Ion selective electrodes Flashcards
Define electrolyte
Ionisable constituent of a living cell, blood or other organic matter. Play a crucial role in normal metabolism
- K: muscle contraction and acid-base
- Na: water balance
- Ca: bone metabolism
- H+ in acid-base balance
Also have direct signalling roles or act as cofactors
Methods to measure electrolytes
- ICP- Mass spec
- Ion chromatography
- Atomic absorption spectrometry
- Flame photometry
- Ion-selective electrodes
Principle of ISE (ion selective electrodes)
ISE part of a group of relatively simple and inexpensive analytical tools commonly used.
- ISE exploits the electrochemical potential (ECP). Sometimes called Galvanic potential
- ISE device use at least two half-cell eletrodes with metals of dissimilar ECP in order to function
- Magnitude of electrochemical siganl generated by an ISE device is proportional to analyte concentration in aqueous solution
- ISE is all about flow of electrons and other charged particles, as well as oxidation and reduction
What is the galvanic series and what are some properties?
Standard electrode potential of metals are arranged in the electrochemical series. The more negative the standard electrode potential the greater the tendency of the elements to oxidise.
- More negative=more reactive
What happens to substances at anode and cathode?
Anode - oxidised
Cathode - reduced
How is the result of the ISE presented?
Electrochemical signal vs conc of analyte
- Produces sigmoidal curve
Describe the ISE set up and briefly what happens
Two half cells with a positively charged ion in the solution (breaker).
- Positively charged ions move into the membrane creating an unbalanced electrical potential.
- This is then balanced by drawing electrons from one cell into the other until charge is balanced.
- Flow of electrons is measured as a current and voltage meter
How can ISE selectively modified?
By use of a carefully chosen selective membrane a simple pH-type electrode can be modified to become specific for certain ionic species.
- Glass membranes originally used for H+ conc, but at low levels they become more sensitive to Na+ conc
- Modification of glass composition enhances this property e.g. corning produces a gas electrode with composition: SiO2 72.2%, Na2O 21.4%, CaO 6.4%
- > to impart Na+ selectivity it was changed to: SiO2 71%, Na2O 11%, Al2O3 18%
To what composition was the glass membrane modified to impart Na+ selectivity?
SiO2=x, Na2O=y, Al2O3=z
A. x=72.2, y=21.4 ,z=6.4
B. x=67 , y= 25, z=8
C. x= 80, y= 15, z=10
D. x= 71, y= 11, z=18
D
What are some ISE available?
pH electrode, Ca2+ -, K+-, Na+-, and F-electrodes
Describe the pH probe.
It was composed into one singular entity.
- Glas membrane only permitted H+ ions through
- When electrode was immersed in test solution the containing H+ the external ions diffuse into the membrane until equilibrium is reached between the external and internal conc
- Thus build up of charge on the inside of the membrane is proportional to numbe of H+ in external solution
Describe the nernst equation
The voltage response over their activity range is described by the nernst equation i.e. it is proportional to the log of the conc of the ion being measured
Assumptions of nernst equation.
Assumes that the liquid junction potential is constant and that the ISE responds to only one species of ions
Standard voltage given by a reference electrode is only correct if there is no additional voltage from a liquid junction potential
Problems of ISE: Liquid Junction potential.
Appear when dissimilar electrolytes come into contact. At this junction, a potential difference will develop as a result of the tendency of the smaller faster ions to move across the boundary more quickly than those of lower mobility. - This means that an electochemical gradient will be set up because one constituent moves faster than the other, e.g. Na and Cl: Cl moves faster than Na into water resulting in build up of negative charge causing seperation of charge between membrane. Charge is junction potential
Why must LJP be minimised?
LJPs are difficult to reproduce, unstable, and seldom known with accuracy; so steps must be taken to minimise them.
How are LJP minimised?
Using a strong electrolye solution as the inner filling solution (e.g. 4M KCl).
How is using a strong electrolye solution as the inner filling solution advantageous.
Minimises LJP by: e.g. 4M KCl
- K+ and Cl ions have nearly equal mobilities and hence form an equi-transferrent solution
- In single junction electrodes, the electrolyte conc is much higher than that of the sample solution thus ensuring that the major portion of the current is carried by these ions
- Small but constant flow of electrolyte out from the electrode thus inhibiting any back diffusion of sample ions
How can the LJP be worsened?
When double junction reference electrodes are used and an additonal problem arises when the filling solutions are not equi-transferrent.
- Doubel junction reference electrodes have stronger tendency to form liquid junction potential
- Liquid junction potential facor in nernst equation is the sum of all the LJPs present in the system
- However magnitude and direction of LJP may not be static during an analysis.
