Aula 3- Breve histórico dos eletrodos, conceitos fundamentais e potenciometria

IntroductionHistoric overview, Classification of (bio)chemical electrodes, definitions of sensors andbiosensors and basic measuring techniques

Fundamental ConceptsElectrical conduction, electrodes, electrolytic cells, faraday’s law of electrolysis, Voltaic orGalvanic cells, The Nernst equation, reference and indicator electrodes, standard electrodepotentials, liquid-junction potentials

Potentiometric Methods and ElectrodesPrinciples of potentiometric electrodes, experimental set-up and instrumentation (Galvanic

ll) I di t l t d 1) t lli l t d fi t ( ti d l t d ) d dcell), Indicator electrodes: 1) metallic electrodes: first (cation or redox electrode), second andthird class or specie; 2) membrane electrodes: glass electrodes, polymer (liquid membranes)electrodes, crystalline and pressed powder solid electrodes, gas electrodes, enzymaticelectrodes (biosensors) practical aspects and applicationselectrodes (biosensors), practical aspects and applications

References1)R. W. Cattrall, Chemical Sensors, Oxford, Oxford Press, 1997

2) A. Evans, Potentiometry and Ion Selective Electrodes, Chichester, Wiley, 1987

3) F. Scholz, Electroanalytical methods-guide to experiments and applications, Berlin,Springer-Verlag, 2009

4) V. A. Gault, N. H. McClenaghan, Understanding Bioanalytical Chemistry- Principles andApplications, Oxford, Wiley-Blackwell, 2009

5) A. P. F. Turner, I. Karube, G. S. Wilson, Biosensors- Fundamentals andapplications,Oxyford, Oxyford University Press, 1987

6) G. G. Guilbault, A. A. Suleiman, O. Fatibello-Filho, M. A. Nabirahni, ImmobilizedBioelectrochemical Sensors, In: D. L. Wise, Bioinstrumentation and Biosensors, New York,Dekker, 659-692,

7) O. Fatibello-Filho et al, Chapter 17 - Electrochemical biosensors based on vegetabletissues and crude extracts for environmental, food and pharmaceutical analysis, In: S.p yAlegret & A. Merkoçi, Comprehensive Analytical Chemistry, Vol. 49-Electrochemical SensorAnalysis”, Amsterdam, Elsevier , 355-375, 2007

Breve histórico sobre os sensoresTabela- Breve histórico sobre o desenvolvimento dos sensores

Ano Tipo de sensor Investigadorquímicos

1888 Eletrodos metálicos/íons W. Nernst1906 Eletrodo de vidro M. Cremer1922 Eletrodo gotejante de Hg J. Heyrovsky1930 Eletrodo de vidro/Corning 015 D. Innes & M. Dole1934 El t d d id N (I) B L l & E Bl1934 Eletrodo de vidro para Na(I) B. Lengyel & E. Blum1936 Eletrodo c/ CaF2 para Ca(II) H.J.C. Tenderloo1956 Eletrodo para Oxigênio L C Clark1956 Eletrodo para Oxigênio L.C. Clark1958 Eletrodo para CO2 W. Severinghaus & A. Bradley1958 Eletrodo de pasta de carbono R.N. Adams1958 Eletrodo de pasta de carbono R.N. Adams1959 Sensor piezelétrico G.Z. Sauerbrey

3

Ano Tipo de Sensor InvestigadorAno Tipo de Sensor Investigador1961 Sensor AgI(s) parafina p/ I- E. Pungor1962 Biossensor para glicose L.C. Clark & W. Lyonsp g y1964 Sensor piezelétrico W.H. King1966 Sensor de LaF3/EuF2 para F- M.S. Frant & J.W. Ross1967 Sensor de membrana liq. p/ Ca2+ J.W. Ross1970 Sensor de membrana de PVC p/ Ca2+ G. Moody et al1970 ChemFET P. Bergveld1974 Calorimétrico (enzimático) K. Mosbach & B. Danielson1975 ENFET J. Janata1979 Biossensor PC c/ ADH e LDH T. Yao & S. Musha1979 Biossensor de tecido Rechnit et al1979 Biossensor de tecido Rechnitz et al1980 Sensor de fibra-óptica J.I. Peterson et al1997 Eletroantenograma J Pickett et al1997 Eletroantenograma J. Pickett et al

? Pâncreas artificial implantável J Jaremko & O. Rorstad

Electrical energElectrical energyNon-electrolytic Electrolytic cellsNon-electrolytic cells (gavanic)

