Page 14 - Application Handbook - TOC

Sum parameter – Total Organic Carbon
TOC determination in particle-containing
samples - Cellulose test according to EN 1484
SCA-130-106
Depending on their origin, different effluents
can contain substantial amounts of insoluble
solids. These include clearly visible
components that, in some cases, sediment
very quickly. This applies particularly to heavy
particles such as sand grains. Suspended
solids such as fibers or flakes are naturally
less likely to sediment. However, they also
cause a certain inhomogeneity of the sample
matrix.
A variant of TOC determination is the
detection of DOC, Dissolved Organic Carbon.
The wastewater sample is filtered through a
membrane filter with a 0.45 µm pore size and
subsequently measured. In contrast to DOC,
TOC (Total Organic Carbon) determinations
must detect the total organic carbon in a
sample, including the insoluble components.
According to DIN 1484, the suitability of a
TOC measurement system for the analysis of
wastewater samples that contain solid matter
must be tested. This is carried out using the
so-called cellulose test (Appendix C of the
above-mentioned standard).
■ Cellulose test according to DIN EN 1484
The cellulose test is based on an aqueous
cellulose suspension with a carbon content of
100 mg/L. This corresponds to 225 mg
cellulose. The particle size ranges from 20
µm to 100 µm. Homogenization may only be
carried out under stirring. Alternative methods
such as ultrasound can break up the particles
and thereby provide erroneous results. Large
particles in particular have a tendency
tosediment rapidly.
Stirring speed is critical for this reason. Very
slow stirring leads to increased particle
sedimentation. Extremely high stirring speeds
lead to inhomogeneous particle distribution
due to centrifugal force. DIN 38402 part 30,
dealing with sample homogenization, serves
in this context as an excellent reference. A
solution should be stirred in such a way that
the vortex will amount to 10% of the filling
level.
For three consecutive injections, the mean
value must lie within the range of 90 mg C/L
to 110 mg C/L (corresponding to a recovery
of 90 – 110 %). The relative standard
deviation (RSD) may not exceed 10 %.