1.1 Methods
Notes:
1. Vial cleaning:
As many household cleaners (e.g. dishwasher detergent) contain reducing substances,
the subsequent determination of Chlorine may show lower results. To avoid any
measurement errors, only use glassware free of Chlorine demand.
Preparation: Put all applicable glassware into Sodium hypochlorite solution (0.1 g/l) for
one hour, then rinse all glassware thoroughly with deionised water.
2. For individual testing of free and total Chlorine, the use of different sets of glassware is
recommended (EN ISO 7393-2, 5.3)
3. Preparing the sample:
When preparing the sample, the lost of Chlorine, e.g. by pipetting or shaking,
must be avoided. The analysis must take place immediately after taking the sample.
4. The DPD colour development is carried out at a pH value of 6.2 to 6.5. The reagents
therefore contain a buffer for the pH adjustment.
Strong alkaline or acidic water samples must be adjusted between pH 6 and pH 7
before the reagent is added (use 0.5 mol/l Sulfuric acid resp. 1 mol/l Sodium hydroxide).
5. Exceeding the measuring range:
Concentrations above:
10 mg/l Chlorine using tablets
4 mg/l Chlorine using liquid reagents
2 mg/l using powder packs
can lead to results showing 0 mg/l. In this case, the water sample must be diluted with
water free of Chlorine. 10 ml of the diluted sample should be mixed with the reagent
and the measurement repeated.
6. Turbidity (can lead to errors):
The use of the DPD No. 1 tablet (method 100) in samples with high Calcium ion
contents* and/or high conductivity* can lead to turbidity of the sample and therefore
incorrect measurements. In this case, the reagent tablet DPD No. 1 High Calcium
should be used as an alternative. Even if turbidity does occur after the DPD No. 3
tablet has been added, this can be prevented by using the DPD No. 1 HIGH CALCIUM
tablet.
* it is not possible to give exact values, because the development of turbidity depends
on the nature of the sample.
7. If ??? is displayed at a differentiated test result see page 262.
Oxidizing agents such as Bromine, Ozone etc. interfere as they react in the same way as
Chlorine.
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