Carbon Dioxide, CO₂

Carbon Dioxide, CO₂, also known as Carbonic Anhydride and Carbonic Acid Gas, was first recognised by van Helmont, early in the seventeenth century, as a gas which differed in properties from air. This chemist showed that the gas is produced by acting on limestone and potashes with acids, by the burning of coal, and by the fermentation of wine and beer; that it is contained in the stomach, in mineral waters, and in caves such as the Grotto del Cane, near Naples; and that it possesses suffocating properties and extinguishes a flame. The name gas sylvestre was given by van Helmont to carbon dioxide, because the gas appeared to be uncondensible (sylvestris = wild). The first scientific investigation of carbon dioxide was made by Black in 1755, who in his "Experiments upon Magnesia-alba, Quicklime, and other Alkaline Substances " showed that this substance is present in a combined state in calcium and magnesium carbonates, whence it may be driven by heat or acids. On account of its presence in solid carbonates Black named this gas fixed air. Bergmann, who in 1774 published an account of carbon dioxide, called it acid of air because of its presence in the atmosphere; Scheele and Priestley showed that in the burning of a candle fixed air took the place of dephlogisticated air; and Lavoisier, in 1775-6, established the chemical nature and composition of the gas by showing that it is produced when mercuric oxide is heated with carbon; whilst Dalton, in 1803, showed that " carbonic acid" contains twice as much oxygen combined with the same quantity of carbon as " carbonic oxide" does, and that these two gases furnish an example of the law of multiple proportions.

In the early ages of the earth's geological history the distribution of carbon dioxide was quite different from what it is now. Then the basic oxides of the earth's crust were combined mainly with silica, whilst the atmosphere was very rich in carbon dioxide. This was because the high temperature of the igneous rocks favoured combination with silica rather than with carbon dioxide. The displacement of carbon dioxide by silica on fusion of the latter with sodium carbonate illustrates the principle. With the aid of atmospheric aqueous vapour, however, a process of weathering of siliceous rocks commenced by which carbonates were formed, and silicic acid, silica, or silicates of less basic metals liberated. The weathering of felspar, by which potassium aluminium silicate is converted into potassium carbonate, aluminium silicate or kaolin, and free silica, is the most familial example of this process. By the aid of rain, and consequent streams and rivers, these carbonates or bicarbonates were carried in solution to the sea.

Meanwhile the sea itself was receiving in solution much carbon dioxide directly from the atmosphere, continual interchange of this gas taking place between the two media. Partly by inorganic decomposition, but chiefly through the agency of marine organisms, the bicarbonates of calcium and magnesium were decomposed, and sedimentary rocks produced from the carbonates thus set free, while the free carbonic acid containing half the carbon dioxide originally removed from the air eventually returned this carbon dioxide to the air, which by means of circulation again became available for weathering. Thus carbon dioxide has been continuously removed from the air and stored up in the crust of the earth in the various forms of limestone rocks and coral reefs. So great has been the storage of carbon dioxide by this means that at the present day there is about thirty thousand times as much of this compound in the sedimentary rocks as in the atmosphere.

Simultaneously with this process a quite different agency was at work reducing the amount of carbon dioxide in the air. The green parts of plants, and especially their leaves, possess the power of decomposing atmospheric carbon dioxide in the presence of sunlight, retaining the carbon and returning the oxygen to the air. The chlorophyll, or green colouring-matter, of the leaves is associated with masses of protoplasm to constitute chlorophyll corpuscles or chloroplasts. The radiant energy absorbed by the colour is employed by this protoplasm to bring about a chemical change which may be represented by the equation:

xCO₂ + xH₂O = C_xH_2xO_x + xO₂.

The significance and importance of this process of photosynthesis in the economy of nature can hardly be overestimated. First, the excessive amount of carbon dioxide originally present in the air was gradually replaced by oxygen; secondly, carbohydrate was produced, whence is derived food for man and beast; thirdly, energy was stored up - e.g. in coal - in the separation of carbon and oxygen, which again becomes available when the products are employed as fuel, and the carbon returns to the air as carbon dioxide. Thus the cycle

carbon dioxide → "organic" carbon → carbon dioxide

is completed. From the point of view of mass it provides the carbonaceous basis of living organisms; from the point of view of energy it furnishes the source of their activities.

