Polarisation titration

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

The order of presentation of the electroanalytical methods will be direct potentiometry with ion-selective electrodes, potentiometric titrations, voltammetry/polarography, polarisation titrations (amperometric and potentiometric), conductometry/coulometry and electrochemical detectors. 

The prime advantage of polarisation titrations is the ability to handle reactions of poor equihbrium constant. This stems from the location of the endpoint by extrapolation to intersection from either side of the endpoint zone. This ability renders the method capable of carrying out titrations not easily possible by the potentiometric method. This characteristic is shared with other titration techniques (e.g., conductometric, photometric). 

Procedure. Place 25.0 mL of the thiosulphate solution in the titration cell. Set the applied voltage to zero with respect to the S.C.E. after connecting the rotating platinum micro-electrode to the polarising unit. Adjust the range of the micro-ammeter. Titrate with the standard 0.005 M iodine solution in the usual manner. 

Dilute solutions of antimony(III) and arsenic(III) (ca 0.0005M) may be titrated with standard 0.002 M potassium bromate in a supporting electrolyte of 1M hydrochloric acid containing 0.05 M potassium bromide. The two electrodes are a rotating platinum micro-electrode and an S.C.E. the former is polarised to +0.2 volt. A reversed L-type of titration graph is obtained. 

EXAMPLES OF AMPEROMETRIC TITRATIONS USING A SINGLE POLARISED ELECTRODE... 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

Amperometric titration is a quick, accurate and convenient method similar to potentiometric and conductometric titration and at the equivalence point there is sharp change in diffusion current. The galvanometer used need not be calibrated. The specific characteristic of capillary does not influence the titration. No polarising unit is used, suitable half cell can be easily used for the purpose. 

Stulik, K., and F. Vydra Electrometric Titrations with Two Polarised Electrodes. Determination of Inorganic Compounds and of Water by Means of Karl Fischer Reagent. Chem. Zvesti 21, 274 (1967). 

Amperometric titrations may also be carried out with twin polarised electrodes. Commonly used are a pair of platinum wire microelectrodes. It is important to remember that, in such a situation, whatever charge is apphed to one electrode will induce the opposite charge in the other. This may at times result in undesirable side reactions. Minimum effective potentials should always be used. In some cases, both the reactant (analyte) in the solution under analysis and the reactant in the titrant may behave reversibly at the electrodes. In some others, only the reactant (analyte) in the solution may act reversibly in others again, only the titrant reactant may act. 

Figure 28 shows amperometric titrations involving two polarised electrodes. Figure 28A represents the oxidation/reduction titration in an acid solution containing Fe2+, and using a standard solution of Ce +. The titration uses twin Pt wire electrodes. This is a system where the reactants both have reversible... 

Figure 27 Amperometric titrations with one polarised electrode.

Figure 28 Amperometric titrations with two polarised electrodes (biamperometric titrations).

The application of a fixed potential of 100 mV between the twin platinum microelectrodes in the solution before the start of the titration will result in a polarised electrode situation and only a minimal current will be observed, since... 

Method Transfer the specified volume of the solution to be titrated to a 200-ml beaker fitted with a stirrer. Introduce the electrodes and apply a predetermined polarising voltage, stirring the solution continuously. The galvanometer will show an immediate small deflection but will then slowly return to zero. Add the specified standard solution slowly from a burette, allowing time for the galvanometer to return to zero after the addition of each increment. 

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