Anodic Stripping Voltammetry at a Tubular Mercury-Covered Graphite Electrode W. Rudolf Seitz Southeast Water Laboratory, National Environmental Research Center-Corvallis, 3060 1
Environmental Protection Agency, Athens, Ga.
Rosemary Jones and Leon N. Klatt Department of Chemistry, University of Georgia, Athens, Ga. 30601
William D. Mason School of Pharmacy, University of Georgia, Athens, Ga. 30607
A procedure was developed for preparing a tubular mercury-covered graphite electrode (TMCGE) for doing anodic stripping voltammetry (ASV) in a flowing system. The TMCGE was evaluated using thallium in the presence of 1 0 - 2 M EDTA to mask other metals. The effects of varying plating potential, plating time, and flow rate on the TI stripping peak were as theoretically expected, and linear calibrations of TI peak height vs. concentration were obtained over the ranges 2 - 1 0 X lO-’M, 2 - 1 0 X 1 0 - 8 M , and 2-10 X 10-9M at plating times of 3, 8 and 30 minutes, respectively. The current at any point in the tubular electrode with a flowing system is proportional to one over the cube root of the distance from the upstream end of the tube. Since the upstream end of the TMCGE has the highest current density of any point in the tube, impurities accumulate in this region causing this part of the mercury surface to deteriorate more rapidly.
Because the toxicity and fate of heavy metals in natural waters may depend on the chemical forms in which the metals occur, analytical methods that resolve these chemical forms at the trace level are desirable and necessary for research and water pollution control. One such method is anodic stripping voltammetry (ASV), a variation of polarography in which the ion or ions of interest are first concentrated by deposition onto an electrode under controlled conditions. The electrode potential is then scanned in a positive direction, and the current peaks due to reoxidation of the deposited metals are recorded. Addition of the plating step extends the sensitivity range of ordinary polarography by severai orders of magnitude. Because ASV measures the concentration of metal that is in a form that will plate out under the conditions chosen for the analysis rather than measuring total metal, it is capable of distinguishing between “bound” and “unbound” metal. Barendrecht has recently reviewed ASV ( I ) . Adapting ASV to a flowing system offers several advantages over conventional stationary electrode systems: possibility of automation, rapid and reproducible convection to the electrode surface, elimination of the inconvenience of concentration depletion in the sample during the plating step of conventional ASV, possibility of measuring only “unbound” metal if the equilibrium between the “bound” and “unbound” forms of the metal is slow, and self-cleaning property of flow systems. (1) E. Barendrecht in “Electroanalytical Chemistry,” A. J. Bard, Ed., Vol. 2 Marcel Dekker, New York, N. Y . , 1967.
840
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1 9 7 3
A tubular flow-through electrode was first reported by Blaedel et al. ( 2 ) . The theory for reversible charge transfer at this electrode was developed and confirmed using the ferricyanide-ferrocyanide couple (3). Although tubular electrodes with graphite, platinum, and mercury-covered platinum surfaces have been constructed (2, 4, 51, none of these surfaces are well suited to ASV. The solid surfaces are subject to changing activity and intermetallic interferences, and the mercury film on platinum is unstable ( I ) . Also, platinum dissolved in mercury can interact with metals plated into the electrode. To obtain a better surface for ASV, the tubular mercury-covered graphite electrode (TMCGE) was prepared. ASV at a mercury-covered graphite surface was first reported by Matson et al. (6) and further studied by Seitz (7). The electrode surface is prepared by polishing a waximpregnated graphite surface and depositing mercury droplets on those areas exposed by polishing. The chemical inertness of graphite toward amalgam-forming metals makes it superior to other mercury substrates for ASV. Although the mercury surface consists of discrete droplets on the graphite rather than a continuous film, its behavior during ASV agrees with theory derived for thin films (8, 9). The usual mercury layer on graphite is so thin that diffusion does not affect peak shape. Stripping peaks are symmetrical, and 100% of the plated sample is recovered. Because the quantity of mercury on the graphite surface is small, amalgam concentrations are correspondingly high for a given quantity of metal deposited. This leads to problems with metals that form intermetallic compounds a t higher amalgam concentrations (IO, 1I ) . Intermetallic compounds interfere with analysis because they either do not strip out a t all or strip out a t their own characteristic potential. To avoid such problems, the TMCGE was evaluated using thallium, which is not known to form any intermetallic species. W. J. Blaedel, C. L. Olson, and L. R. Sharma, Anal. Chem., 35, 2100 (1963). W. J. Blaedel and L. N. Klatt, Anal. Chem., 38, 879 (1966). T. 0. Oesterling and C. L. Olson, Anal. Chem. 39, 1543 (1967). W. D. Mason and C. L. Olson. Anal. Chem., 42, 548 (1970). W. R. Matson, D. K. Roe, and D. E. Carritt, Anal. Chem., 37, 1594 (1965). W. R. Seitz, Ph.D. Thesis, M.I.T.. Cambridge, Mass., 1970. D. K. Roe and J . E. A. Toni, Anal. Chem., 37, 1503 (1965). W. T. de Vries and E. Van Dalen, J. Electroanal. Chem., 8, 366 (1964); 9, 448 (1965); and 14, 315 (1967). W. Kemula and 2. Kublik, Advan. Anal. Chem. Instrum., 2. 123 (1963). A. G. Strornberg and V. E. Gorodovykh. Russ. J . lnorg Chem., 8, 1234 (1963).
