Chem_Geek: Heeeeeeelllllllpppppppp!!!

[LexPlayer]

Although I have not previously posted, I wanted to thank all the great people on these Boards.

Here is the situation: I incurred my first green algae problem on 7/4 and "accidentally" dumped too much chlorine and shock into the pool (only 18' above ground round). When the water finally cleared, I tested the water and discovered excessive pH (8.4) and, of course, Cl. I used a dilution method with OTO and estimated a level of 50+ ppm. I adjusted the pH with muriatic acid. The rest of the numbers appear ok according to the test strips (could not find a "real" kit anywhere). The only problem was the excessive chlorine. However, the weather indicated that it was going to be over 90 degrees in 2 days, so I investigated a chlorine neutralizer. A local pool company sold a 10 pound jug to me, but I don't know what the exact chemical is. My best guess is sodium sulfate since the label indicated that 3 oz would reduce 10,000 gallons by 1 ppm. Since I needed to lower an estimated 40 ppm, I played it conservative and used about 3.3 pounds (I calculated 5.75 or so).

Here's the problem. The test strips (HTH 6-way) show 2 ppm. A diluted OTO 3/1 = 16+ ppm and a diluted OTO 5/1 = 12 or so.

I have no idea what the actual chlorine level is.

So, Chem_Geek or anyone else:

1. Now that my water is full of sulfates, which one of the test methods is giving an accurate measurement?

2. Also, assuming I used sodium sulfate, I was wondering what the chemical reaction was. (I thought I smelled something, sulfur dioxide?)

Thanks to all for the great info, I keep a cheat sheet on how to deal with all the water problems.

[waterbear]

The chemical in most chlorine removers is sodium thiosulfate (photographers hypo).
Your pH was NOT high, that was interference from your high chlorine levels. Never adjust pH when chlorine levels are high since that will always cause your pH to read high falsely.

I assume your test strips are testing free chorine. OTO tests total chlorine and it is very possible the TC is much higher than your FC if you were killing algae. Dilutions introduce error into the test. The more you dilute the more the error.

[LexPlayer]

The chemical in most chlorine removers is sodium thiosulfate (photographers hypo).
Your pH was NOT high, that was interference from your high chlorine levels. Never adjust pH when chlorine levels are high since that will always cause your pH to read high falsely.

I assume your test strips are testing free chorine. OTO tests total chlorine and it is very possible the TC is much higher than your FC if you were killing algae. Dilutions introduce error into the test. The more you dilute the more the error.

Thank you Waterbear. I had read one of your posts where you indicated the false highs when the chlorine is high. Since the phenol red contained a chlorine neutralizer, I thought it was accurate. I am guessing, if the OTO test is somewhat accurate after dilution, that the chlorine is between 12 - 20 ppm. The bottle says that there is FC reading within 10 seconds, and a TC thereafter after continuous development. In your expert opinion, is that an inaccurate statement? [edit: various websites clearly state OTO only measures TC)

I'm not sure exactly what chlorine neutralizer it is, sodium something, but I wonder if the TC reading is affected by it. That's why I was wondering about the chemical formula, I believe salt is a by-product of the reaction and I did not know if that would throw the reading off.

I guess I better find some test strips that measure both FC and TC

[waterbear]

Even phenol red with sodium thiosulfate built into the formula is not going to be accurate much above 10 ppm of chlorine. Taylor has probably the best pH reagent around and theirs is only good up to about 15 ppm.

In theory OTO will measure the Fc within the first few seconds and then give you a TC reading in about a minute but in practice this is not reliable. Also, I would not use test strips either. There are just too many problems with them. Invest in a good test kit that uses the FAS-DPD titration test for free chlorine and combined chlorine. I recommend the Taylor K-2006.

[LexPlayer]

One other oddity in my experience. Prior to adding the chlorine neutralizer, the test strip was deep purple and the OTO dilution (even at 10%) was orange in color. After I put the neutralizer in, test strip was white and OTO was clear (maybe a faint, faint yellow).

After waiting a few hours, the results were again the same and I figured I used too much neutralizer . I decided to start over and added .25 gal of 10% HTH chlorine. According to bleachcalc, that is an increase of 3.3 ppm (assuming the neutralizer in the water is done neutralizing). Well the strip is right around 3 ppm, but the OTO is off the chart, deep yellow, even at 1:2 dilution (but not at 5:1).

Anyway, thanks Waterbear for the quick and insightful reply.

Lance

[waterbear]

Once again, are your strips just testing FC? It is possible to have very high TC and the FC low. I suspect that you have a lot of ammonia or organics in the water and a lot of CC has formed. Don't know if the thiosulfate would add to that but it might.