41 () ANALYTICAL CHEMISTRY. 



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, phenolphthalein, 

 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. (See colored Plate VII.) 



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. 



Phenolphthalein solution. 1 gramme of phenolphthalein is dissolved in 50 c.c. 

 of alcohol and water added to make 100 c.c. 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 indi- 

 cator for ammonia. Carbonates must be titrated in boiling solution to drive 

 off carbon dioxide. 



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

 thin, tropseolin D, or Poirier's orange 3 P), the sodium or ammonium salt of 

 dimethylamido-azobenzol-sulphonic acid (CH 3 ) 2 N.C 6 H 4 .N.NC 6 H 4 .SO 3 H, is dis- 

 solved in 1000 c.c. of water. To the solution is carefully added, with constant 

 stirring, jj sulphuric acid, in drops, until the liquid turns red and just ceases 

 to be transparent; it is then filtered. 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. It should be used in 

 cold solutions. 



Hcematoxylin solution. 0.2 gramme of hsematoxylin ( ( a vegetable coloring- 

 matter derived from hsematoxylon) is dissolved in 100 c.c. of alcohol. The alka- 

 line solution has a purple color which is changed to yellow or orange by acids. 



Rosolic acid solution. 1 gramme of commercial rosolic acid (chiefly C 20 H ]6 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. 