A secondary objective of this research was to show that thallium can be analyzed by ASV if EDTA is added to mask lead and cadmium, which would otherwise interfere. Because thallium is toxic (22) and quite soluble in water as Tl(1) it could be a serious water contaminant at the trace level.
REFERENCE
,
WORKI!.-
N2
COUNTER
\T
,
U c
l
EXPERIMENTAL Apparatus. Graphite cylinders % in. (0.64 cm) in diameter and 0.4 to 0.5 cm long, were impregnated with ceresine wax under vacuum and press fitted between two Teflon (Du Pont) disks in a holder similar to t h a t described by Mason and Olson (5). A 0.078-in. (0.198 cm) diameter hole was then drilled through the graphite and Teflon. The smooth bore through the press-fitted Teflon and graphite minimizes turbulence in the solution flowing through the electrode and prevents other surfaces from contacting the solution. The inside of the graphite tube was cleaned with a pipe cleaner soaked in ethyl acetate to expose bare graphite for mercury deposition and further polished with a dry pipe cleaner. The present cell was changed in two ways from that reported previously (5) to prevent trace metal adsorption on glass and to permit convenient access to the cell. A platinum wire counter electrode and a Ag/AgCl reference electrode were slip fitted into a Teflon cap t h a t screws into the top of the cell, replacing the rubber septum. Also, the Tygon sleeve fittings were eliminated by machining the cell to fit Chromatronix fittings, which were connected to Vs-in. i.d. Teflon tubing. The flow system is illustrated in Figure 1. The peristaltic pump (Harvard Apparatus, Model 1201) was placed downstream from the cell to pull solution through the TMCGE, thus minimizing the possibility of oxygen entering the cell by diffusion through the Tygon tubing required by the pump. Both channels of the pump (180" out of phase) were used to keep the flow forward a t $1 times and prevent solution in the upper area of the cell from backing into the TMCGE. The Tygon tubing was Y4 X inch. When working with thallium, solution going through the flow system was recycled into a calibrated 500-ml sample bottle to keep the total sample volume constant. The method of standard additions could then be used for calibration. The flow was turned off a t the end of the timed plating step and the background current was allowed to decay for about 30 seconds before starting the positive voltage scan. To facilitate background current decay, the electrode potential was often manually adjusted to less negative potentials during the 30-second waiting period before scanning. In all experiments, the voltage scan rate was 1 volt/minute. Electrode potential was controlled by a Heath chopper-stabilized polarograph. Current-voltage curves were recorded on a Moseley 7035B X-Y recorder. Reagents. Diluted sea water was chosen as t h e supporting electrolyte because it has considerably lower concentrations of copper, lead, and cadmium than commercially available reagent grade salts (13). Although sea water does not offer any advantages for thallium analysis, it was used because of projected work with copper, lead, and cadmium. No background problems were encountered with the thallium concentrations used in this study. The electrolyte concentrations for thallium were 45% sea water and 5% 0.2M IVaZEDTA. Potassium hydroxide was added to adjust the p H t o 6 t o preclude interference by hydrogen evolution during the deposition of thallium. A 0.10M T1 standard solution was prepared from T l N 0 3 by direct weight. Other standards were prepared by dilution. All standard solutions were stored in polyethylene bottles. Standard additions of T1 to the cell were made with a Grunbaum pipet. Oxygen was removed by continuous purging with high purity nitrogen.
