Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 545

to an electrical signal and is used in spectrometers. A second

example is a piezoelectric crystal-based sensor that converts

a mechanical force to an electrical charge translatable to a

potential. The piezoelectric effect is reversible; an electric

charge will cause a mechanical dislocation in the crystal.

Another example of a physical sensor is a platinum resis-

tance thermometer where the resistance of a platinum wire is

altered by a change in temperature.

Chemical sensors are devices that allow the analyte or

target material through one of its specific chemical param-

eters to ultimately generate an energy signal, usually electri-

cal, in a transducer through the agency of a selective chemical

or physical chemical reaction. A transducer is a material

structure inside of which or on whose surface the specific

chemical or physical chemical reaction takes place leading

to the generation of the energy signal. Thus, there are two

parts to the chemical sensor, the interface zone or area where

the selective reaction takes place and the usually non-specific

transducer.^8 Figure 2 illustrates, functionally, the parts of a

chemical sensor.

An example of a chemical sensor is a potentiometric

electrode. Here the selective chemical reaction, the redox

reaction of the analyte, is in equilibrium at the electrode sur-

face imposing a potential that is proportional to the loga-

rithm of the concentration of the analyte as described by the

Nernst equation. For example, a copper electrode in a solu-

tion of copper ions will take on a potential in response to the

concentration of copper ions. The logarithm of the copper

ion concentration is proportional to the electrode potential.

Another illustration of a chemical sensor is an amperometric

electrode, where a current arises due to the redox reaction of

the analyte when the electrode is at the appropriate poten-

tial. The concentration of the analyte is proportional to the

magnitude of the current. A platinum electrode maintained

at the redox potential for the silver/silver ion redox system

will detect the concentration of silver ions. A membrane

electrode is another type of chemical sensor. The fluoride

electrode consists of a lanthanum fluoride (LaF 2 ), thin, crys-

tal membrane. On the outside surface, the sample side of

the membrane, the fluoride ions, F^ ^ , from the sample are

attracted electrostatically to the lanthanum ion, La^3 ^ , at the

surface of the membrane to form a complex. The complexed

entities do not penetrate very deeply into the surface. The

amount of F^ ^ complexed is a direct function of its activity

(see Section III,B,2, a ) and represents a selective physical

chemical reaction. A membrane potential arises because

the opposite side of the membrane is exposed to a standard

activity of F^ ^ giving a net difference in potential between

the two sides. The membrane potential is the non-specific

electrical signal of the sensor.

Signal-Conditioning Circuits These circuits modify the

signal produced by the sensor so as to provide an accurate

representation of the sensor signal with optimal electrical

characteristics to drive the readout device. In Figure 1 a

number of signal conditioning modes are given and can be

Measurand

Sample property Input

Signal

Sensor

Energy transducer Transducer

Signal

Signal conditioning

Signal modification

Output

Signal

Readout

Types

Light

Absorption

Emission

Thermal

Conductivity

Temperature

Heat capacity

Electrical

Redox

Electrolytic conductivity

Ionic activity

Mechanical

Mass

Density

Viscosity

Surface tension

Nuclear

(X-ray, b.g)

Emission

Absorption

Light

Photocell

Photographic plate

Thermal

Thermocouple

Katharometer

Thermister

Bolometer

Electrical

Electrode pair

Electrodes, AC system

Membrane electrode

Mechanical

Balance force transducer

Force transducer

Hydrometer

Viscosity pipet

Nuclear

Ionization tubes

Scintillation counters

Photographic plates

Cloud chamber

Semiconductor detectors

Amplification

Arithmetic operation

Chopping

Comparison to reference

Digitization

Rectification

Stabilization

Analog

Meter

Oscilloscope

Recorder

Digital

Nixie display

Point plotter

Printer

Tape, paper, or

magnetic

Criteria

Band width

Noise figure

Sensitivity

Signal-to-noise

Time constant

Error

Hysteresis

Nonlinearity

Scale

Zero displacement

Instrument

FIGURE 1 Diagram of instrumental functions. Reprinted from Ref. (4), p. 1442 by courtesy of Marcel Dekker, Inc.

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