Hello Sir, why we are using only Disodium Tartarate for the calibration of Karl-Fischer titrator?
Moisture content of disodium tartarate is theoretically fixed. Sodium tartarate dihydrate, is actually Sodium tartarate containing two molecules of water held in crystal structure. On heating these water molecules are released. Since the amount of water is fixed ( calculated 15.66% of mol. wt.), it can be used for checking the karl fischer apparatus.
Also, it is safe, easily available.