Hydrogen : Hydrogen From Alkalies

A solution of caustic soda or potash readily dissolves zinc or aluminium on warming, with evolution of hydrogen and formation of a soluble zincate or aluminate:

Zn + 2KOH = K₂ZnO₂ + H₂.

2A1 + 2NaOH + 2H₂O = 2NaAlO₂ + 3H₂

Hydrogen prepared in this way is pure, and has no smell.

Expt. 7. - Heat 25 gm. of granulated zinc with a 30 per cent, solution of caustic soda in a flask, and collect the hydrogen. The action is more rapid if iron filings are added: these are unchanged, and probably form a galvanic couple with the zinc.

Aluminium turnings may also be dissolved in dilute caustic soda solution by warming.

Pure hydrogen is evolved from the negative electrode by the electrolysis of a warm solution of recrystallised barium hydroxide, in a hard glass U-tube, with nickel electrodes

Fig: Pure hydrogen production

Preparation of pure hydrogen by the electrolysis of barium hydroxide solution and absorption in metallic palladium.

This is sealed to U-tubes containing pieces of caustic potash, followed by tubes of pure phosphorus pentoxide, to dry the gas.

The hydrogen is first passed over heated platinum gauze, when oxygen from air-leaks and diffusion is burnt to water, which is taken up in the drying train. A little nitrogen is left, which is removed by passing the gas into an exhausted bulb containing heated palladium foil. This readily absorbs more than 600 times its volume of hydrogen on cooling, but does not absorb nitrogen or any other gas. The residual nitrogen is pumped out of the bulb, and the latter then heated to dull redness, when pure hydrogen is evolved.

Nearly pure hydrogen may be prepared by electrolysing dilute sulphuric acid with an anode composed of a pool of zinc amalgam, which absorbs the nascent oxygen liberated at the anode:

Zn + O = ZnO; ZnO + H₂SO₄ = ZnSO₄ + H₂O.

Protein crystallography
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Hydrogen
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