What Is a Junction Potential

Whenever dissimilar electrolyte solutions are in contact, a voltage difference called the junction potential develops at their interface. This small voltage (usually a few millivolts) is found at each end of a salt bridge connecting two half-cells. The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we usually do not know the contribution of the junction to the measured voltage. 

1 M NaCl solution was placed in contact with a 0.1 M NaN03 solution. Which side of the junction is positive  

Solution Because [Na+] is equal on both sides, there is no net diffusion of Na+ across the junction. However, Cl- diffuses into NaN03, and N03 diffuses into NaCl. The mobility of Cl is greater than that of N03, so the NaCl region will be depleted of Cl-faster than the NaN03 region will be depleted of N03. The NaNOs side will become negative, and the NaCl side will become positive. 

Ion-selective electrodes, discussed in the remainder of this chapter, respond selectively to one ion. These electrodes are fundamentally different from metal electrodes in that ion-selective electrodes do not involve redox processes. The key feature of an ideal ion-selective electrode is a thin membrane capable of binding only the intended ion. 

Because the junction potential is usually unknown, Eeel is uncertain. 

Mobilities of ions are shown in Table 15-2, and several junction potentials are listed in Table 15-3. Because and CP have similar mobilities, junction potentials of a KCI salt bridge are slight. This is why saturated KCI is used in salt bridges. 

1 M NaCl solution is placed in contact with a 0.1M NaN03 solution. The concentration of Na is the same on both sides of the junction, so there is no net diffusion of Na from one side to the other. The mobility of CF is greater dian that of NOJ, so Cr diffuses away from the NaCl side faster than NOJ diffuses away from NaNOs. Which side of the junction will become positive and which will become negative Explain your reasoning. 

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