Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 577

The accuracy of automatic titrators depends on the con-

centration of the analyte and the sensitivity of their electrode

sensing systems. Accuracies are 0.1% for a 10^ ^2 N solution

and 1% for the limiting concentration of 10^ ^3 N.

Titrimetric methods are given in Standard Methods^2 for

the following parameters—carbon dioxide, cyanide, COD,

sulfite, and ammonia.

(2) Amperometric titrations^50

Amperometric titrations may utilize one or two polar-

izable micro electrodes configured differently, electrically.

These several electrode systems will lead to differently

shaped titration curves. Titration curves show the current

flow as a function of the volume of titrant.

A single polarized electrode may be a DME, a solid-

state electrode of carbon, platinum or other noble metal or

a rotating noble metal electrode. The potential imposed on

the indicating electrode is such that the limiting current, i L ,

is obtained. As in voltammetry, the current obtained is pro-

portional to the concentration of the electroactive species.

Therefore as the titration proceeds, the diffusion current, i d ,

changes. The shape of the titration curve depends on the elec-

troactivity of the titrant and the analyte as seen in Figure 30.

In general two straight lines are obtained and their interpo-

lated intersection indicates the endpoint.

Two polarized microelectrodes result in titration curves

whose shapes depend on the reversibility of the electrode

reactions of the titrant and the analyte. When there is a

reversible reaction for the analyte, (I 3 /I), and an irreversible

one for the titrant, (S 2 O 0 /S 4 O 6 ), the result is a “deadstop”

endpoint where the current ceases to flow. When analyte and

titrant show reversible electrode behavior, e.g., Fe^2 ^ /Fe^3 ^

and Ce^3 ^ /Ce^4 ^ , respectively, the titration curve shows no

current flow at the endpoint. There is current flow on either

side of the endpoint.

(3) Conductometric titrations^95

The instrumental apparatus and conductance cells for

conductometric titrations are given in Figures 31 and 32.

FIGURE 30 Amperometric titration curve.

Volume of reagent Volume of reagent Volume of reagent

Diffusion current Diffusion current Diffusion current

Only analyte reduced Only analyte reduced Analyte and reagent reduced

(a) (b) (c)

In Figure 33 a precipitation and several acid–base titration

curves are illustrated. Since ions are detected by the elec-

trodes, any change in ionic concentration in the analyte solu-

tion during the titration is the basis for a conductometric

titration. In a neutralization reaction two ions, hydronium,

H 3 O^ ^ and hydroxide, OH^ ^ , are removed to form union-

ized water, H 2 O. Two ions, silver, Ag^ ^ and chloride, Cl^ ^ ,

form a precipitate of silver chloride, thereby decreasing the

conductivity of the solution in the course of the titration.

Analogously, the formation of an unionized complex can be

the basis for a conductometric titration.

(d) Conductometric methods^96

Conductometric methods detect ionic species in solu-

tion. The conductivitivity of a solution indicates the pres-

ence of ionic species and can, under certain conditions, be

used to estimate the concentration of the dissolved electro-

lyte. Conductance is also used in conductometric titrations

(see the previous section III,B, 2, c, (3) ). Solutions contain-

ing electrolytes conduct electricity and obey Ohm’s law.

Electrical resistance or conductance is measured by placing

the solution between two electrodes and using a Wheatstone

bridge to carry out the measurement.

Conductivity, C n , is reciprocally related to resistance, Rs,

so that C n  1/Rs. Units for Rs and C n are ohms and mhos

(ohms^ ^1 ), respectively. (The IS unit for C n is siemans, S.)

Resistance and conductivity of a solution are sensitive to the

dimensions of the volume of solution included between the

plate electrodes (see Figure 32). Conductivity is proportional

to the area of the electrodes and reciprocally related to the

distance between them.

By normalizing conductivity to a given dimension, a

cube that is one centimeter on its edge, and designating a new

parameter, specific conductance, C sp in mhos/cm, the follow-

ing calculation can be made,

C sp  (1/R m )(d/ar)  (1/R m )(k c ) mhos/cm (40)

where R m is the measured solution resistance in the conductiv-

ity cell, d and ar are the distance between and the area of the

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