GASTRIC JUICE AND CHANGES IN STOMACH. 163 



more than traces, as suggested by the qualitative tests above, 

 the process becomes more difficult. Before describing the 

 details of a method something must be said about the indica- 

 tors themselves, as an understanding of their nature and be- 

 havior is necessary for much that is to follow. 



THEORY OF INDICATORS. 



The indicators employed in acidimetry and alkalinity are all weak acids 

 or weak bases themselves, and in general much weaker than the acids or 

 bases in the determination of which they are employed. These indicators, 

 as acids or bases, form salts with the bases or acids to be titrated; it is 

 on the peculiar properties of these salts that the value of the indicators 

 depends. As is well known the change in " reaction " in employing an 

 indicator is accompanied by a change in color. This change in color is 

 accounted for in two general ways. In terms of the dissociation hypothe- 

 sis the indicators themselves, being very weak acids or bases, are but 

 slightly dissociated into ions. The weak acid indicators in neutral or acid 

 solution exhibit the color of the undissociated substance. Phenol-phthalein 

 as acid or undissociated body is colorless. Methyl orange in neutral or 

 alkaline solution is still a basic body and is yellow. These indicators rep- 

 resent two distinct types; the first is an extremely weak acid in normal 

 condition, while the latter or its related body dimethylaminoazobenzene is 

 a very weak base. All the other common indicators may be ranged be- 

 tween these two in properties and behavior. Now if we add an alkali to 

 phenol-phthalein a salt is formed. For example, with sodium hydroxide 

 the sodium salt of the coloring matter is produced and such salts, even 

 with the weakest acids, are dissociated into metallic ion and colored acid 

 ion. The red color of the alkali solution of phenol-phthalein is then due 

 to the liberated red ion. The undissociated colorless molecule may be 

 represented by the formula OCOCeHtCCCeHiOHyz, while the acid radical 

 or red ion may be given the formula OCOGH^CCeH^OEOCelLO. With 

 relatively strong alkalies this ion is always set free. This is not the case 

 with weak bases or very weak alkalies, because very weak bases and very 

 weak acids do not form stable salts with each other in aqueous solution. 

 Salts formed in this way suffer a decomposition by water which is called 

 hydrolysis, as illustrated by this equation : 



BA + H 2 O = BOH + HA. 



B represents a weak basic radical and A a weak acid radical. With the 

 weak base we obtain the undissociated HA (weak acid) instead of the 

 ion A. Phenol-phthalein is not at all an indicator for weak bases and a 

 very poor one for weak alkalies like NH 4 OH. A considerable excess of 

 such bodies must be used to overcome the hydrolyzing power of the water 

 solvent. 

 The case is different in testing for or measuring weak acids by aid of 



