The Carr index (Carr's index or Carr's Compressibility IndexPodczeck, Fridun & Brian E. Jones, eds. 2007. ''Pharmaceutical Capsules''111.) is an indicator of the

compressibility In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...

of a

powder A powder is a dry solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms ''powder'' and ''granular'' are sometimes used to distinguish se ...

. It is named after the scientist Ralph J. Carr, Jr. The Carr index is calculated by the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

C=100\frac

, where

\rho_B

is the freely settled

bulk density In materials science, bulk density, also called apparent density, is a material property defined as the mass of the many particles of the material divided by the bulk volume. Bulk volume is defined as the total volume the particles occupy, includ ...

of the powder, and

\rho_T

is the tapped bulk density of the powder after "tapping down". It can also be expressed as

C=100(1-\rho_B/\rho_T)

. The Carr index is frequently used in

pharmaceutics Pharmaceutics is the discipline of pharmacy that deals with the process of turning a new chemical entity (NCE) or an existing drug into a medication to be used safely and effectively by patients. The patients could be either humans or animals. Ph ...

as an indication of the compressibility of a powder. In a free-flowing powder, the bulk density and tapped density would be close in value, therefore, the Carr index would be small. On the other hand, in a poor-flowing powder where there are greater interparticle interactions, the difference between the bulk and tapped density observed would be greater, therefore, the Carr index would be larger. A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability. Another way to measure the flow of a powder is the Hausner ratio, which can be expressed as

H=\rho_T/\rho_B

. Both the Hausner ratio and the Carr index are sometimes criticized, despite their relationships to flowability being established

empirical Empirical evidence is evidence obtained through sense experience or experimental procedure. It is of central importance to the sciences and plays a role in various other fields, like epistemology and law. There is no general agreement on how t ...

ly, as not having a strong theoretical basis. Use of these measures persists, however, because the equipment required to perform the analysis is relatively cheap and the technique is easy to learn.

References

General Bibliography

Particulates Pharmaceutics {{pharmacology-stub