Nash Reagent Method
Definition
The EU reference colorimetric method for free formaldehyde in cosmetics: formaldehyde reacts with ammonium acetate and acetylacetone to produce the yellow 3,5-diacetyl-1,4-dihydrolutidine chromophore, measured at 412 nm. Sensitivity approximately 1 mg/kg (1 ppm) free formaldehyde.
- Type
- Colorimetric method
- Reference status
- EU reference method for free formaldehyde in cosmetics
- Reagents
- Ammonium acetate and acetylacetone
- Detection wavelength
- 412 nm
- Sensitivity
- About 1 mg/kg (1 ppm) free formaldehyde
Common questions
Why does the Nash method target 'free' formaldehyde specifically rather than total formaldehyde?+
Cosmetic preservatives such as certain formaldehyde-releasing agents keep formaldehyde bound until it slowly releases during product use, and it is this released, unbound form that poses the sensitisation risk regulators are concerned with, so the method is designed to quantify only the free fraction rather than formaldehyde still bound in a releaser compound.
What does the yellow chromophore in this reaction actually indicate?+
The formation of 3,5-diacetyl-1,4-dihydrolutidine, a yellow compound produced when formaldehyde reacts with the ammonium acetate and acetylacetone reagents, has an absorbance that is proportional to the formaldehyde concentration present, which is what the 412 nm measurement quantifies.
Can the Nash method be fooled by other aldehydes in a cosmetic formulation?+
Some other aldehydes can potentially cross-react in colorimetric formaldehyde assays, so laboratories confirming borderline or regulatory-critical results often follow up with a more specific chromatographic method to rule out interference before reporting a final free-formaldehyde value.
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