ANALYST, AUGUST 1990, VOL. 115 1103 Determination of Formaldehyde Using a Kinetic- Spectrophotometric Method Part II.* Oxidation of N-Methyl-substituted-1,4-phenylenediamines With Hydrogen Peroxidet Nicholaos P. Evmiridis, Nicholaos C. Sadiris and Miltiades 1. Karayannis Laboratory of Analytical Chemistry, Department of Chemistry, University of loannina, loannina, Greece A kinetic - spectrophotometric method is proposed for the determination of HCHO based on its ”catalytic” property to oxidise N,N-dimethylphenylenediamine (DMPDA) by H202. The sensitivity at optimum conditions with this method is 1.3-fold greater than the corresponding oxidation of p-phenylenediamine (PDA). The condensation reaction step leading to the Bandrowski Base is much slower with DMPDA than with the PDA oxidation process.Other analytical parameters remain virtually unchanged and the detection limit lies in the range of a few p.p.m. However, the N,N,N’,N’-tetramethyl-I ,4-phenylenediamine is not oxidised by H202 even in the presence of HCHO. Keywords: Formaldehyde determination; oxidation; N-methyl substituted derivative; kinetic - spectro p ho tom e tric method Aldehydes are considered to be one of the major pollutants present in the air; they are very irritative to the skin, eyes and mucous membranes.’ The principal source of aldehydes in the atmosphere is the partial oxidation of organic matter including fuel. Aldehydes from the chemical industry also pollute the air, especially HCHO which is used as a raw material in many industrial processes. 192 Various methods have been reported for the determination of aliphatic aldehydes in the atmos- phere.3-7 However, the higher volatility and the stability of HCHO in polluted air renders it the main aldehyde com- ponent and, therefore, makes the demand for a selective method of HCHO determination in the atmosphere far more important.The enhanced “catalytic” activity of HCHO in the oxidation of p-phenylenediamine (PDA) with hydrogen peroxide (H202) is well known”.’) and both a spectrophotometric method7 and a kinetic - spectrophotometric method10 based on this property have been proposed for HCHO determina- tion. Both methods are simple and selective for HCHO, but the kinetic method is much quicker and has a detection limit of 20-30 p.p.m. A reaction path has been suggested’” as follows: oxidation oxidation QH2 QH condensation oxidation Q~Hz-QO Q” Q 1 oxidation hydrolysis where: QH2 = p-phenylenediamine; Q = p-phenylenedi- imine; Q“ = benzoquinone; and Q3H2 - Bandrowski Base.The intermediate product QH is coloured and readily soluble in water. This product is considered to be a “Wurster Salt” as it is intensely coloured and is formed from the oxidation of p-phenylenediamine according to the following reaction. 11-13 * For Part I of this series. see Anufyst, 1987, 112. 831. t Presented at the 3rd International Symposium on Kinetics in Analytical Chemistry, Dubrovnik-Cavtat, Yugoslavia, 25-28 September, 1989. t t etc. The rate of formation of the coloured “Wurster Salt” product is a good basis for the determination of formaldehyde.However, the condensation step on the reaction path pro- duces coloured products which interfere and therefore must be avoided by eliminating the condensation step. This can be done by increasing the stability of the diimine compound; p-quinonediimines with alkyl groups substituted on the nitrogen atom are slightly more stable than the unsubstituted compounds14 while the N,N’-diary1 analogues are definitely more stable. Elimination of the condensation step, apart from removing the interfering products, can also increase the sensitivity by decreasing the rate of QH conversion. The sensitivity can also be increased by finding a QH product with a higher molar absorptivity. In this work the catalytic effect of HCHO in the oxidation of N-methyl substituted p-phenylenediamines is presented. This catalytic effect is compared with that of p-phenylenediamine and is followed by changes in pH, concentration of reagent, oxidant, HCHO and temperature. Finally, a more sensitive method for HCHO determination is proposed using N,N- dimethyl-l,4-phenylenediamine as reagent.Experimental Reagents and Standard Solutions All reagents were of analytical-reagent grade, unless specified otherwise, and the water was distilled. Standard solutions of p-phenylenediamine, oxidant, and formaldehyde as well as the preparation of buffer solutions have been described elsewhere. 