390
Anal. Chsm. 1983, 55,390-391
using the CP technique, provided quantitative spectral data a long measurement time was needed* The fa value was 0.70 when a pulse repetition time of 30 s was used. In conclusion, the maximum fa value obtained a t various contact times in the CP/MAS experiments should be closest to the real value. Thus, the optimum values for contact time and pulse repetition time must be determined for each sample to obtain the most reliable measurement of fa. LITERATURE CITED Pines, A.; Gibby, M. G.; Waugh, J. S. J. Cbem. Pbys. 1973, 5 9 , 569. VanderHart, D. L.; Retcofsky, H. L. Fuel 1976, 55, 202. Bartuska, V. J.; Maciei, G. E.; Schaefer, J.; Stejskai, E. 0. Fuel 1977, 56. 354. Retcofsky, H. L.; VanderHart, D. L. Fuel 1978, 5 7 , 421. Resing, H. A.; Garroway, A. N.; Haziett, R. N. Fuel 1978, 5 7 , 450. Maciei, G. E.; Bartuska, V. J.; Miknis, F. P. Fuel 1978, 57, 505. Ziim, K. W.; Pugmire, R. J.; Grant, D. M.; Wood, R. E.: Wiser, W. H. Fuel 1979, 58, 11. Maciei, G. E.; Bartuska, V. J.; Miknis, F. P. Fuel 1979, 58, 391. Ziim, K. W.; Pugmire, R. J.; Larter, S. R.; Aiian, J.; Grant, D. M. Fuel 1981, 60, 717.
(IO) Yoshida, T.; Nakata, Y.; Yoshida, R.; Ueda, S.;Kanda, N.; Maekawa,
Y. Fuel 1982, 61, 824. (11) Yoshida, T.; Narita, H.; Yokoyama, S.;Yoshida, R.; Maekawa, Y., submitted for publication in Fuel. (12) Johnson, L. R. F.; Jankowski, W. C. "Carbon-I3 NMR Spectra"; Wiiey: New York, 1972. (13) Schaefer, J.; Stejskai, E. 0.; Buchdahi, R. Macromolecules 1977, 70, 384.
Tadashi Yoshida* Yosuke Maekawa Government Industrial Development Laboratory, Hokkaido 2-17 Tsukisamu-Higashi, Toyohira sapporo 061-01, Japan Teruaki Fujito JEOL Ltd. 1418 Nakagami, Akishima Tokyo 196, Japan RECEIVED for review June 7,1982. Accepted October 21,1982.
Reagent Stability in the Modified Pararosaniline Method for the Determination of Formaldehyde Sir: The selectivity and sensitivity of the pararosaniline method make it the preferred procedure for determining the formaldehyde content of dilute aqueous solutions or of the atmosphere ( I ) . However, when performed in the usual way the method suffers from the significant drawback that a poisonous tetrachloromercurate (TCM) solution has to be used to stabilize the sulfite reagent ( 2 , 3 ) .References have recently appeared in the literature concerning a modified pararosaniline method without mercury ions which is used to determine both sulfur dioxide and formaldehyde in air. When determining sulfur dioxide in the atmosphere, formaldehyde is used to stabilize the sample solution ( 4 , 5). When determining formaldehyde, however, no stabilizer is added to the sulfite solution ( I ) . This correspondence describes a study of the stability of sulfite reagents in pararosaniline methods. EXPERIMENTAL SECTION Reagents. The concentrations of the various components of the reagents were chosen in such a way that the final concentrations in the solutions used for measurement would always agree with those of Lahmann and Jander (2). Except for the concentration of the hydrochloric acid, these concentrationsdo not differ from those reported by Miksch et a1 ( I ) . The tetrachloromercurate-sulfite (TCMS) solution was made up by dissolving 100 mg of sodium sulfite in 100 mL of TCM solution. The TCM solution consisted of 5.8 g of sodium chloride and 13.6 g of mercuric chloride dissolved in 1L of water. The pararosaniline (P) solution consisted of 160 mg of pararosaniline (Merck) in 100 mL of 2.88 M hydrochloric acid solution; this reagent will keep for several months (3). The sulfite (S) solution consisted of 200 mg of sodium sulfite heptahydrate (NazS0,.7H20)dissolved in 100 mL of water. The combined TCMS-pararosaniline (TCMSP) reagent was prepared by mixing equal volumes of TCMS solution and P solution. The sulfite-pararosaniline (SP)solution, Le., the combined reagent without the mercury ions, consisted of 200 mg of sodium sulfite heptahydrate and 160 mg of pararosaniline in 200 mL of 1.44 M hydrochloric acid solution. The standard formaldehyde solutions were always freshly prepared from a 37 wt % formaldehyde solution (Baker) in a 0.005 M hydrochloric acid solution. The solution strength was determined by a potentio-
