Why Do Adjusting The Ta Move The Ph So Much.?

[footie]

I forgot to ask this question when I first balanced my water but I suppose better late than never.

My water right out of the tap read 7.0pH but the TA was only 40, after adjusting this up to between 100-120 it took a sizeable amount of dry acid to bring the pH back down to 7.4.........why?

[Mikey_in_NY]

Adding dry acid lowers PH AND TA, so you need to recheck your TA level, which I suspect is now less than 100.

There's a lot of potential variables here, but for what it's worth to keep my PH around 7.5 I need to maintain TA at about 50-55, any higher than that and the PH rises. I'm using the Dichlor/chlorox method, and have a 24hr ozonator.

[reeffreak]

I forgot to ask this question when I first balanced my water but I suppose better late than never.

My water right out of the tap read 7.0pH but the TA was only 40, after adjusting this up to between 100-120 it took a sizeable amount of dry acid to bring the pH back down to 7.4.........why?

Because pH is the overall acidity / alkalinity of solution (your water). Increasing TA is essentially adding a base in the form of calcium (needed for proper water chemistry), (often baking soda or soda ash) which will naturally raise pH. The acid brings the pH back down. This is the non-chemist version. We do a lot of this in reef aquaria, however our ideal pH is somewhere around 8.43, which usually means we can stabilize TA and pH at the same time with the addition of various calcuims (hydrated lime, Soda Ash, sometimes calcium chloride). Since hot tubs require lower pH, using an acid to offset a pH swing created by a TA adjustment is expected.

in other words Total Alkalinity is just that - TOTAL (an absolute). pH is a ratio. I believe you can theoretically have a high TA with a low pH if you added enough of an acid. TDS will continue to climb as you pour chemicals in the tub however.

[quantumchromodynamics]

pH is a measure of the hydrogen ion concentration. It is an inverse and logarithmic relationship. The more hydrogen ions, the lower the ph. The fewer hydrogen ions, the higher the pH.

Some of the bicarbonate ions combine with hydrogen ions to become carbon dioxide dissolved in water. This raises the pH by removing the free hydrogen ions. Some of the carbon dioxide off-gasses, which causes more bicarbonate ions to combine with more hydrogen ions, which further raises the pH.

HCO₃^- + H⁺ < > CO₂ + H₂O

To reduce the rate of carbon dioxide production and its subsequent off-gassing, you need to reduce the bicarbonate ion concentration and/or the hydrogen ion concentration. You do this by lowering the TA and/or raising the pH.

You should only raise your TA until your pH gets to the correct range. If your pH goes too high, then you have raised your TA too much. Next time raise the TA in 10 ppm increments until your pH is right.

[footie]

Thanks for this.

I suppose I got hung up on getting the right TA without paying enough attention on the steadily rinsing pH until it was out of control. I will re-check the TA and see where it now is that I have finally got the pH right.

There is much more work in maintaining the water of a tub compared to a pool, probably that ratio of 20:1 water quantity coming into play.

[chem geek]

Just remember that TA is a SOURCE of rising pH itself since anything but an extremely low TA (at usual pH) has the water be over-carbonated. When carbon dioxide outgasses due to this over-carbonation and aearation such as jets in a spa, the pH rises (with no change in TA since carbonic acid is removed).

The following is a table of the equilibrium amount of TA at various pH (this is carbonate alkalinity so the TA does not account that from CYA then that adds around 10 ppm to TA at a pH of 7.5) at hot spa temps (104ºF):

pH ... Equilibrium TA (no CYA) ... Equilibrium TA (with 30 ppm CYA)

7.0 ...... 2.8 ppm ................... 9.7 ppm

7.2 ...... 4.5 ppm ................... 12.5 ppm

7.5 ...... 9 ppm ..................... 18.5 ppm

7.8 ...... 18 pmm .................... 28 ppm

8.0 ...... 29 ppm .................... 40 ppm

Fortunately, TA higher than the above doesn't have the pH rise very quickly.

With your water out of the tap, you probably could have just run the jets and the pH would have risen somewhat without having to add anything to adjust it or the TA. Just note that if you use Dichlor, it is net acidic so will need higher TA, but if you use bleach (after initially using Dichlor), it is closer to pH neutral when accounting for chlorine usage/consumption (which is acidic) so is better with a lower TA to prevent the pH from rising.

[footie]

Just remember that TA is a SOURCE of rising pH itself since anything but an extremely low TA (at usual pH) has the water be over-carbonated. When carbon dioxide outgasses due to this over-carbonation and aearation such as jets in a spa, the pH rises (with no change in TA since carbonic acid is removed).

The following is a table of the equilibrium amount of TA at various pH (this is carbonate alkalinity so the TA does not account that from CYA then that adds around 10 ppm to TA at a pH of 7.5) at hot spa temps (104ºF):

pH ... Equilibrium TA (no CYA) ... Equilibrium TA (with 30 ppm CYA)

7.0 ...... 2.8 ppm ................... 9.7 ppm

7.2 ...... 4.5 ppm ................... 12.5 ppm

7.5 ...... 9 ppm ..................... 18.5 ppm

7.8 ...... 18 pmm .................... 28 ppm

8.0 ...... 29 ppm .................... 40 ppm

Fortunately, TA higher than the above doesn't have the pH rise very quickly.

With your water out of the tap, you probably could have just run the jets and the pH would have risen somewhat without having to add anything to adjust it or the TA. Just note that if you use Dichlor, it is net acidic so will need higher TA, but if you use bleach (after initially using Dichlor), it is closer to pH neutral when accounting for chlorine usage/consumption (which is acidic) so is better with a lower TA to prevent the pH from rising.

I'm running Bromine, so where does that leave me?

[quantumchromodynamics]

I'm running Bromine, so where does that leave me?

It depends on what chemicals you're using. Bromine tabs are acidic, so would require a higher TA. The primary point to remember is that the TA needs to be where it keeps the pH stable. If the pH is constantly too high, then the TA is too high. If the pH is constantly too low, then the TA is too low. When the TA is just right, then the pH will stay in the correct range with very little movement.