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A saturated solution of potassium nitrate (KNO3) is commonly chosen as the electrolyte in a salt bridge. The principal reason for this choice is that

1. the mobility of K+ exceeds that of NO3-
2. the mobility of NO3- exceeds that of K+
3. K+ and NO3- have almost equal ionic mobilities
4. KNO3 dissolves very readily in water

Correct answer: K+ and NO3- have almost equal ionic mobilities

Solution

The salt-bridge ions must carry charge across the boundaries without setting up an extra potential of their own. This requires that the cation and anion have nearly identical transport numbers (mobilities). For KNO3, the mobilities of K+ and NO3- are very close, so the liquid junction potential is minimized. High solubility helps maintain a high ion concentration but is not the fundamental reason.

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