DST is primary standard and it is stable up to 150 degree Centigrade. It release free water molecules easily when reacts with KF reagent.DST is easily soluble in methanol. This DST has 15.66% water molecules so we can easily calculate and standardize the KF reagent. We use the factorof DST the compound which has less than 1% of moisture.Water i.e 100% water molecules. But we must use 100% pure water there should absent dissolved salts and anymore. This method is not 100% accurate like DST. So generally We use the factor of water for the compound which has more than 1% of moisture.1water molecule approximately equal to 5mg of KFR.