if you do not know the crystal water content, you can dry the sodium carbonate in the baking oven at 150 - 200° C for about 1 - 2 hrs. Then it will be anhydrous. Sodium carbonate will not decompose. In my formulation for D-72 there is 3.1 g Metol, 45 g sodium sulfite anhydr., 12 g hydroquinone, 67.5 g sodium carbonate anhydr. and 1.9 g potassium bromide for a total volume of 1 litre.