Analytica Chimica Acta, 159 (1984) 397-400 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Short Communication

EFFECT OF CELL GEOMETRY ON CONDUCTANCE MEASUREMENTS IN FLOW CELLS

DOUGLAS TAYLOR and TIMOTHY A. NIEMAN*

School of Chemical Sciences, University of Illinois, Urbana, IL 61801 (U.S.A.)

(Received 12th October 1983)

Summary. Solution resistance is monitored as the spacer thickness in the flow cell is increased. Different trends are observed for electrode pairs in series on the same side of the cell, directly opposing each other in opposite sides of the cell, and diagonally opposing each other in opposite sides of the cell. For the diagonal and series arrangements, anomal- ously high resistances are measured for narrow spacers.

Conductimetric monitoring of flowing streams is becoming widespread. It has found use in ion-chromatography [l] , in a flow-injection system [ 21, in ion-selective electrode monitoring [ 31 and in critical micelle concentration studies [ 41. In many flow-cell designs, the principal goal has been to minimize the cell volume without degrading performance. Recent work in this labora- tory on bipolar-pulse conductimetric monitoring of ion-selective electrodes in flowing streams has shown unusual effects on individual measurements and shapes of working curves depending on the relative location of the electrodes [ 51. This phenomenon seemed to deserve further study in order to learn general principles relevant to flow-cell design for conductimetric measure- ments. This communication presents results obtained when the distance between electrodes was varied for three different relative placements, and two different sizes of electrodes.

Experimental Instrumentation. A computer-controlled conductance instrument based

on the bipolar-pulse method (BICON) was used. The theory of BICON and description of the instrument can be found elsewhere [ 3, 6, 71. The instru- ment allows very fast measurement (30 PS) and signal-averaging for precise readings (0.1% relative standard deviation). Solutions to be monitored were fed into the cell by a Sage Model 355 syringe pump. The flow rate was nominally 1 ml min-‘. A flowing stream was used only to minimize any possible heating. Resistance measurements in static solutions gave identical results.

The flow cells were similar to those used previously [ 51. The basic design of the cell is presented in Fig. 1. The cell design was originally developed

0003-2670/84/$03.00 0 1984 Elsevier Science Publishers B.V.

398

Fig. 1. Schematic of the conductance flow cell. For detail, see text.

to allow a variety of counter-electrode geometries for use with commercially available (and therefore relatively large) ion-selective electrodes. Although the resulting cell volume is larger than would be desirable for simple con- ductimetric monitoring in flowing streams, this cell was very useful for the present study because it offered the flexibility of studying several electrode geometries and spacings. In one cell, the electrodes were stainless steel, approximately 8 mm in diameter. A second cell had small platinum rods which were about 1 mm in diameter, but all other cell dimensions were the same. Electrodes 1 and 2 are 1.0 cm center to center. The cells were made from Plexiglas and teflon. Teflon was used for the spacers which ranged in thickness from 1.6 to 25 mm. The cell cavity cut in the spacers was approxi- mately l-cm wide and 2-cm long with rounded ends. The nominal cell volume then varied from 0.3 to 4.8 ml for the 1.6 to 25 mm spacers. The same spacers were used for both the steel and platinum electrodes. Because the platinum electrodes were smaller than the steel electrodes, it would have been possible to have reduced cell volumes to 50 to 800 ~1 for the same spacer thicknesses. To distinguish between the three possible combinations, the following labels are used: electrodes 1 and 2 were the series, 2 and 3 were the opposing, and 1 and 3 were the diagonal configurations.

Procedure. Potassium chloride solution (0.010 M) was fed into the cell and measurements were taken with the instrument; the pulse height was 5.0 V and the pulse width 10 ps. An individual data point was the average of 128 readings. Ten of these individual points were then averaged to get the final resistance value for a given spacer. This was done for all possible electrode arrangements before a new spacer was inserted.

Potassium chloride (analytical grade; Malhnckrodt) was dissolved in water (from a Continental/Millipore Milli-Q reagent grade water system) to make the 0.010 M solution.

Results and discussion Figure 2 shows the trends of the solution resistance vs. spacer width for

the three configurations with the stainless steel electrodes and the platinum electrodes. The resistances with platinum were higher because of the smaller area. There was no systematic variation in precision with spacer thickness.

