I had the same problem. I thought about using another compound other than the carbonate. I m studying chemistry atm and theoretically it doesn t make a difference. The only purpose for the carbonate is so that it washes the residual lye sticking to the dmt after freeze precipitation. The carbonate, forms an ionic/polar solution in water. thus the residual NaOH(being ionic it is only soluble in polar solutions) migrates to the carbonate solution and fres itself from the dmt. it doesn't matter whether its a carbonate or a bicarbonate, as long as it is soluble in water and forms an polar solution. have a good day