Electrochemical Sensors for Process Stream Monitoring - American

Electrochemical Sensors for Process Stream Monitoring Peter L. Bailey Electronic Instruments Ltd. Richmond, Surrey, UK

T h e r e are several aims to be met in t h e design of an electroanalytical system to be used for the continuous analysis of dissolved species in aqueous process streams. T h e system must be capable of being set u p and operated by relatively unskilled personnel and must run u n a t t e n d e d for long periods, preferably at least 30 days. T h e indicated analytical m e a s u r e m e n t should be of the required accuracy and precision over the whole expected range of both sample composition and t e m p e r a t u r e and pressure. Moreover, the system must have some means of preventing fouling and poisoning of the sensor and associated e q u i p m e n t by suspended solids, algal growth, or chemical attack. T h e running costs of the system should also be minimized. Relatively few of the many electroanalytical techniques now available are capable of adaptation to such' stringent requirements, b u t of those t h a t are, the most widely used are direct potentiometry, conductivity measurement, and the simplest forms of voltammetry. T h e modification of each of these three techniques to process stream monitoring will be described in turn. Among t h e species and parameters commonly measured are p H , dissolved ammonia, chlorine and oxygen, redox potential, conductivity, and concentrations of sodium, nitrate and sulfide ions. Frequently, it is more important for the system to monitor changes in these parameters t h a n to measure actual concentrations.

Potentiometry For potentiometry, two electrodes are required, an indicator electrode

(an ion-selective electrode or a redox electrode) and a reference electrode. These electrodes may in some cases be p u t directly into t h e sample stream. However, if the sample has to be pretreated before presentation to the electrodes or has a varying temperature or pressure, a small p a r t of the sample may be bled off into a monitor for analysis under more controlled conditions. T h i s monitor will usually bring the sample to a constant t e m p e r a t u r e and pressure, add reagents to mask or remove contamin a n t s t h a t would interfere with the electrode response, and buffer the sample to a p H within the working range of the ion-selective electrode. T h e sample is t h e n p u m p e d past t h e electrodes a t a constant flow rate. In nearly all continuous m e a s u r e m e n t s of process streams with ion-selective electrodes, the monitor is used because most of the ion-selective electrodes currently available are intolera n t of the conditions in typical untreated samples: the electrode t h a t is the exception to the norm is the p H glass electrode. T h u s for p H measurements, except of low conductivity waters, such monitors are not necessary, and the electrodes may be p u t directly into the sample stream using a cell of the type shown in Figure 1. This is because the p H glass electrode, which is the best behaved of all ion-selective electrodes, suffers virtually no interference from normal sample constituents and may sustain blockage of p a r t of the membrane surface without losing performance or suffering p e r m a n e n t damage. Its response is also virtually insensitive to variations in pressure and

698 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978

all b u t the most severe variations in sample flow rate, unlike most other ion-selective electrodes. Glass electrodes for such applications are usually made with a domed or flat end instead of the conventional bulbous shape, which is much weaker and can accumulate sample debris around the shoulder of the bulb. These domed or flat-ended electrodes are also easier to clean in situ. Scaly deposits such as of lime or similar inorganic materials are usually removed by means of a mechanical wiper arm t h a t passes across the m e m b r a n e like a windscreen wiper (see Figure 1). T o maximize the lifetime of t h e electrode, the frequency of wiping is reduced to t h e minimum compatible with keeping the m e m b r a n e clean. For keeping the m e m b r a n e free from organic solids or for preventing fine suspended material settling on and fouling the m e m brane, an in situ ultrasonic cleaner is generally more effective. In some applications it is difficult, because of a particularly d a m p or aggressive environment, to maintain a high impedance between t h e signal line and earth in the cable from the m e a s u r e m e n t cell to a remote meter. A satisfactory solution is to build an impedance converter or amplifier into or on top of the electrode; this converts the response of the glass electrode into a low impedance signal for transmission to a display unit or recorder. T h e design of the reference electrode in such a system is just as imp o r t a n t as the p H electrode. T o maintain a small and stable liquid junction potential across the tip of the reference electrode and hence maintain a 0003-2700/78/0350-698A$01.00/0 © 1978 American Chemical Society

Instrumentation

kjstem body

pH Electrode Mechanicol cleoner Temperoture compensotor Reference electrode

stable pH response, a fast-flowing junction is used. This also helps to prevent particulate matter in the sample clogging the junction. To produce this fast flow (typically 0.1-1 mL/day) over extended periods of unattended operation, a large volume sealed electrode is used or, alternatively, a conventional electrode fitted with an external reservoir of electrolyte. This external reservoir will pressurize the electrode and keep up a steady flow in those applications in which the sample line is under pressure or liable to pressure variations. This is important since if any sample gets forced back into the reference electrode, the potential can rapidly become unstable and the reference element may be poisoned.

