acid determinations. No other potential interferences were noted. The quantitative aspects of the procedure were studied in considerable detail. C-l-14C-palmitic, oleic, linoleic, linolenic, and stearic acids were added to smoked filter pads and to condensate to test the extraction efficiency. Essentially 100% recovery was obtained with overnight soaks. Complete recovery was also noted after as little as 1 hour of soaking with periodic vigorous agitation using a Vortex mixer. Pyridine has been found to be a particularly effective solvent for this analysis. Not only is pyridine an ideal silylating medium (13) but, a t the sample concentrations used, it provides a very nearly true solution of condensate rather than the inhomogeneous slurry encountered using most other solvents. The reliability of the derivatization procedure was examined from several standpoints. TPM containing 8-12 wt % water and acetone suspensions of condensate containing 5-10 wt % water were found readily analyzed without requiring drying. Identical results were obtained regardless of whether the extract was derivatized by heating for 30 minutes or by standing overnight at room temperature. The silyl derivatives are seen to degrade slowly, however. An approximately 16% reduction in peak size was noted 48 hours after derivatization when the sample was allowed to stand under ambient conditions. It is thus recommended that the sample be analyzed the same day it is derivatized. No such difficulty was noted with storing the underivatized extracts. The ability to reliably derivatize "wet" samples suggests that a sufficient excess of reagent is available to react with both the water and silylateable functions of all of the smoke components. Varying the silyl reagent: condensate extract ratio from 1:4 to 1.5:l provided the same final result thus confirming the availability of sufficient reagent at the 1:2 ratio employed for the routine work. Table I further illustrates the reliability of the procedure. Added quantities of palmitic acid approximating 75% of
Received for review August 29, 1973. Accepted December 14, 1973. Research sponsored jointly by the National Cancer Institute and the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
(13) Alan E. Pierce, "Silylation of Organic Compounds," Pierce Chemical Co.. Rockford. Ill., 1968, p 7.
(14) S Bellin, Abstracts 15th Tobacco Chemists Research Confereme Philadelphia, Pa . October 1961
the original amount present are easily recovered. Agreement between spiked and unspiked samples following correction for the added acid also suggests that no major interfering component is present. Table 11 summarizes typical results obtained with the routine application of this procedure. Analyses of the TPM delivered by more than 30 types of experimental cigarettes were consistently accompanied by the aqreement illustrated for the 1R1 cigarette. It is not possible to correlate results of the condensate analysis with published results of fatty acid deliveries because the characteristics of the experimental cigarette were not defined. The data for TPM should be generally comparable to that presented by Hoffmann ( I O ) for the blended cigarette. Values of 0.37 and 0.13 wt % for palmitic and stearic acids agree reasonably well with the values of 0.47 and 0.23 wt % reported by Hoffmann. Analysis of a standard blended cigarette using this procedure yielded values for palmitic and stearic acids of 160 and 73 pg/g tobacco smoked-in excellent agreement with the values 152 and 75 pg/g tobacco smoked reported ( ! O ) for a blended cigarette. Agreement on the unsaturated acids is poorer-results for the three acids using this procedure are consistently lower than the sum reported by Hoffmann. The relative quantities of palmitic and unsaturated acids found using this procedure are, however, in agreement with that reported earlier by Bellin (14). The silyl procedure reduces the quantitative estimation of free fatty acids in smokes to a routine task. The convenience of the procedure can greatly facilitate future surveys of the role free fatty acids may play in experimental tobacco carcinogenesis or smoke quality.
Advantage of Short Controlled Drop Times for Alternating Current Polarography in High Resistance Solvents D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
In analytical applications of polarography, nonaqueous solvents possess a number of advantages over aqueous solutions (1-3). To date, alternating current (ac) polarography has rarely been employed in nonaqueous solvents, principally because of the high resistance which usually characterizes such solvent systems ( 3 ) , and which gives rise to analytically undesirable iR drop effects like wave distortion and nonlinear calibration curves. Another undesirable consequence of ohmic potential loss (iR drop) in ac polarography is that it decreases the effectiveness of phase-sensitive detection at discriminating against the (1) R. Takahashi, ialanta. 12, 1211 (1965).
