Thermo Fisher Scientific C101 Colorimeter - Manual (Page 16)

Measurement: Chlorine, Free & Total
C401/301/201Colorimeter
12
Alkalinity:
Greater than 250ppm CaCO
3
may cause interference. To remove this
interference, neutralise a separate sample to pH6-7 using 1N Sulphuric
Acid. Note the volumes used and add the same volume of 1N Sulphuric
Acid to the sample before carrying out the above test procedure. Use a
multiplication factor to correct for the dilution of the sample.
Hardness:
No effect at less than 2000ppm CaCO
3
.
Monochloramine:
Monochloramine will cause a gradual increase in the Free Chlorine result
over time. Take the test result within 1 minute of adding the reagent
sachet to avoid these errors.
Oxidants, including Bromine, Chlorine Dioxide, Iodine and Ozone:
These will interfere with the test method at all levels. It is recommended
that an alternative test method is used if more than any two of these
species are present.
Oxidised Manganese or Oxidised Chromium:
All levels will cause interference. To remove this interference:
1. Test a 10ml sample as described in the above procedure.
2. Adjust the pH of a separate 10ml sample to pH6-7 using 1N Sodium
Hydroxide or 1N Sulphuric Acid.
3. Add 1 drop of 20% Potassium Iodide Solution; swirl to mix and then
wait 60 seconds.
4. Add 3 drop of 5g/l Sodium Arsenite Solution and swirl to mix.
5. Use this as the sample in the above test procedure and make a note
of the result. Use a multiplication factor to correct for the dilution of
the sample.
6. Subtract the result obtained in step 5 from the result obtained in step
1 to give the true result.
7. Correcting for dilution: To correct for any reagent added to the
sample to neutralise it, a dilution correction factor must be used:
Final Volume ÷ Initial Sample Volume = Correction Factor
The final volume can be calculated by adding together the initial