15 января, 2016 
Pokraskin 1. Litmus Paper. This is an indicator for all acids and bases, weak or strong. Acids turn it red, bases blue. In preparing litmus. paper, only the best litmus is used. The cubes, which contain from 50 to 90 per cent of gypsum, are broken up and extracted once with alcohol. The dye is then dissolved in water, 4 to 5 grams per liter, and pure filter paper is soaked in the solution. The sheet is suspended on a cord to dry and is then cut into thin strips. For red litmus paper, a few drops of acetic acid is added to the solution, and for blue litmus paper, a few drops of ammonia. The less pronounced the color of the paper, the more sensitive it is.
2. Congo Red Paper. This is a reagent for strong acids. It is turned to a pure blue by mineral acids and to a violet color by strong organic acids. The paper is prepared by soaking filter paper in a warm solution of 0.5 gram of Congo red in 1 liter of water containing 5 drops of acetic acid, and drying in a clean place.
3. Thiazole Paper (Mimosa Paper). This indicator is used to test for free alkali. It is colored a pure red by alkalis and is far better for this purpose than Curcuma. The paper is prepared as described for Congo red paper, except that the acetic acid is omitted. Ammonia has no effect on this paper unless it is in very concentrated solution.
4. Phenolphthalein Paper. This paper is turned red by alkalis, including ammonia and sodium carbonate solutions, but not bicarbonates. It can be used to advantage for differentiating among the alkalis. The paper is prepared by soaking filter paper in a hot solution of 1 gram of phenolphthalein in 1 liter of water.
5. Starch-Iodide Paper (Nitrite Paper), This is used to test for it blue violet, larger amounts turn it brown. This paper should be spotted and not streaked. Pure starch (10 grams) is rubbed with a small amount of water, and 1 liter of boiling water is added with thorough stirring. After the solution has cooled, 2 grams of potassium iodide is added and sheets of clean filter paper are soaked in it and allowed to dry in a clean place. This paper is very sensitive, giving a definite reaction nitrous and hypochlorous acids. Small amounts of oxidizing agents turn
with a 1 per cent hydrochloric acid solution containing one drop of 1 N nitrite solution per liter.
6. Lead Paper. This reagent for hydrogen sulfide, is prepared by soaking filter paper in a solution containing 5 grams of lead nitrate per liter, and drying in air free from H2S. Instead of this paper, one can use paper moistened with ferrous sulfate or lead acetate solution.
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