The Carr index (Carr's index[1] or Carr's Compressibility Index[2]) is an indicator of the compressibility of a powder. It is named after the scientist Ralph J. Carr, Jr.

The Carr index is calculated by the formula , where is the freely settled bulk density of the powder, and is the tapped bulk density of the powder after "tapping down". It can also be expressed as .

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 .

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.

References

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  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. ^ "General Chapters: BULK DENSITY AND TAPPED DENSITY".
  4. ^ Kanig, Joseph L.; Lachman, Leon; Lieberman, Herbert A. (1986). The Theory and Practice of Industrial Pharmacy (3 ed.). Philadelphia: Lea & Febiger. ISBN 0-8121-0977-5.

General Bibliography

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