The basic principle of ion-selective electrode is that the potential developed is due to a fix ion, and not affect (actually very little effected) by the change in concentration of other ions. Now what we need here is a selective membrane which can give me a pass for only certain ions and not to other.
In this electrode, a suitable non-porous membrane separates two solutions, containing similar ions of different concentration, and acts as an electrochemical membrane. The remarkable property of such electrodes is that a potential is developed on either side of membrane. Therefore E b = E2 – E1 where the E’s are developed at the respective membranes.
Can we use the ion-selective electrode in place of ‘Concentration cell’? Actually that is not such a obvious question but what do you think? If we say that the two sides of the membrane is of same ions and the uniform distribution is present then the equation will be,
Eb = 2.303RT/nF * log([C2]/[C1])
Now this is very similar to the next kind of cell we are going to talk so, better hold your thoughts, but before going to next part I think we should dig a bit more deep, to know what is the mechanism the membrane knows the difference in two sides of the membrane, and there are some requirement for this kind of electrode –
- It should not be affected by the action of oxidizing and reducing agents.
- It should have sufficient mechanical strength.
- It should have low electrical resistance.
- It should not be attached by organic solvents.
There is multiple kind of Ion-selective electrode and have similar kind of working principle, so we will discuss the most commonly find
Glass electrode: Ag, AgCl(s)|HCl(0.1 M) |Glass Membrane
These electrodes are responsive selectively towards hydrogen ions. Glasses of multiple variety are used which has higher electrical conductance than ordinary glass is used. They can be used for any given pH.
Mechanism- It consists of a thick walled glass tube with a very thin glass bulb placed at the bottom. The thickness of the bulb is 0.01-.03 mm. Generally, glass is found to have high electrical resistivity.
Special glass containing 22% Na2O, 6% CaO & 72% SiO2 of low melting point this glass can specially sense hydrogen ions. The bulb containing 0.1M HCl and an Ag-AgCl electrode is immersed into a solution.
The glass bulb in the bottom has some specific features. Both in and out are covered with layer of gels. Electrode reactions –
H+ + Na+Gl– < ———- > Na+ + H+Gl–
Occurs in and out but since we already know the concentration of the inner part so, from that we can conclude that how much of that ion concentration is present out. Here also we are measuring the Electrode potential with taking in part as reference.
There are some important points in the mechanism-
- Na+ is allowed to exchange with H+ ions and amount of Na+ replaced is dependent on the hydrogen ions present in the surrounding solution. No ions by pass the bulb.
- The coated wire is connected for conductivity purposes.
Advantages and Limitations-
- It is portable and compact.
- Unlike other systems, the equilibrium is easily attained.
- It gives precise and accurate determination of [H+], even in case of very dilute solutions.
- Unlike the Potentiometric method, a few drops of the solution are enough for measurement.
- It is a stable electrode system and can be used in presence of strong oxidizing and reducing agents.
- It can be used in the presence of viscous biological fluids.
- It can detect and estimate H+ ions in presence of other types of ions.
Glass electrode is not free from limitations, few are –
- The glass membrane is very thin and fragile, and so it has to be handled with at most care.
- Even through care is taken to select the glass of relatively high electrical conductance; it offers fairly high electrical resistance. The current generated is supposed to be of the order of 10-12 A, which ordinary voltmeters cannot measure. Hence, costlier and highly sensitive electronic equipment like VTVM has to be used.
- Fluoride ions in the sample attack the glass surface and alter the composition of the membrane.
Abhishek kumar jha
(Chemistry at Utkarshini)
downlordable file –1-108-electrochemistry-ion-selective-electrode