Direct Conductometry and Conductometric Titration
Conductance measurement can be utilized for analytical purposes. It may be direct
conductometry or conductometric titration.
Direct conductometry
Conductance of a solution lack of specificity, hence has limited application however the
technique is useful for the following.

To determine the solubility and solubility product of sparingly soluble salt,
degree of ionization and ionization constant of weak acid/base.

To determine overall ionic content of the solution

Determination of purity of potable water

Checking purity of water

Regeneration of ion exchange column

Conductometric detector is used gas chromatography in determination of
herbicide and pesticide.
Conductometric Titration
Conductometry is one of the oldest and simplest electrochemical methods of analysis which
is based on measurement of electrical conductance of electrolyte solutions. When electric
field is applied to the solution, ions experience a force that attracts them towards the
oppositely charged electrode. Measurement of conductance afford us to determine the
concentration of electrolyte through conductometric titration
The end point of certain titration can be located by conductometric method and such
titrations are called conductometric titrations. During titration one of the ions is replaced
by other ion and invariably these two ions have different conducting power. If conductance
is plotted against the volume of the titrant added, the nature of the curve before the end
point will be different from the curve after the end point. On extrapolation of these two
curves, they intersect at a point, which corresponds to the end point of the titration. This is
the basic principle of conductometric titration.
In order to locate the end point accurately, the solution should not contain
extraneous electrolytes and the volume change during titration should not be very large.
Therefore, the concentration of titrant is in general 10 to 100 fold more concentrated than
the titrant. For accurate measurement of the volume of titrant high precision microburette
is employed.
Neutralization Reaction
1.Titration of Strong Acid with Strong Base
Consider the titration of HCl with NaOH. Let the conductivity cell be dipped in a solution of
HCl, and from the burette NaOH is added. At the very beginning there are only H + and Cl–
ions produced by ionization of HCl.
HCl

H+ + Cl–
When NaOH is added the H+ ions are removed due to the neutralization reaction
H+ + Cl– + (Na+ + OH–)

H2O + Na+ + Cl–
The above equation shows that the H+ ions have been replaced by Na+ ions. Since the
mobility of Na+ ion is much less than that of H+ ion, hence there will be a net decrease in
conductance as the titration progresses.
After the end point, with the addition of NaOH, the conductance increases sharply due to
the addition of Na+ and OH– ions to the solution.
If we plot the conductance against the volume of NaOH added we get a curve as shown in
the following diagram.

The descending portion of the curve AB represents the conductance before the
equivalence point and the ascending portion CD represents the conductance after the
equivalence point. On extrapolation of these two curves, AB and DC, they intersect at the
point E, which is the end point of the titration.
Titration of Weak Acid with Strong Base
If weak acid, such as acetic acid is titrated with sodium hydroxide.
At the beginning there will be a few hydrogen and acetate ions produced by the partial
ionization of acetic acid. When a few drops of NaOH are added, a slight decrease in
conductance will take place due to the neutralization of free hydrogen ions.
But as the titration progresses, the unionized acetic acid reacts with sodium hydroxide
producing completely dissociated acetate and sodium ions.
CH3COOH + (Na+ + OH–)

CH3COO– + Na+ + H2O
Owing to this, there will be slow net increase in conductivity and this continues till the end
point.
After the end point, the addition of excess of NaOH, leads to the sharp rise in conductance
due to the addition of sodium and highly mobile hydroxyl ions. The variation of conductivity
with addition of sodium hydroxide is shown in following figure.

The slight decrease in conductivity, at the beginning of titration is due to the neutralization
of free hydrogen ions which is produced by partial ionization of weak acid. The slight
curvature near the end point is due to the hydrolysis of the salt, the sodium acetate. Hence
actual end point is located by extrapolation of the curves BC and FD to the point E, the end
point.
Titration of Weak Acid with Weak Base
If weak acid, such as acetic acid is titrated with weak base like ammonium hydroxide, the
following type of conductometric curve will be obtained.

The acid being weak the conductance of the solution at the beginning is very low. With the
progress of titration, conductance increases up to the end point due to the formation of
completely dissociated salt, the ammonium acetate. After the end point there is an addition
of weak base which remain unionized, hence conductance remains constant.
Titration of Strong Acid with Weak Base
If strong acid such as HCl is titrated with weak base as ammonium hydroxide, the following
type of the curve will be obtained.

