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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
T I M E
Figure 4. Memory effects. (a) 0.2 ppm Fe in diluted mineral acid, steady state undetermined, analysis fails; (b) 0.4 ppm Fe in diluted acid, steady
state undetermined, analysis fails again; (c) norreproducible acid blank (erroneous signal equivalent to that of = I ppm Fe), steady state undetermined; (d) 0.2 pprn Fe in deionized water, steady state attained, analysis succeeds: (e) 0.4 ppm Fe in deionized water, steady state attained, analysis succeeds again; (f) microsampling memory transients (acid blank) accompanying sudden acidity increase; interference decays rapidly with trial number until it reaches the small, reproducibly constant value seen in (9). This small value is near or below the limit of detection imposed by system noise; (9) greatly reduced acid blank (signal equivalent to or less than that of -0.1 ppm Fe) encountered when using microsampling; (h) 0.4 ppm Fe microsampling replicates in acid, analysis succeeds (when the small memory signal (9) is subtracted). Similar memory improvements were observed for the microsampling determination of silicon in petrified wood digest (in HF) by atomic absorption spectrometry represent a n interference disadvantage heretofore not mentioned in the literature for the microsampling approach. T h e opposite is true of the second type of memory (Figure 3, parts c and d showing large, steadily increasing, nonreproducible memory signals) which presents a more serious problem with steady-state sampling. This problem is dramatically reduced (see Figure 4) by the microsampling approach. T h e steady-state nebulizer determination of silicon and iron in digested complex materials is frequently plagued by the second memory type (not due to reagent contamination) and can be seen to be greatly improved by t h e small sample approach even if large amounts of sample are available. Figure 4 also shows t h a t the first memory type (transients) generally occurs only when the sample acidity is suddenly raised (parts d, e, and f). T h e figure also shows t h a t this memory becomes very small and reproducibly constant a t a
value near or below t h e limit of detection for microsampling (parts f and 9). This dramatic reduction in memory occurs even at high acidity for microsampling if the sample acidity is maintained relatively constant thereafter. No attempt was made to identify the source of this acid memory; it was simply observed to be heavily pronounced for Fe and Si when steady-state nebulizer sampling was employed with t h e Varian Techtron AA-5. This normally high acid memory is subsequently observed in the present studies to be very aptly reduced and controlled using the microsampling nebulizer technique.
C 0NCLUS IO NS T h e new microsampling cone for capillary pneumatic nebulization represents a technique of similar concentration sensitivity, improved absolute sensitivity, similar precision, improved sample size requirements, improved analysis time, reduced memory problems, and improved tolerance to high salt content materials in comparison to normal steady-state aspiration approaches. Microsampling cones are found to be very desirable for premixed burner nebulizer systems used in flame emission and atomic fluorescence as well as atomic absorption spectrometry.
LITERATURE CITED (1) H. T. Delves, Ana/yst(London), 95, 431 (1970). (2) H. L. Kahn, G.E. Peterson, and J. E. Schallis, At. Absorpt. Newsl., 7, 35 (1968). (3) H. Massmann, Spectrochim. Acta. far7 8.23, 215 (1968). (4) T. S. West and X . K. Williams, Anal. Chim. Acta, 45, 26 (1969). (5) J. Y. Hwang, P. A. Ullucci, and S. B. Smith, Am. Lab., August (1971). (6) M. B. Denton and H. V. Malmstadt, Anal. Chem., 44, 241 (1972). (7) K. W. Olson, W. J. Haas. Jr., and V. A. Fassei, Anal. Chem.,49, 632 (1977). (8) R. C. Fry and M. 8.Denton, Anal. Chem., 49, 1413 (1977). (9) F. J. Schmidt, and J. L. Royer, Anal. Lett., 6 , 17 (1973). (IO) K . G.Brodie, Am. Lab.. March, 73 (1977). (1 1) E. Sebastiani, K. Ohls, and G. Riemer, Fresenius' Z. Anal. Chem., 264, 105 (1973). (12) D. C. Manning, At. Absorpt. Newsl., 14, 99 (1975). (13) H. Berndt and E. Jackwerth, Spectrochim.Acta, Par7 8,30, 169 (1975). (14) T. T. Gcrsuch, Analyst (London),84, 135 (1959).