RESULTS AND DISCUSSION TMCGE Preparation. The quantity of mercury plated onto graphite is critical for the preparation of satisfactory ASV electrodes. If too little mercury is deposited, coverage of the graphite is incomplete and electrodes tend to deteriorate rapidly; if too much mercury is deposited, the (12) "Water Quality Criteria." McKse and Woif. Ed., California Water Quality Control Board Publication No. 3A, 1963, pp 285-6. (13) W . F. Fitzgerald. Ph.D. Thesis, Woods Hole Oceanographic Institute and M.I.T.. Cambridge, Mass.. 1969.
I
Y
Figure 1. Diagram of flow system used for ASV at a tubular mercury covered graphite electrode
surface forces holding the mercury on the electrode are not strong enough to keep the mercury from balling and falling off the graphite. Coverages of from 2 x 10-7 to 5 x mole Hg/cm2 give the most satisfactory results (7). In conventional cells, a known mercury coverage can be achieved by adding a known quantity of mercury to the cell followed by exhaustive plating. This procedure is not applicable to a tubular electrode. When plating from a stream flowing through a tubular electrode, the current density is not uniform along the length of the tube. Instead, it is high a t the upstream end of the tube and decreases continuously toward the downstream end, corresponding to the buildup of a diffusion layer as solution moves along the tube. Blaedel e t al. (2,3) derived an equation for diffusion controlled electrolysis current a t constant flow through a tubular electrode:
I
=
5.31 x l o 5 n C D 2 1 X 2 3 V / 1 3
(1)
where I is current, mA; n is the number of electrons in the charge transfer; C is the concentration, M ; D is the diffusion coefficient, cm2/sec; X is the tube length, cm; and V f is the volume flow rate, cm3/sec. This is equivalent to
I,
=
5.31 x l o 5 n C D 2 ' 3 x 2 Vi''
(2 )
where x is the distance from the upstream end of the tube of length X,and I , is the current carried by the part of the tube between x and upstream end of the tube. Differentiating this expression with respect to x , yields
bl(x)
ot x - 1 ' 3 (3) bX This means that the current density at any point x along the tube is inversely proportional to the cube root of the distance from the upstream end of the tube. Figure 2 illustrates the theoretical variation in current density with position and compares it with the average current density. Because a homogeneous covering of mercury cannot be obtained by plating from a flowing solution, plating from a quiescent solution is necessary. Counting coulombs is satisfactory for determining the quantity of mercury on the electrode only if the current efficiency is known. An empirical procedure was therefore used for plating the proper amount of mercury. The color of the electrode provides a rough estimate of the mercury coverage, a dull mole Hg/cm2 X 100%. gray indicating a level of 2 X The amount of time required to deposit the requisite mercury layer was determined using an externally pol-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973 * 841
--
*
FLOWING
SYSTEM
AVERAGE
CURRENT
c
a
z
W
n
c z w a
a
3
W
2 4
0
POSITION
ALONG TUBE
i
X
I
Figure 2. Variation of current density with position at constant flow through a tubular electrode of length L compared with the
average current ished graphite surface, since the plated mercury cannot be determined directly within the tube. In a quiescent solution of 0.02M Hg2* in sea water a t pH 2 , 20-30 seconds were required to deposite 2-5 X 10-7 mole Hg/cm2. The TMGE was then prepared by pumping the same plating solution into the tube, stopping the flow, and plating for 20-30 seconds, After plating the mercury, the TMCGE was held at -0.20 volt u s . Ag/AgCl for 20-30 minutes with 50% sea water flowing through. This “conditioning” process reduces any HgzClz formed on the electrode during the plating. When a mercury surface became unsuitable for further work, it could be wiped off with pipe cleaner, recleaned with ethyl acetate, and replated. Because of rapid electrode aging (discussed below), fresh mercury surfaces were prepared daily. TMCGE Deterioration. After a period of use, the accumulation of foreign metals in the mercury surface will cause deterioration of the electrode. Nickel and cobalt, for example, react with mercury to form compounds that cannot be stripped out. Many intermetallics