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Chromotropic acid–formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid
Talanta 60 (2003) 171 /176 www.elsevier.com/locate/talanta
Chromotropic acid formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid /
E. Fagnani, C.B. Melios, L. Pezza, H.R. Pezza * Instituto de Quı´mica-UNESP, P.O. Box 355, CEP 14801-970 Araraquara, SP, Brazil Received 4 November 2002; received in revised form 3 February 2003; accepted 3 February 2003
Abstract The procedure for formaldehyde analysis recommended by the National Institute for Occupational Safety and Health (NIOSH) is the Chromotropic acid spectrophotometric method, which is the one that uses concentrated sulphuric acid. In the present study the oxidation step associated with the aforementioned method for formaldehyde determination was investigated. Experimental evidence has been obtained indicating that when concentrated H2SO4 (18 mol l 1) is used (as in the NIOSH procedure) that acid is the oxidizing agent. On the other hand, oxidation through dissolved oxygen takes place when concentrated H2SO4 is replaced by concentrated hydrochloric (12 mol l1) and phosphoric (14.7 mol l 1) acids as well as by diluted H2SO4 (9.4 mol l 1). Based on investigations concerning the oxidation step, a modified procedure was devised, in which the use of the potentially hazardous and corrosive concentrated H2SO4 was eliminated and advantageously replaced by a less harmful mixture of HCl and H2O2. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Formaldehyde; Chromotropic acid; Spectrophotometry; Dissolved oxygen
1. Introduction Formaldehyde (CH2O, methanal, formic aldehyde, oxomethane) is uniquely important because of its widespread use and toxicity and it is recognized as one of the most important air pollutants. Due to large volume production and usage of formaldehyde and its possible exposurerelated health effects, much concern has arisen
* Corresponding author. Fax: /55-16-222-7932. E-mail address: [email protected] (H.R. Pezza).
over the sensitivity and accuracy of analytical methodology for this compound. Small amounts of formaldehyde and formaldehyde-releasing compounds are commonly analyzed by spectrophotometric methods [1 /3] and, one of these, the chromotropic acid (CA) method, especially through its P&CAM 125, P&CAM 235 [4] and 3500(2) [5] versions, was established as an international reference method and, despite the advent of more sophisticated techniques, it is still widely used because it is simple, sensitive, inexpensive and very selective. The major drawback presented by the CA method has been the use of
0039-9140/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-9140(03)00121-8
172
E. Fagnani et al. / Talanta 60 (2003) 171 /176
hot concentrated sulphuric acid, which is potentially hazardous and corrosive. Attempts to replace the aforementioned acid by less hazardous ones, e.g. glacial acetic, concentrated hydrochloric and phosphoric acids, were unsuccessful. It was reported, without details, that the reaction’s sensitivity was greatly reduced as compared with that observed by employing concentrated H2SO4 [6,7]. In order to reduce the hazard brought about by concentrated H2SO4, the use of a 50% (v/v) solution of that acid has been recommended [8], but, again, significant loss of sensitivity was encountered by adopting this procedure, along with other drawbacks [8]. The chemistry of the color reaction between CA and formaldehyde, in strongly acidic media, is not yet known with certainty. The most often-quoted reaction path [7,9,10] involves a two step process, as shown in Scheme 1, where (I) would be responsible for light absorption at lmax /570/ 580 nm [7,9,10]. According to Feigl [9], H2SO4 participates in both steps as dehydrant and, in the last one, also as oxidant, being reduced to sulfurous acid. It is
perhaps worth noting that H2SO4 rarely acts as an oxidizing agent [11]. An alternative structure proposed for the chromogen arising from the reaction of CA and formaldehyde is (II) [7,8]:
The nature of this chromogen has never been unambiguously proven but evidence has been obtained [7] using NMR techniques and calibration line studies to support the hypothesis that the chromogen formed in the analytical procedure has a mono-cationic dibenzoxanthylium structure (II) and not the p-quinoidal one (I) that is commonly cited. A likely mechanism for the reaction is presented [7], involving a final oxidation step.
