When a strong acid is titrated with a strong base or vice versa, the pH throughout the titration can be calculated by determining the concentration of the remaining hydronium or hydroxide ions.

For example, 50 mL of 0.10 M hydrochloric acid dissociates into 0.10 M hydronium and chloride ions with an initial pH of one.

If 25 mL of 0.10 M sodium hydroxide is added, the new concentration of hydronium ions can be calculated by subtracting the total moles of hydroxide ions from the total moles of hydronium ions and dividing it by the total volume—75 mL or 0.075 L.

50 mL of 0.10 M hydrochloric acid contains 0.0050 moles of hydronium, while 25 mL of 0.10 M sodium hydroxide has 0.0025 moles of hydroxide ions. Substituting these values into the equation, the hydronium ion concentration equals 0.033 M. Therefore, the pH of the solution has risen to 1.48.

If the addition of sodium hydroxide is continued up to 50 mL, all the hydronium ions from hydrochloric acid get neutralized, and the equivalence point is reached when the pH rises to seven.

Above the equivalence point, the pH of the solution is determined by the hydroxide ions, as the hydrochloric acid has been completely neutralized.

If 70 mL of sodium hydroxide solution containing 0.0070 moles of hydroxide ions is added into the solution, the final concentration of hydroxide ions can be determined by subtracting the initial moles of hydronium ions from the total moles of hydroxide ions and dividing it by the total volume of the solution.  

As the final hydroxide ion concentration is 0.017 M, the pOH and pH of the solution equal 1.78 and 12.22, respectively.