Ion selective electrodes Flashcards
A technique used to measure the ACTIVITY of a SPECIFIC ion in solution is known as
Ion selective electrode
What type of ion can be used in an ion selective electrode analytical technique?
Ions may be simple ions such as H+, Ca2+, Na+ etc or can be ionic or ionisable drugs.
State the Pharmaceutical Applications of ion selective electrodes
- Directly measure ions like Ca2+, K+, Cl- in body fluids (Blood, Plasma, Serum, Sweat) – Monitor health e.g.
Test for dehydration in athletes sweat as they workout
Cystic Fibrosis can be diagnosed by analysis of sweat - More specialised ion selective electrodes are being developed to look for drugs (see paper at the end).
Explain How ion selective electrodes work
- Based on electrical conductivity: 2 types
In Metals the electronic current is carried by electrons
In Liquids the electronic current is carried by ions - Electrodes are the interface between the liquid and metal state
- At the interface there is an exchange of electrons in one direction or another (depending on how easily the different materials give or receive electrons).
- The ability of a metal or ion to give or receive electrons is know as an electrode potential
Facts about ISE
An Ion Selective Electrode measures the potential of a specific ion in solution. (The pH electrode is an ISE for the Hydrogen ion.) This potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes will depend upon the activity of the specific ion in solution (related to concentration), therefore allowing us to assay for the ion
State the advantages of ISE
Many applications
Modern ISEs are very sensitive (to ppb 1mg/1L levels)
In some cases can be used rapidly (by simply dipping)
They are unaffected by sample colour or turbidity
They measure activity not concentration
Define Activity with respect to ISE
Activity is a measure of the amount of available ions not the total amount of ions (its concentration).
Facts about pH electrodes
The most well know ISE
Essential component is a special sensitive glass membrane which allows the passage of H+ ion, but no other ions
What happens When the electrode is placed in an solution containing H+ ions,
they diffuse through the membrane until an equilibrium is reached between the external and internal environments affecting the charge inside and leading to a change in voltage
Know the Nernst equation
see slide 9
State facts about the selective electrode
By changing the material at the interface you can change the selectivity of the ion selective electrode to look for ions of your choice
Glass electrodes
All solid state electrodes (inorganic solids e.g.F electrode)
Polymer based ion selective electrodes designed to only allow certain shaped or charged molecules through.
How to calibrate ISE
Looking at the Nernst equation it is clear that a plot of EM vs. In a should be linear.
To Calibrate you can immerse the electrodes in a series of solutions of known concentration/activity
Then probe an unknown solution and use your plot to determine activity.
Calibration graph of ISE
The calibration graph tends to curve off at some point of lower activity when the levels are too low to measure over background ions which may interfere.
The calibration graph will also tends to curve off at high activity
State the Limitations of use of Ion selective electrodes
Although it can have a linear response over 4 to 6 orders of magnitude precision is rarely better than 1%
Electrodes can be fouled by proteins or other organic solutes
Electrodes respond to activity of uncomplexed ion so ligands must be absent or masked in some way if you want to measure concentration.
Interference by other ions
ISEs are not completely ion selective –they are sensitive to other ions to some extent true or false
True
