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Anal. Chern. 1986, 58,2848-2850
Direct Colorimetric Determination of Formaldehyde in Textile Fabrics and Other Materials Joseph Chrastil* and Robert M. Reinhardt
U.S. Department of Agriculture, Southern Regional Research Center, New Orleans, Louisiana 70179
A colorimetric method for direct determlnatlon of formaldehyde In textlle fabrics and other materlals Is described. Color development and breaking formaldehyde bonds of the analyzed material occur simultaneoudy In the same reaction mlxture without destruction of the materlal. The method Is based on the color reaction of formaldehyde with indole-3acetic acid or tryptophan. Common inorganic salts, higher allphatlc aldehydes, carbohydrates, amino acids (except tryptophan), and many other organic compounds do not react and do not Interfere with the color reaction. Some interferences have been exhibited by acetaklehyde and glyoxal. The method was simole, accurate, and relatively insensitive to the reaction conditions. Only very small amounts of material are needed, and the reactlon proceeds at room temperature. Different klnds of polymeric materials have been analyzed successfully (cotton, wool, plastks, collagen, wood, and furs). Most of the dyed fabrics or other materlals could be analyzed in the same manner because under the reaction conditions the dyes were not extracted in the reaction mixture.
Total formaldehyde in textile materials is usually determined by breaking the bonds that bind it to the substrate in a strong acid and analyzing the extract by different methods (for example, titration, gravimetry, chromotropic acid, or polarography) ( I ) . In other methods the released free formaldehyde is distilled into a separate trap in which it can be determined by various methods (1-7). These procedures often require higher temperatures and are usually laborious, time-consuming, and/or subject to possible losses during the distillation. Additionally, it is diffult to modify these methods for small samples. We have developed an accurate and simple direct colorimetric method in which there is no need for distillation, long heating, or other laborious procedures. The method was successfully applied to different textile and other polymeric materials containing free and/or bound formaldehyde.
EXPERIMENTAL SECTION Materials. Formaldehyde (37.6%)and H2S04(96.1% ) were from J. T. Baker Chemical Co., NJ (analyticalreagents). All other chemicals were from Sigma Chemical Co., MO (highest available purity). Total Formaldehyde: To approximately 10 mg of a sample (less than 10 mg with more than 2% HCHO content) in a 20-mL test tube were added, first, 4 mL of 0.04% (w/v in 8% ethanol) indole-3-aceticacid (IAA) and then 2 mL of 90% (w/w in water) H2S0,, and the solution was immediately mixed. After the solution was allowed to stand for 3 h at room temperature with occasional mixing, 5 mL of methyl cyanide (or acetone) was added to dissolve the cloudiness. The absorbance of the supernatant liquid from the test tube (centrifuging or filtration is usually not necessary) was read in a cuvette at 440 nm vs. H,O in a Shimadzu UV-vis double-beam spectrophotometer. Reaction blanks without samples were prepared and analyzed in the same manner. All samples were prepared and analyzed in triplicates. From each sample an average reaction blank (from triplicates) was subtracted, and the formaldehyde content in the cuvette (supernatant) was read from the standard curve in milligrams per liter. Finally, the
formaldehyde content in the sample was expressed in percent (w/w).
When tryptophan (TRY) (0.04% in 1% H,SO,) was used instead of the IAA, the color was read at 450 nm and the addition of methyl cyanide (or acetone) was not necessary because the colored product was soluble in the reaction mixture and the solution was clear. Standard Curve. A standard curve was obtained with formaldehyde solutions containing 0, 1.5, 3, 6, 7.5, 10.5, 15, 18, 22.5, and 25.5 F g of formaldehyde/mL. The color was developed in the same manner as with the samples. Blank was zero formaldehyde. All standard solutions were prepared in triplicates. Total Formaldehyde by Distillation Method. The samples were analyzed by the modified distillation method described by F. C. Wood (2-5), which is conventionally used in our Reearch Center for the determination of bound and/or free formaldehyde. Formaldehyde in the distillate was analyzed by chromotropicacid as described by W. J. Roff (6, 7). This method, which had almost 100% recovery of formaldehydefrom the textile samples, was used for comparison with the method described in this paper.
