Changed to 10% liquid chlorine from 7.55% Clorox...is my pH observation coincidence?

[Insaneoctane]

So I’m just over one month into the dichlor to bleach method and have been loving it! I’ve been using 7.55% Clorox and with my total alkalinity at 70, and have been very happy with very stable pH of 7.6. But this weekend I switched to 10% liquid chlorine. Yesterday was my weekly SLAM protocol and I adjusted for the slightly stronger chlorine and added 0.9 cups to elevate my FC to about 16 (my CYA is 35). I checked a little later as the family was interested in getting in... I overshot it a tad with FC of 17, but the pH was up at 8.0+. Acid demand took 2 drops it get to 7.6. So either I just never checked pH right after SLAMing, or the 10% made a pH impact. Admittedly, I don’t normally SLAM right before getting in, but thats how it worked yesterday. I put in about 1 TBSP of pH down product to get it down to 7.8 (I was worried about getting in at 8.0) and we got in. This afternoon it’s down to 7.6.
I’ve read that while the chlorine will raise the pH, as it’s consumed it works out to be approximately net zero pH impact. This might have been true if I had tried to get in the next day instead of right away? Have I just never noticed a pH spike with the addition of SLAM quantity bleach? Since I added a pH down product, will my pH be too low once the FC gets consumed? Seems like the usage of chlorine from getting into the spa would have naturally lowered the pH, but getting in at 8.0+ seems unwise, no? Any advice appreciated!

 

[mknauss]

Liquid chlorine raises pH when initially added. You are correct as it goes through the process of oxidizing organics and reverting to primarily salt water, the pH result is essentially zero.

Also be aware, that your pH test method is not accurate if your FC is above 10 ppm. Unless you are using a calibrated pH test probe.

It will not harm you to get in a slightly elevated pH water. Low pH can lead to dry skin and eyes.

 

[JamesW]

but getting in at 8.0+ seems unwise, no?

a pH at 8.0 or even 8.3 is not a problem.

Large increases in FC from liquid chlorine will have a noticeable impact on pH.

High FC can combine with the pH reagent and create a false high reading.

Any FC over 10 can cause a false reading on the pH test.

What is the TA?

 

[Insaneoctane]

a pH at 8.0 or even 8.3 is not a problem.

Large increases in FC from liquid chlorine will have a noticeable impact on pH.

High FC can combine with the pH reagent and create a false high reading.

Any FC over 10 can cause a false reading on the pH test.

What is the TA?

TA is 70

So, based on the numbers above, would you think just getting in without any pH adjustment would have been fine?

 

[mknauss]

Yes

 

[Insaneoctane]

Liquid chlorine raises pH when initially added. You are correct as it goes through the process of oxidizing organics and reverting to primarily salt water, the pH result is essentially zero.

Also be aware, that your pH test method is not accurate if your FC is above 10 ppm. Unless you are using a calibrated pH test probe.

It will not harm you to get in a slightly elevated pH water. Low pH can lead to dry skin and eyes.

Any way to confirm it was safe to get in? Would checking the pH before the SLAM have been the right approach and then not worry about the pH effect of adding 0.9c of 10% chlorine since it would start dropping once the FC started getting used?

 

[mknauss]

Always check your pH prior to adding liquid chlorine.

The pH you measured after adding chlorine to that level FC would not have been accurate.

 

[mgtfp]

Did you reduce your pH down to 7.2 before you added chlorine to get to SLAM level?

Adding enough bleach to get from let's say 4ppm to 17ppm will increase your pH (temporarily) from 7.6 to about 8.5. Once that chlorine has been used up again, pH will get more or less back to where it was before (the whole chlorination cycle is pH-neutral). But while you are at SLAM level you are at increased risk for metal staining and calcium scaling.

That's why it's important to reduce your pH before SLAMming. At SLAM level you are blind to the actual pH, because anything above 8 or 8.2 reads the same anyway, and because of the high FC interference with the pH test.

With TA 70 it probably won't stay for long at 8.5 as that's above equilibrium pH regarding CO2 equilibrium with atmospheric CO2, and aeration will actually lead to more CO2 dissolving into the water, reducing pH towards equilibrium pH at about 8.3. Because of that, and because you where not maintaining SLAM level for a long time, pH probably would have come down pretty quickly anyway.

 

[Insaneoctane]

Did you reduce your pH down to 7.2 before you added chlorine to get to SLAM level?

Adding enough bleach to get from let's say 4ppm to 17ppm will increase your pH (temporarily) from 7.6 to about 8.5. Once that chlorine has been used up again, pH will get more or less back to where it was before (the whole chlorination cycle is pH-neutral). But while you are at SLAM level you are at increased risk for metal staining and calcium scaling.

That's why it's important to reduce your pH before SLAMming. At SLAM level you are blind to the actual pH, because anything above 8 or 8.2 reads the same anyway, and because of the high FC interference with the pH test.

With TA 70 it probably won't stay for long at 8.5 as that's above equilibrium pH regarding CO2 equilibrium with atmospheric CO2, and aeration will actually lead to more CO2 dissolving into the water, reducing pH towards equilibrium pH at about 8.3. Because of that, and because you where not maintaining SLAM level for a long time, pH probably would have come down pretty quickly anyway.

Interesting information. I’ve been enjoying not touching my pH for about a month as it’s been really stable at 7.6 every time I checked. So, are you suggesting I should drop my pH prior to SLAM? Won’t I be using far more product dropping pH and then raising once the FC gets consumed and drops it more naturally?

 

[mknauss]

Not at all. You are calling what you did a 'SLAM'. However, that is not what you did. You added a single dose of chlorine. That is not a SLAM. So no, you should not drop your pH prior to adding the single dose of chlorine. You should monitor your pH by testing it before adding the chlorine. And adjust if at 8 or above.

 

[mgtfp]

Put it like that: That's what's recommend in the SLAM process, as it ensures that you won't increase the risk for metal staining or calcium scaling. If there are no metals in the water, the first risk is low. If the SLAM is short, just to eliminate CCs in a spa, the CSI will for a short period of time be much higher (roughly by about the pH change). Depending on where the levels where before, it may or may not be a problem.

If someone has high CH fill water and is struggling to keep CSI in range at normal pH, then a weekly high pH burst might eventually worsen a scaling problem, I reckon. If CH is not very high, then a short weekly burst will probably not do much harm.

Make sure that no metals get into the water, certainly no copper based algaecides - that could turn into a metal staining problem, eventually.

But for a "real" SLAM that might take a few days or even weeks, especially in a pool, I would always recommend to adjust pH to 7.2 prior to SLAMming.

 

[Insaneoctane]

Not at all. You are calling what you did a 'SLAM'. However, that is not what you did. You added a single dose of chlorine. That is not a SLAM. So no, you should not drop your pH prior to adding the single dose of chlorine. You should monitor your pH by testing it before adding the chlorine. And adjust if at 8 or above.

Thank you guys for your very helpful replies. You’re right that I’m referring to my single dose of chlorine to SLAM levels as SLAM, which isn’t necessarily a SLAM. I try to hit that level at least once a week as my standard protocol.

 

[mknauss]

That's fine. Doubt it is necessary.

Take care.