Application note 47: Determination of True Density
by Dynamic Vapour Sorption

By Majid Naderi, Manaswini Acharya, Armando Garcia and Dan Burnett

The versatility and impact of Dynamic Vapour Sorption (DVS) for the true density measurements of solid powder materials have been investigated. True density measurements on different amounts of Lactose and Avicel (microcrystalline cellulose) samples were successfully determined using DVS and a gas Pycnometer for comparison.

true density

Introduction

True density is a fundamental parameter required for characterising many solid materials and slurries including: monitoring of structural rearrangement and porosity of carbon and ceramics after calcination, monitoring the composition of pharmaceutical active and excipient ingredients, determination of the reproducibility when blending building materials and determination of the crystallinity of plastic films and polymers as well as pigmented coatings

[1-2]. True density has historically been determined by liquid displacement using for example, kerosene or naptha. However, gas rather than liquid, is the preferred medium since it is cleaner, faster and more amenable to automation, and does not require the rigorous thermostatting associated with liquid techniques [3].

Based on the gas displacement technique, the DVS powder density method measures the weight of the sample in two gases of known density. The most accurate results are obtained when the gas is moisture-free, non-reactive and behaves as ideally as possible. For this reason, helium is chosen for most applications. However, other inert gases such as nitrogen may be used as well. The measurement and calibration are fully automated and the internal temperature control of the DVS allows measurements to be performed isothermally, thus making it independent of room temperature fluctuations.

This application note outlines how DVS instruments can be used to measure the true density of different amounts of Lactose (Sigma Aldrich) and Avicel (PH 101, FMC Corporation). The density values obtained from the DVS instruments are compared against those obtained by a gas Pycnometer for the same materials.

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