Atomic Weights of Carbon, History
In his original table of atomic weights, published in 1803, Dalton gave 4.3 as the atomic weight of carbon, but altered this in 1808 to 5. Prout, in 1815, wrote C=6, and Gmelin attributed the same value to the equivalent of carbon. Thenard, in 1826, accepted for the atomic weight C=12.25 (H = 1); Meissner, in 1834, C = 4.24 (H = 1); Kuhn, in 1837, C = 6.1 (H = 1).
This confusion was removed by the recognition of Avogadro's theory, and the fact that never less than 12 parts by weight of carbon are present in a molecular proportion of any of its volatile compounds; though Wanklyn, in 1894, unsuccessfully attempted to revive the value C = 6, on the ground that Russian kerosene contained hydrocarbons in whose molecules there would appear to be present halfatoms of carbon, e.g. C_{7½}H_{15}, if the atomic weight of carbon were made equal to 12.
Although the specific heat of carbon varies for the different allotropes, and also varies in the case of each allotrope with temperature, it approaches a constant value, which is the same for diamond and graphite, at about 600° C. Thence, if C = 12, the atomic heat of carbon becomes 5.5, a value comparable with that of analogous elements.
In the Periodic Table there is a vacancy for an element exhibiting the same general properties as carbon, the atomic weight of which lies between those of boron (11.0) and nitrogen (14.01).
The methods employed in the accurate determination of the atomic weight of carbon have been many and varied, but they ail fall into one of two categories, namely, (1) physical methods, based upon determining the densities of gaseous compounds, and (2) chemical methods, depending upon the combustion of carbon or the analysis of its compounds. 
Physical Methods of Carbon Weight CalculationOn the assumption that Avogadro's theory is rigidly true, and therefore that oxygen and carbon monoxide undergo no change of volume when they are separately converted into carbon dioxide, the molecular weights of these three gases can be compared by determining their densities: so that the atomic weight of carbon can be calculated if that of oxygen is known.
Wollaston, in 1814, first calculated the atomic weight of carbon in this manner from the density determinations of Biot and Arago, namely:
Density of oxygen (air = 1) = 1.10359 Density of carbon dioxide = 1.51961
Hence, atomic weight of carbon (O = 16.00) = 12.063.
Berzelius and Dulong, in 1819, obtained the following values:
Density of oxygen (air = 1) = 1.1026 Density of carbon dioxide = 1.5245
Hence, atomic weight of carbon (O = 16.00) = 12.24.
They confirmed this value for the atomic weight of carbon by a chemical method (vide infra). On the authority of Berzelius this value was accepted for a number of years, though R. D. Thomson, in 1836, obtained the figure C = 12.000 by a density determination. Wrede, however, in 1842, subsequently to the chemical methods of Liebig and Redtenbacher in 1841, pointed out that carbon dioxide is more compressible at ordinary temperatures than corresponds with Boyle's law, and that oxygen therefore yields less than its own volume of carbon dioxide, the density of which is correspondingly high. He estimated the ideal density of carbon dioxide to be 1.52037, that of oxygen 1.1052, and that of carbon monoxide 0.96779; whence by comparison of densities he obtained the following results:
From the ratio CO_{2}:CO, atomic weight of C = 12.023 From the ratio CO:O, atomic weight of C = 12.021 From the ratio CO:O, atomic weight of C = 12.022 the mean value being C = 12.022 (O = 16).
The above results, however, are merely of historical interest.
Modern investigators have, like Wrede, recognised the approximate character of Avogadro's theory, and devised methods whereby gaseous densities may be utilised for the determination of exact molecular weights. Of these methods, D. Berthelot's "method of limiting densities" and Guye's "method of critical constants" have been explained in vol. i of this series. Leduc has devised a third method, the "method of molecular volumes," which is in principle identical with that of Berthelot, although the actual methods of calculation adopted by Leduc are somewhat different. Leduc has determined the densities of carbon monoxide, carbon dioxide, acetylene, and oxygen at N.T.P. relatively to that of air, and also measured the compressibilities of these gases. His density results, when that of oxygen is unity, are as follow:
CO = 0.87495; CO_{2} = 1.38324; C_{2}H_{2} = 0.8194;
the values for the oxides of carbon being in excellent agreement with Lord Rayleigh's data for the same gases. Leduc's method of calculation leads to the following results:
Molecular weight of CO = 28.005, whence atomic weight of C = 12.005 Molecular weight of CO_{2} = 44.005, whence atomic weight of C = 12.005 Molecular weight of C_{2}H_{2} = 26.026, whence atomic weight of C = 12.005
where O = 16 and H = 1.00762; and the same method of calculation gives for methane the results: Molecular weight of CH_{4} = 16.036, whence atomic weight of C = 12.006, when Baume and Perrot's value for the weight of a "normal" litre of the gas is adopted, viz. 0.71680 grammes.
