Nitric acid was used in the dissolution step since other acids contribute F, C1, P, or S blanks which precludes their determination in the sample. The In and Re were probably present as nitrates in the dried doped graphite. Polyethylene vessels were employed instead of Teflon to avoid a fluorine blank. Methanol was added to lower surface tension of the solution, permitting thorough wetting of the graphite. The proportion of liquid to graphite was kept in the range 1.5 to 2 ml of internal standard solution per gram of powdered graphite in order that a slurry be formed with no liquid left over. Excess liquid, when evaporated, would leave a thin layer of crystals on the walls of the container and on the surface of the bulk of the graphite ( 5 ) resulting in inhomogeneities. Graphite has the desirable property of being inert and, hence, not behaving as an inorganic ion exchanger. Therefore, provided that even wetting of the graphite has been achieved, the graphite slurry contained homogeneously dispersed internal standard. Quick freezing followed by freeze-drying was used to prevent the formation of crystalline layers of internal standard and to eliminate any chromatographic effects resulting from the drying step as reported earlier ( 5 ) . The doped graphite, when removed from the freeze-drying apparatus contained approximately 0.1 t o 0.5% moisture which was removed by heating a t 60 “ C for 3 hours in a vacuum oven. The relative standard deviation of photoplate exposures from the linear relationship between exposure value and calculated ion density (10) was used as an indicator of homogeneity. Typical values of that RSD for the internal standard lines lI3In, I1~In,le5Re, and IS7Re in the blend of W-1 basalt and doped graphite were approximately 5% based on experience with over 300 photoplates. Since the sensitivity of the Q2 emulsion has been quoted to fluctu-
ate from 5 to 60% within the same plate (12), the homogeneity of the internal standard is expected to be much better than that measured by photoplate detection. Morrison and Colby (6) have shown that electrode sampling precisions on the order of 3% are possible by using electrical detection and an electrode pressed from a mixture of pure graphite and an optimized synthetic sample prepared by solution doping of graphite. Since it is possible to achieve precisions on the order of 3% using optimized synthetic samples, it can be concluded that the main problem in preparing a homogeneous final mix is rendering both real samples and doped conducting medium homogeneous before they are blended together. The running of replicate analyses on different batches of doped graphite-sample mixtures constitutes a test of the reproducibility of the method. Comparison of sensitivity factors (SF) obtained in this laboratory from triplicate analyses of each of three of W-1-doped graphite mixtures showed that, for most elements, the SF agreed to better than &8% RSD (range 3 to 20%) within each triplicate run and to better than & l o % RSD (range 4 to 15%) among the averages for the three batches. Of necessity, these precisions reflect in part an uncertainty due to the unpredictable variation of sensitivity known to exist for the Q2 emulsion. Hence, the reproducibility of doping levels in different batches of doped graphite is expected at least to be better than 10% and probably lies in the 3 to 5% range. Received for review July 30, 1973. Accepted December 21, 1973. (12) A J Ahearn, in ’ Recent Developments in Mass Spectroscopy K Ogata and T Haykawa, Ed , University Park Press, Tokyo, 1970,
pp 150-7
Split Crystal Ion Selective Membrane Electrodes Ronald Wawro and G. A. Rechnitz Department of Chemistry, State University of N e w York, Buffaio, N. Y 14214
Just as double beam techniques have greatly extended the usefulness of spectrophotometry, true differential measurements should offer advantages over conventional potentiometry with ion selective membrane electrodes. In a previous paper, we described ( I ) the necessary electronic instrumentation for differential potentiometry but relied upon separate membrane electrodes to serve as indicator and reference electrodes. Such an arrangement already offers certain advantages but does not allow realization of the full potentialities of the differential technique. We now describe a novel split crystal membrane electrode (Figure 1) which permits almost perfect matching of electrode characteristics and allows “double beam” measurements to be made by simultaneously operating one side of the split electrode as a reference and the other as the indicating electrode. Aside from the convenience of having matched electrodes, this new system has the important advantage of minimizing the need for restandardizntion with changes in conditions or in prolonged use. Tihis desirable feature results from the fact that the split sensor is formed from a single membrane which undergoes ( 1 ) M J D Brand and G . A Rechnitz, Ana/ Chem 42, 1659 (1970)
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1974
uniform aging with time, specific use, or temperature changes. Although the general principle of the split membrane approach should be applicable to many membrane electrodes, we selected the AgZS/AgI type iodide sensor for detailed study because of our previous experience with this electrode (2, 3) and its known reliability. When used in the differential mode, the split membrane sensor eliminates the need for an external reference electrode and should be useful for continuous monitoring, automated analysis, and process control.