Scheme 1.
E. Fagnani et al. / Talanta 60 (2003) 171 /176
However, the oxidizing agent is not mentioned [7]. Finally, these authors state that concentrated H2SO4 appears to be the optimal acid to use and that no simple modification would seem feasible in order to avoid the use of concentrated H2SO4 in the analytical method [7]. In this paper, investigations concerning the oxidation step associated with the CA spectrophotometric method for formaldehyde determination were carried out. Based on these investigations, a modified procedure was devised, in which the use of concentrated sulphuric acid was eliminated and advantageously replaced by a mixture of HCl and H2O2, without significant loss of sensitivity.
2. Experimental 2.1. Apparatus A Hewlett Packard Model HP8453 spectrophotometer with 1 cm matched silica cells was used for all absorbance measurements. Micropipettes Brand and Eppendorf were used to measure the smaller volumes in the experiment. A fast oxygen detection system from Hanna Instruments mod, HI 9142 was used to study the oxygen consumption in the reactional medium. A gas meter GF 2200 HR, from Gilmont Instruments was used to monitor the flux of gases in the saturation of solutions. All experiments were performed in a thermostated room (25.09/0.1 8C). 2.2. Reagents . All reagents utilized were of analytical grade (Carlo Erba, Merck or Mallinckrodt Co.). For the preparation of the solutions and samples, deionized water (Milli-Q plus) and grade ‘A’ glassware were used throughout. Nitrogen was used to deaerate solutions where required. The solutions were flushed with pure N2 until the O2 concentration was 4.1 /105 mol l1 or less, as determined by an official method [12]. In many experiments, air- and oxygen-saturated solutions were employed for which the deter-
.
.
.
.
173
mined oxygen concentrations [12] were 2.3 / 10 4 and 8.6 /104 mol l1, respectively. Formaldehyde stock standard solution, 1000 mg l 1, prepared by appropriate dilution of commercially available analytical-reagent grade formaldehyde solution (37%, Mallinckrodt Co.), standardized by a AOAC method [13]. The standard formaldehyde solutions used for constructing the calibration graph were freshly prepared by appropriate dilution of the stock solution with water. Chromotropic acid (disodium salt dihydrate, C10H6O8S2Na2 ×/ 2H2O, Merck): a 5% (m/v) aqueous solution was freshly prepared. Hydrogen peroxide (2.5 /10 2 mol l1): prepared from perhidrol 30% by convenient dilution and standardized as described in the literature [14]. Sulphuric acid solution: (50% v/v; 9.42 mol l 1), prepared in the usual way, from the concentrated acid (96%).
2.3. Recommended procedure 2.3.1. Calibration curves 2.3.1.1. NIOSH procedure. The basic National Institute for Occupational Safety and Health (NIOSH) procedure was followed with modifications as described by Georghiou et al. [7]. Curves with concentrated sulphuric, phosphoric and hydrochloric acids and diluted sulphuric acid were made under oxygen-, air- and nitrogen saturated solutions. Calibration graphs are prepared by plotting absorbance against formaldehyde concentration for each acid. 2.3.1.2. Proposed method using hydrochloric acid and hydrogen peroxide. A calibration curve is prepared as follows: transfer 630 ml of formaldehyde working standard solution (comprising 0.80 / 4.80 mg of formaldehyde) into 25 ml glass tubes, add 300 ml of 5% CA solution, 70 ml of 2.5 /102 mol l 1 H2O2 and 4.00 ml of 12 mol l 1 HCl (under stirring). The tubes are sealed with PTFE tape and heated for 1 h in a steam bath (100 8C). Afterwards, they are cooled at 25 8C. The absor-
174
E. Fagnani et al. / Talanta 60 (2003) 171 /176
bances are recorded at 575 nm (b/1 cm) against the reagent blank.
