Wash the prawns. De-shell and devein them. Marinate the prawns and keep aside for 10 minutes.
In a mixing bowl, mix the 2 flours (rice flour and all-purpose flour). Add black pepper powder and red chili powder and salt and mix.
Add water and make a thick paste or batter.
Add the baking soda and mix well.
Heat oil in a kadai/pan. Test if the oil is heated by dropping a drop of batter into the oil. If the drop of batter rises and gets fried immediately, it means that the oil is ready for deep-frying.
Coat the marinated prawns with the batter and gently drop them into the heated oil in batches of 4 to 5 prawns depending on the size of the pan.
Gently turn the prawns to the other side when they turn golden and let them get cooked.
Deep fry them for 3 to 4 minutes till they turn brownish.
Take them out of the kadai/pan and drain them on a tissue paper.