RECEIVEDfor review October 3,1977. Accepted June 19, 1978. This research was supported in p a r t by the Office of Naval Research and by a n A. P. Sloane Foundation Research Fellowship (to M.B.D.).
Flow-Through Electrochemical Cell with Open Liquid Junction W. J. Blaedel" and
Z.Yim
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53 706
We report t h e construction and characterization of a flow-through electrochemical cell with open liquid junction, which permits measurements in the steady-state mode. T h e cell is of compact design, simple to use, and has good wash characteristics. Most flow-through type cells employ porous plugs (I),microcracks (2), membranes (3),or agar-impregnated frits ( 4 ) t o effect contact between sample and reference solutions, and hence are prone to clog and are difficult to clean, especially when surfactants or proteins are involved. Cells with open liquid junctions have been used to alleviate such difficulties (5). Our design is of the open liquid junction type with the reference and working electrodes situated at channels that form the arms of a Y, and with the flowing reference and sample solutions merging at a junction downstream from both electrodes (see Figure 1). Cell performance is characterized by examining t h e cathodic reduction of K3Fe(CN)6.
EXPERIMENTAL Figure 1 is a schematic diagram of the cell, which is machined from Plexiglas. The main body of the cell is 1.5 in. thick and 1.5 in. in diameter, with 2-mm diameter flow channels drilled as shown. There are two wells to accommodate the working electrode (platinum) and reference electrode (silver/silver chloride electrode, SSCE). Inlet and outlet ports are all threaded to accommodate Cheminert fittings. The removable working electrode consists of two platinum tubular electrodes (each 2 mm in diameter, 2 mm long) separated by a thin layer of nonconducting epoxy. Such a design can have various applications, anodic stripping voltammetry with collection (ASVU'C) (6), for example. When the two electrode leads are connected together as is the case here, it works as a single electrode with an effective length of 4 mm. The potting with nonconducting epoxy (Epotek 349, Epoxy Technology, Watertown, Mass.) of the platinum disk inside the Plexiglas rod, the drilling of the flow
0003-2700/78/0350-1722$01.0010 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
y Figure 1. Schematic diagram of the cell. (A) Inlet port for flowing sample solution, (B) outlet port to waste. (C) port for filling reference electrolyte, (D) inlet port for flowing reference electrolyte, (E) retainer ring, (F) platinum tubular electrode, (G) O-ring, (H) hole for bridging contact to reference electrode, (J) junction for merging of solution streams, (K) Teflon washer, (L) anodized silver wire, (M) retainer ring screws, and (N) copper wire leads
channel through the Plexiglas rod and the platinum disk, and the polishing of the electrode surface are done as described previously ( 4 ) . The electrical contact between the platinum electrode and the copper wire lead, N, is via conducting epoxy (Epotek 410E). A hole is drilled through the Plexiglas to accommodate the copper wire lead: N, and after contact was made, the hole is subsequently filled with nonconducting epoxy. The working electrode, guided by the O-ring, G. when inserted into the well, is sealed by the Teflon washer. K, and bolted down tightly with three screws through retainer ring E, which rests on the shoulder of the working electrode. The inlet port, A, is fitted with a short length of glass tubing, which is inserted into the sample solution. The flow of sample solution through the tubular working electrode is by siphon action, flow rate control being achieved by pressure head and a stopcock preceding a rotameter. The removable reference electrode consists of five turns of a 24-gauge silver wire wrapped around a Plexiglas rod, and coated with AgCl in the usual manner ( 7 ) . The electrode is held in place in its well in the s-ime way as the working electrode. The space between the wall of the well and the slightly undersized Plexiglas rod constitute: the reference electrolyte compartment, and is sealed by the Teflon washer, K, and the