Filtration - wtw PhotoLab S12-A Operating Instructions Manual

3 Sample Preparation
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 higher than 100. In the event that yet higher 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 distilled water;
D = 100, dilution number 1+ 99
F
Step 2: Take 5 ml of the above solution and make up to 100 ml;
D = 20, dilution number 1+19
F
The dilution factor for the total dilution is calcu lated by multiplying the
individual dilutions:
D = D D
x
F total F1 F2
Simplified procedure
Dilutions up to 1:10 can also be prepared without volumetric flasks in a
glass bea ker, measuring the volumes of the sample and the dilution water
using a pre viously calibrated positive-displacement pipette (see table for
instructions).
Desired Volume of
dilution sample
[ml]
1:2 5
1:3 5
1:4 2
1:5 2
1:10 1

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 deter mined is not
contained in the sus pended material, in which case a sample decom-
po si tion 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 photo meter (see Function description, "Device set-up/
Correction function"); in such cases it is not necessary to filter the sample
before analysis.
As a measure to distinguish between dissolved and undissolved water-
borne sub stances, the water sample can be filtered through a simple
paper filter. Following the recommendations stated in the refe rence
methods, membrane filters with a pore size of 0.45 µm are required for
fine filtration.
Release 01/2009
= 100 20 = 2000, dilution number 1+1999
x
Volume of
dist. water
[ml]
5
10
6
8
9
Dilution Dilution
factor number
2 1 +1
3 1 +2
4 1 +3
5 1 +4
10 1 +9
photoLab ® Series
11