Voltammetry. Voltammetry. An Introduction in Theory. Metrohm Ltd. CH-9100 Herisau Switzerland

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1 An Introduction in Theory Metrohm Ltd. CH-9100 Herisau Switzerland

2 Overview What is? Electrode Types Voltammetric Techniques Measurement Modes Calibration Techniques Advantages of Summary

3 = Volt-Am(pero)-Metry Voltage ramp applied to electrode Current measured I = f(u) Method first described 1922 by Heyrovsky

4 Instrumentation in voltammetry Me + + e - Me 746 Voltage Source (Potentiostat) I U polarographic cell

5 Principle of voltammetry Pb ions are reduced and dissolved in mercury + - Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb2+ +2e - Pb 2+ Pb 0 Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+ Pb 2+

6 Principle of voltammetry resulting curve I quantitative information U qualitative information U

7 Principle of potential increased U current measured I Pb Cd metal ions are reduced and dissolved in mercury. ΔΙ time Reduction to metal (amalgam formation)

8 3 Electrode system Auxiliary electrode Platinum (Pt) Glassy Carbon (GC) Current I Working electrode MME RDE Potential U Reference electrode Ag/AgCl/KCl 3 mol/l Provides stable reference potential

9 Reference electrode provides stable reference potential Ag/AgCl/KCl 3 mol/l Double junction system with exchangeable electrolyte

10 Auxiliary electrode Current flows between working and auxiliary electrode Two types available: Platinum (Pt) Glassy Carbon (GC) Depending on application!

11 Working Electrodes MME Multi Mode Electrode RDE Rotating Disc Electrode SMDE DME HMDE GC Au Ag Pt UT

12 Rotating Disk Electrodes

13 MME Multi Mode Electrode mercury needle tapper capillary Hg drop

14 DME Mode Dropping Mercury Electrode Needle open Continuous mercury flow

15 SMDE Mode Static Mercury Drop Electrode Formation of small drops Constant drop size

16 HMDE Mode Hanging Mercury Drop Electrode 1 drop for 1 voltammogram

17 Multi Mode Electrode Tapper DME Needle SMDE HMDE Capillary Hg drop New Drop Stable surface One drop! Droplife

18 Selection of Working Electrode Metrohm application Application Bulletin Application Note Application Work approved methods only limited optimizations required Theoretical considerations Concentration of the analyte Half wave potential of the analyte

19 Type of Application Standard electrode: MME Organics: DME or SMDE Metal Traces: HMDE Special applications: RDE Platinum electrode: Organic additives in plating solutions (CVS) Gold electrode: Hg, As UT* with Hg film: Cd, Pb, Cu UT* Hg free: Cr, W, Ni UT: Ultra Trace Graphite Electrode

20 Concentration of the Analyte DME SMDE HMDE & RDE Traces ppm Low ppm ppb ppt

21 Half wave potential of the analyte Maximum possible working ranges of different electrode types Mercury Oxidation of electrode Glassy Carbon Gold Platinum H + + e - H Potential (V)

22 Voltammetric Techniques Polarography uses the dropping mercury electrode (DME, SMDE) ppm level (Stripping) uses a stationary electrode (HMDE, RDE) ppb level with deposition

23 Polarography Pb 2+ Pb 0 direct reduction or oxidation at the electrode limited sensitivity universal technique

24 Voltammetric Techniques Polarography Stripping U U Pb Pb I Cd I Cd ΔΙ ΔΙ Electrolysis time time Reduction to metal (amalgam formation) Reduction to metal (amalgam formation) Oxidation to ion (determination)

25 Anodic Stripping (ASV) For amalgam forming elements (Zn, Cd, Pb, Cu ) 1. Deposition (metal reduction) Cd 2+ Cd 0 (Hg) 2. Determination (stripping) Cd 0 (Hg) Cd 2+ CSV

26 Adsorptive Stripping (AdSV) For non-amalgam forming elements (e.g. Ni, Co, Fe, Cr ) 1. Deposition (adsorption) Ni 2+ (DMG - ) 2 Ni 2+ (DMG - ) ads 2 2. Determination (stripping) Ni 2+ (DMG - ) ads 2 Ni DMG red DMG: Dimethylglyoxime, complexing agent

27 Stripping Properties 2 step measurement electrochemical deposition (enrichment) determination (stripping step) High sensitivity For trace analysis (down to ppt range) Mainly for metal analysis. PSA

28 Measurement Modes = modulation of potential ramp Purpose: Increasing sensitvity Decreasing background signals Increasing selectivity Separation of faradayic and capacitive current

29 Sampled Direct Current: DC potential ramp resulting curve U time U voltage step wave height drop time half wave potential time

30 Differential Pulse: DP potential modulation resulting curve U pulse amplitude I voltage step drop time U time peak height peak potential

31 DC and DP I quantitative information I U quantitative information U qualitative information U Modes

32 Electrolyte in VA Correct electrolyte is most important : see Application Bulletins Purpose of the electrolyte: Increases the conductivity Adjusts the ph Complexes the analyte Increase selectivity (masking of interfering elements)

33 Electrolyte in VA Peak potentials of different metals depending on the electrolyte NH 4 Ac/HAc Co Ni Zn Cd Pb Cu Hg Hg NH 3 /NH 4 Cl Mn Zn Co Ni Cd Pb Cu Hg Hg Na tartrate Mn Co Zn CdPb CuHg Hg Potential (V)

34 Concentration evaluation Peak evaluation Calibration Standard addition External calibration (calibration curve)

35 Peak evaluation I height Peak maximum Basepoints Baseline Peak height Peak area U basepoints baseline

36 Calibration Curve Voltammograms of standards Calibration Curve peak height c1 c2 c3 conc

37 Sample with Cal. Curve Voltammograms of sample Calibration Curve peak height conc c sample

38 Standard Addition Voltammograms Standard Addition Curve additions peak height -cs Add1 Add2 conc sample E Sum

39 Calibration Techniques External Calibration Measure standard solutions Calculate calibration curve Measure sample Standard Addition Measure sample Measure additions to sample solution

40 Why?

41 Advantages of No problem with high salt concentrations Speciation possible: e.g. Cr 3+ -Cr 6+ free / complexed metal Low detection limits: sub-ppb Capital outlay: Low price Low running costs: only N 2 No extensive laboratory infrastructure

42 Advantages of Versatility: inorganics and organics Anions Official methods (DIN,ISO,USP, ASTM, etc.) Alternative and complementary method to AAS/ICP Automation: 80 Samples plus all buffers and standards Calibration in every run: Variable Standard Addition if required.

43 Summary Most important points

44 What you should know about VA! VA: Voltam[pero]metry I = f(u) Electrodes AE: Pt, GC RE: Ag/AgCl/KCl 3 mol/l WE: MME (DME, SMDE, HMDE) or RDE Measurement Modes DC, DP, AC1/AC2, SQW, CV, PSA

45 What you should know about VA! Methods Polarography: Liquid electrode (DME, SMDE) Stripping (HMDE, RDE) Anodic Stripping Adsorptive Stripping Cathodic Stripping Electrolyte (buffer) Allows signal enhancement Increase selectivity Allows masking of elements

46 What you should know about VA! Concentration Evaluation Peak evaluation Calibration Standard Addition Technique Calibration Curve Technique Advantages of

ANALYSIS OF LEAD IN SEAWATER

ANALYSIS OF LEAD IN SEAWATER BY DIFFERENTIAL PULSE POLAROGRAPHY Introduction Electrochemical methods of analysis can be used for the quantitative analysis of any electroactive species any species that