are formed between two metals deposited in mercury, e.g. Cu-Zn, that also will not strip out. Other metals, like silver, have oxidation potentials so positive that they will not strip out before oxidation of the mercury film. Because only a small amount of mercury is present on a good electrode (2-5 x lo-’ mole Hg/cm2), a significant concentration of impurities can build up rapidly. With a conventional ASV cell, impurities will be uniformly distributed throughout the electrode, but in a tubular electrode, they will be more concentrated at the upstream end where current density is higher because of the uneven current density distribution (Figure 2). Since the Hg surface on graphite consists of discrete droplets rather than a continuous film, concentrations cannot be equalized by diffusion. As a result, the upstream end of the TMCGE deteriorates more rapidly than a mercury-covered graphite electrode in a conventional cell. Use of other electrode configurations could remove the non-uniform current distribution; however most of these would not be as convenient to use in a flow system. Peak Height us. Plating Potential. The effect of deterioration is best observed by measuring peak height as a function of plating potential for one of the amalgam-forming metals. Figure 3 shows two experimental plots for thallium-one for a freshly prepared mercury surface, the other for a deteriorated surface (several weeks use). At a a42
ANALYTICAL CHEMISTRY, VOL. 45, NO. :,
M A Y 1973
- 0.2
- 0.4
P L A T I N G POTENTIAL LV. VI PEAK P O T E N T I A L )
Figure 3. Peak height vs. plating potential for thallium with a freshly plated TMCGE and a badly deteriorated TMCGE. The magnitude of the peaks at the deteriorated TMCGE has been increased 5 X relative to the peak heights at a fresh surface 50% Seawater, 2.1 ml/min. 4 X 10-’M thallium, 6-min plate
Table I . Peak Height YS. Plating Timea Plating time, minutes
Peak height. FA
Peak ht/plating time, FA/ minute
3 6 9 12 15
0.39 0.80 1.15 1’.53 1.98
0.130 0.133 0.128 0.127 0.132
50% sea water, -1.10 volts plating potential, 4 X IO-’M TI, 2.1 m l / min.
freshly prepared surface, the peak height us. plating potential curve corresponds to the theoretical prediction for a reversible deposition reaction ( I ) . The deteriorated surface gives similar behavior at plating potentials close to the peak potential, but the overall sensitivity is less. At more negative plating potentials, the peak height continues to be a function of plating potential indicating that there is a significant overvoltage for thallium deposition on a deteriorated surface. Signs of deterioration can be detected in the peak height us. plating potential curve after a few days use. For this reason, fresh mercury surfaces were prepared daily. Characteristics of the Thallium Stripping Peak. Thallium ASV behavior at the TMCGE agrees fairly well with the theory for thin film mercury electrodes. The peak shape is symmetrical with half-width of between 90 and 100 mV. Theory predicts symmetry with a half-width of 74 mV f9), and in a stationary cell, the observed halfwidth is 78 mV (7). The increased half-width at the TMCGE is probably related to the variation in amalgam concentration along the tube and non-uniform current density during the stripping step leading to distortion of the peaks by cell resistance effects. Peak shape is independent of electrode deterioration and cannot be used as an indicator thereof.
I
0.55
c 2
I w I
u 3
I -
,\
-0 I
-0 5
-0 3
v
VI
-0 7
,
np/Agci
Figure 4. Actual data for thallium showing peak height as a
function of concentration from 2-70
X 10-8
0
I
2
molar.
45% Seawater, 5% 0 2 M EDTA, 8-minute plate, -1.05 volts plating potential, 8.4 ml/minute
3
4
FLOW
-09
RATE,
5
6
7
8
ml/mii
Figure 5. Peak height vs. flow rate. The number next to each point is the percentage total thallium plated at that flow rate 50% Seawater, 6-minute plate, 4 X lO-’MTI, -1.10 volts plating potential. dashed line is the theoretical curve
Table Ill. Peak Height YS. Concentration Table II. Reproducibilitya Run No.
Peak ht., p A
[Til M
Peak height, p A
Peak ht./concentration ( X l O - ’ )
1 2 3 4 5
1.24 1.24 1.24 1.22 1.24
2 x 10-7 4 x 10-7 6 X a x 10-7 i o x 10-7
0.37 0.74 1.17 1.42 1.82
0.1 a5 0.1 a5 0.195 0.178 0.182
a 50% seawater, -1.10 volts plating potential, 4 X 1O-’M Ti, 8-minute plate, 2.1 ml/min.