The third component of the cell, as shown in Figure 1, is the temperature compensator. The signal from this compensator, together with the isopotential setting on the meter (2), allows the pH meter to compensate the pH reading for changes in temperature. Both the reference electrode and the temperature compensator may have bodies made from one of the more chemically resistant plastics in order to make them more robust and to permit a simpler and more troublefree construction. Plastics that have been used include ABS, TPX, glassfilled polypropylene, and some of the fluorinated polymers. For the pH electrode, glass still provides the cheapest and most reliable stem material compatible with the glass membrane: the

electrodes may, however, be strengthened by sleeving this stem with a plastic or metal tube. With this type of cell, in which the electrodes are immersed directly in the sample line, the electrodes must be removed periodically for calibration in buffers. However, if used in a nonaggressive sample, the electrodes may be sufficiently stable to need calibration at only weekly or even less frequent intervals. Redox measurements may also be made in an in-line cell such as that shown in Figure 1 by replacing the pH electrode with a domed or flat-ended noble metal electrode, usually a platinum electrode. The same cleaning systems as before or alternatively chemical or electrochemical cleaning meth-

ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 · 699 A

Electronic Unit

Liquid Handling Unit

Constant Head Unit

Flow Valve

Pretreatment Reagent

Peristaltic Pumps

Ion-Selective Electrode

Amplifier

Meter

Alarm Controller

Automatic Calibration Module

Reference Electrode

I Flow Cell Inlet Waste

Sample Overflow

Standard Solution

? Exchanger Recorder

Figure 2. Schematic diagram of a monitor using ion-selective electrode

ods may be used. Where the sample reacts with t h e metal of t h e electrode, for example, causing a film of oxide to be formed, anodic or cathodic cleaning in situ (depending on the nature of the surface coating) is most effective. These cells are used in such applications as the monitoring of chromate in plating effluents during t r e a t m e n t (Pt electrode) a n d for control of chlorine during the chlorination of cyanide wastes [when an Au electrode is better (2)]. For process stream applications, ion-selective electrodes other t h a n the p H electrode are normally used in monitors. These are designed to cater for all the peculiar sensitivities of ionselective electrodes a n d to allow the measurements to be made reliably in an industrial environment. In order t h a t the response time of the system is short and there is the minimum loss of the species to be measured in the sample line between the sampling point and the monitor, t h e monitor is installed as close as possible to the sampling point. Until recently, this has generally m e a n t t h a t the monitor had to be encased in a steel or rigid plastic housing, so t h a t it could withstand the drastic changes in environmental conditions t h a t may occur in a plant, a n d be splash-proof or even rain-proof. Such protection is costly to provide, and when several monitors are to operate close together, it is now common to build t h e m into a small portable cabin. This cabin is air-conditioned to provide o p t i m u m conditions for the operation of the moni-

tors, whatever the plant environment. T h e components of t h e monitors are rack or shelf m o u n t e d for easy maintenance and routine checking. If the process stream contains suspended material, the abstracted sample must be passed through a filter unit to prevent clogging of the monitor due to buildup of deposits in the narrow tubing or in t h e m e a s u r e m e n t cell. Filtration also helps to reduce the concentration of material t h a t might foul or poison the electrode membrane. This latter point is particularly i m p o r t a n t since the membranes of ion-selective electrodes are not sufficiently strong to withstand automatic mechanical or ultrasonic cleaners. After filtration the sample is passed through a unit, such as a constant head unit, so t h a t the sample intake to the monitor is a t a constant pressure. T h e flow rate is fast to keep the response time of t h e monitor short. A fixed ratio of a p r e t r e a t m e n t reagent is mixed with t h e sample stream; this ensures t h a t the electrode sees the species to be measured against a cons t a n t background of inert electrolyte buffered to the p H for o p t i m u m electrode response in t h a t sample. Also, any species t h a t might interfere with the electrode's response or reduce the activity of the species to be measured is either masked or removed by the reagent. T h e formulation of such reagents is discussed in ref. 3. T h e monitor is kept at a constant t e m p e r a t u r e so t h a t the sample and m e a s u r e m e n t cell are both in thermal equilibrium. This avoids the thermal drift t h a t is