I I I I I I

0 5 IO 15 20 25 Spacer thvzkness (mm)

I I I I I 0 5 IO 15 20 25

Spacer thxkness (mm)

Fig. 2. Variation of measured resistance with spacer thickness for different electrodes: (A) stainless-steel electrodes of ca. 8 mm diameter; (B) platinum electrodes of ca. 1 mm diameter. Electrode geometry; (0) series, (0) opposing, (a) diagonal.

The relative standard deviation for all measurements was 0.5% or better. The shapes of the curves and the magnitudes of the measured resistances were independent of the solution flow rate and the flow characteristics of the cell. The same values were obtained in measurements with flowing and static solutions.

Theory predicts the measured resistance to be proportional to d A-‘, where d is the distance between electrodes and A is the electrode area. The opposing configuration gave the expected results. As the electrodes were moved further apart, the resistance increased monotonically. The observed increase is more nearly linear for the small platinum electrodes than for the large steel electrodes. The other two arrangements behaved differently than originally expected. It was believed that because electrode spacing and area for the series configuration never changed, the resistance measured should be constant. However, as Fig. 2 shows, this is not the case until the opposite wall is relatively far away. The large electrodes show a much larger deviation at close distance than the small electrodes.

The diagonal electrodes show behavior similar to each of the other two configurations. The diagonal electrodes behave like series electrodes at small spacings, then they act more like the opposing electrodes at large spacing. This behavior is reasonable considering the inter-electrode distances. For a 1.59-mm spacer, the distances for the series, opposing, and diagonal pairs are 10.0 mm, 1.59 mm and 10.13 mm; the series and diagonal pairs are nearly the same distance apart, and show nearly the same resistance. For a 25.4-mm spacer, the distances for the series, opposing, and diagonal pairs are 10.0 mm, 25.4 mm, and 27.3 mm; then it is the opposing and diagonal pairs that are nearly the same distance apart and show nearly the same resistances. The effective electrode area (the projected geometric area) decreases for the diagonal pair as the spacer thickness decreases. One can calculate the projected electrode area as a function of spacer thickness and calculate the product, d A-‘. Such a calculation correctly predicts that for

400

the cell geometry used, the resistance for the diagonal pair should pass through a minimum at around 10-15 mm.

For the series electrodes, and perhaps somewhat for the diagonal position, the effect of the opposite wall on the electric fields would explain the data. When the walls are close, the field tends to run into the wall, but at larger spacings, the electric field can follow its natural path. This “wall effect” was also noticed with a solution in a beaker. If the positions of the rods used for the electrodes were changed relative to the wall (but not to each other), the measured resistance changed.

Calibration plots of log resistance vs. log concentration were prepared for 10d-10-2 M potassium chloride using all three electrode configurations (stainless-steel electrodes) in cells with 1.6- and 12.7-mm spacers. For these conditions, the absolute magnitudes of the resistances were very different, as given in Fig. 2A, but the shapes of the log-log plots were the same. From lo-’ to 10” M, the plots were linear and parallel, but flattened out below lo3 M.

Electrode placement within a cell and electric field effects can make a substantial difference to the signal from a cell. For the small spacing, the opposing configuration gives the largest signals (current measured is in- versely proportional to resistance). Because larger signals need less ampli- fication, and this usually means less noise, the opposing configuration is the best. Considerable reduction of cell volume can be made if the cell is designed with only opposing electrodes of smaIl area. Such a cell can be made with a total volume of 10 ~1 or less, and is thus very attractive for detection in flowing streams.

REFERENCES

1 T. Stevens, J. Davies and H. Small, Anal. Chem., 53 (1982) 1488. 2 Yokogawai Electrical Works, Japanese Patent, 83 55, 857 (1983). 3 C. R. Powley, R. F. Geiger and T. A. Nieman, Anal. Chem., 52 (1980) 705. 4 D. Taylor and T. A. Nieman, Anal. Chem., 56 (1984) 593. 5 C. R. Powley and T. A. Nieman, Anal. Chim. Acta, 152 (1983) 173. 6 K. J. Caserta, F. J. Holler, S. R. Crouch and C. G. Enke, Anal. Chem., 50 (1978) 1534. 7 R. F. Geiger, Ph.D. Thesis, University of Illinois, 1983.