Adding sodium bicarbonate

[bobobi-wan]

when I add sodium bicarb to raise alkalinity,it raises ph which triggers my ph monitor to lower the ph which is the same chemical that lowers alk.usually I turn off the pump to let the s/bicarb do it's thing.is there a better way to add the s/bicarb so that I don't have to turn the pump off?sometimes I forget to turn the pump back on.

[chem geek]

The pH rises from baking soda mostly due to the locally high TA where you add it that causes carbon dioxide to outgas. So if you are able to add it much more slowly and over a return flow for thorough mixing or perhaps slowly into the skimmer with a funnel effect, you might be able to reduce the rate of pH rise upon addition.

In theory, if you were to add baking soda with perfect mixing and no outgassing of carbon dioxide, then increasing the TA from 80 to 115 would only raise the pH from 7.5 to 7.6. Any additional rise in pH is due to the CO2 outgassing upon addition. Dumping the baking soda into the water quickly makes the surface of the water very high in TA and leads to fast carbon dioxide outgassing which causes the pH to rise.

If you are adding baking soda regularly, then try it the two ways I described -- slowly over a return flow at the deep end and slowly into the skimmer with the pump on high -- and let us know if you notice any differences in the amount of resulting pH rise.

Richard

[bobobi-wan]

I always mix with water then dump it in towards middle(the spas)but will start putting it in your way,premix and put in skimmer.thanks,I'll let you know mon. when I add more.I keep it at 70 because my ch is around 420-450.is alkalinity lost thru evaporation?

[chem geek]

No, but evaporation might accelerate some of the carbon dioxide outgassing and that has the pH rise faster though aeration is a more powerful effect for this. However, when you add acid either directly or indirectly through net acidic chlorine sources then the pH and TA are lowered so the net effect is a lowering of TA over time (if nothing else increases it).

The bottom line is that acid (or acidic chlorine sources) lowers TA, but it also lowers pH so it takes carbon dioxide outgassing to have the TA lower with no change in pH. The net result is a removal of carbonates from the pool and bicarbonate is the largest component of TA.

Evaporation and refill add whatever is in the fill water to the pool, so TA can increase in that way.