Instructions Using the Chromotropic Acid Assay Method

Formaldehyde Sampler Analysis Using
the Chromotropic Acid Assay Method
Analysis Instructions
863 Valley View Road, Eighty Four, PA 15330 USA
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This method is a variation of the original NIOSH Method 3500 for analyzing passive samplers for formaldehyde (SKC Catalog Nos. 526-100, 526-200, and 526-
201). Samples should be analyzed within 30 days of exposure.
MATERIALS NEEDED:
Spectrophotometer (Capable of holding 13 x 100 mm glass tubes and reading absorbance at 580 nm.)
Centrifuge (Capable of 1500 G to spin 13 x 100 mm tubes)
Hot Water Bath (Capable of 95 degrees C)
Oven (Capable of drying glassware at 110 degrees C)
Vortex Mixer
Pipets (Various 0.5 - 10.0 ml)
Volumetric Flasks (Various)
Tube Racks
Test Tubes
(centrifuge tubes) (13 x 100 mm glass - Fisher)
Polyethylene Film
PE Caps for Tubes (KimKaps)
CHEMICALS NEEDED:
Sulfuric Acid, Concentrated (Mallinkrodt AR)
Sodium Bisulfi te, Granular (Baker Analyzed)
Chromotropic Acid, Disodium Salt, Dihydrate (Baker Analyzed)
Formaldehyde Standard, 4000 mg/L (Hach Voluette)
Formaldehyde-Free Deionized Water
Hot Tap Water (55 degrees C minimum)
PROCEDURE:
It is imperative to avoid sources of formaldehyde, general aldehyde, alcohol and organic contaminations and that the method follows all good analytical lab prac-
tices. Although the chromotropic acid procedure is generally free from many interferences, strict attention to detail is mandatory if good results are to be obtained.
Analyze samples within 30 days of exposure.
ANALYTICAL CURVE:
Preclean all glassware to be used in the analysis by rinsing 2x with hot tap water followed by 2x with DI water. Dry the glassware in the oven for
1 hour and keep covered with plastic until ready to use. The glassware is good for one day.
Make 100.0 ml of a 2.5% (w/v) stock sodium bisulfi te solution with DI water. Make fresh daily.
Make a 400.0 μg-ml stock solution of formaldehyde solution using the Hach standard with DI water. This solution is good for one month at
4 degrees C.
Make 10.0 ml of an 8.8% (w/v) solution of chromotropic acid solution with DI water and store protected from the light. Make fresh daily.
Make a series of working formaldehyde standards as follows. Take 10 ml of the 400 μg/ml stock solution and dilute to 100 ml in DI water. Use this
standard to make calibration stndards according to the following table:
Flask size* 2.5% Bisulfi te stock soln. 40 μg/ml formaldehyde std Formaldehyde (μg/ml)
50 ml 5 ml 0 Blank
200 ml 20 ml 0.5 ml 0.1 μg
100 ml 10 ml 0.5 ml 0.2 μg
100 ml 10 ml 1.0 ml 0.4 μg
50 ml 5 ml 1.0 ml 0.8 μg
50 ml 5 ml 2.5 ml 2.0 μg
50 ml 5 ml 4.0 ml 3.2 μg
50 ml 5 ml 5.0 ml 4.0 μg
* make up to the mark with DI water
In duplicate, pipet 2.00 ml of each blank/standard into a 13 x 100 mm test tube (centrifuge tube).
Add 0.10 ml of chromotropic acid to each tube.
Add 3.00 ml of sulfuric acid slowly and with care, using appropriate protective measures, to each tube, cap and vortex for 5 seconds.
Heat these solutions in the water bath for 15 minutes. Make sure that the liquid level of the bath exceeds the level in the test tubes.
Cool the standards for 30 minutes in air or more quickly in a cold water bath, and vortex for 5 seconds. Read the values at 580 nm in the spectropho-
tometer. Graph concentration in μg/ml against absorbance. The slope should be between 0.24 and 0.28, the intercept 0.000 ± 0.010, and the regression
coeffi cient should be better than 0.995.