ON SOME N.S.W. TAN-SUBSTANCES. 261 



B. One gramme potassium permanganate to one litre of water. 



C. One gramme tannic acid to one litre of water. 

 Standardisation of Indigo. 20 c.c. of Solution A (Indigo), are 



run from a burette of narrow calibre into three-quarters of a 

 litre of distilled water (or the purest water obtainable), contained 

 in a Berlin dish of about a litre capacity, that is to say, with a 

 diameter of about 8|- inches. The contents of the dish are 

 rendered uniform by gentle agitation with a porcelain spoon,* 

 which is far preferable to the glass-rod usually recommended. 

 The solution of permanganate (B) is then run in, and care should 

 be taken to always perform the process in precisely the same 

 manner, as uniformity is of vital importance. The method I 

 have found to give the most uniform results is to run in 10 c.c. 

 of the permanganate (the liquid in the dish is agitated with the 

 spoon after every addition for five or six seconds) to begin with, 

 then 1 c.c. at a time until the liquid becomes of a yellowish-green. 

 The permanganate has now to be added with the utmost caution, 

 two or three drops at a time as the liquid becomes of a more 

 decided yellow colour. Then one drop is added at a time, with 

 much stirring, until the indigo is perfectly oxidised, when the 

 liquid assumes a pure yellow colour. It is of the highest 

 importance to note this end-reaction with precision, for unless 

 the exact point be noted important errors will creep into the 

 analysis. Fortunately Kathreiner has pointed out that pure 

 yellow liquids under the above circumstances show a pinkish 

 tinge on the circumference of the liquid. With a little practice 

 this pink colour may be detected immediately it appears. Over- 

 head light, and not too much of it, is best. 20 c.c. of indigo 

 should require about 17 c.c. of the permanganate. I may mention, 

 as an instance of the uniformity of results obtained by a uniform 

 method of working, that working with two separately made 

 batches of indigo solution, and two separately made batches of 

 permanganate solution, the whole of nine determinations made 

 gave 16.8 or 16.9 c.c. in each case. 



Method of Making the Decoction. 

 Five grammes (usually) of finely divided bark, &c, are placed 

 in a glass vessel (preferably a 40oz. beaker or flask), about half a 

 litre of boiling waterf added, and the vessel placed on the water- 



* Since the above was written I have seen a paper by Mr. Procter, in 

 the Journ. Soc. Chem. Incl., 1886, in which he discards the glass-stirrer, 

 on account of the uncertainty of the results obtained by its use, and 

 substitutes a " disc-stirrer." 



f The advantage of using boiling water is that complete exhaustion of 

 the material takes place. Its disadvantages are that it decomposes the 

 tannic acid slightly, though at the same time it decomposes a little of 

 the phlobaphene which is then returned as tannic acid. It is my 

 intention to deal separately with this subject. 