101104 ANALYST, AUGUST 1990, VOL. 115 N,N-Dimethyl- 1,4-phenylenediamine (DMPDA) solution. A 1% m/v aqueous solution was freshly prepared by dissolving 1 .0000 g N,N-dimethyl-l,4-phenylenediammonium dichloride (Merck) in water and diluting to 100.0 ml.N ,N,N’-N’-Tetramethyl-l,4-phenylenediamine (TMPDA) solution. A 1% m/v aqueous solution was freshly prepared by dissolving 1 .0000 g N,N,N’,N’-tetramethyl-l,4-phenylene- diammonium dichloride (Merck) in water and diluting to 100.0 ml. Working solutions of lower concentrations were prepared by dissolving smaller amounts of the solid reagent salt in water and diluting to 100.0 ml. Apparatus All spectrophotometric absorbance versus time graphs were obtained with a Pye Unicam SP 6-200 spectrophotometer equipped with a thermostated 1.5 cm i.d. cylindrical cell. The rate graphs were recorded on a Linear 1200 recorder. The cylindrical cell was thermostated with a Julabo F40 thermo- stirrer.All absorbance spectra were obtained with a computerised Hitachi 100-80 double-beam UV - visible spectrophotometer. Titration curves were obtained by using a Radiometer ABU 80 autoburette and a Radiometer PHM 83 autocalibrated pH meter equipped with a Radiometer combination pH elec- trode. During the titration procedure, the solution was stirred with a Gallenkamp magnetic stirrer. Methods The absorbance versus time graphs and the absorbance spectra were obtained as described in previous work. 10 The determination of HCHO proposed here follows the procedure described in previous work10 except for the 560-nm wavelength used in this work. Results Titration Curves Titration curves of the reagent hydrochloride salt were recorded and the equivalence points (first and second) were obtained as well as the pK1 and pK2 values of the ammonium species, these are shown in Table 1.Only small differences in the pH values of these titration curve characteristic points are observed between the different reagents. Absorption Spectra The absorption spectra of the reagents are shown in Fig. 1. The corresponding “Wurster Salts” of the reagent compounds are obtained by oxidising the reagents under such conditions that there is negligible formation of other coloured products (i. e., high oxidant concentration, low reagent concentration, acidic pH). Fig. 1 also shows the absorption spectra of the “Wurster Salts.” Absorption spectra were also obtained during the progress of the oxidation reaction by using a quick scanning method and solutions of 0.1% m/v reagent, 500 Table 1.Information deduced from titration curves. Temperature, 35 “C; cSalt, 0.05 M Titration equivalenc points/pH pK values Compound (dihydrochloride salt) First Second pK1 pK2 p-Phen ylenediamine 4.43 8.60 2.73 6.0 N,N-Dimethyl-l,4- N,N,N’ ”Tetrame thyl- phen ylenediamine 4.37 9.30 2.66 6.20 1,4-phenylenediamine 4.37 9.40 2.63 6.30 p.p.m. formaldehyde and 30 or 1.2% m/v Hz02 over an extended range of pH values. From the changes in absorbance of the spectra with the progress of the reaction, it was found that TMPDA shows a slow increase in the “Wurster Salt” characteristic band from 330 to 670 nm. However, after a long period of time the Wurster Salt characteristic bands start to decrease in intensity. In contrast, the PDA and DMPDA reagents, initially show a rapid increase (depending on the pH) in the intensity of the Wurster Salt band, in the visible region, with time, followed by a decrease (significantly quicker than that observed with TMPDA) at longer times but the region from 330 to 400 nm is increasing, even at times where the visible region of the band is decreasing.Influence of Variables on the Initial Rate in the Presence of Formaldehyde The rate of increase of absorbance with pH (Fig. 2), with H202 and reagent concentration (Fig. 3), with temperature (Table 2) and with formaldehyde concentration (Table 3) is investi- gated for the reagents studied in this work and at conditions as shown in the figures and tables. The wavelengths 450,500 and 560 nm shown in Fig. 2 are the maxima of absorption spectra that correspond to PDA, DMPDA and TMPDA, respect- ively.The rate of absorbance increase is higher around pH 8 when TMPDA is the reagent, but for the other two reagents is around pH 6.0 for both H2O2 concentration levels. It is