572 Analyst, August, 1973, Vol. 98, +$. 572-579 Potentiostatic Coulometric Determination of Vanadium, Vanadium - Manganese and Vanadium = Iron Mixtures and the Influence of Chromium on the Process* B Y E. BISHOP AND P. H. HITCHCoCKt (Chemistry Departmeizt, University of Exetev, Stocker Road, Exeter, EX4 4QD) Earlier voltammetric work permitted the mass and charge transfer kinetic parameters of the vanadium system to be calculated for diverse media and platinum electrode pre-treatments, and command potentials to be selected for potentiostatic determination of vanadium alone and in certain combinations with other steel-forming elements. A simple coulometric cell and an adaptation of a commercial potentiostat are described. Current integration by strip-chart recorder is too inaccurate and so RC integration is discussed.Philbrick SP456 amplifiers refused capacitive feedback, but very satisfactory results were given by Solartron AA 1023 amplifiers. The design of a very high quality integrating capacitor from S.T.C. polystyrene elements is described; leakage and drift tests were very satisfactory. Pre- treatments of electrolytes and electrodes are discussed. Vanadium(V) is determined at -0.128 V in acetate buffer and at f0-247 V in 2.0 M sulphuric acid, in the latter with a relative standard deviation of 0.27 per cent. and a 95 per cent. confidence level result of (1.008 to 1.011) x 10-1 M compared with 1.012 x 10-1 iu for a standard solution. Chromium(V1) suppresses all reduction at pH 4.0, and is reduced simultaneously with vanadium in sul- phuric acid.Manganese(VI1) is reduced to manganese(II1) in the first step at +Om7 V at pH 3-5 and manganese(II1) and vanadiuin(V) are simultaneously reduced in the second step a t -0.12 V. The separation of iron(II1) is possible a t +O-9 V but impracticable; simultaneous reduction a t +0.25 V in 2.0 M sulphuric acid followed by re-oxidation oi the iron(I1) at + 1.0 V is recom- mended. A VOLTAMMETRIC study of the vanadium(V) - vanadium(1V) system in various media has been made in pure supporting electrolyte1 and also in mixtures with chromium(VI), man- ganese(VII), iron(II1) and combinations thereof . 2 Examples of current-efficiency calculations for the reduction of vanadium(V) have been reported, and a basis has been laid for the selection of conditions for the potentiostatic determination of vanadium(V) alone or in certain ad- mixtures2 Amperostatic coulometric determinations of vanadium have been reported in which such intermediates as ~opper(I),~ i r ~ n ( I I ) , ~ t i t a n i ~ r n ( I I 1 ) ~ ~ ~ and tin(I1)' were used, and Israel and Meites8 made a potentiostatic determination at mercury electrodes, but platinum electrodes do not seem to have been used for the potentiostatic determination of vanadium, either alone or in combination with the elements mentioned.Attempts so to do are described in this paper. EXPERIMENTAL Reagents and certain of the apparatus have been described el~ewhere.~ Potentials are quoted veYsus the standard hydrogen electrode (S.H.E.). COULOMETRIC CELL- The coulometric cell is shown in Fig.1. The anolyte and catholyte must be separated and diffusion of sample out of, or deleterious species into, the working compartment, in this instance the cathode, must be prevented. With the finest porosity sintered-glass separator, the auxiliary electrolyte was sucked into the working compartment by the stirrer. The junction was therefore immobilised with agar gel in saturated potassium sulphate solution. Tests showed, during a determination, no detectable migration of sample into the auxiliary compartment. As it is a protein, agar could act as a poison or deactivator for the working electrode,1° but no such action attributable to the agar occurred. * Presented at the Second SAC Conference, Nottingham, 1968. t Present address: Ever Ready Co. (G.B.) Ltd., Central Research Laboratory, St.Ann's Road, 0 SAC and the authors. London, N15 3TJ.BISHOP AND HITCHCOCK 573 Fig. 1. The coulometric cell and electrodes : C, cell consist- ing of a 400-ml beaker with top sawn off and ground flat; L, machined Perspex lid approp- riately drilled ; R, rubber sleeve holding the auxiliary elec- trode compartment; AE, auxiliary electrode, platinum gauze, JM 72050 ; RE, reference electrode (S.C.E.) with remote junction; WE, working electrode, platinum gauze, JM 72020; S, porosity 4 sintered-glass junc- tion plugged with agar gel; and M, PTFE-coated magnetic stirrer