INTERREACTIONS OF TOXIN AND ANTITOXIN 8l 



Now in general it is only free ions which enter into chemical 

 combination. For example, on adding HC1 to NaOH the posi- 

 tively charged H ion combines with the negatively charged OH 

 ion to form water, and the positively charged Na ion combines 

 with the negatively charged Cl ion to form NaCl. The two 

 substances HC1 and NaOH are strongly dissociated, and hence 

 the combination between the two is almost complete. This is an 

 example of the combination of a strong acid and a strong base ; 

 where it was expressed in older phraseology, the substances have 

 a strong affinity for one another. It is to this type that Ehrlich 

 conceives the union of toxin and antitoxin belongs. 



When two substances dissociate but slightly e.g., acetic acid 

 and alcohol or boracic acid and ammonia the reaction takes 

 place in obedience to different laws (the law of "mass action" 

 of Guldberg and Waage). When we ^dd alcohol and acetic 

 acid we get ethyl acetate (ester) and water ; but if we take 

 equivalent combining quantities of the two primary substances, 

 the reaction is not complete, as it is in the case of HC1 and 

 NaOH. On the contrary, the solution will still contain free 

 alcohol, free acetic acid, ester, and water. This is due to the 

 fact that the process is reversible. Acetic acid and alcohol 

 combine on the one hand to form ester and water, and ester 

 and water combine on the other, and dissociate into free acid and 

 free alcohol. 



In the first case, in the combination of a strong acid and a 

 strong base, the reaction is a simple one. If we take 100 com- 

 bining equivalents of NaOH, and add to it 10 combining equiva- 

 lents of NaOH, 10 parts of the alkali will be neutralized, and so 

 on, the whole of the alkali being neutralized when 100 combining 

 equivalents have been added. In the second case the reaction is 

 more complicated, and is expressed by the law that the products 

 of the concentrations of the substances on one side of the equa- 

 tion, divided by the product of those on the other, is a constant 

 (which varies in different reactions, and can be determined by 

 experiment). Thus in the case given : 



Concentration of acid x concentration of alcohol 



= constant. 

 Concentration of ester x concentration 01 water 



It follows that when we add a relatively small amount of acid 

 to a given volume of alcohol, it is practically all used up to form 

 ester, much free alcohol remaining ; but each succeeding addition 



6 