( 2 ) A. L. Woodson and D . E. Smlth, Ana/. Chem.. 42, 242 (1970) (3) D. E. Smith. C.R.C. Crit. Rev. Anal. Chern, 2, 247 (1971)
double-layer charging current ( 4 ) . Phase-sensitive detection is important in analytical applications of the ac k c h nique because it provides considerable irnDrnvament in the detection limit and the precision of current measurement at low depolarizer concentrations. If significant iR drop is present, the ideal 90" phase relationship between the charging current and the applied alternating voltage no longer holds, and complete separation of the faradaic and charging current components cannot be obtained. In recent years, much effort has been expended in trying to minimize ohmic potential loss in voltammetric (4) D. E. Smith, in "Eiectroanalytical Chemistry," A. J. Bard, Ed.. Marcel Dekker, New York, N.Y., 1966, Vol. l , Chap. l . ( 5 ) E. R . Brown. T. G. McCord. D. E. Smith, and D. D . DeFord. Anal. Chem.. 38, 1119 (1966). A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6 , M A Y 1 9 7 4
763
techniques (5). Undoubtedly the most successful approach has been that of employing operational amplifier, threeelectrode potentiostats incorporating positive feedback iR compensation (6, 7 ) . Although polarographs with threeelectrode configuration are relatively common, instrumentation with positive feedback circuitry is still rather specialized and beyond the reach of many analytical chemists. Recently ( 8 ) , phase-sensitive detection has been successfully used in conjunction with the rapid polarographic technique. An interesting result of this investigation was the observation that phase-sensitive detection was more effective under rapid conditions ( e . g . , t = 0.16 sec) than a t a normal drop time of several seconds. That is, as the drop time was decreased, the faradaic-to-charging current ratio increased. The explanation given for this observation was that the iR drop due to uncompensated resistance was lower with the rapid method. Previous results (8) were obtained in a solution of low specific resistance (ca. 5 ohm cm). The present paper examines the possibility of extending the rapid phase-sensitive technique to high resistance solvents where the ohmic potential loss is more severe. In particular, it was of interest to establish whether or not useful ac waves could be obtained in such a medium without the need for additional iR compensating circuitry.
THEORY The basis of the rapid polarographic method is that mercury drops are mechanically dislodged from a capillary at a short time interval ( t ) .Because of back pressure due to surface tension, the flow rate of mercury ( m ) does not remain constant during the growth of a drop (9). Thus, for a given column height, m will also decrease when short drop times are achieved in this manner (Table I). Since the radius ( r ) of the DME (assuming spherical electrode geometry) is given by (IO)
where d is t h e density of mercury, it follows that drops of small radius (or small surface area) can be obtained with the rapid method. Calculated values of r for the same capillary under conventional and rapid conditions are compared in Table I. For the rapid method, the assumption that the drop is spherical throughout its life is no longer a good approximation, and the actual drop size will differ from the calculated value. Cover and Connery ( 2 1 ) have discussed the breakdown of sphericity assumptions for very short, mechanically controlled drop times. For a three-electrode cell with the DME between the reference and counter electrodes, cell resistance ( Rcell) is given by (12)
RceI, = where tion.
p
P
is the resistivity (specific resistance) of the solu-
(6) E. R . Brown, D. E. Smith, and G . L. Booman, Ana/. Chem., 40, 1411 (1968). ( 7 ) E. R. Brown, H . L. H u n g , T. G . McCord, D. E. Smith, and G . L . Booman, Anal. Chem., 40, 1424 (1968). ( 8 ) A . M. Bond and D. R. Canterford,Ana/. Chem., 44, 1803 (1972). ( 9 ) G . S. Smith, Trans. Faraday. SOC.,47, 63 (1951). ( l o ) J . Heyrovsky and J . Kuta, "Principles of Polarography," Academic Press, New York/London. 1966, p 35. ( 1 1 ) R. E. Cover and J. G. Connery, Anal. Chern., 41, 1797 (1969). ( 1 2 ) W . E. Thomas, J r . , and W. B. Schapp, Ana/. Chem.. 41, 136 (1969).
764
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 6, M A Y 1974
Table I. Comparison of Drop Time, Flow R a t e of Mercury, and Calculated Drop Radius for Conventional and Rapid Techniques Technique
Conventional Rapid
t, s e c o
m, mg sec-1
3.47
2.39 2.19
0.16
a
rcaiod, m m b
0.53 0.18
a Measured at open circuit in 1M NaClOa with column height of 38 cm. Spherical drop is assumed.
