Neutral couplers

Sulfonated amines are dissolved as their alkali salts in water, and acetate solution, equivalent to the free mineral acid in the diazo solution plus that formed in the coupling reaction, is added. The diazo solution is added to the coupler solution with stirring. If necessary, the mixture is partially or completely neutralized with soda to accelerate the coupling. 

For coupling in acid solution, the coupler is dissolved in water as its alkali salt, and hydrochloric acid is added until the mixture is just acid to Congo red. The resulting suspension is added to the diazo solution and the mixture is stirred in the cold until dye formation is completed. If the reaction is very slow, it can be accelerated by neutralizing the excess mineral acid with sodium acetate. The reaction mixture must always be kept weakly acid to Congo red, however. 

Amines which are diflBcultly soluble or insoluble in water, but soluble in dilute mineral acid (most of the unsulfonated monoamines), are dissolved in dilute hydrochloric or sulfuric acid and mixed with the diazo solution as described above. If coupling proceeds slowly under these conditions, the nuxture can be neutralized with sodium acetate or soda, but must be kept fairly acid to Congo red in order to keep the coupler in solution. Since acid is set free in the coupling reaction ... 

The addition of alcohol or some other water miscible solvent for the coupler is recommended when very sluggish coupling reactions are involved. It is then possible to have the solution neutral or acid (acetic) from the start. If the coupler tends to form a diazoamino compound (e.g., m-toluidine), the coupling reaction is always carried out in as strongly acid solution as possible. 

Uses Hydrotrope, catalyst coupler and solubilizer for liq. detergents anticaking aid in dry neutralization catalyst in org. reactions (e.g, esterification)... 

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