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Grade: 12
         Q. the liquid junction potential aries due to accumulation of charges at the junction of two solutions as a result of relative speed of ions & thereby developing an electrical double layer ; how does it  results in decrease in emf of cell ?  plz reply ; in book (p . Bahadur) it is given as point but not given any explaination . plz explain
9 years ago

Answers : (1)

879 Points

Dear priya,

Liquid junction potential occurs when two solutions of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one. The rate of diffusion of each ion will be roughly proportional to its speed in an electric field. If anion diffuses more rapidly than the cation, it will diffuse ahead into the dilute solution leaving the later negatively charged and the concentrated solution positively charged. So an electrical double layer of positive and negative charges will be produced at the junction of the two solutions. So at the point of junction, a difference of potential will develop because of the ionic transfer. This potential is called liquid junction potential or diffusion potential. The magnitude of the potential depends on the relative speeds of the ions.

The liquid junction potential can not be measured directly but calculated. The Electromotive force (EMF) of a concentration cell with transference includes the liquid junction potential. Ewithout transference = RT/F . ln(a2/a1) where a1 & a2 are activities of HCl in the two solutions. Ewith transference = t – RT/F . ln (a2/a1) where a2 & a1 are activities of HCl solutions of right and left hand electrodes respectively and t – be transport number of Cl– Liquid Junction potential = Ewith transference– Ewithout transference = (t – 1) RT/F . ln (a2/a1)

The liquid junction potential interferes with the exact measurement of EMF of a chemical cell. So it should be eliminated or at least its effect should be minimized. The general practice to eliminate the liquid junction potential is to place a salt bridge consisting of usually a saturated solution of Potassium chloride (KCl) and Ammonium nitrate (NH4NO3) with Lithium acetate (CH3COOLi) between the two solutions constituting the junction. When such a bridge is used, the ions in the bridge are present in large excess at the junction and they carry almost the whole of the current across the boundary. The efficiency of KCl/NH4NO3 is connected with the fact that in these salts, the transport numbers of anion and cation are the same.


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Askiitians Expert

Sagar Singh

B.Tech, IIT Delhi

9 years ago
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