?
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, Joyce 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 US. SCE (Rl)
4Fe02H-
+ ClOz- + 2H20 -+4Fe03Hz- + C1k‘
=
1.7 X lo8 1. mole-’ sec-I
+ HzO + 2e- FI Fe + 30HEl/?
(R2)
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 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) H. Gerischer and W. Mehl, 2. Elektrochem., 59, 1049 (1955). (2) D. Schumann, J . Electroanal. Chem., 17, 45 (1968). (3) D. E. Smith and H. R. Sobel, ANAL.CHEM., 42, 1018 (1970). (4) L. Gierst, L. Vandenberghen, and E. Nicholas, J . Electroanal. Chem., 12,462 (1966).
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
30 Y
(ra’) 20
IO
0
-10
Figure 1. A . DC polarogram of 3mM NaCIOz in 0.1M NaOH saturated with Fe(II1) B. Phase-resolved ac polarogram of the same solution, measured with a 10-mV top-to-top 5-Hz sine wave All curves in A and B compensated for solution resistance, and measured at drop age of 5.0 sec, temperature 25°C. Dashed lines: same in 0.1M NaOH. Dotted lines: same in 0.1M NaOH saturated with Fe(II1)
are indicated horizontally. All diffusion coefficients are taken to be equal. After substitution of elf' = 0 into Equations 20 and 21 of ref. (5), and using
where
{
= ( R + Z)/(2wD)’/Z
F {(l G = ((1
K>> 1 (3) the following expressions are obtained for the in-phase (YF’) and quadrature (YF”)components of the faradaic impedance
(5) H. Moreira and R. de Levie, J . Electroanaf. Chem., in press.
+ k2/02)1’2+ k / w ) ‘I2 + - k/w)”2 k2/w2)1/2
(7) (8) (9)
In Equation 6, c’ denotes an average interfacial concentration. The rate constants k , E, and k’ are defined in scheme R4. The other symbols are quite conventional and all have been defined in ref. (5). When reactions R3 are very fast, Equations 1 and 2 reduce to
Y,’
=
y,”
=
FZAC2’ (2wD)”*(3
+ + 1)
- F)
RRr 1) F’Ac~’(2wD)’” (3 - G)
(10)
(11)
which is equivalent to the result given for the reversible case by Smith and Sobel [ref. (3) Equations 18-25]. In Equations 4 and 5, the first terms on the right hand side represent the faradaic admittance in the absence of ClOz-, ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
785
I
1 4 0 Kn
B: z,,
0.8pF
;Q;;
0.6
4t
D: 2 ,
z d
0
0.4
2 3 3
I
I
I
-0.6
-0.4
1
I” I 2 3Mn -012 Figure 2. A . Faradaic admittance calculated from Equations 4 and 5 with RCt= 3.35 MQ, 3 . ~ ~=’ 1~ sec-1’2, and k = 5.1 X lo5sec-I Calculated points at 5,10,and 25 Hz are indicated by solid circles, whereas crosses indicate experimental data at those same frequencies, in 3 mM NaC102 and 0.1M NaOH saturated with Fe(III), at -1.45 V us. SCE -30
-20
I
-10
B. Electrode impedance plane calculated from Equations 4 and 5 with k C d i = 0.38 pF a: R,t = 3.35 MO, 3- u’” = 1 sec-’’2
=
5.1 X
lo5sec-I,
r r
6 : Ret = 1 MO, u’” 3.35 sec-1’2 c : R C t= 335 KQ, 3- ~ 1 =’ ~10 sec-112 d : R C t= 33.5 KQ, wl’* = 100 sec-1’2
C. Corresponding complex capacitance plane representations D. Corresponding faradaic impedance plane representations Decadic logarithms of angular frequencies (w in rad sec-1) are indicated with the curves
whereas the second terms indicate the additional admittance due to the regeneration reaction R2. Comparison of curves in Figure 1 in the presence or absence of (2102- immediately shows that the first terms on the right hand side of Equations 1 and 2 are negligible compared to the second terms. Furthermore, the pseudo first-order rate constant k has a value ( 4 ) of the order of 5 X lo5 sec-1 for 3mM NaC102, whereas the highest frequency used is 25 Hz (w = 157 rad sec-I). Consequently, k/w >> 1, F = (2k/w)I‘2 >> 1 and G = 0. Thus, Equations 1 and 2 reduce to
