O S A YEW I’IZOCESS FOIt Y.’lIb; b~X’l’lLlCTIOKOF 1NJ)IGO‘I’IN FROM COMMKItC I B L INJIIGO. __-
13Y TlIOi. 31. X ~ I K ~ A ~ .
h few years ago I hac1 occnsion to use indigotill a s pure as 1’0ssible arid in considerable quaiitities, for iiidustrial purposes, ant1 endeavored to obtain it froni ootnnicrci:tl indigo by some of tlie well known methods. l3F:ventuallyI adopted the following process, which is easy of execution aiitl gi yes :L satisfactor? yield. Commercial iiidigo, finely groiiiid :mcl intimate11 niixed with about a n eqtial weight of zinc (lust is spread i n layers, about a n incli deep, 011 thin boards, ant1 introduced into ;L ste;im cliest SO constructed t h a t it c : ~ n1)c closcd a i r tiglit, and provided with ;t stccrm pipe f o r tlic introtluction of steam i t n d one for its exit : also with an aperture f o tlic ~ introduction of a solution of su1phui.ous acid. T h e indigo mixture is :~lsoscreeiieil. to prevent colidetisccl steam dropping upoii it. \Vlicii these :irr:ingements :ti’(’ complete tlie air is driveli o u t by a ixpid cnrretit of sie:tni : tlieii tlie steam valve is i i w r l y closctl, ii little stearn bcing :tllowc~l t o pass tlirougli the spparatus, so a s to 1)reveiit tlic ciitraiicc of air. Xcxt the solution of sulplinrous acid is introtiuced, a little at a time, so as to saturate tlic! stcani it) t h e cliest with sulpliur dioxidr, :ind it is maintsinctl in this c011d ition whilri t h e retluctioii of tlie indigo is going on. The reaction hctwcclr t h e ziiic, sulpliiir clioxiclc :nid indigotin goes 011 mpidly, arid i i i ;tbout one or two hours t h e indigotin should be coinplctely reduced. I t then presents ;t dirty yellow o r greenislr yellow color, and when i t dries, which it quickly does on taking from tlie steam cliest, is not liable to rapid oxidation, b u t may be kept for several days without mucli change. F r o m tlie mixture the indigo white may be rccwvered in a n extraction a p p ratus by means of wood spirit, or by digestion in flasks filled to the neck with thc same solvent. On exposure of t h e solution in shallow vessels to the air tlie indigotin is precipitatecl pure arid crystalline. T h e yield is large, and probably it theoretical one when all necessary precautions are employed. :302
