The Spectroquant
®
test kits can be kept for up to
three years when stored cool and dry. A few tests
have a shorter shelf-life, 18 or 24 months, or must
else be stored in the refrigerator. The exception here
are all COD cell tests, which must be stored pro-
tected from light; then the three-year shelf-life is
guaranteed.
The expiry date is printed on the outer label. The
shelf-life may become shortened when the reagent
bottles are not reclosed tightly after use.
2. Photometric Test Kits
2.2.7 Shelf-life of the Reagents
The taking of samples is the first and
most impor-
tant step
on the way to obtaining the correct ana-
lysis result. Not even the most exact method of
analysis can correct any mistakes made in the taking
of the sample. The objective of the sampling proce-
dure is to gain a sample with a representative com-
position. The most important precondition for
gain-
ing a representative sample
is the identification of
the suitable sampling site. Here it must be borne in
mind that the solution to be investigated can display
varying concentrations in different places at different
times.
In sampling, a distinction is made between manual
and automatic methods. In many cases a true picture
of the average composition of the sample can be
obtained only once several individual samples have
been collected; this can be done manually or with an
automatic sampler.
Clean plastic containers with a volume of 500 or
1000 ml are suitable for collecting samples. They
should be rinsed several times, under vigorously sha-
ken, with the water to be investigated, and then filled
free of air bubbles and immediately closed tightly.
The containers must be protected against the effects
of air and heat and then be forwarded for the further
analytical steps as soon as possible. In exceptional
cases, preservation measures in the form of short-
term refrigeration at + 2 to + 5 °C and chemical
conservation can be taken.
3. Sample Preparation
3.1 Taking Samples
Sample preparation covers all the steps necessary
before the actual analysis can be performed.
Parameter
Preservation
COD
+ 2 to + 5 °C max. 24 h
or –18 °C max. 14 days
N compounds:
analyze immediately, only in
NH
4
-N, NO
3
-N, NO
2
-N
exceptional cases + 2 to + 5 °C
max. 6 h
P compounds:
short-term storage, no
PO
4
-P, P total
preservation; with nitric acid
to ph 1, max. 4 weeks
Heavy metals
short-term storage, no
preservation; with nitric acid
to ph 1, max. 4 weeks
3.2 Preliminary Tests
Correct measurement results can be obtained only
within the measuring range specified for each indivi-
dual parameter. When dealing with sample solutions
of an unknown concentration, it is advisable to
establish whether the sample concentration is indeed
within the specified measuring range, ideally roughly
in the middle of the range.
Preliminary tests enhance the analytical reliability and
make the determination of the necessary dilution
ratios in the case of high concentrations easier.
Merckoquant
®
Test Strips
are very well suited for
preliminary tests.
VII
Release 07/03
