If a filament control relay is available, the absolute pressure protection relay can be combined with the pressure burst protection of the filament. Pressure burst protection turns the filament off regardless of pressure range when a sudden burst in pressure causes a rapid meter reading deflection, and opens the filament control relay, To combine these two safety features, the equipment to be protected is simply connected across both relays, which are connected in series. Now either a sudden pressure burst or gradual increase in pressure to a specified absolute level will cause shutdown of external equipment.
This circuit in the above indicated mode has worked successfully for the past 14 months in two applications. Granville-Phillips Series 236 Model 02 ion gauges and controllers protect a CEC 21-104 Mass Spectrometer and a CEC 21-613 Residual Gas Analyzer from pressure burst and operation above 1 x Torr. During this period there has never been a circuit failure, and the equipment has been successfully protected. Received for review August 7, 1972. Accepted December 11, 1972.
Vacuum-Line Electrochemical Cell for Electrosynthesis C. D. Sc.hmulbach and T. V. Oommen Department of Chemistry, Southern lllinois University, Carbondale, lil. 6290 7
Experience has shown that a vacuum-line electrochemical cell is superior to a conventional cell for the handling of moisture-and-air sensitive compounds. A recent paper described the use of such a cell (1) (hereafter called the Anderson Cell) for polarography, coulometry, and cyclic voltammetry. The Anderson Cell is not suitable for electrosynthetic work, however. The chief limitations of the Anderson Cell for electrosynthesis are: (i) a small capacity of the electrolysis compartment (-10 ml), and (ii) the absence of a separate anode compartment. A conventional H-cell f2), by contrast, has none of these disadvantages, but is not ordinarily attached to a high vacuum line. We have designed a cell that incorporates the useful features of both the Anderson Cell and a conventional cell. For convenience in description, only electrochemical reduction at the working electrode is discussed. The cell is equally suitable for electrooxidation. The cell assembly is shown in Figure 1. The cathode and anode compartments are separated by sintered glass frits and by a middle arm that can be connected to the vacuum line. The cathode compartment can handle 100150 ml of electrolyte. In electrolysis experiments, the working electrode is a mercury pool. The design of the cell enables easy switching from polarography to coulometry or electrolysis. The base electrolyte is contained in ampoule A, and the electroactive species, in ampoule B. Ampoule C contains the anolyte which is usually the base electrolyte itself. The reference electrode may be one of the standard electrodes such as the SCE or the Ag/Ag+ electrode, but for operation under vacuum conditions, the reference electrode must be placed in a separate compartment as it is in the Anderson Cell. On the other hand, a platinum wire dipping in the base electrolyte may be used as a quasi-reference electrode (PQRE) which can be placed in the catholyte, near the DME. The use of PQRE as a reference electrode is recommended where other electrodes are not suitable, though it is known that this electrode is not as highly reproducible as the others (3). The (1) J. L. Mills, R. Nelson, S. G. Shore, and L. E . Anderson, Anal. Chem.. 43, 157 (1971). (2) J F. Coetzee, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1956. (3) D. J. Fisher, W.L. Belew, and M.T. Kelley in "Polarography 1964." Vol. 1 1 , G. H. Hills, Ed., interscience, New York, N.Y., 1966, p, 1043.
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auxiliary electrode may be an inert electrode like platinum, or a sacrificial metal electrode such as silver, which was used in our study. The latter has the advantage of preventing generation of oxidizing gases resulting from anodic reactions, thereby avoiding the mixing problems that arise from gas pressure developing in the anode compartment. The selection of an electrode material is also made to avoid undesirable oxidation processes involving the base electrolyte or solvent. The resistance between the anode and working electrode was 300 ohms for a 0.1M solution of the base electrolyte tetrabutylammonium iodide in acetonitrile. This resistance is sufficiently low so as not to restrict coulometry studies with the Electroscan 30. Operation of the Cell. The cell inc1;ding the filled ampoules, electrodes, and magnetically stirred mercury pool is connected to the vacuum line and evacuated. The amount of mercury that drops from the DME during the evacuation is small. The cell is now isolated from the vacuum line, ampoules A and C are opened to the cell, and their contents are poured into the cell compartments until the solution level in the three compartments is equalized. Pure, dry nitrogen is then introduced into all three arms of the cell through the vacuum line to nearly atmospheric pressure. Nitrogen is added also to the tube containing the reference electrode through its nitrogen inlet. The cell may then be isolated from the nitrogen supply. The height of the mercury reservoir is adjusted for a drop-time of 4 to 6 seconds. G polarographic scan on the supporting electrolyte is made before introducing the electroactive species from ampoule B. The electroactive species is then transferred to the cell. A small known volume of the solution containing the electroactive species is first introduced from the graduated ampoule B to obtain a polarogram and then a larger known volume of the electroactive species is admitted for coulometry and electrolysis. Stop, cock G should remain closed during electrolysis. If pres sure differences develop and the solution level become! unequal in the separate arms of the cell during electrolysii because of gas liberation, provision should be made for thc relief of excess pressure or the nitrogen pressure should bc varied to compensate for pressure differences. A t the en( of electrolysis, the contents of the cell are transferrec through stopcock D to evacuated flasks without contact ing the atmosphere. The progress of electrolysis is moni
U
,2cm
.
