The Orenda Calculator™ gave two acid doses for lowering pH and alkalinity. Why?

When lowering both pH and total alkalinity, the Orenda Calculator™ gives two acid doses. Here's why.

The Orenda Calculator™ chemical dosing is based on the difference between the left side (current readings) and the right side (desired readings). When lowering both pH and alkalinity, the calculator shows both results, giving two different acid doses.

We get this question often, which is why we have the answer in the informational pop-ups for both Decrease pH and Decrease Alkalinity.

When tapped, any underlined words in the Orenda app will show a pop-up for more information.  When tapping Decrease pH, for instance, this appears:

Adjust alkalinity before pH

Total Alkalinity (TA) determines the acid dose necessary to reduce pH, and also determines how high the pH can naturally rise on its own (the pH ceiling). Unless you're raising alkalinity and lowering pH in the same pool visit, focus on alkalinity first.

Lowering alkalinity will also lower pH, because acid neutralizes bicarbonate (HCO₃^-) and converts it into carbonic acid (H₂CO₃). Carbonic acid is a small percentage of dissolved carbon dioxide_(aq).

H₂O + CO₂ ⇄ H₂CO₃

Acid adds a Hydrogen (H⁺) to bicarbonate, neutralizing it (alkalinity decreases), which increases the amount of dissolved CO₂ in the water (pH decreases). We can thank the carbonate alkalinity buffering system for this.

So when you see two acid doses, only use the alkalinity dose (assuming it's no more than 20 ppm reduction!!). The pH will be reduced with the same dose.  The Orenda Calculator™ only knows left to right; it does not know your intentions.