Anal. Chern. 1983, 5 5 , 7445-1448
(coDolvmer).26875-02-5: AI-10 (reDeatineunit), 29087-9!5-4:ALlO (cipoiymerj, 25895-32-3; BTDA-ODA ccopolymer),26875-71-8; BTDA-ODA (repeating unit), 24991-11-5; BTDA-MPDA (copolymer), 25300-72-5; BTDA-MPDA (repeatingunit), 25868-65-9; BTDA-PMA-ODA (copolymer), 85629-22-7; ODPA-ODA (copolymer), 85629-23-8; ODPA-ODA (repeating unit), 52325-20-9.
LITERATURE CITED (1) Sroog, C.E. "Encyclopedia of Polymer Science and Technology"; Wiley: New York, 1969;Vol. l l , pp 247-272. (2) Mlejnek, 0.; Cveckova, L. J . Chromatogr. 1974, 94, 135.142.
r445
(3) Schlueter, D.; Singia, S. Anal. Chem. 1977, 49. 2349-2353. (4) Haslam, J.; Will& H.; Squirrell, D. "Identification and Analysls of Plastics", 2nd ed ; ILIFFE Books: London, 1972;pp 311-312. (5) Ing, H.; Manske, R. J. Chem. Soc. 1926, 2348. (6) Jones, J. J . folym. Sei., Part C IB69, 22, 773-784. (7) Dine-Hart, R.; Parker, D.; Wright, W. Br. folym. J . 1971, 3 , 226-234. (8) Robb, E.; Westbrook J. Anal. Chem. 1963, 35, 1644. (9) Bailv. J. Anal. Chem. 1967. 39. 1485. (io) Fleser, L. "Experiiments In Organic Chemistry"; D. C. Heath and Co.: Boston, MA, 1957;pp 199-200. (11) Rogers, F. E. Chnm. Absfr. 1968, 68, 304199.
RECEJYED for review October 13,1982. Accepted April 4,1983.
Determination of Fluoride at Low Concentrations with the Ion-Selective Electrode Erik Kissa Jackson Laboratory-Chemicals Wilmington, Delaware 19898
and Pigments Department, E. i! du Pont de Nemours and Company,
The fluoride ion selective electrode (ISE) developed by Frant and Ross (1) is a sensitive and selective tool for the determination of fluoride ion activity (1-9). The response of the electrode has been reported to be Nernstian above M F- (10) or 1.5 X lo-" M F- in NaF and 5 X lo4 MF- in buffered IVaF solutions (11).The Nernstian response has been extended to lou0 M fluoride (9, 12) with cations strongly complexed by fluoride. However, the electrode response has been slow and sluggish at low fluoride ion concentrations (6, 13-15). The drift of electrode potential has necessitated frequent calibration (3, 10). The kinetics of the fluoride ISE response have been investigated (16-26) and its dynamic response has been shown to result from four processes: ion diffusion, reaction, L a 3 dissolution, and calibration drift (15). The procedures used for the determination of fluoride ion activities and concentrations have differed in several important aspects. The fluoride ISE has been conditioned and stored in a buffer solution (14) or in a buffered fluoride solution (2, 3,10).Barnard and Nordstrom (27) obtained a stable electrode potential in 12 to 18 min by soaking the electrode in a standard solution of a concentration similar to that of the analyte. Between immersion in analytes, the electrode is customarily rinsed with water (3), is rinsed with a buffer solution, or is not rinsed (15). The reported precision of fluoride determination, expressed as the standard deviation, decreases with the decreasing fluoride Concentration: 0.4% relative in the lov3M range (IO), 0.8% relative in the 10-1 to lo4 M range (8),about 2% relative to 3 X M range, 4% relative a t IO-' M, in the 5 X and 10% relative a t 5 X M fluoride (28, 29). We had to determine lo4 or M P in solutions produced by combustion of organic compounds in an oxyhydrogen torch. The excessive electrode drift, long response times, and inadequate prcxision necessitated development of the methods described in this report. Problems associated with the instability and drift of the fluoride ion selective electrode have been resolved by (a) limiting th,e fluoride concentration to which the electrode is exposed to a 0.01 to 0.1 mg of F-/L or 0.05 to 1.0 mg of F-/L concentration range, (b) measuring the electrode potential in the analyte by approaching equilibrium in the same direction from a higher potential to a lower potential, and (c) keeping the temperature of the solutions constant within f0.2 OC or closer. The fluoride concentration in the analyte is adjusted to the concentration range of the ion-selective electrode by
dilution or fluoride (addition,or fluoride is determined by (;he analyte addition method.
