POTENTIOMETRY - AMPEROMETRY

8/10/2019 POTENTIOMETRY - AMPEROMETRY

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KELUARKAN SELEMBAR KE


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Tuliskan apa yang Saudara rasa kurang dari perkuliah Tuliskan masukan Saudara untuk perkuliahan

kedepannya


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MINGGU LALU

Anoda (up ways) Katoda (down ways)The cathode in an electrochemical cell isthe electrode at which reduction occurs

The anode is the electrode atwhich an oxidation takes


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PRINSIP DASAR : SHE

Gas H2 (1 ba)r

2H+ + 2e - H2E 0 = 0.0 V

mV

Test electrode

catalytic Pt surface in acidic solution (1M)


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PRINSIP DASAR : SHE


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Types of Electrochemical CellsElectrochemical cells are either galvanic or electrolytic

Convert electrical energy into chemical energy

Electrolytic

GalvanicConvert chemical energy into electrical energy


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GalvanicElectrolyt


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[Cu 2+ ] increases with time [Ag +] dec

E right E left


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The Nernst Equation

Complete Nernst Equation for cell

If the net cell voltage + spontaneousin the forward direction

If the net cell voltage + spontaneousin the opposite direction

Electrons flow toward more positive potential

Cd2+

Cd(s)

0.01 M

e - e - e -

mV


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The Nernst Equation

[Oxidized]]Reduced[log303.2

[Oxidized]]Reduced[ln 00

nF RT E

nF RT E E

Ket E = Potential at temperature T E 0 = Standard electrode potential (25C, 1.0M)R = Ideal gas constantF = Faradays constantn = number of electrons transferred

Complete Nernst Equation for cell

If the net cell voltage + spontanein the forward direction

If the net cell voltage + spontanein the opposite direction


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The magnitude of the potential difference between the two electrodes is 0.412 V independent of which electris considered the left or reference electrode.

However, no matter how the cell is arranged, the spontaneous cell reaction is oxidation of Cu and reductionof Ag 1, and the free energy change is 79,503 J


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POTENSIOMETRI -AMPEROMETRI


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POTENTIOMETRY

analysis based on measuring the potential of electrochemical without drawing appreciable current (constant current or zerocurrent) .

A reference electrode is a half-cellhaving a known electrode potentialthat remains constant at constanttemperature and is independent ofthe composition of the analytesolution

An indicator electrode has a pthat varies in a known way wvariations in the concentration of aanalyte.

By convention, the reference electrode is always treated as the left-hand electrode in potentiometricmeasurements


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ELECTRODE

Calomel ReferenceElectrodes

Silver/Silver ChlorideReference Electrodes

Reference Electrodes Indicator Electrode

Metallic Indicator El Electrodes of the Electrodes of the

kind , Inert redox electr

Membrane IndicatorElectrodes


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The Calomel Electrode

Calomel paste (Hg 0/Hg 2Cl2)

Saturated KCl

Liquid junction

mVTest electrode

Hg2Cl2 + 2e - 2Hg0 + 2Cl-

E 0 = 0.268V

consist of mercury in contact with asaturated with mercury (I) chloridealso contains a known concentration ofchloride


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RE : SILVER SILVER CHLORIDE ELECTRO

this electrode is prepared with eithea 3.5 M potassium chloride solution

they can be used at temperatures gr60C, while calomel electrodes can

Silver wire

Saturated

KCl + AgNO3

Liquid junction

mVTest electrode

AgCl +e - Ag0 + CE 0 = 0.222V


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IE : METALLIC INDICATOR ELECTRODE Electrodes of the Second Kind

Metals not only serve as indicator electrodes for their own cations but also respondto the activities of anions that form sparingly soluble precipitates or stable complexeswith such cations

The potential of a silver electrode, for example, correlates reproducibly with the activity ofchloride ion in a solution saturated with silver chloride

in a solution saturated with silver chloride, a silver electrode can serve as an indicator electrode ofthe second kind for chloride ion


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IE : Membrane Indicator Electrodes

Membrane electrodes are sometimes called p-ion electrodes (functions, such as pH, pCa, or pNO3)

The Glass Electrode for Measuring pH

contains two reference electrodes: the external calomel electrode and the internalsilver/silver chloride electrode

the thin glass membrane bulb at the tip of the electrode that responds to pH the concentration (and the activity) of protons inside the membrane is constant.


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R e f 1

R e f 2

++

+

+

+

+

++

E cell = E Ref(1) + E lj E Ref(2)


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IE : Membrane Indicator ElectrodesThe Compos i t ion and St ruc tu re o f Glass Membranes

exhibit excellent specificity to hyup to a pH of about 9.

At higher pH values, the glass beresponsive to sodium as well as tocharged cations


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K+ ion-selective electrode

K+

N

O

O

O

N O

O

O

N

O

O

O

N

O

O

O

NO

O

O

N

O

O

O

H

H

H

H

H

H

Valinomycin is an antibiotic thathas a rigid 3-D structurecontaining pores with dimensionsvery close to the un-hydratedradius of the potassium ion.Valinomycin serves as a neutralcarrier for K+.


