By Timothy Banas • Updated Mar 24, 2022

In analytical chemistry, titration is a reliable method for quantifying the alkalinity of an unknown solution. Alkalinity represents the solution’s capacity to neutralize acid and is essentially the opposite of acidity. By carefully adding a titrant of known concentration to the sample until a color change indicates equivalence, we can compute the solution’s pH with a straightforward set of calculations.

Step 1 – Record the Drop Count

After completing the titration, note the total number of drops required to reach the endpoint. For example, 40 drops of 1 M hydrochloric acid (HCl) may have been needed to neutralize 0.5 L of the unknown solution.

Step 2 – Convert Drops to Milliliters

Because a water‑based solution delivers roughly one milliliter per 20 drops, divide the drop count by 20 to find the acid volume used.

Example: 40 ÷ 20 = 2 mL

Step 3 – Convert Milliliters to Liters

Divide the milliliter value by 1,000 to express the volume in liters.

Example: 2 ÷ 1,000 = 0.002 L

Step 4 – Calculate Moles of Acid

Multiply the volume (in liters) by the acid’s molarity to determine how many moles of H⁺ were added.

Example: 0.002 L × 1 M = 0.002 moles

Step 5 – Convert to Equivalent Moles of H⁺

Adjust for the acid’s stoichiometry by multiplying by the number of H⁺ ions per molecule. For HCl this factor is 1; for H₂SO₄ it would be 2.

Example: 0.002 moles × 1 = 0.002 equivalents

Step 6 – Determine [OH⁻] in the Unknown

Divide the equivalent moles by the volume (in liters) of the original solution to find the hydroxide ion concentration.

Example: 0.002 ÷ 0.5 L = 0.004 M

Step 7 – Compute pOH

Take the negative base‑10 logarithm of the [OH⁻] value to obtain pOH.

Example: pOH = –log(0.004) ≈ 2.4

Step 8 – Convert to pH

Subtract the pOH from 14 to yield the solution’s pH.

Example: pH = 14 – 2.4 = 11.6 The unknown solution thus has a pH of 11.6.