◆ Determination of Bulk Density and Tapped Density

The determination of bulk density and tapped density is a method for measuring the bulk density of powdered drugs in a loose state, with corresponding packaging conditions defined accordingly. Loose packing is defined as the state obtained by pouring a powder sample into a container without any consolidation. Tapped packing is defined as the state reached when the container filled with powder is repeatedly dropped from a specified distance at a constant dropping rate until the apparent volume of the sample in the container remains nearly constant.

The bulk density of a powder is the ratio of the mass of an untapped powder sample to its volume, including the contribution of the intergranular void volume. Therefore, the bulk density depends on the density of the powder particles and the spatial arrangement of the particles within the powder bed. Most densities are expressed in grams per milliliter (g/mL), even though the SI unit is kilograms per cubic meter (1 g/mL = 1000 kg/m³), since measurements are performed using graduated cylinders. It may also be expressed in grams per cubic centimeter (g/cm³).

The flow and packing characteristics of a powder depend on sample preparation, handling, and storage — i.e., the state of sample treatment. Particles can pack into a range of volumes; furthermore, the density of the powder bed may change due to even the slightest disturbance. Consequently, the bulk density of a powder is often difficult to measure accurately and reproducibly, and the method used must be stated when reporting results.

The bulk density of a powder is determined by:

Methods 1 and 3 are preferred.

Sufficient powder for the test is passed through a sieve with an aperture of 1.0 mm or greater, breaking up any agglomerates that may have formed during storage if necessary; care must be taken to avoid altering the nature of the material.

Accurately weigh approximately 100 g of the test sample (m) to the nearest 0.1 g, and gently introduce it into a dry 250 mL graduated cylinder (readable to 2 mL) without compaction. Carefully level the powder without compacting it, and read the unsettled apparent volume (V₀) to the nearest graduated unit if necessary.

Calculate the bulk density in grams per milliliter using the formula:

In general, replicate determinations are advisable for establishing this property.

If the powder density is too low or too high such that the unsettled apparent volume of a 100 g test sample is greater than 250 mL or less than 150 mL, a 100 g sample cannot be used. A different mass of powder must therefore be selected such that its unsettled apparent volume is between 150 mL and 250 mL (apparent volume ≥ 60% of the total cylinder volume); the mass of the test sample is specified in the expression of results.

For test samples with an apparent volume between 50 mL and 100 mL, a 100 mL graduated cylinder readable to 1 mL may be used; the cylinder volume is specified in the expression of results.

The apparatus (1) (Figure 3.01-1) consists of an upper funnel fitted with a 1.0 mm sieve. The funnel is mounted on a baffle box containing four glass baffles over which the powder slides and bounces as it passes. A funnel at the base of the baffle box collects the powder and delivers it into a cup mounted directly below. The cup may be either:

Allow excess powder to flow through the apparatus, using a minimum of 25 cm³ of powder for the cylindrical cup and 35 cm³ for the cubic cup. Carefully scrape off excess powder by moving a spatula smoothly across the rim of the cup, holding the spatula vertically to prevent packing or removal of powder from the cup. Remove any material adhering to the sides of the cup, and determine the mass (m) of the powder to the nearest 0.1%.

Calculate the bulk density in grams per milliliter using the formula:

where V₀ is the volume of the cup.

Record the average of three determinations using three separate powder samples.

The apparatus consists of a 100 mL cylindrical stainless steel container, with dimensions as shown in Figure 3.01‑2.

Sufficient powder for the test is passed through a 1.0 mm sieve, breaking up any agglomerates formed during storage if necessary, and allowed to flow freely into the measuring container until it overflows. Carefully scrape off excess powder from the top of the container as described in Method 2.

Determine the mass (m₀) of the powder by subtracting the previously determined mass of the empty measuring container.

Calculate the bulk density (g/mL) using the formula:

Record the average of three determinations using three separate powder samples.