Ion Selective Electrodes Flashcards
What is an electrolyte?
IN PHYSIOLOGY the ionized or ionizable constituents of a living cell, blood, or other organic matter. An atom that has lost or gained one or more electrons.
A number of electrolytes play crucial roles in normal metabolism and physiology
Potassium in muscle contraction and acid-base balance
Sodium in water balance
Calcium in bone metabolism
Hydrogen ions in acid-base balance
As well as having direct signalling roles themselves, they may also act as co-factors for enzymes and hormones
Measuring levels of electrolytes can give vital clues to diagnosis, prognosis and help with therapy monitoring
What is the principle of an ISE?
Ion-Selective Electrodes (ISEs) are simple and inexpensive.
There are several types available, including potentiometry, amperometry (the basis of the Clark PO2 electrode), coulometry, A.C. conductivity and capacitance.
These physical properties are measured in order to determine concentration, activity (more or less the same thing) or partial pressure of analytes in clinical biochemical samples.
The devices employ at least two electrodes, one reference, unaffected by solution and measuring electrode.
Ion selective electrodes (I.S.Es.) are potentiometric devices i.e. they are based on null current measurement, using a high impedance volt meter, of differences in electrode potential between a measuring electrode incorporating the ion-selective membrane and a reference electrode such as:
Ag;AgCl / 0.1 M HCl (silver/silver chloride) - used to measure chloride electrodes
Hg;Hg2Cl2 / 3.5 M KCl (Calomel) - used to measure pH
What is the calomel electrode?
Glass electrode is the measuring electrode, containing potassium chloride solution and a silver chloride tip. The right hand side is the reference electrode containing a mercury chloride tip and potassium solution.
What is potentiometry?
Potentiometric methods are based upon measurements of the potential of electrochemical cells in the absence of appreciable currents (an equilibrium measurement, therefore, the Nernst equation is applicable)
The Nernst Equation is the basis of electrochemistry:
When a cell is at equilibrium, ΔE = 0.00 and the expression becomes an equilibrium constant K, which bears the following relationship:
n ΔΕ°
log K = ——–
0.0592
where ΔΕ° is the difference of standard potentials of the half cells involved. A battery containing any voltage is not at equilibrium
All equipment is simple: an indicator electrode, a reference electrode and a potential measuring device
Importance in environmental and medical applications e.g.
Electrolytes Na+, K+, Cl-, Li+
pH
blood gas analysis (O2, CO2), etc.
What is the ion selective membrane and why is it important?
An ion-selective membrane is a multi-layer barrier that doesn’t allow ions to pass across.
BUT they may migrate into the membrane, thus establishing a charge separation termed a Membrane Potential (uses ion selective membranes sound 1 micron thick - not specific for one analyte, but SELECTIVE with reduced interference).
How does the electrode work?
If its at equilibrium nothing is changing, so we measure at a point when it is changing until it reaches equilibrim so we can measure how hard it is to reach eg. On the graph, this is the section where there is most change (nearly vertical section). Positive charges are absorbed into membrane, a powerful force tries to restore equilibrium by neutralising - the charge needed to restore neutrality is proportional to the amount the barge changed by - measuring maximum current flow to restore equilibrium. Then the flow stops.
What is the glass pH electrode?
The most essential component of a pH electrode is a special, sensitive glass membrane that permits the passage of hydrogen ions, but no other ionic species
When the electrode is immersed in a test solution containing hydrogen ions (H+) the external ions diffuse through the membrane until equilibrium is reached between the external and internal concentrations
Thus there is a build up of charge on the inside of the membrane which is proportional to the number of hydrogen ions in the external solution
Silver chloride electrode where the half cells are housed in the same container, no longer two separate electrodes dipped into the solution - can fit into a smaller container. Internal reference solution with a small liquid junction and an ion selective membrane with the silver/silver chloride electrode.
Drive is to make these smaller so less blood is needed.
Charge in the membrane of the silver/silver chloride causes a flow of electrons to balance the charge. This is measured until it reaches equilibrium. The composition of the membrane is therefore vital for determining whether the electrode is selective.
The membrane composition is the crucial element in determining the selectivity of the electrode towards a particular ion.
Try to be selective only to one ion, but specificity is difficult to achieve.
Note: all elements have an electric chemical potential - the basis for the choice of material that goes into the electrode. It is this background potential difference that we overlay the effect of the electrolyte in solution. Metals are highly electrochemical active.
How is the Nernst equation used in ISEs?
