1198
Anal. Chem. 1982, 5 4 , 1198-1200
magnesium. The percent recovery of calcium and magnesium appeared to be independent of the relative composition of the standard solution and river water mixtures. This suggests a lack of selective sorption for the metals by the column. A subsequent injection of EDTA solution (0.5 mL of 0.1 M EDTA) stripped 340 pg of calcium and 100 pg of magnesium from the column and indicated that metals from previous water samples had accumulated on the column. These levels were about 10-fold higher than those observed in the untreated river water filtrate (34 pg of Ca and 9 pg of Mg per injection). After EDTA treatment, the recoveries of metals from five subsequent river water injections ranged from 30 to 80% of injected levels for calcium and from 40 to 90% for magnesium. These data suggest that columns should be equilibrated with samples, similar to those to be analyzed, for effective recovery of metals. A small amount of metal bleed, observed for calcium (0.15 f 0.04 pg of Ca mL-l eluate, N = 4) and magnesium (0.05 f 0.02 pg of Mg mL-l eluate, N = 4) after several equilibration injections of river water, indicated the weak nature of the column-metal association. The above chromatographic results demonstrate that DWSEC with specific detectors can fractionate and measure forms of dissolved elements in natural water filtrates. Separation mechanisms for metal components by DWSEC appear to involve both molecular size and component polarity. When sample components are ionized, and salts and buffers are absent from the mobile phase, attraction or repulsion of ions by the column can affect DWSEC fractionation behavior (15). Selective enlargement of ions by hydration may also affect resolution of constituents. Because anions are generally larger in size than metal cations, size fractionation of metal components by DWSEC probably depends strongly on the nature of the complexing anions. The identical speciation patterns we observed for calcium and magnesium suggest that the anions complexing the two metals in our water sample had the same composition. ACKNOWLEDGMENT We thank J. M. Malczyk for preparing sample filtrates, D. W. Morse, J. E. Grimes, and W. R. Burns for help in sample collection, A. M. Corbin for technical assistance in the laboratory, and B. J. Eadie and D. Scavia for reading the manu-
script. The Columbia National Fishery Research Laboratory, U.S. Fish and Wildlife Service, Columbia, MO, provided the ICAP detector. LITERATURE CITED (1) Uden, P. C.; Quimby, 9. D.; Elllott, W. G. Anal. Chim. Acta 1978, 101, 99-109. (2) Gast, C. H.; Kraak, H.; Poppe, H.; Maessen, E. J. M. J. J. Chromatogr. 1979, 185,549-561. (3) VanLoon, J. C. Anal. Chem. 1979, 51, 1139 A-1150 A. (4) Parks, E. J.; Brinkman, F. E.; Blair, W. R. J. Chromatogr. 1979, 185, 563-572. (5) Morita, M.; Uehlro, T.; Fuwa, K. Anal. Chem. 1980, 52, 349-351. (6) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1223-1227. (7) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1227-1231. (8) Taylor, L. T.; Hausler, D. W.; Squires, A. M. Science 1981, 213, 644-646. (9) Synder, L. R.; Klrkland, J. J. "Introduction to Modern Liquid Chromatography, Second Edltlon"; Why-Interscience: New York, 1979; 663 PP. (10) Gelotte, 9. J. Chromatogr. 1960, 3, 330-342. (11) Stumm, W.; Morgan, J. J. "Aquatlc Chemistry an Introduction Emphasizing Chemical Equilibria in Natural Waters"; Wiley-Intersclence: New York, 1970; 583 pp. (12) Urano, K.; Katagurl, K.; Kawamoto, K. Water Res. 1980, 14, 74 1-745. (13) Frayley, D. M.; Yates, D.; Manahan, S. E. Anal. Chem. 1979, 51, 2225-2229. (14) Strong, A. E.; Eadie, 9. J. Limnol. Oceanogr. 1978, 23, 877-887. (15) Koropchak, J. A.; Coleman, G. N., University of Georgia, Athens, GA, personal communlcatlon. (16) Saito, Y.; Hayano, S. J. Chromatogr. 1979, 177,390-392.
Wayne S. Gardner* Peter F. Landrum Great Lakes Environmental Research Laboratory/NOAA 2300 Washtenaw Avenue Ann Arbor, Michigan 48104 Dennis A. Yates Environmental Trace Substances Research Center and Department of Chemistry University of Missouri Columbia, Missouri 65211
RECEIVED for review September 24, 1981. Accepted February 22, 1982. The Columbia National Fishery Research Laboratory, U.S. Fish and Wildlife Service, Columbia, MO, provided partial research support for D. A. Yates. This is Contribution No. 274 of the Great Lakes Environmental Research Laboratory.
