7. Measuring Free Chlorine in the presence of Manganese and Chloramines
Disinfection techniques and treatment strategies used in the drinking water industry have steadily increased in complexity and effectiveness. Mixed oxidant systems, ozone, chlorine dioxide, chloramines, peroxide, membranes and ultraviolet lamps are now routinely used to meet the challenges presented by pathogens and other microbial substances being found in source water supplies. These challenges coupled with water scarcity and the increased emphasis to use recycled water present new challenges for test methods, especially those designed to monitor disinfectant residuals. For the most part, methods for determining chlorine have remained unchanged with manufacturers focusing their new product development efforts instead on techniques to determine disinfectant levels faster and more efficiently without focusing on determining the species more accurately.
Manganese interference has been a problem in free chlorine analysis for decades. Sodium arsenite is commonly used to determine the level of manganese interference. While this can be effective, it does require the analysis of a second sample and generates a hazardous waste solution. Chloramines, both inorganic monochloramine and dichloramine, and the organochloramines can also break through in free chlorine determinations causing an over-estimation of the disinfectant residual present.
The New Indophenol Method for Free Chlorine (FreeChlor F)
Work has recently been completed on a method for determining free chlorine in situations that have traditionally been difficult to analyze. The method based on the indophenol reaction gives one a new tool for determining free chlorine levels directly in samples having manganese or chloramine interference. This eliminates the need to know if manganese or chloramines are present in the sample and prevents the over-estimation of the actual free chlorine concentration. The method is also applicable to applications where free chlorine concentrations have been difficult to accurately determine. These applications include kinetic studies in forming chloramines, mixing efficiency studies and in breakpoint chlorination determinations.
Oxidized forms of manganese (Mn4+, Mn7+) react immediately with the DPD indicator used for free chlorine. This immediate reaction makes its presence in the sample not obvious to the analyst. The formed pink color is stable and indistinguishable from free chlorine. Chloramines also react with DPD Free Chlorine indicator, but in a different manner. Their presence is sometimes obvious to the analyst because the color developed is often 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 monoimine 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.
When should one use the indophenol method for determining free chlorine?
And when should the DPD method continue to be used?
To answer these questions, facilities should determine the following. 1) If they have any of the situations described above which could affect chlorine concentration values. 2) If the interference level is significant. 3) What would be the impact of a false-positive free chlorine value on treatment processes and water quality. 4) If there are costs associated with the disposal of the arsenite waste solution that could be eliminated. To help illustrate the best use of this method, the following examples compare DPD and indophenol method results at utilities that have suspected interference issues.
This utility from Northeastern US has a manganese problem with levels running 0.2 to 0.4 mg/L. The impact of free chlorine test results is shown in the graph.
The DPD Free Chlorine values are significantly over-estimating the free chlorine concentrations which could result in inadequate microbial protection. Additionally, treatment changes or decisions based on the elevated chlorine values may be impacted. The indophenol method would be recommended at this utility. Arsenite disposal issues would be eliminated.
A utility in Western US draws water from a reservoir that has high manganese episodes in late summer or early fall during reservoir turnover. The manganese levels in the samples tested were very low at ~ 0.03 mg/L. DPD and the indophenol methods for free chlorine were compared.
The difference between the DPD Free Chlorine Method and the Indophenol Method were insignificant at these manganese levels. Either method would be appropriate for determining free chlorine levels. However, determining when the higher levels of manganese reappear results in the need to do the additional testing for manganese. In this case using the indophenol method would eliminate the need to do manganese testing.
Figure 3 is an illustration of chloraminated water that required further treatment to remove manganese. Free chlorine was added to mitigate the manganese. This also converted the water to a free chlorine system by taking the chloraminated water through breakpoint chlorination. The DPD Titration method and the Indophenol method were used to analyze the treated water for free chlorine. The DPD titration values were adjusted using sodium arsenite to compensate for manganese interference. Even with the arsenite compensation, the DPD values were higher than the indophenol values. It is important to note that in this analysis situation we are dealing with two possible interferences in the free chlorine determination; manganese and chloramines. The waters were also tested for monochloramine levels using Monochlor F Reagent. See Figure 4.
Several of the samples showed trace levels of monochloramine. While this may not appear significant, this would also indicate that higher concentrations of dichloramine and trichloramine concentrations are likely present as those are the next steps in the breakpoint chlorination reactions before a full free chlorine water is produced. It is suspected that the presence of these chloramines is responsible for the higher values found with the DPD FAS titration. Samples such as these can be difficult to interpret until one understands the water matrix and the components that may be present. Arsenite was used to compensate for the manganese, but in this instance where the water is mostly through breakpoint, we still can see low levels of chloramines being picked up by the DPD Free Chlorine indicator.
Method 10241 using the indophenol method for free chlorine is now available at www.hach.com or by calling 1-800-227-4224.