Introduction:

We are familiar that an indicator is employed to define the end point of a reaction in the titrimetric analysis. Though, if no visible indicator is available, the detection of the equivalence point can often be accomplished in other manners like: Potentiometric titration, Coulometry titration, amperometric titration and so on.

Principle of Potentiometric Titration:

In a Potentiometric titration, the potential of an indicator electrode is evaluated as a function of the volume of titrant added. The equivalence point of reaction will be revealed via a sudden change in potential in the plot of e.m.f. readings against the volume of titrating solution; any technique that will identify this abrupt change of potential might be employed. One electrode should maintain a constant, however not essentially known, potential; the other electrode that points out the changes in ion concentration, should respond fast. All through the titration, the analyte solution should be comprehensively stirred.

In a simple arrangement for the manual Potentiometric titration, there is a reference electrode (example: a saturated calomel half-cell) and an indicator electrode. The solution to be titrated is generally contained in a beaker fitted by a magnetic stirrer. Whenever titrating the solution which needs exclusion of air or atmospheric carbon-dioxide, a three or four necked flask is employed to enable nitrogen to be bubbled via the solution before and throughout the titration.

The e.m.f. of the cell having the initial solution is found out, and relatively large increments (1 to 5mL) of the titrant solution are added till the equivalence point is approached; the e.m.f. is determined after each and every addition. The approach of the equivalence point is pointed out by a rather more fast change of the e.m.f. In the vicinity of the equivalence point, equivalent increments (example: 0.1 or 0.05 mL) must be added. Adequate time must be allowed after each and every addition for the indicator electrode to reach a reasonably constant potential (+1 - 2 mV) before the subsequent increment is introduced. Some points must be obtained well beyond the equivalence point. To evaluate the e.m.f., the electrode system is generally joined to a pH meter which can function as a millivoltmeter in such a way that e.m.f. values are recorded. Used as a millivoltmeter, pH meters can be utilized by almost any electrode assembly to record the outcomes of many various kinds of Potentiometric titrations, and in most of the cases the instruments had provision for connection to a recorder in such a way that a continuous record of the titration outcomes could be obtained generally in the form of a titration curve.

Location of End Point in Potentiometric Titration:

In general speaking, the end point of a titration can be most simply fixed by observing the titration curve, comprising the derivative curves to which this gives rise, or by observing a Gran's plot. Whenever a titration curve has been acquired - that is, a plot of e.m.f. readings obtained by the normal reference electrode-indicator electrode pair against the volume of titrant added, either through manual plotting of the experimental readings, or by appropriate equipment, plotted automatically throughout the course of titration - it is in general be of similar form as the neutralization curve for an acid, that is, an S-shaped curve. The central part of the curve and clearly the end point will be positioned on the steeply rising part of the curve; it will in fact take place at the point of inflexion. Whenever the curve exhibits a very clearly marked steep part, however one can give an estimated value of the end point as being midway all along the steep part of the curve, it is generally preferred to use analytical (or derivative) processes of locating the end point. Analytical methods comprise in plotting the first derivative curve (?E/?V against V), or the second derivative curve (?²E/?V² against V). The first derivative curve provides a maximum at the point of inflexion of the titration curve, that is, at the point, while the second derivative curve (?²E/?V²) is zero at the point where the slope of the ?E/?V curve is the maximum.

The Gran's plot method is a comparatively simple process for fixing an end point. Whenever a sequence of additions of reagent are made up in a Potentiometric titration, and the cell e.m.f. E is read after each and every addition, then if antilog (EnF/2.303RT) is plotted against the volume of reagent added, a straight line is acquired which, whenever extrapolated, cuts the volume axis at a point corresponding to the equivalence point volume of the reagent; plotting is simplified if the special semi-antilog Gran's plot paper is employed. The particular benefit of this process is that the titration need not be pursued to the end point to permit a straight line to be drawn, and the greatest precision is accomplished by employing results over the last 20 percent of the equivalence point volume.

