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Which Test Kit Is Right?


macgd016
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... OTO or tablet based kits ... OTO bottle kit ... OTO chlorine ... DPD tabs ...

... FAS DPD test ...

Umm, sorry I have to ask, but the terms are just to short to get good search results for and I can't ever seem to see them written out. What do OTO, FAS and DPD each stand for?

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OTO is Orthotolidine which is a chemical that turns shades of yellow in the presence of halogens (chlorine, bromine, iodine). It cannot differentiate between Free Chlorine (that is available to sanitizer and oxidize) and Combined Chlorine (that has combined with organics such as ammonia and has become inactive) so it only measures Total (Free+Combined) chlorine. However, it is a useful test since it does not bleach out at high chlorine levels like the DPD test so it will tell you if there is chlorine present or not, it just won't tell you if it is 'active" or 'used up' chloirne. The OTO test is used with a color comparator that has different yellow color blocks. It can give a rough estimate of very high chlorine levels (shock levels turn it deep yellow to orange and very high chlorine levels such as used for killing black algae will turn it brown).

DPD is N, N-diethyl-paraphenylenediamine which is a chemical that turns shades red in the presence of chlorine. This test CAN differentiate between Free Chlorine and Total Chlorine so the Combined Chlorine can be determined by subtracting the Free Chlorine reading from the Total Chlorine reading (TC-FC=CC). It is used with a comparator with red color blocks. However, it is not useful for testing chlorine levels above 10 ppm because it "bleaches out" (turns colorless) at chlorine levels above 10 ppm. It is the most common test for chlorine or bromine at normal bathing levels but is useless for testing shock levels or pools with high stablilizer.

FAS-DPD stands for Ferrous Ammonium Sulfate and N, N-diethyl-paraphenylenediamine. These two chemicals are used in a titration test for Free and Combined chlorine. It is not a color matching test so it can even be done by those who are colorblind. In this test DPD powder is added to the sample to produce a pink color and the sample is titrated with a FAS solution (drops are added one by one and the number of drops added are counted) until the sample turns colorless. (This is the same type of titration method that is used for TA and CH tests) The test can work on FC levels up to about 50 ppm and will directly give a Combined Chlorine reading and can have a precision as great a .2 ppm! It is the preferred way to test chlorine or bromine since it is definitive, foolproof,easy, and precise.

As far as getting search results. the terms you want to search for are OTO, DPD, and FAS-DPD if you are looking for info on chlorine testing for pools and spas since these are the terms that are used, not the chemical names.

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FAS-DPD stands for Ferrous Ammonium Sulfate and N, N-diethyl-paraphenylenediamine. The test can work on FC levels up to about 50 ppm

I have been told by Lamotte that their FAS DPD, like all DPD based tests, is only accurate to around 8ppm as the DPD starts to bleach out at concentrations above this.

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It doesn't matter if the DPD powder/tablet bleaches out at high chlorine levels. At least with the Taylor test using powder, you see a "flash of pink" which just tells you to add more powder. I would assume that with the Lamotte test that if you saw a brief color around the tablet that went away, you could add another tablet, though perhaps that's not as obvious as with the Taylor powder. After adding more DPD so you get a color, you can then titrate using FAS as normal and get an accurate result. The "bleaching out" is a reversible chemical equilibrium so as you add more FAS-DPD drops the color of the sample may get even darker before it eventually gets faded eventually to clear.

There are many, many people who use the Taylor FAS-DPD in this way during shocking to get very high chlorine readings, though usually using smaller sample sizes (5 ml or 10 ml instead of 25 ml) to save on FAS-DPD reagent.

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I only know what I have been told by Lamotte who basically told me that in their experience the testing of FC above 7.5 or 8ppm is inaccurate due to bleaching. I asked if this could be overcome in any way and they suggested their Insta-TEST® Wide Range Total Chlorine & pH which test to 50ppm.

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  • 1 month later...

