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METHOD WOK THE ASSAY OF INUItiV.
METHOD FOR THE ASSAY O F INDIGO ‘1 ENSAT\ I L1.E. The determinatiori of i n d i p Lluc, u r indigotin, in indipo, presents various difficultieb. ‘Yhe p r u c e ~ e i ,i i ) . itoe art‘ long, and bubject to considerable error. Th:. irietlioda w-ii1t.h tiepctici upon t h r reductioil and subsequent measured (,\idation of indigo. require the d i m ination, previously, of :ill othc r rc>iiic2it)le Irodie-. ti) iiisiire accuracy-an operation h t h l o i y arid ttldious. f h e ruethod ~y formation of eulpindigotiue and it. cistimation by :I standardized permanganate solution, always give.:: too high rrsulta by reason of the preseiice uf other oxidisable bociic.. . For seveial years the zuthor i i a h U b d j, method by mblimation, which has been uriiforiiilj- iatisiacruq. Indigo blue sublimes readily and, bj- a cn\ily if the bottolrl of the tray 10 quite flat, with no rounding towards the sides. Sublime on an iron plate, ;it f i ~ b raising t the heat gradually to avoid burrirng. When t h e surmce of’ t h r indigo I > covered over with a shining layer of cryrtals, turn down up311 the plate :t piece of Russia iron beni into the form of a Hat arcbh, t h e highest point of which is about 1 em. abovt ( h e p l a ~ e ,a r i d ‘i little loriqer than the tray. Lowwr the tieat at the stme time tlirlt ri-w m . t i i \ put 011, ab the temperature ri8es rapidlj The purple vapors of inctigotin are iiow given off, :iportion eondensmg u p i n the under sides of the arch. Raise tlie heat slowly, and enough t o maintain a constant sublimation ot indigotin. By raising the arch the progress ot the work is seen. For a 50%indigo the tinit! required is 30-40 minutes ; but boft, Java indigo must be bublimed with more caution, and hornetirnm require 137 C H d R L l i h
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METEOD FOX TEE ASSAY OF INDIGO.
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two hours. The last crystals of indigotin are easily seen upon the the dark colored surface of the residue, When all have diesppeared, remove the tray, cool in a dessicator, and weigh. The loss i n weight isindigotk. Observe that the heat be no greater than is required to sublime the indigo blue ; and that no yellowish vapors appear, which would indicate the destruction of the residue, leaving only ash. If the bottom of the tray is 0at and everywhere touches the plate, the sublimation goes on regularly, except in case of very rich indigos, already mentioned, when care must be exercised t o prevenk burning. Results by this method are constant within # of 1 per cent., but the author has frequently made re-determinations with variations of only half that error. A little practice enables one to leave the Sublimation with only occasional attention, and three or four determinations may be carried on at once under the same arched cover, For commercial and industrial purposes this method appears to have decided advantages. Its rapidity is in great contrast to the other methods which admit of perhap8 two determinations in a day, while in point of accuracy it is not wanting. Laboratory 4.5 Kilby st., Boston, 5th May, 1884.
