Indicator (Litmus Paper) and Test Papers

Indicator and Test Papers

Indicator and test papers are strips of filter paper of suitable dimension and grade impregnated with an indicator or a reagent that is sufficiently stable to provide a convenient form of the impregnated substance. Commercial samples of indicator and test papers are available and may be used. Those required in the assays and tests of the Pharmacopoeia may be prepared as described in the following paragraphs.

Treat strong, white filter paper with hydrochloric acid and wash with water until the last washing does not show an acidic reaction to methyl red. Then treat with dilute ammonia solution and wash again with water until the last washing is not alkaline to phenolphthalein. Dry the paper thoroughly and saturate it with the proper strength of the indicator solution or reagent solution and dry carefully in still air by suspending it from glass rods in a space free from acid and other fumes.

Cut the paper into strips of suitable size and store in well closed, light-resistant containers, protected from moisture.
Lead Acetate Paper: Prepare from lead acetate solution and dry the impregnated paper at 100°, avoiding contact with metal.
Litmus Paper: Use red litmus paper or blue litmus paper, as appropriate.
Litmus Paper, Blue: Boil 10 parts of coarsely powdered litmus under a reflux condenser for 1 hour with 100 parts of ethanol (95 percent), decant the ethanol and discard. To the residue add a mixture of 45 parts of ethanol (95 percent) and 55 parts of water. After 2 days, decant the clear liquid. Impregnated strips of filter paper with the extract and allow to dry. Complies with the following test.
SENSITIVITY: Immerse a strip, 60 mm x 10 mm, in 100 ml of 0.002 M hydrochloric acid. On shaking, the paper turns red within 45 seconds.
Litmus Paper, Red: To the extract obtained in the preparation of blue litmus paper add 2 M hydrochloric acid dropwise until the blue solution turns red. Impregnated strips of filter paper with the solution and allow to dry. Complies with the following test.
SENSITIVITY: Immerse a strip, 60 mm x 10 mm in 100 ml of 0.002 M sodium hydroxide. On shaking, the paper turns blue within 45 seconds.
Mercuric Chloride Paper: Smooth white filter paper, not less than 25 mm in width, soaked in a saturated solution of mercuric chloride, pressed to remove the superfluous solution and dried at about 60° in the dark. The grade of filter paper is such that the weight is between 65 and 120 g per sq.m; the thickness in mm of 400 papers is approximately equal, numerically, to the weight in g per sq.m.
Store in a stoppered bottle in the dark. The paper which has been exposed to sunlight or to the vapor of ammonia affords a lighter stain or no stain at all when employed in the limit test for arsenic.

Starch Iodate Paper; Starch-iodate Paper: Immerse strips of filter paper in 100 ml of the iodide-free starch solution containing 0.1 g of potassium iodate. Drain and allow drying protected from light.
Starch Iodide Paper; Starch-iodide Paper: Prepare from a mixture of equal volumes of starch solution and 5 percent w/v solution of potassium iodide in water and dry the impregnated paper protected from light. Complies with the following test.
SENSITIVITY: Mix 0.05 ml of 0.1 M sodium nitrite with 4 ml of hydrochloric acid and dilute with water to 100 ml. Deposit one drop of the resulting solution on the paper; a blue spot appears.
Titan Yellow Paper: Impregnate filter paper with titan yellow solution and dry at room temperature.

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