Electrolytic cells

Non-Spontaneous Spontaneous process

pprocess

ΔG 0ΔG < 0ΔE > 0

ΔG > 0ΔE < 0

Chemical energyΔE > 0

PotenciometryI 0

Electrolysis CoulometryP l h /V l

5I ~ 0 Polarography/Voltammetry

Electrogravimetry

Electroanalytical methods

Interfacial methods

Bulkmethodsmethods methods

Static methodsI 0

Dinamic methodsI 0

Conductometry(G 1/R)

Conductometric Ti iI=0 I > 0 (G = 1/R) Titrations

Potentiometry Potentiometric ConstantPotentiometry(E)

Potentiometric titrations

Constant current

Coulometric Titrations(Q = It)

Electrogravimetry(wt)

Controlled potentialConst.

electrode ( )

ElectrogravimetryAmperometric i i

Voltammetry[ I f (E) ]

electrode potential

coulometry (Q = ∫ 1 idt

6

titrations[ I = f (E) ](Q = ∫01 idt

Esquema de (Bio)sensor químico 

7

Detectores eletroquímicos

Potenciométricos Amperométricos

Medidas realizadas sem a passagem de corrente

Medidas de corrente elétrica realizadas sob a aplicação passagem de corrente

elétrica

Baseada na mudança de t i l d fí i d

de um potencial elétrico constante

Baseada nas reaçõespotencial da superfície do eletrodo de trabalho

Baseada nas reações oxidação e redução que ocorrem na superfície do

eletrodo

8

eletrodo

2 Fundamental Concepts2. Fundamental Concepts

11

Electrical conduction

Materiais

Isolantes

Eletrô nicos Iô nicos

Condutores

Metais, Óxidos Inorgâ nicos,Polí meros Condutores

Soluç õ es de Eletró litosCristais Dopados

I = dQ/dt

e-

Electronic conductor

12Ionic conductor

Condutores Eletrônicos e Iônicos

Eletrônicos: Obedecem a lei de Ohm (E = IR)E = Diferença de Potencial (volts) devido ao movimento f ç ( )

de elétronsR = Resistência (ohms) do condutor à passagem de Correntep gI = Corrente (amperes)

Iônicos: Obedecem a lei de Ohm para pequenos l d valores de corrente

E Dif d P t i l ( lts) d id i t E = Diferença de Potencial (volts) devido ao movimento de íons

R = Resistência (ohms) do eletrólito à passagem de corrente13

R = Resistência (ohms) do eletrólito à passagem de correnteI = Corrente(amperes)

Reações de Oxi-Redução

Transferência de elétrons de um reagente para outro

2 Ag+ + Cu(s) 2Ag(s) + Cu2+2 Ag + Cu(s) 2Ag(s) + Cu

Esta reação pode ser realizado por dois caminhos fi i t dif tfisicamente diferentes

Caminho 1: Colocar os reagentes em contato diretoCaminho 1: Colocar os reagentes em contato direto

Cuo CuoCu

Ag+ Ag+

A o

14

Ago Cu2+

Célula Eletroquímica

Caminho 2: Separar os reagentes em um arranjo apropriado

e- e-

Eletrodo de Cobre Eletrodo de PrataPonteSalina

(KCl sat.)

[Cu2+] = 1.00 mol/L [Ag+] = 1.00 mol/L

Componentes de uma Célula Eletroquímica

Cu(s) Cu2+ + 2e- Ag+ + e- Ag(s) Ânodo (oxidação) Cátodo (redução)

C p C q• 2 condutores imersos em uma solução contendo eletrólitos (eletrodos)• 1 condutor eletrônico externo para permitir o fluxo de elétrons

15• 1 condutor iônico para evitar o contato direto dos reagentes e permitir o fluxo de íons

Célula Eletroquímica – Movimento de cargas

e-e-

e-

e- e-

e

e e

e- Cu2+ NO3 e-

Oxidação Redução

Cl-

e-

Cu2+SO4

2-

SO42-

-Ag+

Ag+

NO3e-

e-

e

e-

e-

e-AgNO3CuSO4

K+

K+Cl-

Cu2+ NO3e e

Interface Eletrodo/solução Interface Eletrodo/solução

g 34

16Voltaic or Galvanic Cell

17

E ll = E t E + EjEcell Ecat – Ean + Ej

18

Schematic diagram showing the t d d h d l t dstandard hydrogen electrode

19

20Voltaic or Galvanic Cell

21

2 2

22Galvanic Cell: Zno + Cu2+ Cuo + Zn2+

Schematic diagramshowing the saturatedcalomel electrode

Schematic diagram showing a Ag/AgCl electrode.

