6 GENERAL METHODS 



by noting the point at which the rider must be placed to make 

 the empty pans balance. 



The point of balance in weighing is always determined by 

 adjusting the weights until the pointer swings equal distances to 

 right and left of zero on the scale. It is never determined by adjust- 

 ing the loads until the pointer remains stationary when released. 



INDICATORS : 



In order to determine the amount of acid present in an unknown 

 solution an alkaline solution of known strength is required; and 

 conversely, in the analysis of a base an acid solution of known 

 strength is required. In both cases the " end-point " of the reac- 

 tion is determined with the aid of a suitable indicator. 



The accuracy of the results depends largely upon the choice 

 of the indicator. All indicators employed for the titration of acids 

 and alkalies are either acids or bases. They behave like weak 

 acids and bases whose ions and unionized molecules have different 

 colors. The color change is due probably to a rearrangement of 

 the molecule to a colored form. This rearrangement is probably 

 due to the dissociation of the molecule, the undissociated molecule 

 not rearranging. Since the indicators are acids of different avidi- 

 ties, that is, since they have different amounts of dissociation, 

 some are weaker than others. Accordingly some are able to form 

 salts in sufficient amounts to give a perceptible color in the presence 

 of more acid than are others which are weaker. 



A neutral solution is one in which there are equal numbers of 

 hydrogen and hydroxyl ions. An acid solution has a preponder- 

 ance of hydrogen ion and an alkaline solution an excess of hydroxyl 

 ion. All indicators do not show changes in color at the true neutral 

 point, but at some fixed figure of acidity or alkalinity, that is at a 

 definite hydrogen or hydroxyl ion concentration. Indicators which 

 change color at the true neutral point are litmus and rosolic acid, 

 while phenolphthalein changes color in a slightly alkaline solution. 

 Congo red, sodium alizarin sulphonate and tropaeolin 00 are 

 examples of indicators which change color in an acid solution. 



Organic acids in general are not sufficiently strong, i.e., do not 

 dissociate with the production of enough hydrogen ion, to cause 



1 Modified from Hawk: Practical Physiological Chemistry, Sixth Edition, 

 1918. 



