Acid-base tritrations

• A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution.
• Titrations can be used for:

  • finding the concentration of a solution
  • identification of unknown chemicals
  • finding the purity of a substance
• Checking purity is an important aspect of quality control. It's essential that pharmaceuticals have a high level of purity - a tiny amount of an impurity can be very harmful
• A standard solution is a solution of known concentration
• A volumetric flask is used to make up a standard solution very accurately
• 100cm3 volumetric flask = ±0.20 cm3
• 250cm3 volumetric flask = ±0.30 cm3
• Preparing standard solution:

  1. The solid is first weighed accurately
  2. solid is dissolved in a beaker using less distilled water than needed to to fill the volumetric flask to the mark
  3. this solution transferred to a volumetric flask. Last traces of the solution are rinsed into the flask with distilled water
  4. The flask is filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark. If too much water added solution will be too dilute, start again.
  5. volumetric flask is inverted several times to mix the solution throughly
• 10 cm3 pipette = ±0.04cm3
• 25cm3 pipette = ±0.06 cm3
• 50cm3 burette = 0.10 cm3
• Each burette reading is measured to the nearest ±0.05 cm3 so the reading always has two decimal places, the last place always being either 0 or 5
• When working out the mean titre, it is important to use only your closest accurate titres:

  • by repeating titres until two agree within 0.10 cm3, you can reject inaccurate titres
  • if you were to include all the titres in the mean, you have lost the accuracy of the titration technique