A more complex chromophoric group is the triple ethylenic grouping: C > C =, the introduction of which was rendered necessary by the discovery of certain coloured hydrocarbons.
The chromophoric groups are rarely strongly acid or basic; on the other hand, the auxochromes are strongly acid or basic and form salts very readily.
Notable differences attend the neutralization of the chromophoric and auxochromic groups.
With basic substances, the chromophoric combination with a colourless acid is generally attended by a deepening in colour; auxochromic combination, on the other hand, with a lessening.
Metals producing colourless salts with acids, is attended by colour changes contrary to those given above, auxochromic combination being accompanied by a deepening, and chromophoric by a lessening of the tint.
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