The absorption spectra of solutions obtained by using concentrated sulphuric or hydrochloric acids are superimposable within the 400/700 nm range, suggesting that the same chromogen is produced in both procedures. The results obtained for the calibration curves carried out with concentrated sulphuric, hydro-
chloric and phosphoric acids, varying the oxygen concentrations, are shown in Fig. 1. It was found that the reaction of CA with formaldehyde, developed as recommended by Georghiou et al. [7], is not dependent on the concentration of dissolved oxygen, as shown in curve 1, Fig. 1, supporting Feigl’s proposal [9] concerning the oxidation step promoted by concentrated (18 mol l 1) H2SO4. However, if that acid is replaced by concentrated hydrochloric (12 mol l 1) or phosphoric (14.7 mol l 1) acids, the results are significantly dependent on the concentration of dissolved oxygen, in addition to the already reported drop in sensitivity [6,7], as shown
Fig. 1. Dependence of oxygen concentration and nature of the concentrated inorganic acid on the CA /formaldehyde reaction. Before heating, the reaction solutions were saturated with O2 ([O2]/8.6/10 4 mol l 1), air ([O2]/2.3 /10 4 mol l 1) or N2 ([O2]/4.1 /10 5 mol l 1). For all solutions, the color reaction was developed as described in [7]. Curve 1: H2SO4 (18 mol l 1)/O2 (j), H2SO4 (18 mol l 1)/air ( /), H2SO4 (18 mol l 1)/N2 (k) (r /0.999; pooled points); Curve 2: HCl (12 mol l 1)/O2; Curve 3: HCl (12 mol l 1)/N2; Curve 4: H3PO4 (14.7 mol l 1)/O2; Curve 5: H3PO4 (14.7 mol l 1)/N2 (r/ 0.998).
Fig. 2. Dependence of oxygen concentration and nature of the concentrated inorganic acid on the CA /formaldehyde reaction. Before heating, the solutions comprising H2SO4 were saturated with O2, air or N2 as described in Fig. 1, and the color reaction was developed as recommended in [7]. For solutions comprising HCl, the color reaction was developed as described in the text and freshly distilled/deionized water was used throughout. Curve 1: H2SO4 (18 mol l 1)/O2 (r/0.999); Curve 2: HCl (12 mol l 1)/H2O2 (r/0.999); Curve 3: H2SO4 (9.4 mol l 1)/O2; Curve 4: H2SO4 (9.4 mol l 1)/N2.
3. Results and discussion
E. Fagnani et al. / Talanta 60 (2003) 171 /176
in Fig. 1, curves 2/5. The same is true if lower H2SO4 concentrations are used (e.g. 9.4 mol l1) cf. curves 3 and 4 of Fig. 2. These facts could explain the lack of reproducibility of the measurements and consequently the non linearity of the calibration curves reported in the literature [8] when 9.4 mol l1 H2SO4 is utilized, which would probably be associated with the lack of control over oxygenation conditions. No color at all appears in the absence of oxygen (e.g. in sodium dithionite rich solutions). Curves 2 and 3 of Fig. 1 show a departure from linearity for a formaldehyde concentration higher than approximately 2.4 mg l 1 and a drop of analytical sensitivity with decreasing of dissolved oxygen content in the reaction medium comprising HCl. Furthermore, departure from linearity is more pronounced for the solutions comprising lower oxygen concentrations (curve 3). Taken together, these observations provide, for the first time, compelling indirect evidence regarding the O2 participation in forming the purple chromogen. The curves obtained by using concentrated and diluted sulphuric acid (in this last case, in oxygenand nitrogen-saturated solutions), along with that obtained by using HCl and H2O2, are presented in Fig. 2. The adopted hydrogen peroxide concentration was found to be sufficient to yield the oxygen needed to promote the oxidation step of the reaction and to avoid the