follower POTENTIOSTAT- A Wadsworth (Southern Analytical) electrogravimetric potentiostat was used for high- current work, modified so that the control potential could be set by means of a 39A pH meter; the command potential could be set much more precisely, and the high input impedance prevented polarisation of the reference electrode, which occurred with the unmodified instrument.CURRENT INTEGRATION- Initial trials were conducted by recording on a strip-chart recorder the decaying current as a function of the voltage drop across a standard resistor placed between the auxiliary electrode and the potentiostat. The current - time integral was obtained by cutting out the section of chart below the current trace, together with a square portion of paper of known area from above the trace, and weighing the paper on an analytical balance. The results were poor on account of the limitations of the recorder and variations in paper density, and the method was abandoned in favour of electronic RC integration by using chopper- stabilised operational amplifiers.574 BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION OF [ArtabySt, VOl.98 There are three basic sources of error in RC integration: (1) the leakage resistance of the integrating capacitor, (2) the insulation of the amplifier summing junction, particularly to high voltages, and (3) the noise and drift of the amplifier itself. Morrison11 has examined some aspects of these errors, particularly (1) and (2). Regarding (S), little could be done; a t the time this work was carried out, only Philbrick SP456 and Solartron AA 1023 amplifiers were available. According to their manufacturers, both amplifiers were “capable of high- accuracy integration,” but neither manufacturer gave actual figures, particularly about the leakage resistance between the power supply and the summing junction.In the event, the Philbrick SP456 chopper amplifiers refused to operate with capacitance in the feedback loop, although they performed normally with resistive feedback. A second amplifier and different values of the input resistance and feedback capacitance were tried, and different earthing arrangements were tested in order to remove any unsuspected earth-loops, also without success. The standard arrangement of a single earth point, and that on the power supply, was adopted. The Solartron amplifiers performed very satisfactorily as integrators, and advantage was taken of their high-voltage output. As in the ramp generator de~ign,~ the current-measuring standard resistor was placed in the lead from the working electrode to the potentiostat, in parallel with a Solartron chopper amplifier in the current follower mode. The block diagram is shown in Fig.2. Calibration of the integrator was effected by substituting a constant-current source12 (either the Solartron AS 1411 or the operational amplifier source) for the cell and potentiostat, and integrating known currents for known times, which were measured by a crystal clock.12 For this purpose, the standard resistor, Rf, was a Cropico RS1 1.0-Q or a Cambridge 0.142 standard immersed in transformer oil, and the IR drop was measured with a Cropico P3 pot en t iometer . Fig, 2. The coulometric circuit: C, 1-pF poly- styrene integrating capacitor (see Fig.3) ; Q, Solartron AA 1023 chopper-stabilised amplifiers driven by Solartron AS 853.3 power supply (& 300 V, 100 mA) ; Rf, standard feedback resistor of current follower (0.1 Q); Ri input resistor to integrator (300 kQ); R1, load resistors (8.2 kQ) ; P, Wadsworth controlled potential apparatus ; V, Venner digital voltmeter; AE, auxiliary electrode ; RE, reference electrode; and WE, working electrode INTEGRATING CAPACITOR- The integrating capacitor is the most critical component in the circuit; it should be large, at least 1 pF, and have a minimum leakage resistance of 1013 Q, for an error of 0.1 per cent. over a 2-hour integration. A commercial source was long sought in vain, until the Development Department of the Capacitor Division of Standard Telephones and Cables Ltd.