3 THE CHROMOTROPIC ACID TEST FOR FORMALDEHYDE

3.1 THE REACTION

The chromotropic acid test is based on the reaction of formaldehyde with a solution of chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulfonic acid) to produce a purple species in solution (West and Sen 1956). The mechanism of this reaction has not been fully elucidated.

The test is carried out at elevated temperatures to increase the emission rate of formaldehyde from the display material under test. The spot test presented here has been adapted from the original quantitative method.

The chromotropic acid solution is placed in the reaction dish (fig. 1). The material under test is added into the reaction flask. The stoppered flask is then placed in an oven at 60�C until the solution in the reaction dish becomes purple.

3.2 FACTORS AFFECTING THE CHROMOTROPIC ACID TEST

1. This test relies on the diffusion of free formaldehyde into the chromotropic acid in the reaction dish. The depth of the reaction dish was found to be important. Using reaction dishes of 2 mm and 9 mm depth and the chromotropic acid in a small test tube of 25 mm depth, it was shown that the optimum depth is 2 mm.
2. The temperature at which the test is undertaken will affect the rate of appearance of the purple color. Trials undertaken at 20�C, 60�C, and 100�C showed that 60�C is the optimum temperature at which to conduct the chromotropic acid test.
3. The weight of the sample under test will influence the time taken for the test solution to turn blue. Trials were undertaken using a range of weights of plywood (as sawdust), as it is often bonded with formaldehyde-based adhesive, which can emit free formaldehyde (Sparkes 1986). The time at which the purple species first appeared was noted. Results are shown in table 3.

The optimum sample size was found to be 2.0 g

TABLE 3 EFFECT OF PLYWOOD WEIGHT ON FORMATION TIME OF PURPLE SPECIES

3.3 RECOMMENDED PROCEDURE

1. Prepare a solution of chromotropic acid (1% w/v) in concentrated sulfuric acid (97% w/w). The solution should be stored below 4�C and used within 2 days.
2. Put approximately 0.2 ml (10 drops) of the chromotropic acid solution into the reaction dish shown in figure 1.
3. Add 2.0 g of the sample under test to the reaction flask.
4. Place the stoppered flask in an oven at 60�C.
5. Examine the flask after 30 minutes. If the solution has turned purple, the sample under test is a source of formaldehyde and is therefore not suitable for use with materials known to be susceptible to deterioration by formaldehyde.