256 ANALYTICAL CHEMISTRY. 



Indicators. In all cases of volumetric determination it is of the 

 greatest importance to observe accurately the completion of the reac- 

 tion. In some cases the final point is indicated by a change in color, 

 as, for instance, in the case of potassium permanganate, which changes 

 from a red to a colorless solution, or chromic acid, which changes 

 from orange to green under the influence of deoxidizing agents. In 

 other cases the determination is indicated by the formation or cessa- 

 tion of a precipitate, and in yet others the final point could not be 

 noticed with precision unless rendered visible by a third substance 

 added for that purpose. 



Such substances are termed indicators. Litmus, phenol-phtalein, 

 methyl-orange, etc., are used as indicators in acidimetry and alka- 

 limetry. Starch paste is an indicator for iodine, potassium chromate 

 for silver, etc. Of indicators, a few drops are in most cases sufficient 

 for the purpose. 



Litmus solution. This is made by exhausting coarsely powdered litmus with 

 boiling alcohol, which removes a red coloring matter, erythrolitmin. The 

 residue is treated with about an equal weight of cold water, so as to dissolve 

 the excess of alkali present in litmus. The remaining mass is extracted with 

 about five times its weight of boiling water, and filtered. The solution should 

 be kept in wide-mouthed bottles, stoppered with loose plugs of cotton to ex- 

 clude dust but to admit air. Blue and red litmus paper is made by impregnat- 

 ing strips of unsized white paper with the blue solution obtained by the above 

 process, or with this solution after just enough hydrochloric acid has been 

 added to impart to it a distinct red tint. 



Phenol-phtalein solution. 1 gramme of phenol-phtalein is dissolved in 100 

 c. of diluted alcohol. The colorless solution is colored deep purplish-red by 

 alkali hydrates or carbonates, but not by bicarbonates ; acids render the red 

 solution colorless. The solution is not suitable as an indicator for ammonia 

 or bicarbonates. 



Methyl-orange solution. 1 gramme of methyl-orange (also known as helian- 

 thin, tropa3olin D, or Poirier's orange III), dimethylamido-azobenzol-sul- 

 phonic acid, (CH 3 ) 2 N.C 6 H 4 .N.NC 6 H 4 .SO 3 H, is dissolved in 1000 c.c. of water. 

 The solution is yellow when in contact with alkaline hydrates, carbonates, or 

 bicarbonates. Carbonic acid does not affect it, but mineral acids change its 

 color to crimson. 



Rosolic acid solution. 1 gramme of commercial rosolic acid (chiefly C 20 H 16 O 3 ) 

 is dissolved in 10 c.c of alcohol, and water added to make 100 c.c. The solu- 

 tion turns violet-red with alkalies, yellow with acids. 



Other indicators used at times in acidimetry are solutions of brazil-wood, 

 cochineal, corallin, eosin, fluorescein, turmeric, etc. 



Titration. This term is used for the process of adding the volu- 

 metric solution from the burette to the solution of the weighed sub- 

 stance until the reaction is completed. We also speak of the standard 



