Paper, Indigo. From a solution of indigo. Decoloured by chlorine.
Paper, Iodine of Potassium. a. From the solution in distilled water. Turned blue by an acidulated solution of starch.
b. From a mixture of a solution of iodide of potassium and starch paste. Turned blue by chlorine, ozone, and the mineral acids, and by air containing them.
Paper, Lead. From a solution of either acetate or diacetate of lead. Sulphuretted hydrogen and hydrosulphuret of ammonia turn it black.
Paper, Litmus. In general, this is prepared from infusion of litmus, without any precaution, but the following plan may be adopted when a superior test paper is desired;—
a. (Blue.) Triturate commercial litmus, 1 oz., in a wedgwood-ware mortar, with boiling water, 3 or 4 fl. oz.; put the mixture into a flask, and add more boiling water until the liquid measures fully 1⁄2 pint; agitate the mixture frequently until it is cold, then filter it, and divide the filtrate into two equal portions; stir one of these with a glass rod previously dipped into very dilute sulphuric acid, and repeat the
operation until the litmus infusion begins to look very slightly red, then add the other half of the filtrate, and the two being mixed together, dip strips of unsized paper into the liquid, in the usual manner, and dry them. Acids turn it red; alkalies blue. The neutral salts of most of the heavy metals also redden this, as well as the other blue test papers that are affected by acids.
b. (Red.) The treatment of the whole quantity of the infusion (see above) with the rod dipped in dilute sulphuric acid is repeated until the fluid begins to look distinctly red, when the paper is dipped into it as before. The alkalies and alkaline earths, and their sulphides, restore its blue colour; the alkaline carbonates and the soluble borates also possess the same property. Very sensitive. An extemporaneous red litmus paper may be prepared by holding a strip of the blue variety over a pot or jar into which 2 or 3 drops of hydrochloric acid have been thrown.
Paper, Mallow. From an infusion of the purple flowers of the common mallow. Affected like ‘dahlia paper.’
Paper, Manganese. From a solution of sulphate of manganese. Ozonised air blackens it.