Physical Properties of Carbon Monoxide

Carbon monoxide is a colourless gas without taste or smell, which is very poisonous. Its density (air = 1) is 0.96702 (Leduc) or 0.96716 (Rayleigh). To the latter density the molecular weight 27.9989 corresponds, whilst CO requires M.W. = 28.002; therefore at atmospheric pressure carbon monoxide is practically an ideal gas. The molecular weight of CO is the same as that of nitrogen (N₂ = 28.02); and in condensibility and other physical properties the two gases closely resemble each other.

The coefficient of expansion of carbon monoxide at atmospheric pressure is 0.0036667, which is that for an ideal gas.

Natterer determined the diminishing compressibility of the gas at high pressures. Compressibility is expressed by the ratio p₀v₀/pv, where p₀ = 1 atmosphere. Some of the values obtained are given in the following table.

p (atm)	p0v0/pv
77	1.000
204	0.965
408	0.900
515	0.810
629	0.727
814	0.623
1133	0.500
1716	0.366
2790	0.261

Carbon monoxide was liquefied by Cailletet in 1877, and by Wroblewski and Olszewski in 1883, who cooled the gas to -136° C. in boiling ethylene and compressed it to 150 atmospheres. It forms a colourless liquid which boils at -193° C. (Wroblewski) or -190° C. (Olszewski) under 1 atm. pressure, and forms a snowlike solid at -211° C. (Cailletet) or -199° C. (Wroblewski). The critical temperature and pressure of carbon monoxide, together with the constants a and b for van der Waals' equation, are:

Critical Temp. °C.	Critical Pressure	a	b	Author
-141.1	35.9 atm.	0.00275	0.001683	Wroblewski
-139.5	35.5 atm	0.00285	0.001723	Olszewski

The critical density of carbon monoxide is 0.3110. The densities of liquid carbon monoxide at different temperatures have been determined by Baly and Donnan. They are expressed by the formula:

d = 0.8558 – 0.0042 (T – 68), where T = abs. temp.

The specific heat of carbon monoxide between 99° C. and 23° C. is 0.2425, and the ratio Cp/Cv is approximately 1.40, the value for a diatomic gas.

The heat of combustion (CO,O) is 68,200 calories, and the heats of formation are as follow:

(C) amorph. + (O) = CO + 29,000 calories,

(C) diamond + (O) = CO + 26,100 calories.

Carbon monoxide is very slightly soluble in water; its solubility at different temperatures is represented by the following equation:

c = 0.032784 – 0.0080094t + 0.000015872t².

Its solubility in other, solvents has been investigated by Just, Skirrow, and Christoff.