Potentiometry

I. Introduction Potentiometry is a method used in electroanalytical, usually to find the concentration of a solute in solution. In potentiometric measurements, the potential between two electrodes is measured using a high impedance voltmeter. In the most common forms of potentiometry, two different types of electrodes are used. The potential of the indicator electrode varies, depending on the concentration of the analyte, while the potential of the reference electrode is constant. Potentiometry is probably the most frequent used electroanalytical method. It can be divided into two categories on the basis of the nature of the indicator electrode. If the electrode is a metal or other conductive material that is chemically and physically inert when placed in the analyte, it reflects the potential of the bulk solution into which it is dipped. Electrode materials that are commonly used for this type of potentiometry include platinum, gold, silver, graphite, and glassy carbon. Another method of potentiometry is potentiometric titration. Potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid, no indicator is used; instead the potential is measured across the analyte, typically an electrolyte solution. In this experiment, potentiometric titration was used together with Silver Nitrate(AgNO 3) as a titrant.

II. Methodology A.) Preparation of Reagents

A.)Dissolve 2.395g of AgNO3 in distilled water and dilute to 1L.

B.) Dissolve 4g NaOH in 100mL distilled water.

B.) Titration Process

Dissolve 0.1-0.2g of [Co(NH33)55]Cl33 in 10mL distilled water.

Titrate with 0.0141M AgNO33 solution. Read the potential after each addition of 1mL titrant.

Locate the endpoint and determine the number of chlorides that are not in the coordination sphere.

Dissolve 0.1-0.2g of [Co(NH33)55]Cl33 in 100mL distilled water.

Dry the [Co(NH33)55]Cl33 by adding 7-8mL NaOH.

The number of chlorides that are found in the primary coordination sphere is the difference between the number of chlorides in step 6 and 3. III. Discussions

Repeat steps 1 -3.

Repeat steps 1 to 7 for [Co(NH33)55Cl]Cl22.

Potentiometric titrations are preferred to manual titrations, since they are more accurate and precise. There are several types of potentiometric titrations for the determination of analytes in photoprocessing solutions include acid-base, redox, precipitation and complexometric. Oxidation-Reduction titration involves the transfer of electrons from the substance being oxidized to the substance being reduced. This kind of titration involves insoluble salts of metals such as mercury, silver, lead and copper and followed potentiometrically. This was the method used and AgNO 3 was used as titrant. [Co(NH3)5]Cl3 and [Co(NH3)5Cl]Cl3 are titrated with 0.0141M Silver Nitrate, AgNO3, in which electrode potential is determined versus the volume of the titrant added. In potentiometric titration, there are many ways in locating the end point. One method, is to create a titration curve. Titration curve is obtained by plotting the successive values of the cell emf on ordinate and corresponding values of volume of titrant added on the abscissa. This gives an S-shaped curve. The central portion of this curve which shows the steeply rising corresponds to the volume for the end point of the titration. Another way to locate the end point is the derivative method. method, the end point can be located precisely from the first derivative and second derivative curves. The first derivative curve involves the plot of slope of the titration curve, ∆ E/ ∆ V, against the volume of the titrant added. Sometimes,

In

this

∆ E/ ∆ V is

plotted against the average volume of the titrant added corresponding to the values of emf taken. The volume on the x-axis corresponding to the peak of the curve is the end point of the titration. In the second derivative, on the other hand, the slope of the first derivative curve ( ∆ 2E/ ∆ V), is determined by plotting it against

slope of the

volume. The point on volume axis where the curve cuts through zero on the ordinate gives the end point. This point corresponds to the largest steepest point on the titration curve and the maximum ∆ E/ ∆ V curve.

The first two method on locating the end point was believed to be used only in symmetric reaction. Another method wherein unsymmetrical reaction can be used in locating the end point is with the use of Gran Plot method. Gran Plot Method or Gran titration is developed by G. Gran in 1952. This method does the numerical manipulation of titration curves into linear straight lines intersecting at the equivalence point.

V O+ V VO ¿

) 10-FEcell/2.30RT = 10-FEcell/2.30RT

γC (V e −V ) VO

Where: VO = initial volume of the solution V = volume of the titrant added Ve = volume of the titrant added C = concentration of the titrant F = Faraday’s Constant R = Gas constant T = Temperature γ = Activity coefficient V O+ V ) 10-FEcell/2.30RT is called as Gran’s function. A VO ¿ straight line will be obtained if it will be plotted against the volume of the titrant. The end point from Gram plot can be obtained by either taking the points before the end point or taking the points after the end point. The results obtained from linear curves are more accurate than from non-linear ones. Simplicity of measurement, simplicity of calculation, versatility and precision are the advantages of using Gran Plot Method.

IV. References ChemBuddy(2009). Mohr Method. Retrieved October 18, 2014 http://www.titrations.info/precipitation-titration-argentometry-chlorides-Mohr

from

Chemicool(2014). Definition of Potentiometry. Retrieved October 18, 2014 from http://www.chemicool.com/definition/potentiometry.html Determination of Chloride using Potentiometry. Retrieved October 18, 2014 from www.asdlib.org/onlineArticles/...Potentiometry/SPECION%203%201.pdf NepaChem. Potentiometry in Analytical Chemistry. Retrieved October 19, 2014 from http://www.nepachemistry.com/2011/03/potentiometry-in-analytical-chemistry.html UCDAVIS Chemwiki. Potentiometric Methods. Rerieved October 19, 2014 from http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0/11_Elec trochemical_Methods/11B_Potentiometric_Methods Wikipedia. Argentometry. Retrieved http://en.wikipedia.org/wiki/Argentometry

October

18,

2014

from

Wikipedia. Potentiometric Titration. Retrieved http://en.wikipedia.org/wiki/Potentiometric_titration

October

Wikipedia. Standard Electrode Potential. Retrieved October http://en.wikipedia.org/wiki/Standard_electrode_potential

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2014 2014

from from