found that there is no dependence of the initial rate of increase of absorbance on the concentration of H202 and HCHO from Fig. 3 and Table 3, respectively, when the reagent is TMPDA. On the contrary, the influence of the concentration of HCHO on the rate of increase of absorbance is high in the range studied when the reagent is either PDA or DMPDA, however, the influence of HzOz concentration is high only up to 6% d v , becoming negligible beyond this value. Finally, from Fig. 3 it can be seen that the dependence of the initial rate of increase of absorbance with the concentration of reagent is almost linear for all reagents studied.The difference in the rate of increase of absorbance between experiments in the presence and absence of HCHO, is given in Table 2 with the conditions as shown. Again no significant 1 .o 0.9 0.8 0.7 a, 0.6 0 C m 0.5 :: a P 0.4 0.3 0.2 0.1 0 270 470 670 hlnm Fig. 1. Absorption spectra of the reagents: A, PDA; B, DMPDA; C, TMPDA; and of their “Wurster Salts”: D, PDA; E, DMPDA; and F, TMPDAANALYS 3.0 - I C .- E 2.0 9 4- I - m .- *.. ‘E 1.0 - 0 , AUGUST 1990, VOL. 115 n- 3 5 7 9 11 PH Fig. 2. Initial rate graphs at various pH values for: 0 and a, PDA at 485 nm; A and A, DMPDA at 500 nm; and 0 and H, TMPDA at 560 nm. Conditions: temperature, 25 “C; cell concentrations of: H202, 3.75% m/v (open symbols) and 0.24% m/v (closed symbols); HCHO, 62.5 p.p.m.; and rcagent, 1.25 X lo-’% m/v Table 2.Difference in initial rate of increase of absorbance between the reaction held in the presence and absence of HCHO versus temperature at 560 nm. in units of AA min-1, pH, 5.7; [reagent], 1% m/v; [H202], 30% m/v Reagent compound Temperature/ “C PDA DMPDA TMPDA 5 0.04 0.02 0.00 15 0.60 1.30 0.00 2s 3.10 4.75 0.40 40 22.0 16.0 0.50 60 99.8 89.7 0.50 Table 3. Effect of HCHO concentration on the initial rates of absorbance Formaldehyde Initial rate/AA min-1 concentration, p.p.m. PDA DMPDA TMPDA 25 0.13 0.15 - 50 0.23 0.2s 0.15 100 0.36 0.45 0.15 250 0.68 0.95 0.15 500 1.23 1.60 0.18 750 1.60 2.05 0.24 1000 1.88 2.55 0.22 change is observed with the change of temperature when the reagent is TMPDA, however, there is a significant change when the reagent is either PDA or DMPDA.Reaction Profiles Fixed wavelength reaction profiles at two H202 concentra- tions (30 and 1.2% m/v) are obtained at various pH values. The reaction profiles obtained at 345 nm are compared to those obtained at the maximum in the absorption spectra in the visible region for each reagent. The comparison reveals that for TMPDA the profiles are almost identical although they may change with pH or H202 concentration; for the PDA and DMPDA reagents at high H202 concentrations, the profile features are not identical and are found at different times during the progress of the reaction, however, at low H202 concentrations the two profiles become almost iden- tical.This observation for the PDA and DMPDA reagents strengthens the previous suggestion, based on the changes in the absorption spectra with time, that the absorbance at 345 nm has contributions from more than one product (Wurster Salt) and that the amount from the other product(s) depends on the pH, and the concentration of H202. 3.0 7 , t .- E q 2.0 I -. 4- - m c .- w .- - 1.0 0 1105 c1-1~0~. % m/v 1.2 6 10 20 30 0.5 1 .o CR, % m/v 0.1 Fig. 3. Initial rates versus reagent concentration (open symbols) and versus oxidant concentration (closed symbols) for: 0, PDA at 485 nm; A and A, DMPDA at 500 nm; and 0 and W, TMPDA at 560 nm. Conditions: temperature. 25 “C; pH, 5.7; cell concentrations of: reagent, 5.0 x lO-3% m/v (closed symbols); H202, 3.75% m/v (open symbols); and HCHO, 62.5 p.p.m.The reaction profiles of PDA and DMPDA are at a maximum at 485 and 500 nm, respectively. The absorbance at the maximum increased with pH up to pH 4.5, however, at a pH of >4.5 the absorbance at the maximum is decreased although the initial rate of Wurster Salt formation is increased. As the maximum is the result of competition of the rate of formation of the Wurster Salt to another product, the above behaviour suggests that the ratio of the two rates decreases with increase of pH beyond a value of 4.5. Discussion From the results obtained it is realised that the behaviour is different for each reagent and is described for each as follows: TMPDA This reagent is oxidised to a QH product, a rather