Comparison of Definitions of Response Times for Copper(I I) Ion Selective Electrodes Sir: Response time is one of the most significant parameters to characterize the nature of ion-selective electrode (ISEs). However, no universally accepted definition of the response time is available. This situation has caused considerable problems in the discussion of the response time (1). Various definitions have so far been proposed ( 2 , 3 ) . t , is defined as time required for the ISE potentials to reach a% of its equilibrium potential after a step change in sample concentration (activity). As a, 50,90,95, and 99 have been proposed. Also, t* was defined in 1975 by IUPAC as the length of time required for the ISE potential to become equal to its steady value within 1 mV (4). I t should be noted that t9,,was recommended later in 1978 (5). The reason for this may be due to the fact that t* gives different values for univalent and divalent ISEs when the electrode response is described by a single exponential or a single hyperbolic equation, even if the value oft, is theoretically the same for both uni- and divalent
ISEs. However, it should be noted that it is not yet sufficiently clarified whether or not the ISE response is represented simply by the equation having one time constant, but rather it has been revealed that multiexponential or hyperbolic terms are needed to simulate many of response curves of ISEs (6). In the latter case, even t , may lose its theoretical background. From a practical point of view, it is difficult to apply these definitions when the final electrode potential is not readily determined. Such is the case with rather slow response time. Actually, the response times have been reported on various ISEs, which cover a very wide time scale ranging from 10 ms to even hours (6). For example, the solid membrane ISEs do not always exhibit response times of 10 ms to second order, but they often give response times of the order of minutes or more (6-11). On the basis of the above mentioned background, we also defined a quantity t ( A t , AE)as a response time. The size of
0 1982 Amerlcan Chemlcal Society 0003-2700/82/0354-1198$01.25/0
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7,JUNE 1982
-"
W
f
j
Table I. Response Times Determined According to Some Conventional Definitions (See Text) [ CUZ+] changes
10-7 + 10-6 M 10-7 -+ 10-5M 10-7 -+ 10-4M
tqol min 0.59 t 0.04 a 0.18 .L 0.01 0.11 -t 0.11
10-7 -+ 10-3 M
1199
0.09 (*0.004)
t95, rnin
tsg, min
min
1.22* 0.07 0.37 f 0.03 0.17 i: 0.01 0.13 I 0.01
4.73 i 0.23 2.81 f 0.20 1.90k 0.07 1.43 i. 0.12
1.83 f 0.10 1.55 i: 0.14 1.53 i: 0.07 1.73 f 0.12
f*
9
LA-
2
3
1 Time,
5
min
--
Table 11. Response Times Determined According to a New Definition (See Text) [ CU*+]
changes 10-7 + 10-6 M
the potential change of ISEs generally decreases as a function of time. Therefore, as shown in Figure 1, it is possible to find a time t(At, AE)when the size of the potential change begins to become at most AE mV during the time At. The difference between t(At, M )and t* is that in the former case the final steady potential of the ISE is not explicitly dealth with, while the latter cannot be defined strictly without determining the final equilibrium potential.
10-7 -+ 10-4M
RESULTS AND DISCUSSION The results of response times for a Cu(I1) ISE according to the conventional definitions obtained from our experimental data are shown iin Table I. The electrode potential at 10.95
1
Figure 2. Typical examples of potential-time responses for Cu(I1) ion I) of the potential selective electrode and the 10% fraction (I remained until the equilibrium potential is attained.
a Each respona;e time value is the mean ( n = 6) * mean error of mean.
EXPERIMENTAL SECTION Electrodes and Chemicals. The ISEs used were Cu(I1) ion selective electrodes Cu-DKK made by Denki Kagaku Keiki Co., Ltd. The solid mismbrane is made of 90 mol % Ag2S and 10 mol % CuS. The refeirence electrode used was a HS-305D (Ag/AgCl, saturated KC1, a double junction type) of TOA Electroics Co. All reagents used were of analytical grade. All solutions were made up from CuS04 and 0.1 M NaN03. Water used was deionized and distilled. Pretreatment and Procedure. The surface of the electrode was pretreated a 3 follows. After a regular surface polish, the electrode was dipped in 0.1 M NaN03 aqueous solution for a long time period so aai to eliminate the surface contamination, the extent of which was continuously monitored by the potential measurement. The solution was repeatedly changed to enhance the effect of cleanliness until the potential corresponding to lo-* M Cu(I1) was finally attained. All concentration changes were done as follows: 'I .O mL of 10" M CuS04 solution was pipetted into 100 mL of a Cu(I1)-freesolution of 0.1 M NaN03. After the electrode showed a steady potential corresponding to the conM Cu(I1) ion, 1.0 mL of the more concentrated centration of solution of CuS04was pipetted. The test solution was constantly stirred with a magnetic stirrer. The whole procedure lasted less than 5 s, so that the delay, compared with the transient time of some minutes, is not very material except for some cases. And also, this time lag is, in any case, a constant factor in all measurements. The data for electrode potentials were collected by a minicomputer controlled on-line ISE measuring system (1%) every 3 s at 200 points. The precision of analog to digital conversion of this syigtem was 0.05 mV. The same experiment for each concentration change was repeated six times. All measurements were performed at 20 i 0.5 "C.
