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4.3
Procedure instructions when
using tablets
When calculating non-directly
determinable parameters from
individual measured values error
propagation based on the pos‐
sible tolerances of the individual
methods must be considered.
1.
Cleaning the cuvettes Many
domestic cleaning agents con‐
tain reducing substances. Use
of these substances leads to
false low readings when meas‐
uring chlorine/bromine/chlorine
dioxide/ozone. To exclude this
measurement error, the glass‐
ware must not cause any
chlorine loss. Therefore the cuv‐
ettes must be stored in a
sodium hypochlorite solution
(0.1 g/l) for one hour and then
thoroughly rinsed with deionised
water.Volle.
2.
For the relevant specification of
free chlorine and total chlorine
use a separate set of cuvettes
for each determination (see EN
ISO 7393-2, section. 5.3).
3.
When preparing water samples
you must avoid the outgassing
of chlorine/chlorine dioxide, e.g.
by pipetting and shaking. This
applies particularly for the dis‐
solved gases chlorine dioxide
and ozone, especially at tem‐
peratures > 30 °C. You must
carry out the analysis immedi‐
ately after taking the sample.
4.
The DPD colour development
takes place at a pH-value of 6.2
to 6.5. Therefore the reagents
contain a buffer for pH value
adjustment. You must ensure
strongly alkaline or acidic water
samples come within a pH
range between 6 and 7 (using
0.5 mol/l sulphuric acid solution
or 1 mol/l sodium hydroxide sol‐
ution).
5.
Concentrations above
10 mg/l chlorine when using
n
tablets
19 mg/l chlorine dioxide
n
when using tablets
ð may lead to results within
the measuring range down
to 0 mg/l. In this case, you
must dilute the water
sample with water free from
oxidizing agent and repeat
the measurement (plausi‐
bility test).
6.
Turbidity (causes incorrect
measurements): For water sam‐
ples with High Calcium content*
and/or high conductivity* when
[DPD no. 1 tablet] the
using the
result can be the rendering
turbid of the water sample and
consequently incorrect meas‐
Analysis methods
23
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