The conductance of the solution will be high because of hydrogen and chloride ions
produced by ionization of HCl. As the titration progresses, highly mobile hydrogen ion will
be replaced by the slow moving ammonium ion, and therefore the conductance decreases
till the end point.
H+ + Cl– + (NH4OH)  NH4 + Cl– + H2O
After the end point, there is an addition of NH4OH which remain unionized, therefore the
conductance remains almost constant.
Titration of a Mixture of Strong Acid and Weak Acid
If a mixture of weak and strong acids is titrated with strong base, the conductometric
titration curve that will be obtained is shown in fig. 11.11.

In the mixture of acids, there will be large number of free hydrogen ions produced by
ionization of strong acid; hence conductance of the solution will be high. When the mixture
is titrated with strong base, the conductance will decrease due to replacement of highly
mobile hydrogen ion with slow moving metal ion and this decrease in conductance will
continue till whole of the strong acid is neutralized. After that, the added sodium hydroxide
starts reacting with un-dissociated acid, thereby there will be slow increase in the
conductance due to the formation of dissociated salt and, this slow increase in conductance
continues till whole of the weak acid is neutralized. After that further addition of alkali
produce a sharp rise in the conductance due to the addition of highly mobile hydroxyl ions.
Thus the titration curve contains two breaks, the first break corresponds to the
neutralization of strong acid and second break corresponds to that of weak acid.
Titration of Dibasic Acid with Widely Different Dissociation Constants
If a dibasic acid such as oxalic acid is titrated with strong base, the conductometric
titration curve will be similar to the curve obtained for the mixture of strong and weak
acids. The oxalic acid is neutralized in two stage.
The first stage
H+ + HC2O4– + [Na+ + OH–]  Na+ + HC2O4– + H2O
The second stage
HC2O4– + [Na+ + OH–]  Na+ + C2O4– – + H2O

The first stage is similar to the neutralization of strong acid while the second stage is
similar to the neutralization of a weak acid. Here, however, the volume of NaOH consumed
in the second end point will be approximately twice the volume consumed in the first end
point.
Precipitation Reaction
Titration of Silver Nitrate with Sodium Chloride
Consider the titration of silver nitrate solution with sodium chloride. At the beginning there
are only Ag+ and NO3– ions in the solution.
AgNO3  Ag+ + NO3–
and conductance is due to silver and nitrate ions. As NaCl solution is added, the following
reaction occurs
Ag+ + NO3 + [Na+ + Cl–]  AgCl + Na+ + NO3–
Thus there is a replacement of Ag + ions by Na+ ions. The mobilities of Ag+ and Na+ ions are
not much different, so conductance of the solution remains almost same till the end point.
After the end point, due to the addition of sodium and chloride ions, there will be sharp
increase in the conductance as shown in the following figure.

Complex formation
An interesting complex formation reaction is the determination of mercury by cyanide
ion. Mercury(II) nitrate a solution as nitrate exist almost as free ion. It may be titrated
with potassium cyanide solution.
( Hg++ + 2 NO3- ) + 2 ( K+ + CN-) = Hg(CN)2 + 2 K+ + 2 NO3Before the equivalence point, one Hg(II) is replaced by 2 K+ ion conductance very only
slightly. After end point there is an addition of K+ and CN- ions and conductivity
increases sharply and end point is determined readily determined from the intersection
of these two lines.
Redox Titration
It is to be noted that conductometric titration is seldom used for the titration involving
redox reaction. In redox titration, besides active reactant there is a presence of large
excess of electrolytes e.g. acids and the change in conductance during titration is often
low, and the location of end point becomes difficult.
Advantages of Conductometric Titrations
Conductometric titration has several advantages over classical titration with the use of
chemical indicator, which may be listed as follow:
Conductometric titration can be performed even for a colored solution since the color of the
solution does not interfere in the measurement of conductance. With chemical indicator
method highly colored solution can not be titrated.
A very weak acid where other method of titration fails can be successfully titrated by
conductometric method. Moreover, very dilute solution can be titrated by conductometric
method.
A mixture of weak and strong acids can be analyzed by a single titration.
No need to take special precaution as the end point approaches since the end point is
located by utilizing the titration readings of several observations graphically. Owing to this
more accurate and reliable result can be obtained by conductometric method.