[VcL 9 



424 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



sistant and grow normally in a suspension of flowers of sulphur 

 at temperatures below 25 ° C. Because of these facts it is logical 

 to assume that the toxic property of sulphur is due to a compound 

 formed by the oxidation of sulphur. Having eliminated the more 

 common oxides and acids of sulphur it was thought that this 

 toxic compound might be one or a mixture of the more complex 

 polythionic acids. At any rate hydrophilic colloidal sulphur con- 

 tains such an acid. The chemistry of hydrophilic colloidal sul- 

 phur has been studied by a number of investigators. Barj r ('20) 

 studied Raffo's soluble sulphur (here termed hydrophilic colloidal 

 sulphur), and came to the conclusion that the substance con- 

 tained not only sulphur but polythionates. He thought the solu- 

 tion was made stable by the presence of small amounts of electro- 

 lytes. Freundlich and Scholz ('22) made a very extensive study 

 of the so-called soluble sulphur and concluded that it was large- 

 ly pentathionic acid. They base their conclusion on the following 



which would take place if nentathionic acid 



sent. 



2NaOH + H 2 Sr,0 8 = Na 2 S 3 6 + S, + 2H 2 

 2 Na,S.O„ + GNaOH = Na 2 S 2 3 -f 4 Na 2 S0 3 + 3 H 2 + 4S 

 or 2Na,S.O, + 6NaOH = 5Na 2 S 2 3 + 3 H 2 0. 



By the aid of this reaction they were able to determine qualita- 

 tively and quantitatively the pentathionic acid. The qualitative 

 test was made by the addition of an alkali which precipitated out 

 the sulphur in the form of a white turbid solution. They state 

 that this test applies only to pentathionic acid, and to no other 

 sulphur compound containing oxygen, such as any of the well- 

 known acids or oxides. The quantitative test is made by treating 

 the colloidal solution with normal NH 4 OH, forming amnonium 

 thiosulphate and titrating this with 0.01N iodine solution. With 

 hydrophobic colloidal sulphur these tests were negative. They 

 designate colloidal sulphur in this form as SA and the form as- 

 sociated with pentathionic acid as S p. When S m is precipitated 

 out of hydrophilic colloidal sulphur it probably becomes S A. 

 Such a change also takes place when pentathionic acid is created 

 with H 2 S. 



According to these workers, sodium thiosulphate and sulphuric 

 acid react as follows: 



Na 2 S 2 3 + H 2 S0 4 = H 2 S 2 3 + Na 2 S0 4 

 H 2 S 2 3 = S0 2 + 3S + H 2 