YF’f=
F +F 4Rct(2t2 + 2 t + 1) - 4Rcd2Y2 + 2t + 1)
6t
(14)
so that 3- can be estimated from the phase angle C#J of the faradaic admittance,
+
1) (15) cotan = -(2{ Such an estimate yields 2< 1 = 1.34 at 5 Hz at -1.45 V us. SCE, which corresponds to rate constants and k‘ of the order of 10-3 cm sec-1, in agreement with the conclusion of Gierst et a/. ( 4 ) that reactions R3 yield a quasi-reversible polarographic wave. Assuming ( 4 ) that k = 5 X lo5 sec-I ’~ and, consequently, for 5mM NaC102,F = ( 2 k / ~ ) is~ known R,,can be estimated from the magnitude of Y,. Figure 2a shows both the experimental and theoretical faradaic admittances, the latter calculated from Equations 1 and 2. Only at frequencies too low to be practicable in ac polar-
+
786
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
ography ( 5 Hz) would the faradaic admittance in the present case approach that of a negative resistance [ref. (3) Figure 11. The usual graphical representations (6) of the observable frequency dependence are shown in Figure 2,b-d. We have mentioned earlier (7) that the present electrochemical system is nonoscillatory-Le., in the absence of additional, external phase-shifting devices-and this can be seen most readily by application of the Llewellyn criterion to the electrode impedance plot (7). All measurements were performed with an instrument described earlier (8) in conjunction with an Ithaco 353 lock-in amplifier. Measurements of Y,” become quite inaccurate at frequencies above 25 Hz, since the double layer capacitance then accounts for more than 90% of the measured quadrature signal. Measurements below 5 Hz are inconvenient with a dropping mercury electrode of conventional drop time. The very narrow frequency range thus available to us does not allow for more than order-of-magnitude estimates of { and R , 1 ,and effectively prevents the accurate determination of ksh or cy. At higher chlorite concentrations, a polarographic maximum develops in the region where YF’ < 0 and interferes with the measurements. The above results clearly demonstrate and quantitatively account for the existence of a faradaic admittance with (6) R. de Levie and L. PospiSil, J . Electroanal. Chem., 22, 277
(1969). (7) R. de Levie, ibid.,25, 257 (1970). (8) R. de Levie and J. C Kreuser, ibid., 21, 221 (1969).
negative in-phase component while the quadrature component is positive. The reverse situation of a negative quadrature component with a positive in-phase faradaic admittance has been reported for the reduction of concentrated Ni(I1) solutions (9). The latter is probably caused by a parallel reaction pathway involving polymeric species, although so far no quantitative interpretation has emerged. In the presence of a negative charge transfer resistance, both in-phase and quadrature components of the faradaic ad(9) R. de Levie and A. A . Husovsky, J. Electroanal. Chem., 20, 181 (1969).
mittance are negative (10). Perhaps such negative values of Y,’ and/or Y,” are, after all, not quite as “unusual” as the title of this communication suggests. RECEIVED for review September 8, 1970. Accepted March 3, 1971. Work supported by the National Science Foundation (Grant G P 8575), the Air Force Office of Scientific Research (OAR, USAF, Grant 68-1344), and the Office of Naval Research. (10) Ibid.,22, 29 (1969).
