Chemical Properties of Carbon Monoxide

Carbon monoxide is an unsaturated compound which unites with oxygen, either free or combined, to form carbon dioxide; and also with various other substances under suitable conditions.

Carbon monoxide can reduce compounds containing oxygen not only at high temperature, as in metallurgy, but also in some cases at low temperatures.

For example, silver oxide and yellow mercuric oxide oxidise carbon monoxide at atmospheric temperature, as well as permanganic acid and ammoniacal silver nitrate; whilst chromic acid reacts similarly at 100° C., and crystallised iodic anhydride at 90° C., being thereby reduced to iodine.

The reaction between carbon monoxide and gaseous oxygen, i.e. the process of combustion of carbon monoxide, has been the subject of much investigation.

Combination of these gases can be brought about by passing the mixed gases over moist phosphorus. Probably hydrogen peroxide or ozone plays a part in the reaction; and either of these substances has the power of oxidising carbon monoxide in presence of palladium. In presence of a heated palladium wire the union of carbon monoxide with oxygen is complete at 300° C. Combination between the two gases also takes place gradually under the influence of the silent electric discharge.

The influence of temperature upon the degree of combination between carbon monoxide and oxygen has been studied by Helier, who obtained the following results:

Temperature ° C	195	302	408	500	566	600	689	855
Per cent, combination	0.13	0.44	3.03	6.2	14.43	21.14	46.38	65.0

These figures are of relative value only, as true equilibrium was not attained. The kinetics of the reaction between these two gases at 570° c. has been studied by Kuhl, who found that the order in which they were introduced into the reaction vessel in presence of moisture, and whether or not carbon dioxide was originally present, had much influence on the velocity of the reaction. Bodenstein and Ohlmer have observed that the reaction is catalytically accelerated by quartz-glass.

The influence of moisture in promoting the combination of these two gases is noteworthy. A mixture of dried carbon monoxide and oxygen is not exploded by a spark, though a mixture of the moist gases is so exploded. If a dried mixture is contained in a long tube into one end of which a little moisture is introduced, the moistened mixture will explode, but the flame dies out as it reaches the dried gases. Dried carbon monoxide and oxygen will unite in the path of an electric spark; the reaction, however, is reversible, and a limit is reached when the rates of combination and decomposition are equal. The dried gases also combine, without explosion or flame, when in contact with a hot platinum wire.

It is clear from the above statements that whilst the presence of water is not always necessary to the union of carbon monoxide and oxygen, its presence greatly facilitates the combustion of the former gas. Thus carbon monoxide burns in moist air with a pale blue flame, but such a flame is extinguished when it is plunged into dry air.

Not only water-vapour itself, but other substances capable of forming water-vapour by combustion, such as hydrogen, hydrogen sulphide, and hydrocarbons, promote the combustion of carbon monoxide; and continued addition of water-vapour to a mixture of carbon monoxide and oxygen accelerates the velocity of explosion until about 4.5 per cent, has been added.

Various explanations of this catalytic action of water-vapour have been given. According to Armstrong the two gases, which are inert in the pure state, require "the formation of a conducting system in which electrolysis can occur," or, according to a later exposition, oxidation takes place "in a circuit composed of the oxidisable substance, conducting water and oxygen." Briefly, Armstrong's view may be represented by the following scheme:



in which the elements of water appear to be partitioned between oxygen and carbon monoxide. Against this view is the fact that when dry cyanogen is exploded with excess of oxygen or burnt in a Smithells separator, whilst the final products are carbon dioxide and nitrogen, carbon monoxide is formed as an intermediate product, and subsequently burns to carbon dioxide in the complete absence of water-vapour.

Traube has put forward the theory that the water molecules yield their oxygen to carbon monoxide at the same time that the remaining hydrogen appropriates a molecule of oxygen to form hydrogen peroxide, thus:

1. CO + OH₂ + O₂ = CO₂ + H₂O₂
2. CO + O₂H₂ = CO₂ + OH₂

Carbon monoxide is, however, directly oxidised by steam at high temperature; and Mendeleeff, who accepted Traube's theory with regard to hydrogen peroxide, consequently gave the following equations, which illustrate his belief that reactions between equal volumes of gases precede all others:

1. CO + OH₂ = CO₂ + H₂
2. O₂ + H₂ = O₂H₂
3. CO + O₂H₂ = CO₂ + OH₂

Traube's theory of the formation of hydrogen peroxide is, however, denied by Wieland, who supports Armstrong's views.

Dixon, who has reviewed the various theories which have been advanced to account for the inertness of dry carbon monoxide in presence of dry oxygen, advances the suggestion that the dissociation of carbon dioxide at the temperature at which carbon monoxide and oxygen would combine accounts for the non-appearance of the former gas; but that in the presence of steam carbon monoxide is oxidised thereby, the liberated hydrogen recombining with oxygen to form steam, which is stable at the temperature of the combustion, thus:

2CO + 2OH₂ = CO₂ + 2H₂
2H₂ + O₂ = 2H₂O

It cannot be said, however, that the function of water-vapour in promoting the combustion of carbon monoxide has been definitely established.