O-ring, G. The flow channel and the hole, H, are both 2 mm in diameter. Electrical contact between the silver wire and the copper wire lead, N , is via conducting epoxy. The hole drilled to accommodate the copper wire lead is then sealed with nonconducting epoxy. Electrolytic connection between the working electrode and the reference electrode is through the flow channel and the hole, H. The reference electrolyte solution (0.1 M KC1 saturated with AgC1) is pumped through the reference channel with a syringe pump (Model 237-2, Sage Instruments, Inc., White Plains, N.Y.) at a convenient flow rate of 0.9 mL/min. The cell resistance measured with a conductance bridge (Model 15B1, A . H. Thomas Co., Philadelphia, Pa.) at 60 Hz is 6 KQ, and is independent of flow. Linear scan voltammograms and stopped-flow current measurements were made with a Sargent XV polarograph. For pulsed-flow amperometry (41, a steady-state technique t o reduce background current, a picoammeter (Model 414S, Keithley Instruments, Cleveland, Ohio) is used, with output connected to a strip chart recorder (Omniscribe, Series 5000, single channel; Houston Instruments, Bellaire, Texas). Solutions are prepared fresh before use with reagent grade chemicals and deionized water (17 M R ,Culligan cartridge water treatment systems, Culligan International, Northbrook, Ill.). Sample solution is deaerated by house nitrogen, which has been passed through a gas filter (Model 020, Deltech Engineering Inc., New Castle, Del.) and a bubbler to saturate it with water vapor. The reference solution is not deaerated, since it meets the sample solution downstream from the working electrode. The platinum
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APPLIED POTENTIAL, VOLTS VS. SSCE
Figure 2. Current-potential curve for 60 pM K,Fe(CN),. Supporting electrolyte: 0.1 M KCI, 0.05 M phosphate buffer (pH 7.4). Scan rate = 0.1 V/rnin. Dotted line is corrected (by subtraction)for background, measured in pure supporting electrolyte loo
I
7
801
i 0
I
0.5 I.o I.5 CUBE ROOT OF FLOW RATE, Crnl/MIN)”3
Figure 3. Dependence of current on flow rate. 1 pM K,Fe(CN),
in
0.1 M KCI, 0.05 M phosphate buffer (pH 7.4). Applied potential, -0.4 V
electrode is pre-treated under flow condition by cycling between +1.0 and -1.0 V vs. SSCE three times at 10-min intervals. All solutions are prepared with 0.1 M KC1 ;and 0.05 SI phosphate buffer at pH 7.4 (1:4 molar ratio mixture of KH2P04and K2HP0,). All measurements are made inside a Faraday cage.
RESULTS AND DISCIJSSION Linear scan voltammograms of K,3Fe(CN)6were obtained a t both 1 m M and 60 p M . Replicate voltammograms were superimposable. Furthermore, a t a constant sample solution flow rate, current was independent of the flow rate of the reference solution. Figure 2 shows the linear scan voltammogram of 60 pM K3Fe(CN),, with and without background correction. E l p 2is -0.105 V vs. SSCE:. T h e stopped-flow voltammogram is steeper and shifted anodically. Eli2being 0.055 V vs. SSCE. T h e curve parameters were calculated according t o the curve-fitting procedure described previously
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
I MIN
Flgure 4. Pulsed flow current (represented by the difference between H and L current tracings) for 1 1 M K,Fe(CN),. Conditions: as in Figure
Flow rates, 4.72 (H) and 0.37 (L) rnL/rnin. The pulsed flow current for the blank solution is represented by the difference between the dotted line and the L current tracings
K,Fe(CN)6 solution and the background electrolyte solution, as predicted by theory (9). In the ferricyanide solution, current measured experimentally at the no-flow condition (see asterisk in Figure 3) is significantly greater than the value extrapolated from measurements a t finite flow rates. This deviation and the nonzero intercept have been observed previously (41, and have been explained as a consequence of axial diffusion. Figure 4 is a reproduction of a pulsed-flow chart record for 1 pM K3Fe(CN)G. A detection limit of 2 X M may be estimated from the figure, if it is defined as equal to the peak-to-peak noise level of the signal. The sensitivity of the cell is calculated to be 0.36 kA/pM cm2 a t a flow rate of 4.72 mL/min.
3.
