Instrumental analysis

Prof. dr hab. Agata Michalska - Maksymiuk

Prof. dr hab. Krzysztof Maksymiuk

Prof. dr hab. Ewa Bulska

Electrochemistry

Absorption/ Emission techniques

Instrumental analysis/ Analiza instrumentalna Aparatus signal dependent analysis

Chemical analysis Procedures leading to determination of chemical quantitative and/or qualitative composition of sample

Analytical signal Signal recorded for analytical system, signal that is different from noise and can be used to determination/ quantification of component.

Classical approach Instrumental approach – potentiometry

The choice of the method is dependent on :

time allowed for analysis , required precision of analysis nature of analyte (what/ what concentration range?) other sample constituents (matrix effect) standards/ law costs …

Analytical signal: potentiometry – change of activity

other techniques – change of concentration

(e.g.: Na+, K+, Ca2+,Cl- accessible range = physiological range)

Current techniques Open circuit techniques

Potentiometry (classical) Voltammetry Amperometry

Equilibrium measurment Non-equilibrum measurement

Free ions only (in the presence of ions in complexess ect)

Free ions + ions forming labile complexes

Signal proportional to analyte concentration

Signal proportional to logarithm of activity of analyte

Sensitivity and detection limit can be altered through change of parameters

Bio-medical applications

Potentiometric methods (open-circuit)

Measurement of potential as a function of:

• solution redox potential • activity of ions in solution

Application mode: • direct potentiometry

• titration

• electrochemical detection

Potentials:

Redox

Membrane (Donnan)

Diffusion

Signal

Signal

Trouble

Equilibrium

Non-equlibrium

Reference electrode

Indicator electrode

Voltmeter

Membrane potential – Donnan potential

Equlibrium between solution and ion-exchanging membrane

membrane solution

M+

M+

M+ M+

A- A-

A-

~X-

~X- ~X-

Mobile ions: M+, A-, Immobile ions (can not leave the membrane): ~X-

Equlibrium M+

m

sDsm

ssmm

ssmm

F

RT

FRTFRT

FRTFRT

][M

]M[ln

]Mln[]Mln[

]Mln[]Mln[

~~

0(s) M

0(m) M

0(s) M

0(m) M

(s) M(m) M

Difussion potential

M+ M+

M+

M+

A- A-

A-

A-

c1 > c2

+ -

2

1lnc

ctt

F

RTLJ

Two electrolyte of different quantitative and/or qualitative composition in contact;

Henderson approximation

pi,i2i

qi,i2i

pi,qi,i2i

pi,qi,iiLJ

cuz

cuzln

)c(cuz

)c(cuz

F

RTΔΦ

Inert electrode

Example: Pt, Au, glassy carbon – GC

[Red]

[Ox]log

nF

RT2.303EE 0

Mn

Oxn+ + ne- = Red

Interferences: other redox system present in solution

Potentiometric redox electrode

Measurement of solution redox potential Redox titration

Applications Indicator electrodes

Sample solution

i+

i+

i+

Membrane

i+

i+ i+

-

-

Membrane potential

roz

mem

mem][I

][IlnRTΔΦ

i

0

ia log

nF

RTEE

'

i

0

i a logsEE

Potentiometric responses

K+/Cl- 59.2 mV

Ca2+/SO42- 29.6 mV

s – slope of potentiometric dependence

Selectivity coeffcient

j

i

z

z

'

jij

pot

ij

'

i

0

i)(aKalogsEE

Nikolsky – Eisenman equation Kijpot – selectivity

coeffcient

Selectivity (good) for „j” log Ki,jpot << 0

log Ki,jpot > 0 jon „j” is strong interferent

disturbs in „i” determination

Sample solution

i+

i+

i+

Membrane

i+

i+ i+

-

-

j+ j+

Detection limit

Typically 10-6 mol/dm3

Membrane electrodes (classification according to the memrbane material): • solid membranes (z membranami stałymi),

• glass membranes (z membranami szklanymi),

• plastic, polymeric membanes containing ionophore and ion-exchanger (z membranami zawierającymi jonofor i wymieniacz jonowy (w matrycy polimerowej))

Membrane material: • insoluble in water;

• fast and equlibrium ion-exchange process on the membrane solution interface;

• ionic conductivity of membrane material, sufficiently high impedance

Solid membranes

Silver salts – low resistivity, High resisitivity - LaF3 - 106 – 108 Ω doping – e.g. Eu(II) added to LaF3

Making Ag/AgCl electrode

Ag

Roztwór FeCl3

Ag Ag/ AgCl

Sensitive to Ag+ / Cl-

Responses recorded in solutions : () Ag+; () Cl- Ks0= [Ag+][Cl-]=10-9.8

S =[Ag+]= [Cl-]= 1.23.10-5 M

Ag+

Cl-

• membrane solubility

• release of ions due to side reactions

W. Morf (ed) Principles of ISEs and membrane transport

AgCl + I- = AgI + Cl-

AgCl + 2 NH3 = Ag(NH3)2+ + Cl-

Ag ׀ AgCl, KCl

KCl : 3.8 (saturated), 1.0 or 0.1 mol/dm3 constant activity of Cl-.

Separation from the sample with liquid junction → diffusion potential.

Low temperature effect.

Interferences:

• ions forming insoluble salts with Ag+

(e.g.: Br-, I-, S2-),

• proteins,

• lilgands complexing Ag+ ions (e.g.: CN-, SCN-).

Ag/AgCl electrode of constant potential

Reference electrode =

Indicator electrdoe of constant potential

E ~ log aCl- (próbki) aCl- = const E = const

Body

Internal

solution

Internal reference

electrode

Membrane: LaF3

membrane: LaF3 + Eu(II)

Internal solution:

0.1 mol/dm3 NaF + 0.1 mol/dm3 NaCl

Internal reference electrode Ag/AgCl

Linear response range: 1 – 10-6 mol/dm3 F-

(typically in acetic buffer)

Interferent: OH-

Fluoride selective electrode

Resistivity 30 MΩ to 600 MΩ Thickness 0.03 – 0.1 mm

Typically 50 µm

Glass membrane H+, Na+, Ag+

Glass electrode

PCV – since 1970

Composition of typical membrane,

% w/w

Plasticizer

PVC

Ionophore

Ion-exchanger

Ionophore

Ion-exchanger

Plasticizer

PVC

Cost of membrane %

Obudowa

Roztwór wewnętrzny

Wewnętrzna elektroda odniesienia

Membrana

ISEs

• Leackage risk

• Required vertical position

• Difficult miniaturization

R. W. Cattrall, H. Freiser, Anal. Chem. 43 (1971) 1905.

Pt

Pt

Shielding

Membrane

Simple construction

Low reproductibility of

potentials

T. Sokalski, A. Ceresa, T. Zwickl, E. Bakker, E. Pretsch, J. Am. Chem. Soc. 119 (1997) 11347

Lowering of ISEs detection limit

Methods of detection limit lowering

Bakker E., Pretsch E. Trends in Anal. Chem. 20 (2001) 11

Constant and low activity of primary ions at the back side of the membrane

Potentiometric trace analysis

Pb2+

A. Ceresa, E. Bakker, B. Hattendorf, D. Günther, E. Pretsch, Anal. Chem. 73 (2001) 343.