Mps Interfering With Cc Measurements?

[simonc]

After using MPS shock in my tub, how long do I need to wait before measuring combined chlorine (CC) accurately?

I have a Taylor test kit. I know I can buy a Taylor reagent that stops the MPS from interfering with the CC test. I went to my local Leslie pool dealer and he told me I didn't need it. All I need to do is wait 4 hours after adding MPS before I test CC. I waited about 24 hours but I got a higher than expected CC reading. As it turns out the dealer does carry that particular reagent, so I would have to order it off the internet and pay shipping charges. All this seems unnecessary if I only have to wait a few days after using MPS shock before I test CC.

So ...

If I add MPS shock once, will all my CC reading be wrong until I refill the tub ... or ... can I measure CC accurately after waiting a certain amount of time (hours/days/weeks) after MPS treatment?

Just learning ...

- Simon

[chem geek]

If there is residual MPS in the water then it will measure as Combined Chlorine (CC). Shortly after addition at high levels, it can even show up as Free Chlorine (FC), but that interference does go away fairly quickly, most likely within an hour. The CC interference normally won't last much more than a day, but it could if there isn't much for the MPS to oxidize in the tub.

If you look at the instructions for the Taylor K-2042 interference remover, you will see that you can actually measure the amount of MPS by measuring the CC difference between not using vs. using the interference remover.

I would say that if you waited 2 days after adding a reasonable (not excessive) amount of MPS, that you could safely measure the CC. Note, however, that there will generally be measurable CC in the hour or so after a soak so you don't want to measure at that time. You want to measure CC before your next soak.

[quantumchromodynamics]

Measure the free and combined chlorine before you add the MPS.

I think that the interference remover is worthwhile for those who use chlorine and MPS.

[simonc]

If there is residual MPS in the water then it will measure as Combined Chlorine (CC). Shortly after addition at high levels, it can even show up as Free Chlorine (FC), but that interference does go away fairly quickly, most likely within an hour. The CC interference normally won't last much more than a day, but it could if there isn't much for the MPS to oxidize in the tub.

If you look at the instructions for the Taylor K-2042 interference remover, you will see that you can actually measure the amount of MPS by measuring the CC difference between not using vs. using the interference remover.

I would say that if you waited 2 days after adding a reasonable (not excessive) amount of MPS, that you could safely measure the CC. Note, however, that there will generally be measurable CC in the hour or so after a soak so you don't want to measure at that time. You want to measure CC before your next soak.

Chem Geek -

As you suggested, when I first joined this forum, I have been using the diclor-then-bleach method. So far it has worked great. The only "problem" that I have been having is my CC level. It's around 2 ppm (+- 0.5 ppm). In the Taylor kit handbook it says that levels above 0.2 can cause "problems". So far I haven't had any "problems". When I first remove the cover, the water has some strong odors. After that, it's OK, but still not the same as a new fill.

When I first started using bleach, I used it to shock the tub. The Taylor handbook says you need FC at least at 10 times the the CC level. My CC was 2. I shocked using 30ppm FC, based on the online pool calculator. After shocking my CC level only fell to 1.5. I tried this a couple times with the same results.

I then decided to try MPS. My CC was again at 2.0. I added 3 level tbls of MPS (about 2 ounces) to my 275 gallon tub. I waited 24 hours, again my CC level was about 1.5. I also tried this a couple of times, with longer wait times before measuring CC, with the same result.

** IS THERE A CALCULATION FOR HOW MUCH MPS IS NEEDED TO BRING 1 PPM OF CC TO NEAR ZERO BASED ON TUB CAPACITY?*** I'm just following the directions on the container. It's not a calculation that I see available on the pool calculator.

Since I'm not experiencing any real "problems" with 2ppm of CC, I've decided not to worry about it until I do another complete tub refill. I have a high level of CYA (>120 ppm) in my tub. I used too much diclor at the beginning of the process because I didn't get my Taylor test kit soon enough. Since then, I've add more CYA because when I'm away on vacation, I use a floater with triclor to maintain FC levels.

The "away on vacation" issue has caused me to look into after market add on salt water chlorine generators that would always keep a minimum level of FC in my tub. Unfortunately, there seems to be very few forum members with much experience with these devices.

Thanks again for your help and expert advice ...

- Simon

[simonc]

Measure the free and combined chlorine before you add the MPS.

I think that the interference remover is worthwhile for those who use chlorine and MPS.

Thanks for responding to my question. As I explain in my response to Chem Geek in this thread, I'm not going to worry about this factor until I refill my tub again. I'm mystified as to why neither FC shock or MPS shock seems to bring my CC levels down significantly. But since I'm not having any significant water quality problems at this time, I'm going to leave CC levels alone at least for now.

- Simon

[chem geek]

Yes, I would just leave the CC alone, especially if you don't notice any smell or irritation from it. It's probably chlorourea or some combined chlorine from some organic you introduce into the tub (say, lotions or something like that). Just make sure you still have FC measurable since that is what will kill pathogens. The CC buildup may be from the slower oxidation since your CYA level is higher. This is something that sometimes happens with the Dichlor-only method after a month or two where not only is there more CC, but the water can get dull looking. On the next refill, try and raise the CYA only to 30 ppm. For vacations that are only a week, you might try just lowering the spa temperature and shocking to 10 ppm FC which with a normal 25% daily chlorine demand would be close to 1 ppm after a week, though that is a little risky if the chlorine demand were higher. With Trichlor, for every 10 ppm FC that it adds, it also increases CYA by 6 ppm.

[quantumchromodynamics]

The high cyanuric acid level could be causing some of the problems. It would be best to drain and refill now.

Alternatively, you could add 30 ppm to 50 ppm bromide from sodium bromide. This would make your tub a bromine tub. Bromine does not combine with cyanuric acid.

[simonc]

Chem Geek & Quantumchromodynamics -

Thanks for all the good info. I'm not going to worry about the measured CC level of around 2 ppm for now. On the next fill I'll try to manage the CYA better. It's been about 10 -12 weeks since my refill was "new" and I started the diclor-then-bleach method. The water still seems very good - clear, no foam, no irritation, chemically well balanced (except for CYA), very little odor (after cover has been off a while). I think the bleach method is making my water last longer & "feel better" than what I was doing before - floater with triclor and a bunch of other chemicals to try to keep the water balanced.

- Simon