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Voltammetry !

Department of Chemistry, Burapha University !

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Common characteristics: !

•  measurement of current as a function of applied potential (over a range of interest) obtained under conditions that encourage polarization of a WE !

•  to enhance polarization, the WE in voltammetry are microelectrodes having surface areas of a few square millimeters or a few micrometers !

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Voltammetry !

Polarography: WE (DME) !‘Dropping Mercury Electrode”!

Terms: !

Voltammetry !

An electrochemical method in which we measure current as a function of the applied potential!

Voltammogram (i-E curve) !A plot of current as function of applied potential!

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Two important points:!

•  Half wave potential (E1/2) is close to E0 for reduction reaction but is usually not identical to that constant!

useful for identification of the component of a solution !

•  Limiting current (il) proportional to analyte concentration (really, activity) !

il = k . CA !

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Current is just measure of rate at which species can be brought to electrode surface. !

Two methods: !

Stirred – hydrodynamic voltammetry !

Unstirred – Polarography (dropping Hg electrode) !

Three transport mechanism: 1. Migration 2. Diffusion 3. convection !

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Applications:!

•  fundamental studies of oxidation and reduction processes in various media!

•  studies of adsorption processes on surfaces!

•  studies of electron-transfer mechanisms at chemically modified electrode surface !

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Modern Polarographic Cell!

electrolyte + analyte solution !

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Electrodes !

Working electrode (WE) !ขวไฟฟาทเกด electrochemical phenomena !

Reference electrode (RE) !ขวททำหนาทเปน reference standard ในการวด คาศกยไฟฟาเทยบกบ WE!

Counter electrode / Auxiliary electrode (AE) !inert material ( cili of Pt wire) ไมมสวนเกยวของใน redox มไวเพอทำใหครบวงจร !

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Reference electrode !

ตองมสมบต nonpolarizable (ไมวาม i ไหลผาน electrode !มากแคไหน คา potential จะไมเปลยนแปลง) !

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E!

SCE Ag/AgCl!

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Electrolyte!

•  inert soluble ionic salt (not react with electrodes) but has conductivity!

•  consist of solvent, supporting electrolyte, and buffer!

•  solvent : H2O!

Why H2O? (Advantages)!

•  สามารถทำใหนำม high degree of purity ไดงาย!•  เปนตวทำละลายไดด!

•  ไม degrade ไปกบเวลา จงสามารถเกบไวได!

•  เสถยรในชวงศกยไฟฟาทกวาง!

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Why H2O? (Disadvantages)!

•  ละลายพวก organic cpds หลายตวไดไมคอยด!

•  นำเปน protic solvent ดงนนจงให H+ ใน !

aqueous solution ทำใหการควบคม pH จงเปนสงสำคญ!

ปรมาณของ supporting electrolyte ทเตม 50-100 !

เทาของตวอยาง!

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หนาทของ supporting electrolyte!

1.  ลดความตานทานไฟฟาของสารละลาย ทำใหความตานทาน!

ของ cell ตำ!2. รกษา ionic strength ใหคงท!

3. กำจด migration current โดยเฉพาะเทคนค polarography หรอ voltammetry !

Buffer system!

เปนสวนจำเปนในการรกษาคา pH ของ protic solvent !

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Microelectrode !

(a) A disk electrode !

(b) A hanging mercury drop electrode (HMDE) !

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Microelectrode !

Dropping Mercury Electrode (DME) !

Static Mercury Dropping Electrode !

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Advantages of DME (compared to planar electrodes): !

•  clean surface generated !

•  rapid achievement of constant current during drop growth!

•  remixing of solution when drop falls !

•  high Hg overvoltage means even metal with high –ve E0 can be measured without H2 formation !

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•  Hg easily oxidized, limited use as anode (E<+0.4V) !

•  expensive!

•  toxic Hg !

•  nonfaradic residual currents limit detection to >10-5 M!

