Ph Increased After Spa Sentry Application

[summerdreamin]

Water professionally tested and pH value was 7.5.

All other values were within guidelines. Spa dealer suggested SpaGuard Spa Sentry (1 qt) to lock in pH. Did that according to instructions. Used spa (two people) for three hours last evening, and chlorinated spa after use. Tested pH today at noon and the level was aprox. 8.2! Are we doing something wrong or is that a normal thing? Do we need to add a decreaser? We thought the Spa Sentry would keep the level the same.

Thanks for any help!

[chem geek]

SpaGuard Spa Sentry is a phosphate buffer and will remove calcium from you water (often making it cloudy until the calcium phosphate is filtered out) in addition to acting as a pH buffer. I suspect that the pH tests were done inconsistently, though it could just be that your Total Alkalinity (TA) was high and caused a pH rise in spite of the pH buffering. Yes, you need to add acid, and with the phosphate buffer in place it will take quite a lot of acid to move the pH.

If you are using test strips, I suggest you get a better test kit, the Taylor K-2006 you can get here.

How did you chlorinate the spa (i.e. what kind of chlorine -- Dichlor or bleach)? Is the chlorine level still high? If so, then the pH may drop some as the chlorine level drops.

[summerdreamin]

SpaGuard Spa Sentry is a phosphate buffer and will remove calcium from you water (often making it cloudy until the calcium phosphate is filtered out) in addition to acting as a pH buffer. I suspect that the pH tests were done inconsistently, though it could just be that your Total Alkalinity (TA) was high and caused a pH rise in spite of the pH buffering. Yes, you need to add acid, and with the phosphate buffer in place it will take quite a lot of acid to move the pH.

If you are using test strips, I suggest you get a better test kit, the Taylor K-2006 you can get here.

How did you chlorinate the spa (i.e. what kind of chlorine -- Dichlor or bleach)? Is the chlorine level still high? If so, then the pH may drop some as the chlorine level drops.

The chlorine level was high when we tested the pH. We use spa Guard chlorinating compound (dichlor). I didn't know that if the chlorine was high the pH would test high also. Should we wait until chlorine in at the residual level before testing pH? We use the Taylor Basic Test Kit K-1000.

Thanks for the help.

[chem geek]

See this post that describes that the pH test can read falsely high if the chlorine level is 10 ppm or higher.

[quantumchromodynamics]

As chem geek noted, SpaGuard Spa Sentry is Phosphate Buffer 36.3 %. If it wasn't properly pH balanced, it could raise your pH.

It should contain Dipotassium phosphate (K₂HPO₄) and Monosodium phosphate (NaH₂PO₄).

pKa for phosphoric acid:

pKa₁ =2.15, pKa₂ = 7.21 and pKa₃ =12.34

Phosphoric acid H₃PO₄.

Based on the pKa values, most of the phosphoric acid will be in the HPO₄^2- form. At a pH of 7.5, 34 % will be H₂PO₄^- and 66 % will be HPO₄^2-.

I would assume that the DiPotassium phosphate (K₂HPO₄) should make up 66 % of the mix and that Monosodium phosphate (NaH₂PO₄) sould make up the other 34 %.

If the product is all DiPotassium phosphate (K₂HPO₄) or DiSodium phosphate (Na₂HPO₄), then the product would cause the pH to rise.

What is your Total Alkalinity?

Note: Depending on how much phosphate you have added, a significant portion of your Total Alkalinity could come from the phosphate. This would make your CSI (Calcite Saturation Index) read artificially high. The CSI only uses the carbonate portion of the Total Alkalinity.

The poolcalculator does account for borates and cyanurates when calculating the CSI. However, it does not account for phosphates. You would need to subtract the phosphate alkalinity from the Total Alkalinity to get the correct CSI.

Since the error in the CSI is going to be make the CSI read higher than it really is, then it is not as much of an issue as if it were to read lower than it really was. Still, it would be good to know the correct CSI.

You can calculate your phosphate alkalinity based on how much of the SpaGuard Spa Sentry phosphate buffer that you have added. How much have you added?