What Is Lowering My Ph?

[matt_s]

1 week ago, ph was 7.2 and alkalinity was 90. Today ph is 6.9, alkalinity is the same. I am using dichlor, and the tub is used almost every day so dichlor added almost every day. Ozone and nature2 also installed.

Is this normal?

[LegsOnEarth]

Dichlor has a pH around 6.8, so it would naturally lower your pH slightly. In addition, the Nature2 system typically is used with potassium monopersulfate (MPS or non-chlorine shock), which also has a low pH and can lead to a decrease. Although you haven't mentioned using it, I would assume you must be, as the Nature2 system requires that you shock with MPS after each soak.

Also, I would advise that you post a full set of chemical test results for your spa water. That would help with troubleshooting immensely.

[chem geek]

Actually, you need to account for chlorine usage/consumption which is net acidic. Even if Dichlor had a 7.5 pH, the pH would still drop from chlorine usage/consumption unless one had enough carbon dioxide outgassing to compensate. For example, with a pH of 7.2 and a TA of 90 ppm as initially indicated, cumulatively adding 15 ppm FC would have the pH drop to 6.94 and the TA drop to 85 (which could still read as 90). If there were no acidity of chlorine usage/consumption, the pH would have only dropped to only 7.15 and the TA would have increased to 95 (from the added CYA).

It's quite possible that more chlorine was added than I indicated and that the pH didn't go down even further because carbon dioxide outgassing helped to compensate for that. However, the TA would have dropped more, but probably that's hidden with +/- 10 ppm test error. Over time though the TA could get dangerously low if one never adjusted it by some chemical addition.

[matt_s]

First of all, I'm not using MPS with my Nature2. Zodiac has instructions for using only dichlor with Nature2.

So, my interpretation is that this is to expect, especially accounting the heavy usage and thus the chlorine usage/consumption. I just have to keep track of the PH and alkalinity and adjust them both accordingly. If I raise my TA first, would that also raise my PH? Or would I need to use both PH and Alk. increasers?

[chem geek]

If you raise your TA (using baking soda) then you may get to a point where the pH is more stable. The TA would then slowly drop over time so you would raise it back up as needed.

When adding baking soda, the pH can rise some, but how much depends on the amount of carbon dioxide outgassing that occurs when you add it -- adding it more slowly into circulating, but not jetted aeration, can help minimize the pH rise.

Realize that using Dichlor-only will build up Cyanuric Acid (CYA) over time making the chlorine less effective so you'll end up needing to change the water at least twice as frequently than if you were to use Dichlor initially and then switch to using bleach. If you use bleach, however, you'll want the TA to be lower.

[waterbear]

First of all, I'm not using MPS with my Nature2. Zodiac has instructions for using only dichlor with Nature2.

Here is a little 'heads up' for you. If you calculate the chlorine levels you are using with straight dichlor use as recommended with the Natue 2 you will find that you are chorinating to the same levels as someone who does not use a Nature 2 cartridge so you are really just wasting your money on putting the cartridge in every 4 months!

When Nature 2 is used with MPS your sanitizer is the MPS which is catalyzed by the silver introduced by Nature 2 at the elevated water temperatures in the spa. Chlorine is not your sanitizer. The chlorine 'shock' is needed on startup and weekly (or more often in heavy usage) to remove the film that accumulates on the ceramic beads in the cartridge that hold the silver nitrate so the silver ions can be released into the water. IF this is not done then the Nature 2 does not work properly as a sanitizer system and the water will 'go south'.

You must maintain a MPS residual in the water at all times for this to be a complete sanitizer system and the water must remain hot at all times also.

If you are using exclusively chlorine the Nature 2 is really doing nothing and you are maintaining what are considered to be in the range of 'normal' chlorine levels and not reduced levels at all!