ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2007, том 62, № 10, с. 1101-1106
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УДК 543
SIMULTANEOUS DETERMINATION OF FORMALDEHYDE AND METHANOL BY FLOW INJECTION CATALYTIC SPECTROPHOTOMETRY
© 2007 r. Xuan-Feng Yue, Zhi-Qi Zhang
School of Chemistry and Materials Science, Shaanxi Normal University Xi'an 710062, China Received 07.04.2006; in final form 19.06.2006
A flow injection method is proposed for the simultaneous catalytic determination of formaldehyde and methanol on the basis of the catalytic action of formaldehyde upon the redox reaction between crystal violet and potassium bromate in phosphoric acid medium and on-line oxidization of methanol into formaldehyde using a lead dioxide solid phase reactor. The indicator reaction is monitored spectrophotometrically by measuring the decrease in the absorbance of crystal violet at the maximum absorption wavelength of 610 nm. A technique based on three sampling loops with a single injection valve is developed. The flow injection system produces a signal of a main peak with two shoulders of the same height. The height of the shoulders corresponds to the formaldehyde concentration, and the height difference between the shoulders and main peak corresponds to the methanol concentration. The detection limit is 0.1 |g/mL for formaldehyde and 1.0 |g/mL for methanol with the sampling rate of 10 samples per hour. The relative standard deviations for eleven replicate determinations of formaldehyde (1.0 |g/mL) and methanol (10 |g/mL) are 1.1% and 2.1%, respectively. The method has been successfully applied to the simultaneous determination of formaldehyde and methanol in some gas samples.
Formaldehyde is ubiquitous in residential and industrial environments and is one of the most important and familiar pollutants. Various methods have been developed for the determination of formaldehyde including chromatography [1-2], electrometry [3-4], fluorimetry [5-6] and spectrophotometry [7-8]. The conventional spectrophotometry and fluorometric methods are widely used but require time-consuming and complicated procedures. Flow unjection analysis (FIA) shows some advantages and has been also applied to the determination of formaldehyde [5, 8, 9].
Methanol is known as a harmful substance for health. The determination of its content is mainly performed by enzymatic method [10] and chromatography [11-12].
The simultaneous measurement of formaldehyde and methanol is of significance and has received much attention in the development of cleaner fuels [13], quality evaluation of airs [14]. Fourier transform infrared (FT-IR) technology was firstly used to the determination of formaldehyde and methanol in automobile exhaust [13], but it is expensive and not suitable for routine analysis. The conventional spectrophotometry is hampered due to its insufficient sensitivity, gas sensor [14] has low selectivity, and gas chromatography [15] needs strict experiment conditions. So, developing a simple, more sensitive and selective alternative is still a great need for the simultaneous determination of formaldehyde and methanol.
Catalytic spectrophotometry methods are attractive alternatives in terms of sensitivity. Yet up to now there has been no catalytic spectrophotometric method for the simultaneous determination of formaldehyde and methanol. This work proposes a flow injection method for the simultaneous catalytic determination of formaldehyde and methanol based on the catalytic effect of formaldehyde [16] and on-line oxidizing methanol into formaldehyde with a lead dioxide solid phase reactor. A technique with three sampling loops and a single injection valve is developed in order to accomplish a signal of the main peak with two shoulders of the same height. The height of the shoulders corresponds to the formaldehyde concentration, and the height difference between the shoulders and main peak corresponds to the methanol concentration. The proposed method has been applied to some gas samples and satisfactory results were obtained.
EXPERIMENTAL
Apparatus. A schematic diagram of the reverse flow injection system is shown in Fig. 1. A model IFIS-C intellectual flow injector (Xi'an Ruike Electron Equipment Corporation, China) was used to set up the flow injection system. A model 501 thermostat (Shanghai Experimental Apparatus Factory, China) was employed to keep the reaction temperature. A model 722 grating spectrophotometer (Shanghai Analytical Instruments Factory, China) with an 18 ^L flow cell
S
Ri
с
R2 R3
A
Шп
RC
—ywwvw
T
- RE
1 D w
Po
Fig. 1. Schematic diagram of the FIA system for simultaneous determination of formaldehyde and methanol. S - sample; C - Mixture of phosphoric acid and manganese sulfate solution as carrier; Rj - Mixture of phosphoric acid and manganese sulfate solution; R2 - potassium bromate in phosphoric acid solution; R3 - crystal violet solution; A -anion exchange column; Pj, P2 - peristaltic pumps; V - injection valve; RC - reaction coil; T - thermostatic water bath; D - spectrophotometric detector; RE - recorder; W -waste.
