What is the equivalence point in a titration?

The equivalence point is where the moles of titrant exactly neutralise the moles of analyte, accounting for stoichiometry. It is a property of the chemistry, whereas the endpoint is where your indicator changes colour. A well-chosen indicator makes the two coincide.

Why is the equivalence pH not always 7?

Only a strong-acid and strong-base titration gives a neutral salt with pH 7. A weak acid titrated with a strong base leaves a basic salt, so the equivalence pH is above 7. A strong acid titrated with a weak base leaves an acidic salt, so the pH is below 7.

How do I pick the right indicator?

Choose an indicator whose colour-change range straddles the equivalence pH. Phenolphthalein suits basic endpoints, methyl orange or methyl red suits acidic endpoints, and bromothymol blue suits neutral strong-strong titrations. The tool suggests one based on the titration type you select.

What are the protons or hydroxides per molecule fields for?

They handle polyprotic acids and polyhydroxide bases. Sulfuric acid releases two protons, so its valence is 2, while calcium hydroxide releases two hydroxides. Setting these correctly keeps the equivalents balance accurate for anything beyond simple monoprotic reactions.

Acid-Base Titration Calculator

Q: Which formula does this calculator use?

It uses the equivalents balance, Ca times Va times za equals Cb times Vb times zb, where C is molarity, V is volume, and z is the number of reactive protons or hydroxides per molecule. Solving for the analyte molarity gives Ca = (Cb·Vb·zb) / (za·Va).

An acid-base titration finds an unknown concentration by reacting a measured sample with a titrant of known strength until neutralisation is complete. The volume of titrant needed, the titre, is the single measurement that unlocks the answer. This calculator turns that titre into a concentration and tells you what to expect at the endpoint.

How it works

Neutralisation is complete when the reactive equivalents of acid and base are equal. In normality form:

Ca x Va x za  =  Cb x Vb x zb

where C is molarity, V is volume, and z is the number of reactive protons or hydroxides per molecule. Rearranging for the analyte concentration:

Ca  =  (Cb x Vb x zb) / (za x Va)

The valence terms za and zb matter for polyprotic acids such as sulfuric acid (z = 2) and polyhydroxide bases such as calcium hydroxide. For a simple monoprotic-monohydroxide reaction both are 1 and the formula collapses to the familiar Ca·Va = Cb·Vb.

Equivalence pH and indicator choice

The pH at the equivalence point depends on the salt that forms. A strong acid neutralised by a strong base leaves a neutral salt, so the pH is 7. A weak acid neutralised by a strong base leaves a conjugate base, pushing the equivalence pH above 7, while a strong acid neutralised by a weak base leaves a conjugate acid and an equivalence pH below 7. Pick an indicator whose transition range brackets that pH so the colour change marks true neutralisation.

Example and notes

Titrating 25.0 mL of an unknown acid against 0.100 mol/L NaOH that requires a 25.0 mL titre, with both valences 1, gives Ca = (0.100 × 0.025 × 1) / (1 × 0.025) = 0.100 mol/L. The equivalence pH and indicator suggestion are idealised guides; for exact endpoint pH you must include the salt concentration and the relevant Ka or Kb.