Public Lab Research note

False positives for Formaldehyde testing

by mathew | July 03, 2015 23:47 03 Jul 23:47 | #12031 | #12031

What I want to do

Understand why used Kitagawa tubes are turning pink in our fridge. We've been storing them in there for a while, and they've been stable-- i.e. if I put a tube in there it has the same reading the next day. Then, starting over the past two weeks they've been getting pinker (exposed) in the fridge.

My attempt and results

Of recent additions to the fridge, the biggest and funkiest is definitely this half-gallon of kimchi I put there. I really like kimchi. You can definitely smell it when you open the fridge.


Kimchi is actively decomposing, My suspicion is that these aromatic decomposition gasses are affecting the tubes. Kitagawa lists these false-positives and cofounding gasses:


Nitrogen dioxide is an intermediate component in anaerobic metabolism, which is likely to be occurring in the kimchi. Acetaldehyde may also be generated by partial metabolization of alcohol, although the lacto-fermentation of kimchi shouldn't generate that, I don't think. Acetaldehyde is also carcenogenic, but at much higher concentrations than formaldehyde.

Questions and next steps

What else do you think it is?

Why I'm interested

confounding factors to formaldehyde testing are important to recognize and potentially account for with other tests.


Even though you have it in the refrigerator I suspect that it is the result of time and perhaps humidity. I have left my used tube lying around in the living room. The indication color is all the way to the end of the measurement area. Any chemist want to comment on the reaction? HCHO+(NH2OH)3.H3PO4->H3PO4+HCN=NOH+H2O

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Time and humidity can be controlled for.

Take 4 to 8 tubes. Do whatever it is you do before putting them in the fridge, but put them in: fridge no tupperware, in tupperware in fridge, in a cabinet no tupperware, in tupperware in a cabinet. 8 tubes would be for testing pairs in the same location which gives you repetitions for more significant results.

If you check all 4-8 the next day and they all turn pink, it is likely (but not necessarily) a matter of time. Non-ziplocs only? Probably humidity or some air contaminant.

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