Encyclopedia of Environmental Science and Engineering, Volume I and II

576 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

titrations. A reference and the appropriate indicating elec-

trode allows the measurement of the potential for each

volume of titrant added. The potential changes in response

to the changes in the ratio of concentrations of oxidized to

reduced species. The Nernst equation can be used to calcu-

late the magnitude of these changes. A common classical

example of a potentiometric titration is an acid-base titration

using pH/reference electrode pair. The pH electrode indi-

cates the hydrogen ion concentration in terms of pH as a

function of volume. The titration apparatus and the resulting

“S” shaped curve is shown in Figure 29. Another example

concerns the complexometric titration of calcium ion with

the titrant, EDTA (the sodium salt of ethylene diamine tet-

raacetic acid). A calcium ion selective and reference elec-

trode are employed. The titration curve, is similar in shape

to the curve in Figure 29. The pCa value is measured by the

calcium electrode as a function of volume of EDTA titrant.

Automatic titrators allow the analyst to carry out a vari-

ety of potentiometric titrations without the consumption of a

great deal of time and effort. However the reaction must not

be too slow and the concentrations of the analyte between

0.1 and 0.001N. Two types of automatic titrators are avail-

able—recording and non-recording. Many commercial

titrators have the following features: A delivery system con-

sisting of a buret with a solenoid operated delivery valve

or a calibrated syringe whose plunger is motor-driven by a

micrometer screw. The potential from the indicator-reference

electrode pair is compared to a set end point potential by an

anticipator, null-sensing amplifier circuit yielding an ampli-

fied error signal. The error signal controls the buret delivery.

Anticipation of the endpoint prevents over titrating the end-

point (see Figure 29).

Recording titrators plot the titration curve and are com-

pletely automatic accepting a number of samples for titration

in a serial manner. Some titrators contain microprocessors

providing computational ability and processing of a number

of samples. The error signal is used to drive the recorder pen

and control buret delivery. Advantageously, unknown sys-

tems may be titrated yielding a curve that can be interpreted

by the analyst.

On/off

Titrate

Speed

Refill

Titrant

Motor-

driven

syringe

Direction selector

(0–14 or 14–0)

Burette

Elect-

rodes

On/off

Magnetic

stirrer

Calibra-

tion

Selec-

tor

On/off

Temperature

compensation

Error

signal

On/off

Starting pot

("end point" )

Amplifier

Switch

Indicator

lamp

Pen

motor

Chart

motor

Chart recorder

pH meter

Potentiometer

Up Down

E

, mV

V, mL V, mL

E

, mV Potentiometer drive on;

buret and chart drive off

Buret and chart drive on;

potentiometer drive off

(a)

(b) (c) (d)

FIGURE 29 (a) An automated, curve-recording titrator. (b) Theoretical titration curve. (c) Recorded curve with endpoint

anticipation. (d) Enlarge portion of (c) with explanation. (From G. Svehla, Automatic Potentiomeric Titrations, p. 176,

Pergamon Press. Copyright © 1978 by Prof. G. Svehla. Reprinted with permission.)

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