Preparation of Carbon Monoxide

The methods available for the preparation of carbon monoxide fall into four categories:

A Partial oxidation of carbon; B Partial reduction of carbon dioxide; C Dehydration of formic acid and its derivatives or of other oxy-acids; D Electrolytic decomposition of a suitable oxyacid.

1. Carbon is oxidised to carbon monoxide when excess of it is heated with zinc oxide, ferric oxide, manganese dioxide, or other reducible oxide. The reactions are, however, of little importance for the preparation of the gas.
2. Carbon dioxide may be reduced to carbon monoxide in various ways:
   
   
   1. By red-hot carbon.
      
      It is well known that red-hot charcoal reduces carbon dioxide according to the reaction:
      
      CO₂ + C ⇔ 2CO.
      
      That the reaction is reversible, carbon monoxide dissociating at high temperature into carbon and carbon dioxide, was shown in 1864 by Deville. The matter was further investigated by Sir Lothian Bell, in 1869, who showed that finely divided metals promote the decomposition of carbon monoxide; and by Boudouard, in 1902, who employed iron, nickel, and cobalt as catalysts, and showed that the equilibrium ratio CO:CO₂ is a function of temperature, the reaction beginning at about 600° C.
      
      The effect of temperature on the equilibrium represented by the above equation has been studied by Rhead and Wheeler, who have obtained the results given in the table on the opposite page.
      
      The equation of equilibrium for the reaction is
      
      19,500/T + ln c1*c1/c2 = k
      
      where T is absolute temperature, c± and c2 are molecular concentrations of carbon monoxide and dioxide respectively, and h is a constant, the mean value for which is 20.39, the gases being left approximately at atmospheric temperature, when equilibrium is attained. The effect of pressure on the equilibrium has also been studied by Rhead and Wheeler.
      
      

      Temperature ° C.	Per cent, by volume
      	CO2	CO
      805	6.23	93.77
      900	2.22	97.78
      950	1.32	98.68
      1000	0.59	99.41
      1050	0.37	99.63
      1100	0.15	99.85
      1200	0.06	99.94

      
      
      Gautier has shown that carbon monoxide is not appreciably dissociated into carbon dioxide and carbon at the melting-point of lava (1250° C.), and therefore may exist in the pyrosphere of the earth.
   2. Metallic zinc may also be employed to reduce carbon dioxide to the monoxide. Thus carbon monoxide is formed when zinc filings are heated with chalk or magnesite:
      
      (Ca,Mg)CO₃ + Zn = (Ca,Mg)O + ZnO + CO,
      
      and likewise when carbon dioxide is passed over zinc dust heated just short of redness in a glass tube.
   Carbon dioxide may also be reduced by means of hydrogen sulphide thus:
   
   CO₂ + H₂S = CO + H₂O + S,
   
   or by leading the vapour of carbon disulphide with carbon dioxide over red-hot copper:
   
   CS₂ + CO₂ + 4Cu = 2Cu₂S + 2CO.
3. The dehydration of formic acid represented by the reaction
   
   HCOOH = CO + H₂O
   
   may be carried out by heating the acid or a suitable salt such as sodium formate with concentrated sulphuric acid, when pure carbon monoxide gas is evolved.
   
   Chloroform, bromoform, and iodoform, being derivatives of ortho- formic acid, may also be made to yield carbon monoxide by suitable means, the course of the reaction being represented thus:
   
   CHX₃ → CH(OH)₃ → CO + 2H₂O.
   
   According to Desgrez, chloroform yields carbon monoxide when treated with cold aqueous potash solution, thus:
   
   CHCl₃ + 3KOH = 3KCl + 2H₂O + CO,
   
   the reaction being accelerated by light. Bromoform is decomposed more slowly in this way, and iodoform not at all.
   
   Silver nitrate, however, aids the decomposition of iodoform on account of the formation of insoluble silver iodide, thus:
   
   CHI₃ + 3AgNO₃ + H₂O = 3AgI + 3HNO₃ + CO.
   
   Hydrocyanic acid, or formonitrile, yields carbon monoxide by the reactions:
   
   HCN + 2H₂O → HCOOH + NH₃ → CO + H₂O + NH₃.
   
   Thus potassium cyanide yields carbon monoxide when heated with concentrated sulphuric acid; but it is usual to prepare this gas by heating finely powdered potassium ferrocyanide, K₄Fe(CN)₆.3H₂O, with eight to ten times its weight of concentrated sulphuric acid, the water in the acid and in the salt being sufficient for the reaction:
   
   K₄Fe(CN)₆ + 6H₂SO₄ + 6H₂O = 2K₂SO₄ + FeSO₄ + 3(NH₄)₂SO₄ + 6CO.
   
   Other carboxylic acids besides formic acid, such as tartaric and citric acids, yield carbon monoxide among the products of their decomposition by means of sulphuric acid; and oxalic acid yields carbon monoxide and carbon dioxide in equal volumes, thus:
   
   COOH-COOH = CO + CO₂ + H₂O.
   
   It is therefore convenient to prepare carbon monoxide by heating oxalic acid with concentrated sulphuric acid, and absorbing the carbon dioxide by passing the gaseous mixture through a solution of caustic alkali.
   
   By an analogous reaction oxalates when heated yield carbon monoxide, leaving a residue of carbonate, as, e.g.:
   
   CaC₂O₄ = CaCO₃ + CO.
4. Carbon monoxide is produced by the electrolysis of malonic acid, which according to Petersen undergoes the following changes:
   
   
   1. CH₂(COOH)₂ = CH₂(COO)₂ + H₂
   2. CH₂(COO)₂ + H₂O = CH₂(COOH)₂ + O
   3. CH₂(COO)₂ = CH₂ + 2CO₂.
   The nascent oxygen then oxidises methylene (CH₂) thus:
   
   CH₂ + 2O = CO + H₂O.