Sodium Carbonate Hand Wash
Definition
Mildly alkaline (about pH 11) wash solution collected in a sealed glass bottle from the suspect's hand or pocket. Alkalinity is needed because phenolphthalein only develops its pink colour above pH 8.2.
- Approximate pH
- About 11 (mildly alkaline)
- Container used
- Sealed glass bottle
- Reason for alkalinity
- Phenolphthalein needs pH above 8.2 to turn pink
- Sample source
- Suspect's hand or pocket
Common questions
Why is glass specified for collecting the hand wash rather than plastic?+
A sealed glass container avoids the risk of the residue chemically interacting with or adsorbing onto plastic surfaces, and glass better preserves the sample's chemical integrity until it reaches the laboratory for phenolphthalein or other trap-case testing.
What happens if the wash solution is not alkaline enough when tested?+
Phenolphthalein only develops its characteristic pink colour in solutions above roughly pH 8.2, so a wash solution that has become neutral or acidic before testing can give a false negative for a chemical trap residue even if the residue is genuinely present.
Is the sodium carbonate wash itself the test, or only the sample-collection step?+
It is the collection and preservation step. The alkaline wash extracts and stabilises any residue on the skin so that the subsequent phenolphthalein or other confirmatory chemical test can be carried out reliably in the laboratory.
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