Tintometer Lovibond Photometer Mode D'emploi page 30

Notes
1. Cell cleaning
As many household cleaners (e.g. dishwasher detergent) contain reducing substances, the
subsequent determination of oxidation agents (e.g. chlorine) may show lower results.
In order to rule out this measurements error, we refer users to ISO 7393 / Part 1 and Part 2:
"The glass appliances should be free of chlorine consumption and used exclusively for this
process (determination of free chlorine and total chlorine). Chlorine consumption-free glass
appliances are obtained by placing them in a sodium hypochlorite solution (0.1 g/l) for 1
hour and then rinsing thoroughly with water."
N.B.: As an alternative to the sodium hypochlorite solution, the cell may also be placed in
chlorinated swimming pool water and then thoroughly rinsed with water before use.
2. Preparing the sample
When preparing the sample, the escape of chlorine gases, e.g. by pipetting or shaking,
must be avoided. The analysis must take place immediately after taking the sample. The
DPD colour development is carried out with a pH value of 6.3 - 6.5. The reagent tablets
therefore contain a buffer for the pH value adjustment. Strongly alkaline or acidic water
must, however, be neutralised before the analysis.
3. Turbidities (lead to errors)
The use of the DPD No. 1-tablet in samples with high calcium ion content (and/or high
conductivity) can lead to turbidity of the sample and therefore incorrect measurements. In
this event, the reagent tablet "DPD No. 1 High Calcium" should be used as an alternati-
ve. Even if the turbidity does not occur until after the DPD No. 3-tablet has been added, this
can be prevented by using the "DPD No. 1 High Calcium-tablet".
4. Exceeding of the measuring range
Concentrations above 10 mg/l of chlorine can produce results within the measuring range
up to 0 mg/l. In this event, the water sample must be diluted and the measurement repeated.
Accuracy of the method
The certified limit (lowest to the determined concentration) is 0.05 mg/l.
The basis laid down is a sample volume of 100 ml.
For spectralphotometric analysis of free chlorine with a wavelength of 510 nm the relative standard
deviation out of 5 parallel determinations is given as 0.97% (Lit. 3).
For spectralphotometric analysis of total chlorine with a wavelength of 510 nm the relative standard
deviation out of 5 parallel determination is given as 2.96% (Lit. 3).
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