and stir until the oxalate has dissolved. Add 90-95 per cent of the required quantity of permanganate solution from a burette at a rate of 25-35 mLmin-1 while stirring slowly. Heat to 55-60 °C (use a thermometer as stirring rod), and complete the titration by adding permanganate solution until a faint pink colour persists for 30 seconds. Add the last 0.5-1 mL dropwise, with particular care to allow each drop to become decolorised before the next is introduced. For the most exact work, it is necessary to determine the excess of permanganate solution required to impart a pink colour to the solution. This is done by matching the colour produced by adding permanganate solution to the same volume of boiled and cooled dilute sulphuric acid at 55-60 °C. This correction usually amounts to 0.03-0.05 mL. Repeat the determination with two other similar quantities of sodium oxalate.
Transfer 25.0 mL of the '20-volume' solution by means of a burette to a 500 mL graduated flask, and dilute with water to the mark. Shake thoroughly. Pipette 25.0 mL of this solution to a conical flask, dilute with 200 mL water, add 20mL dilute sulphuric acid (1:5), and titrate with standard 0.02 M potassium permanganate to the first permanent, faint pink, colour. Repeat the titration; two consecutive determinations should agree within 0.1 mL.
A metallic peroxide, such as sodium peroxide, can be analysed in similar manner, provided that care is taken to avoid loss of oxygen during the dissolution of the peroxide. This may be done by working in a medium containing boric acid which is converted to the relatively stable 'perboric acid' upon the addition of the peroxide.
To 100 mL of distilled water, add
5 mL of concentrated sulphuric acid, cool and then add 5 g of pure boric acid;
when this has dissolved cool the mixture in ice. Transfer gradually from a
weighing bottle about 0.5 g (accurately weighed) of the sodium peroxide sample
(handle with care) to the
well-stirred, ice-cold solution. When the addition is complete, transfer the
solution to a 250 mL graduated flask, make up to the mark, and then titrate 50
mL portions of the solution with standard 0.02 M permanganate solution.