oxidation of chromotropic acid to the corresponding quinone and/or formaldehyde to formic acid. Very good analytical results were achieved when concentrated HCl was used in combination with hydrogen peroxide (see curve 2, Fig. 2). Based mainly on this observation, a modified CA method for the determination of formaldehyde was proposed in which the hazardous H2SO4 is replaced by a mixture of hydrochloric acid and hydrogen peroxide. This method proved to be equivalent in color developing time and temperature, simplicity, inexpensiveness and reproducibility */with only about 5% loss in sensitivity */as compared with widely accepted CA procedures [4,5,7]. The slope of that line leads to an apparent molar absorptivity of (1.719/0.02) /104 mol l1 cm 1 for the chromogen, which is close to that reported for the reaction developed with concentrated H2SO4,
175
i.e. 1.8 /104 mol l 1 cm 1 [7] and (1.839/0.03) / 104 mol l1 cm 1 (this laboratory). In principle, the O2 consumption could be directly monitored in the solutions where the analytical reaction is carried out. These solutions, however, contain large excess (100 /600 fold) of CA relative to microgram amounts of formaldehyde (see curve 2, Fig. 1), thereby precluding the use of even the most sensitive O2 sensors. An experiment was, therefore, carried out with a much larger formaldehyde amount (0.032 mmol), where the same CA and HCl concentrations were maintained, permitting direct measurement of O2 concentration during the reaction course (before and after heating) by employing a waterproof oxygen sensor, mod HI 9142, from Hanna Instruments, Germany. The results obtained can be seen in the Table 1. The consumption of O2 in the reaction tube was greater than that observed in the blank tube confirming that, even though there is an oxygen loss due to heating, oxygen is consumed in the chemical reaction. This is in keeping with what was observed in the spectrophotometric measurements, i.e. the oxygen participates in the reaction when it occurs in a concentrated hydrochloric acid medium. In these experimental conditions, consumption of O2 was, therefore, undoubtedly confirmed.
4. Conclusion The results of the present work provide a significant contribution regarding the oxidation step of the reaction between formaldehyde and chromotropic acid in a strongly acidic media. Feigl’s hypothesis [9] was confirmed for the first
Table 1 Determination of oxygen consumption Tube
([O2]before
Blank Reaction
12.29/0.3 16.19/0.2
a
heating/[O2]after heating)
Triplicate determinations.
(mg l 1)a
176
E. Fagnani et al. / Talanta 60 (2003) 171 /176
time, namely that for the reaction in concentrated sulphuric acid this acid acts as oxidizing agent (step 2, Scheme 1); furthermore, the reaction does not depend on the concentration of dissolved oxygen. However, when concentrated hydrochloric- and phosphoric acids or diluted sulphuric acid (9.4 mol l 1) are used, the participation of dissolved oxygen is crucial, as the results are significantly dependent on its concentration. Recognition of the important role of oxygen in the reaction promoted the development of the proposed analytical procedure, in which the use of concentrated H2SO4 was eliminated and replaced by a mixture of HCl and H2O2. Preliminary experiments have shown that the utilization of concentrated phosphoric acid and hydrogen peroxide can also be successfully carried out, with changes in the procedure and appropriate heating. Investigations along these lines are currently in progress.
Acknowledgements We would like to thank FAPESP, CNPq and CAPES Foundations (Brazil), for financial support.
