Ion selective electrodes Flashcards
What is ion selective electrodes
technique used to measure the ACTIVITY of a SPECIFIC ion in solution
- May be simple ions or ionic/ionisable drugs
What are the pharmaceutical application of ISE
Measure ion in body fluid to monitor health and test for dehydration in athletes or CF
More specialised ion selective electrodes are being developed to look for drugs
How do ISE’s work
It is based on conductivity:
- 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
The ability of a metal or ion to give or receive electrons is know as an electrode potential
what does ISE measure
An Ion Selective Electrode the potential of a specific ion in solution- eg pH electrode is for hydrogen ions
The potential difference between the two electrodes will depend upon the activity of the specific ion in solution
Measured against refence table
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
What is the difference between activity and concentration
Activity is a measure of the amount of available ions not the total amount of ions (its concentration).
How does a pH electrode work
a special sensitive glass membrane which allows the passage of H+ ion, but no other ions
they diffuse through the membrane until an equilibrium is reached
How do electrode become selective
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
>All slolid state
>Polymer based ion selective: allows certain shape or charge molecule thriugh
How are ISE’s calibrated
immerse the electrodes in a series of solutions of known concentration/activity
Then probe an unknown solution and use your plot to determine activity.
The graph curves off at high and low levels of activity
What are the limitations to 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
What is a selectivity co-efficient
Unfortunately ISEs are not completely ion selective –they are sensitive to other ions to some extent
Selectivity coefficients (K) define the ability for the ISE to distinguish between different ions
if K(IJ) = 0.001/log k= -3 then the ion selective electrode is 1000 x more sensitive to I (what we want to analyse) than J (the interfering ion)
more negative the log K the greater the selectivity
What are some recent developments of ISE
Direct potentometry of sweat electrolytes
>Hydration of athletes – improving performances since dehydraton can undermine both mental and physical performance
Clozapine:
