Modi®cation of a Commercial SMDE Electrode to Improve the Precision of Electrochemical Measurements D. Gonza Âlez-Arjona,* + G. Lo Âpez-Pe Ârez, + E. Rolda Ân, + and J. D. Mozo ++ + Department of Physical Chemistry, University of Seville, E-41012, Seville, Spain; e-mail: dgonza@cica.es ++ Department of Chemical Engineering, Physical Chemistry and Organic Chemistry, University of Huelva, Huelva, Spain Received: December 22, 1999 Final version: February 29, 2000 Abstract A modi®cation of the commercial PARC 303A SMDE has been developed to improve the precision of electrochemical measurements by using an easy integrated electrical design. The original remote operation for the Dislodge=Dispense cycle has been modi®ed allowing an entire time control of the signals involved in this process. Highly reproducible experimental data can be obtained using this procedure. Keywords: Static drop mercury electrode (SMDE), AC impedance, Polarography 1. Introduction Mercury drop electrodes are widely used mainly because a renewable and reproducible surface can be obtained. In fact, some recent publications describe new mercury drop electrode designs [1±3]. Nevertheless, the 303A SMDE, from Princeton Applied Research (PAR) EG&G developed in the early eighties, is one of the more commonly used in electrochemical experi- ments as working electrode. It is able to produce a renewable mercury drop electrode whose area remains unchanged during the measuring process, avoiding the drop growing associated problems of classical dropping mercury electrodes (DME). This fact improves the sensitivity in pulse electrochemical techniques because the base line current is considerably reduced. The main advantage of these electrode systems is that they can be remote controlled via digital signals, generated in an easy and reliable way using personal computers. The 303A SMDE has been employed as an enhancement of an automatic system for the measurements of electrode impedances [4]. Other changes have also been made in this measurement system using an compatible IBM PC 486 instead of the old fashioned Apple II, a new I=O interface based on 82C55 PPI± 82C54 timer-counter [5, 6] and a GPIB from National Instrument [7] to con®gure and acquire experimental data from the Solartron 1286 electrochemical interface and 1250 frequency response analyzer. There are two methods for obtaining a new drop of mercury using a 303A SMDE. In remote mode, the drop is produced by an automatic Dislodge=Dispense cycle triggered by a logical low signal (0 V) in the Dislodge or Dispense line. In manual mode, a new drop is obtained by pressing the pushbutton Dislodge which knocks the capillary, and then the Dispense button, which keeps the valve open allowing the mercury to ¯ow through the capil- lary. The size of the drop is controlled by the time that the valve is kept open. Both pushbuttons are placed on the 303A SMDE front-panel. The remote mode gives a reasonable precision for experi- mental measurements, but a noticeable increase in the precision is obtained by using the manual procedure to generate a new drop. This fact is independent of the strength adjustment for drop dislodgement in its normal range of operation. The aim of the present article is to describe a simple and easy setup to carry out a remote control for 303A SMDE in order to increase the reliability of the experimental measurements. This method emulates the manual operation by using digital signals, which, conveniently modi®ed, can trigger the electric contacts of the front panel pushbuttons. 2. Procedure A detailed schematic of the main board is supplied in the operation manual of the electrode system in page VI±3. All the operations of the 303A SMDE are controlled by the integrated 8 bits microprocessor MC68705 from Motorola [8]. Figure 1 shows the electrical design of the present modi®cation performed in the 303A SMDE. Bracket notation refers to the nomenclature used in the main board schematic, above mentioned. Figure 2a shows the timing diagram of the standard cycle for Dislodge=Dispense operation. The cycle starts, in the original remote mode, with a falling edge in the DISL or DISP lines. It produces a positive pulse of ca. 25 ms in the PB1 line of the integrated microprocessor, that activates the knocker ampli®er circuit to remove the drop. The PB2 line controls the valve circuit to dispense mercury. This line is lowered after ca. 42ms of the initial falling edge. The drop size is directly related to the time that this line is at low level, controlled by the three positions switch, named ``Drop Size'', located on the front-panel (Small, Medium and Large). The moderate precision achieved in the experimental measurements using the PAR remote control seems to be a consequence of the short delay between the Dislodge and Dispense signals. To avoid the use of this standard remote cycle, the process to generate a new drop is accomplished sequentially by a couple of electrical signals that emulate the manual opera- tion of the front-panel pushbuttons. To generate the DISP and DISL pulses, two miniature low power relays (Omron SPCO G2E=24 V) are used to physically ground the integrated microprocessor PA7 (DISL) and PA6 (DISP) lines, see Figure 1. The power supply (22 V) is taken from the rear-panel connector (pin 6). The relay activation is 1143 Electroanalysis 2000, 12, No. 14 # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 1040-0397/00/1410±1143 $17.50.50=0