Acid-Base Titrations

Acid-Base Titrations

  • A titration is a technique used to determine the concentration of a solution by reacting it with a solution of known concentration, known as the titrant.
  • An acid-base titration involves the gradual addition of an acid to a solution of a base, or a base to a solution of an acid, until the acid and base have reacted completely. This point is known as the equivalence point.
  • The equivalence point of an acid-base titration can be visually identified by using a pH indicator. This is a chemical that changes colour at a certain pH.

Indicators and the End Point

  • The end point of a titration is the point where the indicator changes colour. This is usually close to, but not exactly the same as, the equivalence point.
  • The ideal indicator to use is one whose pH range includes the pH at the equivalence point of the titration.
  • Phenolphthalein, which is colourless in acid and pink in alkali, and litmus, which is red in acid and blue in alkali, are commonly used indicators.
  • More accurate titrations use a pH meter instead of an indicator to determine the pH at the equivalence point, because this eliminates any subjectivity of colour perception.

Calculations in Titrations

  • The molar concentration of the titrant and the volume added to reach the end point are used to calculate the moles of titrant used.
  • The stoichiometry of the acid-base reaction can then be used to calculate the moles of the substance being titrated.
  • From this, you can determine the molar concentration (in mol/dm^3) of the solution being titrated.
  • It is important to recognise that while some titration reactions have a 1:1 stoichiometry, others do not. Always check the balanced equation to ensure correct calculations.

Standardising Titrants

  • Sometimes, the exact concentration of the titrant is not known. In this case, it can be standardised by titrating it against a solution of known concentration, known as a primary standard.
  • Primary standards must be very pure, stable in air, and of known molar mass. Examples include anhydrous sodium carbonate and potassium hydrogen phthalate.

Remember: Precision, accuracy, and careful technique are crucial in titrations. Every drop counts!