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idation of Brilliant cresyl blue by bromate. The reaction is monitored photometrically by measuring the decrease in absorbance of the dye. Formaldehyde in the ...
Fresenius J Anal Chem (1999) 363 : 376–379

© Springer-Verlag 1999

O R I G I N A L PA P E R

Ali A. Ensafi · S. Abassi

Sensitive reaction rate method for the determination of low levels of formaldehyde with photometric detection

Received: 26 May 1998 / Revised: 30 September 1998 / Accepted: 3 October 1998

Abstract A simple and rapid method is proposed for the determination of ultra trace amounts of formaldehyde. It is based on the catalytic effect of formaldehyde on the oxidation of Brilliant cresyl blue by bromate. The reaction is monitored photometrically by measuring the decrease in absorbance of the dye. Formaldehyde in the range of 0.005–2.300 µg/mL can be determined with a limit of detection of 0.003 µg/mL. The relative standard deviation for ten replicate measurements of 1.5 µg/mL formaldehyde is 0.1%. The method was used for the determination of formaldehyde in real samples with satisfactory results.

They are either not sensitive enough [11], are subject to interference from other compounds [18, 21], or have a high limit of detection [11, 19, 20]. To our knowledge, there are no reports on the use of a catalytic effect of formaldehyde for its determination. Proposed in this paper is a rapid, highly sensitive and selective method for the determination of formaldehyde using the Brilliant cresyl blue-bromate system. It is suitable for the determination of ultra traces of formaldehyde in real samples. The applications of the method to the determination of formaldehyde in chemical industrial wastewater and melamine formaldehyde resins are reported.

Introduction Experimental Formaldehyde is one of the exceptionally important air pollutants in residential and industrial environments. Exposure to formaldehyde has caused intense concern because it is an irritant giving rise to dermatitis, eye irritation, respiratory irritation, asthma, and pulmonary edema [1, 2]. It has the potential to react with hydrochloric acid to form bis(chloromethyl)ether, a known carcinogen [3, 4]. Industrial exposure to formaldehyde occurs mainly in the woodworking and garment industry using formaldehyde based resins. Because of its widespread use and adverse health effects, interest in improved analytical methodology for the determination of formaldehyde is high. Various methods have been developed for the determination of formaldehyde including GC [5, 6], HPLC [7, 8], voltammetry [9, 10], chemiluminsecence [11], fluorimetry [12, 13], and spectrophotometry [14–17]. The spectrophotometric methods are most widely used. However, they are not very sensitive and are subject to numerous interferences by phenols, alcohols and cyclohexanone [14– 17]. Only few reports have been found for the determination of formaldehyde with kinetic methods [11, 18–21].

A. A. Ensafi (쾷) · S. Abassi College of Chemistry, Isfahan University of Technology, Isfahan, Iran

Reagents Analytical-reagent grade chemicals and doubly distilled water were used throughout. Formaldehyde stock solution (1.00 mg/mL) was prepared by diluting 0.5 mL of 37% formaldehyde solution (Merck) to 1 L with water and was standardized by using the sulfite method [21]. Formaldehyde standard solutions were prepared daily from the stock solution by appropriate dilution with water. Bromate solution (0.5 mol/L) was prepared by dissolving 7.455 g of NaBrO3 (Merck) in water in a 100 mL volumetric flask. BCB solution (1.25 × 10–3 mol/L) was prepared by dissolving 0.1040 g of BCB (Aldrich) in water and diluting to 250 mL with water in a volumetric flask. Apparatus A probe photometer (Methrom, Model 662) was used to measure the absorbance change at 630 nm. A thermostat water batch (GallenKamp, Griffin BJL-240-W) was used to keep the reaction temperature at 30 °C. Recommended procedure All solutions were kept in a 30 °C thermostat water batch for 30 min before initiation of the reaction. To a series of 25 mL volumetric flasks 3.0 mL of 4.21 × 10–4 mol/L BCB solution, 1.0 mL of 1.70 mol/L sulfuric acid and different amounts of formaldehyde (less than 5.75 mg) were added. The contents were diluted to ca.

377 23 mL with water. Then 1.7 mL of 0.10 mol/L bromate solution was added and the solution diluted to the mark with water. The zero time was taken as the moment at which the last drop of bromate solution had been added. Then the solution was transferred to a 50 mL beaker and the decrease in absorbance as a function of time (∆As), was measured against a water reference for 0.5–2.0 min from initiation of the reaction at 30 °C. The blank reaction was performed according to the same procedure without addition of formaldehyde and the change in absorbance was labeled as ∆Ab. For each sample and blank, three determinations were made and the average signals were used. Determination of free formaldehyde in melamine formaldehyde resin and industrial wastewater Steam distillation was used to remove the free formaldehyde from melamine formaldehyde resin. For the determination of formaldehyde about 0.2 g of the resin was exactly weighed and suspended in water. The free formaldehyde was separated by steam distillation collecting 250 mL of distillate. Then the formaldehyde content was measured by the recommended procedure with the standard addition method. For the analysis of wastewater, the water was filtered (using Whatman paper No.1) and the formaldehyde content was measured by the standard addition method.

