-
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,
a
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-
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Figure 3. First and second derivative of potentiometric acid-base titration Sample is 920 pmoles of perchloric acid in 50 ml of distilled water, rate of titrant addition IS9 l pmole/sec Curve a,low-pass output curve b, band-pass or first-derivative output, curve c, high-pass or second-derivative output Cutoff frequency is 0 39 Hz
magnitude of the rms noise is essentially independent of the cut-off frequency and has an average value of about 0.1 mV; this corresponds to an input standard deviation of f 7 0 pV/sec a t 00 = 0.2 Hz. The frequency domain of the noise shifts to lower frequency as 00 is decreased. Since the magnitude of the noise inherent in the amplifier is essentially independent of GO, the optimum results with a noisy input signal will be obtained with 00 set to as low a value possible commensurate with the desired 99% response time ( = 5 / W o ) . This is a result of two factors. First, the effective gain of the band-pass filter increases as G O is decreased (see Equation 7). Second, noise in the input signal will be filtered more effectively, the smaller is 00. The second derivative or high-pass output was not studied in detail because the quantitative measurement of the second derivative is seldom of interest. However, the end point of an automatic potentiometric titration is frequently located by determining the inflection point in a titration curve. The original along with the first and second derivatives of a constant rate acid-base titration are shown in Figure 3. These curves were obtained from three individual titrations. The inflection point is quite evident and the second derivative curve has the expected shape (8). The second derivative curve is not significantly noiser than the
(8)L. Meites and H. C. Thomas, "Advanced Analytical Chemistry," McGrawHill, New York. N.Y., 1958,p 45.
Figure 4. First and second derivatives of a thermometric acid-base titration All conditions identical to Figure 3 except sample volume is 30 ml. Curve a, original unfiltered titration curve; curve b, first derivative: curve c, second derivative. Cut-off frequency is 0.74 Hz
first derivative curve, and we estimate that the inflection point can be located within f0.005 pH/sec2. Care must be exercised in using the active filter in very rapid titrations since some phase lag does occur. Thus, the titrant delivery system should be calibrated by titrating a standard with the same setting of 0 0 as that used for the unknown. The effect of a decrease in response time is to elongate the titration curve and shift the end point to longer time. A very rapid rise time does not distort the shape of the titration curve but does permit more noise to pass through the filter. We did not encounter any serious difficulty in adjusting 00 for the titration curve. One pretitration generally suffices to obtain an appropriate setting of 00. Various derivative thermometric titrations are shown in Figure 4. Since we wanted to compare analyses via two methods on essentially identical samples, the total temperature change in the thermometric titrations were extremely large (-0.5 "C).Much lower concentrations could be determined with the apparatus used in this work. The first derivative titration has the expected step functional form. The shape of the second derivative curve is in qualitative agreement with the first derivative. The width of the second derivative spikes which indicate the start and end of the titration is fixed by 00 since no initial or end-point curvature is present in the original titration curve. RECEIVED for review March 4, 1974. Accepted May 3,
1974. This work was supported in part by a grant (GM 17913) from the National Institutes of Health and one of us (R.H.C.) was supported by a fellowship from the Graduate School of University of Georgia.
Ion Selective Electrodes Responsive to Anionic Detergents Taitiro Fujinaga, Satoshi Okazaki, and Henry Freiser Departments of Chemistry of the faculty of Science, Kyoto University, Kyoto, Japan, and the University of Arizona, Tucson, Ariz. 8572 1
One of the major problems in attacking environmental pollution is the absence of adequately sensitive, reliable, specific, and convenient analytical methodology. In particular, determination of trace level organic contaminants in 1842
water such as that of the detergents is rather difficult because of the absence of functional groups of highly distinctive characteristics. Two of the authors have devoted special attention to contamination due to such surface active
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
Z
O
O
L
A
1
3
2
I
5
4
- L O G COYCEYTRATION
Figure 2. Potential response curve of p-toluene sulfonate ion electrode
Figure 1. Configuration of electrode A: Coaxial cable; 8:Teflon body; C: Glass tube; 0:Pt tip
agents ( I ) , now one of the most urgent problems in environmental pollution. It has been recently demonstrated that the ion selective electrode approach can be applied to the determination of organic as well as inorganic ions ( 2 ) . The possibility of extending this to the problem of detergent analysis is most attractive. The recently developed coated-wire ion selective electrode provides an inexpensive and convenient way to do this ( 3 , 4 ) .
