Reagents:
(1) phenolphthalein working solution (1 part stock solution with 4 parts ethanol)
(2) 3% hydrogen peroxide in a dark bottle
Phenolphthalein stock solution:
(1) 2 grams reagent phenolphthalein and 20 grams potassium hydroxide are mixed in 100 mL of distilled water.
(2) The solution is refluxed with 20 grams of powdered zinc for 2 hours, until the solution is clear (phenolphthalein in a reduced state).
(3) The stock solution is placed in a dark bottle with a small amount of zinc powder in the bottom. The solution is stored in the refrigerator until use.
Technique:
(1) Take a sample of the material to be examined (liquid solution, cloth, etc).
(2) Add 2 drops of the phenolphthalein working solution to the sample and observe for a color change.
(3) If a color change does not occur within a few seconds, place 2 drops of the hydrogen peroxide in the same area and observe for a color change.
(4) A maximum of 20 seconds should be used for the observing of a color change.
Interpretation:
• A pink color will rapidly appear in the presence of blood.
• The test is positive for human or animal blood.
• The test is nondestructive to the blood.
Performance:
• The test is positive with dilute blood solutions. Cox found that the test could detect blood diluted 1 in 10,000. Others report sensitivity to lower dilutions, but these may reflect testing of solutions under ideal conditions. The color will take longer to appear if the blood is more dilute.
• The test is relatively specific and is not affected by plant peroxidases. It may give weak positive reactions with fresh green bean, potato or horseradish.
• A color change occurring after 20 seconds is probably a false positive reaction.
