Electrochemistry for analytical purposes. Examples for water analysis Dr Riikka Lahtinen

Transcription

1 Electrchemistry fr analytical purpses Examples fr water analysis Dr Riikka Lahtinen

2 Electrchemistry Based n RedOx-reactins: Reductin: receive electrn(s) Oxidatin: give away electrn(s) Electrchemistry is study f hetergeneus redx-reactins where the xidatin and reductin reactins take place at the surface f the representative electrdes.

3 Gibbs free energy and ptential Ptential is the inherent capacity fr cming int being. Electric ptential: at a pint, the wrk required t bring a charge frm infinity t that pint in the electric field divided by the charge. SI derived unit f electric ptential is vlt V=J C -1. Reductin f cpper(ii) Cu 2+ +2e - Cu And -> there is a direct relatinship between the Gibbs free energy f transfer and the ptential E.

4 G G RT ln Q G nfe E RT E ln Q, missä Q ai nf i i

5 Cnductivity

6 Cnductivity as a measure fr water quality Ohm s law: E=RI Nn-specific: measures ttal amunt f ins. Cnductivity= R -1 Unit: siemens per cm (S/cm)

7 Ptentimetry

8 A simple electrchemical system Smething t measure ptential Electrde where xidatin takes place: ANODE E cell Electrde where reductin takes place: CATHODE Slutin I Slutin II separatr -will let ins thrugh Minimun reguirements fr an electrchemical measurement: tw electrdes in inic cntact and a device (ptentimeter) t measure the ptential cnnected t the electrdes.

9 Electrchemical cell in equilibrium H 2 E cell Platinum Gld Fe 2+ Fe 3+ [H + ]=1 M ANODE Cathde: Fe 3+ + e - Fe 2+ Ande : ½ H 2 H + + e - prus membrane CATHODE Ttal reactin: ½ H 2 +Fe 3+ H + + Fe 2+

10 It is a virtual equilibrium Cu Pt H +,H 2 Fe 3+,Fe 2+ Au Cu The ptentimeter has very high impedance -> n (r practically n) current ges thrugh the system -> a real equilibrium can nt be reached.

11 Galvani ptential Galvani ptential is an electric ptential difference between pints in the bulk f tw phases. It is measurable nly when the tw phases have identical cmpsitin (e.g. tw cpper wires). It is the difference f inner electric ptentials in tw phases,.

12 Equilibrium ptential f the cell Cu Pt H +,H 2 Fe 3+,Fe 2+ Au Cu a b s e g a In equilibrium, there will be fllwing phase equilibriums: phase a phase b, phase b phase s, phase s phase e, phase e phase g ja phase g phase a. When tw phases are in equilibrium, their electrchemical ptentials are equal. Electrchemical ptential is defined by: ~ F i i i where ( i RT ln ai )is the chemical ptential, z i the charge f i and the Galvani ptential. z i

13 Phase equilibria (I): phase a phase b ~ ~ b a e e phase b phase s the platinum electrde) ~ s b s ~ 1 ~ H e H2 2 phase s phase e The liquid-liquid cntact (prus membrane, salt bridge), here it is assumed that the Galvani ptential is equal n bth sides: e

14 Phase equilibria (II): e g e 2 3 ~ ~ ~ Fe e Fe phase e phase g (the gld electrde) phase g phase a a g e e ~ ~ And using the definitin f electrchemical ptential: a b a b e e F H H e H H a a RT F 2 1/ 2 2 ln 2 1 ) ( b b s ln ) ( Fe Fe e Fe Fe a a RT F g e g g a ag a e e F ' '

15 The Nernst equatin H Fe H Fe H H Fe Fe a a a a RT F / ' ln 2 1 ) ( a a H Fe H Fe Cell a a a a F RT E E / ln Cmbining the equatins: Which can be presented in the mre familiar frm as as the Nernst equatin

16 Standard ptential f the cell In Nernst equatin, the E is the value f the cell ptential when all activities f the reacting species are 1. 1/ 2 RT a 3 Fe ah 2 E Cell E ln F a 2 Fe ah Standard ptential f the cell can be diveded int tw E E Fe 3 / Fe 2 E H / H 2 where FE H 1 / H H H crrespnding Pt H +,H Fe 3 / Fe Fe Fe FE 3 crrespnding Fe 3+,Fe 2+ Au

17 Standard ptential f the cell Cmbining the half-reactins gives E Fe 3 Fe 2 F H 1 2 H 2 G nf The standard ptential f the cell gives the wrk necessary t transfer ne electrn between the gld electrde and the redx-cuple (Fe 3+ /Fe 2+ ) in slutin, n a scale defined by the left-side electrde at standard cnditins.

