How to test High Chlorine

[fcfrey]

I just read the entire thread started by DonnaElvira on this page and I have to ask ----- How are we testing for a high Cl level? I use a Taylor DPD test kit which, as I have read many times on this site, is not capable of reading above 5 PPM. Other than the "shotglass" method, is there a test kit out there that I should get. Again, with the bleaching action of high CL levels and the comparator only going to 5. I'm stumped!!!!

When I do shock how can I tell if I've reached breakpoint chlorination if I can't test that high of a level accurately? It takes about 24 hours to turn the pool over with the 29000 gallons at 70 GPM through a DE Perflex filter. At the end of the first day the Cl is off the chart but where???? Do I really need to know? Ben's Best Guess Cl for CYA dictates a given level. So, again, How do I know where I am?

Latest test results are:

FC 1.5
TC 1.5
CC 0
TA 90
CYA 50
pH 7.7
CH 230
Temp 82
Phosphates < 100
29000 Vinyl IG
Crystal Clear ---- No Problems and don't want any!!!!!!!!!!

I am making the transition to BBB so I am keeping close track of things.

[waterbear]

Get an FAS-DPD test. That will accurately test high levels of chlorine. The Taylor K-2006 uses FAS-DPD. So does Ben's kit. The Taylor K-2005 is DPD.

[fcfrey]

How high is this "FAS-DPD test capable of seeing. What does FAS mean and how is it different from DPD?

[waterbear]

How high is this "FAS-DPD test capable of seeing. What does FAS mean and how is it different from DPD? The DPD test uses 3 reagents. The first is a buffer solution, the second is DPD (N,N-diethyl-p-phenylendiamine) which turns pink in the presence of free chlorine. The color is compared on a comparator block to get the FC reading and then the third reagent, potassium iodide is added to get the TC reading once again using the comparator block. The FC reading is then subtracted from the TC reading to get the CC. The biggest drawback to the DPD test, besides the inaccuracy of the human eye and color comparator blocks, is that DPD will bleach out at chorine levels above about 10 ppm which could lead you to believe that there is NO chlorine when, in fact, the chlorine levels are very high. The sample can be diluted to overcome this but you will lose precision in the reading.

The FAS-DPD test (it stands for Ferrous Ammonium Sulfate- DPD) is performed by adding a buffered DPD poweder to the water sample (10 ml). It will turn pink if chlorine is present. The sample is then titrated with the Ferrous Ammonium Sulfate solution until the pink color disappears and the sample turns colorless. The number of drops is counted and this is multiplied by 2 to get the FC readings (each drop= .5 ppm FC). Potassium iodide is then added and if any CC is present the solution will turn pink again. The sample is then titrated again and the drops counted...once again each drop= .5 ppm CC. If you want a TC reading you add together the FC and CC. This test will not beach out like the DPD test. If the sample bleaches out when the DPD powder is added you simply add more until the pink color stays. This way it is possible to test well over 20 ppm (possibly up to 50 ppm) FC. Also if the water sample size is increased to 25 ml the test becomes accurate to .2 ppm per drop instead of .5 ppm per drop.

Hope this explains it without confusing you too much.
Check out this page on the Taylor Technologies website for more info
http://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=11

[fcfrey]

WOW ---- Great reply ---- Thanks --- That really clears things up (No pun intended) I am constantly seeing something different when I look at the comparator tube --- This would be much more precise and leave nothing to interpretation. Thanks Again.