Although electrode geometry is highly questionable for drops of very small radius, Cover and Connery ( 1 1 ) found a linear relationship between cell resistance and reciprocal of drop radius under such conditions. Since fundamental harmonic ac current and charging current are both directly proportional to electrode radius squared (4, 13), iR drop due to uncompensated resistance is proportional to the product of the resistivity of the solution and the electrode radius, i.e.,
iR p r (3 ) Thus, for a particular supporting electrolyte, the iR drop is expected to be lower with the rapid method. Under these conditions, the ideal phase relationship between the charging current and the applied alternating voltage will be more closely approached, resulting in better separation of the faradaic and charging currents if phase-sensitive instrumentation is employed. This advantage of the rapid technique with respect to phase-sensitive detection could not be achieved by producing short drop times as a result of increased column height, since mean electrode radius is independent of column height (IO). This was not made clear in a previous communication (8). EXPERIMENTAL Polarographic studies were carried out in May and Baker analytical grade acetone using 0.1M tetraethylammonium perchlorate (TEAP) as supporting electrolyte. TEAP was prepared by neutralizing a solution of tetraethylammonium hydroxide with 20'30 HClOI. After recrystallization from water and drying in a vacuum oven at 60 "C, the TEAP was stored over silica gel. Reagent grade lithium perchlorate trihydrate, sodium tetrafluoroborate, and potassium iodide were used without further purification. Polarograms were recorded with a PAR (Princeton Applied Research) Model 170 Electrochemistry System. All measurements were made with phase-sensitive detection and at an applied alternating voltage of 10 mV (peak-to-peak).A silver/silver chloride (0.1M LiC1, acetone) reference electrode, separated from the test solution by a salt bridge containing 0.1M TEAP, and a tungsten wire auxiliary electrode were used. Short drop times of 0.16 sec were obtained with a Metrohm Polarographie Stand E354. Rapid and conventional polarograms were recorded using the same capillary with the same head of mercury. Solutions were thermostated a t 25.0 f 0.1 "C and were deaerated with argon which had been presaturated with acetone vapor. Specific resistance measurements were made with a Philips Conductivity Bridge PR 9500.
RESULTS AND DISCUSSION The importance of iR compensation at normal polarographic drop times in high resistance solvents is well established (3, 7), and is further illustrated in Figure 1 for the reduction of sodium(1) in acetone. With the application of iR compensation, by means of the positive feedback loop incorporated into the instrument used, the charging current contribution is almost negligible, even at a frequency as high as 1 KHz (Figure l a ) . However, with(13) P. Delahay (1952).
and T.
J.
Adams,
J.
Arner. Chern. Soc.,
74, 5740
1.
-is
-20,
Figure 3.
,
-1'8
VOLT
n
,
-2.0
@lApCl
Conventional and rapid ac polarograms of 1
X 10-3M
lithium(1) in 0.1M TEAP/acetone, recorded without iR compen-
sation -1'6
-1'8
VOLT
-1% VI
-13
AgIAgCI
Figure 1. Conventional ac polarograms of 1 X 1 0 - 3 M sodium(1) in 0.1M TEAPIacetone, recorded (a) with iff compensation, and (b) without if? (compensation Drop time = ca 2 sec; frequency = 1 KHz
Drop time: (a) ca. 2 sec, (b) 0.16 sec. Frequency = 1 KHz
Table 11. R a t i o of Measured Faradaic C u r r e n t to Measured Charging C u r r e n t (i, /ic) under Various Experimental Conditions ii/ic
w
Ibl
Electroactive species"
KU) Na (1) Li(1)
Technique"
Conventional Rapid Conventional Rapid Conventional Rapid
wit,h LR compenuation
without iR compensation
115 110
10 70
80 80 10 40
4 40 1 20
a 1 X 10-JM solution in 0.1M TEAP/acetone. bFrequency = 1 KHz. Drop time: ca. 2 8ec (conventional); 0.16 sec (rapid). VOLT
V I
*s/Aga
