difference in Taylor chlorine tests

[mohawk]

I have a regular taylor chlorine test that measures up to 5ppm that I have been using for awhile. I received in the mail a new fas-dpd chlorine test kit. When I use the reg. one it may measure 3ppm of chlorine in my pool and the other will say I have 28ppm. The directions say to use 2 dipper fulls of dpd powder. I assume that means 2 of the round end fulls of powder. What do you think I am doing wrong? Which should I believe?

[chem geek]

How many drops of titrant does it take to have the sample go from pink to clear? If you used a sample size of 10 ml, then you need to multiply the number of drops by 0.5 (or divide the number of drops by 2) to get the Free Chlorine (FC) level. If you used the sample size of 25 ml, then you need to multiply the number of drops by 0.2 (or divide the number of drops by 5) to get the Free Chlorine (FC) level.

If you counted 28 drops and used a 25 ml sample size, then that would be 5.6 ppm FC which is certainly higher than 3, but is probably the accurate number.

Richard

[mohawk]

I used 14 drops and multiplied by 2-I don't know why. I'll get out the calculator and figure it out. Thanks for the help. Denise

[CarlD]

Also, you don't need two scoops of FAS-DPD powder. You just need enough to turn the water dark pink--like rose wine. Adding more doesn't change the reading you get, it just wastes the powder. But if it doesn't turn that dark pink unless you add two scoops, then, of course, use the two scoops (unless you have no chlorine, in which case it won't change color at all).

[mohawk]

Thanks for the info. That's good info that the directions don't give. Denise

[chem geek]

I used 14 drops and multiplied by 2-I don't know why. I'll get out the calculator and figure it out. Thanks for the help. Denise

So, 14 drops in a 25-ml sample size would be 14 * 0.2 = 2.8 ppm FC which is essentially the 3 ppm FC seen on the DPD test! At least things are consistent.