Table 2. Several metal salts were used to precipitate the lakes:
calcium–aluminium, potassium–aluminium, copper sulphate
pentahydrate and magnesium sulphate heptahydrate.
Table 2 shows a similarity between the yield of the copper lake
and conventional aluminium lakes; values ranged from 11% to
15%, involving characteristic colours (Fig. 6b). Similar yields have
been obtained by other authors (Fajardo Feo, 1986). Moreover, the
addition of the magnesium salts produced a substantial decrease in
yield, (about 5%), which is lower than those obtained for the common
lakes; but this lake had a different shade of colour. Its chromatic
coordinates determined by standard colourimetric pattern
(CIE1931) are presented in Table 2; all colours were in the violet
and pink range. Fig. 11 shows spectra obtained by infrared spectrophotometry
for the lakes obtained compared with the carminic acid
standard. A slight displacement of the 1569 nm peak of the lakes
spectra can be observed relative to the carminic acid standard spectra.
This effect could be attributed to the H substitution of the hydroxyl
group bonded to C5 of the dye by each metallic ligand.