The Kastle-Meyer test
is a forensic presumptive blood test, where the chemical indicator phenolphthalein is used.
A dry sample is collected with a swab or filter paper. First a few drops of ethyl alcohol, then a few drops of phenolphthalein and finally a few drops of hydrogen peroxide are dripped onto the sample. If the sample turns pink then it is a positive test. This test is nondestructive to the sample, which can be kept and used in further tests at the lab. This test has the same reaction with human blood and animal blood so further testing would be required to determine which one it is.
In the reaction, only phenolphthalein and hydrogen peroxide react with the haemoglobin in the blood; the alcohol behaves as a solvent. As a peroxidase, haemoglobin catalyses reactions of the following type:
- ROOR' + electron donor (2 e-) + 2H+ → ROH + R'OH
(where R and R' are any functional chain). In the Kastle-Meyer test, it is phenolphthalein that acts as the donor of the two hydrogen atoms; phenolphthalein is colourless, but the phenolphthalein ion formed by the removal of these two hydrogen atoms is pink, which is what effects the colour change in this test. The reaction takes place in this way:
- Hb (catalyst) + H2O2 + C20H14O4 → Hb (catalyst) + 2H2O + C20H12O42-