Wine Analysis

End point

• Different indicator solutions are used throughout analytical procedures that change colour at a specific pH or pH range.
  
  This colour change is referred to as the end point or equivalence point.
  
  
• The end point usually occurs close to a neutral pH of 7.
  
  
• When titrating a strong acid (HCl or H₂SO₄) with a strong base (NaOH) (and visa versa) the colour change (end point) of the indicator solution usually occurs within one drop and is quite distinct and easy to determine. (see graph)
  
  
• When titrating a weak acids (tartaric acid, malic acid or volatile acids) with a strong base (NaOH) the colour change of the indicator solution occurs over a wider pH range and is less distinct.
  
  Some experience is required to distinguish the end point in this case or a pH meter to determine the end point can be used. (see graph)
  
  
• The titration performed during SO₂ analysis and during the standardisation of your NaOH solution are of the strong acid/strong base type. The mixed indicator and phenolphthalein indicator respectively, are adequate for end point determination without the need for a pH meter.