EXPERIMENTAL SECTION Apparatus. The Iluoride ion selective electrode, Orion Model 94-09A, was used in combination with a double junction electrode, Orion Model 90-01. 'The cell potentials were measured with the Orion Ionalyzer, Model 901. The Orion electrode holder (Catalog No. 13-641-814)was provided with a stop to keep the immersion depth of the electrodes constant. Two sets of electrodes were u s e d one set was expcxsed only to solutions containing 10-100 pg of F / L , and the other to solutiionscontaining 50-1000 pg of F / L (0.05-1.0 mg of F-/L). All volumetric flasks and beakers (50 mL) used were made of "Nalgene". Agitation was provided by a thermally insulated magnetic stirrer operating at a constant speed. Reagents. The standardizing solutions were prepared by successive dilutions of a stock solution containing 11.10 g of reagent-grade NaF (dried to a constant weight at 125 OC) per liter. The standardizing solutions and analytes contained an acetic acid-sodium acetate buffer (2% by volume) prepared as follovvs: Add to 2800 mL of water 480 mL of acetic acid, reagent grade, and 500 mL of 30% sodium hydroxide solution, made of ACS certified, electrolytic NaOH pellets. Dilute to 3800 mL with water and adjust to pH 5.0 to 5.2 with sodium hydroxide. TISAB I1 (without, CDTA) or TISAB I11 (3) were used for electrolyte-containing analytes. RESULTS AND DISCUSSION Electrode Potentiometry. The equilibration time needed to attain a stable electrode potential decreases with increasing fluoride concentration and stirring rate, and increases wilh the increasing concentration change resulting from successive immersions (Figure 1). If the concentration difference is small, the equilibriuim time can be reasonably short even a t low fluoride concentrations. We have restricted, therefore, the exposure of the fluoride ion selective electrode to solutions differing less than 10 or 20 times in fluoride concentration. By limiting the fluoride concentration range and using a programmed immersion sequence, the equilibration time in the analyte has been reduced to 3 to 7 min even at fluoridle concentrations as low as 20 wg of F-/L. This is the lowc?r practical limit for determining fluoride conveniently, becaupe the electrode response is no longer linear below 20 fig of F-/L (lo4 M F-) (Figure 2). If the millivolt reading indicates that the fluoride concentration in the analyte exceeds the concentration range of the fluoride ion selective electrode (20-100 kg of F / L or 100-1000
0003-2700/63/0355-1445$0 1.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
1446
Table I. Sequential Program of Fluoride Determination external standard method electrode electrode A B F - concn (mg of F-/L) of standardizing solution used for electrode storage
0.02
analyte addition method electrode electrode A B
0.10
0.02
-
0.10
Sequential Program for F- Concentration Determination (I) conditioning concn, mg of F-/L time, min (11) reference solution concn, mg of F-/L time,a min (111) analyte time, min
0.01 3
0.05 3
0.01
0.02 l + l O
0.10
0.025
l+lO
l+lO
+ loa
0.05 3
3
0.10 1 + 10
record mV 10 min record mV 10 min after adding analyte after adding analyte Concentration Range of F - Determined 0.02b 0.10b 0.025 0.10 0.10c 1.oc depends on analyte volume added 1
lower limit, mg of F-/L higher limit, mg of F-/L
1 -t l o a
a One minute in 40 mL of solution, discard solution, 1 0 min in another 40 mL of solution, record mV reading. If Fconcentration of the analyte is below this limit, add NaF solution to increase F- concentration. If F- concentration is above this limit, dilute analyte with diluted buffer ( 2 0 mL/L).
