Preparation
Hydrogen peroxide may be prepared from sodium peroxide or barium peroxide, in a laboratory.
From sodium peroxide
Sodium peroxide is added to an ice-cold dilute solution of sulphuric acid (20 %), in small doses at a time.
Sodium sulphate is removed by cystallization, and a dilute solution of H2O2 is obtained.
From barium peroxide
A paste of BaO2.8H2O is prepared and added gradually to ice cold dilute sulphuric acid. The solution is kept well-stirred throughout.
Barium sulphate precipitates out leaving behind a dilute solution of hydrogen peroxide. In this method, phosphoric acid can also be used in place of sulphuric acid.
An alternative way is to pass a current of carbon dioxide through a suspension of barium peroxide in cold water.
Manufacture
Hydrogen peroxide is generally manufactured by the electrolysis of 50% sulphuric acid containing ammonium hydrogen sulphate. It is also prepared by the auto-oxidation of 2-ethyl anthraquinol.
By the electrolysis of ammonium hydrogen sulphate
Hydrogen peroxide can be prepared on a large scale by the electrolysis of ammonium hydrogen sulphate dissolved in excess of sulphuric acid, using platinum electrodes and a high current density. In order to have high current density (i.e., current strength per unit area of the electrode) at the anode, the area of the anode should be small. In this process, ammonium persulphate is produced by oxidation at the anode while hydrogen is liberated at the cathode.
The solution containing ammonium persulphate is heated at 43 mm pressure when it hydrolyses, yielding hydrogen peroxide.
Hydrogen peroxide along with water distils over. The aqueous solution (30-40% H2O2) is concentrated by fractional distillation under reduced pressure, to 85-90%. Water, being more volatile, passes over first. The above process recovers ammonium bisulphate, which is used again.
By autoxidation of 2-ethylanthraquinol
The modern process for the manufacture of H2O2 is based on the autoxidation of 2-ethylanthraquinol. This method, involves alternate oxidation and reduction steps of 2-ethylanthraquinol. In this process, a 10 per cent solution of the compound is prepared in a mixture of benzene and an alcohol containing 7-11 carbon atoms.
Air is passed through the mixture. The resulting 2-ethylanthraquinone is then reduced by passing hydrogen under a pressure of 1 to 3 atmospheres at 4oC. This gives back 2-ethylanthraquinol. The hydrogen peroxide formed in this reaction is extracted with water, and the dilute solution of H2O2 is concentrated to obtain H2O2 of the required strength.
Concentration of Hydrogen Peroxide
The hydrogen peroxide obtained from any of the above methods is dilute. It is further concentrated by:
Evaporation
The dilute solution of hydrogen peroxide is carefully evaporated on water bath at 70°C. The concentration of H2O2 increases to about 50%, by this method.
Freezing
The dilute solution of hydrogen peroxide is cooled in a freezing mixture. Water present in hydrogen peroxide solution separates out as ice. The-ice is removed. This leaves behind a solution richer in hydrogen peroxide.
For most commercial purposes, concentration is carried over upto 60 per cent only.Distillation under reduced pressure
The partially concentrated hydrogen peroxide solution obtained above is further concentrated by distillation under reduced pressure (15 torr at 35°C). However, a more concentrated solution can be prepared, if necessary, by repeated distillation over concentrated sulphuric acid.
Removal of last traces of water
The above solution of hydrogen peroxide is placed in a vacuum desiccator containing concentrated sulphuric acid. The water vapours are absorbed upto 90%. Finally, higher concentration is achieved by cooling the above solution till it begins to crystallize. The crystals being richer in hydrogen peroxide are separated and remelted. Now, fractional distillation under reduced pressure gives a concentration of 91%.
Strength of hydrogen peroxide solution
It is customary to indicate the strength of a sample of hydrogen peroxide solution in terms of the 'volume of oxygen at NTP that one volume of hydrogen peroxide gives on heating'. When the grade of H2O2 in volumes is given, its percentage strength can be calculated as follows:
For example, we have a sample of H2O2 marked as 30 volume.Then, by definition of '30 volumes H2O2 means that 1 volume of this grade H2O2 gives 30 volumes of O2 at NTP.
So, 1 litre of 30 volume H2O2 gives 30 litre of oxygen at NTP
We find that, 1 mol of O2 = 2 mol of H2O2
Thus, 30 volume H2O2 contains 2.678 mol per litre of this solution.Therefore the Molarity of 30 volume sample of H2O2 = 2.678
So, the strength of H2O2in the 30 volume sample= Molarity x Molar mass = 2.678 x 34 g L-1 = 91.1g/L = 9.11%
'30 volume' sample of H2O2 actually contains 9.11% of H2O2Physical properties of hydrogen peroxide
- Pure hydrogen peroxide is a pale-blue syrupy liquid with a bitter taste.
- It is soluble both in water and alcohol.
- Its density is 1.469 g/mL.
- It melts at 272.4 K (- 0.7°C) and boils under 760 mm of Hg pressure at 423.2 K (150°C) with violent decomposition. However, it can be distilled under reduced pressure (= 26 torr) at 69.2°C.
- It is diamagnetic.
- It behaves as an excellent solvent for electrolytes, due to its high dielectric constant. (93.7 for pure H2O2).
Structure of hydrogen peroxide
Each H-O-O bond angle is about 94.8%, while the angle between the planes containing the hydrogen atoms is 111.5°. In the crystal, the angle between the plan containing hydrogen atoms gets reduced from 111.5 to 90.2° on account of hydrogen bonding. The two oxygen atoms are joined by a single electron-pair bond.
