Ta And Automatic Acid Doser

[jack in oz]

Hi,

another complete newbie question. My TA was measured by my local pool store as 65. I was told to add 5kg of sodium bicarbonate to bring TA up into the ideal range.

Do I have to disable my peristatic acid dosing pump after adding the sodium bicarbonate? If so, for how long?

Rgds,

Jack

[quantumchromodynamics]

If you need to add acid on a regular basis, then you don't want to add baking soda. A TA of 65 ppm is OK as long as your CSI is high enough. You want to maintain a CSI as close to zero as possible to protect your plaster from dissolving and to keep your cell from scaling.

The Saturation Index is a good guide – water should have a saturation index value in the range of –0.3 to +0.5. This range does not actively promote either scaling or etching.

Reference

What are your pH, calcium and cyanuric acid levels?

[jack in oz]

Thanks for the reply.

My pH/chlorine controller requires an absolute minimum TA of 80, but prefers 100-140.

yesterday's water test from my pool store gave the following results:

pH 7.8 (the readout on my controller says it's 7.5- it might need recalibrating)

calcium 90 (they said to leave this alone for a few months until the cement render had finished curing)

Cyanuric acid 45

TA 105 (it was 65 before I added the 5kg of bicarb- sorry for the confusion!)

I'm unfamiliar with CSI so I'll read the document you linked.

[quantumchromodynamics]

You can calculate your CSI (Calcite Saturation Index) using the pool calculator. Right now your CSI is about -0.45.

I recommend that you increase your CSI a bit by raising your calcium to about 175 ppm. Once you get your TA and pH stable, you should keep your CSI between -0.2 and 0.0. You want it as close to 0.0 as possible to protect your plaster from dissolving. Check your cell periodically for scale buildup and lower the CSI a bit if you are getting excessive scaling.

The TA (Total Alkalinity) needs to be where it keeps the pH stable. If your pH is constantly rising, then your TA is too high. If your pH is constantly falling, then your TA is too low. Your TA is just right when your pH stays stable with very little addition of acid or pH increaser. High TA causes the pH to rise.

I recommend that you keep your pH between 7.7 and 7.9. You might want to consider adding 50 ppm borates to help buffer your pH. You should increase your cyanuric acid level to about 60 to 80 ppm.

How much acid are you going through per week?

[jack in oz]

How much acid are you going through per week?

It's early days yet so I don't know. My system uses a peristaltic pump to add a 1:5 dilution of muriatic acid as required to maintain the set pH. I'll replace the 15L acid drum with a smaller container so I can measure how much acid is being used. What would you consider to be a reasonable weekly acid consumption for a 50,000L pool?

Thanks for the info so far. It's a steep learning curve and the pool shop people don't seem to undertsand the underlying chemistry.

[quantumchromodynamics]

The key is to find the correct TA for your pool. Don't worry about what your pH controller says it wants you to keep the TA at. It's biased anyway. It wants you to keep adding sodium bicarbonate so that it has something to do. Here is what's happening.

NaHCO₃ + HCl --> NaCl + H₂O + CO₂

Baking soda + muriatic acid --> salt + water + carbon dioxide

By adding sodium bicarbonate and hydrochloric acid, all you are doing is neutralizing both and wasting money. Moreover, it's also increasing your salt level, which is already too high.

Don't worry about trying to keep your TA at any predetermined level. Allow it to go wherever it needs to to achieve a stable pH. What matters is your CSI. By balancing the other parameters, you can achieve perfect balance.

I recommend that you add 50 ppm borate to help buffer the pH. Increasing the cyanuric acid will help you reduce your cell run time and it will also provide a pH buffer.

Once you get your chemistry properly balanced, you should not need much acid at all.

[jack in oz]

OK, I'll go and get some borate and cyanuric acid today.

Cheers.

[quantumchromodynamics]

To add 50 ppm borates, you will need 14.3 Kg of boric acid.

http://www.chemistrystore.com/search.cgi?keywords=boric+acid

http://www.proteampoolcare.com/index.php/products/enhancers/supreme-plus/

[jack in oz]

How much will 14kg of boric acid reduce my pH by?

[quantumchromodynamics]

Boric acid has a pKa of 9.14, so it's not very acidic at all. How acidic it is depends on how close your pH is to 9.14. The farther away (lower) from 9.14 that your pH is, the less acidic the boric acid is. Therefore, it is self limiting on how much of an effect it can have on your pH.

For example, at a pH of 7.6, 97 % of the boric acid will stay as boric acid B(OH)₃ and only 3 % will become the tetrahydroxyborate ion B(OH)₄^-, which should lower your pH by about 0.19.

B(OH)₃ + H₂O < > B(OH)₄^- + H⁺

Basically, you can consider it to be mostly pH neutral with only a slight decrease in pH.

You could add 50 ppm borates by adding 13.9 Kg of boric acid and 0.65 Kg of Borax (to offset the acidity of the boric acid), but it shouldn't be necessary if your pH normally runs high anyway. The Proteam Supreme Plus is mostly boric acid with a little bit of Borax to make it pH neutral.

[chem geek]

The answer depends on your starting pH, TA, etc., but adding 14 kg of Boric Acid to 50,000 liters with a starting pH of 7.5, TA of 100, results in a drop in pH to 7.3. That is, there isn't very much of a drop in pH when using boric acid. The reason is that it is a rather weak acid where it mostly remains as boric acid (the pKa is around 9.1 accounting for ionic strength).

Adding boric acid to get borates is close to pH neutral. If you were instead to use 20 Mule Team Borax or similar sodium tetraborate product, you would need to add a lot of acid along with the borax.

[jack in oz]

Thanks again!