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Oxides And Oxy-acids Of Chlorine : Hypochlorous Acid


This acid is known only in solution. On distillation the solution breaks up into water and the anhydride of the acid, Cl2O. A solution of the acid is obtained by shaking chlorine water with yellow precipitated mercuric oxide:

2Cl2 + 2HgO + H2O = HgCl2,HgO + 2HOCl.

The liquid is distilled and a dilute solution of hypochlorous acid collects in the receiver.

An interesting reaction is the formation of hypochlorous acid by the direct oxidation of hydrochloric acid, discovered by Odling in 1860: HCl + O = HOCl. A current of air is passed through concentrated hydrochloric acid in a wash-bottle, and then through potassium permanganate solution in a retort heated on a water bath; hypochlorous acid distils over.

Hypochlorous acid is most conveniently prepared from bleaching powder. When dissolved in water, this decomposes into chloride and hypochlorite: 2Ca(OCl)Cl = CaCl2 + Ca(OCl)2. If a clear solution of bleaching powder is treated with the calculated amount of 5 per cent, nitric acid, added slowly from a burette whilst the liquid is kept well stirred, hypochlorous acid is set free: Ca(OCl)2 + 2HNO3 = Ca(NO3)2 + 2HOCl. The liquid is then distilled, and a dilute solution of hypo-chlorous acid is obtained.

Hydrochloric acid reacts with hypochlorous acid with liberation of free chlorine: HCl + HOCl <=> Cl2 + H2O. If, therefore, an excess of any acid capable of liberating hydrochloric acid from calcium chloride is added to bleaching powder, or its solution, the whole of the chlorine is expelled as such:

Ca(OCl)2 + CaCl2 + 2H2SO4 = 2CaSO4 + 2H2O + 2Cl2.

Free hypochlorous acid is produced by the action of chlorine on a solution of a hypochlorite:

KOCl + Cl2 + H2O = KCl + 2HOCl.

This reaction probably occurs in two stages, as follows:

  1. H2O + Cl2 <=> HCl + HOCl <=> H۰ + Cl' + HOCl.
  2. OCl' + H۰ = HOCl.


If chlorine is passed through a suspension of sodium bicarbonate or precipitated calcium carbonate in water, or a solution of sodium sulphate or phosphate, hypochlorous acid (not a hypochlorite) is formed:

2Cl2 + H2O + CaCO3 = 2HOCl + CaCl2 + CO2. This reaction probably proceeds in two stages:

  1. Cl2 + H2O <=> HCl + HOCl <=> h۰ + Cl' + HOCl;
  2. 2H۰ + CO3'' <=> H2CO3 <=> CO2 + H2O; H۰ + SO4'' <=> HSO4, etc.


The hypochlorous acid produced is too weak to decompose the carbonate with formation of a hypochlorite. The function of the carbonate, etc., is to remove the hydrochloric acid as fast as it is produced, and so to prevent reaction (i) coming to a standstill.

Hypochlorous acid in solution is pale golden yellow, or colourless when dilute. It is 0.02 per cent, ionised in N/10 solution. The dilute solution is fairly stable in the dark: concentrated solutions decompose on heating or exposure to sunlight, with evolution of oxygen and chlorine and formation of some chloric acid:

  1. 2HOCl = 2HCl + O2;
  2. HCl + HOCl = H2O + Cl2;
  3. HOCl + 2O (nascent) = HClO3.


The decomposition is accelerated by platinum black, manganese dioxide, or cobalt oxide. Hypochlorites on heating with the latter oxide in alkaline solution rapidly evolve oxygen: 2NaOCl = 2NaCl + O2. With concentrated acids they evolve chlorine.

Hypochlorous acid precipitates silver nitrate solution:

3HOCl + 3AgNO3 = 3HNO3 + 2AgCl + AgClO3.

Chloric and perchloric acids give no precipitate with silver nitrate.

The acid dissolves magnesium with evolution of hydrogen: Mg + 2HOCl = Mg(OCl)2 + H2. Iron and aluminium evolve hydrogen and chlorine; copper, nickel, and cobalt evolve chlorine and oxygen. With hydrogen peroxide the acid and its salts evolve oxygen:

OCl' + H2O2 = Cl' + H2O + O2.

Hypochlorous acid is a powerful oxidising and bleaching agent, due to the liberation of nascent oxygen: HOCl = HCl + O.

Expt. 3. - Add caustic soda to a solution of manganous sulphate. A white precipitate of manganous hydroxide is formed: MnSO4 + 2NaOH = Mn(OH)2 + Na2SO4. Add sodium hypochlonte solution. The precipitate becomes brown, and is converted into hydrated manganic oxide: Mn(OH)2 + NaOCl + H2O = Mn(OH)4 + NaCl.
Expt. 4. - To a solution of chrome alum add excess of NaOCl solution and boil. A yellow solution of sodium chromate, Na2CrO4, is formed:

2Cr(OH)3 + 3NaOCl + 4NaOH = 2Na2CrO4 + 5H2O + 3NaCl.
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Oxides And Oxy-acids Of Chlorine
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