References [1] E. Sawicki, T.R. Hauser, S. McPherson, Anal. Chem. 34 (1962) 1460. [2] A.A. Cardoso, E.A. Pereira, Anais Assoc. Bras. Quı´m. 48 (1999) 63. [3] K.A. Sakiara, L. Pezza, C.B. Melios, H.R. Pezza, M. de Moraes, Il Farmaco 54 (1999) 629 (References therein). [4] NIOSH Manual of Analytical Methods, second ed., vol. 1, US Department Health, Education, and Welfare, Publ. (NIOSH) 77-157-A, 1977. [5] NIOSH Manual of Analytical Methods, fourth ed., US Department Health, Education, and Welfare, method 3500(2), 1994. [6] C.E. Bricker, W.A. Vail, Anal. Chem. 22 (1950) 720. [7] P.E. Georghiou, C.K. Ho, Can. J. Chem. 67 (1989) 871. [8] A.D. Pickard, E.R. Clark, Talanta 31 (1984) 763. [9] F. Feigl, Spot Tests in Organic Analysis, seventh ed., Elsevier, Amsterdam, 1966, pp. 434, 435. [10] E. Jungreis, Spot Test Analysis, second ed., Wiley, New York, 1997, pp.125, 126. [11] M. Hudlicky´, Oxidations in Organic Chemistry, ACS Monograph 186, Am. Chem. Soc., Washington, DC, 1990, p. 20. [12] Association of Official Analytical Chemists, Official methods of Analysis of AOAC international, vol. 2, 16th ed, AOAC International, Arlington, 1995, p. 5. [13] Association of Official Analytical Chemists, Official methods of Analysis of AOAC international, vol. 2, 16th ed, AOAC International, Arlington, 1995, p. 99. [14] O.A. Ohlweiler, Quı´mica Analı´tica Quantitativa, vol. 2, third ed, LTC-Livros Te´cnicos e Cientı´ficos Editora SA, Rio de Janeiro, 1982, p. 190.
Chromotropic acid formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid /
E. Fagnani, C.B. Melios, L. Pezza, H.R. Pezza * Instituto de Quı´mica-UNESP, P.O. Box 355, CEP 14801-970 Araraquara, SP, Brazil Received 4 November 2002; received in revised form 3 February 2003; accepted 3 February 2003
Abstract The procedure for formaldehyde analysis recommended by the National Institute for Occupational Safety and Health (NIOSH) is the Chromotropic acid spectrophotometric method, which is the one that uses concentrated sulphuric acid. In the present study the oxidation step associated with the aforementioned method for formaldehyde determination was investigated. Experimental evidence has been obtained indicating that when concentrated H2SO4 (18 mol l 1) is used (as in the NIOSH procedure) that acid is the oxidizing agent. On the other hand, oxidation through dissolved oxygen takes place when concentrated H2SO4 is replaced by concentrated hydrochloric (12 mol l1) and phosphoric (14.7 mol l 1) acids as well as by diluted H2SO4 (9.4 mol l 1). Based on investigations concerning the oxidation step, a modified procedure was devised, in which the use of the potentially hazardous and corrosive concentrated H2SO4 was eliminated and advantageously replaced by a less harmful mixture of HCl and H2O2. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Formaldehyde; Chromotropic acid; Spectrophotometry; Dissolved oxygen
1. Introduction Formaldehyde (CH2O, methanal, formic aldehyde, oxomethane) is uniquely important because of its widespread use and toxicity and it is recognized as one of the most important air pollutants. Due to large volume production and usage of formaldehyde and its possible exposurerelated health effects, much concern has arisen
* Corresponding author. Fax: /55-16-222-7932. E-mail address: [email protected] (H.R. Pezza).
over the sensitivity and accuracy of analytical methodology for this compound. Small amounts of formaldehyde and formaldehyde-releasing compounds are commonly analyzed by spectrophotometric methods [1 /3] and, one of these, the chromotropic acid (CA) method, especially through its P&CAM 125, P&CAM 235 [4] and 3500(2) [5] versions, was established as an international reference method and, despite the advent of more sophisticated techniques, it is still widely used because it is simple, sensitive, inexpensive and very selective. The major drawback presented by the CA method has been the use of
0039-9140/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-9140(03)00121-8
172
E. Fagnani et al. / Talanta 60 (2003) 171 /176
hot concentrated sulphuric acid, which is potentially hazardous and corrosive. Attempts to replace the aforementioned acid by less hazardous ones, e.g. glacial acetic, concentrated hydrochloric and phosphoric acids, were unsuccessful. It was reported, without details, that the reaction’s sensitivity was greatly reduced as compared with that observed by employing concentrated H2SO4 [6,7]. In order to reduce the hazard brought about by concentrated H2SO4, the use of a 50% (v/v) solution of that acid has been recommended [8], but, again, significant loss of sensitivity was encountered by adopting this procedure, along with other drawbacks [8]. The chemistry of the color reaction between CA and formaldehyde, in strongly acidic media, is not yet known with certainty. The most often-quoted reaction path [7,9,10] involves a two step process, as shown in Scheme 1, where (I) would be responsible for light absorption at lmax /570/ 580 nm [7,9,10]. According to Feigl [9], H2SO4 participates in both steps as dehydrant and, in the last one, also as oxidant, being reduced to sulfurous acid. It is
perhaps worth noting that H2SO4 rarely acts as an oxidizing agent [11]. An alternative structure proposed for the chromogen arising from the reaction of CA and formaldehyde is (II) [7,8]:
The nature of this chromogen has never been unambiguously proven but evidence has been obtained [7] using NMR techniques and calibration line studies to support the hypothesis that the chromogen formed in the analytical procedure has a mono-cationic dibenzoxanthylium structure (II) and not the p-quinoidal one (I) that is commonly cited. A likely mechanism for the reaction is presented [7], involving a final oxidation step.
