Senzori Potentiometrici

8/10/2019 Senzori Potentiometrici

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Senzori

potentiometrici.Electrozi ion-

selectivi.Curs 03


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selectivi. Senzori potentiometrici. Electrozi ion-

selectivi.Aparitia potentialului in celula de masura la

curent zero

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Senzori potentiometrici. Electrozi ion-selectivi.

Aparitia potentialului in celula de masura lacurent zero

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Potentiometric Methods1.) Potentiometric Methods: based on measurements of thepotential of electrochemical cells in the absence ofappreciable currents (i = 0)

2.) Basic Components:a) reference electrode !ives reference for potential

measurementb) indicator electrode "here species of interest is

measuredc) potential measurin! device

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Electrozi ion-

selectivi cumembrana solida

Membrana monocristalsubstanta cristalinasticla

#in punct de vedere construnctiv$arianta %totul solid&varianta %membrana&


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EIS cu membran solid

#in aceast' clas' de ES e*emplul cel mai bine cunoscut +l constituie electrodul desticl' pentru m'surarea p,-ului. rup'rile schimb'toare anionice din structuramembranei de sticl' apar ca urmare a defectelor din matricea de Si / i datorit'vacan1elor cationice provocate de constituien1ii sticlei diferi1i de Si/. 2n stratul desticl' hidratat' de cca 3-400 nm !rosime ce se formeaz' la suprafa1a membranei desticl' activ' concentra1ia de !rup'ri schimb'toare este relativ mare (cca. 5 - 40 M).Astfel se e*plic' domeniul de lucru e*cep1ional de +ntins pentru electrodul de sticl'de p,.

#ac' se admite c' diferen1a de poten1ial care apare +ntre cele dou' solu1ii

adiacente membranei de sticl' este rezultatul unui proces de schimb ionic localizatla cele dou' interfe1e membran'-solu1ie cuplat cu un proces de difuzie prinmembran' atunci +nsumarea acestor contribu1ii conduce la urm'toarea e*presiepentru poten1ialul electrodului de sticl' 6748

9 = 90: (;)ln6i.?potAi.ai(4zi)8 (4@)

unde A semnic' ionul primar (i = 4) z i - sarcina ec'rui ion determinant depoten1ial ?potAi - coecientul de selectivitate poten1iometric' iar ; i 90 au

semnica1iile cunoscute. Pentru electrozii de sticl' coecientul de selectivitate poten1iometric' este dat

de relatia

?potAB= (uBuA).CAB (4D)

unde CAB este constanta de echilibru pentru reac1ia de schimb ionic care descrieprocesul de interferen1'

(4(zB) Ba : (4zA) Amembran' (4zB) Bmembran' : (4zA) Aa (4F)

iar uAi uBsunt mobilit'1ile ionilor respectivi +n faza membran'.


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EIS cu membran solid

;ela1iile (4@) - (4F) sunt valabile i +n cazul ES cu membrana

relizat' dintr-un schimb'tor de ioni solid. Pentru ES cu membran'solid' din monocristal (de e*. Ga>5) sau policristalin' (e*. A!/S A!/S-A!H etc.) procesul de difuzie prin membran' ind e*trem de lent nu

Ioac' un rol notabil +n apari1ia poten1ialului de electrod. #e aceeapentru aceti electrozi coecientul de selectivitate poten1iometric'are o e*presie simplicat' 67/8

?potAB= (JA!H)(JA!K) (4L)

unde J reprezint' produsul de solubilitate al s'rurilor !reu solubileimplicate +n func1ionarea electrozilor pe baz' de s'ruri de A!. Seobserv' c' +n acest caz coecientul de selectivitate poten1iometric'se va identica cu constanta de echilibru a reac1iei de schimb ioniccare st' la baza apari1iei fenomenului de interferen1'

A!Hmembran' : Ka A!Kmembran' : Ha (/0)

;ela1ia (4L) permite deci o estimare rapid' a valorii coeentului deselectivitate pentru ES din s'ruri !reu solubile. Aceast' ipotez' afost vericat' e*perimental 6758 !'sindu-se o e*celent' concordan1'

+ntre coecien1ii de activitate calcula1i i cei m'sura1i (!. L).


