The intense red colour and the stability of the product of reaction between ferrous salts and αα′ dipyridyl has remained a striking instance of complex salt formation since the discovery of this dipyridyl by Blau (1888). Although for obvious reasons this compound would seem a good reagent for the colorimetric estimation of iron, it has never been used to any extent for the purpose, without doubt because there are other more sensitive and accessible reagents. It seems, however, to possess unique advantages for the study of iron in connection with living material, and on its account it was thought advisable to bring αα′ dipyridyl, as a reagent for iron, to the notice of biologists. αα′ dipyridyl was first prepared by Blau (1888) by distilling the copper salt of α picolinic acid; the yield is small, but this method has the advantage of readily yielding a pure product. Recently Hein and Retter (1928) have found a relatively simple method of preparation by heating pyridine in a sealed tube with an oxidising agent such as ferric chloride, which removes hydrogen without the formation of water. This method though more economical needs greater manipulation to produce the substance in a state of purity. Smith (1926) isolated the same substance by the oxidation of sodium pyridines with dry air above 100° C. The ferrous complex salts were examined by Werner (1912) who showed that they could be resolved into two optical isomers. The ferrous salts of αα′ dipyridyl have the formula [Fe(C
10
H
8
N
3
)
3
]X
2
(Blau, 1898) where X is a monovalent acid radicle, and from the fact that the optical isomers do not racemize at once (Werner, 1912) dissociation into free ferrous salts must be relatively slight. They differ from the normal ferrous salts in not being eaily oxidised, in fact only powerful agents such as permanganate or chlorine oxidise them to a blue compound. This latter compound, which should be the ferric complex salt, is not obtained directly from ferric salts and αα′ dipyridyl and passes very easily into the red ferrous salt.
Studies on the Organic Analytical Reagents. V-VI. V. Ferrous Complex Salts of cr-Picolinic Acid and Quinaldic Acid.