Titrations

• To prepare a 250cm3 standard solution:

  • weigh sample in boat
  • transfer/wash all sample into beaker
  • re-weigh boat
  • dissolve sample in distilled water
  • Add to volumetric flask with washings
  • Make up to 250cm3 mark
  • SHake/invert
• Titrations are a method of finding out the concentration of a solution
• The first titre always gives a rough value
• When approaching the rough value, the acid must be added dropwise
• Alkali must be added until there is a permanent colour change, not just a temporary one
• Titrations are a method of finding the concentration of a solution. One of the reagents must be a standard solution
• There are two ways for carrying out a pH titration:

  • Use a pH probe to measure the change in pH as a base is added to an acid
  • add an indicator and add the acid until the colour changes
• During a titration the pH does not change in a linear manner. There are sections which are almost horizontal and sections that are vertical
• The starting point off a titration curve depends on the strength of the acid. A strong acid starts at about pH 1, whereas a weak acid starts at around pH 3
• The end point of a titration curve depends on the strength of the base. A strong base ends at about pH 13. A weak base ends at about pH 9
• The vertical section of a pH titration graph is called the equivalence point
• The equivalence point is exactly when enough acid has been added to neutralise the base
• The end point is the exact volume of acid or base which needs to be added to cause an indicator to change colour
• A suitable indicator changes colour somewhere on the vertical section of a pH titration curve
• Phenolphthalein changes from colourless to red at a pH of 10
• Methyl orange changes from red to yellow at a pH of 4.5
• The half neutralisation point is half way between 0 and the equivalence point on a pH titration curve