a major source of inaccuracy in laboratory beaker measurements. T h e other function performed by the monitor is the pumping of the treated sample past the electrode at a constant and high velocity; the importance of this has been emphasized by several workers {4,5). T o compensate for errors t h a t would be caused by electrode drift, t h e monitor often incorporates a self-calibration facility. This consists of a timing device t h a t periodically activates a change-over valve, which automatically exchanges the sample for a standard solution of the species to be measured, and a suitable electronic circuit t h a t corrects for any a p p a r e n t drift in the s t a n d a r d potential of the cell. A diagram of a typical monitor, showing the various components of both the liquid handling and the electronics units, is given in Figure 2. T h e ion-selective electrode and reference electrode used in such a monitor do not require special modifications and may be of the type normally used in the laboratory. Because the reference electrode is m o u n t e d downstream from the ion-selective electrode, contamination of the sample from bridge solution leakage cannot affect the readings. T h e bridge solution may therefore be chosen to give the minimum liquid junction potential and is usually a strong potassium chloride solution. As a result of working in an environm e n t with closely controlled temperature, steady flow rate, constant addition of reagent, and regular s t a n d a r d -

ization, the accuracy and precision achieved by the electrodes are much better than those generally achieved for determinations of samples in bea kers. Monitors based on these electrodes have been used in a wide range of ap plications, some of which are given in Table I. For the ammonia monitor an ammonia-sensing membrane probe is used. In this case, no reference elec trode is needed since this is incorpo rated in the sensor.

Β

V8 Conductivity 5 R

Conductivity Measurement

Conductivity measurement is often the preferred method for monitoring a process stream, especially for those samples in which the species being monitored is the major ionic species with a changing concentration. It is simple to install and operate and has low maintenance costs and a high reli ability. Examples of its application in clude the monitoring of detergent strengths in solutions used for auto matic bottle washing, control of limedosing for flocculation in potable water treatment plants, control of the first stage carbonization in sugar re fining, regulation of fertilizer concen trations in horticultural feed waters, and monitoring of concentrated acids. An industrial sensor or cell for con ductivity measurement consists of a tube made of an insulator in which are embedded electrodes made of a noble metal, often platinum, or graphite. This cell is fitted directly into the pro cess stream to be monitored. For mea surements of concentrated acids, plat inum electrodes in a specially lined glass and/or PTFE cell are suitable. For the very simplest measurements in clean solutions, a two-electrode cell may be used. Either a constant alter nating current or a constant alternat ing voltage is applied to the elec trodes, and the resultant voltage drop or current is measured. The conduc tivity is then deduced by knowledge of the cell constant (dependent on the geometry of the cell) and Ohm's law. The constant voltage mode is more frequently used since the current mea-

12 3 4

Figure 3. In-line conductivity cells A : F o u r - e l e c t r o d e c e l l a n d c o n t r o l c i r c u i t . B: S i x - e l e c t r o d e c e l l

sured is directly proportional to the required conductivity. Two-electrode cells are, however, often unsatisfactory for continuous flow monitoring because any buildup of resistive deposits on the electrodes is recorded as a decrease in the con ductivity of the sample. Various types of automatic mechanical cleaners have been used but are not fully satisfacto ry. Wear of the electrode surface soon occurs and can affect both the insula tion of the cell and the cell constant. A better answer is provided by the four-electrode cell, the use of which gives measurements less vulnerable to the effects of cell fouling. Again, both the constant current and con stant voltage methods may be used. A design showing the cell in the con stant voltage mode is shown in Figure 3A. The current passed between elec trodes 1 and 4 is regulated such that the voltage drop between electrodes 2 and 3, V2 3, remains constant at all times. V2 3 is amplified by Ai and compared by A2 to a reference voltage: the difference between them drives the fixed frequency current generator G. The current flowing is proportional to the sample conductivity (6). A

source of inaccuracy with these cells is the leakage of current, some of which inevitably flows through the sample to surrounding metal pipework instead of entirely between electrodes 1 and 4. To counteract the effect of this leakage, two further electrodes may be introduced (6) as shown in Figure 3B. Electrode 5 is maintained at the same potential as electrode 4 to prevent current flow from electrode 4 in that direction. Any leakage thus occurs from electrode 5, and a return path for the current is provided to electrode 1 via electrode 6. The dis tances between electrodes 4 and 5, and 5 and 6 are similar to that between electrodes 1 and 4, since this minimiz es leakage between electrodes 4 and 5. The measurement may be compen sated for changes in temperature by a thermistor, in series with electrode 1, which is embedded in the cell. Voltammetry

In industrial process monitoring, there is little room for the theoretical ly satisfactory but practically trouble some mercury electrode either of the dropping, hanging drop or film types favored by laboratory polarographers.