Figure 2. Rapid ac polarograms of 1 X 10-3M sodium(1) in 0.1M TEAP/acetone, recorded (a) with iR compensation, and (b) without if? compensation Drop time = 0.1 6 sec; frequency = 1 KHz
out iR compensation (apart from that provided by a conventional three-electrode potentiostat), not only is the ac wave much smaller in height but considerable charging current is recorded (Figure 1b). Under these conditions (i.e., without iR compensation), phase-sensitive detection is not very effective a t discriminating against the charging current because of the large ohmic potential loss. On the other hand, with the rapid technique, phase-sensitive detection provides excellent discrimination against the charging current whether or not iR compensation is applied (compare Figures 2a and 2b). Furthermore, there is little decrease in peak current in the absence of iR compensation. These results confirm that the ohmic potential loss can be decreased significantly by using a short controlled drop time. A more dramatic illustration of this advantage of the rapid technique is shown with the lithium(1) reduction process, which departs considerably from reversibility in acetone. Above about 100 Hz, the peak current becomes independent of frequency ( 1 4 ) , whereas the charging current increases directly with frequency ( 4 ) . Thus, at high frequencies, the ac wave recorded without iR compensation is of limited analytical use even if phase-sensitive detection is employed (Figure 3a). However, by simply using drops of much smaller radius, a well defined wave is ob( 1 4 ) D. R. Canterford and A . S. Buchanan, unpublished work, 1972
tained without the need to resort to positive feedback iR compensation (Figure 3b). As previously discussed (8), it is often the ratio of the measured faradaic current to the measured charging current ( & / i C )rather , than the absolute magnitude of the faradaic current, that determines the detection limit of a particular polarographic technique. With ac polarography, the faradaic and charging currents show the same dependence on electrode radius ( 2 3 ) .Therefore, in a comparison of the conventional and rapid techniques, this ratio gives a comparison of the effectiveness of phase-sensitive detection and thus, indirectly, of the effectiveness of iR compensation. Values of obtained for alkali metal ions in acetone under various experimental conditions are listed in Table II. With the conventional technique, there is a marked decrease in the if/& ratio if positive feedback iR compensation is not employed. This is not unexpected because, a t a frequency as high as 1 KHz, the large current flowing results in a large ohmic potential loss. With the rapid method, however, the decrease is much less. Thus, in the absence of positive feedback iR compensation, the rapid technique gives a much more favorable G/ic ratio than the conventional technique. Reduction of lithium(1) provides the interesting result that, under certain conditions, the rapid method is more effective at decreasing the influence of iR drop than is positive feedback circuitry at a normal drop time of several seconds. Although the above results refer to a nonaqueous solution (resistivity of 150 ohm cm), it is obvious that the rapid technique could be applied to aqueous solutions with low supporting electrolyte concentrations. This ANALYTICAL CHEMISTRY, VOL. 46, NO. 6, M A Y 1974
765
would be an advantage in trace analysis where supporting electrolyte purity is often a problem. Since iR drop is proportional to the product of solution resistivity and drop radius (Equation 3), the iR drop could be maintained at an acceptably low level for solutions of much higher resistivity by using drops of smaller radius than those used in the present work. The advantage of being able to record ac polarograms
without the need for positive feedback iR compensating circuitry, coupled with the fast scan rates of potential used (8), makes the rapid phase-sensitive technique attractive for routine analysis in nonaqueous or other high resistance solvents. Received for review June 29, 1973. Accepted December 10, 1973.
