DPD bromine vs. OTO bromine

[Alfred Beachport]

With the brand new DPD kit from Leslie's (store replacement for my expired Spanish one), I tested my bromine spa a few minutes after activating with MPS.

The reading initially indicated lower than expected (based on an OTO test I had just done on the same sample), but slowly (over a few minutes) got darker and darker until it settled down about a notch (2 ppm) higher than the OTO test.

I read that MPS can throw off the DPD's free chlorine test; can it affect the total chlorine/bromine test? If not, why don't my tests agree?

And what's up with the indication creeping up? Is that normal? I've read that darkening OTO usually indicates CC, but this is DPD.

 

[Alfred Beachport]

Tested 12 hours after MPSing, and both OTO and DPD agreed that there was 5ppm bromine in the water.

It looks (to me) like MPS can throw off a DPD FC reading. Anyone else?

I guess that DPD test is going to be mostly good for testing OPP (other people's pools). I'll see what it says next time I try activating with a cup of bleach.

 

[chem geek]

You are correct that MPS will affect the DPD test as well as the FAS-DPD test as it will register mostly as CC though if you want long enough it appears as FC (in general, you want to measure FC without waiting too long as any CC will slowly show up as FC -- it just happens a little faster with MPS).

The MPS will break down in sunlight so after a full day of sun the reading should get back to normal. Of course, if your spa were exposed to that much sunlight, you wouldn't want to use bromine since sunlight breaks down bromine as well.

There is an interference remover called the Taylor K-1520 here for getting rid of the MPS for the DPD test and there is the Taylor K-2041 here for getting rid of it in the FAS-DPD test. However, some people have reported problems with using these interference removers so they don't seem to work reliably.

Richard

 

[CaOCl2]

There is an interference remover called the Taylor K-1520 here for getting rid of the MPS for the DPD test and there is the Taylor K-2041 here for getting rid of it in the FAS-DPD test.

In both cases it's the same reagent, R-0867.