Frustrating Ph Vs Ta Issue

[Hillbilly Hot Tub]

It really isn't rocket science but it is some basic chemistry.

Thanks water bear. This helps with the rising PH in my tub. I have a 24 hour circ pump which does not cause much airation, but my ozone runs 24/7. We have been debating whether to reprogram it to run shorter cycles, weighing pros and cons...love the lack of chemical smell and very little chemical use, but it will cause damage to the pillows (I use a spa balnket to protect the cover) and now to add to the list, fighting with the PH.

I will ask, why does the PH/Alk stay ok for 2 and a half months, then I have issues with PH rising and Alk dropping? Any ideas? I am guessing it is to do with the TDS rising or contaniments getting in the tub.

[chem geek]

Hillbilly Hot Tub,

waterboy in this post didn't see the kind of pH rise that you did from adding Alkalinity Up / sodium hydrogen carbonate / sodium bicarbonate / baking soda. The CO₂ outgassing does cause the pH to rise, but to see if there's something wrong with the specific product you are using you'd have to measure the pH before you added it and then again afterwards after some mixing and equilibration time, probably 30 minutes should be fine. If you instead measure from one day to the next, then the outgassing can have an effect to hide what's going on from chemical addition. Even so, the kind of pH rise you saw is unusually large unless the TA was high and there was lots of aeration. If you continued to see this sort of pH rise after adding acid to lower the pH again, then aeration is the main cause, not chemical addition of bicarbonate.

My comments on the order of chemical addition or what to adjust first depends on whether you are too low or too high in pH and what types of chemicals you use. Probably the main reason to adjust TA first is that its measurement changes most rapidly -- that is, if you test for it again you'll see an immediate effect. With pH, changes occur more slowly after chemical addition due to the slowness of the equilibrium between dissolved carbon dioxide and carbonic acid. So if you adjust pH first and don't wait long enough, then you won't be measuring the right TA value as it will also change as the pH changes though the effect is smaller.

The reality is that if one had a proper calculator (like the spreadsheet I made), one could know exactly how much acid/base and alkalinity increaser to add with the order not making a bit of difference.

Richard

[waterboy]

The reality is that if one had a proper calculator (like the spreadsheet I made), one could know exactly how much acid/base and alkalinity increaser to add with the order not making a bit of difference.

Richard

Is it possible to get a copy of your spreadsheet?

[Hillbilly Hot Tub]

If you instead measure from one day to the next, then the outgassing can have an effect to hide what's going on from chemical addition. Even so, the kind of pH rise you saw is unusually large unless the TA was high and there was lots of aeration. If you continued to see this sort of pH rise after adding acid to lower the pH again, then aeration is the main cause, not chemical addition of bicarbonate.

Richard

I do usually test the next day since I bring the sample to the store to run through the computer program. I use the tub nightly with all 3 pumps on and all the air open to the jets, plus the 24 hour ozone, so there probley is a lot of aeration which causes the PH rise.

[chem geek]

Is it possible to get a copy of your spreadsheet?

The link to the spreadsheet is here, but it is not for novices. If you put in Initial and Goal values and then click on "Calculate Acid/Base/Buffer", then it will tell you how much acid or base to add and how much baking soda, but if you are trying to lower the TA then it will tell you how much CO₂ to outgas which is pretty meaningless -- you have to follow the TA lowering procedure to do that. I also provide a "Calculate Acid/Base/TA" button for the more common case where you want to adjust the pH and have flexibility in the TA since you can drive yourself nuts trying to adjust both and the TA only has to be approximately in the ballpark.

The spreadsheet is easiest to use when entering in amounts of chemicals and then clicking on "Calculate pH/TA" to see the result. Also, if you see the big red "Recalculate" word, then the results are not correct and you need to click on the "Calculate pH/TA" again. It usually works correctly right away unless you've got something weird like very low TA that is technically impossible (given other parameters like CYA and pH) and makes the "Carbonate Alkalinity" line 16 go negative.

The next best thing to the spreadsheet is The Pool Calculator, but it uses simpler formulas for the simultaneous effects of pH and TA so might not be correct (though might be OK for smaller adjustments). This online calculator is good for single adjustments, however. The Calcite Saturation Index roughly tracks my spreadsheet (and the Taylor watergram) as well, though is also slightly simplified.

Richard