X
3. Sample preparation
The manner in which the sample is pretreated en-
ables the three proportions to be distinguished from
each other. This can be illustrated using a copper-
containing wastewater sample as an example.
Decomposition converts the substance to be deter-
mined into an analyzable form. In most cases, de-
composition agents take the form of acids in com-
bination with oxidizing agents; in exceptional cases
(e.g. in the determination of total nitrogen) an alkaline
decomposition is more effective. The type of decom-
position procedure used depends on the analyte to
be determined and the sample matrix.
The ready-to-use sample-decomposition products
Spectroquant
®
Crack Set
10 and 20 are suited for
the preparation of the sample materials for the deter-
minations stated in the table below.
The decomposition processes are carried out in the
thermoreactor
(capacity: 8 /12 decomposition cells)
at 120 °C or, respectively, 100 °C. Details regarding
the heating times and further treatment can be
found in the package inserts contained in the
Spectroquant
®
Crack Set
packs.
In the event that the sample to be analyzed is a high-
ly contaminated material (high proportion of organic
substances) or water-insoluble samples, decomposi-
tion using concentrated acids and other agents is in-
dispensible. Corresponding examples are described
in the
Application Compendium
(obtainable from
your local Merck representative; see Appendix),
a collection of analytical specifications for real
samples.
The necessity for decomposition can be checked
according to the following diagram:
For wastewater with a consistent composition, this
check as a rule need be carried out only once. It is,
however, advisable to check the result periodically.
Filtration
Filtration
Decomposition
Decomposition
Example
Total content
Dissolved proportion
Dissolved proportion
Solid Substances
Cu(OH)
2
Complexes Cu-EDTA Complexes Cu-EDTA
Ions Cu
2+
Ions Cu
2+
Ions Cu
2+
Result A
Result B
Result C
Proportion:
Ionogenic
= C
Complex
= B – C
Solid Substances
= A – B
Total content
= A
Determination of
Sample preparation with
Phosphorus total*
Crack Set 10 / 10 C**
Chromium total*
Crack Set 10 / 10 C
[ = sum of chromate
and chromium(III)]
Metal total
Crack Set 10 / 10 C
[ = sum of free and
complex-bound metal]
Nitrogen total*
Crack Set 20
* The decomposition reagents are already contained in the packs of the
respective cell tests.
** Decomposition cells are included in the pack; empty cells are required for
the decomposition for Crack Sets 10 and 20.
Decomposition
Measurement
Result A
A and B
idential?
No decomposition
necessary
Yes
Procedure
Measurement
Result B
Procedure
No
Decomposition
necessary
Release 07/03
