Table I. Fluorescence and Phosphorescence Characteristics of Carbazole and Several n-Alkylcarbazoles Range of linearity PhosphoExcitation Emission of analytical Limit of detection rescence maximum, nm5 maximum, nma curve, decade* (a/mP lifetime,d Species Solvent Fluore Phosf Fluore Phosf Fluor Phos Fluor Phos sec Carbazole 0.001 7.80 EtOH 340 341 360 436 4 4 0.0003 Carbazole 0.001 7.2 360 435 Cyclohexane 340 297 4 4 0.0005 n-Methylcarbazole 0.001 8.4 EtOH 346 336 360 437 4 4 0.ooO8 n-Methylcarbazole 7.5 0.001 Cyclohexane 346 298 360 431 4 4 0.0005 n-Ethylcarbazole 340 369 437 4 4 0.001 0.001 7.8 EtOH 339 n-Ethylcarbazole 0.001 8.1 Cyclohexane 340 298 364 433 4 4 0.0008 2-Methylcarbazole 442 4 4 0.001 0.001 8.1 333 357 EtOH 346 2-Methylcarbazole 332 356 443 4 4 0.001 0.001 7.5 Cyclohexane 346 a Maxima are uncorrected for instrumental characteristics. Accuracy of wavelength setting is f 2 nm. The range of linearity could be considerably greater because the highest measured concentration was still on the linear portion. Limits of detection were taken as those concentrations producing a signal twice the background. Shortest measureable lifetime is 0.5 sec. e Measurements taken at 298 “K. f Measurements taken at 77 “K. Lifetimes are taken as time for phosphorescence to decay to lie of the original signal. All decays were exponential and precise to k0.2 sec.
Procedure. Measurements of spectra, analytical curves, limits of detection, and phosphorescence lifetimes were made according to procedures previously discussed (IO). The clean-up procedures and precautions described by Zweidinger, Sanders, and Winefordner (11)were also followed. The limit of detection was defined as that concentration resulting in a signal of two times the variation in the luminescence background of the solvent.
(10) J. D. Winefordner, W. J. McCarthy, and P. A. St. John,
Phosphorimetry as an Analytical Approach in Biochemistry, Chapter in D. Glick, “Methods of Biochemical Analysis,” Vol. 15, Interscience, New York, N. Y . ,1967. (11) R. A. Zweidinger, L. B. Sanders, and J. D. Winefordner, Anal. Chirn. Acta, 47,558 (1969).
RESULTS
The fluorescence characteristics at 298 OK and phosphorescence characteristics at 77 OK of carbazole, n-methylcarbazole, n-ethylcarbazole, and 2-methylcarbazole in ethanol and in cyclohexane are listed in Table I. N o actual analyses of carbazoles in cigarette smoke were carried out, but because of the low detection limits for the carbazoles, the use of fluorimetry and phosphorimetry for quantitative measurement of n-alkylcarbazoles in a rather small amount of cigarette smoke seems possible.
RECEIVED for review November 16, 1970. Accepted January 21, 1971. Research was carried out as part of a study on the phosphorimetric analysis of drugs in blood and urine, supported by a U. s. Public Health Service Grant (GM-11373-08).
New Spectrophotometric Method for Determination of FormaIdehyde B. W. Bailey and J. M. Rankin DiGision of Laboratories and Research, New York State Department of Health, New Scotland Avenue, Albany, N . Y . 12201
THEMOST COMMON of the many methods which have been devised for the determination of formaldehyde in air are the chromotropic acid method ( I ) , the 6-amino-l-naphthol-3sulfonic acid or J-acid method (Z), and the 3-methyl-2-benzothiazolone hydrazone (MBTH) method (3). All three exhibit excellent sensitivity and are applicable over a wide range of concentrations. The chromotropic acid and MBTH methods are spectrophotometric procedures with a Beer’s law range of 0.24-4.0 pg HCHO/ml and 0.05-0.92 pg HCHO/ml, respectively, in (1) A. P. Altshuller, D. L. Miller, and S. F. Sleva, ANAL.CHEM., 33, 621 (1961). (2) E. Sawicki, T.W. Stanley, and J. Pfaff, Anal. Cltirn. Acta, 28, 156 (1963). (3) E. Sawicki, T. R. Hauser, T.W. Stanley, and W. Elbert, ANAL.