Scheme 1.
E. Fagnani et al. / Talanta 60 (2003) 171 /176
However, the oxidizing agent is not mentioned [7]. Finally, these authors state that concentrated H2SO4 appears to be the optimal acid to use and that no simple modification would seem feasible in order to avoid the use of concentrated H2SO4 in the analytical method [7]. In this paper, investigations concerning the oxidation step associated with the CA spectrophotometric method for formaldehyde determination were carried out. Based on these investigations, a modified procedure was devised, in which the use of concentrated sulphuric acid was eliminated and advantageously replaced by a mixture of HCl and H2O2, without significant loss of sensitivity.
2. Experimental 2.1. Apparatus A Hewlett Packard Model HP8453 spectrophotometer with 1 cm matched silica cells was used for all absorbance measurements. Micropipettes Brand and Eppendorf were used to measure the smaller volumes in the experiment. A fast oxygen detection system from Hanna Instruments mod, HI 9142 was used to study the oxygen consumption in the reactional medium. A gas meter GF 2200 HR, from Gilmont Instruments was used to monitor the flux of gases in the saturation of solutions. All experiments were performed in a thermostated room (25.09/0.1 8C). 2.2. Reagents . All reagents utilized were of analytical grade (Carlo Erba, Merck or Mallinckrodt Co.). For the preparation of the solutions and samples, deionized water (Milli-Q plus) and grade ‘A’ glassware were used throughout. Nitrogen was used to deaerate solutions where required. The solutions were flushed with pure N2 until the O2 concentration was 4.1 /105 mol l1 or less, as determined by an official method [12]. In many experiments, air- and oxygen-saturated solutions were employed for which the deter-
.
.
.
.
173
mined oxygen concentrations [12] were 2.3 / 10 4 and 8.6 /104 mol l1, respectively. Formaldehyde stock standard solution, 1000 mg l 1, prepared by appropriate dilution of commercially available analytical-reagent grade formaldehyde solution (37%, Mallinckrodt Co.), standardized by a AOAC method [13]. The standard formaldehyde solutions used for constructing the calibration graph were freshly prepared by appropriate dilution of the stock solution with water. Chromotropic acid (disodium salt dihydrate, C10H6O8S2Na2 ×/ 2H2O, Merck): a 5% (m/v) aqueous solution was freshly prepared. Hydrogen peroxide (2.5 /10 2 mol l1): prepared from perhidrol 30% by convenient dilution and standardized as described in the literature [14]. Sulphuric acid solution: (50% v/v; 9.42 mol l 1), prepared in the usual way, from the concentrated acid (96%).