23

calomel electrode

24

25

26

27

Relationship between the potential of an Fe3+/Fe2+ half cell relativeRelationship between the potential of an Fe3+/Fe2+ half-cell relativeto the reference electrodes. The potential relative to a standardhydrogen electrode is shown in blue, the potential relative to ay g , psaturated silver/silver chloride electrode is shown in red, and thepotential relative to a saturated calomel electrode is shown ingreen.

28

Junction Potentials

Fig Origin of the junction potential between a solution of 0 129

Fig. Origin of the junction potential between a solution of 0.1 M HCl and a solution of 0.01 M HCl.

Potential of Junction

Potencial de Junção Líquida: Origem e Cálculos

Devido a diferença de mobilidade dos íons que transportam pelajunção (ponte salina)junção (ponte salina)Max Planck (Ann. Phys., 39, 161 (1890); 40, 561 (1890)

sendo Zi = número atômico

ti = número de transporte

bilid d iô i ( 2 1V 1)31

µi = mobilidade iônica (cm2 s-1V-1)

Caso 1Caso 1P Henderson (Z Physik Chem 59 118 (1907); 63P. Henderson (Z. Physik. Chem., 59, 118 (1907); 63,325 (1908)): Cálculo da junção líquida entre 2soluções com concentrações diferentes (C1 e C2) massoluções com concentrações diferentes (C1 e C2), masde mesma valência

Ag │ AgCl │ MCl(C1)│ │ MCl(C2) │AgCl │Ag

Ej = 0,0591 (2 t+ -1) log C1/C2

32

Caso 2Caso 2Lewis & Sargent (J. Am. Chem. Soc., 31, 363 (1909)):Cálculo do potencial de junção líquida entre 2eletrólitos univalentes diferentes de mesma

Ag │AgCl │ M Cl(C)│ │ M Cl(C) │AgCl │Ag

concentração com um íon comum

Ag │ AgCl │ M1Cl(C)│ │ M2Cl(C) │AgCl │Ag

Onde Λ é a condutância iônica equivalente a diluiçãoinfinita de cada eletrólitoa de cada e e ó o

34

3. Potentiometric Methods and Electrodes

35

Potentiometric Methods and electrodes: Principles

In potentiometry the potentialp y pof an electrochemical cell ismeasured under staticconditions. Because no currentor only a negligible currentflows through theflows through theelectrochemical cell.

36

Potenciometria

E = E* + 0,059 / n log [ox] / [red] Equação de Nernst

Metallic Indicator Electrodes

1a)Electrodes of the First Kind or Class responds to theactivity of Mn+activity of Mn+

Examples: Ag, Hg, Cu, Pb in contact with Ag+, Hg2+, Cu2+, Pb2+Pb2+

Cu2+(aq) + 2 e Cuo Eo = 0.34 V

ECu2+/Cu = EoCu2+/Cu + 0.0592/2 + log [Cu2+]

Ecell = Eind – Eref + Ej

39

1b) Redox Electrodes

Pt, Au and Pd electrodes

40

Potentiometric Methods: Redox Electrodes

The platinum electrode generally form a monolayer of PtO on its surface that causes

Treatment of platinum electrode

The platinum electrode generally form a monolayer of PtO on its surface that causes

changes in its potential and also delay its response,

Th l d b k d bThe electrode can be attacked by:

a) Oxidizing solutions containing Cl- with formation of PtCl4-;

b) Natural waters or other oxidant solutions with formation of PtO;

c) Cr(II) in acid medium reduces H+ by formation of adsorbed H2 on Pt

Pre-treatment of Pt electrode

a) Mechanical with Al2O3 or CeO2 or diamond powder;

b) Chemical (alcoholic KOH , sulphochromic, aqua regia or Ce(SO4)2 solution) ;

c) Electrolytic: a) Cathodic - remove the oxide layer; b) Anodic - remove adsorbed H2

41

Potentiometric Titration

42

Potentiometric Methods and electrodes: Electron activity and pE

43

Potentiometric Methods and electrodes: Electron activity and pE

Continuing...

