Specifications

49
Sartorius WDS 400. Selective Detection of Surface Water,
Capillary Water and Water of Crystallization
Water, not moisture
Thermogravimetric methods, such as the
oven drying method, use the weight loss of
a sample to determine the total content of
all volatile components and not, however,
the pure water content. As a rule, the latter
task is performed using electrochemical
techniques that are based on the principle
of coulometry (coulomb = electric charge).
The most commonly known methods are
coulometric Karl Fisher titration for solid
and liquid samples and the phosphorus
pentoxide method for trace analysis of
gases. However, both methods require
complicated equipment; moreover, KF
titration necessitates the use of additional
chemicals in order to perform an analysis.
The WDS 400 Water Detection System from
Sartorius combines these three standard
methods into a high-resolution and easy
procedure for selective detection of water
in solids and pastes.
Get all three in one
The WDS 400 adopts the principle of
convection heating from the oven drying
method in order to drive out the entire
moisture from a sample. A ceramic disc
coated with extremely hygroscopic phos-
phorus pentoxide P2O5 completely absorbs
the water from the resulting gas mixture
and bonds water molecules to phosphoric
acid H3PO4 on the disc surface in a
chemical reaction.
By coulometry, i.e., by an electric current
generated at the ceramic disc, phosphoric
acid is broken down into phosphorus pen-
toxide P2O5, hydrogen H and oxygen O.
Based on Faraday’s law, it is known how
much current is necessary in order to split
off all hydrogen atoms from a chemical
compound. Thus, the WDS 400 uses the
amount of electric current to calculate the
quantity of water driven out of a sample.
Highly accurate and selective
This combination method works so accu-
rately that it is even possible to detect one
single microgram of water. Beyond that,
the WDS 400 enables water fractions to be
differentiated according to surface water,
capillary water and water of crystallization
(the latter is chemically bound water).
Easy operation
All the user has to do is just weigh-in a
sample. The WDS 400 does not require
any complicated handling of detection
reagents, many of which are toxic.
For measurement of the water content,
the user can choose the type of carrier gas,
either nitrogen (Class 5.0) or room air. For
using room air, the WDS 400 has a built-in
pump and a drying unit.
PC Printer
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Balance
Dry
nitrogen
Oven up to 400°C Nitrogen + H
2
OP
2
O
5
H
2
O
cell
Dry nitrogen with
H
2
+ O
2
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Electronics