Direct Conductometry and Conductometric TitrationConductance 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 TitrationConductometry 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+ + H2OOwing 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– + H2OAfter 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– + H2OThe 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 ReactionTitration 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 NO3-Before 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.