44

Potentiometric Methods and electrodes: Principles Electron activity and pE

Table. Standard EMF`s and pEO for some common redox couplesp p

45

Potentiometric Methods: Electrodes of the Second kind

The electrode of the second kind responds to the activity of another specie in

equilibrium with Mn+ (precipitate or stable complexes) Generally this species areequilibrium with Mn+ (precipitate or stable complexes). Generally this species are

anions.

h i l f A l d i l i f A + iEx: the potential of a Ag electrode in a solution of Ag+ is:

This first kind electrode in presence of AgI has the following half-cell equation:

Where:

46

Potentiometric Methods: Electrodes of the Second kind

Another examples: Ag/AgCl(s), Ag/AgBr(s), (Hg/HgY2-).

Where:

47

Potentiometric Methods: Electrodes of the Second kind

Electrode of the second kind: metal-metal oxide electrodes

Ex: Ti, Pb, Nb, Sb, W when passivated chemically responses to H3O+ (pH).

Ex:

Ex:

Potentiometric Methods: Electrodes of the third kind

Electrode of the third kind responds to the activity of another Cation in specific

conditions Some times when an electrode of the second kind is subjected to a redoxconditions. Some times when an electrode of the second kind is subjected to a redox

reaction and another reaction, such as a solubility reaction (involving more than two

reactions) an electrode of the third kind is foundreactions) an electrode of the third kind is found.

Ex:

Where :

Electrode Membranes

50

Origin of a membrane potential

If the smaller ions are able to diffuse through the membrane butthe larger ions cannot, a potential difference will develop betweenthe two solutions. This membrane potential can be observed byintroducing a pair of platinum electrodes.

5151

52

Potentiometric Methods: Membrane potential

A typical potentiometric electrochemical cell equipped with an ion-selectiveelectrode.

The electrochemical cell includes two reference

electrodes: one immersed in the ion-selective

electrode’s internal solution and one in the

sample. The cell potential, therefore, is:Variable

K- Constant

The analyte’s interaction with the membrane generates a membrane potential if

K Constant

53

The analyte s interaction with the membrane generates a membrane potential if

there is a difference in its activity on the membrane’s two sides.

Potentiometric Methods: Glass Selective Electrodes

Fritz Haber (1901): discovered that there is a change in potential across a

glass membrane when its two sides are in solutions of different acidity.g y

Harber & Klemensiewicz (1909): First glass electrode + Nernst equation;Harber & Klemensiewicz (1909): First glass electrode + Nernst equation;

Mc-Innes & Dole (1930): Corning 015 (commercial);

Lengyel & Blum (1934): Na+ electrode;

Nikolsky & Eisenman (1960-1975): Alkaline and alkaline-earth

electrodes;54

electrodes;

Potentiometric Methods: pH Glass Electrode

Reference electrolyteelectrolyte

Inner solution

Sample solution

E1 = Outer potential of the glass membrane (a1);E2 = Asymmetry potential:

solution

glass thickness;

asymmetry;

wearing;E3 = Inner potential of the membrane (a1‘);E = Inner reference electrode potential (a -);

Glass membrane (0.2-0.5 mm)Gel layer (10-4 mm)

55

E4 = Inner reference electrode potential (aCl-);

E5 = Outer reference electrode potential;E6 = Junction potential;

Potentiometric Methods: Glass Selective Electrodes

Schematic representation of the

reactions in a glass membrane

aq = aqueous solution;

sg = surface gel ;g g

g = gel layer;

v = dry glass layerv dry glass layer.

56

Potentiometric Methods: Glass Selective Electrodes

57Schematic representation of the atomic structure of (a) soda silica glass; (b) soda aluminosilica glass

Potentiometric Methods: Combined pH Glass Electrode

Screw Cap

Filling port

Connecting plugConnecting plug

Reference elementLead-off electrode

Reference electrolyte

Diaphragm

58Internal buffer

Membrane

An early Beckman pHmeter

Arnold Beckman59

Arnold Beckman (1934 -1939)

Potentiometric Methods: Glass Selective Electrode

Asymmetry potential and pHmeter calibration

61

Potentiometric Methods: Glass Selective Electrodes

pHmeter calibration Dependence of the factor pre-Nernstian with T

K is found using buffer solutions

62

63

64

65

Potentiometric Methods: Glass Ion-Selective Electrodes

Corning 015 (first commercial) = 22% Na2O, 6% CaO and 72% SiO2. When

immersed in an aqueous solution for 7 h, the outer approximately 10 nm of the

membrane’s surface becomes hydrated, resulting in negatively charged sites, —SiO–.