Gas Chromatographic Analysis of Samples Containing Both Volatile and Nonvolatile Organic Components L. E. Philyaw,’ A. E. Krc, and M. J. O’Neal2 Houston Research Laboratory, Shell Oil Company, P. 0. Box 100, Deer Park, Texas 77536 TEMPERATURE AND PRESSURE PROGRAMMING are widely used to increase the range of applications of gas chromatography (1-3). These techniques make gas chromatography applicable to a wide variety of mixtures containing components with a broad range of boiling points. However, these techniques do not provide a means of direct analysis of samples which contain nonvolatile residue. Thus, in general, gas chromatography is limited in applications to materials like crude oils containing asphalt, various biological mixtures, and mixtures containing inorganic residue like shale and coal. Some indication of the amount of the nonvolatile portion of such samples can be obtained by the use of a known amount of an internal standard ( 2 ) . However, this approach is often not very accurate, and it is frequently impossible to find standards which are not interfered with by the sample components (as in crude oils which contain a continuum of components as a function of boiling point). These considerations led to an investigation of the possible separation and selective determination of nonvolatile organic constituents directly in the gas chromatographic equipment. EXPERIMENTAL
The apparatus developed for the purpose of determining nonvolatile residue during a gas chromatographic analysis is shown in Figure 1. The equipment basically consists of a programmed temperature column with the effluent passing into a furnace containing a copper oxide tube to convert the organic components to carbon dioxide so as to eliminate response factors in thermal conductivity detection (4). The system is arranged so that the column can be backflushed to remove any low volatility material that vaporizes but does not Present address, Celanese Chemical Co., P. 0. Box 58009, Houston, Tex. 77058. * To whom correspondence should be addressed. (1) J. Griffiths, D.
James, and C. Phillips, Analyst, 77, 897 (1952). (2) F. T. Eggertsen, S. Groennings, and J. J. Holst, ANAL. CHEM., 32,904 (1960). (3) A . Zlatkis, D. C. Fenimore, L. S. Ettre, and J. E. Purcell, J . Gas Cliromatogr., 3,75 (1965). (4) M. C. Simmons, L. Taylor, and M. Nager, ANAL. CHEM., 32, 731 (1960).
pass completely through the column and as a means for raising the injection port temperature and introducing oxygen in order to oxidize nonvolatile residue remaining in the injection port. The zone into which the sample is injected can be heated to 700 “C for oxidation of the nonvolatile residue. The outer portion of the injection port is water-cooled to prevent decomposition of the silicone rubber injection port septum. A slipstream of preheated carrier gas (helium) introduced immediately inward from the water jacket prevents back condensation of sample in the cooled area. It is necessary to use an oxygen scrubber (tube packed with copper wire) to remove the excess oxygen during the backflush/ residue-combustion operation so that the excess oxygen does not interfere with the COzpeak due to the residue oxidation. In making an analysis, the injection port furnace is set at a temperature high enough to vaporize the volatile components, but below the temperature at which pyrolysis occurs. In the case of crude oils and similar petroleum fractions, an injection port temperature of about 350 O C is satisfactory. The volatile portion is chromatographed using programmed temperature in the usual manner up to the maximum permissible column temperature (300-350 “C for the heavier silicone column packings). The carrier gas flow through the column, combustion tube furnace, and injection port is reversed by means of the backflush valve. The material backflushed from the column passes through the injection port into the backflush combustion tube furnace where it is converted to carbon dioxide and water and passes into the drier/detector system. The direction of flow through the drier/detector is unaltered from the forward flow conditions. After backflushing the column, the injection port temperature is raised to 700 “C over a period of approximately three minutes to vaporize and/or thermally decompose any heavy residue left in the injection port. This step removes most, but not all, of the nonvolatile residue in samples like crude oil. Any coke or other very heavy material remaining in the injection port is recovered by introducing oxygen into the injection port by mixing it with the helium slipstream. Excess oxygen remaining after oxidation is removed by the copper scrubber which is also located in the combustion furnace. The elution of the nonvolatile residue in three separate portions is useful in characterizing certain types of samples since some are easily vaporized with little coke formation and others form a large fraction of coke. This approach also serves to prevent overloading the detector. Some heavy ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
787