EXPERIMENTAL Apparatus. Potential measurements were made using an Orion Model 801 digital meter. For some evaluation experiments an Orion Model 90-01 reference electrode was also used. Differential measurements were made with our dual high input impedance amplifier (1) connected to a Varicord Model 43 potentiometric recorder. Sample and reference flow streams were introduced using a Model 975 Syringe Infusion Pump (Harvard Apparatus Co., Millis, Mass.) (2) J. D . Czaban and G . A . Rechnitz. Ana/. Chem.. 45, 471 (1973). (3) H . Thompson and G . A . Rechnitz. Chem. instrum., 4, 239 (1972)
Table I. Calibration of Split M e m b r a n e Electrodes I and I1 US. a Single Junction Reference Electrode Potential readings, mV Electrode I
Electrode I1
[I-1,M
Expt A
Expt B
Expt A
Expt B
10 -5
-92.7 -149.7
-85.3 -145.4 -203.6 -259.9 -316.3
-93.6 -150.0 -207.6 -264.11 -320.9
-86.0 -145.9 -202.9 -260.6 -317.1
56.9
57.7
in-* 10-1
-207.1
-263.8 -321.2
Calibration sllope/(mV/decade)
57.5
w'aused
57.7
to record the potential differences as well
as to
measure
It:sponSe times.
RESULTS AND DISCUSSION The matched readings of the two flow-through iodide Electrode Construction, Coprecipitated AgI/AgSS powder was prepared and pressed into a wafer thin disk membrane (13-mm diameter by 0.7-mm thickness) as previously described (2). The disk membrane was then split to give two semicircular membranes which were embedded together in one Bio-Plastic mold (3).The two half membranes were embedded side by side but not in contact as shown in Figures 1 and 2. A flow channel was drilled through each split membrane and connected to polyethylene tubing of appropriate diameter: The flow channels were polished with diamond powder to improve the response of the electrodes. Internal contact to the membrane was made by drilling a hole at the edge of the plastic body to the membrane and attaching a piece of Plexiglas tubing (Figure 2). AgN03 (0.1M) was placed in the side arm to provide liquid contact with the membrane. A piece of silver wire attached to a BNC connector completed the circuit for each arm of the electrode. Procedure. Each split electrode membrane was first calibrated in a flowing stream against an external reference electrode using a series of KI solutions of ionic strength adjusted to 0.3M with KNOa. The two electrode halves were calibrated simultaneously atroom temperature (21 f 1"C). In the differential mode, a solution of known iodide cancentration was passed through one of the memhrane halves which served a8 the reference electrode. Solutions of varying iodide concentrations were passed through the other membrane (sample electrode), and the potential differences between the two electrodes were measured with the differential amplifier. The recorder
membranes are shown in Table I where the data from two simultaneous calibration runs are given. The readings at each iodide concentration agree t o within 1 millivolt. T h e critical factor in determining matched potentials does not depend so much on the electrode membrane, but rather on the internal electrical contact-ie., the filling solution, silver wire contact, and B.N.C. connections must be identical. The silver wire leads had to be lightly sanded t o remove any coatings and good tight fits of the silver wires into the B.N.C. connections were essential for matched readings. day to day with the indicated difference being the largest found. This difference is the noncumulative drift that norITiallv results from such external fatctors as temDerature
r
bt4.C.
/'""""""
I"
r Figure 2. Schematic diagram of fiow-through split membrane iodide electrodes
i
i
Figure 3. Dynamic response of iodide split membranes I and I 1 to decade changes in iodide concentralion ANALYTICAL CHEMISTRY, VOL. 46, NO. 6, MAY 1974
807
Table 11. Differential Analysis Using Split Crystal Electrode Actual I - concn., M
5 . 0 X lo-* 2 . 0 x 10-2 1 . 2 x 10-2 4.8
x
1-3
1.9 x 10-3
Measured
I - conc., M
+
(5.0 0.2) X ( 2 . 1 =t0 . 1 ) x (1.2 0.1) x (5.2 x 0.2) x ( 2 . 1 =t0.1)x
*
10-2
10-2 10-3 10-3
changes, junction potentials, and membrane conditioning. Although the potential readings varied, the response of the two electrodes remained matched and the response slope remained constant (57.3 f 0.4 mV) for each calibration. This is an important factor for possible use of the split electrode system as a continuous monitor. The system may be used over a long period of time since the response of the matched membrane electrodes will not be affected by normal changes in external factors. The electrodes change in exactly the same manner and, thus, the need for frequent standardization is eliminated. We have used the electrodes over a period of 3 to 4 weeks with no deterioration in performance. In the dual flow differential experiments, the same iodide solution was initially passed through both electrode membranes. Because the two membranes are matched to