(light path, 10 mm) was used to measure the absor-bance of indicator at 610 nm. The results were recorded by means of an automatic balance recorder (Shanghai Dahua Instrument Factory, China). Except for the pump tube (Tygon), PTFE tubing (0.9 mm i.d.) was used thoughout the manifold. A DQ-1A gas analyzer (Jiangsu Jiangfen Electroanalytical Instrument Co. Ltd., China) was employed to absord a certain volume of gas.
Reagents. All chemicals used were of analytical reagent grade or higher (Xi'an Chemical Reagent Plant, China). Doubly deionized water was used throughout.
Formaldehyde stock solution (100 |g/mL) was prepared from the 37% reagent and standardized with io-dimetry [17] before use. The concentration of methanol stock solution was 500 |g/mL. Working standard solutions were prepared freshly by appropriately diluting the stock solution.
Crystal violet (indicator) solution (0.001 M) and potassium bromate solution (0.20 M) were correspond-
ingly prepared by dissolving the required amount of reagent in water.
Phosphoric acid solution (4.0 M) was prepared from the 85% reagent.
Manganese sulfate (1 mg/mL) solution was stored air tightly.
Model - 717 resin (Xi'an Electric Power Resin Plant, particle diameter 0.3-1.2 mm) was purified in a traditional way, the last processing step was to wash the resin with water until the effluent remained pH 7.0. The processed resin was then filled into a mini-column (5 cm in length and 2 mm in diameter).
The solid phase reactor was prepared by filling a glass tube (2 mm in inner diameter and 20 cm in length) with lead dioxide granules and stuffed with fiberglass at both ends. The lead dioxide granules were made as in [18] and cut into granules of 0.5 mm size.
Procedure. The configuration of the injection valve of the proposed method is shown in Fig. 2. When the valve is at filling position (Fig. 2a), the sample was driven through sampling loop Lj, L2, solid phase reactor B and sampling loop L3 in sequence, and methanol in the sample passing through the reactor B is partly oxidized to formaldehyde. When the valve is at injecting position (Fig. 2b), the sample in L2, L3 and Lx is driven in sequence to mix with indicator reaction mixture and detected. By measuring the absorbance at the maximum absorption wavelength of crystal violet (610 nm), a signal of the main peak with shoulders of the same height on either side is obtained (Fig. 3). The height of the shoulders (AAj) corresponds to the formaldehyde concentration and the height difference (AA2) between the shoulders and the peak corresponds to the methanol concentration.
RESULTS AND DISCUSSION
Optimization of the flow injection system. Considering the potential interference of anions from sample solution, an anion exchange column A was connect-
(a)
D
C
S + R1
Filling
W
D
C
(b)
S + R1
Injecting
W
Fig. 2. Configuration of the injection valve for Fig. 1.
Lx, L2, L3 - sampling loops; B - lead dioxide solid phase reactor; A - block; C, S, R^ D, W - the same as in Fig. 1.
P
3
SIMULTANEOUS DETERMINATION OF FORMALDEHYDE AND METHANOL
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ed in sampling flow to remove interferences on-line. A configuration of injection valve with three sampling loops was developed (Fig. 2). At injecting step of this configuration a sample zone having passed the solid phase reactor B was sandwiched between two other sample zones without passing reactor B, such that less diffusion of the sandwiched sample zone was obtained. This increased the sensitivity for the determination of methanol. An advantage of the configuration was the following. There was no solid phase reactor all along in the stream to the detector such that a stable baseline was obtained and the life of solid phase reactor was lengthened.
The variables investigated were volumes of sampling loop Lj, L2 and L3; size of lead dioxide granule and length of solid phase reactor; flow rate and reaction coil temperature. The conditions used in these experiments were as follows: C, mixture of 100 |g/mL of manganese sulfate and 0.01 M of phosphoric acid; Rx, mixture of 1 |g/mL manganese sulfate and 0.1 M phosphoric acid; R2, mixture of 0.1 M of phosphoric acid and 0.1 M of potassium bromate; R3, 5.5 x 10-5 M of crystal violet; S, a mixture of formaldehyde (10 |g/mL) and methanol (50 |g/mL); RC, a reaction coil of 200 cm in length and 0.9 mm in diameter; flow ratio of S to Rx, 9 : 1.
The volume of sampling loops was a critical factor for the appearance of the peak with shoulders and the sensitivity for determination of methanol was affected by L3. Experimental results indicated that no apparent shoulders were observed when the volume of Lx was less than 500 |L and L2 less than 700 |L. The determination sensitivity for methanol increased with increasing the volume of L3 up to 200 |L. The dispersion coefficients of formaldehyde and methanol in the system were 2 and 3.4, respectively,
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