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What is moisture content?
Moisture content is usually expressed as the percentage of water mass in a solid, liquid or gas.
When the sample is a gas or partially liquid, it can also be expressed as a percentage of water mass versus
sample volume. Furthermore, for gases, moisture content can also be referred to as hygroscopic moisture or
humidity.
Since moisture content can be classified in various ways and with various names depending on the state of
the sample, the evaluation and handling of measurement data requires special care.
Moisture mechanically adhered to a material’s (sample’s) surface can be called adhesive moisture, free water
or hygroscopic moisture. Furthermore, under certain conditions (such as pressure, temperature, volume, etc.),
moisture absorbed by a material can be called absorbed water or equilibrium moisture content. Finally, water
chemically bonded to a material at the molecular level is referred to as water of crystallization or hydrated
water, while this moisture is referred to as bonding moisture or combined water.
Note
Sodium tartrate dihydrate has theoretically known moisture and is a typical standard sample for the
measurement of moisture content using a moisture analyzer. It is a by-product of the production and
preservation of alcohols such as wine and has been used by people for ages.
Using the classifications above, this material is a hydrate with water of crystallization. Sodium tartrate
(molecular formula: Na
2
C
4
H
4
O
6
, rational formula [-CH(OH)COONa]
2
; molecular mass: 194.0517) and two
water molecules (2H
2
O, molecular mass 36.03056) combine chemically to form sodium tartrate dihydrate
(molecular formula: Na
2
C
4
H
4
O
.
2H
2
O, rational formula [H
2
O.CH(OH)COONa]
2
/ molecular mass 230.0823).
Heating sodium tartrate dihydrate releases the two water molecules and changes the material to sodium
tartrate (
anhydrate)
.
Sodium tartrate dihydrate’s melting point is 150
℃
. At room temperature, it is stable and does not release the
water of crystallization in its molecules. When the temperature rises to 200
℃,
intermolecular bonds other than
water of crystallization start break. Accordingly, when using sodium tartrate dihydrate as a test sample for a
measurement with a heated-air moisture analyzer, the heating temperature should be greater than 150
℃
but
less than 200
℃
to ensure that only water of crystallization is evaporated.
Therefore, the moisture percentage (moisture content) of sodium tartrate dihydrate can be theoretically
obtained from the ratio of the molecular weight of the two intermolecular water molecules and the sodium
tartrate dihydrate, as shown in the formula below.
How a moisture analyzer measures moisture content
Moisture content can be measured by various methods, including drying, the Karl Fischer method, the
dielectric method, the infrared absorption method, a neutron analyzer and the crystal oscillation method. Of
these methods, heating and drying and the Karl Fischer method are most frequently used in laboratories,
while the infrared absorption method and dielectric method are mainly used in processing.
In the heating method, the sample is heated for a period of time at or over the sample’s transpiration
temperature to dry the sample to evaporate its water. The moisture content is acquired as the reduction in
A. Measurement
1. Moisture content
2. Method of measurement
■
Basics
(36.03056/230.0823) x 100
≒
15.66%