Note 13 — Chapter IX · COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

Up to the year 1840, or thereabout, acids were not distinguished by their basicity. Graham, while studying phosphoric acid, H3PO4, and Liebig, while studying many organic acids, distinguished mono-, bi-, and tribasic acids. Gerhardt and Laurent generalised these relations, showing that this distinction extends over many reactions (for instance, to the faculty of bibasic acids of forming acid salts with alkalis, KHO or NaHO, or with alcohols, RHO, &c.); but now, since a definite conception as to atoms and molecules has been arrived at, the basicity of an acid is determined by the number of hydrogen atoms, contained in a molecule of the acid, which can be exchanged for metals. If carbonic acid forms acid salts, NaHCO3, and normal salts, Na2CO3, it is evident that the hydrate is H2CO3, a bibasic acid. Otherwise it is at present impossible to account for the composition of these salts. But when C=6 and O=8 were taken, then the formula CO2 expressed the composition, but not the molecular weight, of carbonic anhydride; and the composition of the normal salt would be Na2C2O6 or NaCO3, therefore carbonic acid might have been considered as a monobasic acid. Then the acid salt would have been represented by NaCO3,HCO3. Such questions were the cause of much argument and difference of opinion among chemists about forty years ago. At present there cannot be two opinions on the subject if the law of Avogadro-Gerhardt and its consequences be strictly adhered to. It may, however, be observed here that the monobasic acids R(OH) were for a long time considered to be incapable of being decomposed into water and anhydride, and this property was ascribed to the bibasic acids R(OH)2 as containing the elements necessary for the separation of the molecule of water, H2O. Thus H2SO4 or SO2(OH)2, H2CO3, or CO(OH)2, and other bibasic acids decompose into an anhydride, RO, and water, H2O. But as nitrous, HNO2, iodic, HIO3, hypochlorous, HClO, and other monobasic acids easily give their anhydrides N2O3, I2O5, Cl2O, &c., that method of distinguishing the basicity of acids, although it fairly well satisfies the requirements of organic chemistry, cannot be considered correct. It may also be remarked that up to the present time not one of the bibasic acids has been found to have the faculty of being distilled without being decomposed into anhydride and water (even H2SO4, on being evaporated and distilled, gives SO3 + H2O), and the decomposition of acids into water and anhydride proceeds particularly easily in dealing with feebly energetic acids, such as carbonic, nitrous, boric, and hypochlorous. Let us add that carbonic acid, as a hydrate corresponding to marsh gas, C(HO)4 = CO2 + 2H2O, ought to be tetrabasic. But in general it does not form such salts. Basic salts, however, such as CuCO3CuO, may be regarded in this sense, for CCu2O4 corresponds with CH4O4, as Cu corresponds with H2. Amongst the ethereal salts (alcoholic derivatives) of carbonic acid corresponding cases are, however, observed; for instance, ethylic orthocarbonate, C(C2H5O)4 (obtained by the action of chloropicrin, C(NO2)Cl3, on sodium ethoxide, C2H5ONa; boiling point 158°; specific gravity, 0·92). The name orthocarbonic acid for CH4O4 is taken from orthophosphoric acid, PH3O4, which corresponds with PH3 (see Chapter on Phosphorus).