Results and discussion Brilliant cresyl blue (BCB) is a dye that can be oxidized with strong oxidizing agents [22, 23] at slow reaction. Formaldehyde can catalyze this reaction at ultra-trace level. In this paper, the catalytic effect of formaldehyde on the oxidation of BCB with bromate was used. The reaction was monitored photometrically by means of the decrease in absorbance of the characteristic band of BCB (630 nm) in acidic media. Effect of variables The effect of various acid types with the same concentration such as sulfuric, hydrochloric, phosphoric and nitric

Fig. 1 Effect of sulfuric acid concentration on the reaction rate. Conditions: formaldehyde, 2.20 µg/mL; BCB, 1.40 × 10–5 mol/L; temperature, 30 °C

Fig. 2 Influence of BCB concentration on the sensitivity. Conditions: formaldehyde, 2.20 µg/mL; H2SO4, 0.068 mol/L at 30 °C

acid was studied. The results show that sulfuric acid gives greater sensitivity. The effect of sulfuric acid concentration on obtaining maximum sensitivity was investigated with 1.40 × 10–5 mol/L BCB and 0.004 mol/L bromate for catalyzed and uncatalyzed reaction at 30 °C (Fig. 1). The results show that by increasing the acid concentration up 0.068 mol/L, the sensitivity increased. On the other hand, higher acid concentrations cause a small decrease of the sensitivity (∆As–∆Ab). This is due to the fact that at higher acid concentrations, the blank reaction becomes faster and thus ∆Ab becomes larger and ∆As–∆Ab smaller. Therefore 0.068 mol/L of sulfuric acid was used for the study. The effect of BCB concentration in the reaction rate was studied with 0.068 mol/L sulfuric acid and 0.0040 mol/L bromate at 30 °C. The results show that by increasing the BCB concentration up to 5.0 × 10–5 mol/L the sensitivity (∆As–∆Ab) increases, whereas a greater amount of the reagent does not affect it (Fig. 2). Thus 5.0 x10–5 mol/L BCB concentration was selected throughout the study. The influence of bromate concentration on the reaction rate was studied in the range of 2.0 × 10–3–8.0 × 10–3 mol/L with 0.068 mol/L sulfuric acid and 5.0 × 10–5 mol/L BCB at 30 °C (Fig. 3). The results show that by increasing the bromate concentration up to 0.0060 mol/L, the sensitivity (∆As–∆Ab) increased, whereas greater amounts of the reagent cause decreasing sensitivity. This effect is due to the fact that at higher concentrations of bromate the blank reaction is so fast that the sensitivity (∆As–∆Ab) decreased. Therefore, 0.006 mol/L bromate concentration was selected for the study. The influence of temperature on the sensitivity (∆As–∆Ab) was studied in the range of 10–50 °C with the optimum reagent concentrations. The results show that with increasing temperature the net reaction rate increased. A temperature of 30 °C was selected due to the simplicity of application and its reproducibility. The effect of ionic strength on the net reaction rate (∆As–∆Ab) was studied with 3.0 mol/L KNO3 solution and optimum reagent concentrations. The results show that a change in ionic strength has little effect in the sensitivity. The optimization procedure for the reagents was also checked with Simplex optimization and the following re-

378 Table 2 Determination of formaldehyde in real samples Sample

Melamine-formaldehyde resin(I) Melamine-formaldehyde resin(II) Chemical industrial wastewater

Fig. 3 Effect of bromate concentration on the reaction rate. Conditions: formaldehyde, 2.20 µg/mL; BCB, 5.0 × 10–5 mol/L; H2SO4, 0.068 mol/L at 30 °C

sults were obtained for optimum conditions after thirteen experiments: sulfuric acid, 0.05 mol/L; BCB concentration, 5.2 × 10–5 mol/L; BrO3–, 0.007 mol/L. As can be seen the agreement with the above values is excellent. Calibration graph and precision The absorbance time graph at 30 °C and 630 nm with the optimum concentration of the reagents was obtained for the catalyzed reaction for different amounts of formaldehyde with the fixed-time method. Measurement was made 0.5–2.0 min from initiation of the reaction, because it provided the best regression, sensitivity and analysis time. Under the optimum condition described above in the concentration range of 0.005–2.300 µg/mL formaldehyde: ∆As = 0.124 + 0.314 C (r = 0.9984, n = 10), where C is µg/mL of formaldehyde. The relative standard deviations for ten replicate determinations of 0.50, 1.00, 1.50 µg/mL were 2.0, 1.5 and 1.0% respectively. The experimental limit of detection (3Sb/m, three times the blank standard deviation divided by the slope of the regression equation) is equal to 0.004 µg/mL. Interference study For analytical application of the catalytic reaction, the fixed-time method is recommended. The influences of Table 1 Interference study for the determination of formaldehyde Species Mn2+, Al3+, Cd2+, Cs+, Sr2+ Zn2+, Cr3+, Mg2+, K+, Na+, ClO3– CO32–, C2O42–, IO3–, CH3COO–, Benzene, urea, benzaldehyde, acetaldehyde, methanol, acetone I–, Cl–, Br –

Tolerance limit (Wion/Wformaldehyde)

2500 10

Formaldehyde found Proposed method (n = 5)

Standard method (n = 5)

(7.2 ± 0.1)% (6.7 ± 0.1)% (0.62 ± 0.01) µg/mL

(7.3 ± 0.2)% (6.7 ± 0.2)% (0.61 ± 0.1) µg/mL

several organic (alcohol and aldehyde) and inorganic compounds were tested using the standard solution of formaldehyde (1.00 µg/mL). The results are given in Table 1. No interference was observed from nitrate, nitrite, hydrogen carbonate, glucose, urea, ammonia, aldehyde and alcohol at 2500-fold excess. Iodide and bromide interfere at concentrations greater than 10 µg/mL. Determination of free formaldehyde in melamine-formaldehyde resin and chemical industrial wastewater The free formaldehyde in melamine-formaldehyde resin was determined using a simple calibration graph and also by the standard addition method. Similar results were obtained by both methods. This shows that a simple calibration graph is adequate for determination of formaldehyde in the resin (Table 2.). The proposed method was also applied to the determination of formaldehyde in a sample of chemical industrial wastewater by using the standard addition method (due to unknown matrix of the sample). The results were compared with a standard method for the determination of formaldehyde using chromotropic acid [24]. The results are given in Table 2.

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