16
O X TIlE METHODS
OF 1SI)IGO TESTIN(:.
that its density allows it to float on water ; and :3d, that the solution has a density slightlp different from t h t of water. I am riot prepared a t present to develop these ideas, whirli would requirc. further experiments to establish. I will confine myself to the well ascertained fact that ;L piece o r camphor wears away much fasier wlien iii motiori than when at rest. This being established, it appears more thaii probable that if a piece of camphor is perfectly free 1‘roiu oily matter it will dissolve i n water more readily than if protected hy a greasy film. The slightest film of this nature, iii contact with campilor, becomes :i saturated oily solution of camphor, and if m y exc what will cover the camphor, the qreasy film will Extend over t,he surl‘ace (If t l i r watcr. TVlien tliings are in this miidition, if an electritietl rod is ciil)petl several times i n the water, every immersion w i l l i*eiiiove x portioii of oily film froin the surface, until finally t h e film O I I each piece of‘ camphor becomes so tliiu that tlie water rc?nc.lies the. iiicccs of caniphor, and tliesc immediatc.ly bworce gifted with inotioii. OS THE NETHODS OF ISIjIGO Tli:S’I’ISG. BY 1 1 E s 1 t - r l r . RAC.i’ll. 11. I r appears s t r m p e , wlien the higli pi,icc of‘ iritligo and its large consumption i n the industrial arts :irv conaitlci.cd, that the methods commonly cinpIoyc(1 in this coiiritry f o r ils i.:i!untion should, from a scientific standpoirit, be so c . ~ ~ i d:ind e inaccurate. Taking t h e a ~ c w g epricc of tliv v:irious gr:i(les of indigo in the iiinrket as a h i s f o r vnlculatioii, it niay be stated tli:it a single per cent. of iiidigotiiie rcprcsentr, t o t I i v wnsuiii(’1’. from t w o to t,n-o and one-half cents for each pouiiil o f good.: ~iut~c*liascd. Under these c,ircunistances it n-oiihl seer11 highly i1c:siixble to ernploy t c s t s as acciirate as possible, evcw tliough tlitB>e should not hi. as time-saving as the grenter niinihhr of wninicLrcia1 tests. Nevertlielt~s\,it is :i fact, t h a t t h e ordinary iiidigo “:inalyscs” are YO wide of reliable results, that giitw work niiplit quite tis well br substituted for t h e m , and this in face of the I‘:ict tli:it ,wc have in the gmvimctric determination b y tlic reduction methods, a means for a closer valuation of indigo tlimi can b e applied in the case of almost any other dyestuff.
ON THE METHODS OF INDIGO TESTING.
17
T h e ordiitary mode of jiidging the quality of an indigo a n d by wbich it is commonly bought and sold, is quite supei-flcial ; and even in experienced hands allows but a rude estimate of its value. The buyer or broker takes a cake of indigo, breaks i t with a knife, and compares it with another of previously known worth as t o color, density, hardness, etc. Upon these points of comparisou he relies fcr a correct valuation. Where a formal analysis is required, the oxidation tests are usually employed ; and i t is to the inadequacy of theRe that I particularly desire to call attention. They are based upon the oxidation of the sulpho acid of indigotine in aqueous solotion b y a variety of agents, with the destrnction of the blue color. T h e different proceRses in use employ potassium permanganate, potassinm bichrdmatc, bleaching powder, or nitric acid, as the case may be. T h e saniple of indigo is dissolved in strong solphuvic acid and the solution brought to a certaiit voliime by the addition of water. A measured quantity i+then decol jrieed with an oxidizing solution which has been standardized upon piire indigotine or some indigo of known value. W h e n the blue color of the indigotilie solutioii has been destroyed and been replaced by R yellow or ochreous shade, t h e oxidatioxi is complete and t h e volume of the reagent solution employed, is noted. Now, if iiidigo contailled merely the coloring principle, indigotine, i i i mixture u ith inorganic mattem, this voluumetric estimation would be both practical and reliable, but as its various other ingredien t+ (which are entirely valueless) also enter into the reaction and consume a quantity of the reagent, the method gives these useless matterb the same valiie ab the iitdigotiiie. The figures vbtaiiicd aro, therefore, always too high, and that i n no uniform ratio, b u t varying with the anionnt and character of the accompanying impurities. 'l'he color which the Litter themselves impart to the solution d s o renders i t very difficult t o determine exactly when the requisite quantity of the r e a g e n t has been added. T h e reason why these ~nctliods,although known to be so faulty, have been retainctl in geiiernl practice, is to be foiirtd in the high figures obtained by them, which are t o the apparent advantage of the dealer. Their scieiitific appearance also is calculated to iittpress the unukilled. They are, in reality, anything but scientific. A good Bengal indigo which actually contained 8 2 . 5 $ indigotilie, showed
7 2 . 5 % when tested by the perniaiigatc piwess, R I