Problems with ISE selectivity
Ion selective membrane are not entirely ion-specifc and can permit passage of some other ions, causing ionic interference. Calc of ionic conc of target analyte is more dependent ona precise value for the potential difference than is the pH value
- 5 millivolts measurement error = 0.1pH units
- 1 millivolt error = 4% in monovalent ion conc
- 1 millivolt error = 8% in di-valent ion concentration
Thus when measuring other ions, it is essential to take extra precautions to minimise drigt and errors in the potential difference value
In a monovalent ion concentration what % or error does 1 millivolt error equal?
A. 2
B. 4
C. 6
D. 8
B
In a divalent ion concentration what % or error does 1 millivolt error equal?
A. 2
B. 4
C. 6
D. 8
D
What is ISE analytical range and sensitivity?
Lower linear range and higher detection limit than the pH electrode
- Curved calibration line in region 10-5 to 10-7 moles/L and few can be used to determine conc below 100nm/L
How can low concentration samples be analysed?
A construct of a calibration curve with several points in order to define the curve more precisely in the non-linear range
- On a log scale rather than linear
How is selectivity dealt with?
ISE shoudl respond to only one species of ion, when ions do interfere with one another e.g. Na+ and K+ an expanded version of the nernst equation, the Nikolskii-Eisenmann equation describes this situtation
Basically describes situation when another ion we are not interested in is present
What are some ISEs?
- Solid- state crystal membranes
- Liquid ion-exchange membranes
- Neural carrier liquid membranes
- Gas sensors
- Enzyme electrodes
What is a solid- state crystal membranes?
Which should be chemically inert, highly insoluble, non porous and physically strong in thin sections.
- Ions move into defects in crystal lattice.
- Active membrane phase consist of single crystal or pressed pellets of appropriate salt, eg. LaF3 doped with a trace of Eu3+ for measuring F in blood or urine
What is liquid exchange membranes?
Consist of an insoluble species e.g. fatty acid dissovled in water-miscible solvent usually stabilised by an inert porous matrix such as cellulose acetate e.g. Ca2+ electrode consisting of the Ca salt of an alkyl phosphate dissolved in di-nitrophenyl phosphonate stabilised by PVC
What is neutral carrier liquid membranes?
e.g. K+ selective memebrane consisting of valinomycin dissovled in a nitro-aromatic solvent and supported in an inert matrix such as PVC or silicone rubber. Structures of these ionophores include cyclic polyethers, spherands and hemispherands
What are gas sensors?
Work on the principle that gases such as CO2 and NH3 can selectively diffuse across a membrane and cause a change in pH which can be sensed by a potentiometric pH electrode
What are enzyme electrodes?
Involve enveloping the electrode with an enzyme causing its substrate to react, e.g. urease, which generates NH3 from urea
- Can be potentiometric or amperometric
- Potentiometric depend on enzymatic generation of an ion such as H+ in resposne to the analyte being measured e.g. in urea sensor where several ion-selective electrodes could be used
Clinical applications
- ISE can be part of an electrolyt, blood gas/acid base, or calcium analyser or as a module of a larger profiling instrument.
- Fluoride, Iodide, ammonium are most requested substances
- Total Ca2+ is measured by ISE
- Also lithium is
- Electrolyte in U&E, Na, K, Cl & HCO3
- Na and K are intimately related to H and to the hydration state
What is pseudohyponatraemia?
Occurs when we use a prior dilution step before ISE measurement. Called an indirect ISE measurement
Assumptions of indirect ISE
All plasma/serum has same water content
Calibrators has typical water content
Problesm with lipaemic or proteinaemic sample is aliquoted for diltion.
Note: aliquot means practice of dividing a sample into one or more smaller portions
Assumption that all plasma/serum has same water content does not hold
When lipid or protein is present it excludes water from the sample causing underestimation in indirect ISE
What are ion selective field effect transistor (ISFET)
Voltage driven devices combine the chemical-sensitivite properties of glass membrane with the high impedance converting characteristics of the metal-oxide-semiconductor field effector transistor (MOSFET)
How can ISFET be modified?
ISFET can be used to fabricate enzyme modified FETS
Advantages of ISE
- Simple
- Can be used in whole blood as no chromatic interferences and not affected unduly by solid particles
- Wide dynamic range
- Usually non-destructive analysis
- Reduced chemical hazards over many conventional analytical techniques
- Overall procedure relatively rapid
Disadvantage of ISE
- Difficult to calibrate absolutely
- Problems with other potentials in system especially junction potentials
- Selectively problems with similar ions
- Linearity problems
- Response time of electrodes can be slow
- Electrodes prone to fouling incl. by serum proteins
