Bromine - knowledge is confusion!

[yo_adrian]

Hello! I've had my hot tub for 5 or 6 years and muddled along with the test strips with reasonable success. My tub has mostly stayed balanced, except the odd times I've missed a few maintenance cycles. I have noticed the bromine smell is stronger than I'd like, so I did some research and found this great forum! I've read up on a bunch of threads and feel a lot more confident and knowledgeable about maintenance now.

One thing I learned right away was that the strips are a pretty bad way to go. I ordered the Taylor K-2106 test kit and have a few questions about bromine testing. I do the test and get the free and combined, do I multiple the result by 2.25? The directions don't specify that for the DPD test. Is the total bromine the sum of the free and combined?

I'm curious what the advantages are using the DPD test. It seems like the test I need is for total bromine, is there an advantage to knowing free vs combined? How do I use this information to better maintain my tub?

Lastly, I've been shocking with MPS and after reading a lot of the posts here I'm thinking about switching to chlorine shock. I have a bottle of Spa 56 that seems like it would be the right thing to use. How do I determine the best shock dosage for my 320 gallon tub?

Thanks in advance!
Adrian

 

[Justhereforthebugs]

Hi Adrian,

The DPD drop-test works for bromine you just have to multiply your result. In what I have seen, bromine has issues with bleaching and reliability at lower concentrations than it does with chlorine. I wouldn't trust DPD drop-test readings at high concentrations of bromine.

DPD can be oxidized by bromoamines, and you do not necessarily need to add potassium iodide to read total bromine. To the best of my knowledge, the Taylor drop-tests for bromine function as total bromine since there is no KI reagent. There are conversations to be had as to how effective bromoamines are as sanitizers, but it seems the industry mostly treats free and total bromine as interchangeable in terms of active sanitizers.

 

[yo_adrian]

Thanks for your reply! I'm not totally clear on what you mean. I put the DPD powder into the sample, then add the 0871 reagent (the bottle doesn't say what it is exactly), then read the first reading. Then I add the 0003 reagent (again, unspecified as to what it is), add the 0871 again and get the second reading. For bromine, is reading 1 the total bromine, and you're saying I don't need to do reading 2?

I realized when I bought my kit they sent me the K-2006c not the K-2106. The label on the box says "chlorine" where the 2106 says "bromine". I think the only difference may be the label, and that I didn't get the instructions for bromine, everything else looks the same. In searching around, it looks like I'm going to multiply the reading by .5 to get my bromine level...

 

[Justhereforthebugs]

In the typical DPD chlorine drop test you are measuring free and total chlorine separately. The R-0870 powder contains your DPD and the R-0871 is the titrating agent. The R-0003 is a potassium iodide solution that you add to get your total chlorine reading. Your free chlorine is considered your active sanitizer. The combined chlorine is a byproduct of your chlorine reacting with organics in the pool that can cause irritation and bad smell.

For most bromine tests the free and combined bromine are both considered to be active sanitizers. The combined bromine/bromoamines will give a response to the DPD, so you will measure total bromine in that test. To my understanding, most of the industry is in agreement about this way of measuring. A few tests use a potassium iodide solution for their bromine-DPD tests but it's not universal.

To convert from chlorine to bromine using the DPD drop-test you use their molecular weights. The conversion factor is 2.25 (159.81 g/mol /70.9 g/mol) to convert from chlorine to bromine. This is often simplified further to just multiplying by 2. You can go the other way by multiplying by .5.

Let's say your reading of your spa is 2.5 ppm in terms of chlorine (you don't need to do use the R-0003). To convert to bromine you would multiply by 2.25 (or 2) to obtain roughly 5.5 (or 5).

If you're more comfortable using just the bromine specific titrant, you could purchase just that reagent as the DPD powder is the same.

 

[yo_adrian]

Yes, thanks. Getting much clearer now. I also gave Taylor a quick call also and they have me the following.
Do the DPD powder and the 0871 dropwise. Multiply by .2 to get the chlorine reading. The multiply that by 2.25 to get the total bromine reading. This is the only reading needed. Doing some quick math I see I can also multiple the dropwise count by .45 to get the bromine reading with one less step. Basically the same as what you said, but clarifying you have to get the chlorine number first. Also pretty darn close to the original way I did it, just multiplying by .5. This gave me a reading that was 10% off .

If you get the bromine specific reagent you multiple by .5. Seems like a very minor difference and not worth changing.

Adrian