Washburn, E.W., Principles of Physical Chemistry (McGraw-Hill Book Co.), (Second Edition, 1921), pp. 380-387.

Prideaux, E.B.R., The Theory and Use of Indicators (Constable & Co., Ltd.), (1917).

Salm, E., A Study of Indicators, !Z. physik.  Chem.!, 57 (1906), 471-501.

Stieglitz, J., Theories of Indicators, !J.  Am.  Chem.  Soc.!, 25 (1903), 1112-1127.

Noyes, A.A., Quantitative Applications of the Theory of Indicators to Volumetric Analysis, !J.  Am.  Chem.  Soc.!, 32 (1911), 815-861.

Bjerrum, N., General Discussion, !Z.  Anal.  Chem.!, 66 (1917), 13-28 and 81-95.

Ostwald, W., Colloid Chemistry of Indicators, !Z.  Chem.  Ind.  Kolloide!, 10 (1912), 132-146.

[Note 1:  Glaser, !Indikatoren der Acidimetrie und Alkalimetrie!.  Wiesbaden, 1901.]

PREPARATION OF INDICATOR SOLUTIONS

A !methyl orange solution! for use as an indicator is commonly made by dissolving 0.05-0.1 gram of the compound (also known as Orange III) in a few cubic centimeters of alcohol and diluting with water to 100 cc.  A good grade of material should be secured.  It can be successfully used for the titration of hydrochloric, nitric, sulphuric, phosphoric, and sulphurous acids, and is particularly useful in the determination of bases, such as sodium, potassium, barium, calcium, and ammonium hydroxides, and even many of the weak organic bases.  It can also be used for the determination, by titration with a standard solution of a strong acid, of the salts of very weak acids, such as carbonates, sulphides, arsenites, borates, and silicates, because the weak acids which are liberated do not affect the indicator, and the reddening of the solution does not take place until an excess of the strong acid is added.  It should be used in cold, not too dilute, solutions.  Its sensitiveness is lessened in the presence of considerable quantities of the salts of the alkalies.

A !phenolphthalein solution! is prepared by dissolving 1 gram of the pure compound in 100 cc. of 95 per cent alcohol.  This indicator is particularly valuable in the determination of weak acids, especially organic acids.  It cannot be used with weak bases, even ammonia.  It is affected by carbonic acid, which must, therefore, be removed by boiling when other acids are to be measured.  It can be used in hot solutions.  Some care is necessary to keep the volume of the solutions to be titrated approximately uniform in standardization and in analysis, and this volume should not in general exceed 125-150 cc. for the best results, since the compounds formed by the indicator undergo changes in very dilute solution which lessen its sensitiveness.