Relative accuracy of Taylor Tests for FAS-DPD vs DPD vs OTO

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M

MPurcell

0
Feb 17, 2017
105
Dahlonega, Georgia
The spa has been in operation for almost a month, and we use it every day, usually twice a day. My wife and I usually soak for about half an hour every evening, and my wife does exercises each morning in the water. I've been learning as much as I can for the past six weeks or so. Before I really understood enough about testing, I decided to go with the DPD test kit k-1004 to conserve on funds. I then also purchased a k-1000 test kit along with CH reagents, thinking I would use the DPD and OT tests alternately, assuming I would get more reliable accuracy from the DPD test. I had been wanting to get the reagents for the FAS-DPD test for a couple of weeks so I finally ordered them this past week, and they arrived in the mail yesterday afternoon. In hindsight I should have purchased the Taylor k-2106 test kit from the beginning.

Once I had completed my first FAS-DPD test, I was very surprised by the results. I first tested with DPD getting ~3 ppm and OTO getting 2 ppm. This was done in strong sunlight, so since the values were low, I think the color matches were pretty close. Thinking that the FAS-DPD test would be somewhere in the same ballpark, I decided to do the 25 ml test for my first test. It took 28 drops giving a result of 14 ppm. I then repeated the test with 10 ml using 12 drops for 15 ppm. I don't know if the k-1004 test kit was old or fresh since I purchased it from someone through Amazon, but the k-1000 test kit should be relatively fresh since I purchased it from TFTestKits. The discrepancy seems greater than one would expect. I've read some old posts discussing the inaccuracy of the two color comparison tests, but I don't recall anyone mentioning a discrepancy this great. I couldn't run additional comparisons because I'm doing a bromine demand test today and the bromine level is too high for the color comparison tests to be useful. But if this level of discrepancy is consistent, then the color comparison tests will be useless, even as a rough guide between more infrequent FAS-DPD tests.

As far as I know, we have been having pretty good success with keeping the water balanced and sanitized. However, if the above holds true, then we have probably been soaking in fairly high concentrations of active bromine, since I have been keeping the bromine level at 4-6 ppm as best I could with the color comparison test kits. Just for background I have listed the other water parameters below.

Temp ~100 (varies 96 - 101)
pH 7.5
TA 80
CH 225
Borates ~50 ppm

Interested in any comments. Thanks, Michael
 
For the OTO test, you really can't trust the color. I never got it to report with any accuracy. My 9ppm FC water sample would look like a 3ppm color on the block and it would never change until I got to very high FC levels. It is simply useless as a quantitative test.

The DPD only drops will give you total
Bromine and, like chlorine, if the bromine level is too high there can be color bleaching. You can add another drop of the R-0001/R-0002 reagents (6 total) and see if the color changes. If it gets darker, then you're getting bleaching.

The DPD-FAS test will the most accurate as you are adding such a large quantity of DPD powder that there is no bleaching effect.


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It's actually bromine adjusted reagent R-0872, so at 25 ml, 1 drop = 0.5 ppm and at 10 ml, 1 drop = 1.25 ppm.

So I guess the OT test is superfluous unless I ever come up with a 0 on the FAS test. With the DPD test, do you think I would get bleaching with 15 ppm bromine, which is roughly equivalent to 7 ppm of chlorine? If I've been running between 4 to 6 ppm on the DPD test and the ratio is similar (3 tested as 15 with FAS), the bromine could be up around 20 to 30 ppm. I guess that *would* cause bleaching.

I'll do some comparative testing over the next weeks to see if I can come up with something useful. I have 2 oz of the reagent, so I'm not worried about using the FAS-DPD test every day for a while, but I'd like to eventually cut it down to once or twice a week.
 
Oh, I forgot they made that special titrant for bromine. Apparently too many people didn't like having to multiply by 2.25 so they made it easier....hehehehe.

Anyway, I think bleach out is fairly rare at the levels you're looking at but it's not hard to test with a few extra drops.

Here's a trick - your water sample in a Taylor high range comparator block (#9056) is 9.5mL, very close to 10mL. So you can use your R-0872 drops with the R-0001/R-0002 to titrate that sample and you should get a number close to what you would see with the DPD powder. It's also a good way to double check your "eyeball" color discrimination.


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Thanks for that trick. I'll definitely try that. It also answers another question I was wondering about. I saw the chemical name of DPD, but I don't know what's in the other reagent, the one of R-0001 or R-0002 that isn't the actual DPD. Curious about that and couldn't find it yet. So the combination of those two agents is about the same as the DPD powder?
 
It's the same chemistry. When chlorine reacts with
N,N-Diethyl-p-Phenylenediamine, it forms a product called Würster dye. FAS reduces the Würster dye back to a colorless amine. 2Fe2+ > 2Fe3+

Reagent liquid r-0001 is phosphate buffer and probably edta.
 
Thanks for the explanation, James.

I tried Matt's trick last night. With the powder, I got 12 drops, 17.5 bromine. When I transferred the liquid from the Taylor high range comparator block it actually measured a little over 10 ml with the 10 drops added. That took 11 drops to clear, so 13.25 bromine. Same ballpark but not exact. I will probably try this again when the bromine is back down in normal range.

This morning I tested twice an hour apart and got a reading of 10 ppm both times with the FAS-DPD test. With the DPD R-0001 and R-0002 reagents, I got readings of 4 and 6. I'm not clear why it would vary when the FAS test did not. I'll keep working with it for the next few days.
 

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The drop count accuracy is +/- 1 drop. You're within the accuracy of the test.

Don't transfer the liquid to another container, just use the high range block and titrate the sample with the R-0872. You have to cap and mix every drop but that's not a big deal.

Also, there's a fill line on the FC/Br side that you fill the comparator up to.
79f031ce4575b249eda1da38060fab69.jpg


You see the "9" line in that picture? That's what you fill the tube up to. Not the top of the comparator. 9 = 9mL (roughly).


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When I saw the picture, I realized we are using different comparator blocks. I have the k-1004 test kit, which uses component #9782 described as "RES COMPAR,BR&CL(HI),DPD/" for the comparator block. It has a fill line, but no numbers on the side. I would assume the sample is the same size though. It's shaped the same as the OT block in the k-1000 kit, and both have pH on the right and Cl Br on the left. I'll try doing the FAS titrant in the block. It might fit if there aren't too many drops.
 
The #9782 is the midget comparator block. I have one but I don't use it. I believe the sample volume of water is more like 4.5mL.

Get a #9056 high range block. It works better in my opinion and allows for direct titration with your R-0872. I think they go for $11 on Amazon.


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