Question

What is the source of

(a) the junction potential in a glass-reference electrode system?

(b) the boundary potential in a membrane electrode?

(c) the asymmetry potential in a glass-membrane electrode?

(d) the potential of a crystalline membrane electrode used to determine the concentration of ${\mathrm{F}}^{-}$?

Short answer

a) In a glass-reference electrode system, the junction potential is caused by the interaction of two solutions with different concentrations.

b) The difference in potential between the surfaces creates the boundary potential in a membrane electrode.

c) The asymmetric potential in a glass membrane derives from the membrane's imperfection.

d) The interaction of the two surfaces of the membrane with different ion concentrations produces the potential of a crystalline membrane electrode. The concentration difference will result in a potential difference, which can be applied to estimate the concentration of ${\mathrm{F}}^{-}$ions.

Step-by-step solution

Part (a) Step 1: Given information

Determine the source of the junction potential in a glass-reference electrode system.

Part (a) Step 2: Explanation

The interference between two solutions containing two distinct electrolytes or the same electrolyte at different concentrations is known as junction potential.

The interaction of two solutions of electrolytes with different concentrations produces junction potential in the glass-referenece electrode system. A salt bridge connects the two solutions. The potential arries on the other side of the bridge.

Part (b) Step 1: Given information 

Determine the source of the boundary potential in a membrane electrode.

Part (b) Step 2: Explanation

The potential that arises due to the difference between the potential at the surfaces is known as boundary potential. Boundary potential depends on the activity of hydrogen ions in the solution.

The boundary potential is given by an expression as shown below.

$Eb=E1-E2=0.0592\log \frac{a1}{a2}$

Here,

• E1 is the SCE constant.
• E2 is the Ag/AgCl constant.
• $a1$is constant for the analyte.
• $a2$ is the constant inside the reference electrode 2.

Part (c) Step 1: Given information 

Determine the source of the asymmetry potential in a glass-membrane electrode.

Part (c) Step 2: Explanation

A glass membrane electrode is made up of a glass bulb filled with a fixed concentration of hydrochloric acid. It's an ion selective electrode of such type.

The potential arises due to the imperfection of the membrane in glass-membrane electrode is known as asymmetric potential. In other words, it is the difference in strain between the two surfaces of the membrane that is created during the manufacture.

Part (d) Step 1: Given information 

Determine the source of the potential of a crystalline membrane electrode used to determine the concentration of ${\mathrm{F}}^{-}$.

Part (d) Step 2: Explanation

The potential is generated in the crystalline membrane electrode by the interaction of the analyte solution with the internal solution. This will result in a charge on the membrane's surface. This charge also causes a potential difference, which is used to calculate fluoride ion concentration.