Ion Selective Electron ISE Flashcards
Potentiometry
Measurement of the electrical potential difference between 2 electrodes that are immersed in electrolyte solutions using a Nerst equation Reference Electrode Measuring Electrode Voltmeter or potentiometer Readout device
Nernst Equation
E = cell potential measured Eo = standard reduction potential N = number of electrons involved in the reaction ared = activity of the reduced species aox = activity of the oxidized species F = faraday (96,487 coulombs) T = absolute temperature R = molar gas constant
Reference Electrodes
Potential is reversible and obeys the Nernst equation
Electrode exhibits a potential that is constant with time
Electrode returns to its original potential after being subjected to small currents
Electrode exhibits little hysteresis or lag with temperature cycling
Ion Selective Electrodes (ISE)
Most common use to measure the electrolytes Only free (unbound) ions are measured using ISE
ISE how is it work?
Electrodes are covered with material that is selective for one ion more than others
Potential: Detectable electrical charge exists between a metal (electrolyte) and its ions in a solution.
ISE advantage
No reagent preparation No standard curve preparation Direct measurement (no dilution) Cost effective (reusable) Fast analysis time Precise measurements Very sensitive Very selective/specific for analyte measured 24 hour state of readiness Easily automated
What are ISEs made of?
Glass Electrode
Solid State (crystal)
Liquid Electrode
Electrode Coated with Enzyme
pH ISE System Requirements:Indicator Electrode
Silver wire coated with AgCl immersed in an internal solution of HCl and placed into a tube containing a special glass with a membrane tip
The membrane tip is only sensitive to hydrogen ions (H+). The glass membranes are manufactured using different complimentary metals: lithium, cesium, lanthanum, barium or aluminum oxides in silicate
When the indicator electrode is placed into the sample, H+ ions move near the tip of the glass causing a potential difference between the sample and the HCl inside the electrode. This is measured as the pH and read by the voltmeter.
Lets look at the glass tip on the indicator electrode: The glass slowly and continually dissolve due to the electromotive force at the glass surface, the result of the continuous exchange of cations. There is no penetration of H+ through the glass. Overtime, the glass tip will eventually need to be replaced – it may take several years.
Although the electrode is highly sensitive for H+ ions, other cations may cause interference, the most common is Sodium ions (Na+).
pH ISE System Requirements: Reference Electrode
Calomel electrode a paste made up of mercury chloride that comes in contact with the liquid. It is important that the reference electrode generate a stable electrical potential. AgCl is another common reference electrode. Reference electrodes must be maintained at a stable temperature and the electrolyte concentration must remain stabile within the solution in order for the electrode to work.
HgCl
Must generate a stable electrical potential
pH ISE System Requirements: Liquid Junctions
Serves as a boundary between 2 different solutions
allows for the electrical connection between the indicator and the reference electrodes. This happens at the tip of both electrodes. Because there are both (+) and (-) ions floating around between 2 different solutions, the junction serves as a boundary between the 2 solutions. The electrical potential produced at the junction must remain minimal when the reference electrode is in solution.
pH ISE System Requirements: Readout Meter
pH of 7.0 = 0 volts for all temperatures
pH ISE System Requirements : Nernst Equation and
Calibration
the equation. used to measure the electromotive force generated by hydrogen ions at the glass tip. Temperature affects this equation, so pH meters are calibrated between 10-60oC.
pH meters must be standardized. Protocol generally follows this algorithm (1) a buffer solution with pH=7 is used to balance the pH meter. (2) a solution that has a different pH replaces the initial buffering solution. If the pH meter doesn’t register the correct pH, it is caught by the slope produced in the Nernst equation.
pH ISE System Requirements: Voltmeter
captures the electromotive forces produced by both electrodes and converts it to volts. At a pH= 7, voltmeter reads at zero volts for all temperatures. Because temperature fluctuations will cause differences in pH readings, an internal buffering solution is used to compensate for those changes.
Commonly used ion-selective electrodes in clinical laboratories.
Sodium (Na+)- Silicate in glass
Lithium (Li+)- Dodecylmethyl-14-crown-4 (ether)
Calcium (Ca++)- Calcium di-(n-decyl)phosphate in- di-(n-octylphenyl)-phosphate
Magnesium (Mg++)- N’N’’N’’’-imin-6,1-hexandiyl-tris-(N-heptyl-N-methyl-malonamide)
Hydrogen (H+)- Neutral H+ carrier tridodecylamine
Chloride (Cl−)- Solvent polymeric membranes
Potassium (K+)- Valinomycin