Potentiometric Microtitration of Sulfate Ion Using a Sodium-Selective Glass Electrode in a Nonaqueous Medium Naoshige Akirnoto Faculty of Pharmaceutical Sciences, Kyoto University, Kyoto 606, Japan
Keiichiro Hozurni Kyoto College of Pharmacy, Kyoto 607, Japan
The potentiometric titration of sulfate with an ion-selective electrode has been carried out using a lead-selective electrode to determine the end point ( I , 2 ) . However, the method is not acceptable for a highly accurate titration of sulfate in any low concentration, because the electrode does not give a curve with a well defined point of inflection that can be related to the equivalence point. On the other hand, the NAS 11-18 and NAS 27-4 (Corning Glass Works, Medfield, Mass.) have been used as representative glass electrodes which respond with high selectivity to monovalent cations. The former is known to have a selectivity order of hydrogen, sodium, and potassium ion; the latter, a reverse order of potassium, sodium, and hydrogen ion (3, 4 ) . They are commercially available under the respective specifications "Sodium-Selective Glass Electrode" and "Monovalent Cation-Selective Glass Electrode," in keeping with their most common application. Their response characteristics for monovalent cations have been examined in detail, but to date very little information has been reported concerning their response to divalent cations. In a study of this latter property, the present authors have discovered a potential break for barium ion when the electrodes were used in sulfate titration. The NAS 11-18 electrode in particular exhibits a sharp potential break. Subsequent investigation with regard to its application to the microdetermination of sulfate has disclosed that it will register a sudden increase in electrode potential in the vicinity of the equivalence point ( 5 ) under the condition of about a 70 vol 9i concentration of some organic solvent a t pH 5-6. However, the measurement of the Nernstian slope by plotting steady-state potentials in nonaqueous solutions of various concentrations of barium ion indicates no electrode response to barium ion, whereas the electrode, when exposed to a sudden change of barium ion concentration, developed a potential instantly which then gradually (1) J. W. Ross, J r . , and M . S. Frant, Anal. Chem.. 41, 967 (1969). ( 2 ) W . Selig, Mikrochim. Acta. 1970, 168. (3) G . Eisenrnan, "Electrochemistry of Cation-Sensitive Glass Electrodes" in "Advances in Analytical Chemistry and Instromentation," C. N . Reilley, Ed., Vol. 4 , Wiley (Interscience), New Y o r k , N . Y . . 1965, pp 213-369. (4) G . Eisenrnan, "Glass Electrodes for Hydrogen and Other Cations," Marcel Dekker. New York. N . Y . , 1967. (5) K . Hozumi and N . Akimoto, Anal. Chem.. 42, 1312 (1970).
766
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1974
dropped off to the initial potential. This phenomenon is similar to the transient response to calcium and strontium ions reported by Rechnitz et al. ( 4 , 6, 7). One must therefore conclude that the potential break in the vicinity of the equivalence point during the titration of sulfate is due to a transient response of the electrode to barium ion. EXPERIMENTAL Apparatus. Automatic titrations were carried out with a Metrohm Potentiograph E 436 (The Metrohm Co., Herisau, Switzerland) set to the 500-mV full-scale range. The glass electrodes used were Corning NAS 11-18 (Cat. No. 476210) and Corning NAS 27-4 (Cat. No. 476220). The double-junction reference electrode was the Orion Model 90-02 (Orion Research Inc., Cambridge, Mass.) with a bridge solution of 0.2M hexamethylenetetramine ("Hexamine") to prevent any diffusion of potassium ion from the electrode to the sample solution. Reagents. A standard solution of 0.005M barium chloride was prepared by dissolving 122 mg of barium chloride dihydrate in 300 ml of water and diluting it t o a liter with isopropanol. A 0.2M Hexamine solution was prepared by dissolving 28.08 grams of Hexamine in water and diluting up to a liter. Procedure. An approximate volume of sample solution equivalent to 0.5 to 5.0 mg of sulfate radical (about 5 ml) is accurately pipetted into a 100-ml beaker and the solution is adjusted to pH 5-6 by adding 1 ml of the 0.2M Hexamine. Acetone and distilled water are added to bring the volume up to 50 ml and a t the same time to make it approximately 70 vol 9'0 in acetone. Both the sensing electrode and the reference electrode are rinsed well with 0.01M hydrochloric acid before immersion in the sample solution. Automatic titration with standard 0.005M barium chloride is then started at a delivery speed of 0.5 ml per min. The equivalence point in the titration is indicated by a sharp point of inflection in the titration curve on the recorder chart. In the event that the sample solution should contain other metal cations besides barium to which the electrode will respond, it is advisable that they be removed prior to the titration by passage of the solution through an ion exchange column. Otherwise, low analytical figures will be obtained.
RESULTS AND DISCUSSION Titration Curves in Highly Nonaqueous Medium. Titration curves of 2.500 ml of 0.005M sulfate obtained with the NAS 11-18 and NAS 27-4 electrodes in 90 vol 70 ace(6) G . A. Rechnitz and G . C. Kugler. Anal. Chem.. 39, 1682 (1967). (7) "Ion-Selective Electrodes," R . A . Durst. E d . . Nat. Bur. Stand. (U.S.).Spec.Pub/., 314, 313 (1969).