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Electrodul de sticla

i.most common e*ample of an SE based on use of !lass membrane that preferentiallN

binds ,:

ii.


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Electrodulde sticla

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Electrodul de sticla.Masuratori

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Electrodul de sticla iii.p, is determined bN formation of

boundarN potential across !lassmembrane

Si O-

Glass Surface

At each membrane-solvent interface a small localpotential develops due to the preferential adsorptionof ,:onto the !lass surface.

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Electrodulde sticla

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iii. p, is determined bN formation of boundarN potentialacross !lass membrane

BoundarN potential dierence (Eb)= E4=E/"herefrom Qernst

EuationEb= R 0.3L/p,

-log aH+(on e*terior ofprobe or

in analNtesolution)

constant

Selective binding of cation (H) to glass mem

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Electrodul desticla


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Electrodul de sticla eroareacida

Errors at lo" p, (Acid error) can !ivereadin!s that are too hi!h

E*act cause not Rno"n - usuallN occurs at p, =0.3

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!l"ali error range - at lo# concentration of h$drogen ions%high &alues of pH'

contri(utions of interfering al"ali metals%li"e )i, *a, +' are compara(le #ith the one

of h$drogen ions n this situation dependence of the potential on pH (ecome non-

linear

.he effect is usuall$ noticea(le at pH / 01, and concentrations of lithium or sodium

ions of 20 moles per litre or more 3otassium ions usuall$ cause less error than

sodium ions

!cidicerror range at &er$ high concentration of h$drogen ions %lo# &alues of pH'

the dependence of the electrode on pH (ecomes non-linear and the influence of

the anionsin the solution also (ecomes noticea(le .hese effects usuall$ (ecomenoticea(le at pH
http://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/Alkali_metalhttp://en.wikipedia.org/wiki/Acidhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Acidhttp://en.wikipedia.org/wiki/Alkali_metalhttp://en.wikipedia.org/wiki/Hydrogen_ion


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Electrodul de sticla Eroare acida

eroare alcalina

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Potential de asimetrie TTT

AvantaIe #ezavantaIe

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Oalibrarea electroduluide sticla

p, 7 p, D TTTTTT

p, L

O lib l t d l i


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Oalibrarea electroduluide sticla


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Potential de asimetrie

Fig. 13.11. so!o"en"ial

!oin".

The isopotential point (usually at pH 7) of a glass electrode is temperature insensitive.

The potential is set at zero here (if different than zero, this is the offset).

The isopotential point (usually at pH 7) of a glass electrode is temperature insensitive.

The potential is set at zero here (if different than zero, this is the offset).


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Electrod de sticla pentru alticationi

i.Ma*imize R,Qafor other ions bNmodifNin! !lass surface

chan!e composition of !lass

membrane puttin! Al/5or B/5in !lass

enhances bindin! for ions other

than ,:

ii.Possible to maRe !lassmembrane electrodes for Qa: ?:Q,7: Os: ;b: Gi: A!:U

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Electrod monocristalde Ga>5

Ga>5crNstal doped "ith Eu>/

mechanism similar to p,electrode "ith potentialdevelopin! at t"o interfaces ofthe membrane from the reaction

the side of the membrane "ith thelo"er a

>-becomes positive relative

to the other surface

Presence of >-analNte pusheseuilibrium ri!ht reduces :vechar!e on electrode surface

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test

refconst

c

c

F

RT

,F

,Fln

+=


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Electrod monocristal deGa>5

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Electrozi cumembrana

solida omo!ena

Electrozi A!/S A!:sau S/-

selectivi Electrozi A!/S-A!H H- selectivi

Electrozi A!/S-MeS Me:- selectivi


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A!/S-A!H

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Senzori Potentiometrici