Potentiometry

Analytical Laboratory II

• Potentiometry is based on the measurement of the potential of an electrode system.

• Potentiometric measurement system consists of two electrodes called reference and indicator electrode, potentiometer and a solution of analyte.

• Potentiometry is an electrochemical analysis method that can be applied where a suitable color indicator is not possible (for example, in dark or very dilute solutions).

• This method can also be used for the analysis of two or more different components.

• The electrode potentials can not be measured absolutely but can be found by comparison with the potential of the reference electrode. Electrochemical Cells;

– Galvanic Cells: Cells that undergo electrical reactions as a result of chemical reactions

– Electrolytic Cells (Electrolysis Cells): These are the cells in which chemical reactions take place as a result of external electric current application.

• Each cell comes from two half-cells that are reduction and oxidation.

• Each half cell is called an electrode.

• Reduction electrode is named as cathode, oxidation electrode is named as anode. The electrodes are connected by a salt bridge from the outside. Electrodes have electrolyte solutions immersed in them.

• Reference electrode is an electrode with potential which is a)independent of concentration of analyte ions andtemperature.

• Potential of an indicator electrode depends mainly on theconcentration of the analyte ions.

• Due to the ohmic resistance (iR) in two-electrode systems,current values do not give a healthy result, so a thirdelectrode is required. This is why the counter electrode isused.

• pH ranged from 0-14 in pHmeter. However, since the glass electrode is used as a measuring electrode during pH measurement, it operates at a certain pH range.

• pH < 1; Asidic Error

• Glass electrodes are thought to be filled with H + centers on the glass surface and can no longer respond to H + changes.

• pH > 10; Alkaline Error

• Electrode H + also starts to respond to the change of alkaline metal ions in the glass structure.

• Glass electrode works between pH 1 and 10 effectively.

Scheme of glass electrode

The potential of the indicator electrode is sensitive to hydrogen ions. the potential is measured in reference to a calomel electrode, e.g. calomel electrode functions as the reference electrode.

Things to watch out for using glass electrode

• Very precise and careful handling is required.

• It should be stored in a saturated KCl solution.

• The electrode should not be immersed in dehydrating solvents such as ethanol, sulfuric acid, and in the glass-soluble hydrophobic acid solution or concentrated alkaline solutions.

• Electrodes should never be washed with organic solvent.

Potentiometric Titration

• Potentiometry is also an electroanalytical method that is also used in titration procedures. In this method, titration is performed without using an indicator. There is no suitable indicator for the titration of some substances.So, titration is performed without using an indicator in this method.

• In the potentiometric titration; the measured potential or pH is plotted against the volume of the titrator.

• The instantaneous increase in pH indicates the equivalence point.

• The first and second derivatives of the titration curve are calculated so that the point of equivalence can be determined more clearly.

Experimental Procedure

• Before starting the experiment, the pH meter should be calibrated with abasic and acidic buffer solution. Acid(10 mL HCl) is transferred to beaker,added 30 mL distilled water to increase volume. The solution is stirredwith magnetic stirrer. First, pH values are recorded with pHmeter. Buret isfilled with standart NaOH solution and titrate with NaOH solutionprepared. Add small increments of titrant and record pH forcorresponding volume of titrant. The instantaneous increase in pHindicates the equivalence point. After reading all the values, a titrationcurve is obtained and found the normality of HCl. The first and secondderivative curves are plotted in order to determine precisely the turningpoint of the curve.

Titration curve

• As a result of the experiment performed, the pH value read is plotted against the volume of each mL base which is first titrated. However, the turning point can not be read accurately in the graph. For this reason, a graphical method is used to derive the derivation.

First-derivative curve

ΔpH/ Δv= pH2-pH1/V2-V1

• The equivalence point may not be clearly defined in this graph. A flat curve can be obtained instead of such a steep curve and the turning point can not be detected. For this reason, it is necessary to take a second derivative:

Second derivative curve

the data changes sign from (+) to (-) at the equivalence point.

REFERENCES

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