•  sometimes produce current maxima for unclear reasons (use maxima suppressor)!

Disadvantages of DME: !

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Impurities ทพบใน Hg !

1. Surface oxide!

กำจดโดยการกรองผานกระดาษกรองทเจาะร!

2. Dissolved base metals เชน Zn Cd จะถก oxidized !ออกจาก Hg ไดโดยการผาน Air เขาไปใน Hg ทอยใน 2 M HNO3 หลงจากนนลาง ทำใหแหง แลวกรอง !

3. Dissolved noble metals กำจดไดโดยการกลน!

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Polarography !

First voltammetric technique !

Differ from hydrodynamic!

•  unstirred (diffusion dominate)!•  dropping Hg electrode is use as working electrode!

•  current varies as drop grows then falls off!

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Instrument!

•  polarographic cell •  electrodes •  polarograph/potentiostat

•  ระบบการกำจดออกซเจน!ออกจากสารละลาย!

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Curve (a): Cd2+ + 2e + Hg = Cd(Hg)!

Curve (b): 2H+(aq) + 2e = H2(g) !

Linear Scan Polarography!

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Diffusion current is the limiting current when the rate of electrolysis is controlled by the rate of diffusion of species to the electrode. !

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Figure 18-4 Sampled current polarograms of (a) 5 mM Cd2+ in 1 M HCl (b) 1 M HCl alone!

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การเกด residual current!

1. Redox reactions of impurities in solution !

2. Charging of Hg drop (non-faradaic current/non-redox current) !

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Faradaic current (due to redox reaction) !

Charging current (due to charging of Hg drops) !

Faradaic current is the signal of interest. Charging current obscures the signal of interest, so we seek to minimize charging current. !

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Oxygen Waves !

Problems with dissolved O2 – must purge solutions !

Figure 18-7 Sampled current polarogram of 0.1 M KCl: (a) saturated with air; (b) after bubbling N2 through to remove O2!

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Current Maxima !

•  เกดจาก mass transport ประเภท convection !

à current จะมากกวา limiting current !

Solutions:!

Gelatin Triton X-100!

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Qualitative Analysis !

•  half-wave potential (E1/2) !

•  E1/2 คอคา potential ของ WE ททำใหเกด i = id/2 !

•  E1/2 มคาคงทสำหรบแตละไอออน และไมขนกบ!

ความเขมขนของไอออนนน ๆ !

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Quantitative Analysis !

(a) ! (b)! (c) !

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Pulse Methods: Improving Sensitivity!

Linear-scan polarography!

Disadvantages:!

•  poor detection limits !

At present, two pulse methods dominate!

1. Normal Pulse Polarography (NPP) !2. Differential Pulse Polarography (DPP) !

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Normal Pulse Polarography !

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Differential Pulse Polarography !

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Comparison of direct current (D.C) and DDP!

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CYCLIC VOLTAMMETRY (CV)

Name Waveform Type voltammetry

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In cyclic voltammetry (CV), the current response of a small stationary electrode in an unstirred solution is excited by a triangular voltage waveform.

In this example, the potential is first varied linearly from 10.8 V to -0.15 V versus a saturated calomel electrode. When the extreme of -0.15 V is reached, the scan direction is reversed, and the potential is returned to its original value of +0.8 V. This excitation cycle is often repeated several times.

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Figure 23-24 (a) Potential versus time waveform. (b) Cyclic voltammogram for a solution that is 6.0 mM in K3Fe(CN)6 and 1.0 M in KNO3.

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Stripping Methods

In anodic stripping methods, the analyte is deposited by reduction and then analyzed by oxidation from the small volume mercury film or drop.

In cathodic stripping methods, the analyte is electrolyzed into a small volume of mercury by oxidation and then stripped by reduction.

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Figure 23-30 (a) Excitation signal for stripping determination of Cd2+ and Cu2+. (b) Stripping voltammogram.