Chlorine levels went higher than add after shocking pool

[donaldm823]

Been using the BBB method since opening my 30000 gal inground concrete pool in May this year. Water has been clear and no algae problems. I have a DE filter and a Nature 2 system so been keeping chlorine low 1-2ppm with no problems and water has been crystal clear since May 2007 (it was clear when I opened the pool also. I last shocked the pool in mid June 2007. 4 days ago we had a big rain (2 inches), but my free chlorine levels (FC) held at 1-2ppm. I usually add 60oz of chlorox each night and FC has been maintaining 1-2 ppm without issue during the day. (my pool gets full sunlight). 2 days ago, my FC dropped during the day to 0.1ppm for no reason-I quickly added chlorox for a 1.5ppm increase and measured it the next morning. Again, the FC dropped to 0 during the night, so I added another 136 oz dose of chlorox plus a 1lb bag of CaHypo that morning (yesterday morning) and measured FC during the day-it was 5 ppm (as expected after this add) and slowly decreased to 3.3ppm by the end of the day (yesterday) (not too bad since sunny all day). So, that evening (last night) I shocked the pool with 4 large jugs (1.4gal) of chlorox or 5.6 gal total which should have been an 11ppm increase.

2 hrs later (last night) I measured FC and it was 22ppm and the pool was cloudy-barely able to see the main drain (8 ft of water). pH also had gone up to 8.2 from the chlorox add. The next morning (this morning) it had decreased down to 16ppm (and pH went down to 8.0). The water this morning has cleared up approx 50% as I can now see the main drain clearly although still some cloudiness in the water. At this point I back flushed the DE filter as the back press had risen from 21psi to 27psi during the night (went up 6psi after shocking).

Question is, where did the chlorine come from? The FC is way higher than I added, so it appears the chlorine came out of some "black hole" after shocking. I have been measuring FC with both OTO Taylor drops and my LaMotte DPD colorimeter, so I think the high chlorine was a true reading as they both compared before and after the shock (had to dilute 1 to 3 after the shock to get a reading within range).

Any ideas why my chlorine rose to 22ppm when I only added enough chlorox for 11ppm? (5.6 gal).

My latest chemistry shows pH a little high (it is coming down as the chlorine level comes down. Here is my chemistry before and after the shock. Pool temp = 85F. I expect pH to continue to decrease so I have not added for pH, plus the water is in balance at this point.

Any suggestions of what happened would be appreciated. I will be adjusting pH as needed after my chlorine comes down due the the sun



Before shock
FC 3.3/TC 3.5
pH 7.6
TA 75
CH 218
CYA 46
water clear, DE press 22psi

After shock
FC 16/TC 16
pH 8.0
TA 80
CH 218
CYA 46
water slightly cloudy, DE press 27psi

 

[JasonLion]

The ColorQ is quite good at measuring chlorine levels, so I don't doubt that your measurements are good.

It doesn't really make much sense, except that one other person recently had the exact same thing happen to him. I am beginning to suspect that there is a batch of extra strong bleach out there. What brand of bleach did you use?

 

[Guest]

First thing, your pH did NOT increase. When chlorine levels are high pH will read highe because of an interferance between high sanitizer levels and the phenol red indicator used for pH tests. Even Taylor's reagent, which is one of the best at reading accurate pH with high chlorine, is not going to be accurate if the chlorine levels are much over 12 ppm. Second thing. You should find that your pH reading is normal after the chlorine levels drop back to normal.

Second, OTO only measures total chlorine, not free chlorine so if you had a chlorine demand it is entirely possible that your chlorine was that high. You might not have had much FC in the pool since your pool turned cloudy. It was probably all TC. You also have to remember that dilutions will decrease the accuracy of the tests . The ColorQ can only accurately measure chlorine up to 10 ppm and if you dilute as much as you did you will introduce a bit of error into the testing. The ONLY way to accurately measure FC and TC as high as you are reporting is with an FAS-DPD titration test. By addind .8 gallon of bleach to raise it to 1.5 then adding another 136 oz and a bag of cal hypo and then adding another 5.4 gallons it is entirely possible that ,while you might not have had much FC your TC would be around 16 ppm and with the dilition error you introduced you might have gotten a reading as high as 22 ppm.

 

[donaldm823]

First thing, your pH did NOT increase. When chlorine levels are high pH will read higher because of an interferance between high sanitizer levels and the phenol red indicator used for pH tests. Even Taylor's reagent, which is one of the best at reading accurate pH with high chlorine, is not going to be accurate if the chlorine levels are much over 12 ppm. Second thing. You should find that your pH reading is normal after the chlorine levels drop back to normal.

Second, OTO only measures total chlorine, not free chlorine so if you had a chlorine demand it is entirely possible that your chlorine was that high. You might not have had much FC in the pool since your pool turned cloudy. It was probably all TC. You also have to remember that dilutions will decrease the accuracy of the tests . The ColorQ can only accurately measure chlorine up to 10 ppm and if you dilute as much as you did you will introduce a bit of error into the testing. The ONLY way to accurately measure FC and TC as high as you are reporting is with an FAS-DPD titration test. By addind .8 gallon of bleach to raise it to 1.5 then adding another 136 oz and a bag of cal hypo and then adding another 5.4 gallons it is entirely possible that ,while you might not have had much FC your TC would be around 16 ppm and with the dilition error you introduced you might have gotten a reading as high as 22 ppm.

ok, Water Bear, your analysis makes this mess make sense!!! THANK YOU. I did not consider analysis errors due to the high chlorine levels plus dilution errors. Appreciate the explanation and I understand the pH will come back so I will hold off any pH corrections. The FC sample at 1200 (6 hrs later) was 11.2ppm (TC was the same) and pH of 7.8, but I still had to dilute 1 to 2 on the colorQ introducing some accuracy errors per your comment.

Is there anything I should do other than wait until chlorine levels settle in and watch for any decrease tonight? The pool is almost totally clear now and I am hoping chlorine will not drop during the night. I guess you think I had some contaminant in the pool yesterday that was eating chlorine-other than watching for FC decreases (and reshocking as needed), is there anything else I am missing?

 

[donaldm823]

The ColorQ is quite good at measuring chlorine levels, so I don't doubt that your measurements are good.

It doesn't really make much sense, except that one other person recently had the exact same thing happen to him. I am beginning to suspect that there is a batch of extra strong bleach out there. What brand of bleach did you use?

Using Chlorox regular-180 oz bottle. The bottle says 6%. Based on Water Bear's comments, the 22ppm may have been analysis errors

 

[Guest]

If it were my pool I would go swimming as soon as the chlorine was below 10 ppm. As an aside, this is another reason why I don't like Nature2. You are running a reduced chlorine level and it was not enough to take care of a chlorine demand problem that occurred. If you had been running your FC at a higher (normal) level for your CYA, about 4 ppm, you probably would not have had this problem.

 

[chem geek]

The other possibility is that it took a while for your Cal-Hypo to dissolve and dissipate. It sometimes dissolves slowly. So maybe it continued to dissolve when you added your bleach and you thought the FC increase was from the bleach but part of it was from the Cal-Hypo. The Cal-Hypo can also make the water cloudy since it adds calcium (and raises pH temporarily, until the chlorine gets used up) -- this cloudiness eventually dissipates as the Cal-Hypo mixes with all the pool water.

Richard