Na+, serve as counter ions. H+ displace the Na+, giving rise to the membrane’s

selectivity for H+.

Corning 015 obeys the following equation:

Ideal application: a pH range of approximately 0.5 to 9;

At more basic pH levels the glass membrane is more responsive to other cations66

At more basic pH levels the glass membrane is more responsive to other cations,

such as Na+ and K+ (alkaline error).

Potentiometric Methods: Selectivity of Membranes

Most membranes are not selective toward a single analyte. Instead, the membrane

potential is proportional to the concentration of each ion that interacts with thepotential is proportional to the concentration of each ion that interacts with the

membrane’s active sites.

zA and zI = charges of the analyte and the interferent;

KA I = selectivity coefficient.KA,I selectivity coefficient.

(aA)e and (aI)e = activities of analyte and interferent;

67If KA,I is 1.0, the membrane responds equally to the analyte and the interferent;

If a membrane shows good selectivity, KA,I << 1.0.

68

Nikolsky’s equation:Nikolsky s equation:

S l ti id dNAS 11-18 from Na+

Na2O 11% (mol)

Seletividade:pH > 7 KNa,K = 10-3

H 7 K 3 3 10 3Al2O3 18%

SiO2 71%

pH = 7 KNa,K = 3.3.10-3

pH < 7 KNa,H > 1

NAS 27-4 from K+

N O 27% ( l)Seletividade:H+ > Ag+ > K+ = NH + > Na+ >Li+ >> Ca2+Na2O 27% (mol)

Al2O3 4%

H+ > Ag+ > K+ = NH4+ > Na+ >Li+ ....>> Ca2+

70SiO2 69%

Potentiometric Titration

Solid-based graphite-epoxy electrodes for potentiometric measurements of pH and acid-base titration

Graphite-epoxy composite

pH range Slope /mV pH-1

Lifetime /mon (det)

Ref.

40% m/m PbO2 1.0 – 11 -58.7 + 0.3 > 8 (> 1200) 1

30% m/m silica gel 2.0 – 13 -40.5 + 0.4 > 12 (> 6000) 2

30% m/m λ-MnO2 2.0 – 13 -53.6 + 0.5 > 4 (1500) 3

30% m/m Fe2O3 1.7 – 12.5 -39.7 + 0.6 > 6 (2000) 4-6

20% magnesium silicate

1.0 – 12.0 -39.2 + 0.3 > 8 (1500) 7

74Cu/Cu2S film acid-base

titrations-59.0 + 0.5 > 3 (400) 8

Potentiometric Methods: Liquid-Based Selective Electrodes

This class of ISEs uses a hydrophobic membrane containing a liquid organic

complexing agent that reacts selectively with the analyte. Three types of organic

complexing agents have been used:

Cation exchangers;

Anion exchangers;Anion exchangers;

Neutral ionophores.

One example of a liquid-based ion-selective electrode is that for Ca2+, which

l ti b t t d ith th ti h di ( d l)uses a porous plastic membrane saturated with the cation exchanger di-(n-decyl)

phosphate.

75

Potentiometric Methods: Kind of ionic exchangersCationic exchangers:Cationic exchangers:

Calcium di-(n-decyl) phosphate: (Ca[PO2 (CH3(CH2)9O)2];Sodium tetraphenylborate: (NaBØ4);Sodium tetraphenylborate: (NaBØ4);

Anionic exchangers:g

Tricaprylylmethylammonium chloride (Aliquat 336) : CH3N[(CH2)7CH3]3Cl ;

Protonated tertiary amine (tri-n-octylamine);y ( y );

Tri-n-benzylamine;Tetraphenylarsonium chloride ((AsPh4)Cl)Tetraphenylarsonium chloride ((AsPh4)Cl)

Neutral exchangers:

Valinomycin;

Crown ether.

76

Preparation of the ionic pair and polimeric membrane

P ti f th ti t i l

a)Cationic Species:

Preparation of the active material

) p

Ionic Pair

a)Anionic Species:

Ionic Pair

Preparation of the polimeric membrane with the active material

2 10 % ( / ) f i i i2 - 10 % (m/m) of ionic pair;60 – 68 % (m/m) of plasticizer DBP (dibutylphtalate), DOP (dioctylphtalate), or

2-nitrofeniloctileter (o-NPOE);30 % (m/m) PVC;dissolve the mixture in 10 mL THF (tetrahydrofuran)

Potentiometric Methods: Liquid-Based Selective Electrodes

First L-ISE selective to Ca2+ developed by Ross (Science, 156, 1378 (1967).

