Indicators and Test Papers
Indicator and test papers are strips of paper of suitable dimension and grade (see Filter Paper, Quantitative, in the section Reagents) impregnated with an indicator or a reagent that is sufficiently stable to provide a convenient form of the impregnated substance. Some test papers may be obtained from commercial sources of laboratory supplies. Those required in Pharmacopeial tests and assays may be prepared as directed in the following paragraphs, by means of the solutions specified, or to meet the tests set forth herein under the individual titles.
Treat strong, white filter paper with hydrochloric acid, and wash with water until the last washing no longer shows an acid reaction to methyl red. Then treat with ammonia TS, and wash again with water until the last washing is not alkaline to phenolphthalein.
After thorough drying, saturate the paper with the proper strength of indicator solutions, and carefully dry in still air, unless otherwise specified, by suspending it from rods of glass or other inert material in a space free from acid, alkali, and other fumes.
Cut the paper into strips of convenient size, and store the papers in well-closed containers, protected from light and moisture.
Cupric Sulfate Test Paper —Use cupric sulfate TS.
Lead Acetate Test Paper —Usually about 6 × 80 mm in size. Use lead acetate TS, and dry the paper at 100, avoiding contact with metal.
Litmus Paper, Blue —Usually about 6 × 50 mm in size. It meets the requirements of the following tests.
Phosphate (Reagent test)—Cut 5 strips into small pieces, mix with 500 mg of magnesium nitrate in a porcelain crucible, and ignite. To the residue add 5 mL of nitric acid, and evaporate to dryness: the residue shows not more than 0.02 mg of PO4.
Residue on ignition—Ignite carefully 10 strips of the paper to constant weight: the weight of the residue corresponds to not more than 0.4 mg per strip of about 3 square cm.
Rosin acids—Immerse a strip of the blue paper in a solution of 100 mg of silver nitrate in 50 mL of water: the color of the paper does not change in 30 seconds.
Sensitiveness—Drop a 10- to 12-mm strip into 100 mL of 0.0005 N acid contained in a beaker, and stir continuously: the color of the paper is changed within 45 seconds. The 0.0005 N acid is prepared by diluting 1 mL of 0.1 N hydrochloric acid with freshly boiled and cooled purified water to 200 mL.
Litmus Paper, Red —Usually about 6 × 50 mm in size. Red litmus paper meets the requirements of the tests for Phosphate, Residue on ignition, and Rosin acids, under Litmus Paper, Blue.
Sensitiveness—Drop a 10- to 12-mm strip into 100 mL of 0.0005 N sodium hydroxide contained in a beaker, and stir continuously: the color of the paper changes within 30 seconds. The 0.0005 N sodium hydroxide is prepared by diluting 1 mL of 0.1 N sodium hydroxide with freshly boiled and cooled purified water to 200 mL.
Mercuric Bromide Test Paper —Place a 50 mg/mL solution of mercuric bromide in dehydrated alcohol in a dish, and immerse in it pieces of white filter paper weighing 80 g/m2 (speed of filtration = filtration time expressed in s for 100 mL of water at 20 with a filter surface of 10 cm2 and a constant pressure of 6.7 kPa; 40–60 s), each measuring 1.5 cm by 20 cm and folded in the middle. Allow the excess of liquid to drain, and allow the paper to dry, protected from light, suspended over a nonmetallic thread. Discard 1 cm from each end of each strip, and cut the remainder into 1.5-cm squares or discs of 1.5-cm diameter. Store in a glass-stoppered container wrapped with black paper.
Methyl Green–Iodomercurate Paper —Immerse thin strips of suitable filter paper in a 40 g per L solution of methyl green, and allow to air-dry. Immerse the strips for 1 hour in a solution containing 140 g per L of potassium iodide and 200 g per L of mercuric iodide. Wash with water until the washings are practically colorless, and allow to air-dry. Store protected from light, and use within 48 hours.
Methyl Yellow Paper —Use a 1 in 2000 solution of methyl yellow in alcohol.
pH Indicator Paper, Short-Range —Use a suitable grade.
Phenolphthalein Paper —Use a 1 in 1000 solution of phenolphthalein in diluted alcohol.
Starch Iodate Paper —Use a mixture of equal volumes of starch TS and potassium iodate solution (1 in 20).
Starch Iodide Paper —Use a solution of 500 mg of potassium iodide in 100 mL of freshly prepared starch TS.
Thiazole Yellow Paper— Use a 1 in 2000 solution of thiazole yellow in water.
Turmeric Paper —Use a solution prepared as follows: Macerate 20 g of powdered turmeric, the dried root of Curcuma longa Linné (Fam. Zingiberaceae), with four 100-mL portions of cold water, decanting the clear liquid portion each time and discarding it. Dry the residue at a temperature not over 100. Macerate with 100 mL of alcohol for several days, and filter.
Sensitiveness—Dip a strip of the paper, of about 1.5-cm length, in a solution of 1.0 mg of boric acid in 5 mL of water, previously mixed with 1 mL of hydrochloric acid. After 1 minute remove the paper from the liquid, and allow it to dry: the yellow color changes to brown. Then moisten the paper with ammonia TS: the color of the paper changes to greenish black.