TOXICITY OF ACIDS TO CILIATE INFUSORIA 463 



fixed amount of HCl is added; this depresses the dissociation of 

 the weaker acid and produces a solution containing chiefly the 

 organic acid molecule, H+ and Cl~. If the anion of the organic 

 acid is toxic, this mixture should be less toxic than the original 

 solution, provided the H ion concentration is not greatly altered. 

 Klocman found that the acetic anion was toxic, but did not 

 try the method with other acids. In the present experiments 

 the method was somewhat modified and was applied to a num- 

 ber of organic acids of the same concentration (0.0002 N). The 

 Ph of each solution was first determined by the use of indicators, 

 then enough HCl was added to increase the Ph slightly, by a 

 known amount when the toxicity of each of these mixtures had 

 been determined HCl was again added, increasing the Ph decid- 

 edly. With each addition the ionization of the organic acid was 

 depressed further, until finally most of it was in molecular form 

 and the H ions were derived chiefly from the HCl. By deter- 

 mining the toxicity of the acids alone and again after each addi- 

 tion of HCl it was possible to follow the effect of the decreasing 

 dissociation of the acids and at every stage to compare them 

 with HCl of the same H ion concentration. The figures obtained 

 in this experiment are recorded in table 4 and expressed graphi- 

 cally in graph 4. 



It will be seen at once that some of the acids become less 

 toxic as the H ion concentration is increased, formic lactic and 

 acetic, for example, in the experiments with Paramoecium. 

 This means that as the anions decrease in number the acid be- 

 comes less harmful. In other cases, addition of HCl increases 

 the toxicity, but only until it is equal to that of HCl of the 

 same Ph. Thus phthalic by itself is more toxic than HCl, 

 but as ionization is depressed approaches it more closely; that 

 is, in graphic form, the curves are at first distinct, but as Ph 

 is increased through the addition of HCl the phthalic approaches 

 the HCl curve and finally coincides with it. The same is true 

 of oxalic, benzoic, citric, acetic, propionic, butyric, valeric. In 

 these cases, too, the anion must be toxic, since depression of 

 ionization does away with the greater toxicity of the organic 

 acid as compared with HCl of the same Ph. Since these acids 



