Search

The behaviour of the calcium-aluminium-alizarate complex on alumina is described. By elution with mixtures of dimethylformamide and water two fractions can be isolated. It has been shown that these fractions are not cis-trans isomers. In all probability the two fractions differ in the charge distribution of the electrons, caused by a difference in water content.

We had reason to doubt the correctness of current opinion that the formula of the calcium–aluminium lake of alizarin (Turkey Red) is Ca2A13Az4 (where Az represents an alizarin molecule minus two hydroxylic hydrogen atoms). We therefore carried out a further study of this complex, the results of which are presented in this paper. The molecular structure of the complex has been established. The complex has been prepared in vitro and purified by washing, crystallisation, and chromatographic separation. The structure has been elucidated by conventional analytical methods, including infrared spectroscopy. The basic compound has been established to be CaAl(OH)(Az)z, in which calcium is bound as an ion and aluminium forms part of a chelate ring with the 9-carboxyl and 1-hydroxyl groups of alizarin. This compound can take up three molecules of water, forming a new compound in which the water molecules are part of the molecular lattice. This new compound can form a hydrate with another three molecules of water.

The molecular structures of some complexes of alizarin have been established. It has been shown that all these compounds have the sane general type of structure and that they consist of a complex anion (a chelate of alizarin with a trivalent metal) with a uni-, bi- or quadri-valent metal cation. Before complex formation, salt formation of the alizarin molecule is necessary, because otherwise complex formation will be very slow.

It has often been assumed that the oil used in the traditional Turkey Red dyeing processes is an essential constituent of the dye complex formed on the cotton fibre. Work is now described which shows that the same complex is formed in vitro and on cotton fabrics mordanted with aluminium salts in the presence of calcium ions, both in absence and in presence of oils, e. g. Turkey Red oil; the formula of the complex is CaAl(OH)(Az)2. Factors governing the deposition of the complex within the cotton fabric are discussed. For level dyeing, fixation of the mordant should take place at pH 8, which is also the optimum dyebath pH; in the dyeing process itself, alizarin should be present in excess of the amount necessary for complex formation.

The preparation of the potassium and calcium salts of alizarin and of 3-nitroalizarin is described. Their structures have been established and compared with those of salts of complex alizarates containing the same metals, which have been described in the preceding parts of this series. It was already known that, prior to complex formation, salt formation must take place. With the knowledge of the structure of these salts, the mechanism of complex formation following salt formation has been established. An intermediate monobasic salt, e.g. potassium 2-alizarate or the acid calcium 2-alizarate, is formed, from which the complexes are produced with the chelating metal ion. During dyeing with Turkey Red (calcium-aluminium lake of alizarin), calcium alizarate is not formed, because the intermediate acid calcium salt reacts more quickly with aluminium hydroxide to form the complex than the calcium atom in the acid salt reacts with the 1-OH group.

The molecular structure of some metal complexes of 3-nitroalizarin, 3-aminoalizarin, and sodium 3-alizarinsulphonate has been established. It has been found that all these compounds have the same basic structure as that of the corresponding complexes of alizarin, as discussed in Part II of this series.