(S.T.C.) suggested a trial of high-quality, computer-grade, polystyrene capacitors, whichAugust, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 575 were then in the experimental stage. These were 0.2 pF -J= 1 per cent. S.T.C. 455/LWA/l12 FR capacitors of 500V d.c. working. They were tested by washing them with ethanol, suspending them in free air in a clean laboratory, charging them to 2 V and connecting them to the input of an E.I.L. 39A pH meter. For all samples, there was no detectable change over 2 hours; thereafter, for five samples the potential began slowly to decrease, at an in- creasing, although modest, rate as dust and moisture were deposited on them and produced surface leakage, and by the natural ionisation of the air caused by radioactive potassium contained in the materials of construction of the building.Another sample increased its charge slightly with time; this behaviour, although unusual, is not unknown, and came within the experience of the S.T.C. engineers, who could not offer a satisfactory explanation of its cause. The test in free air did not give a measure of leakage resistance but did show that it was satisfactorily low. The final design of the composite capacitor is shown in Fig. 3. Five capacitors, after thorough cleaning in ethanol, were mounted close-packed in parallel so as to give a capacitance of 1 pF. The remainder of the heavy machined material was thoroughly scrubbed with soap and water, washed with water and acetone and baked out before mounting.After assembly, the three-way tap was used for evacuation of the vessel and for leakage testing, and then to admit nitrogen dried over phosphorus(V) oxide. The evacuation and flushing operations were repeated many times, and the vessel was finally filled with dry nitrogen. Nitrogen was chosen as it has a higher excitation energy than argon. The whole assembly was protected from the atmosphere by a polythene bag. The terminals were connected to a Vibron pH meter and the capacitor was charged to 2.0 V. Readings were taken at intervals over a period of 300 hours, and showed that the combined leakage of capacitor, co-axial cable and the meter was 2.8 x 10l2 $2. If the leakage of meter and cable be taken as 1013 $2 (which is probably low), then the capacitor leakage resistance is about 3-9 x 1012sz* LIMITS IMPOSED ON INTEGRATION BY LEAKAGE- Consider the capacitor to be charged to 80 V in a determination.The leakage current at the end of the run is 80/(3-9 x 10l2) = 20 PA. An input current can therefore be summed to within 0.1 per cent. with a feedback capacitor leakage of 3.9 x lOl2Q. To the question of the loss that could be expected to occur after charging to 80 V and leaving for 1 hour, at such a large leakage resistance a sufficiently accurate answer is given by .. EC t I k a k = - where E is the loss in voltage, C is the capacitance and t is the time that remains before reading. Solving for E gives a value of 0.07 V, which represents an error of 0.09 per cent. in 1 hour, In tests of the completed integrator shown in Fig.2 , the drift was about 30 pV s-1, which is less than the manufacturer’s specification of 50 pV s-l for a unity gain integrator. Drift invariably increases with increase in the size of the input resistor, which explains the desire for a large capacitance. In the example of integrating for 1 hour to reach a final output of 80 V, the amplifier drift could cause an error as large as 0.108 V, or 0.13 per cent. GENERAL PROCEDURE FOR A COULOMETRIC DETERMINATION- The supporting electrolyte and any other necessary reagents were added to the working compartment. The large platinum-gauze electrode was pre-treated as required, and mounted in the cell. The auxiliary electrode and electrolyte were placed in the auxiliary compartment and the tip of the probe of the reference cell salt bridge was positioned near the bottom of the working electrode, as in Fig.1. This position was found to be the optimum both for reducing the IR drop in the command voltage and for minimising current noise occasioned by the stirring. After de-aeration of the electrolyte, a pre-electrolysis was performed until a constant current (the residual current) was attained. The integrator capacitor was dis- charged and the residual current integrated for a known time. The electrolysis was stopped, the integrator re-set and the sample added. Electrolysis was then continued until a constant current was again attained, the value of the current and the integrator voltage were noted and electrolysis was stopped. The integral of sample plus residual current was corrected for the residual current.The interpretation of the residual current correction is a severe limiting factor in potentiostatic co~lometry.