Effects of the working electrolyte (cyclodextrin type and pH) on the separation of aromatic sulphonic acids by capillary zone electrophoresis

The effects of the composition of the working electrolyte on the capillary electrophoretic separation of various naphthalenesulphonic acids used as intermediates in production of synthetic dyes were investigated. Berate buffers should be preferred to phosphate buffers and the working electrolytes should contain cyclodextrins for a successful separation. alpha-, beta-, and gamma-cyclodextrin were compared as the additives to the working electrolyte. Best separation of strong unsubstituted isomeric naphthalene sulphonic acids can be achieved in electrolytes containing a mixture of beta- and gamma-cyclodextrin. beta-cyclodextrin additive is also necessary for separation of the aminonaphthalenesulphonic acids present in technological samples of 6-amino-1-hydroxynaphtalene-3-sulphonic acid and 1-amino-8-hydroxynaphtalene-3,6-disulphonic acid (I and H acids). The pH of the berate buffer used as the working electrolyte affects the ionisation of these weak acids which is enhanced at higher pH. Optimum separation is obtained in berate buffers with beta-cyclodextrin at pH=8.5 or 9. To speed up the elution, elevated pressure can be applied across the capillary. Under these conditions, quantitative analysis of technological samples can be obtained in 15-25 min.