Table 1. Examples of Applications of Monitors Using Ion-Selective Electrodes Species monitored

Na+ F~

Concn range

1-200 >*g/L 0.1-10mg/L

Type of sample

Boiler feedwater Drinking wat0f sup; >


Typical prelreatment reagent

NH3 CHaCOOH/CHsCOONa

Major Interferences

H+ Ai , F©

Silver Electrode Retaining Screw

Silver Oxide Paste

Thermistor O-Ring

Probe Body

Electrolyte Pt Auxiliary Electrode Porous Ceramic Former

Ag/AgCI Reference Electrode

FEP Membrane

Figure 4. V o l t a m m e t r i c cells A: Haller N 2 H 4 cell. B: Mackereth 0 2 cell. C: Kane and Young cell for 0 2 and Cl 2

Generally, only the solid noble metal electrodes are robust enough and give sufficiently reproducible results in the plant environment, although it is recognized t h a t in some applications their performance is not ideal. Two main types of voltammetric cell are used for on-line monitoring: a) cells with an exposed working electrode (for CI2, N 2 H 4 , etc.) and b) membrane-clad cells (for O2 and CI2). T h e industrial versions of cells of type (a), in which the working electrode has the sample flowing directly over it, are usually of substantially different construction from their laboratory counterparts. T h i s is because the major problem with these cells is t h a t it is necessary, for most applications, to incorporate some method for keeping the working electrode free from deposits, growths, surface films, etc., which affect the m e a s u r e m e n t sensitivity. Several m e t h o d s are available for maintaining the electrochemical activity of the working electrodes. These include mechanical, chemical, and electrochemical cleaning. It is sometimes necessary to use more t h a n one of these methods simultaneously. T h e electrode is mechanically cleaned by continuously or periodically b o m b a r d -

ing it with a jet of an abrasive grit such as alumina or by wiping the electrode with a moving arm as with mechanical cleaners for p H electrodes. For chemical cleaning t h e sample flow is periodically alternated with a chemical cleansing agent, often a strong acid. T h e third procedure, of electrochemical cleaning, is the same as previously referred to in connection with redox electrodes. A current high in relation to the m e a s u r e m e n t current is periodically passed through the cell between the working electrode and the counter electrode, in such a direction as to remove, by oxidation or reduction, surface films of compounds (oxides, sulfides, etc.) of t h e metal forming t h e working electrode. These cleaning problems would be lessened if the sample were to be chemically treated to reduce the level of interferents before they reached the electrode. However, because of t h e high flow rates normally required (often 200 m L - 1 L/min), bulk sample t r e a t m e n t is not economically practicable. For hydrazine analysis, however, some sample t r e a t m e n t is essential because the electrochemical oxidation of hydrazine, which is the reaction giving rise to the current, only takes place in strongly alkaline media and


not a t the near neutral p H of the boiler waters t h a t constitute the majority of samples. A sensor design (7) t h a t solves this problem without t r e a t m e n t of t h e whole mass of sample is the two-electrode Haller cell shown in Figure 4A. T h e platinum wire anode is wound onto a porous ceramic former through which strong sodium hydroxide solution seeps; a constant pressure of this solution is maintained by an external reservoir. This ensures t h a t t h e thin film of sample in contact with the anode is kept at a high p H , much higher t h a n the bulk of t h e sample. T h e counter electrode is of silver and is immersed in a silver oxide paste in the tip of the cell. T h i s is consumed by t h e reaction and must be periodically replaced. No electrode cleaning is necessary because the samples are very clean. Both mechanical and chemical cleaning have been found to be essential in the design of a chlorine analyzer for the monitoring of free chlorine in treated cooling waters, particularly when the source of the water may be brackish (8). In this analyzer a platin u m working electrode and silver counter electrode are used. T h e platin u m is kept clean with recirculating alumina grit, together with a periodic