~~576 [Analyst, Vol. 98 PRE-ELECTROLYSIS OF ELECTROLYTES- In instances in which the pre-electrolysis described above is known to deactivate the working electrode, or when an impurity can be removed by electrosorption, potentiostatic purification was conducted in a second sealed vessel with a platinum-black working electrode BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION OF Scale - 0 1 2cm Scale I I I I I 0 1 2 3 4cm 1 r I r - I L " I '\ W - I I I \ \ \ H L I I Fig. 3. Sectional view of integrating capacitor: C, S.T.C. 455/LWA/112 FR polystyrene capacitors (0.2 pF f 1 per cent. 600 V d.c. working, five in parallel) ; N, 2 BA terminal nuts; S, platinum glass to metal seal; T, three-way greased tap; I, machined PTFE insulators; X, neoprene O-rings; and W, copper wire, 22 s.w.g.All parts are made of brass unless otherwise specifiedAugust, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 577 of geometric area 100 cm2. The all-glass cell carried ground-glass joints9 lubricated with distilled water, and bearing electrodes, a nitrogen bubbler and a glass siphon tube, the other end of which was led to the coulometric cell, After electrolysis to a satisfactorily small residual current, the “cleansed” electrolyte was forced into the voltammetric or coulometric cell by closing the nitrogen outlet so that electrolyte was forced out of the siphon tube.The siphon could be broken by an outlet tap at its highest point. The first few portions of electrolyte transferred were used to rinse out‘ the receiving cell, and were then discarded. POTENTIOSTATIC DETERMINATION OF VANADIUM (V)- Medium-From the voltammetric study,l the saturated potassium sulphate - acetate buffer at pH 4.0 and 2 . 0 ~ sulphuric acid are both suitable media for the determination. Both show well defined vanadium(V) limiting-current regions of sufficient potential range to permit the choice of a command potential that keeps the desired reaction well separated from background reactions so that a good current efficiency could be expected. Furthermore, in these media the ageing of the working electrode did not seriously affect the limiting-current region of the vanadium(V) wave.At intermediate hydrogen-ion concentrations, the deter- mination is impracticable, if not impossible, because severe deactivation of the electrode would so severely prolong the reduction that background corrections would be much too large to be reliable. Electrode pre-treatment1,2-Method (b)lv2 was selected and slightly modified on account of the large area of the working electrode; the previously cleaned electrode is immersed in 1.0 M sulphuric acid, anodised for 30 s at 100 mA cm-2 (i.e., 12-5 A) then cathodised for 30 s at 1 O O m A cm-2. The cycle is repeated once more, and the electrode thoroughly washed with water. That this modification made no difference to the electrode behaviour in the two media was confirmed by repeated voltammetric scanning, Selection of method (b) was made on the following grounds.Method ( a ) , reduction with iron(I1) in sulphuric acid, had not proved particularly successful, and was suspect in respect of transfer of trace amounts of iron(I1) or iron(III), trapped in the gauze meshes, into the test s01ution.l~ Method (e), oxidation, was deemed unsuitable because the charge-transfer process was slowed down by this treatment, and an error would be introduced by the reduction of the oxide film, although a correction could be applied for the latter. Methods ( b ) , (c) and (d) are very similar, but method (d) , simple cathodisation, was considered less suitable because such electrodes appear to be prone to cumulative adsorption of some species in the vanadium sample on repeated use.Such an impurity is known to be removed by anodisation, and so method (b) was chosen, in preference to method (c), stripping and cathodisation, as being simpler and faster. Method (c) remains appropriate for the initial cleaning of dirty electrodes. Determinations-The results of determinations in the two chosen media with electrodes pre-treated by method (b) are collected in Table I, and show that results obtained in 2-0 M sulphuric acid are substantially better than those for the buffer at pH 4.0. This effect arises from two causes. First, different integration methods were used, and the strip-chart record- ing - weighing method for group D is not as accurate as the electronic method. The second cause is more fundamental and important. In acetate buffer, the background current, which was substantial, changed during the determination, but the exact manner of the change is not