Fas/dpd

[DONNIE]

I know its critical to fill test water to proper levels and try to do the drops as exact as possible etc.....but how critical is the measurement of the DPD powder? That little scoop can be interpreted many ways.

Donnie

[JohnT]

I know its critical to fill test water to proper levels and try to do the drops as exact as possible etc.....but how critical is the measurement of the DPD powder? That little scoop can be interpreted many ways.

Donnie

Doesn't appear to be at all critical.

[chem geek]

You only need to add enough scoops to produce a consistent pink color. So if for some reason you have very high chlorine levels > 25 ppm FC, then you may find that adding the powder shows a flash of pink and then either turns brown or clear. If you then add more powder, you should get the pink and can then continue the test to titrate (count drops) until the sample becomes clear.

Richard

[CarlD]

Yeah, it's not critical--if you use too much it doesn't seem to affect the ability of the titration fluid to give an accurate reading.

I've been using it for years and you only need enough to turn the water nicely pink, no more. If it's less than two scoops, fine.

However, if you have NO chlorine, it won't turn pink at all. This is a good time to double-check with a standard OTO test kit. Ben's pH test cell also has the OTO test for chorine.

[DONNIE]

Thanks. guys. I get consistant results with 2 scoops but Im always trying to fill the scoop "just right" by taping any excess out of the little depression on the scoop.

Donnie

[CarlD]

Thanks. guys. I get consistant results with 2 scoops but Im always trying to fill the scoop "just right" by taping any excess out of the little depression on the scoop.

Donnie

Don't bother! Use less if you can--the less the better, as long as it turns pink. That way you get more tests out of it.

But be really careful to keep the powder as dry as possible. NEVER put an even slightly damp scoop in it.

[fcfrey]

Carl and Richard
Please don't yell but I do have to disagree.

When I ran several tests and adding enough to turn it "nicely pink" gave different results than if you use the "prescribed 2 scoops". I called Taylor and they told me that their test requires 50 milligrams of DPD powder which is 2 of their blue scoops. I get very consistent results using this method.

IMHO it's like baking a cake add too much flour it gets tough, too many eggs will make it fall.

Do what you want but I think the manufacturer of the test should know what is best. Even though they do want to sell reagents.

[DONNIE]

But how much is a scoop? A heaping scoop? A level scoop? A scoop that just fills the depression on the spoon?

[fcfrey]

I used a "mini" electronic scale (for reloading ammunition), which measures from 50 grams down to .001 Gram. I found that if I squashed the filled scoop against the inside of the container and then slide it up out of the powder, it gives a packed, nearly level, scoop which is consistently 25 milligrams. Obviously I use 2 scoops (50 milligrams)

Please keep in mind that I use the small blue scoop supplied by Taylor not the larger scoop that is in Ben's kit.

I think, and am sure most every one will agree, the most important part is being consistent with your methods. You will get consistent results which is more important than if you are .1 or .2 ppm off in the result.

In cooking and pool chemistry I admit to being a bit anal about following the directions. That way the recipe comes out right!!

[chem geek]

Out of curiosity, I tried my own experiment and used 1, 2 and 4 scoops and got within 1 drop of the same results with quick consecutive tests, but I did notice variation with time -- mostly declining. There were several factors that could have contributed to the decline with time (mostly that I used a single container that was filled with pool water which was probably warming up and outgassing and getting partly contaminated even though I rinsed the measuring test tube with tap water between tests -- and yes, tap water has chloramine so that didn't help), but I kept going back to different scoop amounts and saw no variation consistent with the number of scoops. If I swirled longer I usually got a slightly lower result than expected (accounting for the slower decline with time). However, I did not try less than one scoop for a "barely visible" test.

I think the main reason that the 50 mg is recommended is that the DPD test is colorimetric in that the intensity of the pink is related to the chlorine concentration AND the amount of DPD powder, so even in the FAS-DPD test the "endpoint" of light pink to clear will be a little dependent on this absolute intensity. Taylor probably assumes a certain typical chlorine level for this test. If you only used one scoop and had very low chlorine levels, you probably wouldn't see much pink to start with so the transition to clear would be hard to catch. Also, higher chlorine levels will bleach out the DPD powder and that could be as low as 12 ppm if only one scoop were used. Two scoops is probably the minimum that works well for this test, but more scoops would certainly not be a problem as even the Taylor instructions (see this link) say to add more scoops if the pink goes away right away (which happens with > 25 ppm FC if you start with two scoops).

Anyway, it's probably best to stick with the Taylor recommendation and at least now we know that a compacted "flat" scoop fill is the accurate amount, but I'm with Carl that it's not critical to get the scoop measurement exactly right. I am not disputing your results, but it seems that there are lots of factors for variation that outweigh at least 1,2,4 scoops. Maybe more (or less) scoops than that starts to give a problem, but I didn't want to use up all my chemicals!

Richard