High Ph, Normal Ta

[Jemill]

Hi all you experts,

The last three fills of my spa the PH climbs off the chart. I reduce it using a commercial PH reducer to acceptable levels of about 7.4. The TA is usually OK although low on the acceptable range, about 80-90. I treat the water daily with about 3 ozs. of Clorox and weekly with 1-2 oz each of Bright & Clear, Enzyme if needed, shock, and a reduced amount of Spa Perfect.

What would cause the PH to be so high each week.

Thanks for any help!

[Nitro]

"What would cause the PH to be so high each week."

Aeration!

Don't try to adjust pH directly. Instead, adjust TA to a level that keeps pH in line. A TA of 80-90 is too high. Lower your TA to round 60 ppm. Also, add borates to your tub. That will help buffer pH.

Check out my Maintenance Guide below for more info.

[cofive]

"What would cause the PH to be so high each week."

Aeration!

Don't try to adjust pH directly. Instead, adjust TA to a level that keeps pH in line. A TA of 80-90 is too high. Lower your TA to round 60 ppm. Also, add borates to your tub. That will help buffer pH.

Check out my Maintenance Guide below for more info.

I have actually taken your advice and only raised the TA to a point where my Ph is say 7.5 I am curious as to why some spa manufactures recommend a TA of 125-150ppm when in many cases it is almost impossible to keep the TA that high and bring the Ph in line. I'm guessing water makeup is a factor but i've never been able to accomplish the manufacture recommendations so I've gone with the "experts" here. Please educate me a bit.

Ken

[The Pup]

I have read this passage a couple of times before...but for some reason today, the light bulb above my head just turned on bright:

"pH will have a tenancy to rise with aeration (i.e. use of jets and air), more so if your TA is high. Although, as long as you're not adding Acid (or anything with a lower pH) to your tub, the pH will not usually drop. Therefore, in hot tubs the problem is normally pH rise (or Drift), because of all the aeration. So the trick is to get the TA high enough to not create an unstable situation, and low enough to not allow pH to rise too much.

Let me repeat the last sentence, because it's the single most important thing to keeping your water balanced. The key to having balanced water, without pH drift, is having the correct TA level. If you find your pH rises too high (>8.0) after using your tub, your TA is too high, and needs to be lowered. If you find your pH is too low and/or your water is continually acidic, your TA is too low and needs to be raised. By fine tuning your TA, you can get your pH perfectly balanced, that rarely needs adjustment."

I would also enjoy learning more about the relationship between TA and Ch.

"SO, what's a good TA then? That depends on your CH."

[chem geek]

Manufacturers tend to recommend the higher TA levels because they assume that you will be using Dichlor as your source of chlorine. Though the addition of Dichor is relatively pH neutral, the consumption of chlorine (mostly from oxidation of the urea and ammonia from your sweat and urine) is acidic so the net result from continued use of Dichlor would be a drop in pH and TA since that is what adding an acid to water does. However, since spas have a lot of aeration and are at high temperatures so that carbon dioxide outgassing is relatively fast, this would cause the pH to rise. The net result of using Dichlor with a TA high enough to have sufficient outgassing is a relatively stable pH, but the TA dropping over time (you then add baking soda to compensate for that).

DICHLOR (with proper high TA)

................................ pH ...... TA

Dichlor Addition .......... 0 ........ +

Chlorine Consumption .. - ........ --

Aeration/Outgassing .... + ........ 0

---------------------------------------

Net Result ................... 0 ........ -

If you instead use a hypochlorite source of chlorine, such as bleach, chlorinating liquid, Cal-Hypo or lithium hypochlorite, then these initially raise the pH upon addition, but as the chlorine gets used up the pH drops back down so that these are essentially pH neutral sources of chlorine. However, remember that the outgassing of carbon dioxide still occurs so that has the pH rise, especially if the TA is higher. The solution is simply to have a much lower TA when you are using hypochlorite sources of chlorine.

BLEACH (with TA too high)

................................ pH ...... TA

Bleach Addition .......... + ........ ++

Chlorine Consumption .. - ........ --

Aeration/Outgassing .... + ........ 0

---------------------------------------

Net Result ................... + ........ 0

BLEACH (with proper low TA)

................................ pH ...... TA

Bleach Addition .......... + ........ ++

Chlorine Consumption .. - ........ --

Aeration/Outgassing .... 0 ........ 0

---------------------------------------

Net Result ................... 0 ........ 0

Total Alkalinity (TA) is mostly a measure of bicarbonate in the water and such carbonates are literally carbonation just like a carbonated beverage. The amount of dissolved carbon dioxide in the water is much higher than the normal amount that would occur from being exposed to the air so the result is that the carbon dioxide outgasses into the air. This outgassing occurs faster when things are further out of equilibrium so occurs faster at higher TA levels, lower pH, and with more aeration. This chart shows how far out of equilibrium the water is with air at various TA and pH levels. However, the rate of outgassing appears to be related to the square of the TA, not linearly as shown in the table, which is why one finds such a powerful effect when lowering the TA by seemingly smaller amounts (say, from 80 to 60). Higher aeration causes faster outgassing because it increases the amount of air-water mixing (surface area of interaction) and is just like vigorously stirring a carbonated beverage which makes it go flat (lose its carbonation) faster.

Though a lower TA will lead to a slower rise in pH when using bleach, you still want some pH buffering in the water so should not go below 50 ppm in any event. For additional pH buffering, you can use 50 ppm Borates -- this being a particularly nice buffer since it is strongest at slowing down a rise in pH. However, you can't just use Borates and have a high TA level because you'd still need to add lots of acid in that case, just less frequently (but total amount per time would be the same so larger amounts added less frequently).

As for the appropriate Calcium Hardness (CH) level, this should generally be adjusted last, after finding the appropriate TA level. After finding a relatively stable pH at a certain TA level, then the CH can be adjusted to have the saturation index be near or a slightly below zero. You can use The Pool Calculator for calculating this (don't forget to include the correct temperature -- higher spa temperatures are more likely to cause scaling).

Richard

[Nitro]

I have found that a TA of 50-60 ppm will reduce pH rise to a minimum, and adding 50 ppm Borates will almost stop it all together. If you keep a low TA it's a good idea to have a higher CH to counteract the low TA. As Chem Geek mentions, The Pool Calculator will show you how they relate to give you a near zero SI, which is what you're after.

Just over four months ago when I refilled my tub last, I adjusted my TA from 100 ppm to ~60 ppm, and my CH to ~200 ppm from 100 ppm. I also added 50 ppm Borates. My pH has been steady between 7.6-7.8 up until a few weeks ago, when it finally drifted up to 8.0. I then added an ounce of dry acid which dropped my TA down to ~55 ppm. My pH is now back to hovering between 7.6-7.8. I believe this should last me until my next change water, sometime this spring.