BTablets & CapsulesPowder Density in Solid Dosage Forms

Both active and inactive (excipient) compoundsare used in the manufacture of solid dosage for-mulations, i.e. tablets and capsules. These pow-ders possess many properties or characteristics andexhibit certain behaviors as a result.

Tapped DensityOne important characteristic is tapped bulk densi-ty, or simply tapped (tap) density: that is, the max-imum packing density of a powder (or blend ofpowders) achieved under the influence of well-defined, externally applied forces. The minimumpacked volume thus achieved depends on a num-ber of factors including particle size distribution, truedensity, particle shape and cohesiveness due tosurface forces including moisture. Therefore, thetap density of a material can be used to predictboth its flow properties and its compressibility(see right sidebar as to how). These are just two ofthe many parameters which are important in theoverall tabletting process - which requires thatloose powders be compacted into a durable solidform with the correct mechanical strength, poros-ity and dissolution characteristics – and in capsule-filling performance.

Compressibility Index & Hausner RatioThe two most commonly used measures of the rel-ative importance of interparticulate interactions arethe compressibility index (often referred to asCarr’s Index [1]) and the Hausner ratio [2] (seeright sidebar). “V” and “D” represent powder vol-ume and density respectively, subscript “o”denotes the initial or untapped state and “f” the

final or tapped state. In free-flowing powders theinitial bulk and tapped densities will be more sim-ilar than in poor flowing powders which yieldgreater differences between the two values.

Autotap & Dual AutotapThe Autotap and two-sample Dual Autotap con-form to USP 616 , for Tapped Density (Method II).The measuring cylinder containing the powdersample is dropped a height of 3mm (1/8”) at a rateof approximately 250 drops per minute. BothAutotaps feature a drop counter which can be setto any value from 1 to 9999. An optional lock-outmechanism is available to prevent tampering witha preset number of counts. Quantachrome’sAutotaps therefore can contribute a vital role inthe development and assurance of solid dosageformulations in the pharmaceutical industry.

References[1] R.L. Carr, (1965) Chem Eng. 72, 163-168 [2] H.H. Hausner, (1967) Int J Powder Metall. 3, 7-13[3] F. Podczeck, (1998) in "Particle-particle Adhesion

in Pharmaceutical Powder Handling" Imperial College Press, London, p108

[4] K.H. Ludde and K. Kawakita, (1966) Pharmazie21, 393-403

For more information about measurement capabilitiesand how to submit an evaluation sample, contactQuantachrome Instruments by phone: (561) 731.4999, fax: (561) 732.9888, email: QC.pharma@@ quantachrome.comor visit www.quantachrome.com.

by: Martin A. Thomas Ph.D., Director of Business DevelopmentQuantachrome Instruments (martin.thomas@quantachrome.com)

Part# 07158 | No images/charts/text can be reproduced/used without permission. © 2005 Quantachrome Corporation

Powder & Porous Materials Characterization

PPaarrttiiccllee SSiizzee AAnnaallyyssiiss?? Is the entire size distribution represented in the small scoopful you just putinto your analyzer? Probably not, unless you split the original sample on aRotary Riffler. Reduce variability in your particle size results...make your next sample a representative one.

Ask about Rotary Rifflers from Quantachrome at QQCC ..pphhaarrmmaa@@qquuaannttaacchhrroommee..ccoomm .

www.quantachrome.com

Quan

tachr

ome

Instru

ments

Dua

l Auto

tap.

The usefulness of the two simple ratiosbelow, both based on the decrease inpowder volume during tapping, is in theirability to predict flowability. The lowerthe number, the more free-flowing thepowder. An increase in the value is pro-portional to adhesion and friction proper-ties of a powder [3], including (attractive)triboelectric charge.

Carr’s Index(Compressibility Index)

100*(Vo - Vf)Vo

100*(Do - Df)Do

=

For More Information:Please avail yourself of the easy access to ourexperts regarding powder and porous materialscharacterization. Quantachrome specializesnot only in tap density apparatus but also truedensity pycnometers plus surface area, poresize and water sorption analyzers.

Contact us at: QC.pharma@quantachrome.com or visit uson our website at www.quantachrome.com.

Pow

der D

ensity E

quatio

ns

Hausner RatioVo

Vf=

Df

Do

The dynamics of the compression ofthe powder mass can be further eval-uated by using the:

Kawakita Equation [4]

Nc =

Na +

1ab

N = number of taps

c =

a = Carr’s Index (Compressibility Index)b = constant related to cohesiveness

and shear strength

Vo - VVo

( (=

=

By plotting vs N, the parameters aand b can be evaluated graphically.

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