INTRODUCTION

Density is universally defined as mass per unit volume but it becomes difficult when experimentation is done with the powders which particles with microscopic cracks, internal pores and capillary spaces. So, on the basis of the determination of densities, different types of densities are: Bulk density, Tapped density, Granular density and True density.

Bulk density is defined as the 'Mass' of the powder divided by the 'Bulk Volume' whereas Tapped density is defined as the 'Mass' of the powder divided by The 'Tapped Volume'. The pharmaceutical powders which have low bulk density and larger bulk volume are known to be as 'light' powders and the 'heavy' powders are those having high bulk density but smaller bulk volume. The experimental determination of Bulk ana Tapped density is dependant on the particle size distribution, particle shape and the tendency of the particle to adhere to each other. The particle may pack in such a way that as to leave large gaps between their surfaces, resulting in a light powder or powder of low bulk density. On the other hand, the samller particles may shift between the larger ones to form a heavy powder or powder with high bulk denensity.By studying the bulk and tapped densities of powders it can be utilized to check the uniformity of bulk chemicals, and to determine proper size of containers, mixing apparatus, and capsules for a given mass of powder.

PRINCIPLE

Bulk Density: The bulk density of a powder is the ratio of the mass of an untapped powder sample and its volume including the contribution of the interparticulate void volume. Hence, the bulk density depends on both the density of powder particles and the spatial arrangement of particles in the powder bed. The bulk density is expressed in grams per mL (g/mL) although the international unit is kilograms per cubic meter (1 g/mL = 1000 kg/m³) because the measurements are made using cylinders. It may also be expressed in grams per cubic centimeter (g/cm³). The bulking properties of a powder are dependent upon the preparation, treatment, and storage of the sample, i.e., how it was handled. The particles can be packed to have a range of bulk densities; however, the slightest disturbance of the powder bed may result in a changed bulk density. Thus, the bulk density of a powder is often very difficult to measure with good reproducibility. Bulk density is not an intrinsic property of a material; it can change depending on how the material is handled. For example, a powder poured into a cylinder will have a particular bulk density; if the cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a higher bulk density (tapped density). The bulking properties of a powder are dependent upon the preparation, treatment and storage of the sample, i.e. how it was handled. The particles can be packed to have a range of bulk densities and, moreover, the slightest disturbance of the powder bed may result in a changed bulk density.

Tapped Density: Tapped density is a type of increased bulk density which is attained after mechanical tapping of the container containing a powder sample.After observing the initial powder volume or weight, the measuring cylinder or vessel is mechanically tapped, and volume or weight readings are taken until little further volume or weight change is observed. The mechanical tapping is achieved by raising the cylinder or vessel and allowing it to drop under its own weight. Tapped density of a powder is the ratio of the mass of the powder to the volume occupied by the powder after it has been tapped for a defined period of time. The tapped density of a powder represents its random dense packing. Tapped density values are higher for more regularly shaped particles (ie, spheres), as compared to irregularly shaped particles such as needles.The tapped density of a powder can affect operations critical to solid dosage manufacturing, including bulk storage, powder flow, and compaction.