This standard specifies a method for the determination of free water in calcium phosphoric acid using the vacuum oven method. The scope of application includes the determination of free water in calcium phosphoric acid. The principle is to subject a sample of calcium phosphoric acid to periodic decompression drying at a certain temperature and vacuum degree to represent free water in terms of weight loss. The required equipment includes an electrothermal thermostatic Vacuum oven (vacuum oven), a vacuum pump and a weighingbottle. The operation steps include weighing the sample, placing it in a pre-dried and constant weight weighingbottle, placing it in a vacuum oven under specified conditions for drying, and then chilling down and weighing after taking it out.