Equilibrium jar method

Equilibrium Jar Method, Colorimetric Determination of Formaldehyde Using Chromotropic Acid Reagent for Product Testing - GPAM 203.6" Georgia Pacific Corporation, 1979. 

Isopiestic determination is one of the most commonly used methods for measuring food aw. In this method a sample of known mass is stored in a closed chamber and allowed to reach equilibrium with an atmosphere of known ERH (or equilibrate with a standard of known aw). In the first protocol (see Basic Protocol), a standard salt solution, for which aw is well established, is used to control this atmosphere. The aw of the sample is then determined by equilibration with the resulting atmosphere. In the second protocol (see Alternate Protocol), the isopiestic determination of aw is accomplished by equilibration of the sample with a reference material, for which the relationship between water content and aw is known. The condition of equilibrium is determined by reweighing the sample at intervals until constant mass is reached. The moisture content of the sample is then determined either directly or by calculation from the reference material s original moisture content and change in mass. Unsaturated salt solutions of known ERH can also be used to equilibrate the samples however, this requires estimation of the ERH of the jars at the end of the equilibration by measuring the exact concentration of the salt solution, which may be tedious. 

For the measurements here, a 1.27 cm diameter probe with a hemispherical cap was used to indent 50g of polymer gel cured in a 5cm diameter glass jar on a Texture Technologies TA.XT Plus Texture Analyzer. Figure 17 shows results for four lightly crosslinked polymer gels of varying equilibrium modulus. As with other characterization methods we have discussed, the measured hardness is strongly dependent on the measurement speed. 

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