2.3. Recommended procedure 2.3.1. Calibration curves 2.3.1.1. NIOSH procedure. The basic National Institute for Occupational Safety and Health (NIOSH) procedure was followed with modifications as described by Georghiou et al. [7]. Curves with concentrated sulphuric, phosphoric and hydrochloric acids and diluted sulphuric acid were made under oxygen-, air- and nitrogen saturated solutions. Calibration graphs are prepared by plotting absorbance against formaldehyde concentration for each acid. 2.3.1.2. Proposed method using hydrochloric acid and hydrogen peroxide. A calibration curve is prepared as follows: transfer 630 ml of formaldehyde working standard solution (comprising 0.80 / 4.80 mg of formaldehyde) into 25 ml glass tubes, add 300 ml of 5% CA solution, 70 ml of 2.5 /102 mol l 1 H2O2 and 4.00 ml of 12 mol l 1 HCl (under stirring). The tubes are sealed with PTFE tape and heated for 1 h in a steam bath (100 8C). Afterwards, they are cooled at 25 8C. The absor-
174
E. Fagnani et al. / Talanta 60 (2003) 171 /176
bances are recorded at 575 nm (b/1 cm) against the reagent blank.
The absorption spectra of solutions obtained by using concentrated sulphuric or hydrochloric acids are superimposable within the 400/700 nm range, suggesting that the same chromogen is produced in both procedures. The results obtained for the calibration curves carried out with concentrated sulphuric, hydro-
chloric and phosphoric acids, varying the oxygen concentrations, are shown in Fig. 1. It was found that the reaction of CA with formaldehyde, developed as recommended by Georghiou et al. [7], is not dependent on the concentration of dissolved oxygen, as shown in curve 1, Fig. 1, supporting Feigl’s proposal [9] concerning the oxidation step promoted by concentrated (18 mol l 1) H2SO4. However, if that acid is replaced by concentrated hydrochloric (12 mol l 1) or phosphoric (14.7 mol l 1) acids, the results are significantly dependent on the concentration of dissolved oxygen, in addition to the already reported drop in sensitivity [6,7], as shown
Fig. 1. Dependence of oxygen concentration and nature of the concentrated inorganic acid on the CA /formaldehyde reaction. Before heating, the reaction solutions were saturated with O2 ([O2]/8.6/10 4 mol l 1), air ([O2]/2.3 /10 4 mol l 1) or N2 ([O2]/4.1 /10 5 mol l 1). For all solutions, the color reaction was developed as described in [7]. Curve 1: H2SO4 (18 mol l 1)/O2 (j), H2SO4 (18 mol l 1)/air ( /), H2SO4 (18 mol l 1)/N2 (k) (r /0.999; pooled points); Curve 2: HCl (12 mol l 1)/O2; Curve 3: HCl (12 mol l 1)/N2; Curve 4: H3PO4 (14.7 mol l 1)/O2; Curve 5: H3PO4 (14.7 mol l 1)/N2 (r/ 0.998).
Fig. 2. Dependence of oxygen concentration and nature of the concentrated inorganic acid on the CA /formaldehyde reaction. Before heating, the solutions comprising H2SO4 were saturated with O2, air or N2 as described in Fig. 1, and the color reaction was developed as recommended in [7]. For solutions comprising HCl, the color reaction was developed as described in the text and freshly distilled/deionized water was used throughout. Curve 1: H2SO4 (18 mol l 1)/O2 (r/0.999); Curve 2: HCl (12 mol l 1)/H2O2 (r/0.999); Curve 3: H2SO4 (9.4 mol l 1)/O2; Curve 4: H2SO4 (9.4 mol l 1)/N2.