Just thought I'd give an update:

I ordered the Lamotte Kit through Sword Scientific. They seemed somewhat confused, not getting their quotes straight and quoting me shipping costs in EUR which they then charged in GBP, but I finally did get the kit shipped to Germany within about 2 weeks at a total cost to me of 144EUR. Steep I must say, but the kit does look impressive. Will get to work with it shortly... :-)

Thanks btw for bearing with my ignorance of abbreviations.

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I don't think you will be disappointed, I have just been out to my house in Spain for three weeks and used the test kit every day, more out of interest than out of necessity though. It is so very much easier to read than test strips or OTO, this for me makes it worth the expense for this alone.

My only criticism of it is that it does not read high chlorine levels well and that there is nothing in the documentation to alert you to this. Other than this it is a fantastic bit of kit.

The FC test gives an unequivocal reading as the pink changes to clear dramatically as the last drop is added, pH is really easy to read using the Comparator slide, the ALK test gives a rapid change from green to red within one drop so the reading is clear. The only test that is slightly unclear is Calcium Hardness, in my case the CH is over 400 and I am not sure whether I should stop adding drops as soon as the colour starts to change to blue or when it has fully changed.

Using the 7022 daily has taught me at least one valuable lesson though, you need to take your FC reading at the same time each day as FC varies during the day, in my case by as much as 3ppm.

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  • 2 weeks later...

After a couple of days worth of experience using the kit mainly to test CH and TA and occasionally the other parameters I must say I am quite impressed and satisfied. The tests are easy to perform and the results are clear cut.

The FC test gives an unequivocal reading as the pink changes to clear dramatically as the last drop is added, pH is really easy to read using the Comparator slide, the ALK test gives a rapid change from green to red within one drop so the reading is clear. The only test that is slightly unclear is Calcium Hardness, in my case the CH is over 400 and I am not sure whether I should stop adding drops as soon as the colour starts to change to blue or when it has fully changed.

During titration I frequently see a color change limited to the area where the drop enters the liquid to be tested, which either resolves on its own or through my swirling the test tube. Sometimes on the last two or three drops a beginning color change can be noticed, which does not resolve. The latter would be what you're describing.

I understand you are supposed to follow through until the color change is complete.

I suppose you could subtract half a drop for the calculation to reflect the early start of a permanent color change, but as with any measurement you have to realize that the result will not be more accurate than the mode of measurement allows. So measuring The TA using a dropwise titration at 20ppm per drop, using 10 drops until the color change is complete, you will get a value of 200ppm which would be exact up to +/-10ppm at best. If you subtract half a drop for early starting color change, this would give you a value of 190ppm. And I would think that this would have to be interpreted with the same absolute exactness of +/-10ppm at best.

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I suppose you could subtract half a drop for the calculation to reflect the early start of a permanent color change, but as with any measurement you have to realize that the result will not be more accurate than the mode of measurement allows. So measuring The TA using a dropwise titration at 20ppm per drop, using 10 drops until the color change is complete, you will get a value of 200ppm which would be exact up to +/-10ppm at best. If you subtract half a drop for early starting color change, this would give you a value of 190ppm. And I would think that this would have to be interpreted with the same absolute exactness of +/-10ppm at best.

If the range between colour change was only one or two drops I wouldn't have an issue but when testing the CH the range between the colour starting to change and fully changed is 400 to 600ppm

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I can't really comment on Leisureteq.

With my Lamotte kit the main issue has been drying the equipment after use. I don't want to get too much humidity in the case. Therefore I recently set up a drying rack using some shishkebab sticks and a leftover wooden block:

testtubedryer.jpg

Better ideas, anyone?

[edit]

As far as getting search results. the terms you want to search for are OTO, DPD, and FAS-DPD if you are looking for info on chlorine testing for pools and spas since these are the terms that are used, not the chemical names.

I hadn't commented on this earlier. The problem with search results is that search engines have a heck of a hard time giving good results on terms as short as these. Some simply refuse searching for 3-letter searchterms.

[end edit]

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