testing chlorine levels when levels are high

[rhythm]

Just wondering about something:

With my test kit (K-2006), if I want to check my chlorine levels during a shock and hold, I'll be using a LOT of the FAS-DPD titrating reagent. Using the 10 ml sample amount, I'd use 40-50 drops if my Cl is between 20-25 ppm. When testing the Cl levels when the levels are this high, is there an alternate way of checking without using so much titrating reagent?

Thanks,
Greg

 

[chem geek]

If you aren't worried about being as accurate, then you can dilute your sample with distilled or filtered water (water that has any chlorine or monochloramine removed from it). If you dilute the 10 ml sample to 25 ml, then each drop will represent 1.25 ppm instead of 0.5 ppm. If you dilute the 10 ml sample to 50 ml, then each drop will represent 2.5 ppm.

Richard

 

[rhythm]

Richard,

Thanks for the suggestion. I'll test some of our filtered water for free chlorine. If none registers, I'll go ahead and dilute my pool water sample with it as you suggested. Being a little less accurate in this case should be fine, since I'm targeting a Cl level a little higher than what is really needed, just to be on the safe side.

Thanks,
Greg

 

[rimshaker]

Richard please correct me if i'm wrong but isn't that backwards? To increase the ppm measurement for each drop, shouldn't the water sample be smaller?

During shock levels I use the Taylor alternate test method where a 5ml sample is used. 1 drop = 1ppm.

A large 25ml sample will increase accuracy where 1 drop = 0.2ppm but I can't think of how that can be very useful anyway.

 

[chem geek]

Well, I was wrong, but not exactly backwards. A smaller UNDILUTED sample size will have each drop representing a larger quantity (as you point out), but if one measures a larger diluted sample (which I was incorrectly suggesting), then each drop represents the SAME quantity as before (because the total amount of substance to react with hasn't changed).

What I should have said was to take a 10 ml sample, dilute it to 25 ml and mix, then take 10 ml of THAT diluted sample to measure (it's that last part I left out). In that case, each drop represents 2.5 times what it would have before, so instead of 0.5 ppm it represents 1.25 ppm for each drop. If you take a 10 ml sample, dilute it to 50 ml and mix, then take 10 ml of that diluted sample to measure, then this is 5 times 0.5 or 2.5 ppm per drop. Your method of just taking a 5 ml sample with no dilution also works to have each drop be 1 ppm FC.

So the two ways to get the drop size to represent more FC are: 1) use a smaller undiluted sample size or 2) dilute the sample and then measure a smaller portion of it.

Thanks for catching the error.

Richard