3. Results and discussion
E. Fagnani et al. / Talanta 60 (2003) 171 /176
in Fig. 1, curves 2/5. The same is true if lower H2SO4 concentrations are used (e.g. 9.4 mol l1) cf. curves 3 and 4 of Fig. 2. These facts could explain the lack of reproducibility of the measurements and consequently the non linearity of the calibration curves reported in the literature [8] when 9.4 mol l1 H2SO4 is utilized, which would probably be associated with the lack of control over oxygenation conditions. No color at all appears in the absence of oxygen (e.g. in sodium dithionite rich solutions). Curves 2 and 3 of Fig. 1 show a departure from linearity for a formaldehyde concentration higher than approximately 2.4 mg l 1 and a drop of analytical sensitivity with decreasing of dissolved oxygen content in the reaction medium comprising HCl. Furthermore, departure from linearity is more pronounced for the solutions comprising lower oxygen concentrations (curve 3). Taken together, these observations provide, for the first time, compelling indirect evidence regarding the O2 participation in forming the purple chromogen. The curves obtained by using concentrated and diluted sulphuric acid (in this last case, in oxygenand nitrogen-saturated solutions), along with that obtained by using HCl and H2O2, are presented in Fig. 2. The adopted hydrogen peroxide concentration was found to be sufficient to yield the oxygen needed to promote the oxidation step of the reaction and to avoid the oxidation of chromotropic acid to the corresponding quinone and/or formaldehyde to formic acid. Very good analytical results were achieved when concentrated HCl was used in combination with hydrogen peroxide (see curve 2, Fig. 2). Based mainly on this observation, a modified CA method for the determination of formaldehyde was proposed in which the hazardous H2SO4 is replaced by a mixture of hydrochloric acid and hydrogen peroxide. This method proved to be equivalent in color developing time and temperature, simplicity, inexpensiveness and reproducibility */with only about 5% loss in sensitivity */as compared with widely accepted CA procedures [4,5,7]. The slope of that line leads to an apparent molar absorptivity of (1.719/0.02) /104 mol l1 cm 1 for the chromogen, which is close to that reported for the reaction developed with concentrated H2SO4,
175
i.e. 1.8 /104 mol l 1 cm 1 [7] and (1.839/0.03) / 104 mol l1 cm 1 (this laboratory). In principle, the O2 consumption could be directly monitored in the solutions where the analytical reaction is carried out. These solutions, however, contain large excess (100 /600 fold) of CA relative to microgram amounts of formaldehyde (see curve 2, Fig. 1), thereby precluding the use of even the most sensitive O2 sensors. An experiment was, therefore, carried out with a much larger formaldehyde amount (0.032 mmol), where the same CA and HCl concentrations were maintained, permitting direct measurement of O2 concentration during the reaction course (before and after heating) by employing a waterproof oxygen sensor, mod HI 9142, from Hanna Instruments, Germany. The results obtained can be seen in the Table 1. The consumption of O2 in the reaction tube was greater than that observed in the blank tube confirming that, even though there is an oxygen loss due to heating, oxygen is consumed in the chemical reaction. This is in keeping with what was observed in the spectrophotometric measurements, i.e. the oxygen participates in the reaction when it occurs in a concentrated hydrochloric acid medium. In these experimental conditions, consumption of O2 was, therefore, undoubtedly confirmed.
4. Conclusion The results of the present work provide a significant contribution regarding the oxidation step of the reaction between formaldehyde and chromotropic acid in a strongly acidic media. Feigl’s hypothesis [9] was confirmed for the first
Table 1 Determination of oxygen consumption Tube
([O2]before
Blank Reaction
12.29/0.3 16.19/0.2
a
heating/[O2]after heating)
Triplicate determinations.
(mg l 1)a
176
E. Fagnani et al. / Talanta 60 (2003) 171 /176
time, namely that for the reaction in concentrated sulphuric acid this acid acts as oxidizing agent (step 2, Scheme 1); furthermore, the reaction does not depend on the concentration of dissolved oxygen. However, when concentrated hydrochloric- and phosphoric acids or diluted sulphuric acid (9.4 mol l 1) are used, the participation of dissolved oxygen is crucial, as the results are significantly dependent on its concentration. Recognition of the important role of oxygen in the reaction promoted the development of the proposed analytical procedure, in which the use of concentrated H2SO4 was eliminated and replaced by a mixture of HCl and H2O2. Preliminary experiments have shown that the utilization of concentrated phosphoric acid and hydrogen peroxide can also be successfully carried out, with changes in the procedure and appropriate heating. Investigations along these lines are currently in progress.
Acknowledgements We would like to thank FAPESP, CNPq and CAPES Foundations (Brazil), for financial support.
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