It may be mentioned that according to Gautier interaction between carbon monoxide and water-vapour takes place at 1200°-1250° C. The reactions of carbon monoxide with hydrogen, water-vapour, and iron and its oxides have been studied by Gautier and Clausmann, and their bearing upon volcanic and geological phenomena and the origin of petroleum has been discussed.

Besides uniting with oxygen, carbon monoxide, as an unsaturated compound, forms addition products with a number of other substances. The fact that this gas combines with chlorine under the influence of light to form carbonyl chloride, but not with bromine or iodine, has already been noticed.

Combination with sulphur vapour to form carbonyl sulphide, COS, takes place at a red heat; hydrogen combines with carbon monoxide to produce formaldehyde under the influence of the silent electric discharge, but by the help of a catalyst such as metallic nickel reduces this gas to methane. Alkali formate is produced by the absorption of carbon monoxide by heated caustic alkali:

CO + KOH = HCOOK,

and also by the action of the gas on alkali hydride, carbon being separated thus:

2CO + KH = HCOOK + C.

By the interaction of lime and carbon monoxide between 350° C. and 400° C. considerable quantities of methane, ethylene, and hydrogen are formed by the following reactions:

2CO + Ca(OH)₂ = (HCOO)₂Ca = (COO)₂Ca + H₂;
2(HCOO)₂Ca + CaO = 3CaCO₃ + CH₄;
4(HCOO)₂Ca + 2CaO = 6CaCO₃ + C₂H₄ + 2H₂.

Carbon monoxide also reacts with sodium methoxide and its homologues at 160° C. to produce the sodium salt of the corresponding carboxylic acid -

CO + CH₃ONa = CH₃-COONa.

One of the most important additive reactions of carbon monoxide, since it is used in the volumetric estimation of this gas, is its combination with cuprous chloride. The conditions of this reaction have been studied by Manchot and Friend, who find that the ratio Cu:CO determines the maximum limit of absorption - the compound CuCl.CO.2H₂O, which crystallises in colourless leaflets, being formed in all cases.

The absorption of carbon monoxide by a hydrochloric acid solution of cuprous chloride increases with decreasing concentration of hydrogen chloride, with lowering of temperature, with increasing concentration of cuprous chloride, and increasing pressure of carbon monoxide.

Ammonia, aniline, etc., may take the place of water in the compound CuCl.CO.2H₂O; otherwise water is necessary. An alcoholic suspension of cuprous chloride, for example, does not absorb carbon monoxide. Cuprous hydroxide does not combine with carbon monoxide, but in presence of-sodium hydroxide is reduced thereby to copper.

A number of interesting syntheses can be brought about by submitting carbon monoxide mixed with various other gases and vapours to the influence of the silent electric discharge:

CO + H₂O = HCOOH (formic acid)
CO + H₂ = HCOH (formaldehyde)
CO + CH₄ = CH₃COH (acetaldehyde)
CO + H₂S = HCOH + S
HCOH + H₂S = HCSH (thioformaldehyde) + H₂O
CO + HCl = HCOCl (formyl chloride)
CO + NH₃ = HCONH₂ (formamide).

According to Kuhlmann, however, ammonium cyanide is formed when carbon monoxide and ammonia are passed together over heated platinum-black:

CO + 2NH₃ = NH₄CN + H₂O;

but Jackson and Northall-Laurie find that ammonium cyanate, which gives rise to urea, is formed thus:

CO + 2NH₃ = OCN-NH₄ + H₂.

Ultra-violet rays also cause combinations between carbon monoxide and other gases.

When carbon monoxide itself is submitted to prolonged electric discharges one or more suboxides of carbon are produced.

Compounds of Carbon Monoxide with Metals; Metallic Carbonyls

In his Presidential Address before the Chemical Section of the British Association in 1896 Dr. Ludwig Mond described a carbide of nickel which he had observed to be formed on some nickel valves; and this observation led to the discovery in 1890 of a volatile compound of nickel and carbon monoxide: nickel carbonyl, Ni(CO)₄, whose formation and decomposition has proved of great practical value in the metallurgy of nickel. The following carbonyls are known to exist: Ni(CO)₄, Co₂(CO)₈, [Co(CO)₃]_n, Fe(CO)₅, [Fe₂(CO)₉]_n, [Fe(CO)₄]_n, [Mo(CO)₆]_n, [Ru(CO)_x]_n; and they will be described under the corresponding metals. Their constitution is still a matter of conjecture.

A potassium carbonyl, K₆(CO)₆, which is explosive, and was formed in the old Brunner process for the manufacture of potassium by heating potassium carbonate with carbon, is probably the potassium derivative of hexahydroxybenzene.