Gurr's Dye Classes

Edward Gurr proposed a classification system based on auxochromes. He considered auxochromes to be of two types and coined the term colligator to name them. The two types are colligators and non-colligators. Colligators functioned as ionising groups, as we now use the term auxochrome. Non-colligators functioned by altering the colour of the dye, but did not have ionising capability. He defined the new term this way:–

"A colligator is a special kind of auxochrome. It can be either basic or acidic in nature. It can be positively or negatively charged. In non-ionic dyes the colligators are uncharged. The function of a charged colligator is to convert a chromogen into a dye-ion and to enable the latter to interact with substances of opposite charge to itself. .....  Auxochromes can now be divided into two classes, colligators and non-colligators. The main function of the non-colligator auxochromes appears to be that of colour modifiers."

 

In Gurr's classification system the acid colligators are:–
SO₃^-      SO₃H      SO₃Na      OH      ONa      OK      COOH      COONa

The basic colligators are:–
N⁺      NH₂      NH

His system is shown graphically below. Note that Group 1, non-ionic dyes, are the ones with uncharged colligators. Type 2 (cationic) and type 3 (anionic) dyes are charged.

 

Reference

Edward Gurr, (1971)
Synthetic dyes in biology, medicine and chemistry
Academic Press, London, England.