IO0
70
w
50
If
40 .J
iLL
P
30
x
,601
I
I
5
10
TIME I M I N I
I
I
15
20
I 25
Flgure 1. The electrode potential of the fluoride ion selective electrode vs. time in 20 p g of F-/L stirred solution after an immersion in 30 pg of F-/L (upper curve) or 100 p g of F-/L (lower curve). The electrode was immersed for 1 rnin in a stirred 20 p g of F-/L solution, which was replaced by a fresh solution before recording electrode potentials.
pg of F-/L), the electrode is withdrawn immediately from the analyte and the fluoride concentration of the analyte adjusted by appropriate dilutions or additions. The concentration restriction applies also to rinsing of electrodes and their storage. The electrodes are stored in a buffer containing fluoride in a concentration correponding to the lower limit of the fluoride concentrations measured. Between immersions, the electrode stems are wiped and the electrode tips gently blotted with tissue paper to remove adhering liquid drops. The electrodes are not rinsed with water or a buffer solution. The electrodes are first immersed in 40 mL of the solution to be measured (analyte or reference solution) which is agitated for a minute and discarded. A fresh 40-mL portion of the solution is then used for the electropotential measurement. The precision of the electrode potential measurement by our procedure depends mainly on temperature constancy. The standard deviations of temperature and the electrode potentials in a buffered solution containing 20 pg of F / L were 0.16 "C and 0.18 mV and 0.05 "C and 0.08 mV, respectively (six determinations each, alternated with electropotential measurements in analytes containing 23 to 42 pg of F-/L). If the temperature is kept within 10.2 "C,the electrode can be operated for weeks without a calibration drift.
20
/ lo
[Io
b
Ib
io
io
410
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
>
>
l-
I-
E
E
z 4
z 4
%z
mV
100
130
140 I
1
I
I
IN 100 g F-/L
IO
'
TIME (MINI
u
E
I
-
110-
120
0
>
80 90 -
'3 'I1
1,447
0
-
LtT-
i
IN ANALYTE
IN 50 gF7L
Figure 3. Determination of fluoride by the external standard method.
Table 11. Precision and Accuracy of the External Standard Method Pg of
F-IL takena 0.5
1.o 3.0 30 50 300
Pg of
standard deviation ( u ) of % relaF"/L tive
F'IL found Electrode A 0.73b 0.04 1.17& 0.07 3.32 0.06 29.5 0.20 49.8 0.37
6.1 6.0 1.8 0.67 0.75
Electrode B 301 1.7
0.57
(x)
a The concentration of F- added to water. 20 mL of water containing 2.00 fig of F- and the buffer were added to 80 mL of analyte. The amount of F- added was deducted from the result.
External Standard Method. The external standard method compares the electrode potential in the analyte with that of a reference solution. The sequential steps of the procedure we shown in Table I and in Figure 3. The electrode is immersed first in a buffered solution, the fluoride concentration of ,which i s half of the lowest fluoride concentration to be measured with the electrode. Reconditioning the ion selective electrode at a fluoride concentration below that of the analytes and standardizing solutions assures that the electrode potential equilibrium is always approached in the same direction and from the higher potential side. The electrode potential is then measured in a standardizing solution representing the lower concentration limit of fluoride. This is followed by electrode potential measurement in the analyte. A buffer is added to the analyte and the solution diluted to a measured volume sufficiently large for two electrode immersions (the first immersion has the purpose of washing the electrode). The potential in the standardizing solution can be assumed to be equal to zero and the fluoride concentration calculated from the potential difference between the potentiids in the standardizing solution and in the analyte (3). If the temperature of the solutions is closely controlled (hO.1 "C),the electrode potential in the standardizing solution is reproducible and the fluoride concentration can be calculated from the potential instead of from the potential difference in t,he analyte. The accuracy and the precision of the determination of' fluoride in low concentrations are shown in Table 11. Analyte Addition Method. The analyte addition method (31) (Table I) eliminates the dilution of analytes to the con-
Figure 4. Determination of fluoride by the analyte addition method.
