90 THE SIMPLER NATURAL BASES 



A. Iodine or iodic acid. The excess of iodine may be removed by 

 shaking with ether and the sensitiveness is then according to Schur 

 [1909] i : 1,500,000. Abelous, Souli6 and Toujan [1905] removed 

 the excess of iodine by means of sodium thiosulphate, but according 

 to Bayer [1909] the reaction, when carried out in this way, is not very 

 delicate and the red colour is not permanent. 



Another modification of the iodine reaction was suggested by L. 

 Krauss[i9O9] who used iodic acid. Subsequently Frankel and Allers 

 [1909], independently of Krauss, employed an equal volume of O'ooi 

 N-potassium bi-iodate and added a few drops of phosphoric acid ; by 

 heating the mixture nearly to the boiling point, the reaction is said to 

 be obtainable at a dilution of I : 300,000. Hale and Seidell [1911] 

 recommend this test, but do not add phosphoric acid. Frankel and 

 Allers consider their test to be quite distinct from that of Vulpian ; 

 they state that at no stage of the reaction is iodine set free, but both 

 Krauss and Ewins [1910] deny this. Bayer [1909] claims to have 

 greatly increased the sensitiveness of the Frankel-Allers reaction 

 by adding sulphanilic acid, which, however, changes the red colora- 

 tion to an orange or yellow one, which is less specific ; Bayer gives 

 I : 5,000,000 as the limiting dilution. 



B. Another oxidising agent, which colours adrenaline solutions 

 red, is mercuric chloride, recommended by Comessatti [1909]. Boas 

 [1909] and Frankel and Allers [1909] could not obtain the reac- 

 tion at all readily, but Ewins [1910] has pointed out that Comessatti 

 used solutions of mercuric chloride in tap water, and that the calcium 

 bicarbonate present in the latter acts as a catalyst ; it may be replaced 

 by solutions of other salts of weak acids. This observation is of con- 

 siderable interest in connection with the discovery of Euler and Bolin 

 that the oxidase from Medicago consists of calcium salts of organic 

 hydro xy-acids. It was moreover already noticed by Vulpian, that the 

 spontaneous coloration of the adrenal chromogen by exposure to air 

 takes place slowly in distilled water, but much more rapidly in tap 

 water. 



Ewins suggests the following conditions for carrying out Comes- 

 satti' s reaction. To I c.c. of adrenaline (i : 100,000) an equal volume 

 of a I per cent, sodium acetate solution is added and then four to five 

 drops of a OT per cent, solution of mercuric chloride in distilled water. 

 A pale rose tint is produced at room temperature in 4 to 5 minutes. 

 Here the sodium acetate solution replaces tap water, in order to secure 

 uniformity. 



C. The most sensitive oxidising agent is probably a persul- 