RESULTS AND DISCUSSION The colorimetric method described in this work is based on the reaction of formaldehyde with indole or carbazole derivatives. From the time of its discovery by Emil Fischer (8-10) it was studied extensively (11-20), but for a long time its analytical applications were limited only to the sensitive qualitative or semiquantitative test for indole derivatives or proteins (21-24). The color was always described as red, violet, or blue, because the colored reaction products obtained in the presence of oxidation compounds (for example, HzOzand Fe3+)were not homogenous products and thus the final color greatly depended on experimental reaction conditions. Additionally, because of the impurities in the substrates (indole derivatives) and evidently also in the reagents (HC1, H2S04,HCHO, ZnC1,) a violet color was often obtained even in the “absence” of supplemental oxidation compounds. Additionally, with the excess of H2O2or FeC13the colored products were destroyed and the violet color disappeared or changed again. These oxidative color reactions have been modified for the determination of indole derivatives and/or formaldehyde, but these colorimetric methods required stringent reaction conditions and were sensitive to oxidative impurities (25, 26). We have found that with analytically pure reagents (H2S04, IAA, TRY, HCHO, and HzO) and in the absence of the oxidation compounds, the reaction of formaldehyde with IAA or TRY always resulted in the formation of a clear yellow color. The color intensity was not linear with the concentration of formaldehyde and a curved relationship was obtained (Figure 1). This was expected because it was an equilibrium reaction of a t least second or third order and large excess of the reagents (IAA or TRY) caused several problems. First, the solubility of TRY or IAA in water mixtures is very limited, and when, for example, acetone and/or methyl cyanide solutions with higher (1-5 g/L) concentrations of IAA have been used instead of lower concentrations in water solutions, the reaction heat with H2S0, was too high and the volatile solvents boiled vigorously. After cooling of these mixtures to room temperature neither the sensitivity nor the
This article not subject to U.S. Copyright. Published 1986 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
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Table I. Analysis of Some Materials Treated w i t h Formaldehyde
material HzO washed cotton cotton cotton cotton cotton
collagen dyed commercial materials cotton-polyester shirting, 5050 cotton-polyester oxford, 5050 cotton-polyester shirting, 50:50 cotton-polyester shirting, 60:40 cotton-polyester shirting, 5050 cotton-polyester shirting, 35:65 cotton chambray cotton denim wool wood (particle) rabbit fur skin
treatment 2% HCHO, 1.5% MgC12 8,% HCHO, 3% MgClZ
10% urea-HCHO 9% dimethyl01isopropyl carbamate, MgClz, citric acid
untreated 1.5% HCHO, 0.5% NaZCO3 unknown unknown
unknown unknown
unknown unknown
THPOH-NH3 precondensate THPOH-NH8 precondensate untreated unknown unknown
%HCHOa
sb
% HCHO'
0.48 1.76 3.50 1.68 0.00 0.37
0.01 0.04 0.04 0.03 0.00 0.01
0.46 1.70 3.43 1.70 0.01 0.35
0.35 0.52 0.32 0.57 1.39 0.61 1.05 1.01 0.02 1.68 0.26
0.01 0.01 0.01 0.01 0.03 0.02 0.02 0.02 0.00 0.04 0.004
0.35 0.50 0.31 0.55 1.39 0.60
.LO4 0.99 0.01 1.69 0.23
aTotal formaldehyde analyzed by IAA method (% w/w). bStandard deviation of the mean. CTotalformaldehyde by distillation method and by chromotropic acid reagent (% w/w). Averages are from triplicates. linearity increased significantly and we did not follow this modification further. Additionally, higher concentrations (over 1g/L of the reaction mixture) of TRY or IAA in acid or alcoholic solutions, respectively, resulted not only in higher reaction blanks but also in the shift of the maximum absorbance caused mainly by the colored reaction products of TRY and/or IAA with HzS04. Simultaneously the sensitivity of the reaction decreased, and with 5 g/L of TRY or IAA in the reaction mixture i t was already very low. This was evidently caused by an inhibitory effect of the reaction of TRY and/or IAA with H2S04,which a t these high concentrations of TRY or IAA overlapped the absorbance of the reaction product with small amounts of HCHO. At the concentration of TRY and/or IAA below 0.5 g/L in the reaction mixture, the blanks were already close to zero ( A = 0.010-0.020) and further decrease of TRY or IAA concentration had no advantage. Naturally, for