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Nebulizer for Eliminating Oxygen from Polarographic Flow Cells Chalm Yarnitzky and Ezra Ouziel' Department of Chemistry, Technion-IsraelInstitute of Technology, Halfa, Israel
Polarographic flow cells represent one type of several electrochemical cells used for monitoring concentrations or for the rapid analytical determination of successive discrete samples, containing electroactive components. The potential of the working vs. the reference electrode is usually kept constant, its value dictated by the analyte. Whenever such a flow cell is used for monitoring column effluents, e.g. in classical (I) or in high performance liquid chromatography (2), its dead volume is one of the most important design parameters. Deaeration of the solvent is usually effected prior to its introduction into the column, i.e., in the bulk solvent container. Whenever the flow is to be used for the analysis of discrete samples, three important factors have to be considered: sample volume, the method of its introduction, and the time required for complete deaeration. The use of commercially available cells, with low sample volumes (2-5 ml, for instance) usually involves the following steps for each sample analyzed: cleaning of the cell and electrodes, rinsing with sample, and deaeration with an inert gas. The Rotacell ( 3 )recently introduced, decreases deaeration time to 5min (for a sample volume of 15 ml). The automated approach suggested by Porter et al. ( 4 ) of spraying the sample on the DME allows the processing of 30 samples per hour, provided previously deaerated samples are employed (sulfite was used, requiring 30-min delay for complete oxygen removal). The present paper describes a simple flow cell, suitable for detection as well as for routine analytical work. A nitrogen activated glass nebulizer, employing a Teflon capillary for aspirating the sample solution (total volume, 2-5 ml; flow rate, 6-20 ml/min), sprays the sample into a nitrogen atmosphere. Oxygen removal is practically instantaneous (within milliseconds of the sample leaving the capillary orifice). The step limiting the sample changeover is the time required for sufficient sample collection in the cell: 20-40 s.
cell after deaeration (bubbling nitrogen through the solution for half an hour). Another portion of the original solution was sprayed into the new cell (Figure 1)and a polarogram recorded after 40 s; the results are shown in Figure 2. Two solutions were chosen for the demonstration of the deaeration efficiency; the first one was 0.1 M potassium chloride dissolved in water; the second, methylene chloride 0.1 M in tetra-n-butylammonium perchlorate and 7.10-4 M in camphorquinone. The removal of oxygen from water was complete when using the standard or the new cell (Figure 2). Reduction waves of oxygen and camphorquinone were recorded after bubbling nitrogen for 30 min through methylene chloride, when the standard cell was used, before and after dissolving the electroactive material in the solution. Clearly, the wave of camphorquinone was increased because of the evaporation of the solvent. When the nebulizer-polarographic cell was used, the wave maintained its correct value and the oxygen wave disappeared. Polarographic Reduction of Cadmium Ions. The reduction of Cd2+ was carried out and recorded, using the two cells mentioned above; no difference is detectable. The time required for sample changeover in the new cell was checked by inserting the tubing first into a supporting electrolyte solution and then into a solution containing Cd2+,
EXPERIMENTAL Three different types of cell have been constructed and the one used to carry out this work is shown in Figure 1.All three consist of a nebulizer, a collector-separator, and the polarographic cell proper. The nebulizer is based on the Venturi effect and is activated by a stream of nitrogen gas at an input pressure of 1000-1500 Torr. This unit is the heart of the system, since it controls the effectiveness of the deaeration. It is suggested that a nebulizer of the kind developed for AA (e.g. Perkin-Elmer model 403, part No. 303-0358)be used. For a home made unit, 0.7-mm i.d., 200-mm length Teflon tubing should be used; its position in the nebulizer must be determined experimentally by polarographic monitoring of the oxygen wave. The collector-separator is placed in the compartment in which the spray is separated from the nitrogen and collected. The solution flows on to the polarographic cell. Polarograms were recorded with a Radiometer PO4 polarograph in conjunction with a Dynamic Compensator ( 5 ) .
RESULTS AND DISCUSSION Reduction Waves of Oxygen. The reduction waves of oxygen were recorded polarographically in a standard H type 2024
Figure 1. A nebulizer-polarographic cell suitable for monitoring chromatographic effluents (A) Nebulizer for effluent a. (8)Nebulizer for effluent b. (C) Collector. (D)Cell.
(E) Reference electrode
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Figure 3. Polarographic current monitored at constant potential (-1 .O V vs. AglAgCl electrode) At point A, the tubing was Inserted in a solutlon containing 0.1 M KCI and M CdCI2. At point B, the tubing was inserted in a solutlon containing 0.1 M KCI. Changeover tlme Is less than 30 s
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Flgure 2. Polarographic
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(a) In the presence of oxygen; (b) after bubbling nitrogen for 30 mln in a standard cell; (c) recorded In the Nebulizer-Polarographic Cell.
while recording the current at the potential corresponding to the cadmium reduction plateau. A changeover time of 40 s is sufficient (Figure 3). Irregularities in the diffusion current resulting from the solution flow are reduced by reducing the nitrogen stream to a minimum (required for keeping an inert atmosphere) during the recording of the polarogram. Evaporation of the Solvent. One of the major problems encountered with oxygen removal is the evaporation of the solvent by the gas flowing through the solution. Frequently, the inert gas is passed through a trap in which it is saturated with the solvent vapor, so that its subsequent passage through the polarographic cell causes no evaporation losses. Since pure solvent is placed in the trap, its temperature must be controlled to prevent its evaporation or condensation in the cell. In order to simplify the technique, it is important to obtain a close estimate of the rate of vaporization prevailing in the nebulizer-polarographic cell, when no trap is used. This is done by assuming that the droplets formed by the nebulizer
are very small, and equilibrium between the two phases (related to the partial pressure of oxygen in both phases and to the partial pressure of the solvent in the gas) is reached instantaneously. It has been proved experimentally in the case of water that 100 ml of nitrogen (the inert gas), measured at 760 Torr, were required to carry over 200 mg of the solution into the cell. If the vapor pressure of water was 17.5 Torr (at 20 "C), 2.3 ml (at 760 Torr, 20 "C) or 1.7 mg of water were carried over with the nitrogen. This means that the maximum error caused by the evaporation was less than 1%.However, since the temperature is lowered by the evaporation, the vapor pressure is decreased and the expected loss is less than 1%.In practice, no change in wave height has been observed. Special care should be taken when using volatile organic solvents such as methylene chloride (-400 Torr a t 18 "C); in 'this case, the loss by evaporation was only 20%because of the temperature drop. From the analytical point of view, the error caused by the change of temperature must be taken into account: it is therefore suggested to saturate the gas with the solvent, to check the temperature of the cell, and to keep it at a certain constant value by adjusting the temperature of the trap. The method would not work if the sample sprayed produced a foam or if the sample constituent itself were volatile.
LITERATURE CITED (1) W. Kemula, Rocz. Chem., 26, 281 (1952). (2) P. T. Klsslnger, L. J. Felice, R. M. Riggin, L. A. Pachla, and D. C. Wenke, Clln. Chem. (Winston-Salem, N.C.), 20, 992 (1974). (3) Elect Ro Cell-ASV, McKee-Pedersen Instruments, P.O. Box 322, Danville, Calif. 94526. (4) S. R. Porter, V. J. Jennings, and J. W. Ogleby, Proc. Anal. Div. Chem. SOC., Nov. 1975, p 285. ( 5 ) Ch. Yarnitzky and Y. Friedman, Anal. Chem., 47, 876 (1975).
RECEIVEDfor review May 12, 1976. Accepted June 25, 1976.
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