Peak height is proportional to plating time as shown in Table I. The peak potential for T1 in 45% sea water, 10-2M NaZEDTA, is between -0.70 and -0.73 volt relative to Ag/AgCl. Some typical thallium peaks are illustrated in Figure 4. Reproducibility. Table I1 lists five T1 peak height measurements for one solution. The relative standard deviation is less than 1%. Electrode-to-electrode reproducibility was not systemically studied, but IpCVf1/3/twas calculated for different surfaces, where I p is the peak current of the stripping peak, t is the plating time, and C and V , are as defined earlier. This factor is constant for a given surface, but was observed to vary by up to a factor of four for different mercury surfaces. A fresh Hg surface requires recalibration. Effect of Flow Rate. Equation 1 predicts that diffusion-controlled electrolysis current at constant flow through a tubular electrode is proportional to the cube root of the flow rate. The plating efficiency and the height of the stripping peak should, therefore, be proportional to the cube root of the flow rate. Figure 5 shows an experimental measurement of thallium peak height as a function of flow rate, along with the theoretical cube root dependence. The theoretical expression is derived for perfect continuous laminar flow. Although in practice the pump generates a pulsating flow and local turbulence probably exists, the agreement of the experimental data with theory is good. One potential source of local turbulence is the mercury droplets protruding from the wax-impregnated graphite. Figure 5 also includes the percentage of total thallium passing through the TMCGE that is plated at the various
3-minute plate, -1.10 volts plating potential, 2 , f ml/min
2 x 10-8 4 x 10-8 6X a x 10-8 10 x 10-8
0.05 0.095 0.14 0.18 0.22
0.25 0.24 0.23 0.22 0.22
8-minute plate, -1.05 volts plating potential, 8.4 ml/min
2 x 10-9 4 x 10-9 6X i o x 10-9 30-minute plate,
0.037 0.082 0.105 0.1a0
i.a5 2.05 1.75 1.ao
- 1.05 volts plating potential, 8.4 ml/min
flow rates. At the lower flow rates, thallium in a given volume increment of solution has more time to diffuse to the electrode surface; therefore, the plating efficiency is higher. The low plating efficiency observed might be thought to adversely affect the sensitivity of the TMCGE; however, the 100% recovery during stripping and the low background current associated with the small electrode area (0.25-0.30 cm2)offset the inefficiency of plating. Response to Thallium Concentration. A plot of peak height as a function of thallium is linear over three ranges-2-10 X 10-9M, 2-10 x 10-8M, and 2-10 x lO-?M (Table 111). The sensitivity of the TMCGE compares favorably with other electrode systems used for ASV. Two-electron transfer systems, e.g., Cd and Pb, are four times more sensitive than the thallium system, which is a one-electron transfer. For a given metal concentration, the current for a two-electron reaction is doubled and the peak is twice as sharp. The theoretical minimum half-width for a stripping peak for a thin mercury film is 7 4 / n mV. The volume of solution required for an analysis depends on the desired sensitivity and the flow rate. From the data in Table I11 shown in Figure 4, a readily detectable peak ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6, MAY 1973
843
can be obtained for 4 x 10P8M T1 (equivalent to 1 x 10-8M P b or Cd) by a 4-minute plating at a flow rate of 8.4 ml/minute. This requires a sample volume of 33.7 ml to go through the TMCGE. The same sensitivity can be obtained by plating for 6Y2 minutes at 2.1 ml/minute or 12 minutes a t 0.42 ml/minute requiring 13.7 and 5 ml; respectively. Interferences. In sea water, lead and cadmium strip out of mercury at potentials close to the stripping peak potential for thallium. EDTA was added to eliminate possible lead and cadmium interferences. The logarithmic formation constants for Pb(II) EDTA, Cd(I1) EDTA, and Tl(1) EDTA are 17, 16, and 5.8, respectively (14). In 45% ( 1 4 ) L. G . Sillen and A. E. Martell, "Stability Constants," Spec. Publ. 17, The Chemical Society. London, 1964.
No.
sea water, 5% 0.2M EDTA, no peak is observed for lO-5M P b or Cd plating for 15 minutes at -1.05 volts at a flow rate of 8.4 ml/min. A small peak was observed for lO-5M copper under these conditions, but the peak appeared around -0.4 volt, sufficiently positive not to interfere with thallium.
Received for review September 25, 1972. Accepted December ll, 1972. One of the authors (R.J.) acknowledges support from NSF Undergraduate Research Participation Grant No. GY-8791. Use of trade names does not imply endorsement by the Environmental Protection Agency or the Southeast Water Laboratory.
