Amperometric Titration, Slides of Pharmaceutical Analysis

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AMPEROMETRIC TITRATION

An amperometric titration or amperometry is concerned with the measurement of current under a constant applied voltage ; and under such experimental parameters the concentration of the ‘analyte’ exclusively determines the quantum and magnitude of the current.

In this particular case, the total current flowing shall remain almost equal to the current carried by the ions that undergoes equal electrolytic migration together with the current caused on account of the diffusion of the ions. Moreover, there is a little contribution of residual current to the total current which is caused due to condenser current and Faradaic current. Thus, we have:

I = I

d

+ I

m

+ I

r where, I = Total current Id = Diffusion current (Current formed due to the mass transfer of electro-reducible ions to the electrode surface by the diffusion force which is proportional to the conc. gradient at the electrode surface-solution interface) Im = Migration current (Current due to the potential gradient at the electrode surface-solution interface, depends on the transport number of cations) Ir = Residual current [Ir = ic+if; ic (Condenser Current) is due to formation of Helmholtz Double layer at the Hg-solution interface and if (Faradaic current) is due to the presence of trace amount impurities in test solution].

If supporting electrolytes (e.g. 0. 1 M KCl) are used then unwanted migration current becomes almost vanished and absence of impurities renders residual current almost zero. Therefore, total current observed is essentially due to only the diffusion current i.e. , I = Id.

PRINCIPLE

▪ In amperometric titration, the analyte is dissolved in an appropriate

volume of a suitable solvent.

▪ The prepared solution is transferred to the titration cell. Then a definite

potential is applied across the indicator electrode but reference

electrode voltage is kept constant.

▪ As a result the analyte diffuse to the indicator electrode surface from

the bulk of the solution and the diffusion current is obtained.

▪ Since the process of diffusion is governed by conc. gradient, the

diffusion current is proportional to the concentration of electroactive

substances present in the solution which is known from Ilkovic

equation –

I

d

= 607nCD

1/

m

2/

t

1/

▪ If some electroactive material removed by

interaction with a reagent titrant, the

diffusion current will decrease.

▪ After each addition of titrant, the change in

current is recorded & plotted against the

volume of titrant.

▪ Plot of data before & after the equivalent

point will give two straight lines of different

slopes.

▪ The end point of titration is established by

extrapolation of two lines up to intersection

Factors affecting Ilkovic equation- concentration, capillary characteristics-drop time and rate of mercury flow, time, etc.

Titration curves vary due to different situations, such as-

1. The titrand (analyte) is reducible but the titrant non-reducible

2. The titrand (analyte) is non-reducible but the titrant reducible

3. The titrand (analyte) and the titrant both are reducible

TITRATION CURVES

TITRATION CURVES Example: The titration of Pb^2 +^ with SO 42 –^ or C 2 O 42 –^ ions. An appreciably high potential is usually applied to yield a diffusion current for lead. From the Figure , one may evidently observe a linear decrease in current because Pb^2 +^ ions are removed from the solution by precipitation. The small curvature just prior to the end-point (or equivalence point) shows the incompleteness of the analytical reaction in this particular region. However, the end-point may be achieved by extrapolation of the linear portions, as shown in the said figure.

1. The titrand (analyte) is reducible but the titrant non-reducible: It represents a titration wherein the analyte reacts at the electrode whereas the reagent does not. In other words, only the substance under titration gives rise to a diffusion current; whereby the electro-active substance is removed from the solution by means of precipitation with an inactive substance.

3. The titrand (analyte) and the titrant both are reducible: It represents an amperometric method wherein the solute as well as the titrating reagent afford diffusion currents and give rise to a sharp V-shaped curve. The end-point may be obtained by extrapolation of the lower-end of the V-shaped portion of the curve as depicted in the figure. Examples: (a) Titration of Pb^2 +^ ion with Cr 2 O 72 – ion. The Figure corresponds to the amperometric titrations of Pb 2 + and Cr 2 O 7 2 – ion at an applied potential more than – 1. 0 V; when both these ions afford diffusion currents at this very potential and the end-point is duly signaled corresponding to a minimum in the curve. (b) Titration of Ni 2 + ion with dimethylglyoxime ion.

INSTRUMENTATION

Three methods, namely:

1. Amperometric titrations with the dropping mercury electrode ,

2. Amperometric titrations with a rotating platinum microelectrode ,

and

3. Amperometric titrations with twin-polarized microelectrodes (or

Biamperometric Titrations or Dead-stop-end-point method).

 The titration-cell Figure (a) essentially comprises of a pyrex 100 - ml, four- necked, flat-bottomed flask. A semimicro burette (B) (graduated in 0. 01 ml), a 2 - way gas-inlet tube (A) to enable N 2 to pass either through the solution or simply over its surface, a dropping mercury electrode (C) and an agar- potassium salt-bridge are duly fitted into the four necks with the help of air-tight rubber stoppers.  The electrical circuit, Figure (b), consists of two 1. 5 V dry cells that provides a voltage applied to the above titration cell. It is duly controlled and monitored by the potential divider (R) and is conveniently measured with the help of a digital voltmeter (V). Finally, the current flowing through the circuit may be read out on the micro-ammeter (M) installed.

The following steps may be carried out in a sequential manner for an amperometric titration :

1. A known volume of the solution under investigation is introduced in the titration cell,
2. The apparatus is assembled and electrical connections are duly completed with dropping mercury electrode (C) as cathode and saturated calomel half-cell as anode,
3. A slow stream of pure analytical grade N 2 gas is bubbled through the solution for 15 minutes to get rid of dissolved O 2 completely,
4. Applied voltage is adjusted to the desired value, and the initial diffusion current (Id) is noted carefully,
5. A known volume of the reagent is introduced from the semimicro burette (B), while N 2 is again bubbled through the solution for about 2 minutes to ensure thorough mixing as well as complete elimination of traces of O 2 from the added liquid,
6. The flow of N 2 gas through the solution is stopped, but is continued to be passed over the surface of the solution gently so as to maintain an O 2 free inert atmosphere in the reaction vessel,
7. The current (μA) and microburette readings are recorded simultaneously, and
8. Finally, the said procedure is repeated until sufficient readings have been obtained to allow the equivalence point to be determined as the intersection of the two linear portions of the graph thus achieved.

 Figure (a) depicts a simple rotating platinum microelectrode which is made out from a usual standard ‘mercury seal’. A platinum wire (length : 5. 0 mm ; diameter : 0. 5 mm) protrudes from the lower end wall of a 21 cm long 6 mm glass tubing, which is bent at an angle approaching a right angle a short distance from the lower end. There are holes (H) in the stem of the mercury reservoir for making electrical contact with it. The mercury reservoir is provided with a flange fitted inward to prevent Hg from being thrown out.  Figure (b) illustrates the electric circuit. The electrical connection is duly done to the electrode by means of a strong amalgamated Cu-wire passing through the glass tubing to the lower end of the Hg covering the sealed-in platinum wire; the upper end of which passes through a small hole made in the stem of the stirrer and dips well into the Hg present in the Hg seal. Subsequently, a wire from the Hg seal is connected to the source of applied voltage. The glass tubing serves as the stem of the electrode that is rotated at a constant speed of 600 rpm.

Advantage of using RPME over DME ▪ Mercury cannot be used as electrode at positive potentials because of its oxidation, RPME is used. ▪ Diffusion current is 20 times larger than DME which allows to measure the small concentration of ion. ▪ The rotating platinum electrode can be used at positive potential up to + 0. 9 volt where as DME can be used only + 0. 4 volt to - 2. 0 volt. ▪ The electrode is simple to construct.