ISO ROYAL SOCIETY OF CANADA 



We have C, x C4 ^ , , 

 -~ — = Constant 



Cj = naught. Cg = naught. 



Thus both have changed to the same extent. 

 Conclusion. C^ is unchanged. 



As we might expect, the number of citrate ions is unchanged. 



And C3XC, ^ ^^^^^^^^ 



C3 is naught C4 is unchanged. 



Conclusion. C5 must have changed to the same extent as C3 

 therefore C5 = zero, which gives us a result that all the iron citrate 

 has disappeared after neutralisation. We might have expected this, 

 because iron citrate is completely dissociated, and iron hydrate being 

 insoluble falls out of solution. 



In determining the acidity of our iron citrate solution we used 

 as much alkali as was necessary to change all the iron into iron hydrate. 

 One gram of iron citrate scales (Merck) has an acidity of + 5.6, but 

 one gram of pure iron citrate, molecular weight 285, a tribasic salt, 

 should have an acidity of + 12.2, consequently our own iron citrate 

 scales contained 5.6 = 45.7 per cent iron citrate. 

 Ï2T2 



To recapitulate — 



a. The formation of the black salt is dependent on the iron ions. 



b. The number of iron ions is equal to the number of hydrogen 

 ions. 



c. The number of hydrogen ions determines the acidity. 



From these three points we may draw the conclusion that irre- 

 spective of the percentage of iron citrate, the intensity of the black- 

 fields in aesculin media stands in direct relation to the acidity, pro- 

 vided that at least -|- 0.6 of the acidity proceeds from the added iron 

 citrate. 



To prove the above wo made the following media and tested 

 them with the colon bacillus. 



Distilled water, 1.5% agar, 0.25% bile salt, 1% peptone, neutra- 

 lised and filtered and added 0.1% aescuUn, final reaction made neutral. 