The response of I.S.Es. (E-ISE) over their activity range displays behaviour compatible with the Nernst equation i.e. it is proportional to the log of the activity of the ion being measured (log ai)
In equation form this is:
E-ISE = Ei 2.303(RT/ZiF) log a Temperature component
I.e. ΔΕ-ISE = k / zi Δlog(ai)
…Ei = a constant composed of certain fixed potentials, R = the Gas Constant, T = the absolute temperature, Zi = the charge of ion i and F = the Faraday Equivalent = 96,491 coulombs, Δ = change
This assumes that the liquid junction potential is constant and that the I.S.E. responds to only one species of ion
Under ideal conditions a monovalent ion would produce an E.M.F. of 59.16 mV at 25oC for a ten-fold change in activity
It must be noted that the standard voltage given by a reference electrode is only correct if there is no additional voltage supplied by a liquid junction potential formed at the porous plug between the filling solution and the external test solution
Why are liquid junctions a problem with ISEs?
Liquid junction potentials (LJPs) appear when two dissimilar electrolytes come into contact. At this junction, a potential difference will develop as a result of the tendency of the smaller and faster ions to move across the boundary more quickly than those of lower mobility.
These problems are doubled when double junction reference electrodes are used and an additional problem arises when the filling solutions are not equi-tranferrent (if the sizes don’t match up, ie using KCl to measure F). Hence, double junction reference electrodes have a stronger tendency to form liquid junction potentials.
The liquid junction potential factor in the Nernst equation is the sum of all the LJPs present in the system. However, the magnitude and direction of LJPs may not be static during an analysis. Dynamic variations in LJPs can be a major source of potential drift and error in measurements.
How are LJPs avoided?
LJPs are difficult to reproduce, unstable, and seldom known with accuracy; so steps must be taken to minimise them.
One method of minimising LJPs is to use a strong electrolyte solution (eg 4M KCl) as the inner filling solution
This has the following advantages:
- K+ and Cl- ions have nearly equal mobilities and hence form an equi-transferrent solution
- In single junction electrodes, the electrolyte concentration is much higher than that of the sample solution thus ensuring that the major portion of the current is carried by these ions
- There is a small but constant flow of electrolyte out from the electrode thus inhibiting any back-diffusion of sample ions.
Water molecules surround ions, making them bigger. Ability to cross the membrane is governed by mass transfer (size of molecule), for example potassium is very positively charged, so very affected by water - bigger non-specific effect caused by water enveloping it - moves slower than something such as Na. As we cant fix this we try to make it negligible by using a storm electrolyte filling solution. Eg using sodium and magnesium together.
How is ISE selectivity changed?
All about the glass used in the membrane - modifying its composition by empirical observation, can favour an analyte.
By use of a carefully chosen selective membrane a simple pH-type electrode can be modified to become specific for a certain ionic species
Glass membranes were originally used for measuring H+ but at low H+ activities they can become more sensitive to Na+ (the so-called “alkaline error”)
Modification of the composition of the glass enhances this property e.g. Corning produce a glass electrode with composition
What are the problems with ISE selectivity?
Lack of specificity problems.
In contrast to the pH membrane, other ion-selective membranes are not entirely ion-specific and can permit the passage of some of the other ions which may be present in the test solution, thus causing the problem of ionic interference.
The calculation of ionic concentration of a target analyte is more dependent on a precise value for the potential difference (pd) than is the pH value:
5 millivolts measurement error = 0.1 pH units
1 millivolt error = 4% error in mono-valent ion concentration [eg Na+]
1 millivolt error = 8% error in di-valent ion concentration [eg Mg2+]
Thus, when measuring other ions, it is essential to take extra precautions to minimise drift and errors in the pd value
What is the analytical range and sensitivity of ISEs?
Most ISEs have a much lower linear range and higher detection limit than the pH electrode
Many show a curved calibration line in the region 10-5 to 10-7 moles/L (between 100 nmol/L and 10 μmol/L ) and very few can be used to determine concentrations below 1x10-7 moles/L (100 nmol/L)
Thus, for low concentration samples, it may be necessary to construct a calibration graph with several points in order to define the curve more precisely in the non-linear range (the linear portion of the graphcan be narrow - not a broad measuring range. Need a system of sample clean up. For v low samples may need to construct a new calibration bracket to be able to measure with precision).
It is more usual to plot a calibration graph using the ionic concentration with a logarithmic scale rather than on a linear axis.
How are ISE selectivity problems addressed?
Although ideally an I.S.E. should respond to only one species of ion, when ions do interfere with one another eg. Na+ and K+ an expanded version of the Nernst Equation, the Nikolskii-Eisenman Equation describes the situation. This can be used to calculate the required selectivity factors.
Describe a solid state crystal membrane.
Solid- state crystal membranes should be chemically inert, highly insoluble, nonporous and physically strong in thin sections. Ions are able selectively to move into defects in the crystal lattice. The active membrane phase consists of a single crystal or pressed pellets of the appropriate salt eg. LaF3 doped with a trace of Eu3+ for measuring F- in blood and urine
