26 PHYSIOLOGICAL CHEMISTRY 



take place at these concentrations, but the pH figures given are 

 approximately correct for HC1 and NaOH and illustrate the 

 point without undue complexity). 



A H-ion concentration of 0.035 per liter may be expressed 

 as 3.5 X 10- 3 , etc. 



From a biochemical viewpoint the hydrogen-ion concentra- 

 tion is of the very greatest importance. On it depend the action 

 of enzymes, the behavior of colloidal solutions (see later dis- 

 cussion) the proper functioning of cells and tissues and other 

 important processes. The determination of the hydrogen-ion 

 concentration can be made accurately by electrochemical meth- 

 ods, but these are too difficult for the average physician. It is 

 possible to make approximate determinations by the use of sub- 

 stances called indicators. These are compounds of various 

 types which have one color in acid solution, another in alkaline. 

 Their use in titration already should be familiar to the student. 

 Now it happens that the indicators differ in the point of acidity 

 (pH) at which they change color. Some change at the neutral 

 point (pH == 7) others at a pH of 4, 5, 6, 8, 9, etc. The reason 

 for this is as follows. The common indicators are themselves 

 weak acids. As the free acid they have one color, as their salts 

 another color. This is believed to be due to an internal rear- 

 rangement of the molecule when in the salt form. Some indi- 

 cators are stronger acids than others. If an alkali is added to 

 a mixture of two acids, one strong and the other weak, the 

 strong acid gets the base and forms its salt. If more alkali is 

 added, finally a point is reached at which the weaker acid begins 

 to get some of the base and form its differently colored salt, and 

 a color change occurs. The weaker the acid (the indicator) the 

 more alkali will have to be added before it can get any of the 

 base. Two examples will illustrate this, methyl orange and 

 phenolphthalein. Of these, the former is the stronger a,cid. 

 When, by the addition of alkali the pH of an acid solution has 

 been brought to 3 or 4, the methyl orange will get some of the 

 base and change color. The weaker acid, phenolphthalein, will 



