Dilution of samples is necessary for two reasons:
●
The concentration of the parameter under investi-
gation is too high, i.e. it lies outside the measuring
range.
●
Other substances contained in the sample inter-
fere with the determination (matrix interference);
false-high or false-low results may ensue.
The following auxiliaries are absolute prerequisites
for the dilution of the sample:
●
Volumetric flasks of varying sizes
(e. g. 50, 100 and 200 ml)
●
Positive-displacement pipette
●
Distilled or DI water
Only dilutions carried out with these auxiliary pro-
ducts are of sufficient reliability in the area of trace
analysis, to which photometry belongs (for the sim-
plified procedure see below).
An important aspect here is that once the volumetric
flask has been filled up to the mark with distilled
water the flask is closed and the contents are
thoroughly mixed.
The
dilution factor (D
F
)
resulting from the dilution
procedure is calculated as follows:
D
F
=
Final volume
(total volume)
Initial volume
(sample volume)
The analytical result is subsequently multiplied by the
dilution factor.
A calculation can be dispensed with when the dilu-
tion is programmed into the photometer. The
dilution
number
(see the table at the right) is entered and
the measurement value is subsequently calculated
correctly and immediately displayed (for settings
see Function description, “Method parameters:
Dilution”).
All dilutions should be made in such a way that the
measurement value lies in the middle of the measur-
ing range. As a rule, the dilution factor should never
be greater than 100. In the event that yet larger dilu-
tions become necessary all the same, then this must
be done in two separate steps.
Example:
Step 1:
Make up 2 ml of sample to 200 ml
with destilled water;
D
F
= 100, dilution number 1+ 99
Step 2:
Take 5 ml of the above solution and
make up to 100 ml;
D
F
= 20, dilution number 1+19
The dilution factor for the total dilution is calcu-
lated by multiplying the individual dilutions:
D
F total
= D
F1
x
D
F2
= 100
x
20 = 2000,
dilution number 1+1999
Simplified procedure
Dilutions up to 1:10 can also be prepared without
volumetric flasks in a glass beaker, measuring the
volumes of the sample and the dilution water using a
previously calibrated positive-displacement pipette
(see table below for instructions).
VIII
3. Sample Preparation
3.3 Dilution
Desired
Volume of
Volume of
Dilution Dilution
dilution
sample
dist. water
factor
number
in ml
in ml
1: 2
5
5
2
1 + 1
1: 3
5
10
3
1 + 2
1: 4
2
6
4
1 + 3
1: 5
2
8
5
1 + 4
1:10
1
9
10
1 + 9
3.4 Filtration
Strongly turbid samples require pretreatment before
they can determined in a photometer, since the effect
of turbidity can result in considerable variations in
the measurement values and in false-high readings.
Care must be taken here to ensure that the sub-
stance to be determined is not contained in the sus-
pended material, in which case a sample decompo-
sition must be carried out.
Compounds that always occur in dissolved form (for
example ammonium, nitrate, nitrite, chlorine, chlo-
ride, cyanide, fluoride, orthophosphate, and sulfate)
permit a previous filtration, even when the sample
solution is strongly turbid.
Weak turbidity is eliminated by the
automatic turbi-
dity-correction
feature built into the photometer
(see Function description, “Device set-up / Correction
function”); in such cases it is not necessary to filter
the sample before analysis.
Release 07/03
