instrumental analysis -...
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Instrumental analysis
Prof. dr hab. Agata Michalska - Maksymiuk
Prof. dr hab. Krzysztof Maksymiuk
Prof. dr hab. Ewa Bulska
Electrochemistry
Absorption/ Emission techniques
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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
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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 …
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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
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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
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Potentials:
Redox
Membrane (Donnan)
Diffusion
Signal
Signal
Trouble
Equilibrium
Non-equlibrium
Reference electrode
Indicator electrode
Voltmeter
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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
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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
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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
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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
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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+
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Detection limit
Typically 10-6 mol/dm3
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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))
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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
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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-
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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
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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
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Resistivity 30 MΩ to 600 MΩ Thickness 0.03 – 0.1 mm
Typically 50 µm
Glass membrane H+, Na+, Ag+
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Glass electrode
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PCV – since 1970
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Composition of typical membrane,
% w/w
Plasticizer
PVC
Ionophore
Ion-exchanger
Ionophore
Ion-exchanger
Plasticizer
PVC
Cost of membrane %
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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
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T. Sokalski, A. Ceresa, T. Zwickl, E. Bakker, E. Pretsch, J. Am. Chem. Soc. 119 (1997) 11347
Lowering of ISEs detection limit
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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.