Time sensitive nature of FAS-DPD testing

[MPurcell]

I haven't seen too much made of this in the forums, but I decided to do a test case today. Using the speed stir I conducted the test by putting the powder in and then without any delay applying the titration drops, a little faster than 1 drop per second. I got a reading of 7 ppm (7 drops). I'm testing for bromine and using the R-0872 reagent with 12.5 ml of water, so 1 ppm per drop. I then repeated the test using a timer. I put the powder in and waited exactly 2 minutes before applying the titration drops as in the first test. The second result was 14 ppm (14 drops). This indicates that even a small delay in starting the drops could throw the results off high. I'm guessing that the ratio would be about the same with chlorine.

My previous results before I obtained the speed stir would have been consistently higher than with the speed stir because, no matter how efficient you get with swirling it takes quite a bit longer to complete the test. It seems that testing procedure is nearly as important as the reagents to get consistent results. Comparing readings from different people may not be comparing apples to apples. Maybe all this is obvious to you experienced folks.

 

[JoyfulNoise]

One thing to remember is that you are measuring different quantities. For bromine, the FAS-DPD test measures both free bromine (hypobromous acid) and combined bromine (monobromamine), so in effect you are measuring total bromine levels. For chlorine, the test measures only free chlorine and then you must add the potassium iodide reagent (R-0003) in order to get the monochloramine to convert the iodine into triiodide which causes the follow-on pink reaction with the DPD dye. In the case of chlorine, very high CC levels can cause the FC portion of the test to read higher over time but the reaction between CCs and DPD is so slow that it doesn't matter in practical terms (and it takes much higher CC levels than what is typically found in a clean pool). One other aspect of the test is that the DPD dye can be oxidized by air and turn pink. It doesn't typically happen because most pool water is fairly low in dissolved oxygen, but an extensive resting period after adding the dye could cause an slight increase in the titrant consumption.

The DPD dye is not perfect. In fact, the percentage by weight of actual DPD in the powder is incredibly small, only about 1 or 2%. The rest of the powder consists of pH buffering chemicals, chelating agents to remove metal ion interferences and an acid to drop the pH to a more acidic level. All of that is done to maximize the accuracy of the dye. DPD all on it own in water would not work well as an indicator.

So, it could be that your tub has some excess combined bromine in it, but I wouldn't think it was as high as what you are reading. I would suggest you call Taylor and ask them about it as they may have a simple solution, such as - "Don't let the test sample sit longer than 2 mins".

 

[MPurcell]

I did talk with the technical support person at Taylor Technologies. He confirmed the time sensitive nature of the DPD dye in the reagent powder. The recommendation is to start adding drops to the sample solution as soon as possible after adding the dye powder to the sample solution, and to add the drops as quickly as is consistent with proper drop size. Even though the test kit instructions do not mention this, he said it should be obvious because the instructions do not say to delay.

Another interesting point when testing water for a hot tub is to wait long enough for the sample water to cool below 90 deg F, as hot temperatures will interfere with the test reagents. He said this is true for all tests.

I also asked about dissolved oxygen in the sample water. He said that aerating the water before taking a sample would not change the FAS-DPD results. On the other hand, taking a water sample when the air jets are running can cause a false high reading of the pH test by one or two tenths of a point. The sample should be taken after the jets are turned off and the water has had a chance to settle.

The DPD dye powder reacts similarly with both bromine and chlorine, so it seems to me there is an implication for completing the combined chlorine portion of the test for pools or chlorine hot tubs. If you titrate a sample of pool water to clear, and then let it stand for a little time without adding any R-0003 it should turn pink again due to the time factor. So it would follow that the combined chlorine portion of the test should be completed as quickly as practical as well. I don't have a pool, so no chlorine samples to test.

 

[JoyfulNoise]

Thanks for contacting Taylor and reporting back what you discussed.

What you're referring to is monochloramine breakthrough which is a known limitation of the DPD test but it is not very relevant. Please see the following document from The Hach Company on Chlorine Testing in Water -

https://www.hach.com/cms-portals/hach_com/cms/documents/pdf/LIT/L7019-ChlorineAnalysis.pdf

The relevant section is on page 15 -

There is considerable controversy about monochloramine interference in the free chlorine DPD test. Some studies (Ref. 3.14) have indicated the percent interference in the free chlorine results can vary from 2.6 to 6.0%, depending on the monochloramine concentration and sample temperature.

The amount of monochloramine must be substantial in comparison to the free chlorine concentration to indicate an interference in the DPD colorimetric free chlorine determination. The reaction of DPD with free chlorine is rapid. If the color is measured within one minute, the monochloramine breakthrough will be minimal. A concentration of 3.0 mg/L monochloramine (as Cl2) will cause an increase of less than 0.1 mg/L free chlorine when using Hach DPD colorimetric tests.

Monochloramine breakthrough is more of a problem in the DPD titrimetric method for free chlorine because of the additional time necessary to perform the test.

So long as you are doing the test within about a minute or so, there's little problem with monochloramine breakthrough. This is one reason why people should seriously consider using a SpeedStir because it greatly increases the speed of the test. Also, the problem of monochloramine break through really isn't relevant in a clean, outdoor pool because the amount of CCs present are almost always less than 400ppb (0.4ppm). There is also the added complexity of the equilibrium among CCs (monochloramine, dichloramine and nitrogen trichloride) and the FC/CYA ratio, see here -

Chloramines and FC/CYA

In general, when testing chlorine with DPD-FAS and following the test protocol as it is intended, the tests are very accurate and repeatable. As Taylor says, DPD is sensitive to both chlorine and bromine and react in the same way, but DPD is much more sensitive to monobromamine than it is monochloramine and so it really can only be considered a Total Bromine test.