Table 111. Accuracy of the Analyte Addition Method 1. Fluoride Concentration Calculated from A E mg of Y m p of F-/L in AE, F-/L error % relative analyte mV found 1 .o 2.0 5.0 10.0
14.6 25.2 42.2 57.5
0.97 2.01 4.92 9.88
-2.7
+ 0.5
-1.7 -1.2
2. Fluoride Concentration Calculated from E mg of mg of error % F-/L in E, F-/L atlalyte mV found relative
1.o 2.o 5.0 10.0
116.2 1106.0 88.8 '73.3
1.002 2.005 4.97 10.06
+ 0.2 +0.3 +0.6 + 0.2
centration range of the fluoride ion selective electrode and minimizes matrix effects since the volume of the anal@ addod is small relative to tlhat of the reference solution. The analyte addition method is based on the same principles as the external standard method: a limited concentration range of the fluoride ion selective electrode, a rigid adherence to a sequential program (Figure 4 ) , and a close temperature control. The electrode potential in a measured volume (usudy 40 mL) of a reference solution is determined, a measured volume of the analyte (up to 4 mL) is added, and the electrode potential recorded again. The fluoride concentration in the analyte is calculated from the resulting potential difference by using a calibration curve or linear regression data
where V, and C, are the volume and F concentration of the standardizing solution, C,is the F concentration in the solution after analyte addition, and C, and V , are the F concentration and the volume of the analyte added. The precision of thle analyte addition method depends on the temperature constancy of the solutions, the volume of the analyte added, and the matrix effects. When the fluoride concentration is calculated from the potential difference resulting from the addition of 4 mL of analyte ( 2 = 0.193 mg of F-/L) to 40 mL of 25 pg of F-/L standardizing solution, the standard deviation is 0.0019mg of P / L or 1.02% relative.
1448
Anal. Chem. 1983, 55, 1448-1451
The accuracy of the method is shown in Table 111.
ACKNOWLEDGMENT The author thanks C. J. Hensler for his helpful comments and Ward R. Gibson for Careful assistance with experimental work.
LITERATURE CITED Frant, M. S.; Ross, J. W., Jr. Science 1986, 754, 1553. Veseiq, J.; Weiss, D.; Stulik, K. “Analysis with Ion-Selective Electrodes”; Halsted Press, Dlvislon of Wiley: New York, 1978. Instruction Manual, Fluoride Electrodes, Model 94-09, Model 96-09; Instruction Manual, Model 901, Microprocesser Ionalyzer; Orion Research, Inc.: Cambridge, MA, 1978. Llngane, J. J. Anal. Chem. 1987, 39, 881. Baumann, E. W. Anal. Chim. Acta 1968, 42, 127. Bock, R.; Strecker, S. 2.Anal. Chem. 1988, 235, 322. Mesmer, R. E. Anal. Chem. 1968, 4 0 , 443. Warner, T. B. Anal. Chem. 1989, 4 7 , 527. Baumann. E. W. Anal. Chim. Acta 1961. 54. 189. Pavei, J.;’Kuebler, R.; Wagner, H. Micfochem. J . 1970, 15, 192. Bazelle, E. W. Anal. Chlm. Acta 1971, 5 4 , 29. Trojanowicz, M. Talanta 1979, 26, 985. Warner, T. B. Prog. Anal. Chem. 1973, 5 , 229. Warner, T. 6.; Bressan, D. J. Anal. Chim. Acta 1973, 63, 165.
(15) Hawkings, R. C.; Corriveau, L. P. V.; Kushneriuk, S. A,; Wong, P. Y. Anal. Chlm. Acta 1978, 702, 61. (16) Veselj? J. J . Nectroanal. Chem. InterfacialElectrochem. 1973, 4 1 , 134. (17) Parthasarathy, N.; Buffle, J.; Monnler, D. Anal. Chim. Acta 1974, 68, 185. (18) Buffle, J.; Parthasarathy, N.; Haerdi, w . Anal. Chlm. Acta 1974, 6 8 , 253. (19) Buffle, J.; Parthasarathy, N. Anal. Chim. Acta 1977, 93, 111. (20) Parthasarathy, N.; Buffle, J.; Haerdi, W. Anal. Chim. Acta 1977, 93, 121. (21) Morf, W. E.; Llndner, E.; Simon, W. Anal. Chem. 1975, 4 7 , 1596. (22) Rangarajan, R.; Rechnitz, G. A. Anal. Chem. 1975, 4 7 , 324. (23) Cammann, K.; Rechnitz, G. A. Anal. Chem. 1978, 4 8 , 856. (24) Llndner, E.; Toth, K. Pungor, E. Anal. Chem. 1978, 48, 1071. (25) Mertens, J.; Van den Winkel, P.; Massart, D. L. Anal. Chem. 1978, 48, 272. (26) Shatkay, A. Anal. Chem. 1976, 48, 1039. (27) Barnard, W. R.; Nordstrom, D. K. Atmos. Environ. 1982, 76,99. (28) Sekerka, I.; Lechner, J. Int. J . Environ. Anal. Chem. 1973, 2, 313. (29) Sekerka, I . ; Lechner, J. F . Talanta 1973, 20, 1167. (30) Phillips, K. A,; Rlx, C. J. Anal. Chem. 1981, 5 3 , 2141. (31) Durst, R. A. Mlcrochim. Acta 1969, 611.