The membrane is placed at the end of a non-conducting cylindrical tube, and is in

p y ( , , ( )

contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in

di-n-octylphenylphosphonate, which soaks into the porous membrane. The inner

78reservoir contains a standard aqueous solution of Ca2+ and a Ag/AgCl reference

electrode.

79

Potentiometric Methods: PVC membrane electrode

PVC b l t dElectric conection

Silicone rubber

PVC membrane electrodedeveloped by Moody et al(Analyst, 95, 910 (1970).

Glass junction

Ag/AgCl electrode

0.1 mol L-1 CaCl solution

PVC tubePVC tube

PVC membrane

80

Potentiometric Methods: Polymeric membrane

Experimental arrangement for casting PVC membrane

81

82

83

84

Fi t C t d i PVC i l ti l t d

Potentiometric Methods: Polymeric membrane with ionic exchanger

First Coated-wire PVC ion-selective electrode

Coaxial cable

Outer conductor

Inner cond ctorInner insulation

Outer conductor

Inner conductor ParaffinPolymeric membrane

ith i i h85Cattrall, R. W. & Freiser, H.; Anal. Chem.; 43, 1905 (1971).

with ionic exchanger

Coated graphite ion-selective electrode

Coated graphite ion-selective electrode

88

Tubular electrode made of graphite-epoxy with a PVC film

Potentiometric Methods: Tubular electrodeTubular electrode made of graphite-epoxy with a PVC film

P ti f th t b l l t d A li ld ith 8 d 10 f iPreparation of the tubular electrode. A = acrylic mold with 8 and 10 mm of innerand outer diameters, respectively, 7 mm of length, a = 1mm diameter hole. B =adaptation of the central conductor of coaxial cable with welded copper plate. C =

8989mold filled with graphite-epoxy paste. D = view of the electrode showing thechannel where it was deposited the PVC membrane.

Potentiometric Methods: Tubular electrode

l

acrylic mold

graphite-epoxy paste

copper plate

coaxial cable

Details of the tubular electrode based

Shielded Connector

on graphite-epoxy PVC film

A = View of the support; B = View of the tubular electrode holder on the

9090support. 1 = Acrylic holders. 2 = Screws, 3 = rings of silicone rubber, 4 =coaxial cable, 5 = polyethylene tubes (input and output of fluid).

Potentiometric determination of saccharin using coated-carbon rod ISEs and a graphite-epoxy ISE

Toluidine blue O saccharinateNH3C OToluidine blue O-saccharinate Ion-pair: 5:30:65 % m/m ion-pair:DBPh:PVC

SH2N N+(CH3)2 SO2

N-

1 21

2 31

m

2 3

6 cm3

4

2 cm

0 5 cm

5

91Coated-carbon rod ISEs (1 and 2) and graphite-epoxy ISE (3)

0.5 cm

FI-Potentiometric determination of saccharin using a tubular ISE

RE

pH R

WC 6 mm

3 2

1

1 mm

TISEW

coatingS

L6 mm 1 mm

2-3 cm

Schematic diagram of the flow system and tubular ISE of carbon rod: 1) PVC membrane with ionic pair; 2) epoxy resin coating;

3) electric connection.

FIA1.0 x 10-4 – 1.0 x 10-2 mol L-1,

Manual 8.1 x 10-5 – 1.4 x 10-2 mol L-1

DL = 8.0 x 10-5 mol L-1

Slope = 53.1 + 0.4 mV dec-1

DL = 6.3 x 10-5 mol L-1

Slope = 58.9 + 0.9 mV dec-1

92Sampling frequency = 40 h-1Lifetime = 9 months (over 1000

determinations)

Transient potentiometric signals for saccharin determination

1 0 x 10-2 1 0 x 10-4 mol L-11.0 x 10 2 – 1.0 x 10 4 mol L 1,DL= 8.0 x 10-5 mol L-1

Slope= 53.1 + 0.4 mV dec-1

Sampling frequency = 40 h-1

Fatibello-Filho, O.; Aniceto, C. Lab. Rob. and Autom., 11, 234 (1999).

Eletrodo para gases (Severinghaus e Bradley)

9494