metric sulfite method (1, 6) or by an iodometric method (6). Procedure. The procedure of Lahmann and Jander (2)was followed. For the analysis 2 mL of TCMS or S with 2 mL of P or 4 mL of the combined reagent TCMSP, or 4 mL of SP, was added to 20 mL of formaldehyde solution. Absorbance was measured at a wavelength of 578 nm and an optical path of 1 cm (Varian 634). The sensitivity, expressed as absorbance units (AU) per microgram of CH20per milliliter of solution under investigation, was calculated from at least three measurement points for different formaldehyde concentrations (at 0.5,1.0, and 1.5 AU) and from one blank measurement (0.09 AU). RESULTS AND DISCUSSION By application of the method described by Lahmann and Jander ( 2 ) with freshly prepared TCMS and P reagents (TCMS-P method) over a period of 20 months (34 measurements) a mean sensitivity of 0.472 AU mL bg-l with a relative standard deviation, V, of 2% and a mean correlation coefficient, r, of 0.999 94 for the calibration curve was found. A precipitate appears in the TCMS reagent after 24 h (3). The sensitivity of the formaldehyde determination with the filtered reagent deteriorates slowly (2% over 48 h). This agrees with the slow decrease in the sulfite concentration which Doklddalovii and Stankova (7) confirmed in this reagent, in spite of the presence of the [HgCl2SO3I2-complex. If the formaldehyde determination is carried out with an S and a P solution as reagents, erroneous results may be obtained. With a freshly prepared S solution, the same sensitivity was measured as with the TCMS-P method (mean value of three measurements: 0.468 AU mL kg-l, V = 3%, r = 0.999 90). The reagents were added to the analysis solution in immediate succession; no influence from the order of addition of the reagents was found. However, when the S solution was added first and the P solution only after 15 min, it appeared that the sensitivity had decreased by about 10%. Also, the relation between absorbance and formaldehyde concentration is not linear any more in that case. This phenomenon is due to the slow formation of hydroxymethanesulfonic acid in the weakly acid solution after addition of the S reagent (5).
0003-2700/83/0355-0390$01.50/00 1983 American Chemical Society
Anal. Chem. 1983, 55, 391-393
Beside this potential source of errors, deviations may result from the very rapid decrease of the sensitivity of the formaldehyde determination which occurs when the S solution is used for a longer period: after 4 days the absorbance does not increase any more upon addition to a formaldehyde solution together with a P reagent. This is due to the decrease of the sulfite concentration owing to oxidation to sulfate in the weakly basic S solution (8). Lyles et al. (3) found that when separate TCMS and P reagents are used, better results are obtained in the determination of formaldehyde concentrations than when using TCMSP combined reagent. In this strongly acidic combined reagent the [HgC12S03]2-complex is not stable. Though no oxidation of the sulfite ion occurs, its concentration decreases by liberation of sulfur dioxide gas. Since in an acidic medium the tetrachloromercurate consequently has no stabilizing effect, it was justified to expect that the stability and therefore the sensitivity pattern in the formaldehyde determination would be the same for the TCMSP solution as for the S P solution. This indeed proved to be the case. When a freshly prepared SP combined reagent is