0
10-7 + 10-5M
10-7
-+
10-3 M
t( 2, 1.5),a min
t(4, 1.5)) min
t( 2, 0.3), min
t(4, 0.3)) min
0.88 * 0.03 0.54 t 0.02 0.51 i 0.04 0.46 * 0.03
1.06 i. 0.04 0.68 f 0.07 0.68 f 0.03 0.66 i: 0.05
2.87 * 0.10 2.54 i: 0.11 2.78 i 0.10 2.91 * 0.13
3.93 ?: 0.24 3.m+ 0.29 3.94 ~t 0.11 4.41 i. 0.15
a t(At, AE): At, min; AE, mV. Each response time value is the mean ( n = 6 ) ?: mean error of mean.
min after the concentration change was assumed to be the fmal equilibrium potential. From the results in Table I, it is concluded that for 1, ( a = 90, 95, and 99), the greater the activity jump, the shorter the response times. Also, the value oft, relatively increases with increasing a (a: 90 99). On the other hand, in the case of t* (time required for the potential to become equal to its final value within l mV), the values were found to be independent of the difference of the concentration changes. In contrast with t*, in the case oft,, the rest of the potential changes expected until the electrode potential reaches the final equilibrium potential are varied in proportion to the magnitude of the concentration jump: for t,, 2.9 mV for a concentration change from to lo+ M, 5.7 mV ( = 2 X 2.9 mV) for a change from to M, 8.6 mV (-3 X 2.9 mV) for a change from to M, and 11.5 mV (= 4 X 2.9 mV) for a change from loT7to lon3M, respectively. These &uations are shown in Figure 2. From the above results, it may be concluded that t* seems to be more suitable than t, for representing the response speed of the ISE in the later part of the response. In practice, it is necessary to know the final value of the electrode potential when an ISE is used to measure the activity of a real sample. In general it is often hard to apply the above mentioned definitions of response times when the final value of the ISE potential is not determined readily. Though in this study the potential at 10.95 min after the concentration change was assumed a5 the equilibrium potential, the potential increased even after that gradually by an order of 0.1 mV. Such a phenomenon is often noticeable in high-precision measurements. This change may be regarded as only an instrumental drift when the response time of the ISE is very fast. However, some ISEs have response times of the order of several minutes or more as mentioned earlier. In these cases it is difficult to determine the value of the final potential without hesitation.
-
1200
Anal. Chem. 1982. 54. 1200-1201
to determine. In the case of t ( A t , AE),it is not necessary to know E , and t , values beforehand. This is a completely different point from all other definitions of response times. The discussion presented here for Cu(I1) ISEs should also hold true for other types of ISEs.
The results of the response times according to a new definition are shown in Table 11. Each magnitude of A t is properly short and AE is small. The results indicate that the dependence of the response time on the magnitude of the concentration jump is very small as compared with those of tW,tB5, etc. For example, the value t (2,0.3) shows that the magnitude of the potential change between 2.8 and 4.8 min after the concentration change is at most 0.3 mV independent of the magnitude of the concentration jump. This result is similar to that oft* and indicates that the response speed of the electrode near the equilibrium potential depends little on the magnitude of the preceding concentration change. Some advantages of the new definition of response time are (1)it is one realistic measure of the practical performance of the electrode, (2) it can be applied to ISEs whose response speed is slow so that the final value is not determined readily, (3) it is concentration independent when At is chosen properly short and AE is small, and (4) it would also be used for some mechanistic discussion on the very final stage of responses where the precise c w e fitting is rather difficult. On the other hand, it has some disadvantages. It does not necessarily indicate precisely to what extent the electrode response approaches the equilibrium state, while t, theoretically gives such information. In addition, this new definition has no direct relation to mathematical formulation of response curves like the time constant in an exponential or a hyperbolic equation as t , does. However, this latter condition for t , holds only for ideal cases where the response curve can be fitted with a single exponential or hyperbolic. If the equilibrium potential E , is easily obtainable, it is no more necessary to define such parameters as t, or t*; instead, the value of E , and t , (time needed to get equilibrium potential) could simply be reported. However, a logical paradox raised here is that we cannot determine t, and t* values without knowing E , and t,, which are not necessarily easy