D. Berthelot, in 1898, calculating by his method and utilising for the purpose the experimental data of Leduc and Rayleigh, arrived at the value C = 12.005, as the most probable mean of a number of values ranging from 12.000 to 12.007. Berthelot's method, however, cannot be very satisfactorily applied to gaseous carbon compound's, since their compressibilities at 0° C. have not been directly measured., and attempts to calculate them from either van der Waals' equation or one of its modifications are not very satisfactory. The above cited value for the density of methane, together with Leduc's determinations of the compressibility at 16° C., give Molecular weight of CH_{4} = 16.039, whence atomic weight of C = 12.0085 while Rayleigh's value for the density of carbon monoxide at N.T.P., viz. 0.87497 (O = 1), and his value for the compressibilitv at about 13° C., lead to Molecular weight of CO = 28.003, whence atomic weight of C = 12.003.
Guye's method of critical constants may be applied to the difficultly liquefiable gases carbon monoxide and methane. The density of carbon monoxide at N.T.P. Is 0.87497 according to Rayleigh, and 0.87495 according to Leduc, when that of oxygen is unity. The mean value corresponds to a weight of 1.25032 grams for a normal litre of carbon monoxide. A similar volume of methane, according to Baume and Perrot, weighs 0.71680 grams. Guye's method of calculation leads to the following results:
Molecular weight of CO = 28.003, whence the atomic weight of C = 12.003 Molecular weight of CH_{4} = 16.034, whence the atomic weight of C = 12.004 Molecular weight of O = 16, whence the atomic weight of H = 1.00762.
In dealing with readily liquefiable gases Guye adopts carbon dioxide as the standard from which to derive the constants in his formulae, and assumes an atomic weight of C = 12.002. Accordingly, the values C = 12.001 and C = 12.003, that may then be derived from the densities of acetylene and ethane respectively, only serve to indicate the consistency of the method of calculation. 
Chemical Methods of Carbon Weight CalculationFrom an analysis of purified naphthalene, Dumas, in 1838, came to the conclusion that the value for the atomic weight of carbon (C = 12.24) obtained by Berzelius and Dulong in 1819, by the density method previously referred to, was too high. Consequently Berzelius analysed lead carbonate and oxalate, converting them to monoxide, and obtained the value C = 12.242, which appeared to justify his previous result. Dumas, however, was unconvinced, and, together with Stas, carried out a series of researches to determine the ratio C:CO_{2} by the combustion of diamond and natural and artificial graphite.
The graphite employed was purified by fusion with alkali, followed by treatment with hydrochloric acid and with aqua regia, and subsequently, after washing and drying, by ignition at a white heat for sixteen to eighteen hours in a stream of chlorine. The graphite or diamond was contained in a platinum boat placed in a porcelain tube and heated in a furnace, and was burnt in a current of carefully purified oxygen, the carbon dioxide being absorbed in potash bulbs. Both graphite and diamond left a minute ash, the weight of which was subtracted from that of the substance taken.
It has been pointed out by Scott that in correcting their weighings for the air displacement of the potash solution, Dumas and Stas. as well as later experimenters, neglected to take account of the expansion of this solution during its absorption of carbon dioxide. Consequently the values obtained by these observers are slightly too high.
Mean value obtained by Dumas and Stas  C = 11.9975 Corrected by Scott C = 11.9938
Erdmann and Marchand carried out experiments similar to those of Dumas and Stas, weighing the carbon dioxide produced in the combustion of diamond and graphite. They made, however, the same omission as the former experimenters, and their result, together with the correction of Scott, was as follows:
Uncorrected – C = 12.0093 Corrected by Scott – C = 12.0054
Later experiments of a similar kind to which the same correction needed to be applied were those of Roscoe, Friedel, and van der Plaats.
