6. Pipette 4-mL aliquots from each sample solution into 25-mL glass-stoppered flasks.
NOTE: Adjust aliquot size to contain between 2 and 20 μg formaldehyde for optimum
absorbance. The calibration graph between becomes non-linear above an absorbance
of ca. 1.0. A.U.
7. Add 0.1 mL 1% chromotropic acid to the flask and mix.
NOTE: This amount of chromotropic acid can react with ca. 40 μg of formaldehyde. Due to this
fact, the range of the calibration curve should not exceed 36 μg (90% of theoretical).
8. Add 6 mL conc. H 2SO4 slowly to the flask. Replace the stopper gently. Gently swirl the solution
to mix.
CAUTION: Mixing of the sample solution with concentrated sulfuric acid is highly exothermic.
9. Heat the solution to 95 °C for 15 min. Allow the solution to cool at room temperature for 2 to 3
hours.
NOTE: Use caution due to the corrosive nature of hot sulfuric acid and the possible pressure
buildup within the flask.