Gone through a LOT of dry acid, but pH test now turning purp

[wes8398]

Sorry to be such a pain, but I'm still having issues with my hot tub's ph. The phenol red test went purple on me the other night, and now it seems that it doesn't matter how much dry acid I add, the test keeps going purple. Bromine levels are a bit high right now (about 14) because it seems that bromine pucks in a CLOSED floater still adds too much activator to the water. So I'm wondering if the bromine levels are what's messing up the ph test. Anyway, I added 3 tablespoons of DA yesterday, tested again tonight (purple again), so added 2 more tablespoons DA. Between these DA additions I've also added enough baking soda to keep my TA ~60.
One thing to mention about the pH test is that it starts out looking like a normal pinkish colour (think 7.4 ish), but after sitting for 30 seconds it begins to slowly turn bright purple. Also, my BBB method pool was recently shocked and is sitting at about 12ppm FC. I did the phenol red test just to see, and it did not react the same as the hot tub's test. Which makes me wonder what's up with my spa water...
Any comments or suggestions? Hopefully this water isn't damaging my tub!! The sooner I can figure this out, the better!

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

Most likely, your bromine is much higher than 14 ppm. The high levels are interfering with the test. You should get your bromine levels down as soon as possible. I would probably do a partial, or full, drain and refill.

How are you testing the bromine?

 

[wes8398]

Most likely, your bromine is much higher than 14 ppm. The high levels are interfering with the test. You should get your bromine levels down as soon as possible. I would probably do a partial, or full, drain and refill.

How are you testing the bromine?

TF-100 DPD drop test. 11 drops to turn clear which is 12.35ppm when corrected for bromine. Levels aren't higher.

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

You could try adding 1 drop of R-0007 (thiosulfate) to the sample to see if that makes a difference. Add the drop first, cap, invert to mix, add the drops of pH indicator and then cap and invert to mix.

Another thing you could do is to add some R-0009 to see what that does. Add the pH indicator first to see what the color is and then add the R-0009 one drop at a time until the pH is in the right range. Cap and invert to mix after each drop. R-0009 is sulfuric acid, which is the same chemical as R-0005 (acid demand indicator). 10 drops of the R-0009 Total Alkalinity titrating reagent are approximately equal to 16 drops of the R-0005 acid demand reagent.

At high levels of bromine, the phenol red (pH indicator) becomes bromophenol red or bromophenol blue.

Either way, I think that once the bromine level comes down, the pH will read normally.

Are you using MPS (non-chlorine shock)?

 

[wes8398]

James, thanks so much for the info and suggestions! I just follow the TF-100 instructions for each test, so I had no idea that the chemicals could be intermingled like you've suggested. What would x number of drops of r-0009/r0005 indicate in terms of how much and what chemicals need to be added? Lastly, does the r0007 overcome the high sanitizer levels, or something else?

Thanks again!!

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

The drop of thiosulfate will neutralize some of the bromine and that should help determine if it's the bromine causing the purple color.

The R-0009 will allow you to perform an acid demand test to see if acid will make any difference. Multiply the number of drops needed by 0.6 to get the number of ounces of 31.45 % muriatic acid needed to lower the pH (assumes a 400 gallon tub).

Post the results back before adding any more acid. These tests are mostly to determine what is causing the test error, not to make a definite determination of where the pH is.

I think that you are going to need to get the bromine level down before you will be able to get an accurate reading on the pH. You can wait for the bromine to come down, you could do a partial drain and refill, or you could use some chlorine neutralizer (which will be sodium thiosulfate or sodium sulfite) from the pool store.

Between these DA additions I've also added enough baking soda to keep my TA ~60.

The fact that you need to add baking soda to keep the TA from going too low indicates that your pH is probably not high and that the pH indicator is being probably being converted to bromophenol red, which has a pH range of 5.2 (yellow) to 6.8 (red). Some of the indicator might be being converted to bromophenol blue, which has a range of 3 (yellow) to 4.6 (blue).

 

[wes8398]

I added 4 tablespoons of dry acid last night, before I read tour advice not to. pH test today, without any other chemicals started looking good but after 30 seconds or so it started getting purple (8.2) again. One drop of 0009 brought it down to a 6.8 looking colour. Redoing the test with an initial drop of 0007 before the phenol red gave nearly the exact same result with the prescribed 5 drops of phenol red.
So... What does this indicate?

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

I think that you need to wait until the bromine is less than 10 ppm before you're going to be able to get a good reading on the pH. It's possible that the pH is high or that there is some other issue, but I think that it's just the bromine. When lowering the PH, you don't want to add baking soda to raise the TA because the baking soda just neutralizes the acid. It's OK for the TA to be as low as 40 if that's where it needs to be to keep the pH stable.

Also, I find that using muriatic acid works better than dry acid for lowering pH or TA. They sell it in quarts, which make it easier to handle.

Another test you could do is to dilute the sample 50/50 with distilled water and then test that for pH.

 

[wes8398]

Sorry, I should have mentioned that total bromine was down to about 8 today as well. I took the floater out a few nights ago and I'm just letting the ozonator do the work...

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

At this point, I'm not sure if it's high pH, or if the color is due to the bromine. It's possible that the pH was high and the bromine was causing the color to be even more towards the purple range. The baking soda that you added would have offset the acid you were adding, so that could account for why the acid didn't seem to be making a difference.

I think that bromine converts phenol red to bromophenol red at a lower molarity than chlorine converts phenol red to chlorphenol red. I'm not sure exactly where the transition point is for the bromine. I would think that the pH test would be OK with the bromine at less than 10 ppm, but it might need to go lower.

I would just drain and refill. That's the fastest and easiest way to get the tub back under control.

 

[wes8398]

James, your help is much appreciated!! Im on my way up to the cottage right now. The tub will be left as-is for 3 days, then I'll test when I get home and post results.
I'm havin a heck of a time with the bromine levels, so I'll be curious to see where it's at after these 3 days left alone. The floater gets the levels way too high after only a few days and with the dial totally closed on it. Without the floater I thought levels were getting too low, so I dont know.
We'll see after these 3 days...

 

[JamesW]

Re: Gone through a LOT of dry acid, but pH test now turning

Also, remember that ozone will increase the bromine levels, so that will account for part of the bromine increases.

 

[wes8398]

Also, remember that ozone will increase the bromine levels, so that will account for part of the bromine increases.

Right. The frustrating part is that it's not enough to sustain levels on its own, but it's too much to be able to leave the floater in it.

 

[wes8398]

Well, things seem to be leveling out now. What I had been using was about 6-8 bromine tabs in the floater. I reduced that to 2 tabs and the bromine level seems much more stable. Likely as a result of that, I think pH is also stabilizing.