 Uncorrected  Corrected by Scott.  Roscoe  12.0029  11.9973  Friedel  12.0112  12.0056  Van der Plaats  12.0031  12.0018 
The mean of the five values obtained by the above observers from the combustion of carbon, after correction by Scott, gave C = 12.0008.
Stas, in 1849, estimated the atomic weight of carbon by passing carbon monoxide over heated copper oxide, and determining the loss in weight of the latter and the weight of carbon dioxide formed. From the relation O_{2}:CO_{2} thus found the value C = 12.0046 (O = 16) may be obtained. Scott has critically examined the above method, and finds it vitiated by a number of experimental errors which have evidently compensated each other to yield this accurate figure.
Another method for determining the atomic weight of carbon which must be noticed is that depending on the ignition of organic silver salts.
Liebig and Redtenbacher, in 1841, obtained the following results from the ignition of (a) silver acetate, (b) silver tartrate, (c) silver racemate, (d) silver malate:
 C_{2}H_{3}O_{2}Ag:Ag = 100:64.6065, whence atomic weight of C = 12.039
 C_{4}H_{4}O_{6}Ag2:2Ag = 100:59.2806, whence atomic weight of C = 12.044
 C_{4}H_{4}O_{6}Ag2:2Ag = 100:59.2769, whence atomic weight of C = 12.049
 C_{4}H_{4}O_{5}Ag2:2Ag = 100:62.0016, whence atomic weight of C = 12.050
Marignac, in 1846, found
C_{2}H_{3}O_{2}Ag:Ag = 100:64.609, whence atomic weight of C = 12.035, and C_{2}H_{3}O_{2}Ag:Ag = 100:64.649, whence atomic weight of C = 11.986;
the latter result following special precautions against loss of silver by spurting. Some results obtained by Maumene were too low and will not be considered here. The general tendency is for all estimations involving ignition of silver salts to be a little too high, owing to slight volatilisation of silver.
Hardin, in 1896, analysed silver acetate and benzoate by dissolving them in water, adding excess of potassium cyanide and electrolysing the solution. From his results the following values may be calculated:
C_{2}H_{3}O_{2}Ag:Ag = 100:64.637, whence atomic weight of C = 12.000 C_{7}H_{5}O_{2}Ag:Ag = 100:47.125, whence atomic weight of C = 12.001
In 1904 Parsons analysed glucinum acetylacetonate and glucinum basic acetate, with the following results:
(C_{5}H_{7}O_{2})_{2}Be:BeO = 100:12.1124 (C_{2}H_{3}O_{2})_{6}OBe_{4}:4BeO = 100:24.698
These equations give the values Be = 9.112 and C = 12.007.
Nevertheless, Scott, in 1909, titrated carefully purified tetramethyl and tetraethvlammonium bromides with a solution of pure silver, and obtained the remarkably high values of C = 12.017 and 12.019 respectively.
Later, however, he obtained the values C = 11.999 and C = 12.002 by experiments on the combustion of naphthalene and cinnamic acid respectively. The description of these experiments is lacking.
Richards and Hoover have determined the value of the sodium carbonatesilver ratio to be
Na_{2}CO_{3}:2Ag = 29.43501:59.91676,
from which it follows that if Ag = 107.88, the molecular weight of sodium carbonate (Na_{2}CO_{3}) is 105.995, whence the sum of the atomic weights 2Na + C = 57.995 (O = 16).
From this the atomic weight of carbon may be determined if that of sodium is known. Assuming the latter to be 22.996, the atomic weight of carbon is 12.003. 
SummaryThe mean value for the atomic weight of carbon arrived at by Scott in 1897, by summarising and correcting the results obtained by the combustion of carbon, was 12.0008. Worthy to be placed alongside this figure are Hardin's mean value, 12.0005, Parsons' value, 12.007, and Richards and Hoover's value, 12.003. Taking into account the values obtained by the physical method, it would appear that the atomic weight of carbon lies between 12.000 and 12.005, the value 12.003 being perhaps the best compromise. Guve in 19056, and Brauner in 1908, advocated the value 12.002.
The figure which appears in the International Table of Atomic Weights for 1917 is C = 12.005. 