Potentiometric titrations, whenever performed manually, can take a considerable time. A number of commercial automatic titrators are accessible for Potentiometric titrations. The electrical measuring unit might be coupled to a chart recorder to generate a titration curve directly. The delivery of the titrant from an automatic burette is joined to the movement of the recorder, providing an auto-titrator. Instruments will as well plot the first derivative curve (?E/?V) and the second derivative (?²E/?V²), and will give a Gran's plot. The most significant characteristic is the facility to stop the delivery of the titrant if the equivalence potential has been accomplished.

Types of Potentiometric Titration:

As by classical titrimetry, Potentiometric titrations comprise chemical reactions which can be categorized as (1) Neutralization reactions, (2) Complexation reactions (3) precipitation reactions and (4) Oxidation-reduction reactions.

A) Redox reaction: Determination using Potentiometry

A redox titration is mainly based on an oxidation-reduction reaction between the analyte and titrant. As there is usually no difficulty in determining an appropriate indicator electrode, redox titrations are broadly employed; an inert metal like platinum is generally satisfactory for the electrode. ↔

The determining factor for redox titration via Potentiometry is the ratio of the concentrations of the oxidized and reduced forms of some ion species. For the reaction:

Oxidized form + n electrons ↔ reduced form

The potential E obtained by the indicator electrode at 25^oC is represented by:

E = E^? +0.0591/n log[ox]/[red]

Here 'E^?' is the standard potential of the system. The potential of immersed electrode is therefore controlled via the ratio of these concentrations. Throughout the oxidation of a reducing agent or the reduction of an oxidizing agent the ratio and thus the potential, changes more fast in the vicinity of the end point of the reaction. Therefore titrations comprising these reactions (example: iron (II) by potassium permanganate or potassium dichromate or cerium (IV) sulphate might be followed potentiometrically and generate titration curves characterized via a sudden change of the potential at equivalence point.

Experiment: Redox Titration of Manganese by Potentiometry

Purpose: To find out the end point of redox titration of manganese via Potentiometry.

Discussion:  The process is mainly based on titrating manganese (II) ions by permanganate in neutral pyrophosphate solution:

4Mn²⁺ + MnO₄^- +15H₂P₂O₇^2- → 5Mn(H₂P₂O₇)₃^3- + 4H₂O

The manganese (III) pyrophosphate complex consists of an intense reddish violet colour, in such a way that the titration should be performed potentiometrically; a combination redox electrode would be employed. With relatively pure manganese solutions, a sodium pyrophosphate concentration of 0.2 to 0.3 M, the potential at the equivalence point can simply be evaluated at pH 6 to 7. However at pH more than 8 the pyrophosphate complex dissociates, therefore the method can't be employed.

Equipment/Materials:

Potassium permanganate, 1 mL graduated pipette, Sodium pyrophosphate, pH Meter, Distilled water, 400 mL beaker, NaOH, Concentrated H₂SO₄ and Combination redox electrode

Experimental Procedure:

Put 150 mL of freshly made sodium pyrophosphate solution (around 12g in 100 to 150 mL water) in a 250-400 mL beaker, adjust the pH to 6 to 7 by adding concentrated sulphuric acid from a 1 mL graduated pipette (make use of a pH meter). Add 25 mL of the manganese (II) sulphate solution and adjust the pH again to 6 to 7 by adding 5 M sodium hydroxide solution. Put the combination redox electrode to the solution. This is now ready for auto titration by the standardized permanganate solution. The end point can be achieved either directly or by using the derivatives. The process can be adapted for manganese in steel or in manganese ores.

Experiment: Redox Titration of Steel by Potentiometry

Purpose: To find out the end point of redox titration of steel via Potentiometry.

Equipment/Materials: Weighing balance, Conc. HNO3, Urea, Steel, HCl and Kjeldahl flask

Experimental Procedure:

Precisely weigh 5g of steel and dissolve it in 1:1 nitric acid by using the minimum volume of hydrochloric acid in a kjedahl flask. Boil the solution down to a small volume with surplus concentrated nitric acid to re-oxidize any vanadium present reduced with the hydrochloric acid; this step is not essential if vanadium is absent. Dilute, boil to take out gaseous oxidation products, allow cool, adding 1 g of urea and diluting to 250 mL. Titrate the 50.0 mL portions as above.

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