18 Standard Hydrgen Electrde (SHE) SHE Reference electrde: the left-hand side electrde f the cell; the electrde against which the ptential is measured. E E Fe 3 / Fe 2 E H / H 2

19 Standard ptential f SHE It has been pstulated, that the ptential f SHE in all temperatures at standard cnditins is zer vlts. E SHE 0 V Nw the ptentials f ther half-cells can be determined by measuring against SHE. These values are usually tabulated as standard reductin ptentials (the electrchemical series). The tendencies f different species in relatin t each ther can be cmpared based n these tables.

20 Example Why is FeCl 3 used t disslve cpper in the prcess f making electrical circuits? Slutin: Standard reductin ptentials: Fe 3+ + e - -> Fe 2+ E =0.77 V Cu e - -> Cu E =0.34 V Let s cmbine these t give a crrespnding reactin: 2Fe e - -> 2Fe 2+ Cu -> Cu e - 2Fe 3+ + Cu ->2Fe 2+ + Cu 2+ E =0.77 V E =-0.34 V E tt=0.43 V G nfe 2ml C ml 0.43 J C 83kJ Spntaneus reactin

21 Ptential f silver/silver chlride electrde E E E E AgCl Ag / Ag Ag / Ag RT F RT F K sa ln a a H ln K s RT F 1/ 2 H2 Cl f ln p 1/ 2 H2 RT F ln a H a RT / Ag E ln K V V Ag / Ag s F Experimentally: 1 M E Cl AgCl / Ag Silver wire is cated (by electrlysis) with silver chlride, and immersed in a chlride slutin. Half-cell reactins: lim c E HCl 0 Ag + + e - Ag ½ H 2 H + + e - RT F ln c H V c Cl

22 Imprtant t nte! E cell=0,77 V E cell=0,57 V The ptential value yu measure is dependent n the reference electrde yu chse fr the measurement! Befre yu start t cmpare any ptential values, it has t be made sure, that they are presented n the same ptential scale.

23 Abslute (vacuum) ptential scale The standard ptential f a redx-cuple n the abslute scale will determined frm a measurement with SHE: E, abs red / x metal slutin M e F S a H Where a H+ is the real chemical ptential f H + and H2 is the standard chemical ptential f gaseus H 2. Frm thermdynamic data: H + (g)+e - (g) H(g) G = ev H(g) ½ H 2 (g) G = ev H + (g) H + (aq) a H+ = ev F 1 2 H 2

24 Abslute ptential scale E, abs H H / ( ) 4.44 V The standard ptential f a redx-cuple n the abslute scale can be calculated frm ptential values n SHE scale : E, abs Re d / Ox E, SHE Re d / Ox 4.44 V Sme references give this value as 4.6 V (r ev).

25 In selective electrdes: Glass electrde first ISE was the glass electrde cntains as the membrane a specific xnium in binding glass thefre the ptential ver this membrane is dependent n the ph E cns tan t RT F ln a a in ut Inner reference electrde Ag/AgClelectrde requires calibratin

26 In selective electrdes liquid membrane electrdes enzyme electrdes slid membrane electrdes (fluride selective electrde cntaining a LaF 3 -membrane belw) Referenceelectrde Ag/AgClelectrde

27 Measurement with ISE s calibratin with knwn cncentratins by calculatins e.g. ph-meter (calibratin with buffer slutins) graphic calibratin curve measure the ptential E f the unknwn sample pssible errr surces? E E RT nf ln Q

28 Instrumental methd: measure a physical quantity that is dependent n amunt f substance rcncentratin is ptential E measured quantity Ptentimetry: measure ptential between galvanic cuples. Calibratin gives cncentratin. cnnectin between measured quantity and cncentratin Nernst: E E RT nf ln Q measuring system determinatin steps 1. reference electrde 2. indicatr electrde 3. smething t measure ptential e.g. vltmeter Calibrate: calculate r calibratin curve Measure: measure ptential E in the slutins, get c thrugh calibratin

29 Vltammetry/ ampermetry

30 The idea behind ampermetric sensrs Simple: the electrchemical current is directly prprtinal t the cncentratin f the electractive species

31 Clark type xygen sensrs Operatin vltage ca. 0,8 V Measure CURRENT which is directly prprtinal t xygen cncentratin Needs calibratin -n xygen and saturated xygen Ref: Falck Current Separatins 16(1) (1997)

32 The prblem with current flwing thrugh an electrde Ampermetric methds measure current as a functin f ptential. E Current flws- > a prblem (E=RI):plarisatin f an electrde i.e. the ptential f the electrde n lnger crrespnds t the equilibrium value.

