Thermo Fisher Scientific C101 Colorimeter - Manual (Page 29)

Measurement: Chlorine Dioxide
C103 Colorimeter
25
Chlorine Dioxide Measuring Hints & Tips
2.7.1 Sample Collection
Chlorine Dioxide determination must take place as soon as possible after
the sample is taken or else low results may be obtained.
Plastic containers should not be used to collect the sample because these
may have a high chlorine dioxide demand which will lead to low reading
results. It is advisable to wash the container with sample water before the
sample is collected.
Do not agitate the sample as this will also lead to low results.
2.7.2 Sample Measurement
If the pink colour forms and then rapidly fades, this is an indication that the
chlorine dioxide concentration may be very high. If this is the case, dilute
the sample with de-ionised water and retest. Multiply the result by the
dilution factor used.
Example: if the sample was diluted 2:1 with de-ionised water (i.e. 1 part
sample water to 1 part de-ionised water), the result is multiplied by 2 to
give the true concentration of the original sample.
Note: Results obtained by this method will not be as accurate as stated in
this manual because some chlorine dioxide will be lost in the dilution
process.
After use, the vial should be cleaned thoroughly to avoid contaminating the
next sample. Do not use household cleaners as these may have a
chlorine dioxide demand which will lead to low results on subsequent
tests.
2.7.3 Interferences
Acidity:
Greater than 250ppm CaCO
3
may cause interference. To remove this
interference, neutralise a separate sample to pH6-7 using 1N Sodium
Hydroxide. Record the volume used and add the same volume of 1N
Sodium Hydroxide to the sample before carrying out the above test
procedure. Use a multiplication factor to correct for the dilution of the
sample.
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 volume used and add the same volume of 1N Sulphuric