TOXICITY OF ACIDS TO CILIATE INFUSORIA 



457 



and hydroxy acids > fatt}^ acids except formic. This is roughly 

 the same as the order of dissociation and indicates that the H 

 ion is important. But if the H ion is the sole factor in deter- 

 mining toxicity, the order of toxicity and the order of dissocia- 

 tion should agree perfectly, which, as the following table shows, 

 is not the case. 



Toxicity and dissociation 



Still clearer evidence should be given by solutions of the vari- 

 ous acids at the same H ion concentration, for if the H ion is the 

 only toxic factor, they must all be equally toxic. Solutions were 

 therefore made (without buffers) of all the acids in the series at 

 four of the following concentrations, viz., Ph 3.5, 3.85, 4.0, 4.15, 

 4.75. The concentrations were determined by means of indica- 

 tors tetrabromphenolsulphonphthalein and methyl red (Clark 

 and Lubs, '17) in the usual way by comparison with acetic acid 

 Na acetate standards. The method is not perfectly exact, but 

 upon determining the Ph of a series of equinormal acids (0.0005, 

 0.0002, 0.0001 N) both colorimetrically and by calculation from 

 the dissociation constants the indicators proved to be accurate to 

 within about 0.1 Ph. 



The results obtained by this method are indicated in table 3 

 and graph 4. It will be seen at a glance that the toxicity of 

 different acids of the same Ph is by no means identical. This 



