When is the Free Chlorine Indophenol reagent recommended over the Free Chlorine DPD reagent?

Document ID

Document ID TE199

Version

Version 6.0

Status

Status Published

Published Date

Published Date 12/11/2018
Question
When is the Free Chlorine Indophenol reagent recommended over the Free Chlorine DPD reagent?
Summary
Using Free Chlorine Indophenol chemistry
Answer
There are several reasons to utilize the Free Chlorine Indophenol chemistry used in Hach method 10241 over DPD chemistry:

1. For determination of residual free chlorine levels in the presence of manganese and other oxidants which interfere with DPD colorimetric, DPD titrimetric, and amperometric methods for free chlorine. For use in potable water, chlorinated drinking water, swimming pool water and treated wastewater effluent.

2. For determination of free chlorine in the presence of chloramines. Chloramines react with DPD Free Chlorine indicator. Their presence will cause the color developed to be unstable and slowly increases with time. The rate of interference is dependent upon the concentration of the chloramine, the structure of the chloramine compound and the sample temperature, which makes it difficult to predict the level of interference in the actual free chlorine value.

The indophenol method for free chlorine uses Freechlor F Reagent Solution to rapidly convert the free chlorine present into monochloramine. The monochloramine formed is determined with Monochlor F Reagent, which is specific for monochloramine. The monochloramine reacts directly with a substituted phenol contained in the Monochlor F Reagent to form an intermediate monoamine compound. This intermediate then couples with additional excess substituted phenol to form the green-colored indophenol. Manganese and chlorinated organic amines do not react with the substituted phenol and do not interfere in the free chlorine determination. A Monochlor F sample blank is used to compensate for any monochloramine present in the original sample.
 
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