DPD vs FAS-DPD

Matt Ush

In The Industry
May 19, 2012
46
0
#1
Hello TFP Team,
I have not yet verified this since the pool is currently closed..I may add some chlorine to tap water to confirm. Taylor offers the K-2005 and the K-2006. The main difference between these two kits is that the 2005 is liquid DPD and the 2006 is powdered DPD with FAS liquid. My question is why can I not combine the two together for a perfect kit?

In the 2005, you add DPD 1 to DPD 2 to develop a shade of pink, which is then compared using a comparator to get your free chlorine level (Not to be confused with orthotolidine tests which are generally yellow in color). Then, to find combined chlorine, you add DPD 3 which would turn the sample more pink giving you a total chlorine reading. You then subtract the two samples to get a combined chlorine reading.

In the 2006, you add two scoops of DPD powder (R-0870) to develop a shade of pink. Then, using R-0871 drop wise, you turn the sample clear and record your result. To find combined chlorine, you add the same chemical as used in the 2005, DPD 3, and turn it clear once again using R-0871.

Why can I not use DPD 1 + DPD 2 to get my pink sample instead of using DPD powder? Now that I have my pink sample using only liquid, I can then either follow the directions for the K-2005, matching the color, or the K-2006, adding R-0871 drop wise etc... You think that would work? My problem is that DPD powder has a questionable shelf life when used frequently. It starts to cake up and turn black. It would also be nice to have two options. Using the comparator for a fast glance, or using the drops for a more accurate answer without having to perform two tests.

Check out the ingredients for DPD 1 + DPD 2 here.
https://www.taylortechnologies.com/MSDSS/0001.PDF
https://www.taylortechnologies.com/MSDSS/0002.PDF

Check out the ingredients for DPD powder here.
https://www.taylortechnologies.com/MSDSS/0870.PDF

See a similarity?
 

JasonLion

LifeTime Supporter
Platinum Supporter
TFP Expert
May 7, 2007
37,879
5
Silver Spring, MD
#2
That doesn't work. The powder is a different chemical than the liquids. You can't do a FAS-DPD test without the powder.

The powder will turn gray and then black, but it is still good. If it clumps up too much, just grind it up a little with a mortar and pestle and it will continue to be usable. It is the FAS-DPD liquid that can go bad, though it should last a good long while if stored in a cool dark place. If you are a pro, only carry around a small container of powder, so you use it up fairly quickly, and keep your main supply, that you replenish the small one from, somewhere where it won't get much exposure to humidity.

In my opinion the DPD test has no properties that make it worth using. If you want something simple and quick use the OTO chlorine test. If you want something precise and with a wide range use FAS-DPD.
 

Matt Ush

In The Industry
May 19, 2012
46
0
#3
My issue with OTO is it doesn't test free chlorine, only total chlorine. Right now, we use the small containers of DPD powder, but going pool to pool, it becomes a hassle and majority of the time I don't necessarily need to know the level down to 0.5. It looks like I will need to keep the DPD 1 and DPD 2, as well as DPD powder in my kit. Sigh..
 

eqbob

Well-known member
Jul 26, 2012
436
0
Central Texas
#4
I started out with a pool service person. He didn't test for anything, much less worrying about which kit he'd use <G>. He sold his business and we inherited another service person whom I then kicked out for a number of reasons. I then used the DPD test for a while, but rangers weren't conducive to knowing exactly where I was., I also want to know the index values of CSI and LSI which require the FAS/DPD degree of accuracy.

Not sure why the FAS/DPD test is more cumbersome or bothersome? It takes the same amount of time and is more precise, so better decision making.

I had some powder get black. I just picked that part out and threw it away (Didn't have a mortar and pedestal even if I had known you could regrind it)

There are plenty of sites where you can get a half pound of the powder and 200 ml bottles of the reagent. That's what I refilled with.