known and therefore cannot be accurately corrected f0r.1~ There was little alternative but to use a mean value of the pre-titration and post-titration background currents, and the error is magnified by the low charge-transfer rate and the long duration of the determination of 2 to 2.5 hours.In both media, graphs of log current veYsz4s time were linear. The increased determination time in the buffer of pH 4 is aggravated by the lower mass-transfer rate constant2 in this medium. POTENTIOSTATIC DETERMINATION OF VANADIUM(V) IN THE PRESENCE OF OTHER METALS- Chromium( VI)-In acetate buffer, chromium(V1) is specifically adsorbed on the electrode surface and quantitatively blocks the vanadium reduction.2 In 2.0 M sulphuric acid, van- adium(V) and chromium(V1) are reduced at virtually identical rates, giving a single wave.2 In this medium, the total of vanadium film chromium can be determined at a command potential of +045 V, but the current efficiency for the reduction of chromium(V1) is unlikely to be high.RESULTS AND DISCUSSION578 BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION O F [Analyst, VOl. 98 TABLE I RESULTS OF THE POTENTIOSTATIC DETERMINATION OF VANADIUM (V) ALONE Group Conditions A Saturated potassium sulphate - acetate buffer a t pH 4.0, electronic integration, command potential -0.128 V B 2.0 M sulphuric acid, electronic integration, command potential + 0.247 V C 2.0 M sulphuric acid, strip-chart record integration, command potential + 0.247 V D Saturated potassium sulphate - acetate buffer a t pH 4.0, strip-chart record integra- tion, command potential -0.128 V Sample takenlmol 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 1 x 10-3 5 x 10-4 1 x 10-4 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 3 x 10-3 5 x 10-3 5 x 10-4 Background current Initial/mA Final/mA 0.2 0.1 0.15 0.1 0.09 0.1 7 0.15 0.15 0.35 0.15 0.05 0.04 0.05 0-15 0.10 0.10 0.15 0.07 0.36 0.41 0.15 0.15 0.24 0.05 0.10 0-05 0.05 0.04 0.10 0.05 0.15 0-08 0.52 0-50 0.47 0.5 1 0.18 0.37 0.34 0.21 0.27 0.44 0.13 0-37 Time1 minutes 130 140 145 84 71 102 99 81 83 160 140 80 76 81 83 75 88 120 110 130 160 117 Relative error, per cent. + 5-1 + 1-54 + 2.04 +O.lS - 0.51 -0.21 + 0.07 - 0.45 - 0.35 -0.87 - 0-10 -0.10 - 0.20 - 0.28 -0.11 - 0.93 - 0.53 - 1.5 -3.1 +3-7 +2*1 - 1.0 The first nine results in section B give a relative standard deviation of 0.27 per cent.For the 95 per cent. confidence level, this value indicated that the stock vanadium(V) solution was (1.008 to 1.011) x lo-' M. Standardisation by means of sulphur dioxide and permanganate against sodium oxalateB gave a value of 1.012 x 10-1 M. Mmganese(VII)-Well separated waves are obtained in the acetate buffer at pH 4~0,~ and on this basis the sequential determination of manganese(VI1) at +0.700V and van- adium(V) at -0.125 V was attempted. As earlier adumbrated,2 the quantities of electricity consumed by the two steps showed that they corresponded first to the reduction of man- ganese(VI1) to manganese(III), at +O-7 V, and second to the simultaneous reduction of manganese(II1) to manganese(I1) and of vanadium(V) to vanadium(1V) at -0.125 V.Repli- cate experiments showed that the results were inadequately consistent, and also that some manganese(1V) oxide was formed by disproportionation of manganese(II1). To depress this latter effect, the pH was decreased to 3-5 by the addition of acetic acid to the medium. Determinations made in this new medium showed that the first stage in the reduction was improved in reproducibility and accuracy, with a relative error of about h0.3 per cent., as shown in Table 11. The second stage in the reduction remained in error by about +%O per TABLE I1 RESULTS OF THE SEQUENTIAL POTENTIOSTATIC DETERMINATION OF MANGANESE(VII) AND VANADIUM(V) IN SATURATED POTASSIUM SULPHATE - ACETATE BUFFER AT pH 3.5 Electronic integration was used Sample takenlmol MnvII vv' 5 x 10-4 2 x 10-3 5 x 10-4 2 x 10-3 5 x 10-4 2 x 10-3 5 x 10-4 4 x 10-3 2 x 10-3 1 x 10-3 2 x 10-3 1 x 10-3 3 x 10-3 i x 10-3 Command potential/V 7 MnvII VV 0.697 - 0.120 0.697 - 0.120 0.697 -0*110 0.697 -0*110 0.697 -0~110 0.697 -0.120 0.697 -0-120 Relative error, per cent.7 MnvI1 VV +0*31 + 1.1 + 0.10 + 1.7 +0.11 - 1.0 -0.21 + 0.8 -0.19 -2.1 + 0.09 - 1.0 - 0.27 + 1.6August, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 579 cent. after correction for the background current. However, the simple determination of vanadium(V) in acetate buffer at pH 4.0 was also