33 Plarisable and unplarisable electrdes j Ideally plarisable electrde: the change in vltage results in n change in current E Blue: ideally plarisable electrde Red: ideally unplarisable electrde Ideally unplarisable electrde: n change in ptential when current ges thrugh the electrde Ideal reference electrde wuld be an ideally unplarisable electrde, silver-silver chlride is quite unplarisable. The real life slutin: three-electrde system

34 Three-electrde system V in CE RE WE V ref V ut Basic idea: t divide reference electrde in tw parts: ne t measure ptential and ne t pass current. Wrking electrde (WE): the electrde where ur reactin f interest takes place. Ptentistat cntrls the ptential f the electrde and measures the current. The ld reference electrde divided int tw Reference electrde (RE): the electrde against which the ptential is measured. Cunter electrde (CE): the electrde thrugh which the current is passing

35 Psitining the electrdes in a three-electrde system (I) Reference electrde and wrking electrde shuld be as clse t each ther as pssible because f irdrp. If there is uncmpensated ir-drp in the system, it will change the E values ( E=RI) Slutin -> finite cnductivity-> resistance -> ir-drp. Often it is difficult t place the reference very clse t the wrking electrde, the slutin t this prblem is the use f a Luggin capillary.

36 Psitining the electrdes in a three-electrde system (II) The cunter electrde shuld be ideally symmetrically psitined in relatin t the wrking electrde Cell fr the three-electrde measurements.

37 The slutin fr electrchemical measurement Our electractive species f interest Slvent t disslve the electractive species Excess f inert electrlyte Electrlyte: cmpund that prduces ins i.e. cnductivity Slvent: Has t disslve the desired cmpund -> water is nt always pssible Has t have high enugh dielectric cnstant s that ins will exist in the slutins -> ins mean cnductivity Supprting electrlyte Base electrlyte Inert electrlyte Supprts i.e. carries the current in the system Ensures that the slutin phase has enugh cnductivity t reduce ir-drp.

38 The need fr a supprting electrlyte Transprt in electrlyte slutin (Nernst-Planck equatin): zif Ji Dici Dici vci RT diffusin cnvectin migratin In usual systems, there is n cnvectin. where J is the flux f in i, D i is the diffusin cefficient f i, is the velcity f the slvent. The imprtance f migratin is dependent n the transprt number f the species. Transprt number states the prprtin f the current an in carries in a slutin. When inert electrlyte is added t the system in excess (e.g. 100 times mre than the electractive species) the transprt number f the electractive species is clse t zer and the migratin term can be neglected. -> analysis f the results is easier

39 The electrdes- WE Wrking electrde Often sme inert metal like platinum r gld, we generally want it just t accept r give electrns and nt react with the slutin in any ther way Other chices wuld be ther inert but very cnductive materials like glassy carbn, graphite r even ITO It is imprtant, that the electrde can be repetitively cleaned If yu want t measure the quantity f the current, the area f the electrde has t be knwn since current is directly prprtinal t area The rughness f the electrde (which might cme frm the cleaning prcedure) affects the area

40 The electrdes- CE Cunter (auxiliary) electrde Inert and very cnductive material Area shuld be much larger cmpared t the wrking electrde -> we d nt want the reactin at this electrde (which reactin, is usually nt knwn and is nt interesting) be the rate determining reactin=smallest current in the system typically platinum cil graphite rd The current in electrchemical measurements is a direct measure f the reactin rate. Electrchemistry is the nly methd which measures reactin rates directly.

41 The electrdes- RE Reference electrdes Stable ptential thrugh ut the measurement Saturated calmel electrde (SCE) is ne, especially in the past, widely used electrde E (saturated KCl, 25 C)= 0,2444 V vs SHE E (3,5 M KCl, 25 C)= 0,250 V vs SHE Nte: the electrde ptential is dependent als n the chlride in cncentratin and pssible junctin ptentials f the system.

42 The electrdes- RE Silver-silver chlride (Ag/AgCl)-electrde is the mst cmmnly used reference electrde nw-a-days Easy t prepare Can be made small E (saturated KCl, 25 C)= 0,199 V vs SHE E (3,5 M KCl, 25 C)= 0,205 V vs SHE Remember t check the ptential against e.g. a cmmercial silver-silver chlride electrde after preparing the electrde, and every nw and then (the ptential difference between tw identical electrdes in the same slutin shuld be zer. The cncentratin f the chlride slutins shuld be the same in that case)

43 Hw abut rganic slvents? Generally, Ag/AgCl- r calmel electrdes can be used Prblems might arise als frm the leaking f chlride ins r water int the the measuring slutin -> very water sensitive rganic mlecules cannt be measured with these electrdes Anther prblem is related t slubility: the salts used in these electrdes might nt be very sluble in rganic slvents -> there might be sme precipitatin frmatin n the electrde diminishing its area r precipitating n the frit.

