H.A. Hudson

High flow-rate cells for continuous monitoring of low concentrations of electroactive species by polarography and stripping voltammetry at the static mercury drop electrode

Abstract A high-capacity flow-through cell which can be used at a maximum flow rate of 300 ml min -1 has been developed for continuous monitoring of electroactive substances. The cell is compatible with the recently developed static mercury drop electrode. Comparative studies with a cell employing a conventional dropping mercury electrode are described. A wide range of polarographic techniques is applied, and it is demonstrated that the static mercury drop electrode improves the limits of detection, that laminar flow conditions are essential for low noise levels of operation, and that solution flow through a sulphite bed is a more effective method of oxygen removal than nitrogen bubbling. The combination of a microprocessor-controlled polarographic system, static mercury drop electrode and high-volume flow cell is very versatile for the determination of trace levels of electroactive species in flow streams. Preliminary results on anodic stripping voltammetry in flow streams are reported.

Kinetic studies of lead(II) uptake by the seagrass Zostera muelleri in water by radiotracing, atomic absorption spectrometry and electrochemical techniques

Abstract Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent.

On-line monitoring with an ion-selective electrode in a high-volume flow-through cell

Abstract A high-capacity poly(vinyl chloride) flow-through cell which can be used at a flow rate of 3000 ml min -1 in a corrosive environment is described for the continuous on-line monitoring of copper ions in plant electrolytes. The copper(II) ion-selective electrode and double-junction reference electrode in the cell are coupled to a voltmeter and microprocessor-controlled instrumentation to achieve on-line monitoring. The system is designed to withstand the severe industrial environments and was field-tested in a copper refinery where continuous monitoring of copper is required. Cell design and results of the investigation on the determination of copper in plant electrolytes are described.

The effects of temperature, salinity and seagrass species on the uptake of lead(II) from seawater by excised leaves

Abstract The dynamics of uptake by the leaves of seagrass species has been investigated by 210 Pb radiotracer techniques. The uptake of Pb from aqueous media per unit area of leaf by the seagrass Zostera muelleri was found to depend on temperature and salinity. Uptake was positively correlated with temperature. Pb uptake was greatest in distilled water and least in salinities exceeding normal seawater. The profile of uptake varied only slightly between three of the species studied, namely Zostera muelleri, Heterozostera tasmanica and Halophila australis , however, uptake by Lepilaena cylindrocarpa was considerably less than the other three species. The variations were examined in relation to microscopic anatomy especially in relation to the epidermis. The results are discussed in relation to possible mechanisms of the uptake of Pb by seagrass leaves. The results are consistent with the view that the seagrass leaves act as passive ion-exchange media during the course of Pb uptake.

Electrochemical reduction of the trimethyllead(IV) cation in seawater

Abstract The need to distinguish between inorganic and organic forms of lead in seawater has been widely recognized. Electrochemical techniques based on reduction at mercury electrodes or anodic stripping voltammetry have been proposed as methods for determining the various forms of lead. However, detailed investigation of the electrode process for the electrochemical reduction of the trimethyllead cation in seawater show that correct data may not necessarily be obtained with all electrochemical techniques. On the short time scale of polarography the proposed reduction mechanism is: (CH 3 ) 3 Pb + + e (CH 3 ) 3 Pb+3 Hg→Pb(amalgam)+3 CH 3 Hg(ads) 3 CH 3 Hg(ads)+3 e →3 CH 3 Hg − Data show that differential pulse polarography can be used to determine simultaneously inorganic lead and the trimethyllead cation, however, with inverse oxidation experiments involving stripping of lead (amalgam) such as reverse normal pulse voltammetry, substantial modification to the electrode process occurs in the presence of methylated mercury. Anodic stripping voltammetry therefore suffers from substantial interference, when using the: Pb(amalgam)Pb 2+ +2 e electrode process for determining the concentration of the trimethyllead cation in seawater.

Battery-operated microcomputer-based electrochemical instrumentation

Abstract There is a need for microprocessor-controlled, battery-operated, analytical instrumentation in environmental and industrial monitoring programmes. The electrochemical techniques of ion-selective electrode potentiometry and voltammetry are ideally suited for this task because of their inherent simplicity and compatability with digital computer-based instrumentation. The task for the future is to build computerized intelligent stand-alone battery-operated devices with comparable performance to their mains-powered laboratory-based cousins.