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Ca++ ion selective electrode

Ca ++

di- p-octylphenyl phosphate

H 3 C

O

P

O

H 3 C

O

O -

H 3 C

O

P

O

H 3 C

O

O -

PVC membrane


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Ca++ ion selective electrode

Ca ++

Neutral carrier

N

O

H 3 C O CH 3

OH 3 C

OH 3 C

N

O

H 3 C O CH 3

O

O

Inert membrane


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IE : Membrane Indicator ElectrodesBou ndary Po ten t i al

exterior of the glass and theanalyte solution

interface between the internaand the interior of the glass


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IE : Membrane Indicator ElectrodesBou ndary Po ten t i al

a 1 is the activity of the analyte solution

a 2 is the activity of the internal solution(held constant)

The boundary potential is then a measure of the hydrogen ion activity (pH) of theexternal solution.


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IE : Membrane Indicator Electrodes

The Glass Electrode Potential

The potential of a glass indicator electrode

The boundary potential,

The potential of the internal Ag/AgCl reference electrode


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IE : Membrane Indicator ElectrodesThe Alkal ine Error

In basic solutions, glass electrodes respond to theconcentration of both hydrogen ion and alkali metal ions

The Ac id Er ro r

the typical glass electrode exhibits an error, opposite insign to the alkaline error, in solution of pH less than about0.5

saturation effect that occurs when all the surface sites onthe glass are occupied with H 1 ions


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Direct Potentiometry

the potential of a cell can then be expressed in terms of thepotentials developed by the indicator electrode, the referenceelectrode, and a junction potential


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Direct Potentiometry

Electrode response is related to analyte activityrather than analyte concentration.

the pH of the standard buffer used for calibration is generallybased on the activity of hydrogen ions


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Kondisi elektroda


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slope: 95-105%offset: (0-15) mV

slope: 90-94%offset: (15-35) mV

slope: 85-89%offset: > 35 mV

Elektroda bagus Elektroda perlu dibersihkan Elektroda bermasalah

Slope dan offset

Stabilitas pengukuran

Kondisi elektroda


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Kalibrasi Dilakukan minimal Sekali per 24 jam sebelum melakukan penguk

Hal yang diperhatikan Offset Slope

Metode Minimum menggunakan 2 pH buffer standar Jika pengukuran dilakukan pada rentang pH lebar gunakan 3-5 pH buffer


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4. Bilas kembali elektroda


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POTENTIOMETRY TITRATION

Potentiometric titration measurem

the potential of a suitable indicator electroda function of titrant volume. more reliable than data from titrations that use chem

indicators particularly useful with colored or turbid solutions

the measurement is based on the titrant volume thatcauses a rapid change in potential near theequivalence point,

1. Acid-base titration pH (glass) elect2. Redox titration inert electrode (Plati3. Argentometry silver electrode


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= ( ( 1( = (

= + 1

2


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Another method : GRANs PLOT

TUGAS : PLEASE FIND THE GRAN PLOT AND HOW TO FIND PKA WGRANS PLOT!


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TERMS

A depolarizer is a species that is easily reduced (or oxidized) andtherefore alters the polarization of the electrode. It helps maintainthe potential of the working electrode at a relatively small constantvalue and prevents reactions that would occur under more reducingor oxidizing conditions.

An electroactive species is one that can be oxidized or reduced aelectrode.

TERMS


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TERMS

A working electrode is the electrode at which the analytical reactoccurs (oxidation or reduction occurs). The working electrode isequivalent to the indicator electrode of two-electrode cells. Metalelectrodes are polarizable electrode because the potentials are easilychanged when small current is flowing.

A reference electrode is an electrode that provides fixed referenc

potential with negligible current flow. A reference electrode (such acalomel or Ag|AgCl) is nonpolarizable electrode, because its potentdoes not vary much unless a significant current is flowing.

VOLTAMMETRY


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VOLTAMMETRY

Auxiliary electrode that icurrent-carrying partner oworking electrode. Current flows between

working and auxiliary

electrodes. Voltage is measured b

the working and refereelectrodes.

WORKING ELECTRODES


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WORKING ELECTRODES

VOLTAMOGRAM


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VOLTAMOGRAM


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Oxygen (pO 2) electrode


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Gas-permeablemembrane

Platinum wire(cathode)

-0.65V

Reference electrode(anode)

Flow cellO2

Reaction at the platinum electrode


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Reaction at the platinum electrode

The amount of current ( e -) is proportional to theconcentration of O 2

O 2 + 2H+

+ 2 e - H 2 O 2

Pt

-0.6 V

The glucose electrode


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The glucose electrode

Glucose + O 2Glucose

oxidase

H2O2 + Gluconic acid

O2 electrode

O2 + 2H-

2e -

(+0.6 V)

AMPEROMETRI TITRATION


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AMPEROMETRI TITRATION

The current at some fixed potential in the limiting current region is

measured as a function of the reagent volume

if no easily reduced or oxidized species is present in the solution electrode is polarized

easily reduced or oxidized species is present in the solution depolarized

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POTENTIOMETRY - AMPEROMETRY