each new chosen concentration of TRY or IAA a new standard curve must be determined. Thus, 0.4 g/L of the TRY or IAA stock solution seemed to be the best optimum. The experimental results confirmed it. Under these experimental conditions the reaction always proceeded smoothly and without problems. The reproducibility of the reaction as described in the Experimental Section was very good not only with pure formaldehyde standards (Figure 1) but also with different formaldehyde-containing materials (cotton, wool, plastics, leather, and wood) (Table I). The coefficient of variation between triplicates was always smaller than 3 % . A great part of this error was caused by pipetting. For example, when the pipetting error was eliminated by weighing the solutions of IAA (TRY), HzS04,and CH3CN or acetone (weighing accuracy, 0.1 mg), the coefficients of variation were less than 1%. When the new method and conventional method were compared, there was no statistical difference (P > 0.96). Under the experimental conditions described in the Experimental Section the reaction was also not sensitive to small changes in concentration of added HzS04 (in the limits 85-92%) and/or temperature (in the limits 20-30 "C). The influence of temperature is shown on an example with HCHO-treated cotton (Table 111). From this example, it was apparent that there was no advantage in developing the color at higher temperatures, because the color always reached the colder equilibrium after cooling to room temperature, and reading the reaction mixtures in the spectrophotometer cuvettes a t higher temperatures, for example a t 80 "C, causes
C
HCHO (rnglreaction mixture)
Figure 1. Standard curve of formaldehyde. The color was developed by IAA or by TRY. The reaction conditions are described in the Experimental Section. Average values are from triplicates. Coefficient of varlation between triplicates was always smaller than 3 % . A = absorbance minus blank. For convenient calculations, the concentration of HCHO, c , is in milligrams per reaction mixture. Thus, because the volume of the analyzed samples can be neglected (only 0.1 % of the total reaction volume), the HCHO content In the sample (in w/w % ) is simply lOOc/wt, where wt is the weight of the sample in milligrams.
other inconvenient complications (temperature dependency of the absorbance, evaporation, etc.). Furthermore, many textile materials (for example, leather or furs) will not stand temperatures over 60 "C. After addition of HzS04 to the reaction mixtures, the temperature shortly increased up to 50-70 "C, depending on the
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table 11. Rate of Color Development with Formaldehyde-Treated Cotton Samples Containing 1.7% Bound Formaldehyde
reaction time, min
color intensity absorbance," %
reaction time, min
color intensity absorbance," %
30
90
180
100
60 90
94 98 99
240 1440
100
120
99
"IAA method. Absorbance in percent is corrected for a blank. speed of mixing, size of the reaction tube, and the amount of formaldehyde. As is apparent from Table 11, although the color developed quickly, 2-3 h was a safe cooling time to stabilize not only the color but also the temperature of the reaction mixture and thus to avoid the problems of reading the color at different higher temperatures. Under the reaction conditions described in the Experimental Section, the reaction was not influenced by the presence of up to 5 g/L common inorganic, nonoxidative salts, cellulose, keratin, collagen,glucose, sucrose, starch, amino acids (except tryptophan), organic acids, ketones, higher aliphatic aldehydes, and other organic compounds (not shown here) except indole, pyrrol, and carbazole derivatives and, of course, soluble proteins that contain tryptophan. Acetaldehyde interfered (about 3040% of the HCHO absorbance, depending on concentration) but glyoxal and glyoxylic acid interfered much less (up to 7 % and 1% of the HCHO absorbance, respectively). In general, in the absence of oxidation compounds the method was suitable for the determination of bound formaldehyde in different kinds of commercial fabrics and materials treated with formaldehyde or formaldehyde-related compounds (Table I). The color developed quickly and after 2-3 h remained stable for at least one or more days. Elevated temperatures increased the rates of color development but influenced very little the final results after cooling (Table 111). Temperatures over 90 "C resulted, in most