INTRODUCTION
The density of particles, powders, and compacts is an important property affecting the performance and function of many pharmaceutical materials. By definition, all density measurements involve the measurement of mass and volume. Mass is determined with an analytical balance and the key to obtaining reliable density values is in the accuracy and precision of measuring volume.
True density is the density of the solid material excluding the volume of any open and closed pores. Depending on the molecular arrangement of the material, the true density can equal the theoretical density of the material and therefore be indicative of how close the material is to a crystalline state or the proportions of a binary mixture. True density measurements can be preformed on APIs, excipients, blends, and monolithic samples such as tablets.C

PRINCIPLE
True density of a powder can be found out by liquid displacement method (or) helium densitometer method. In the present experiment liquid displacement method is used. The principle in liquid displacement method is the volume of liquid displaced by the powder is equal to true volume of the powder. One condition is that the solid powder should be insoluble in the liquid and should be denser than the liquid used in liquid displacement method.
True volume is the actual volume occupied by the powder particles. It does not include inter and intra particle spaces. The true volume of a powder particle depends on type of atoms in the molecule, arrangement of atoms in the molecule and arrangement of molecules in the substance. True density of a powder is used for qualitative analysis and to calculate porosity of powders.

True density = M/TV = Weight of powder/ Volume of liquid displaced by powder
Four important factors in the determination of true density are:

1. The pore size of a solid substance
   
2. The permeability of the solid substance
   
3. The molecular size of the penetrating medium
   
4. The possibility of chemical or physical interaction between the solid substance and the penetrating medium.