ph suddenly rose out of control

[chovaz]

I just filled my tub (2000 litres) and tested the water. Alka was extremely high, over 250. ph was 7.8. According to the tables to drop Alka 100 units I put in 500g of phDown and ran the jets for an hour. The Alka dropped down to about 130. However, the ph has suddenly shot up off the charts over 8.0.

Total hardness is around 210.

Is there any plausible explanation for this?

I've proceeded to add another 60g of ph Down and will re-test in another hour but it has me baffled how that could happen.

[chem geek]

It's working exactly as it is supposed to.  Acid lowers both pH and TA while aeration forces carbon dioxide to outgas which causes the pH to rise with no change in TA.  A TA of 130 ppm will still cause the pH to rise.  In fact, if you were to use a hypochlorite source of chlorine, your TA would need to be very low to prevent the pH from rising.  If you plan to use Dichlor-only, then a TA of around 80 ppm is probably about right.  It depends on how you will be using your spa -- whether you run aeration jets.

[chovaz]

Thanks. The spa is bromine, so no chlorine source. I always thought that higher TA was what made ph difficult to change.

So I should just continue to add phdown until the TA and pH finally get to recommended levels. It seems that my TA is going to bottom out to zero before my pH starts to balance at a lower level. Then raise TA back up to 80-120 to keep Ph where it is?

[chem geek]

Stop adding acid when your TA is where you want.  Then just aerate to get the pH up where you want.  Aeration won't raise the TA so you don't want to overshoot the TA going down.

With bromine tabs, those are net acidic so a TA of 80 ppm will probably work well.  If you find the pH tends to drop over time, you can always raise the TA later.