262 ON SOME N.S.W. TAN-SUBSTANCES. 



bath for two or three hours, brought up to a brisk boil over a 

 Bunsen burner, decanted on to a filter, fresh boiling water added 

 and the process repeated until nearly a litre of water has passed 

 through the filter. The nitrate is then made up to one litre. 

 Uniformity in exhaustion of materials is most desirable, and 

 some of the methods given in books lack thoroughness. The 

 method of preparing the material for decoction has been given in 

 former papers. 



The colours of extracts (decoctions) have been determined from 

 a litre contained in a 40oz. beaker. 



The Process proper. 



20 c.c. of indigo solution (A) are run into the Berlin dish 

 containing J litre of water, and 10 c.c. of decoction (see p. 261), 

 are then run in and carefully stirred. [It is worthy of note that 

 partly because the decoctions are frequently frothy, and because 

 a comparatively small quantity is used, it is very desirable to use 

 a delicate burette of fine calibre.] 



Potassium permanganate (B) is then run in with the usual 

 precautions to prevent overshooting of the end-reaction. The 

 quantity required is marked, say P. (See Schedule.) A second 

 determination, under precisely similar conditions, is marked Q. 

 The difference between P and Q should not be more than .1 c.c. 

 P and Q are added together, and the mean taken, which we 

 may call P. B therefore represents the quantity of standard 

 permanganate required to oxidise 10 c.c. of decoction + 20 c.c. 

 of indigo. 



But we already know the quantity of permanganate requisite 

 to oxidise the indigo. Call that M. P - M = N = ' quantity of 

 permanganate requisite to oxidise 10 c.c. of decoction alone. 



We now come to an important point which may be made 

 ewparentliese. When P is more than twice M, that is to say 

 when the quantity of permanganate required to oxidise 10 c.c. of 

 decoction + 20 c.c. of indigo, is more than double the quantity 

 required to oxidise 20 c.c. of indigo alone, it has been found by 

 experience, that the strength of the decoction has to be reduced 

 until P is less than twice M.* This of course can readily be 

 determined by a preliminary experiment, which is readily 

 accomplished in practice. 



* Davis (The Manufacture of Leather, p. 157), following Kathreiner, 

 Dingl. Polyt. Journ., ccxxviii., 54, states, " The only thing of importance 

 is that the quantity of indigo used in titration requires at least 1.5 times 

 the quantity of potassium permanganate of that which is necessary for 

 the oxidation of the oxidizable substances to be determined." See also 

 Blyth, Foods, 333, and others who copy this statement. But I find as the 

 result of experiment (at least with Australian barks), that if the process 

 be conducted with the precautions I have given, it is only necessary that 

 E should be less than twice M. 



