Whether you're a large system or small, surface water or groundwater, free chlorine or total, monitoring at the entry point or in the distribution system, water quality monitoring of disinfection residual is an important part of the job. Disinfection residuals are vital to protecting public health and must be reported to the state to show compliance with drinking water regulations. If your go-to monitoring equipment is a Pocket Colorimeter, here are some important things to keep in mind when monitoring chlorine residual.
- My pocket colorimeter is flashing “2.2”; what does that mean? Some pocket colorimeters have the option of low range and high range. For chlorine values less than 2.0 mg/L, it’s best to use low range for the greatest accuracy as high range is subject to variable levels of interference. But
if the instrument consistently reads 2.2 or flashes 2.2, the high range procedure needs to be followed. Using the special high range sample cell, follow the normal steps of zeroing the sample prior to adding the reagent. For Pocket Colorimeters, you’ll add two 10-mL doses of reagent to a 5-mL sample and read. It’s always good practice to review (or re-review) the procedure as a reminder of the proper sequence for analyzing samples. 
Have you checked your glassware lately? Dirty glassware can lead to erroneous readings. Hold the glassware up to a white sheet of paper. Does the glassware look clear or can you see a black film build up? If so, it might be time to purchase some new glassware. You can also try an at home remedy with a 50/50 mix of white vinegar and water. Leave the glassware to soak overnight and rinse out in the morning. Do not use a scrub brush or abrasive cleaning utensil of any kind as that may scratch the glassware.- Free? Total? There’s a difference? There is a difference and you want to make sure you are analyzing for the correct chlorine based on your monitoring schedule. Free chlorine reagent reacts with free chlorine in the sample, and should be read immediately. Total chlorine reagent reacts with ALL the chlorine present in the sample and requires a 3 minute hold time before reading. If your system uses chloramines or receives water from a chloraminated system, then you need to monitor for TOTAL chlorine. Performing a free chlorine residual reading for a chloramine system, will result in a reading of little to no chlorine. This is because most of the free chlorine has bonded with ammonia to produce chloramine. If you operate a free chlorine system and use total chlorine reagent, you won’t see a significant difference, but generally the total will be higher as it has reacted to ALL the chlorine in the sample, and not just the free chlorine. The important thing is to make sure you are using the correct reagent that corresponds to your monitoring schedule.
- Is your reagent current? Check expiration dates. Make sure your reagents used for analyzing and reporting residuals to the state are current. A lot of systems out
there like to use the Swiftest Dispensers. Make sure you know what the expiration date is on the DPD, especially if you’ve acquired the product from a secondary market.
➽ Aspen Coombs, PE, Senior Field Engineer