stable “Wurster Salt,” with an intense blue colour.The rate of oxidation is slow and is independent of both H202 and HCHO concentration (Fig. 3 and Table 3). However, the oxidation rate is significant at ca. pH 8.0 (Fig. 2) where the reagent is in the uncharged form. The “Wurster Salt” slowly decomposes to other products which have previously been observed by Boozer and Hammond.15 As the oxidation proceeds indepen- dently of H202 concentration, it is thought that the oxidation process proceeds with the dissolved oxygen, this assumption is strengthened from the observation that the rate is identical with that in the absence of H202. PDA, DMPDA Both the reagents are oxidised by H202 in the presence of HCHO to their “Wurster Salts” both being red though of different tints for each reagent.The rate of “Wurster Salt” formation and their stability depends on the pH value, the rate of formation being at a maximum at pH 6.0 (Fig. 2) where the singly charged species are equal to the uncharged (Table 1). The “Wurster Salt” formation rate is dependent on the H202 (in the presence of HCHO) and HCHO concentrations (Fig. 3, Table 3). The decomposition of the “Wurster Salt” is accompanied by an increase in absorbance at the 350-nm wavelength, the reaction profiles at wavelengths of 345 and 485 nm (500 nm) are also different. At the optimum pH a coloured precipitate is formed which is denser when the reagent is PDA. Furthermore, the reaction profiles in the absence of HCHO are similar to those of TMPDA, but are very different from those obtained in the presence of HCHO.1106 ANALYST, AUGUST 1990, VOL.115 Table 4. Rates of DMPDA oxidation by H202 in the presence of interfering ions and their formaldehyde equivalence (cDM~DA = 0.1%; cH2O2 = 30%; T = 25 “C; pH = 5.7; h = 560 nm) Formaldehyde Concentration, Slope/ equivalence, Ion p.p.m. A min--l p.p.rn. S03’- . . . . . , 5000 0.06 4.5 NO?- . . . . . . SO00 1.00 77.0 NO3- . . . . . . 5000 0.32 24.6 S2- . . . . . . 5000 0.60 46.0 HCHO (standard) 50 0.64 50.0 ~~ Table 5. Rates of DMPDA oxidation by H202 in the presence of aldehydes, and their formaldehyde equivalence (cDMPDA = 0.1% ; C H ~ O ~ = 30%; T = 25 “C; pH = 5.7; h = 560 nm) Formaldehyde Aldehyde Slope/ equivalence, (500p.p.m.) A SKI p.p.m. Formaldehyde . . . . 0.0230 500 Acetaldehyde .. . . 0.0023 50 Benzaldehyde . . . . 0.0017 40 Reaction Mechanism Investigation The oxidation of p-phenylenediamines involves the formation of “Wurster Salts” through the process of one electron transfer according to the reactionls.’h: R 2 N 0 ( J R 2 - R z i e NRz-etc. The oxidant molecule can be a peroxy-radical which is formed conventionally by the action of oxygen on aldehydes or hydrocarbons. The formation of peroxy-radicals has been suggested8 as the oxidant in oxidations of organic compounds by H202 in the presence of HCHO. However, in this investigation of the oxidation of PDA, DMPDA, and TMPDA by H202 in the presence of HCHO, although an intense “catalytic” property of HCHO was found for PDA and DMPDA no “catalytic” property of HCHO was found for TMPDA.This loss of apparent “catalytic” property coincides with the absence of hydrogen atoms attached to the N atoms of the organic molecule. This is an indication that the peroxy- radical formed specifically attacks the hydrogen atoms attached to the nitrogen, while the oxygen diradical can attack other sites on the organic molecule. From the data presented under Results it is seen that: (i) The “Wurster Salt” of TMPDA is stable, it is formed by the action of dissolved oxygen and is only very slowly oxidised; (ii) the “Wurster Salts” of PDA and DMPDA are less stable, they are oxidised to quinone-diimines which are unstable and can undergo condensation, hydrolysis and redox reactions; (iii) the ratio of the rate of formation to the rate of conversion of the relevant “Wurster Salt” increases with pH up to a pH of 4.5 and then decreases, although the rate of formation continues to increase up to pH 6.0. From this evidence it may be suggested that the optimum rate of peroxy-radical formation is obtained at ca.pH 6.0; also the rate of “Wurster Salt” conversion is increased with an increase in pH, either because the concentration of uncharged QH2 species increases with pH, or because the condensation step is catalysed by OH-. Analytical Application From an analytical viewpoint it