A battery-powered, computerized instrument for the field-based determination of fluoride by the ion-selective electrode technique

Abstract Battery-powered complementary metal-oxide semiconductor (CMOS) electronic devices are used to develop computerized instrumentation based on an ion-selective electrode for environmental monitoring in the field. The development of CMOS-based instrumentation for the determination of fluoride in drinking water, river water, lake water and sea water is described. The instrument is portable and completely field-programmable. Under software control, the equilibrium potential and fluoride concentration, based on a double standard-addition method, are calculated. The associated temperature, date and time form part of the data-logging record. Data obtained in the field are shown to compare satisfactorily with those obtained on samples which were stored and returned to the laboratory for examination by conventional mains-powered instrumentation. Accuracy is good and precision is only slightly poorer than that of mains-powered laboratory instrumentation.

Development and application of computerized battery-powered field programmable data acquisition systems

Abstract A computer-based, battery powdered, data acquisition system is described, which forms part of a field-based chemical system for use with sensors generating a voltage. The equipment has been developed using complementary metal oxide semiconductor (CMOS) “low power” integrated circuit technology and is based on a multiplexed bus. One 2k-byte erasable programmable read only memory (EPROM) controls the data acquisition process. Another 2k-byte EPROM contains the monitor program, while a 2k-byte random access memory stores the data before and after manipulation. A microprocessor unit provides all the control signals used by the instrument and data are collected on a real-time basis. The electronics are housed in a self-contained box to which the sensors and an independent keyboard can be connected. The instrument can be linked with as many as eight sensors. With the use of battery-operated CMOS technology, the measurement noise level is low and analytical techniques can be used to determine low concentrations of variables in the field. The viability of the instrument has been demonstrated by obtaining extensive data on the measurement of temperature and pH. Field-based potentiometric titrations, ion-selective electrode applications and studies on battery life are also reported.

Continous monitoring of copper and cadmium in zinc plat electrolyte usinga microprocessor-based batter-operated data acquisition system, multiple ion-selective electrodes and redundancy principles

Abstract Monitoring of both copper and cadmium is essential in the electrolytic production of zinc because these elemetns must be removed in order to improve the efficiency of the zinc electro-refining process. A new high-volume flow-through cell with an assembly of four ion-selective electrodes (four copper, four cadmium or two copper and two cadmium electrodes), reference electrode and temperature probe coupled with microprocessor-based instrumentattion can be used for this purpose. Determinations can be done continuously for 72-h periods without maintenance in an on-line mode. when multiple electrode determinations and redundncy principles are implemented. A microcomputer system incorporating low-power CMOS technology with multichannel and multiplexing capabilities is used for data acquisition. The use of the battery-powered data acquisition system provides excellent signal-to-noise ratios, meets the special demands of the harsh industrial environment, and is preferable to conventional mains-powered monitoring systems based on ion-selective electodes.

Development and application of a fully automated, battery-operated, computerized field-based fluoride monitor

Abstract An automated, portable, battery-operated, computerized field-based monitor for the determination of fluoride based on the use of ion-selective electrode (ISE) potentiometry has been developed with the aid of low-powered complementary metal oxide semiconductor (CMOS) devices. The whole analytical cycle involving the rinsing of the cell, sampling, stirring, dosing of standards to the sample and data acquisition and manipulation is under microprocessor control. The modular instrument consists of pumps, valves, a flow-through cell containing a reference electrode, a fluoride ISE, a temperature probe and a stirrer, a microprocessor with a real-time clock, a pump-valve-stirrer interface, a portable terminal and a 12-V lead-acid battery to power all the instrumentation. The software for the application and monitoring functions for the instrument is written at assembler level and programmed into a CMOS erasable programmable read only memory (EPROM). The instrument is currently designed to determine fluoride in natural and fluoridated waters and is based on a double standard addition method, although the monitor can be modified easily to suit other appropriate ISE systems. Instrumental performance was evaluated with synthetic and real water samples both in the batch and continuous modes. The monitor can be used to carry out on-line fluoride determinations of water samples continuously for 1 week without anyone being attendance.