the present invention is concerned with the modification of the sodium salt of a known optical brightener to obtain a white crystalline material . the commercially available 4 , 4 &# 39 ;- bis -- stilbene - 2 , 2 &# 39 ;- disulphonic acid is neutralized with a sodium base such as caustic and exposed to temperatures above about 65 ° c . as an aqueous slurry containing some undissolved disodium salt . the water solubility of this salt may be inhibited by the addition of an electrolyte and the product may be recovered by spray drying .