How to Do Titration Calculations

••• undefined undefined/iStock/GettyImages

A titration is a technique used to work out the concentration of an unknown solution based on its chemical reaction with a solution of known concentration. The process usually involves adding the known solution (the titrant) to a known quantity of the unknown solution (the analyte) until the reaction is complete. To calculate the concentration of the analyte, you measure the volume of titrant used.

Things You'll Need

  • Titrant
  • Analyte
  • Erlenmeyer flask
  • Burette
  • Acid-base indicator
  • Calculator
  1. Prepare the Concentrations

  2. Place the analyte in an Erlenmeyer flask (a conical flat-bottomed laboratory flask with a narrow neck). Place the titrant in a burette (a graduated glass tube with a tap at one end).

  3. Mix the Concentrations

  4. Add the titrant to the analyte until the endpoint is reached. This is often indicated by a color change, for example by adding a few drop of phenolphthalein, a commonly used acid‑base indicator, which changes from pink in alkali to colorless in acid.

  5. Calculate the Molarity

  6. Use the titration formula. If the titrant and analyte have a 1:1 mole ratio, the formula is molarity (M) of the acid x volume (V) of the acid = molarity (M) of the base x volume (V) of the base. (Molarity is the concentration of a solution expressed as the number of moles of solute per litre of solution.)

    If the ratio is not 1:1, use a modified version of the formula. For example, if 35 ml of 1.25 M hydrochloric acid (HCI) is needed to titrate a 25 ml solution of sodium hydroxide (NaOH) to the equivalence point, you can work out the concentration of NaOH using the 1:1 ratio formula, because hydrochloric acid and sodium hydroxide have a 1:1 mole ratio (one mole of HCl reacts with one mole of NaOH).

    Multiply the molarity of the acid by the volume of the acid (1.25 x 35). Then take this answer (43.75) and divide it by the volume of the base (25). The answer is 1.75 M, which is the molarity of the base.

    Tips

    • A titration calculation is a simple formula used to work out the concentration (in moles) of one of the reactants in a titration using the concentration of the other reactant. Titrations are usually carried out on acid-alkali reactions, to determine what volumes of the acid and alkali are required to create a neutral solution. They may involve a strong acid with a strong base, a weak acid with a strong base, or a strong acid with a weak base.

References

About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.