ACKNOWLEDGMENT The authors gratefully acknowledge S. Fujiwara for his support toward this study. LITERATURE CITED Fleet, B.; Ryan, T. H.; Brand, M. J. D. Anal. Chem. 1974, 46, 12-15. Rangarajan, R.; Rechnitz, G. A. Anal. Chem. 1975, 47, 324-326. Mertens, J.; den Winkle, P. V.; Massart, D. L. Anal. Chem. 1976, 48,
272-277. IUPAC Pure Appl. Chem. 1976. 48, 127-132. IUPAC I n f . Bull. 1978, NO. 1 , 69-74. Shatkay, A. Anal. Chem. 1976, 48, 1039-1050. Umezawa, Y.; Nagata, M.; Sawatari, K.; FuJlwara, S. Bull. Chem. SOC. Jpn. 1979, 52, 241-242. Alexander, P. W.; Rechnltz, G. A. Anal. Chem. 1974, 46, 250-254. Rechnitz, G. A.; Kresz, M. R.; Zamochnlck, S. B. Anal. Chem. 1966, 38, 973-976. Rechnitz, G. A.; Kresz, M. R. Anal. Chem. 1966, 38, 1786-1788. Blaedel, W. J.; Dlnwlddie, D. E. Anal. Chem. 1975, 47, 1070-1073. Sawatari, K.; Imanishi. Y.; Umezawa, Y.; Fujlwara, S. Bunseki Kagaku ~ 7 8 ~ 2 180-183. 7 ,
’
Present address: Department of Chemlstry, College of General Education, The University of Tokyo, Komaba, Meguroku 153,Japan.
Isamu Uemasu’ Yoshio Umezawa* Department of Chemistry Faculty of Science The University of Tokyo Hongo, Tokyo 113, Japan
RECEIVED for review December 7, 1981. Accepted March 8, 1982.
-. .. . . . ... Gas rnase memiluminescence ot Arsine Mixea with wzone A
A.
-
Sir: We have observed a room-temperature gas phase chemiluminescence between arsine (ASH,) and ozone. Arsine was generated via sodium borohydride reduction of arsenic trioxide using a small amount of Nz carrier and allowed to react with a 2% ozone/oxygen mixture produced by passing pure oxygen through an electrodeless discharge. The gases mixed in a Pyrex reaction vessel producing a bluish white chemiluminescence. A 0.25-m McPherson monochromator with an EM1 9526B photomultiplier and Keithley electrometer were used to obtain the spectra. Figure 1 shows a low-resolution spectrum of the chemiluminescence of arsine with ozone observed a t a reduced pressure of 22 torr. The UV bands from 300 to 331 nm arise from the As0 y system (A22 X21T) (I). The origin of the diffuse apparent continuum is unknown, but by analogy to the similar phosphorus “phosphorescence” discussed by VanZee and Khan (2) it may be due to an excimer, thus A s O ( ~ ~ )A s 0 e (ASO)~* 2As0 hu
-
+
-
+
The UV bands of As0 have previously been observed in diffusion flames (3-5) and discharge systems (6-8). In only one of these studies, however, was a similar visible continuum observed (3). The suggestion of the (ASO)~* excimer to account for the visible continuum is based on three considerations. Firstly, if the excimer has a bound excited state and a dissociative ground state, the excimer emission would be
I
I .
-
expected to be a continuum, red shifted relative to the forbidden A s O ( ~ ~ T X2n)transition which would occur with 0 0 at 3378 A (7). Secondly, the spectral characteristics are very similar to those found for the chemiluminescent reaction of P4with moist oxygen examined by VanZee and Kahn (2). On the basis of their experimental evidence, the analogous (PO),* excimer was assigned as the emitting species in the continuum. Finally, we have obtained experimental evidence consistent with this proposition. Spectra have been taken at higher pressures in which the continuum increased in intensity relative to the UV band emission. In a flow tube constructed to study the kinetics of this chemiluminescence we have found that the intensity of the visible bands increases markedly with increasing pressure and also that the maximum visible intensity can occur down stream from the initial mixing zone. A model involving excimer formation can be constructed which has these properties.
-
RESULTS AND DISCUSSION The primary technique for analysis of arsenic compounds is reduction to arsine using sodium borohydride followed by atomic absorption spectrometry (AAS) (9-11) or atomic fluorescence spectrometry (AFS) (12). The determination of arsenic by AAS of its near-vacuum UV resonance line has been reported to encounter difficulties with stability of the light source as well as absorption interfence from oxygen-containing
0003-2700/82/0354-1200$01.25/00 1982 American Chemlcal Society
