Having doubts about my pH testing!

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J

joelq

0
Mar 29, 2007
54
Hi all - long time Pool Solutions member, but first time TFP poster (longtime lurker).

I've had my IG 11K Salt pool since August of 2006 and have been testing my water daily (at least pH and Chlorine) since startup with a Taylor K-2005 test kit. I've noticed over the last 2-3 months that my pH likes to hang around 7.8 for about 5-6 days before it finally creeps up to 8.0. It will shoot from 7.2 to 7.6 in a matter of 2 or 3 days, then slow down as it approaches the 7.8 level. I started thinking, this isn't too bad - I'll just run my pool at a high pH, no problem.

I started having doubts a few weeks ago - and I'm not sure if I'm just misreading my pH colors, or if something else is going on. Last week, I watched my pH sit at 7.8 for 5 days, and finally to 7.9. In reading my pH test results, I have always considered 7.8 to be reddish, and if it starts to get a pinkish hue, I always considered it to be past 7.8, but if it wasn't as pink as the 8.0 color standard I always considered it to be 7.9. I dumped 32 oz of acid which should have lowered my pH to 7.2 (according to BleachCalc, but also confirmed by my historical acid usage).

After letting the pump run for about 3 hours, I tested my pH again, only to be baffled to get a 7.5 pH reading! Now the doubts are swimming all through my head! :)

- was I misreading my pH test and my pH was really well into the 8's? If so, does anyone have any tips for reading the pH test more accurately?
- is there something else that could cause the acid to somehow not be as strong? I keep my borates level at 50 ppm. Do the borates somehow cause more acid to be needed to lower the pH a determined amount?
- I haven't checked the % concentration of the acid that I have, but this is the same acid I have always purchased at Lowes (around $9 for a pack of 1-gallon jugs)

Any ideas would very much be appreciated! :)
 
Another thing to try is adding an extra drop or two of reagent during the pH test. The color will get darker, but the shade won't change. Good sanity check when you aren't sure.
 
Thanks for the replies, everyone! As for the chlorine levels affecting the pH tests, I try to always have my FC at 5 ppm, and that was the reading last week as well.

I'll definitely be double-checking the concentration on my muriatic acid.

Thanks!
 
tagprod said:
Could high chlorine make the PH test really low?
Click to expand...

Not exactly. High chlorine levels will convert phenol red to chlorphenol red, which measures PH in a lower range. With enough FC, any PH above 6.8 would read close to the regular PH 8.2 color, but it isn't really the same color, so it could be misread as various things. The converstion to chlorphenol can also happen only part way, resulting in various mixed shades that can be difficult to interpret.
 
JasonLion said:
tagprod said:
Could high chlorine make the PH test really low?
Click to expand...

Not exactly. High chlorine levels will convert phenol red to chlorphenol red, which measures PH in a lower range. With enough FC, any PH above 6.8 would read close to the regular PH 8.2 color, but it isn't really the same color, so it could be misread as various things. The converstion to chlorphenol can also happen only part way, resulting in various mixed shades that can be difficult to interpret.
Click to expand...

So if you shocked a pool at 15 PPM chlorine and the PH is reading 7.0 the next day it really is low - right? This was with a Taylor K2005 test kit.
 

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Hi all - I just checked the acid that I have and it reads "31.45% hydrochloric acid", so I guess I still have the "regular strength" acid.

I'm left to assume now that the reason adding 32 oz of acid didn't give me the ~0.8 ppm drop in pH and instead got ~0.4 ppm is because my actual pH was higher than I thought.

Does anyone have any pointers for accurately reading the pH test?
 
A nice bright white surface behind the sample helps a great deal. I hold it up in front of the wall of the shed, which is white, with sun shining directly on the wall. You can also use a sheet of paper in sunlight or a light box (if you have one).
 
Hi all - just a quick update, I got my pH down to 7.6 yesterday, so I thought of doing a test and trying to get it down to 7.2. According to BleachCalc, I need 8.8 oz of acid to drop my ph 0.2 ppm (11K gallons), so I added 20 oz of acid.

Three hours later, I re-tested my pH and got a reading of 7.4 when it "should" be 7.2. Hmmm. I've confirmed I'm using 31.45% acid. Can anyone think of anything that could be causing my acid to not be as strong?

My reagents are about a year old, but I keep them indoors in a cabinet. Could they possible be bad already?

Thanks!
 
How much your PH changes depends on several factors, not just the size of your pool. The TA can have a particuarly large impact on how much you need to use to change the PH. Very few of the calculators correct for the TA level, mine doesn't and BleachCalc doesn't. Just add more acid till the PH gets to where you want it to be.
 
A couple of observations....
How muuch your pH will move is partially dependant on how high your TA is. If the TA is high you will need more acid to get the pH to move down then if it is lower. If your TA is higher now it is normal for the same amount of acid to not cause as much as a pH change.

Borates create a secondary pH buffer and will bring the pH to about 7.6-7.7 very quickly and lock it there for a spell so what you are seeing with the pH is normal for a pool with borates. You will actually get better pH stability with your SWG if you lower your TA to 70-90 ppm, adjust the pH to 7.6 and not any lower and add acid to lower it back to 7.6 when it hit 7.8. Also, if you adjust your SWG output so your FC is about3-4 ppm you will get better pH stability since ther will be less outgassing of CO2 caused by the hydrogen bubbles generated in the cell. Make sure your CYA is about 70 ppm.
 
Hi all - thanks very much for the replies.

Buggsw - I'm pretty sure of the volume of my pool, at least within a couple hundred gallons, as I checked the before-and-after water meter readings when the PB started filling the pool.

It also makes me feel better that this rapid rise to 7.7 pH or so is normal for a pool with borates.

I never posted my latest complete numbers, but here they are:

FC: 4
CC: 0
pH: 7.8
TA: 80
CH: 330
CYA: 70
Salt: 3000
Borates: 50

Thanks, everyone!
 
waterbear said:
...Also, if you adjust your SWG output so your FC is about3-4 ppm you will get better pH stability since ther will be less outgassing of CO2 caused by the hydrogen bubbles generated in the cell. Make sure your CYA is about 70 ppm.
Click to expand...

Waterbear - I just now went back and re-read your post. Quick question on the FC of 3-4ppm - with a CYA of 70 ppm wouldn't I be on the low end of the "best-guess" for target FC? I've been trying to keep my FC in the 5 range because of this.

Thanks!
 
The best guess chart was developed for manually chlorinated pools. It has been found through empirical observation that salt pools can run at a lower FC level and be problem free.
 
So, I was wondering about my acid concentration, which turns out to be only 14.5%. If I can't find the higher concentration is it unwise to double the amount of acid I add when lowering my ph? It's says in the instructions I have for adding chemicals that I should never add more than a pint of acid in a 24 hour period for my pool's capacity. But now I'm wondering if that is solely meant for the 31% concentration. The charts and directions I have don't mention the concentration. I don't want to add too much, though and risk damaging anything.
 

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