RECEIVEDfor review September 27, 1982. Resubmitted February 28, 1983. Accepted April 4, 1983. Research and Development Division Publication No. 594.
Determination of Benomyl by Reversed-Phase Liquid Chromatography Gunter Zwelg”’ and Ru-yu Gao2 Sanitary Englneering and Environmental Health Research Laboratory, University of California, Richmond Field Station, Richmond, California 94804
In support of the field research being conducted by this laboratory on the exposure to pesticides by fruit and crop harvesters, it became necessary to develop an analytical method for the fungicide benomyl (methyl [l-[(butylamine)carbonyl]-1H-benzimidazol-2-yl]carbamate).Because these field studies generated a large number of samples, the requirements for the proposed analytical scheme were speed and simplicity without sacrificing accuracy. Since the matrixes to be analyzed were relatively devoid of interferences (cotton gloves, surgical patches, and plant leaf surfaces), elaborate cleanup steps would probably not be needed. Analytical methods for benomyl and its metabolite carbendazim have been recently reviewed (I).Kirkland (2) and Bleidner (3) in their scheme converted benomyl in soil and plant tissues to carbendazim by acid treatment and analyzed carbendazim by HPLC on a strong cation-exchange column. Chiba and Veres ( 4 ) distinguished between carbendazim and benomyl by first converting carbendazim to the corresponding n-propylcarbamoyl derivative with n-propyl isocyanate (PIC) a t low temperatures and stabilizing benomyl by the addition of n-butyl isocyanate (BIC). They then resolved the resultant mixture by HPLC on a silica gel column. Farrow et al. (5) recognized the instability of benomyl in organic solvents and recommended refluxing plant and fruit extracts with HCl, thereby causing the quantitative conversion of benomyl to carbendazim. The latter was analyzed by HPLC on silica gel or reversed-phase HPLC on an ODS column. Cabras and co-workers (6) reported on the separation of carbendazim and benomyl from other fungicides by reversed-phase HPLC but apparently did not recognize the Visiting Scholar from the U.S.Environmental Protection Agency, Washington, DC. Visiting Scientist from Nankai University, Tianjin, People’s Republic of China. 0003-2700/83/0355-1448$01.50/0
instability of benomyl in organic solvents. The method which will be described here is based on the discovery by Chiba et al. (4, 7,8) that benomyl is converted to carbendazim in many organic solvents to varying degrees of completion. Of the solvents they studied, ethanol, dioxane, and methanol effected the conversion to carbendazim to 92% or greater. Quantitative conversion in methanol was achieved in 5.6 h. As will be shown in this paper, acetonitrile is a suitable solvent for the extraction of benomyl and carbendazim from cotton gauze and foliage and, at the same time, an effective solvent for the rapid quantitative conversion of benomyl to carbendazim, reaching completion in l to 3 h depending on temperature.
EXPERIMENTAL SECTION Materials. Authentic samples of benomyl and carbendazim were obtained from the EPA Reference Standard Repository, Research Triangle Park, NC; carbendazim was also obtained from the Agrichemicals Department, E. I. du Pont de Nemours & Co. All solvents used throughout were HPLC grade (“Baker Analyzed” or equivalent). Water for HPLC solvents were first passed through a Milli-Q water purification system. Mobile-phase solvents were further purified and degassed by filtering them through a Millipore filter in vacuo just prior to use. High-Performance Liquid Chromatography Apparatus. Waters Model 6000A solvent delivery system; WISP automatic sample processor; Waters Data Module with Automatic Integrator; Model 450 variable wavelength detector; RP-18 Spheri 5, Brownlee Labs. bonded reversed-phase column (25 cm X 2 mm, i.d.). Ultraviolet Spectrophotometer. Spectronic 2000, Bausch & Lomb. Analytical Scheme for Benomyl and Carbendazim. Standard solutions of benomyl (2 wg/mL to 12 wg/mL) and carbendazim (1.3 fig/mL to 7.9 wg/mL) are prepared in acetonitrile. Fresh solutions of benomyl are kept at room temperature for 3 h or 40 OC for 1h before use. This waiting period can be decreased even further, because the benomyl-to-carbendazim 0 1983 Amerlcan Chemical Society
