YSI 600DW-B Sonde - Manual (Page 298)

Frequently Asked Questions About YSI Drinking Water Systems Appendix H
How accurate is the colorimetric DPD check of accuracy?
Commercial colorimeters for DPD analysis are very accurate when used properly. However, there can be
errors in the determination which result either from user technique or water chemistry. For example, there
can be errors in the DPD determination which are caused by dirty cells during the analysis processes of
zeroing the instrument or taking a reading. In addition, reagents which are out of date and thus have lost
their potency could cause errors. To avoid problems of this nature, always make sure that the colorimeter
cell has been wiped clean with lens cleaning paper both prior to taking the zero reading and after addition
of reagent prior to taking an actual reading. Also make certain that the reagent pills or packets are within
their shelf-life limitation. Since the analysis is so easy to perform, you should always perform repetitive
analyses of the flow cell effluent until subsequent readings agree within the specification of the colorimeter
– typically 0.05 mg/L.
The DPD analytical procedure for free chlorine is also susceptible to interferences from any oxidizing
species other than free chlorine in the water which might oxidize the DPD reagent. If these non-chlorine
oxidizing species are present, then they will produce color in addition to that from the free chlorine and this
will result in a DPD chlorine value which is erroneously high. Fortunately, the rate of color formation is
usually relatively slow compared to that from free chlorine which is effectively instantaneous. Therefore,
you can usually determine if extraneous oxidizing species are present in the water sample by reading the
color immediately after reagent addition and then again after about 2 minutes. If the value does not change
significantly, then the color is likely to be due to free chlorine only; if the reading after 2 minutes is
significantly different, then it is likely that other color-forming oxidants are present and the free chlorine
reading could be in error.
YSI Incorporated Drinking Water Monitoring Systems Operations Manual H-12