Chem_Geek: Heeeeeeelllllllpppppppp!!!

[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.

[chem geek]

Thiosulfate is supposed to reduce chlorine directly and not form combined chlorines. There are many possible reactions, but one of them is the following:

Na2S2O3•5H2O --> 2Na+ + S2O3(2-) + 5H2O

2S2O3(2-) + HOCl --> S4O6(2-) + OH- + Cl-

There are other reactions that take the thiosulfate all the way to sulfate or even sulfur, but I believe these are less likely. None of them form combined chlorine -- all produce chloride ion. The oxidation potentials are as follows:

2S2O3(2-) --> S4O6(2-) + 2e- ..... Eo = -0.08V

S2O3(2-) + 6OH- --> 2SO3(2-) + 3H2O + 4e- ..... Eo = +0.571V
2x( SO3(2-) + 2OH- --> SO4(2-) + H2O + 2e- ) ..... Eo = +0.93V
--------------------------------------------------------------------------
S2O3(2-) + 10OH- --> 2SO4(2-) + 5H2O + 8e- ..... Eo = +1.501V

The above reaction would result in the following with chlorine:

S2O3(2-) + 6OH- + 4HOCl --> 2SO4(2-) + 5H2O + 4Cl-
or equivalently
S2O3(2-) + 4HOCl + H2O --> 2SO4(2-) + 6H+ + 4Cl-
Thiosulfate Ion + Hypochlorous Acid + Water --> Sulfate Ion + Hydrogen Ion + Chloride Ion
or
S2O3(2-) + 4OCl- + H2O --> 2SO4(2-) + 2H+ + 4Cl-

which is very acidic. My experience from using thiosulfate drops in high chlorine (but < 10 ppm so as to not significantly affect the pH test directly) water and its affect on the pH test is that the reaction is alkaline, not acidic, so I believe these latter reactions don't occur. If we calculate actual oxidation potential for the 8 electron reaction then RT/nF = 8.314472*300/(8*96485.3415) = 0.00323 while ln(10^(14-7.5)^10) = -149.7 so the actual E = +1.501 - 0.00323 * 172.7 = 1.502 - 0.483 = +1.019V. This is still a very likely reaction so perhaps the rate of the reaction is slow (high activation energy?).

Richard

[LexPlayer]

Thiosulfate is supposed to reduce chlorine directly and not form combined chlorines. There are many possible reactions, but one of them is the following:

Na2S2O3•5H2O --> 2Na+ + S2O3(2-) + 5H2O

2S2O3(2-) + HOCl --> S4O6(2-) + OH- + Cl-

There are other reactions that take the thiosulfate all the way to sulfate or even sulfur, but I believe these are less likely. None of them form combined chlorine -- all produce chloride ion. The oxidation potentials are as follows:

................

Richard

Between your post and the chemical anlayis on this page, my eyes have gone buggy!

If Professional Pool Operators of America (PPOA) is correct, I would think that Sodium Sulfite is a better option than Sodium Thiosulfate beause there is no free Sulfur floating around "sulfinating" the water up.

I assume the Sodium sulfate (or Glauber's salt, sodium sulfate decahydrate, Na2SO4·10H2O per wikipedai) is just diluted into the water.

Anyway, I still don't know what my chlorine level is, but I'm trying to find a DPD test kit locally.

[waterbear]

Do yourself a favor and forget the DPD kit and get an FAS-DPD kit. It's easier, more accurate, and can test much higher chlorine levels with an accuracy as great as .2 ppm!

[CarlD]

Evan's right--you are just wasting money--and you'll have to get a FAS-DPD test kit sooner or later anyway.

DPD only goes to 15ppm before it bleaches out--looking like you have NO chlorine!

[LexPlayer]

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.

Yes, the strips only measure FC. But here's the thing, after initially adding the neutralizer, the OTO measured zero. After adding only .25 gal chlorine, the OTO measured 4.0 even with a 3:1 dilution. That's seems too high, even for TC.

Anyway, I added a pinch of the neutralizer to a bucket of pool water to remove excess chlorine and then test pH. The result was a deep purple. I didn't think thiosulfate was supposed to do that. I might have to call the manufacturer to find out what the heck they sold me.

PS- I read somewhere that thiosulfate was used to neutralize chlorine for testing and sulfite was used for actual chrlorine reduction in large municipal pools, etc.

Richard, I appreciate the detailed response. Although I have a degree in chemical engineering (15 years ago), it will take me a little while to comprehend the analysis.

[waterbear]

All the chlorine neutalizers sold in pool stores are sodium thiosulfate, according to the MSDS that I have seen. There might be some that are not but I have not come across them.