Question

why are oh ions discharged in preference to so4 ions

Answer 1

Actually this happens preferentially. The OH- ions are more easily discharged than the sulfates and nitrates. Anions Discharge chart is as follows:

SO42-

NO3-  

Cl-

Br-

I-

OH-

The ease of discharge increases down the chart and so OH- discharges easily compared to sulfates and nitrate up the chart and hence difficult to discharge. There is possibilty that they can discharge in a molten state but most nitrates start decomposing when molten.

Hope it helps :)