United States Patent ( 19 ) Tzikas et al . ( ii ) Patent Number : 5 , 041 , 539 45 Date of Patent : ( 54 PREPARATION OF AOUEOUS

The invention relates to a process for the preparation of concentrated storage-stable aqueous solutions of yellow azoand azoxystilbene dyes which are obtained by self-condensation of 4-nitrotoluene-2-sulfonic acid in the presence of an alkali metal hydroxide, by converting the sparingly soluble alkali metal salt of the dye into a water-soluble ammonium salt by double cation ex change via a lipophilic ammonium salt, which process comprises carrying out the double ion exchange in a two-phase system of nitrobenzene/water and using, for the first ion exchange a dior trialkylamine containing a total of 12 to 40 carbon atoms and, for the second ion exchange, a mono-, dior trialkanolamine containing 2 to 4 carbon atoms in each alkyl moiety, The concentrated storage-stable dye solutions so ob tained are particularly suitable for dyeing paper.