Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 583

are illustrated. A single arm electrolytic cell is also avail-

able.^105 Unstable reagents, namely, bromine, dipositive silver,

titanium(III), chlorine, etc., can be conveniently generated and

utilized in this technique. In Table 11 a number of applications

are listed. The ability to utilize these unstable reagents expands

the horizon of tritrimetry and analytical methodology.

(3) Measurement of radioactivity 106 – 8

a. Introduction

Radioactive species in water and wastewater samples

arise from natural and anthropogenic sources. The radiation

emitted from radioisotopes is deleterious to all life forms

and has led to the development of a number of radiochemi-

cal methods. Standard Methods^2 includes procedures for

the detection of tritium, radium, and radioactive species of

cesium, iodine, strontium, and uranium, as well as, the gen-

eral measurement of the amount of ionizing radiation.

Several kinds of ionizing radiation emanate from

radioisotopes. They include gamma and x-rays, beta rays

(electrons and positrons), and alpha rays (helium nuclei).

Ionizing radiation, when penetrating matter, causes the

formation of ions that are dangerous to life forms leading

to cancer in people and animals. Gamma and x-rays are at

the high frequency ( 10 6 Hz) end of the electromagnetic

spectrum. The former arise from decay events occurring in

the nucleus and are more energetic higher frequency than

x-rays that arise from rearrangement of electrons surround-

ing the decaying nucleus. Both kinds of radiation are highly

penetrating due to their high energy, and therefore are quite

dangerous. Gamma and x-rays are emitted as continuous

spectra.

Beta particles are of two types; negatively charged

electrons or negatrons and positively charged positrons.

They arise from decay processes in nuclei. A positron has a

very short life time and is annihilated on collision with an

electron to form two gamma photons. Beta particles have

a greater penetrating ability than alpha particles and are

poorer at ionizing matter. Since beta particles are scattered

in air, their range is difficult to assess. Beta particles, emit-

ted during nuclear decay, yield a continuous spectrum of

particle energies.

Alpha particles are the nuclei of helium atoms and have

a positive charge of two. They are the product of the decay of

natural isotopes, such as uranium, radium, and others. Alpha

particles have high energies and, therefore, high ionizing

power. Their penetrability, however, is low, about 5–7 cm in

air. Alpha particles emitted from radioisotopes do not yield

continuous spectra. The particles are monoenergetic or have

a group of several discrete energies.

b. Instrumentation

Ionizing radiation interacting or colliding with matter

causes the production of charged particles or energetic spe-

cies; measurement of these entities is the basis of radiation

detectors. Two phenomena are utilized in detecting ionizing

radiation: namely, ionization of gases and solids to yield an

ion current pulse and the excitation of crystals to provide a

luminescence pulse. The ionization of gases is used in ion-

ization chambers, proportional counters, and GM (Geiger

Muller) tubes. Semiconductor radiation devices produce

electron–hole pairs on the impingement of ionizing radia-

tion. Crystal and liquid scintillation detectors are employed

for insoluble and soluble samples, respectively. An instru-

ment for radiation measurement consists of several modules:

namely, the detector responding to the radiation, circuits that

count pulses, and pulse height analysis circuits that discrimi-

nate between the energies of the pulses.

The nature of the pulse must be understood. In a number

of detectors the pulse height of the radiation event is pro-

portional to the energy of the particle or radiation and inde-

pendent of the applied potential, whereas in other detectors

the pulse height is independent of energy. The pulse of pho-

toelectrons or luminescence has a pulse width due to the

random nature of the collision process. This fact leads to

small statistical differences in the pulse heights for a series

of radiation events with the same energy. The smaller the

pulse width the greater the resolution of the detector.

Radiation detectors operate under two modes, pulse and

mean level. Of interest here is the pulse type that detects the

interaction of radiation with the detector as a unique event.

These pulses are counted and measured in counts per unit

time, usually, counts per minute, cpm. The type of radiation

(beta, gamma, etc.), the frequency or wavelength and the

intensity of the radiation can also be determined.

ANODE

DELIVERY

TIP

CATHODE

DELIVERY

TIP

GLASS WOOL

TO GENERATOR

ELECTROLYTE SUPPLY

RESERVOIR

PL ELECTRODES

(SPIRALLY WOUND)

FIGURE 37 Generator cell for external production of

titrants for coulometric titrantions. (Adapted with per-

mission for reference 104. Copyright 1931 American

Chemical Society.) For example in acid/base titrimetry: (a)

generation of OH at the cathode: 2H 2 O  2e → H 2 

2OH (b) generation of H at the anode: H 2 O → 1/2O 2 

2H  2e.

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