Carr index explained

C =100 (\rho _{T-\rho} _{B})/(\rho _{T}) , where \rho _{B} is the freely settled bulk density 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 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.^[3] A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability.^[4]

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 empirically, 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.

General Bibliography

Book: Mark Gibson . Pharmaceutical Preformulation and Formulation: A Practical Guide from Candidate Drug Selection to Commercial Dosage Form . CRC Press . Boca Raton . 2001 . 1-57491-120-1.

Notes and References

1. Bowker, Michael I. & P. Heinrich Stahl. 2008. "Preparation of Water-Soluble Compounds through Salt Formation." In Camille Georges Wermuth, ed. The Practice of Medical Chemistry, pp. 747–766. Burlington, MA: Elsevier, p. 756.
2. Podczeck, Fridun & Brian E. Jones, eds. 2007. Pharmaceutical Capsules111.
3. Web site: General Chapters: BULK DENSITY AND TAPPED DENSITY.
4. Book: Kanig, Joseph L. . Lachman, Leon . Lieberman, Herbert A. . The Theory and Practice of Industrial Pharmacy . Lea & Febiger . Philadelphia . 1986 . 3 . 0-8121-0977-5.