Y
/
I I
2
4
3 -LOG
5
6
CONCENTRATION
Figure 3. Potential response curve (A) and surface tension-concentration plot (B) of lauryl benzene sulfonate ion electrode
EXPERIMENTAL Reagents. Sodium alkyl benzene sulfonates were synthesized by conventional methods and recrystallized three times from benzene. Sodium alkyl sulfonates were also synthesized and recrystallized from ethanol. All other chemicals used were of reagent grade quality. The ion association complexes were prepared by the following procedures. A 50% (v/v) solution of methyltricaprylammonium chloride (Aliquat 3368) in decanol was shaken with 0.1M aqueous solution of the sodium salt of the detergent anion to synthesize the quaternary ammonium salt, and the emulsion, if present, was centrifuged for 3 minutes a t 6000 rpm. These procedures were repeated a t least 6 times or until chloride ion was no longer detected (as silver chloride) in the aqueous phase. The electrode coating mixture was prepared by making a 3:l mixture of 10% polyvinyl chloride dissolved in cyclohexanone and decanol solution of the complex. Apparatus. The electrode assembly, shown in Figure 1, was similar to that described earlier (3, 4 ) . The tip of a fine platinum wire (0.7 mm in diameter) was fused to make a ball of ca. 1.5 mm in diameter. The exposed part of the platinum wire was dipped several times in the electrode coating mixture. The electrode was initially conditioned by soaking it for 30 minutes in a 10-4M standard solution of the anion to be measured and stored in air a t room temperature when not in use. It was reconditioned by soaking in the standard solution for 5 minutes immediately before use. Surface tension measurements of detergent solutions were carried out with a Du Nouy tensiometer.
RESULTS AND DISCUSSION The response time of the electrode varied with the concentration, but the equilibrium steady potential was usually achieved within 2-3 minutes and was reproducible to fO.l mV. Figure 2 shows the potential response curve to the concentration of p - toluene sulfonate anions. A linear response (1) T. Fujinaga and S.Okazaki, Jap. Anal., 14, 832 (1965). (2) C. J. Coetzee and H. Freiser. Anal. Chem., 41, 1128 (1969). (3) R . W. Canrall and H. Freiser, Anal. Chem., 43, 1905 (1971). (4) J. T. Davies and E. K. Rideal, "interfacial Phenomena," Academic Press, New York. N . Y . , 1961.
/
2
3
4
-50
5
-LOG CONCENTRATiOY
Figure 4. Potential response curve of isolauryl benzene sulfonate ion electrode
was obtained to the logarithmic variation of concentration with the slope of 57 mV;log a from 10-lM to 2.5 X lO-4M, although the electrode can be used down to 10-5M after calibration. With laurylbenzene sulfonate (LBS), the potential response curve to the concentration of LBS is steeper than that expected from the Nernst equation in the concentration range between 1 X 10-3M and 2 X 10-5M, and when there the concentration of LBS is greater than 1 X lo-", is a sharp break in the potential response curve (see curve A in Figure 3 ) . Because we suspected that the discontinuity and the potential response curve for LBS represented a change of the solution rather than a failure in the electrode, we postulated that the break in the curve arose when the LBS began
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
1843
Table I. Behavior of Detergent-Responsive Electrodes CMC Electrode
Slope, mV
Concn range of linear response, M
Usable concn range, M
Electrode, x 10-3~
Surface tension, 10 -3M
... 1.7 1.5 ... 10
4001
Table 11. Selectivity Ratios for Interfering Anions with LBS Responsive Electrode
G-o-cY
145
L
1
-200-
:
1
'0
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3
2
-LOG
Figure 5.
-
4
Ki
Chloride Sulfate Nitrate Perchlorate Acetate Lauryl sulfate Lauryl sulfonate p-Toluene sulfonate
0.012 0.006 0.93 0.81
0.59 1.36
0.81 0.75
5
CONCENTRATION
Potential response curve of lauryl sulfate ion electrode
to associate in micellar aggregates. Solutions or surfactants will form such polymeric aggregates a t fairly well defined concentrations, known as the critical micelle concentration (CMC). One of the critical ways of observing CMC is to measure the surface tension of a series of solutions of different concentrations ( 4 ) .A sharp break in the surface tension concentration curve is observed a t CMC. It will be noted from comparison of curves A and B in Figures 3, 4, and 5, that the surface tension and electrode response curve breaks coincide, thereby demonstrating the validity of the use of electrode measurements for the determination of CMC (Table I). The ion selective electrode method is not only more rapid and convenient than the conventional surface tension approach, but it can be also argued that the potential reading of the electrode in solutions having concentrations higher than CMC still reliably measure monomer concentrations. Thus, the use of the anion detergent responsive electrode provides a new and useful tool for studies of behavior of these interesting colloids. The LBS electrode, not
1844
Interfering anion
surprisingly, is not very selective with respect to other anionic detergents of similar molecular weight. The interferences of this electrode were studied in the previously described (5) manner with results as shown in Table 11. It is interesting to note that the more commonly encountered inorganic ions of chloride and sulfate are relatively noninterfering. The selectivity ratios for lauryl sulfate and lauryl sulfonate are reasonably close to unity, which suggests that even though LBS could not be determined in a mixture of the three detergents, that the electrode could be used to give a good approximation of the total concentration of the three together.
ACKNOWLEDGMENT The authors are grateful to the Japanese Society for the Promotion of Science both for sponsoring a visit of one of them (H.F.) to Kyoto University during which time this work was initiated, as well as for the continuing support of this research.
RECEIVEDfor review April 15, 197. Accepted June 27, 1974. (5) H. James, G. Carmack, and H. Freiser. Anal. Chem., 44, 856 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
