Catalysts of Organic and Inorganic Origin. 



309 



but in the presence of hydrogen peroxide gives a reddish-brown at once. 

 With the watery solution of the tartrate the colour sequence is also given. 

 The presence of free nitric, sulphuric, hydrochloric, citric, or tartaric acids 

 prevents or delays the production of an oxidation colour sequence with 

 ferric- chloride, but this takes place readily in the presence of 10 per cent, 

 oxalic acid. With soluble copper salts (sulphate, acetate, chloride), alcoholic 

 paraphenylenediamine darkens directly without showing any colour sequence, 

 but if the solutions are dilute, and salt and hydrogen peroxide are present, a 

 partial colour sequence from brown to violet or purple is shown. 



With the watery solution of ursol tartrate no colour change is given with 

 copper acetate, sulphate, or chloride in the presence or absence of sodium 

 chloride. With the sulphate and acetate an apparent colour sequence of 

 green to brown is given on the addition of hydrogen peroxide, but this is 

 partly due to the fact that the peroxide gives a greenish colour with copper, 

 and ursol tartrate slowly browns in the presence of hydrogen peroxide. 

 With copper chloride, however, a violet or purple tinge ultimately appears, 

 and a full colour sequence (green, brown, violet, or purple) is given with 

 copper sulphate and copper acetate in the presence of salt and hydrogen 

 peroxide. Jf, however, the solutions are very dilute the colour change is 

 slow, and is direct to brown. 



Gruss* suggests that the direct oxidation to brown is due to molecular 

 oxygen, and that the colour sequence is the result of the action of atomic 

 oxygen. The data given above, however, yield no support to this view. 

 The colours produced seem to depend to some extent upon the relative 

 degrees of dilution and intensity of action. Colour may be intramolecular 

 or extramolecular, i.e. due to the absorption or modification of light rays at 

 the surfaces of molecules or of molecular aggregates. In the latter case if 

 the peculiar aggregation is broken up when the material is in solution the 

 colour may disappear or be modified. It is quite possible that the colour 

 sequence with paraphenylenediamine is the result of temporary molecular 

 aggregations during the process of oxidation which react differently to light 

 rays, and whose production depends more upon relative mass action than 

 upon any other factor, this determining molecular aggregations -of material in 

 various stages of oxidation. 



Ursol Tartrate Test for Lignin. 



This delicate and striking reaction is best shown with boiled or dead 



tissues by placing them in the watery solution. It is a reaction comparable 



with the phloroglucin test, and is shown in the absence of free oxygen, acid, 



* Loc. cit., p. 11. 