44 Reference electrdes fr rganic slvents Silver-silver in electrde Silver wire in silver in (AgNO 3 )cntaining slutin in rganic slvent (e.g. acetnitrile, THF, DMSO) Pseud-reference electrdes A metal wire immersed in the studied slutin (platinum r silver) Prvide a cnstant ptential, which is unknwn and dependent n the cmpsitin f the slutin An internal standard shuld be used t tie the measured ptentials t the SHE scale IUPAC recmmends the use f Ferrcene/Ferrcinium cuple In practice, after the measurement f the electractive species has been carried ut, Ferrcene is added t the system and the its xidatin ptential is measured. The assumptin is, that this ptential is the same in different systems (E (water)= 0,40 V, E (MeCN)= 0,69 V, E (DMF)= 0,72 V, E (DMSO)= 0,68 V all vs. SHE)

45 Definitin f signs fr ptential and current Psitive ptential means that the wrking electrde ptential is made mre psitive cmpared t the reference electrde, i.e. it will be easier fr it t accept electrns. By definitin, the andic current resulting frm the xidatin f sme species in slutin is psitive. Therefre, cathdic current, the reductin current, is negative. Be careful when reading literature, nt everybdy respects these definitins... Nte: Zer applied ptential des nt mean, that the ptential f the wrking electrde wuld be zer, and the current may r may nt be zer at zer applied ptential.

46 Reactins at electrdes Three steps: 1. Flux f reactants twards the electrde (diffusin) 2. Interfacial electrn transfer reactin 3. Flux f the prducts frm the surface twards the slutin (diffusin) Irreversible reactin: the mass transfer f the reactants and prducts is rapid cmpared t the electrn transfer reactin. Nte, this has nthing t d with the chemical reversibility f the reactin. Reversible reactin: the electrn transfer is rapid cmpared t the mass transfer rate. The reactin is diffusin- r mass transfer limited. Quasi-reversible: electrn transfer and mass transfer take place in cmparable time scales.

47 Steady-state current-ptential curve Current as a functin f ptential is recrded. Diffusin limited current: I nfac lim, andic Half-wave ptential: RT D E 1/ 2 EOx/ red ' ln nf D Frmal ptential R D R R O If the rati f the diffusin cefficients f the reduced and xidiced species are the same, the half-wave ptential will give the frmal ptential f the redx species. And if the slutin is dilute, we can assume that the frmal ptential and standard redx ptential are clse t each ther.

48 Cyclic vltammetry In cyclic vltammetry, the ptential is changed frm sme initial value E i t sme ther value E s and back while the current is being measured. The current as a functin f ptential is presented. E s E i t = 0 t = t s slpe= kulmakerrin scan = rate v

49 Cyclic vltammgram Cyclic vltammgram f a reversible system, this culd be e.g. Ferrcene Andic peak current Cathdic peak current E 1/2 (andic) peak ptential E p Half peak ptential E p/2 These are values that shuld be defined frm the measured cyclic vltammgrams.

50 Cyclic vltammetry- the parameters Peak current j p nf nFc O, 0 v D RT O Relatin f half peak ptential t half-wave ptential (n is the number f electrns transfered): RT E p / 2 E1/ / n nf E p = 59/n mv at 25 ºC fr a reversible reactin (this is the criteria fr a reversible reactin)

51 Example:Cyclic vltammetry in envirnmental Pb analysis WE: carbn fibre micrelectrde (7 m radius) RE: silver/silver chlride Fast scan cyclic vltammetry (scan rate 400 V s -1 ) Pb 2+ adsrbs n carbn fibre - >precncentratin -> imprved limit f detectin (2 ppm) Tested fr real strm water samples. Idea is t be able t d fast in situ measurements with n sample preparatin. Reference: Yang et al. Anal. Chem. (2013)

52 Differential Pulse Vltammetry Same measuring system as in cyclic vltammetry. A series f regular vltage pulses superimpsed n the ptential linear sweep r stair steps. The current is measured immediately befre each ptential change, and the current difference is pltted as a functin f ptential. By sampling the current just befre the ptential is changed, the effect f the charging current can be decreased. The determined values: peak ptentials fr andic and cathdic peaks In rganic slvents, an internal standard used.

53 Example: pesticides in water with carbn paste electrde (CPE) Analyte: nenictinid WE: tricresyl phsphate based CPE RE: SCE CE: platinum Brittn-Rbinsn buffer (ph 7) as supprting electrlyte Analysed river water and cmmercial pesticide Actara

54 Example: pesticides in water- DPV respnse Reference: Papp et al. J. Serb. Chem. Sc. 75(5) (2010)

55 Instrumental methd is Current I measured quantity Vltammetry: cntrl ptential and measure current between tw electrdes. Usually calibratin gives cncentratin. cnnectin between measured quantity and cncentratin Directly prprtinal measuring system determinatin steps 1. WE 2. RE (and CE fr threeelectrde mde) 3. Ptentistat t cntrl ptential and measure current. Calibrate: calculate r calibratin curve Measure: Measure (limiting) current, get c thrugh calibratin