Determination of Phenols and Aromatic Amines by Direct Titration with Bromine in Propylene Carbonate Richard D. Krause and Byron Kratochvil
Department of Chemistry, Universityof Alberta, Edmonton, Alberta, Canada J6G 2G2 Propylene carbonate is used as a medium for bromine substitution reactions. A series of aromatic amines and phenols were determined with accuracies of about 1% and precisions of a few ppt. A base such as pyridine must be present to accept protons released in the substitutions. Advantages include rapidity of the reactions, solubility of reactants and products, and convenient standardization of bromine with solutions of bromide. The log formation constant of Br3- in propylene carbonate at zero ionic strength is 7.37.
Bromine substitution reactions are widely used for the quantitative determination of phenols and aromatic amines (1). The procedure commonly involves addition of excess bromate-bromide reagent to an acidified aqueous solution of the sample, addition of iodide after a suitable reaction time, and titration of the liberated iodine with standard thiosulfate solution. Alternatively, a strongly acidified solution of the sample can be titrated with standard bromate-bromide reagent until free bromine is observed visually f2), spectrophotometrically (3), or by a spot reaction on starch-iodide paper. The bromate-bromide method requires use of aqueous solutions, which limits its applicability to water-soluble phenols and amines. Ingberman, investigating glacial acetic acid as solvent, found that bromination of phenol was complete in 20 minutes if catalytic amounts of pyridine were added ( 4 ) . Huber and Gilbert titrated several phenols in 90% glacial acetic acid-10% pyridine directly with 0.15M bromine in glacial acetic acid (5). End points were determined by constant-current potentiometry. Ashworth, "Titrimetric Organic Analysis," Vol. I , Interscience, NewYork, N . Y . , 1965, p 135; Vol. I I , p214.
(1) M. R. F.
(2) /bid., Vol. I , p 119. (3) P. B. Sweetser and C. E. Bricker,Anal. Chem., 24, 1107 (1952). (4) A. K. Ingberrnan,AnaLChem., 30, 1003 (1958). (5) C. 0. Huber and J. M. Gilbert, Anal. Chem., 34, 247 (1962).
844
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6 , MAY 1973
This paper describes the ure of propylene carbonate (PC) as a solvent for a number of analytical bromine substitution reactions. Among the advantages of PC are its resistance to chemical attack by halogens (6) and its high dielectric constant, 65, which increases reaction rates. It has a wide liquid range (-49 to +242 "C), is colorless, odorless, and nontoxic, and is not appreciably hygroscopic. Its principal disadvantages are that it hydrolyzes fairly rapidly in the presence of strong acids or bases, and that it has a moderately high viscosity (2.5 cP). Further information on properties and reactivity is available from the supplier (6). Most phenols and aromatic amines, as well as their bromination products, are soluble in PC. In many instances, a base must be present to accept protons displaced by bromine in the substitution step. Several unusual reaction stoichiometries were observed.
EXPERIMENTAL PC (Jefferson Chemical Co.) was distilled at a pressure of 0.01 mm mercury on a 48- by 1-in. vacuum-jacketed column packed with nichrome helices (Podbielniak size-C Heli-Pak), The vacuum-jacketed still head contained a solenoid-operated glass valve set a t a 1O:l reflux ratio. The purity of the distilled PC was monitored by ultraviolet spectroscopy; the fraction kept had an absorbance of less than 0.3 at 250 nm in a 1-cm quartz cell, measured against a distilled water blank. In a typical distillation, the first 800 and last 200 ml of a 2000-ml charge were discarded. Liquid chromatography (Chromasorb W packing) of the fraction retained revealed one unidentified small impurity peak just ahead of the major peak. Solid phenols and aromatic amines were sublimed once at reduced pressure and stored in a desiccator. Aniline and p-phenetidine were distilled on a 30-cm Vigreux column at atmospheric pressure. Tetraethylammonium bromide was recrystallized from ethanol and dried 12 hr at 90 "C under vacuum. Other chemicals were reagent grade. Titrations were performed on a Metrohm Model E436 automatic titrator equipped with a 5-ml buret. Bromine titrant was stored in a low-actinic glass flask fitted with a Teflon (Du Pont) stopper. Venting the flask to the atmosphere only during withdrawal of ti(6)
Propylene Carbonate Technical Bulletin, Jefferson Chemical Compan y , Houston, Texas.