CHEM., 33,93 (1961). 782
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
aqueous solution. The J-acid method is a spectrofluorometric procedure. Its range of applicability is 0.001-0.2 pg HCHO/ ml, making it by far the most sensitive in use for the determination of formaldehyde. All three of these methods, however, lack selectivity (2). The MBTH method, which shows only minimal differentiation between formaldehyde and the higher aliphatic aldehydes, causes the latter to function as a strong positive interference to the determination of formaldehyde. Both the chromotropic acid and the J-acid methods show a negative interference for the higher aliphatic aldehydes and, therefore, produce anomalously low values for the formaldehyde concentration in aldehyde mixtures. The method of formaldehyde determination proposed here is based on the catalytic effect of formaldehyde on the hydrogen peroxide oxidation of p-phenylenediamine. This effect
,200
E 2
Figure 2. Calibration curve for formaldehyde z in the concentration range 0.5-2.5 pg/ml
W 0
z
a
m
V
(L
0
m m
a
300
400
500
60 0
Pathlength = 1.0cm;X = 485 nm; pH = 5.6; p phenylenediamine = 0.01 % w/v; hydrogen peroxide = 1.5%wiv. Measurements were made 20 minutes after the addition of peroxide
“0°
4
.5
h (nm)
2.5
pg HCHO / m l
Figure 1. Spectral study of pphenylenediamine (Curve A ) and Bandrowski’s base (Curve B).
was first described by Woker ( 4 ) in 1914 and was later employed by Feigl and Frank ( 5 ) as a qualitative method for the detection of trace amounts of formaldehyde. The effect of formaldehyde on the oxidation reaction is twofold. First, the product of the reaction in the presence of formaldehyde is a dark complex imine bis(2‘,5’-diaminophenyl)benzoquinone diimine (Bandrowski’s base) rather than the polymeric quinoidal substances usually formed during the oxidation reaction. Second, the reaction proceeds at a much accelerated rate. The overall effect is the rapid formation of a product whose absorbance reflects the amount of formaldehyde originally present in the mixture. The investigations described in this communication show this reaction to be applicable to the quantitative determination of formaldehyde at low concentrations in air with a relatively high degree of selectivity. Results are presented of investigations undertaken to determine the optimum reaction conditions, the range of analytical utility, and the effects of possible interferences. EXPERIMENTAL Instrumentation. The spectrophotometric investigations reported here were performed using a Beckman Model DU spectrophotometer. Reagents. All reagents were analytical grade and were used as received. p-Phenylenediamine-a 1 solution was prepared by dissolving 1 gram p-phenylenediamine dihydrochloride in distilled water and diluting to 100 ml. Hydrogen peroxide-30 H 2 0 2solution was used without further dilution. Formaldehyde-4Ox USP HCHO was diluted as needed. Spectral Study. Absorbance spectra were run for the reaction products obtained when p-phenylenediamine is oxidized with hydrogen peroxide in the presence of formaldehyde and for p-phenylenediamine alone. The spectra (Figure 1) in the wavelength interval 30g600 nm show absorbance maxima at 392 nm for the diamine and at 485 nm for the diimine product (Bandrowski’s base). Reaction Time. A study was made of the catalyzed and uncatalyzed reaction rates by measuring the absorbance as a function of time for both reactions at 485 nm. These measurements showed that the formaldehyde catalyzed reaction proceeds quite rapidly and is essentially complete in 10 minutes. In the absence of formaldehyde, however, the
z
z
(4) G. Woker, Ber., 47, 1024(1914). (5) F. Feigl, “Specific Selective Sensitive Reactions,” Academic
Press, New York, N. Y., 1949.
1.5
reaction was slow and showed no indication of being complete during the 20-minute time interval of the study. pH Dependence. Solutions containing 5 pg HCHO/ml were treated with p-phenylenediamine and hydrogen peroxide, and their pH was adjusted to values in the range 3.3-8.3. Absorbance values were measured against reagent blanks and indicated that the absorbance is quite sensitive to changes in pH and that maximum absorbance is obtained at a pH of about 5.6. A citric acid-phosphate buffer (pH 5.6) was used for the control of pH during the remaining experiments. Effect of p-Phenylenediamine Concentration. A series of solutions containing 5 pg HCHO/ml were treated with pH control buffer, hydrogen peroxide, and varying amounts of p-phenylenediamine, Absorbance measurements were then made against reagent blanks. It was found that a p-phenylenediamine concentration yielding a 20-fold reagent excess with respect to formaldehyde is sufficient to provide maximum absorbance. Effect of Hydrogen Peroxide Concentration. Absorbance measurements were made on solutions containing 5 pg HCHO/ml after treatment with pH control buffer, p-phenylenediamine, and varying amounts of hydrogen peroxide. It was found that maximum absorbance is obtained with a hydrogen peroxide concentration yielding a 3000-fold reagent excess with respect to formaldehyde. RESULTS Analytical Procedure and Calibration Curves. The results of these investigations were used to derive an analytical procedure for the quantitative determination of formaldehyde in aqueous solution. This procedure is outlined in the following method used in the preparation of calibration curves: Varying amounts of a 5 pg/ml solution of formaldehyde were transferred to a series of 100-ml volumetric flasks. The following reagents were added in the order given: 10 ml pH 5.6 buffer; 1 ml 1 w/v p-phenylenediamine solution; 5 ml 3 0 x w/v hydrogen peroxide solution. The flasks were then filled to volume and mixed. Twenty minutes after the addition of peroxide, absorbance measurements were made at 485 nm against reagent blanks. These values were plotted against formaldehyde concentration. The resulting curves (Figures 2 and 3) demonstrate agreement with Beer’s law in the concentration range 0.05-2.5 pg HCHO/ml. Interferences. The investigation of possible interferences was conducted with regard to possible chemical interferences and the problem of selectivity. Since the reaction involves oxidation, oxidizing or reducing agents which exist as atmospheric contaminants resulting from processes similar to those producing formaldehyde were first examined. In this category sulfur dioxide, nitrogen dioxide, and hydrogen sulANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
783
?