used, the same sensitivity is found as with the TCMS-P method (mean value from five measurements over a period of 2 months: 0.480 AU mL pg-l, V = 2%, r = 0.999 96). However, after a period of 6 h the sensitivity of formaldehyde determination with this reagent has decreased by 2%, after 1day by lo%, and after 4 days by 30%Thus, whenever it is undesirable to use the environmentally harmful tetrachloromercurate reagent in the determination of formaldehyde by the pararosaniline method, a freshly prepared sulfite-pararosaniline combined reagent is the best alternative. ACKMO WLEDGMENT The authors are grateful to E. Weijers, Analytical De-
39 1
partment, Research and Patents Division, DSM, for performing a number of experiments. Registry No. P, 569-61-9; Na2S03,7757-83-7;TCM, 1402434-1; Hg, 7439-97-6; Formaldehyde, 50-00-0. LITERATURE C I T E D (1) Miksch, Robert R.; Anthon, Douglas W.; Fanning, Leah 2.; Hollowell, Craig 0.; Revzan, Kenneth: Gianvllle, Jacqueline Anal. Chem. 1981, 53, 2118-2123. (2) Lahmann, E.; Jander, K. GesundJng. 1968, 89, 18-21. (3) Lyles, G. R.; Dowling, F. B.; Blanchard, V. J. J. Air. Pollut. Control Assoc. 1965, 75, 106-108. (4) Dasgupta, Purnendu K.; DeCesare, Kymron; Ullrey, James C. Anal. Chem. 1980, 52,1912-1922. (5) Dasgupta, Purnendu K. Anal. Chem. 1981, 53,2084-2087. (6) Walker, J. F. "Formaldehyde", 3rd ed.; R. E. Krieger Publishing Co.: Huntington, NY, 1975. (7) DokiBdaiovB, J.; Stankova, 0. Mlkrochim. Acta 1965, 4, 725-728. (8) Reinders, W.; Vies, S. I . Recl. Trav. Chlm. Pays-Bas 1925, 44, 249-268.
Arnold T. J. M. Kuijpers* Analytical Department Research and Patents Division, DSM Post Box 18 6160 MD Geleen, The Netherlands Jos Neele
Laboratory of Environmental Chemistry National Institute of Public Health Post Box 1 3720 BA Bilthoven, The Netherlands
RECEIVED for review May 24, 1982. Accepted September 20, 1982.
Low-Temperature Filter Paper Phosphorescence Sir: During an investigation of X-ray excited low-temperature (XLTP) and X-ray excited room-temperature (XRTP) luminescence of drugs (I), it was noticed that a dilute solution of p-aminobenzoic acid was enhanced (compared to room temperature measurements) by a factor of 10 when the sample was cooled to 90 K. In the case of theophylline with no heavy atom present, a luminescence signal was seen for XLTP (at 90 K) when no luminescence signal could be seen at room temperature unless a heavy atom perturber was present. Because of the considerably greater detection power of UV excited phosphorescence compared to X-ray excited phosphorescence ( I ) , w e felt an evaluation of UV-excited low-temperature filter paper phosphorescence (LTFpP) would be interesting and a comparison with UV-excited room-temperature filter paper phosphorescence (RTfpP) was worthwhile, especially because of the recent research into the analytical uses of RTfpP (2-6). It should be noted that Miller et al. (7,8)have described a thin-layer phosphorime!ter for scanning thin-layer chromatographic plates at 77 IK:. However, in their work (8), the sample (5 pL) was separated by TLC, the TLC plate was dried 0003-2700/83/0355-039 1$01.50/0
Table I. Comparison of the Signal Enhancement Xatios and the LOD Ratios of RTFpP and LTFpP concn, ILTFpPI LODRTFpPL compound /dmL I R T F p P LoDLTFpP privine HCI 200 9.4 10 6-methylmer10 84 8.0 captopurine L-tryptophane 1 2.4 1.7 chloroquine 2 12.5 6.7 diphosphate p-aminobenzoic 1 21.3 6.0 acid a The luminescence intensity was measured at the emission maxima for each compound. Signals given in this table have been corrected for background emission. The values for this ratio were taken from Table 11. and wrapped around the sample holder drum, the plate was sprayed with a solvent, like ethanol, until it was wet, and the drum was inserted into its compartment, filled with liquid 0 1983 American Chemlcal Society
