Combine 23 g of diphenylamine, 9.5 g of sulfur and 3.6 g of anhydrous ammonium chloride. Heat the product at 200°C hydrogen sulfide rapidly starts to form stop heating when no hydrogen sulfide comes off. Pulverize the mixture and dissolve it in hot ethanol, filter and collect the liquid, evaporate and recrystallize multiple times in ethanol. Less of diphenylamine is used than the stoichiometric amount needed because removing diphenylamine from the phenothiazine is very hard.
(C6H5)2NH + 2 S = C12H9NS + H2S