wash in hydrochloric acid. A potential is applied between the two electrodes to maintain the platinum at the cor rect potential for the chlorine reduc tion. Other chlorine m e a s u r e m e n t cells t h a t have been successful include the gold/copper cell which, acting galvanically, requires no externally ap plied potential. T h e gold working elec trode will dissolve very slightly in the chlorine-containing samples, a n d this helps to keep it clean. Membrane-clad cells are similar in principle to other voltammetric cells except t h a t the working electrode is separated from the sample by a gaspermeable hydrophobic m e m b r a n e . T h i s restricts their application to those dissolved gases t h a t may be measured voltammetrically, in prac tice, oxygen and chlorine. T h e great advantage of these sensors, which has m a d e t h e m the preferred type of sen sor for these gases in most process control applications, despite the slight increase in response time caused by the m e m b r a n e , is t h a t t h e working electrode is protected from fouling and poisoning. T h e m e m b r a n e also improves the selectivity of the mea s u r e m e n t by preventing ionic redox species in the samples [e.g., Cr(VI)] from interfering. For particularly dirty samples the sensor is incorporated in

a flow-chamber giving him sample ve locity across the m e m b r a n e . This re duces t h e rate at which material col lects on the sensor. Two typical constructions of m e m brane-clad voltammetric cells are shown in Figure 4. T y p e Β is a twoelectrode (Ag/Pb) galvanic cell as orig inally reported by Mackereth (9). T h i s design involves a particularly large cathode t h a t makes the cell both sen sitive and robust. It is widely used in the water industry. For boiler feed water analysis, a version of this cell has been developed (10) in which its sensitivity is greatly increased by use of a much more permeable m e m b r a n e . T h e cell is built into a monitor flow system to ensure a high sample veloci ty across the m e m b r a n e . T h e monitor may be used to measure oxygen con centrations in the range 0-20 Mg/L. T h e other type of cell depicted (Type C) is a three-electrode cell (11). T h i s cell has the advantage of an easi ly replaceable F E P m e m b r a n e and may be used for either oxygen or chlo ride analyses in both aqueous and gas eous process streams. It may be made selective for chlorine in the presence of oxygen by lowering the cathode po tential to approximately 0 V, a poten tial at which t h e oxygen is not re duced. Applications t h a t have been

High Speed Spectroscopy

described include t h e monitoring of dissolved oxygen in the brine/lime slurry wastes from t h e ammonia soda process (11). T h e sensitivity of the chlorine cell in aqueous samples is low unless the sample is strongly acidified to convert all chlorine species to dis solved chlorine gas. Acknowledgment T h e author t h a n k s Electronic In s t r u m e n t s Limited for permission t o use Figures 1, 2, 3, and 4B. References (1) G. Mattock, "pH Measurement and Ti tration", Heywood, London, England, 1961. (2) G. Mattock, Trans. Soc. lustrum. Technol, 16,173 (1964). (3) P. L. Bailey, "Analysis with Ion-Selec tive Electrodes", Heyden, London, En gland, 1976. (4) P. Van den Winkel, J. Mertens, and D. L. Massart, Anal. Chem., 46, 1765 (1974). (5) E. L. Eckfeldt and W. E. Proctor, ibid., 47, 2307 (1975). (6) D. Warmoth and K. Porter, Kent Tech. Rev., 19,17 (1977). (7) J. F. Haller, U.S. Patent 2,651,612 (8th Sept. 1953); I. R. Weingarten, U.S. Pat ent 3,694,338 (26th Sept. 1972). (8) R. J. Baker, Ind. Water Eng., 6, 20 (1969). (9) F.J.H. Mackereth, J. Sci. Instrum., 41, 38 (1968). (10) M. Riley and P, L. Bailey, Kent Tech. Rev., 11,7(1974). (11) P. O. Kane and J. M. Young, J. Electroanal. Chem., 75, 255 (1977). (12) J. E. Harwood, Water Res., 3, 273 (1969).

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Write or call for our OMA brochures: Princeton Applied Research Corp., P.O. Box 2565, Princeton, NJ 08540 609/452-2111

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Peter Bailey is the factory general manager at the factory of Electronic Instruments Limited in Richmond, where their potentiometric sensors PRINCETON APPLIED RESEARCH and pH meters are made. His re search interests include the develop EG&G COMPANY AN ment of gas-sensing membrane probes 421 and ion-selective electrodes.

Circle # 1 6 3 for A d d i t i o n a l I n f o r m a t i o n O n l y .


Electrochemical Sensors for Process Stream Monitoring - American

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