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How to Lower the pH of a Solution?

Published in pH Adjustment 2 mins read

The most common and direct way to lower the pH of a solution is by adding an acidic substance.

Adding an acid introduces hydrogen ions (H⁺) into the solution. An increase in the concentration of H⁺ ions directly corresponds to a decrease in pH, as pH is a measure of the hydrogen ion activity in a solution (specifically, the negative logarithm of the hydrogen ion activity).

Understanding the Mechanism with Acids

As stated in the reference, using acid lowers both the pH and alkalinity in water. When you add acid to a solution, particularly one with buffering capacity like water containing carbonates and bicarbonates, the acid reacts with these substances.

Here's how it works:

  • Acids donate hydrogen ions (H⁺).
  • These H⁺ ions react with alkaline substances present in the solution.
  • In water, common alkaline substances providing alkalinity are carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions.
  • The added H⁺ ions convert these alkalinity components into carbonic acid (H₂CO₃).
  • Specifically:
    • H⁺ + CO₃²⁻ → HCO₃⁻ (Carbonate converted to Bicarbonate)
    • H⁺ + HCO₃⁻ → H₂CO₃ (Bicarbonate converted to Carbonic Acid)

The reference highlights that acid lowers pH at the expense of alkalinity. This happens because you need more dissolved carbon dioxide (which forms carbonic acid in water) to lower the pH significantly, and acid does not contain CO₂, but rather converts existing alkalinity forms into carbonic acid.

This process consumes the alkaline substances that were buffering the solution, effectively reducing its alkalinity and allowing the pH to decrease more readily as more acid is added.

Practical Considerations

When lowering pH using an acid:

  • Type of Acid: The specific acid used (e.g., sulfuric acid, muriatic acid, citric acid) depends on the application and desired outcome.
  • Concentration: The concentration of the acid matters significantly; highly concentrated acids are very potent.
  • Gradual Addition: Acid should typically be added slowly and mixed well to avoid localized pH extremes and allow for proper reaction and measurement.
  • Monitoring: Continuous or frequent monitoring of the pH is crucial to avoid overshooting the target pH level.

In summary, adding an acid introduces H⁺ ions, which directly lowers pH and reacts with alkaline components in the solution, like carbonates and bicarbonates, converting them into carbonic acid and reducing alkalinity, further impacting the pH.