Standard solution of KNO3 is used to make salt bridge because

For a good salt bridge, the velocities of ions should be the same. If the velocities are not the same, then the ions would not be able to neutralize the charge equally in both half cells. Consequently, an opposite force would be developed that restricts the flow of current. For KNO3 , the velocities of K+. and are nearly the same. Therefore, it is used to make salt bridges.