DPD Powder

[rightnitro]

I have only ever used the smallest amount that turns the sample pink, which is just a tiny scoop, not even heaping.

I think the "expiration" of these chemicals is a fake thing. I am still using my kit from 2011, and compared to the one I got 3 weeks ago, it gives the same exact results for every test.

 

[Mr Bruce]

The R-0871 can absolutely expire. Years ago I was using an out of date bottle and it was weaker, leading to more drops to go from pink to clear, indicating FC was much higher than it actually was.

 

[rowiac]

The tftestkits.net instructions that came with my TF-100 kit do not even mention the 25ml test--only the 10ml, so now I see that this comes from the Taylor kit instructions. For the 10ml test, I follow the tftestkits instructions, which is one slightly rounded scoop of powder. So, I have never used the 25ml method and I don't think I need better than 0.5ppm resolution in my results. That would just be a waste of reagents.

With the SpeedStir, I can tell right away if my level is about right based on how dark the pink color is initially. After testing enough times, you get an idea of what it should look like.

 

[duraleigh]

In the presence of VERY. VERY high FC, the dpd powder will "flash" pink and then turn back to clear. 2 scoops is needed to get the pink to stay until you can use the R870 but that is about the only time you need more than one scoop.

The density of the pink is not terribly important.....holding the dropper bottle vertically and allowing the drop to form and then fall off the tip (don't squeeze) is an important part of correct technique.

An equally important technique is to continue until the last drop you use results in ABSOLUTELY no FURTHER color change. Then subtract that last drop and you have the correct endpoint.

I think the "expiration" of these chemicals is a fake thing.

Nope. Everything eventually gets old and performs poorly (me included). A one year date is a nice number you can use to make sure your tests will always be accurate, Sure, some of the chemistry can go well beyond that date but using expired chemistry to save money is penny wise and pound foolish. TFP teaches precision and accuracy and expired chemistry just doesn't fit in to the scheme of things........the money you save is just not worth having doubts about your testing.

 

[rowiac]

An equally important technique is to continue until the last drop you use results in ABSOLUTELY no FURTHER color change. Then subtract that last drop and you have the correct endpoint.

I use the light on the SpeedStir to help determine the last drop. There will be a very slight pink tinge left and one more drop removes it. I do not subtract that last drop. It sounds like you would go one more drop to make sure there is no further change. If you do that, how does it impact the CC test that follows? After adding 5 drops of R-0003 (per TFTestkits instructions) is the number of drops of R-0871 still accurate?

Also I think I need to improve my dropping technique--I squeeze the bottle a little bit.

 

[frogabog]

Also I think I need to improve my dropping technique--I squeeze the bottle a little bit.

For me, the drops come out on their own at first, then they stop and I have to squeeze. If I waited for the drops to come on their own I'd be there for quite a while. Although now that I think about it probably if I turn the bottle over and start again I'd get the same effect as when I first begin dripping.

Hmmm... seems I can figure things out on my own!!

 

[JoyfulNoise]

Folks, you have it wrong. You don’t need two scoops because the FC is low, you need the two scoops to ensure that there’s enough DPD to react with all of the chlorine. If you have too little DPD, you can get a false-low reading. The powder is a mixture of chemicals, not just DPD. In fact, the DPD component only makes up less than 2% of the overall mixture. The rest are pH adjusters and buffers and some metal chelating agents to avoid interference.

I’ve never been an advocate of the “one scoop is fine” philosophy. I use 2 scoops ALL the time.

 

[frogabog]

To be fair, the TF-100 does say one heaping scoop. I never do a level scoop, but the amount of heaping has been inconsistent (for me) and all the varying kinds of heaping I've tried works normally. Shrug...