wtw PhotoLab Spektral Operating Instructions Manual page 6

2. Photometric Test Kits
2.1.2 Spectroquant
The principle behind the reagent tests is that the
reagents necessary for the colour reaction are com-
bined in the form of liquid concentrates or solid-
substance mixtures. In these tests, a few drops of the
respective reagent concentrate are added to, for
2.2 Notes for Practical Use
2.2.1 Measuring Range
The intensity of the colour of a solution, measured as
the absorbance, is proportional to the concentration
of the respective analyte only within a specific range.
This measuring range (effective range) is electro-
nically stored in the photometers for each individual
test kit (see overview table in section 1.3).
Below the specified measuring range, either a
different cell or else another procedure must be
used. The lower limit of the measuring range either
takes the form of nonlinearity of the calibration curve,
as shown in the figure below, or else is given by the
limit of detection. The limit of detection of a given
analytical method is the lowest concentration that
differs from the zero concentration with a defined
probability (e.g. 99%).
The upper limit of the measuring range is charac-
terized by there no longer being any linear correlation
between analyte concentration and absorbance. In
such a case the sample must be diluted accordingly
so that it lies ideally in the middle of the effective
range (least-error measurement).
Concentration
IV
®
Reagent Tests
Art. Method
14752 Ammonium
14558 CT Ammonium
14544 CT Ammonium
14559 CT Ammonium
14828 Chlorine
14557 CT Fluoride
14553 CT Copper
14767 Copper
14551 CT Phenol
14831 Silver
example, 5 ml of sample. This means that there is no
need to dilute the sample, which in turn enhances the
sensitivity of the detection. The making up of a
sample in a volumetric flask to a defined volume usual
in conventional photometry can be dispensed with.
In photometry it is conventional practice to measure
against the reagent blank value. Here the analysis is
carried out "blind", i.e. without any analyte added.
Instead of the sample volume, the corresponding
quantity of distilled or DI water is used. This reagent
blank value is already electronically stored in the
system photometers, meaning that a separate mea-
surement is not necessary. It is possible, however, to
enhance the accuracy of the determination at the
measuring-range limit by measuring against a self-
prepared reagent blank solution (for adjustment see
Function description, "Blank-value correction").
There are, however, also cases in which the colour
intensity of the solution and thus the absorbance
drop off again at very high analyte concentrations.
These exemplary cases are listed in the table below.
The values indicated in the display are correct up to
the concentrations specified in the third column, and
false measuring values are obtained above these
concentrations. In such a case it is necessary to con-
duct a plausibility check by running preliminary tests
using test strips or dilution.
Correct indica-
tion of result up
to sample conc.
25 mg/l
250 mg/l
100 mg/l
5000 mg/l
30 mg/l
4 mg/l
25 mg/l
25 mg/l
100 mg/l
5 mg/l
Colour change
turquoise instead of green
turquoise instead of green
turquoise instead of green
turquoise instead of green
yellow instead of red
brownish-yellow instead of violet
light blue/turquoise instead of blue
light blue/turquoise instead of blue
weakening of colour
no change (flocculation)
Release 05/04