(3):the formal electron exchange rate constant (h"')is 0.00274 f 0.00074 cm/s; the transfer coefficient (CY)is 0.56 f 0.08; and E"' is 0.14 V. These values, except for a , are comparable to values obtained previously with a rotating disk electrode (8) and with a turbulent tubular electrode ( 3 ) . T h e significant difference in cy (reported as 0.378 f 0.07 (8) and as 0.380 f: 0.03 ( 3 ) )were ascribed to differences in pre-treatment of the electrode. Diffusion limited currents of 1 pM K3Fe(CN), and of background electrolyte solution were measured at various flow rates a t a n applied potential of -0.40 V. A cube root dependence on flow rate (Figure 3) was found for both the l 1M
LITERATURE CITED H.F. Osswald, P. C. Meier, and R . E. Dohner, "Advances in Automated Analysis", Technicon International Congress,
W. Simon, D. Ammann,
Vol. 1, 1976, pp 59-62. J. G. Schindler. Biomed. Tech.. 22. 235-244 (1977). W. J. Blaedel and G.W . Schieffer, J . Hectroanal. Chem., 80, 259-271 (1977). W. J. Blaedel and D.G. Iverson, Ana/. Chem. 49, 1563-1566 (1977). A . H. Brand and R. J. M. Rao, U. S. Patent 3,853,732 (1974). G. W. Schieffer and W . J. Blaedel, Anal. Chem., 49, 49-53 (1977). D. R. Sawyer and J. L. Roberts, "Experimental Electrochemistry for Chemists", Wiley, New York, N.Y., 1974. W. J. Blaedel and G.A. Mabbott, Anal. Chem., 50, 933 (1978). W. J. Blaedel and L. N. Klatt, Anal. Chem., 38, 879 (1966).
RECEIVED for review April 10, 1978. Accepted June 19, 1978. This work has been supported in part by a grant (No. C H E 76-15128) from the National Science Foundation.
Accuracy of the Hydrogen Ion Selective Glass Electrode E. P. Serjeant" and A. G. Warner Faculty of Military Studies, University of New South Wales, Duntroon, A.C.T. 2600, Australia
A glass electrode, selected on t h e basis of its ability to exhibit a consistent Nernstian response between the p H values for 0.05 m potassium hydrogen phthalate (pH 4.008) and the equimolal phosphate buffer 0.025 m potassium dihydrogen phosphate, 0.025 m disodium hydrogen phosphate ( p H 6.858) a t 25 "C ( I ) , was used in the cell glass electrodelaqueous solution chlorideJAgC1;Agin order to evaluate the accuracy t h a t can be expected when any correctly functioning glass electrode is used in such a cell and how this accuracy varies with concentration. For this purpose, solutions were chosen such t h a t the acid ( 2 ) or sodium ion ( 3 ) errors of the glass electrode were not likely to be significant over the range of concentrations studied. This study seeks also to explain the observations of previous workers (4-6) using cells of this type who have reported a slight but linear drift of cell emf with time t h a t cannot be associated with the initial equilibrium period which occurs immediately after an electrode transfer. This drift has been attributed to small changes in t h e asymmetry potential of the glass electrode. Measurements using the cell glass electrodelaqueous solution + chlorideJAgC1;Ag are closely analogous to p H measurements if the acidity function p(aHycl) is invoked. Effectively a two-cell system is used in which the electrodes are common to both cells. One cell contains a solution whose acidity function is known, p(aHyCl)s,and the other a solution whose acidity function value is to be measured, p(aHyCl),. If the chloride ion molalities in the two cells are respectively m, and m,, the change in cell emf, AE, on transferring the
+
0003-2700/78/0350-1724$01 .OO/O
electrodes between the two (7) is related to p(aHy& by
where k is the Nernst constant (0.06916 at 25 "C). The standard chosen for this work was 0.02 m hydrochloric acid (p(aHycl)= 1.815 a t 25 "C (8)) and therefore Equation 1 can be written as
EXPERIMENTAL Potassium chloride and sodium chloride were each purified according to the general method of Pinching and Bates (9),with the exception that the final fusion process was replaced by heating the salts at 225 "C under vacuum (0.1Torr) for 1 h. Potassium dihydrogen orthophosphate and disodium hydrogen orthophosphate were recrystallized twice from water and dried at 120 "C. Solutions of hydrochloric acid were prepared from the constant boiling acid (9). These were standardized by gravimetric analysis and their molal concentrations checked subsequently by potentiometric titration against purified ( I O ) 2-amino-2hydroxymethyl-l,3-propanediol(Tris). All solutionswere prepared in ion-free water stored under nitrogen. The pH measurements used in the initial selection of the glass electrode were made on a Vibron Electrometer Model 33B in conjunction with a pH Measuring Unit Model C-33B (Electronic Industries Ltd., Richmond, Surrey, England). The output from the electrometer 1978 American Chemical Society
