112 



EFFECT OF CHEMICALS ON BACTERIA 



and thus prevents the poison from diffusing into the vital part. 

 The burning of sulphur in a dry atmosphere has little if any eft'ect 

 upon bacteria, but in the presence of moisture there is formed 

 sulphurous acid which is a rather efficient disinfectant. 



The germicidal property of salts of the heavy metals, acids, and 

 alkalies is governed in a large measure by the degree of ionization. 

 Mercuric chlorid in water is a good disinfectant, but in alcohol has 

 little or practically no germicidal properties. The addition of sodium 

 chlorid to mercuric chlorid increases the solubility of the latter and 

 yet decreases its germicidal power. This is due to the fact that there 

 is formed a double salt: 



2NaCl + HgCh NasHgCU 



+ + 



This is poorly dissociated by steps into Na + NaHgCU, Na + 



Hg + 4C1. The number of Hg ions formed is very small, 

 therefore, in the presence of sodium -chlorid. 



As a general rule the addition of a common negative ion decreases 

 the number of ions of the metal going into solution. If mercuric 

 chlorid is shaken with water, the salt dissolves until there is an 

 equilibrium between the solid phase and the undissociated molecules 

 in solution. As the molecules dissociate, the equilibrium is dis- 

 turbed and more of the solid dissolves to restore it, until a second 

 equilibrium is established between the ions and the molecules. 

 These equilibria may be expressed by the equation 



[Hg"] X 



[HgCh] 



= a constant; 



since the concentration of the undissociated molecules is constant. 

 So long as there is any undissolved salt, the equation becomes 



Hg X Cl = a constant. 



If we add NaCl, the Cl will increase the concentration of the Cl 

 ions which will combine with the Hg, giving HgClo, which will 

 crystallize out. Moreover, in this case the NaCl combines with the 

 HgCl2 giving the NaoHgCU which greatly decreases the Hg ions in 

 solution. The effect of this on the disinfecting power of different 

 dissociated salts of mercury on anthrax spores is indicated in the 

 following from Paul and Kronig: 



