Using the formaldehyde test kit FM-1 (catalog number 2183100), why does the sample turn blue when there is no formaldehyde in the sample?
Sample turns blue when there is no formaldehyde present
When sodium sulfite from the Formaldehyde 1 reagent is added to deionized or distilled water, the pH changes to approximately 9.8. Because the thymolphthalein indicator changes from clear to blue at a range of 9.3 to 10.5, the deionized or distilled water sample will turn blue even though no formaldehyde is present. Just one drop of sulfuric acid will turn this solution clear again. Although it may appear that the test indicates the presence of formaldehyde, the kit was not designed for levels below 0.05%, and cannot be used as a presence/absence test for formaldehyde.