FAS-DPD powder question

Hi all,

I have a quick question about the amount of powder needed to run the test. I know I've read on here that if the test sol'n turns pink after one scoop then there's no need to add a second scoop.

But... My powder must be really fresh or something (it's a new kit) because I get a very clear pink color with 1/2 scoop or less of powder. In fact, when I had my chlorine levels up to over 12 ppm, I got a bright pink color sol'n with just a couple "bits" or "grains" of the powder.

So, should I be using a minimum of one scoop to run my test... Or just the amount needed to make the sample pink? It doesn't seem to get more pink by adding more powder, at least not significantly so, after that half-scoop. But I thought maybe there needed to be a minimum concentration or something....?

Thanks.

Honestly, with the DPD-FAS test, once you have color, you don't need more DPD.

The amount of DPD powder will slightly affect how clearly you see the transition at the endpoint of the test where more powder results in a sharper endpoint (the chlorine oxidation of DPD dye to a colored form is an equilibrium mostly towards the colored dye but not completely). Are you using a 10 ml sample size or a 25 ml sample size? With a 25 ml sample size, one full scoop is usually needed to see a decent transition, but your half scoop might be OK for a 10 ml sample size. You can certainly experiment and see if it makes any difference for you since you're the one doing the measurements. Odds are it will only change your result by one drop at the most so not a huge deal.

And, just as a reminder, with 10ml, one drop represents 0.5 ppm of FC and CC when testing.
So, as Chem_geek says, you won't be off by more than 0.5.