Water activity measurement in solvent- based
toothpastes with glycerine preservative
Water activity is a useful quality control parameter in both water and
solvent-based toothpaste products. However, with the market leaning
towards greater production of solvent based, low aW toothpastes, the
glycerine preservative can be a barrier to obtaining the most reliable
readings unless the correct meter, with filtering and resilience to nonaqueous volatiles is used. Only the unique electrolytic principle of the
Novasina sensor system has these benefits. Additionally, a consistent
and well-defined standard operating procedure needs to be employed to
ensure a consistent test method and regular regime of calibration and
maintenance of the water activity meter.
THE BENEFITS OF THE ELECTROLYTIC MEASURING PRINCIPLE:
Resistive electrolyte cell
The Novasina LabMaster ‘Toothpaste’ is
specially configured to manage the high levels of
solvent used in modern low- range aw toothpastes.
Capacitive measurement
Dew point mirror
Filtration of non-aqueous volatiles
The problems associated with water activity testing in low-range aw toothpastes are
that many contain non-aqueous additives, such as glycerine. Solvents can cause
sensor damage or give erroneous readings, especially if a dew point type of water
activity meter is employed because the solvents will normally reach dew point
temperature before the available water (equilibrium relative humidity). The
Novasina electrolytic type sensor with specialist alcohol and chemical filters should
be used to protect the sensor element from exposure to solvents and this is often
achieved using a combination of either activated carbon fibres or silver manganese
powder that absorb any non-aqueous volatiles as they evaporate into the air head
space above the sample.
Sensor filter configuration of the LabMaster
Calibration salt capsules used with the LabMaster
Sensor
eVALC filter
eVC21 filter
mesh
retainer clip
Sample
Consistency of sample preparation & test method
Samples should be prepared in a consistent way and homogenised where
necessary. A standardised test procedure (SOP or similar) is beneficial to achieve
a consistent test method. Most solvent-based toothpaste products do not require
any special sample preparation as the material is already fully homogenised, for
presentation to the aW meter. Equilibrium times are normally 10 – 30 minutes and
a stability level setting of around 5 minutes per maximum drift of 0.001 aw is
suitable. Regular calibration intervals with acceptable tolerance criteria should be
set within the SOP. Normally a tolerance of +/-1% erh (0.01 aW) is achievable.
Saturated salt capsules can be used and have the ability to be individually
traceable calibrated against primary standards to ISO 17025 methods.
Lab no. 0669
Novatron Scientific Ltd, Novatron House, 46 London Road,
Horsham, West Sussex RH12 1AY
Tel: +44 (0)1403 754416 Fax: +44 (0)1403 754480
email: [email protected] website: www.novatron.co.uk