Figure 3. Calibration curve for formaldehyde in the concentration range 0.054.25 kg/ml
/ E z
2
.IO = 10.0 cm; X 485nm; pH = 5.6; p - v, phenylenediamine 0.01
Pathlength =
w/v; hydrogen peroxide = 1.5 wjv. Measurements were made 2 minutes after the addition of peroxide .O 5
.I 5 pg H C H O / m l
.25
Table I. Relative Response of Acetaldehyde and Benzaldehyde Formaldehyde, equivalence Aldehyde, 2 ,ug/ml Absorbance wdml Formaldehyde 0.112 Acetaldehyde 0.011 0.2 Benzaldehyde 0.001 0.002 fide were examined. Of these, only sulfur dioxide showed any interference at a 100-fold excess. Further investigations showed that the interference by sulfur dioxide is readily removed by treatment with dilute hydrogen peroxide prior to the determination of formaldehyde. With regard to selectivity, acetaldehyde and benzaldehyde were selected as representative aliphatic and aromatic alde-
hydes and were determined by the procedure outlined above. The results of these determinations, given in Table I, show that although both of these substances produced a positive response, it was significantly below that obtained for formaldehyde. Application to Determination of Formaldehyde in Air. Since many of the sampling methods used in air pollution studies involve the absorption of the contaminating species into an aqueous solution of known volume (6), the analytical method described above can find direct application to the determination of formaldehyde in air. One simply removes a suitable aliquot of the absorbing solution and carries it through the prescribed procedure. The formaldehyde concentration in the analytical solution is obtained by comparing its absorbance to that of standards run simultaneously and is used to determine the total weight of formaldehyde in the absorbing solution. This value, together with the flow rate and time interval of sampling; allows for the calculation of formaldehyde concentration in the air being sampled. CONCLUSIONS
The above results demonstrate that the catalytic effect of formaldehyde on the hydrogen peroxide oxidation of p-phenylenediamine can be used as a sensitive and selective method for the determination of formaldehyde in aqueous solution. Furthermore, they show that the method proposed can be utilized in a simple analytical procedure for the determination of formaldehyde in air. RECEIVED for review October 22, 1970. Accepted January 14, 1971. (6) “Selected Methods for the Measurement of Air Pollutants,” U. S. Dept. of Health, Education, and Welfare, Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio, 1965.
Alternating Current Polarograms with Unusual Phase Angles Robert d e Levie, J o y c e C. Kreuser, and Hector Moreira Department of Chemistry, Georgetown University, Washington, D . C . 20007
FARADAIC ADMITTANCES of unusual sign have been studied before, especially those resulting from a negative charge transfer resistance. Theoretical studies by Gerischer and Mehl ( I ) , Schumann (2), and Smith and Sobel (3) have amply demonstrated that coupled heterogeneous or homogeneous chemical reactions can also result in faradaic admittances of unusual sign. It is in connection with the recent work of Smith and Sobel (3) that we want to report on the Fe(I1)catalyzed reduction of CIOz-. Gierst, Vandenberghen, and Nicholas (4) showed that, in 0.1M NaOH, the following mechanism applies
+ e-
Fe03H2-
Fe02H-
+ OHEl/? 2 -0.96 V
4Fe02H-
US.
SCE (Rl)
+ ClOz- + 2H20 -+4Fe03Hz- + C1k‘
=
1.7 X lo8 1. mole-’ sec-I
+ HzO + 2e- FI Fe + 30HEl/?
(R2)
H. Gerischer and W. Mehl, 2. Elektrochem., 59, 1049 (1955). D. Schumann, J . Electroanal. Chem., 17, 45 (1968). D. E. Smith and H. R. Sobel, ANAL.CHEM., 42, 1018 (1970). L. Gierst, L. Vandenberghen, and E. Nicholas, J . Electroanal. Chem., 12,462 (1966). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
-1.43 V
US.
SCE (R3)
They indicated that the two-electron reduction step R3 is quasireversible in the dc polarographic sense, so that the theoretical expressions of Smith and Sobel (3) would not quite be applicable to the present case. In the potential region of interest, around -1.45 V US. SCE, reaction R1 can be considered to proceed infinitely fast. Furthermore, the concentration of C l 0 ~ -used is SO much larger than that of the sparingly soluble Fe03Hz(-5 x 10-6M) that reaction R2 can be considered to be pseudo first-order. Consequently, the over-all mechanism can be simplified to
.I + I D k
(1) (2) (3) (4)
784
Fe02H-
AI
I.
b + e- -* Az + 2e- e A3 m
(R4)
7 In this scheme, diffusion is denoted by a vertical, doubleheaded arrow, whereas chemical or electrochemical reactions