in error by this amount, as shown in Table I, so the error in the sequential determination was not unexpected.The two-step reduction of manganese(VI1) was not predicted from the voltammetric curves2 because the mass-transfer rate constant was not independently known for the conditions used. The voltammograms2 showed only a single step for manganese(VI1) reduction, and although the limiting current could be measured to within 1 per cent., knowledge of one other parameter would be needed in order to calculate the n-value of the wave. A determina- tion on this mixture was not attempted in 2.0 M sulphuric acid, because the voltammograms2 showed that the wave separation was inadequate for good separation efficiency, although the sum could be determined with good accuracy and efficiency.Imn(lll)-The voltammogram2 suggests that, by using electrodes activated by methods ( b ) or (C),~ it should be possible to reduce vanadium(V) at a command potential of +O-9 V without reducing an appreciable amount of iron(II1). Even if some iron(I1) were formed, it would act as an intermediate and reduce vanadium(V) chemically. Experimentally, it was found that, in 2 . 0 ~ sulphuric acid, the current decayed rapidly to about one third of its initial value during the first 10 minutes and thereafter the decay became much slower. A graph of log current veysus time for the first part of the reduction was not linear, but convex to the time axis. This result indicated that the electrode surface was being deactivated t o the type of surface used to record scan 2 of Fig.4 in reference 2. Although it remained possible to reduce the vanadium(V) without appreciable reduction of iron(II1) by continuing the potentiostatic electrolysis until the current decreased to an acceptable residual value, the time required would be so excessively long that the background correction would be considerable and the determination would be of poor accuracy. Alternatively, it is possible to reduce both vanadium(V) and iron(II1) simultaneously and quantitatively in 2.0 M sulphuric acid at +0.25 V, and then to re-oxidise the iron(I1) at + 1.0 V without any risk of re-oxidising the vanadium(1V) or the solvent, and so obtain the vanadium concentration accurately by difference. Unfortunately, the latter part of this procedure could not be attempted, as the only high-current potentiostat available at the time would operate only in the cathodic mode. CONCLUSIONS The potentiostatic determination of vanadium(V) is possible in the two media selected, and the results obtained in 2-0 M sulphuric acid by using electronic integration are both precise and accurate.The sequential potentiostatic determination of manganese(VI1) and van- adium(V) in acetate buffer at pH 3.5 is possible, although the mechanism is unexpected in that the first step is reduction to manganese(III), which is then reduced to manganese(I1) simultaneously with the reduction of vanadium in the second step. Chromium(V1) can be reduced simultaneously with vanadium(V) , but not sequentially. Reduction of vanadium(V) in the presence of iron(II1) is possible but not practicable, and it is better to reduce both and then to re-oxidise the iron(I1); manganese can be added to this combination. We are deeply grateful to Imperial Chemical Industries Limited for a research grant extending over 3 years. We thank the Capacitor Division of S.T.C. Ltd. for the gift of polystyrene capacitors from which the integrating capacitor was constructed. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. REFERENCES Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 553. Meier, D. J., Myers, R. J., and Swift, E. H., .J. Amer. Chem. S O ~ . , 1949, 71, 2340. Furman, N. H., Reilley, C. N., and Cooke, W. D., Analyt. Chem., 1951, 23, 1665. Kennedy, J. H., and Lingane, J. J., Analytica Chinz. Acta, 1958, 18, 240. Lingane, J. J., Ibid., 1956, 15, 465. Anson, F, C., and King, D. M., Analyt. Chem., 1962, 34, 362. Israel, Y., and Meites, L., J . Electvoanalyt. Chem., 1964, 8, 99. Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 465. -- , Ibid., 1973, 98, 475. Morrison, C. F., “Generalised Instrumentation for Research and Teaching,” Washington State Bishop, E., and Riley, M., Analyst, 1973, 98, 305. Meites, L., and Moros, S. A., Analyt. Chem., 1959, 31, 23. Bishop, E., and Riley, M., Analyst, 1973, 98, 416. I , Ibid., 1973, 98, 563. -- University, Pullman, 1964. Received February 19th, 1973 Accepted March 20th, 1973