HYDROGEN-ION CONCENTRATION 



33 



timate the H-ion concentration of the body fluids with very remarkable 

 accuracy, provided certain precautions are taken to circumvent the 

 disturbing influence which the protein and salts in these fluids may have 

 on the color change. 



To understand this use of indicators, it is important to bear in mind 

 that one solution reacting neutral to one indicator may have a H-ion 

 concentration which differs very markedly from that of another solu- 

 tion reacting neutral to another indicator. This is because indicators 

 react to different H-ion concentrations. A solution that is neutral to 

 phenolphthalein has a PH of about 9, whereas one neutral to methyl or- 

 ange has a P H of about 4. This can be very clearly shown by titrating 

 a solution of phosphoric acid with decinormal alkali. After a certain 

 amount of alkali has been added it will be noticed that methyl orange 

 changes from red to yellow, but after it has changed and is therefore 

 alkaline as judged by this indicator, it still remains distinctly acid to- 

 wards phenolphthalein (shows no red tint) even though considerably 

 more alkali is added. The methyl orange is, therefore, itself unrespon- 

 sive to weak acids such as remain after the greater part of the phos- 

 phoric acid has been neutralized by the alkali. 



The series of indicators which has been employed for this purpose is 

 given in the accompanying table, along with the P H limits through which 

 they change in color. 



LIST OF INDICATORS 



These dyes may now be obtained in this country. 



(W. M. Clark and H. A. Lnbs.) 9 



