Indicators.—The tincture of litmus used to show when the solution is exactly neutral is called an indicator, and many materials are used in a similar way in different analytical processes. Thus the indigo solution in Löwenthal's process is an indicator. A more useful indicator than litmus for tannery purposes is Dr. Lunge's "methyl orange," which is indifferent to carbonic acid, and may therefore be used in the cold with solutions of alkaline carbonates; which are much more easily made and preserved than those of the caustic alkalies necessary with litmus. It is very sensitive to mineral acids, but not equally so to organic. It may be obtained of Messrs. Mawson and Swan, of Newcastle; and as a minute quantity only must be used for each test, it is really cheaper than litmus, and a few grm. will last a lifetime. It must be dissolved in water, and not more than 2 or 3 drops taken for each titration. (Titration signifies an estimation by means of a standard solution.) Other indicators will be named in connection with the analytical methods in which they are used.
Fig. 9.
Instruments.—To practically carry out analysis by standard solutions, measuring glasses are required. One or more flasks marked in the neck to hold exact quantities ([Fig. 9]), one at least, holding 1 litre, are indispensable. One or two graduated cylinders ([Fig. 10]), holding 100 c.c., and divided into tenths of c.c., are very useful, and it is well also to have one holding a litre, and provided with a stopper ([Fig. 11]). This is called a "test mixer," but is not absolutely essential.
Fig. 10.
Fig. 11.
Fig. 12.