7 cm
8cm
I
Figure 1. Vacuum-line electrochemical cell
tored by a current us. time decay curve or by periodic checks of concentrations by polarography. An, Electrolysis Experiment. Titanium tetrachloride has been electrolytically reduced in acetonitrile in a conventional cell by Kolthoff and Thomas ( 4 ) . Both Tic14 and its reduction products are very air-sensitive and therefore form an ideal system to test the effectiveness of the present cell. We have used the cell also for the polarographic, coulometric, and exhaustive controlled potential electrolysis study of gallium trichloride in acetonitrile (5). For the present purpose, however, we shall describe only the work done with TiC14. About 0.38 gram (1.31 mmoles) titanium tetrachloride was condensed into 20 ml of acetonitrile/O.lM tetrabutylammonium iodide (TBAI), in ampoule B. Ampoules A and C contained 100 ml and 30 ml, respectively, of the base electrolyte, CHsCN/TBAI. These solutions were prepared by standard vacuum-transfer techniques and purification methods. A silver anode and PQRE reference electrode were used. The background polarogram indicated that no impurities were present; the residual current increased from zero to 2 WAin the voltage range 0 to -2.8 V (SCE). With Tic14 introduced, polarographic waves were found a t -0.90 V, -1.25 V, and -1.85 V (aq. SCE). The ratio of differences in wave height for successive waves was 1.00:2.20:0.84. Half-wave potentials reported by Kolthoff are -0.77 V, -1.20 V, and -1.86 V and the ratio of differences in wave heights is 1.00:2.18:0.90. The second and third waves correspond to two separate paths for Ti(0) while the first wave is due the reduction Ti(1V) Ti(II1) ( 4 ) . Controlled potential electrolysis a t to Ti(1V) -1.0 V removed the first wave in an hour and the color of the solution turned from yellow to blue-green. Further exhaustive reduction was carried out a t -1.3 to -2.0 V. The solution became dark brown and a brown precipitate formed. After six hours, the current dropped from 70 mA
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(4) I . M . Kolthoff and F. G. Thomas, J . Electrochem. SOC, 111, 1065 ( 1964). (5) T. V . Oomrnen and C.D.Schmulbach, results to be published.
to less than 1 mA and the electrolysis was stopped. The contents of the cell were transferred through stopcock D to an evacuated flask. The solid residue was filtered in vacuum and the pale brown solution was analyzed for C1content, by gravimetric estimation as silver chloride. It was found that 2.2 moles of C1- were produced for every mole Tic14 reduced. This clearly indicated that the majority of Ti(1V) had been reduced only to Ti(I1) but that a small quantity was reduced further. It is known that TiC12-2CH3CN is a dark brown compound of very low solubility in acetonitrile (6).Its formation in the present 2ecase may be represented as TiC14 + 2CH3CN TiC1*.2CH3CN, + 2C1-. The apparent reduction of Ti(1V) beyond Ti(I1) may be accounted for by the disproportionation of Ti(II) complex (71, which is present in small concentrations, to Ti(0) and Ti(I1I) or Ti(1V) followed by further reduction of Ti(III)/Ti(IV) species. Our results thus confirm previous studies in which reduction of Ti(1V) to Ti(I1) was indicated but no titanium metal was produced in detectable quantities despite efforts to achieve exhaustive reduction ( 4 ) . The usefulness of this cell is electrosynthetic work with nonaqueous systems where rigorous exclusion of air and moisture is necessary has been demonstrated. Contamination from trace impurities in the inert gas is reduced to a minimum because the inert gas is used in small amounts to provide an inert atmosphere and a suitable pressure rather than in large amounts necessary for continuous purging. The simplicity of operation and flexibility of the cell have already been described.
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Received for review September 11, 1972. Accepted November 20, 1972. The generous support of Southern Illinois University’s Office of Research and Projects is gratefully acknowledged.
( 6 ) G. W . A . Fowles and T. E. Lester. Chem. Commun.. 1 9 6 7 , 4 7 . ( 7 ) T. C. Franklin and ti V. Seklernian, J Inorg. NucI. Chem.. 12, 181 (1959).
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