Washing Soda caused PH spike after AA Treatment

[kfpool]

I am coming out of my AA treatment and added 1/2 of the amount of washing soda that was recommended by pool math. Checked PH 30 minutes later and the reading is higher than expected. Based on pool math the PH after the addition should have been in the 6.6 to 6.8 range; however, I am getting a reading of 8.4. Is it normal to see that over-shoot like that?
I am using a calibrated quality PH meter and compared the reading to the color test. My meter is accurate.
Should I wait a while or add muriatic acid to get the PH back down to 7.0 -7.@?

 

[ajw22]

Chemicals suggested for raising TA. Their effects are:

• Baking Soda = big TA change, small pH change
• Borax = big pH change, small TA change
• Soda Ash/Washing Soda = big pH change, big TA change.

The best way to raise pH is by aeration of the water. That will raise your pH gradually while not affecting your TA.

If TA is 50 or above, bring your pH up to 7.4 or so using 20 mule team Borax.....soda ash is not a good choice as it will raise the pH and raise the TA a lot.

If TA is below 50, then bring the TA up to 50-60 using baking soda and THEN raise your pH to around 7.4 using 20 Mule Team Borax.

 

[kfpool]

I followed the ascorbic acid treatment instruction & the ph overshot with 1/2 the recommended chemicals. It clearly states to add washing soda 1st. I asked how or if I should react to the overshoot of ph. I am having trouble finding the answer to that in your response.

 

[ajw22]

Lower the pH with MA.

Don't become slave to the robot. PoolMath is giving suggestions and there are often better alternatives for your situation.

What was your pH and TA before you added the Washing Soda?

 

[kfpool]

PH was right at 5, I pool math recommended adding 16 lbs of washing soda. I roughly went 1/2 of their recommendation trying to ease up on the target and added 9 lbs over about 45 minutes.
TA was at 20. the AA instructions said to start with washing soda to get the PH close then finish leveling the TA. Washing Soda should bring up PH and to a lesser extent TA.

What I really need to know is did I test too quick after adding the soda & if I should respond now with MA.
This is my 2nd home with a pool & my 2nd AA treatment. (I seem to have a propensity toward buying homes from uninformed pool owners). I always add chemicals at 1/2 of the recommended dose unless I am absolutely sure what the result will be which is why I am surprised by the overshot PH. The AA treatment I went through on my previous pool was problematic to an extent & I want to proceed cautiously to try to avoid a repeat of that.

 

[PoolStored]

What I really need to know is did I test too quick after adding the soda & if I should respond now with MA.

How soon after adding the washing soda did you test?

 

[mgtfp]

PH was right at 5, I pool math recommended adding 16 lbs of washing soda.

pH calculations are difficult, they require simultaneous solving of a number of equilibrium equations. PoolMath doesn't actually solve these equations, but uses approximations that work quite well within normal pool water parameters.

At pH 5, PoolMath's suggestions need to be taken with a few grains of salt. Much better to adjust pH incrementally. Add a little, retest, add more. Repeat if required.

In the Effects of Adding, there is a disclaimer about the accuracy of pH calculations when calculating the effect of pH-changing chemicals. Unfortunately, there is no disclaimer in the main section.

 

[mgtfp]

I just had a look into your PM logs, where you logged your pH as 4.7. If that were true, then your TA should have come out as pretty much zero. The indicator dye of the TA test is simply a pH-indicator that changes colour around pH 4.5.

Is your pH-meter 3-point calibrated, including pH 4?

 

[mgtfp]

I suspect that your pH was more like 6. 9lbs of washing soda would get pH up to 8.4 from there, and TA up to 120 (from 20).

 

[kfpool]

Thanks. I continued checking last night & added MA to get my PH back into check. As someone who spent the 1st10 years of my career maintaining boiler water for a large coal powerplant (Using the same basic methodology we use here) I knew better than to add that much at once or to trust a PH reading that far out of range. I pretty well knew I had made a mistake as soon as I made it. I felt like I knew how to correct it but after making the overdose mistake, I started questioning myself.
All is in spec now except chlorine & I am SLOWLY dosing to bring that back into spec as well.