is observed that the maximum initial rate occurs at ca. pH 6.0 for PDA and DMPDA, and therefore this is the pH value of maximum sensitivity for HCHO determination. Considering the two reagents DMPDA is the most sensitive and therefore the best for the determination of HCHO.The method is found to give linear absorbance versus time graphs. Formaldehyde calibration graphs with the following con- ditions: pH, 5.7; concentration of: reagent, 0.1% m/v; hydrogen peroxide, 30% d v , have been approximated to linear equations and the parameters of intercept a and slope 6, calculated as follows: aDMpDA = 0.16 A min-1 (SD = 0.05), apDA = 0.15 A min-l (SD = 0.05) ~ D M P D A = 2.5 X 10-3 A min-1 p.p.m.-l (SD = 1 x 10-J), bPDA = 1.9 X lO-3A min-1 p.p.m.-l (SD = 1 X lop4) The correlation coefficients for DMPDA and PDA were found to be 0.995 and 0.992, respectively. The method, as shown by the standard deviation of the intercept combined with the value of the slope, shows that the detection limit is ca. 20-30 p.p.m. for both reagents in the absence of interferences.The method is convenient and with the aid of impingersI7 it can be used for analysing samples of polluted air. Oxidising or reducing agents present as atmospheric contaminants are potential interferents. Table 4 shows the effect of the interference from such species. Furthermore, an experiment was carried out to determine the selectivity between alde- hydes. Table 5 shows the results obtained from the selectivity experiment. In this experiment DMPDA (0.1% m/v) was oxidised by H202 (30% m/v) in the presence of 500 p.p.m. of the specific aldehyde at a pH of 5.7 and at a temperature of 25 “C. The results are fairly close to those previously obtained with PDA.10 Conclusion Comparing the DMPDA method with that of PDA we conclude that: (i) at optimum conditions the sensitivity of the method is ca.1.3-fold higher; (ii) the interference from the formation of the Bandrowski Base is much lower; (iii) the detection limit, although lower, is still in the range of a few p.p.m.; and (iv) the other analytical parameters, i.e., selectiv- ity, valid range of calibration graph, interference from other reductants or oxidants involved in atmospheric air, are the same. This investigation indicates that by slightly increasing the stability of the transient quinone-diimine products of the HCHO “catalysed” oxidation reaction of the p-phenylene- diamine derivatives, a considerable increase in the sensitivity of the method is obtained. Therefore, it is reasonable to suggest that compounds that form quinone-diimine products of high stability during similar oxidations can increase the sensitivity of the method to a level such that the detection limit reaches the p. p.b. range. Further investigations are in progress and it is hoped that a more sensitive reagent will be developed in the near future. 1. 2. 3. 4. 5. 6. 7. 8. References Patty, F. A., “Industrial Hygiene and Toxicology,’’ Volume 11, Second Edition, Interscience, New York, 1963. p. 1959. Leithe, W., “The Analysis of Air Pollutants,” Ann Arbor Science Publishers, Ann Arbor, MI, 1973, p. 229. Jacobs, M. B., “The Chemical Analysis of Air Pollutants,” Interscience, New York, 1960, p. 267. Rayner, A. C., and Jephacott. C. M., Anal. Chem., 1961,33, 627. Sawicki, E . , Hauscr, T. R., Stanley, T. W., and Elbert, W., Anal. Clzem., 1961, 33, 93. Hauser, T. R . , and Curnrnins, R. L.. Anal. Chem.. 1964, 36. 679. Bailey, €3. W., and Rankin, J . M., Anal. Chem.. 1971,43,782. Woker, G., Chem. Ber., 1914, 47, 1024.ANALYST, AUGUST 1990, VOL. 115 1107 9. 10. 11. 12. 13. 14. Feigl, F.. Editor “Spot Tests in Organic Analysis,” Seventh Edition, Elsevier, Amsterdam. 1966. Evmiridis, N. P . , and Karayannis, M. I . , Analyst, 1987, 112, 831. Rumpf, P., and Trombe, F., J . Chem. Phys., 1938, 35, 110. Rumpf, P., and Trombe, F., Compt. Rend., 1938, 206, 671. Michaelis, L., Schubert, M. P . , and Granick, S . , J. Am. Chem. Soc., 1939. 61, 1981. Adams. R., andNagarkatti, A. S . , J. Am. Chem. Soc., 1950, 72, 4601. 15. 16. 17. Boozer, C. E.. and Hammond, G. S . , J . Am. Chem. Soc., 1954, 76, 3861. Boozer, C. E., and Hammond, G. S . , J . A m . Chem. Soc., 1955, 77, 3233. Pickard. P., and Clark, E. R., Taluntu, 1984, 31, 763, Nom-Reference 10 is to Part I of this series. Paper 91 052 76 C Received December 12th, 1989 Accepted March 3rd, 1990