cases, in partial or total destruction of the analyzed samples. Thus, it is not recommended to heat the samples to higher temperatures. The simplest way to develop the color was by quick mixing of the sample in the reagent (IAA or TRY) with H2S04(the mixture warmed up to some extent) and leaving the mixture a t room temperature. In most cases, dyed materials such as fabrics or leather could be analyzed in the same manner because the common dyes were not extracted into the reaction mixture. The reaction with TRY was less sensitive than the reaction with IAA. The maximum absorbance was at a slightly different wavelength (450 nm), and the yellow reaction product was soluble in the reaction mixture up to the relatively high concentrations of formaldehyde. Thus, the addition of methyl cyanide or acetone was not necessary. The higher sensitivity with IAA was probably caused by the low solubility of the reaction product in the reaction mixture, which evidently moved the reaction equilibrium to the right. The advantage of the TRY reaction was the higher analytical purity of the commercial TRY as compared to that of IAA. The blanks with TRY were lower than the blanks with IAA, and the TRY stock solutions were much more stable. But when commercial IAA was recrystallized, the
Table 111. Color Development at Different Temperatures with Formaldehyde-Treated Cotton Samples Containing 1.7% Bound Formaldehyde"
temp, "C 4 25 50
60 70 80
absorbance, 70 after 1 h after 3 h 90
98 100
94 98 102 105 110
100 101 102 103
The color was developed for 30 min at the designated temperature and then kept at room temperature for 1 and/or 3 h. The resulting absorbances are expressed in percent. Averages are from triolicates. ~~
~~
reaction blanks were 5 0 4 0 % lower and stock solutions of IAA were more stable. For most determinations we have used IAA because of its greater sensitivity. The IAA stock solutions were prepared fresh every week. A wide variety of textile and other polymeric samples (Table I) containing bound and/or free formaldehyde (cotton, wool, polyester, dyed cotton, dyed wool, dyed polyester, collagen, wood, or furs) gave accurate and reproducible results. Under the reaction conditions described in the Experimental Section these materials did not dissolve in the reaction mixture but formaldehyde was immediately released and reacted with the reagents. The recovery of HCHO from the textile materials was the same as with the distillation method, close to 100%. The structure of the colored reaction products is not known, and we shall try to elucidate the mechanism of these reactions and the structure of the reaction products in our future work. The method described in this work is simple, requires only very small amounts of material, and can be easily executed in microscale arrangements. Registry No. HCHO, 50-00-0.
LITERATURE CITED Walker, J. F. Formaldehyde, 3rd ed.; Reinhoid: New York, London, 1968; Chapters 16-18. Wood, F. C. J . SOC.Chem. Ind., London 1933, 52, T33-34. de Jong, J. I . Red.: J . R . Neth. Chem. SOC. 1953, 72, 653-654. Steeie, R.; Gidding, L. E., Jr. I n d . Eng. Chem. 1956, 4 8 , 110-114. Mehta, P. C.; Mehta, R. D. Text. Res. J . 1980, 30, 524-532. Roff, W. J. J . Text. Inst. 1956, 47, T309-316. Andrews. B. A. K.; Reinhardt, R. M. Adv. Chem. Ser. 1985. No. 210, 83-100. Fischer, E. Justus Lieblgs Ann. Chem. 1888, 236, 116-151. Fischer, E. Justus Lieblgs Ann. Chem. 1887, 242, 372-383. Fischer, E. Ber. Dtsch. Chem. Ges. 1688, 19, 2988-2991. Voisenet, E. Bull. SOC. Chim. Fr. 1809, 37(4), 736-742. Homer, A. Biochem. J . 1913, 7, 101-116. Perkin, W. H., Jr.; Robinson, R. J . Chem. SOC. 1919, 115, 967-972. Fearon, W. R. Biochem. J. 1920, 14, 548-564. Kermack, W. 0.;Perkin. W. H.. Jr.; Robinson, R. J . Chem. SOC. 1921, 119, 1602-1640. Burr, G. 0.; Gortner, R . A. J . A m . Chem. SOC. 1924, 46, 1224-1246. Wadsworth, A,; Pangborn, M. C. J . Biol. Chem. 1936, 116, 423-438. Harvey, D. G.; Miller, E. J.; Robson, W. J . Chem. SOC. 1941, 153-159. Voisenet, E. Bull. SOC. Chim. Fr. 1905, 33(3). 1198-1215. Konto, K. 2.Physioi. Chem. 1906, 48, 185-186. Rosenheim,. 0. Biochem. J . 1906, 1. 233-240. Bergeim, 0. J. Biol. Chem. 1917, 32, 17-22. Komm, E.; Boehringer, E . 2. Physioi. Chem. 1922, 124, 287-294. Komm,. E.; Boehringer, E . 2.Physiol. Chem. 1924, 140, 74-79. Chrastil. J.; Wilson, J. T. Anal. Biochem. 1975, 63, 202-207. Chrastil, J. Anal. Biochem. 1978. 72, 134-138.
RECEIVED for review April 18, 1986. Accepted July 15, 1986.
