Chromotropic Acid Test
Definition
Confirmatory colour test for formaldehyde (and hence indirectly for methanol). Chromotropic acid (4,5-dihydroxy-2,7-naphthalenedisulphonic acid) plus concentrated sulphuric acid plus formaldehyde gives an intense violet colour.
- Target analyte
- Formaldehyde, the oxidation product of methanol
- Reagent
- Chromotropic acid plus concentrated sulphuric acid
- Positive colour
- Intense violet/purple
- Use case
- Screening spirits for methanol adulteration
Common questions
Why test for formaldehyde instead of methanol directly?+
The sample is first oxidised, converting any methanol present to formaldehyde, because formaldehyde reacts far more selectively and visibly with chromotropic acid than methanol does on its own, giving a cleaner colour endpoint.
Can ethanol interfere with the result?+
Ethanol oxidises to acetaldehyde rather than formaldehyde under the same conditions, so a properly controlled test distinguishes genuine spirit from methanol-contaminated liquor, though poor oxidation control can still blur the distinction.
Is a positive colour test enough for a court report?+
No, a colour test is used for rapid field or preliminary screening; a confirmed methanol finding for evidentiary purposes is backed by a quantitative method such as gas chromatography.
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