(1) Red and blue Litmus paper. - This is used as an indicator for all weak and strong bases and acids. It is turned red by acids and blue by alkalis.
Preparation: Best quality litmus should be used. The cubes, containing 50-90 % of calcium sulphate, are ground up and extracted once each with benzene and with alcohol. 4 or 5 Gms. of the substance are then dissolved in a litre of water, and pure filter-paper is soaked in the solution.
To dry the paper it is hung up on threads, and the sheets are then cut up into strips. For red litmus paper a few drops of acetic acid are added to the solution, whilst ammonia is used for the blue paper. The less pronounced the coloration of the paper the more sensitive it is.
(2) Congo Paper. - Used for strong acids. It is rendered a pure blue by mineral acids and violet by strong organic acids.
Preparation: 0.5 gms. concentrated Congo Red are dissolved in a litre of water and 5 drops acetic acid are added. Filter-paper is soaked in the warm solution and allowed to dry in a clean place.
(3) Thiazole Paper (Mimosa Paper). - Used for free alkali. It is coloured a pure red by alkalis, and is much preferable to Turmeric. It is prepared in the same way as Congo paper, except that the acetic acid is omitted. Ammonia is without influence upon this paper even in high concentrations.
(4) Phenolphthalein Paper. - It is turned red by alkalis. It reacts with ammonia and with sodium carbonate, but not with bicarbonates. It may be used with advantage for the more accurate types of analysis.
Preparation: 1 gm. Phenolphthalein is dissolved in 1 litre of hot water, and filter-paper is soaked in the hot solution.
(5) Starch Iodide Paper (Nitrite Paper). - Used for nitrous acid and for hypochlorites. It becomes bluish-violet with a trace of oxidizing agent, and deep brown with excess. Care must be taken that the paper is merely touched with the drop of solution and that the glass rod is not scraped across it.
Preparation: 10 gms. of pure starch are ground up with a little water and the paste is then poured into a litre of boiling water with good stirring. After cooling, 2 gms. potassium iodide are added, and pure filter-paper is soaked in it and allowed to dry in a clean atmosphere. This paper will indicate clearly in a 1 % hydrochloric acid the addition of a single drop of normal nitrite per litre; it is thus extremely sensitive.
(6) Lead Paper. - Used for detecting hydrogen sulphide. Preparation: Filter-paper is soaked in a solution of 5 gms. lead nitrate per litre, and is then dried in an atmosphere free from sulphuretted hydrogen. Instead of this paper one may use paper soaked in a solution either of ferrous sulphate or of lead acetate.