Taylor Drop Test Kit, Chlorine (free/combined)/Monopersulfate, FAS-DPD, 1 drop = 0.2 ppm

Taylor Drop Test Kit, Chlorine (free/combined)/Monopersulfate, FAS-DPD, 1 drop = 0.2 ppm

In recent years, professionals in the pool and spa industry have progressed from using orthotolidine (OT) to N,N-diethyl-p phenylene-diamine (DPD) when determining chlorine levels with color-matching tests.

The OT method only measures total chlorine (the sum of active and spent sanitizer) which makes maintaining the correct residual a guessing game. Because of this, regulatory authorities do not permit OT testing in commercial pools. In addition, orthotolidine contains hydrochloric acid, making it more costly to ship than DPD and therefore more costly to buy.

Unlike OT, the DPD method will distinguish between free available chlorine and total chlorine. By subtracting the free chlorine reading from the total chlorine reading, the amount of combined chlorine in the water can be known. Combined chlorine is not an effective sanitizer. Its presence causes eye and mucous membrane irritation and the characteristic “chlorine” odor of a poorly maintained pool.

Combined chlorine is eliminated by superchlorination. Calculations for the breakpoint dosage depend on knowing the level of combined chlorine in the water, which is why the DPD method is superior to the OT method for testing chlorine-sanitized pools. However, bromine is an effective sanitizer in all its forms. Because of this, either OT or DPD may be used to test bromine pools and spas.

The latest trend in commercial pools with chlorine sanitizer has been the FAS-DPD titration method, which can measure free and combined chlorine as low as 0.2 ppm (using a 25 mL sample size) and as high as 20 ppm (using a 10 mL sample size).

To get the free chlorine reading, a buffered DPD indicator powder is added to the water sample. It reacts with the chlorine to produce the pink color characteristic of the standard DPD test. Ferrous ammonium sulfate (FAS) titrating reagent is then added until the pink color permanently disappears, signaling the endpoint.

The distinct change from a vibrant pink to no color at all eliminates the need for color matching. This feature comes in handy when testing samples with high levels of sanitizer because the user does not have to distinguish between relatively close printed-color gradations.

The second half of the FAS-DPD test determines the amount of combined chlorine present. It too involves turning the sample from a vibrant pink to a colorless endpoint.

Chlorine will interfere with most tests for potassium monopersulfate, since both are strong oxidizers. Pools utilizing certain alternative sanitizers, such as mineral purification systems, rely on monopersulfate to destroy organic contaminants, but even the low residual of chlorine maintained in these pools makes getting a true monopersulfate reading problematic.

In cooperation with DuPont, Taylor has developed a method to distinguish between the levels of free chlorine, combined chlorine, and the monopersulfate compound in the water. Taylor’s drop-test kit K-1518 contains Deox Reagent to eliminate monopersulfate interference in the chlorine test. It employs FAS-DPD to determine free and combined chlorine levels.