INDICATORS 523 



which will not interfere with it in any shape or, form but which will 

 tell when the acid has been completely neutralized, or, rather, when 

 a very small amount of the alkaline solution has been added in 

 excess, is called an indicator. In general, therefore, an indicator is a 

 substance which by a change of color or a precipitate formed or in 

 some other visible manner will indicate the end point of a reaction. 

 It is always best to use indicators with daylight illumination, because 

 artificial light frequently makes the color reaction less characteristic, 

 and, therefore, confusing. 



The following are the formulae for some of the most commonly 

 employed indicators used in the titration of acids and alkalies : 



Dimethylamidoazobenzol. This is a coal-tar derivative (anilin stain) 

 and is used in the proportion of 0.05 gr. in 100 c.c. of 95 per cent, 

 alcohol. It is yellow in neutral and alkaline solutions and red in acid 

 solutions. It is particularly useful in the titration of strong mineral 

 acids, and is generally used in the determination of hydrochloric acid 

 (in gastric juice, natural or artificial, in investigating the effect of 

 gastric juice upon pathogenic bacteria). A few drops of the indicator 

 are added to 10 c.c. of the gastric juice. The fluid in the presence 

 of HC1 assumes a red color, y^ sol, NaHO is then added from a 

 burette and each c.c. of the normal solution used in neutralization 

 is equal to 0.00365 gram of HC1. 



Cochineal. This substance is prepared from the cochineal louse 

 (Coccus cacti cochinelifera), living on certain species of cactus. It is 

 the substance from which the carmine used for staining tissues is 

 also derived. Three grams of cochineal are extracted in the cold with 

 250 c.c. of 25 per cent, alcohol. This cochineal tincture assumes a 

 violet color in the presence of alkalies and a yellow-red color in acid 

 solutions. It is, like diamethylamidoazobenzol, used in the titration 

 of strong mineral acids. 



Litmus. This substance is of vegetable origin and derived from 

 several species of lichens. It is sold in commerce in the form of small 

 cubes or larger cakes. In order to prepare a good indicator used for 

 general laboratory purposes, or for the preparation of litmus-lactose 

 or litmus-glucose agar and gelatin, it is necessary to boil the cubes 

 with three or four changes of 95 per cent, alcohol. This extracts a 

 dirty violet substance from the commercial article. After purification 

 with alcohol the cubes are soaked in water until the latter assumes 

 a dark blue color. The fluid is then drawn off and dilute sulphuric 

 acid is added to it until a deep violet color is produced. In order to 

 secure the proper color it is necessary to take a few c.c., dilute 

 strongly with distilled water, and examine in a test-tube. When the 

 dilute fluid has a reddish-violet tint, almost a cherry red, a sufficient 

 quantity of sulphuric acid has been added to the original watery 

 extract. Litmus tincture or paper is stained blue in alkaline, red in acid 

 solutions. A tincture stained slightly blue in an alkali on standing 

 frequently turns red, and it therefore must be made blue again before 



