Difference between Photometer and Titration Method readings

Hello all, I'm a swimming pool inspector and recently my employers bought a Palintest photometer. When I tested the pools for Free Available Chlorine/Total Chlorine/Combined Cl, the photometer has been giving me a 0.2 ppm Combined Chlorine reading more than the titration method.

For example, I'll get 0 Combined on the titration method, and 0.2 ppm on the photometer, or 0.3 ppm on the photometer and 0.1 ppm when I titrate. I'm wondering if the photometer is just more accurate, but I'm a bit concerned because these readings were taken on pools that historically have no Combined Chlorines (via titration method).

Does anyone have any experience switching over to photometers from titration, and can shed some light on my readings?

Hi Jennie, and welcome to the forum!

I can't give you an answer to your questions but I know that there are at least 4 or 5 folks on this forum that can. The traffic around here is just starting to pick up, though, so don't give up if it takes a day or two to get some responses!

Hi Jennie;

A comparative study was done on a variety of kits a few years back, and the bottom line was, pool testers (all methods) are not very accurate. I've been trying to get a copy of the actual study. I'll try again. Either way, I'll follow up on this shortly. I do have a news report of that test.

However, if you are still connected with the UCLA school of EH, can you possibly get a "test the testers" project going?

Anyhow, back soon.

Ben / PoolDoc

Thanks for the replies! I'll bring up the possibility of testing the testers to the bosses and see if they're open to it.

OK, I reached the person who had the info, and unfortunately, there was not one definitive report.

1. The most detailed study (1,000 pool water samples with 6+ kit types) was done informally by Que Hales of PoolChlor several years ago. I should have the news report soon, but it may be 6 months (after the pool season!) before I the actual test data.
2. Another study, paid for by the pool plasterer's trade association, was done at the Cal-Poly pool center, but it costs $500+ to get a copy, and does NOT include data or methods, only conclusions.

All chlorine indicators (OTO, DPD, syringaldazine) have some interferences. The use of a non-chlorine shock (often potassium monopersulfate) will always produce a false combined chlorine reading, and will sometimes, I seem to recall, produce an erroneous free chlorine reading.

Richard Falk (Chem_Geek, here and on other forums) probably has detailed info on this -- I'll ask him to look at this thread.

The titration method is only accurate to within one drop which for a 25 ml sample size would be 0.2 ppm. When you add add the last drop that extinguishes the pink color during the FC portion of the FAS-DPD test, it can over-shoot by up to one drop's equivalent especially if the sample was a faint pink when you added the last drop. So usually, the CC reading in a FAS-DPD test will read from 0 to somewhat less than 0.2 ppm higher for FC and at the same time 0 to somewhat less than 0.2 ppm lower for CC than the "true" reading such that the sum of these two errors equals 0.2 ppm (assuming there is both FC and CC to read -- if the CC is really zero, then an over-shoot won't have any error for CC as it will still read 0). Though one could try using a 50 ml sample size, the resulting resolution of 0.1 ppm isn't really accurate because the pink transition is so faint. How are you getting a 0.1 ppm reading when you titrate?

As for non-chlorine shock (MPS), it reads as TC in the OTO test, FC in the FAS-DPD test and CC in the DPD test.

waterbear may have more info regarding a comparison of titration vs. photometric in his experience.

Why do you care about this 0.2 ppm difference? A CC of 0.2 ppm or less is perfectly acceptable and within existing code standards. Now if you are at a higher CC, then this difference may be more of a problem for you and the titration kit would probably let you pass more readily, assuming the inspector also uses a titration kit.

Richard, Jennie *is* the inspector!

Thanks for all the input!

PoolDoc: I would really like to see that news report when you find it.

ChemGeek: The pools I was testing were not previously shocked with MPS. The Palintest photometer we use does have a reagent to offset the effects of MPS on FAC though; it's called OxyStop and we do use it when some of our pools have been recently shocked.

The 0.1 CC reading I got was a very faint pink titration reading. The photometer read 0.3 CC so the difference was about 0.2 CC.

If there is that 0.2 margin of error, it does make sense and I guess that means our photometer is just a bit more accurate. I am concerned about the 0.2 ppm difference because most of my operators still use the titration kit, and I don't want to hold them to eliminating the CC to 0 if they can't even read it on their kits. We advise our operators to eliminate all CC even if it's below legal threshold just so they are aware of it and the CC levels don't get out of hand.