The decomposition of atmospheric carbon dioxide by green plants was observed by Priestley, Senebier, and Ingenhouss before the close of the eighteenth century, and Mayer and Helmholtz showed the importance of the reaction as a means by which energy is stored. By throwing a continuous spectrum upon a filament of alga impregnated with bacteria which are stimulated to active movement only in presence of free oxygen, Englemann proved that the energy absorbed from white light by chlorophyll is employed by the plant in photosynthesis. The zones of activity of the bacteria were found to coincide with the positions of the absorption bands of the chlorophyll spectrum.

With regard to the actual process of carbon assimilation, Baeyer advanced the theory that formaldehyde is the first product, and that this substance polymerises; thus:

CO₂ + H₂O = HCOH + O₂
6H-COH = C₆H₁₂O₆.

According to Baeyer the first reaction takes place in two stages, thus:

CO₂ = CO + O
CO + H₂O + O = HCOH + O₂;

but Erlenmeyer assumes the production of formic acid and hydrogen peroxide, thus: CO₂ + 2H₂O = H-COOH + H₂O₂

which then interact, giving formaldehyde, water, and oxygen, thus:

HCOOH + H₂O₂ = HCOH + H₂O + O₂.

A somewhat different view was expressed by Bach in the equations

3H₂CO₃ = 2H₂CO₄ + HCOH
H₂CO₄ = CO₂ + H₂O₂.

Erlenmeyer's views find experimental support in the work of Usher and Priestley, who have obtained formaldehyde and hydrogen peroxide from aqueous carbon dioxide in presence of chlorophyll, formic acid appearing as an intermediate substance. The same observers have shown that this photosynthesis can be copied outside the green plant by the use of a suitable enzyme, but that for the formation of starch living non-chlorophyllous protoplasm is necessary. These observations are hardly in accord with that of Herzog, to the effect that an extract of green leaves made like Buchner's extract of yeast has no power of carbon assimilation, but harmonise with the view that starch is not derived from photosynthetic sugar, but is a degradation product of protoplasm.

The proportion of carbon dioxide now present in the atmosphere is very nearly 3 parts in 10,000 by volume, or 0.03 per cent.

In the following table are included a few of the more important and recent determinations of the amount of carbon dioxide in the atmosphere.

The total amount of carbon dioxide in the air is estimated at 22 billion (2.2×10¹²) tons, and corresponds to the presence of 600,000 million tons of carbon.

The sources of atmospheric carbon dioxide are respiration of man and animals, and to a less degree of plants; combustion, fermentation, and putrefaction of carbonaceous substances, frequently subterranean.

The soil is constantly evolving carbon dioxide; part of this is undoubtedly of volcanic origin, and part results from chemical change taking place nearer the surface of the soil. For example, small worms exhale as much carbon dioxide as human beings, weight for weight, and in view of the vast numbers of these and other lowly forms of life inhabiting the soil, it is easy to understand that the quantity of carbon dioxide evolved is considerable.

Other sources of carbon dioxide are mineral springs, volcanic vents and fissures, and the calcination of carbonates, e.g. lime-burning. It was estimated by Boussingault in 1844 that Cotopaxi emitted more carbon dioxide annually than was produced by combustion and respiration in the city of Paris.

The accumulation in the atmosphere of the carbon dioxide derived from these various sources is prevented by the process of carbon-assimilation carried out in sunlight by green plants, and by the atmospheric weathering of siliceous rocks, which still continues. The relative magnitude of these two processes is not known; nor is it known how much carbon dioxide is annually removed from the air by their means, though T. C. Chamberlin estimates this amount to be 1.62×10⁹ tons. Neither is it known whether the proportion of carbon dioxide in the air is now quite stationary, or whether it is very slowly increasing or diminishing. It may be supposed, however, that with the advance in human population and civilisation, and the accompanying depletion of forests, the proportion of carbon dioxide must be slowly increasing.

It has been shown by Arrhenius that, owing to the power of absorbing terrestrial radiations possessed by carbon dioxide, an increase in the proportion of this gas in the atmosphere would increase the mean temperature of the latter. Thus Arrhenius calculates that if the quantity of Carbon dioxide in the air were increased threefold the mean temperature of the Arctic regions would rise 8°-9° C., and that glacial periods may be accounted for by a loss of atmospheric carbon dioxide. Moreover, according to Hogbom, it is the intermittent volcanic activity of the globe that disturbs the otherwise balanced proportion of carbon dioxide in the air; hence it would follow that varying volcanic activity is the determining cause of varying secular climate. This theory has not, however, found general acceptance, because it is known that variations in the amount of atmospheric aqueous vapour have a great, and probably preponderating, influence on terrestrial radiation.