Why can't some salts be used in the salt bridge of specific voltaic cells? I had an exam question in reference to a galvanic cell with $\ce{Pb{(s)}}$ as the anode in $\ce{Pb^2+}$ solution and platinum cathode in a solution of  $\ce{Fe^2+}$ and  $\ce{Fe^3+}$:  


  Identify an appropriate electrolyte to use in the salt bridge.


I gave $\ce{KNO3}$ which was correct. However, the marking criteria specified that sulphate and chloride salts were not usable.  

Why are these salts not usable? Is there any way to tell which salts would be usable for other galvanic cells?
The main reason would be that sulphates and chlorides can form precipitates in solution. Potassium nitrate essentially never forms in precipitates in solution by going off the solubility rules. The lead in the cell would form precipitates with the sulphate and chloride, and thus it would be ineffective as a salt bridge.