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Chemistry involved in the aspiration/oxidation method of SO₂ analysis.

Free Sulphur Dioxide (FSO₂)


• Orthophosphoric acid is added to the juice or wine sample to lower the pH.
  
  The much lower pH, shifts the equilibrium of all the Free SO₂ (FSO₂) forms towards the gaseous, molecular SO₂ form. (see equation)
  
  This gaseous form of SO₂ is able to be aspirated across to react with the hydrogen peroxide (H₂O₂) solution held in the pear shaped flask. (see diagram)
  
  
• The hydrogen peroxide in the pear shaped flask reacts with the SO₂ aspirated across to form sulphuric acid (H₂SO₄) (see equation).
  
  
• H₂SO₄ (sulphuric acid) is non volatile and will remain in solution during the aspiration.
  
  
• The increase in acidity and the corresponding decrease in pH, caused by the formation of H₂SO₄ (sulphuric acid), affects the state of the indicator solution added, with a corresponding colour change from olive/green to purple/blue.
  
  
• NOTE THAT: as FSO₂ is removed there is a corresponding shift in equilibrium between the FSO₂ and the BSO₂ (Bound SO₂).
  
  This shift results in some of the BSO₂ being measured as FSO₂.
  
  The 15 min aspiration time at a flow rate of 1L/min ensures the removal of all FSO₂ without a excess of BSO₂ being incorporated into the measurements.
  
  This is the reason why the aspiration time and the flow rate are important when determining free SO₂.
  
  The flow rate and aspiration time is not as important when analyzing BSO₂ or TSO₂ (total SO₂)
  
  
Bound Sulphur Dioxide (BSO₂)


• Once all the free SO₂ has been removed and determined, the BSO₂ (bound sulphur dioxide) concentration can be determined, by applying heat to the remaining juice/wine solution, to break the bonds between the SO₂ binding compounds and SO₂. Once the SO₂ is liberated as FSO₂ all the above mentioned chemistry applies.
  
  
• When all the FSO₂ present has been converted to H₂SO₄ (sulphuric acid), the solution can be titrated.
  
  
Total Sulphur Dioxide (BSO₂)


• Total sulphur dioxide (TSO₂) can be calculated by adding the determined concentration values for FSO₂ and BSO₂
  
  or
  
  
• TSO₂ (total sulphur dioxide) can be measured, on a fresh sample, by preparing as for FSO₂ determination and proceeding as for BSO₂ determination.
  
  
The Titration


• The titration is performed using a 0.01 M NaOH (sodium hydroxide solution), as the titrant.
  
  The equivalent quantity of NaOH (sodium hydroxide) reacts with an equivalent quantity of H₂SO₄ (sulphuric acid) forming one molecule of water (see equation).
  
  
• When practically all of the H₂SO₄ has been reacted with the added NaOH, to form water, the near neutral pH will turn the indicator solution back to the original olive green colour of the H₂O₂ solution (i.e. the end point)
  
  
• The amount of TSO₂, BSO₂ or SO₂ present can be related back to and can be calculated from the amount of H₂SO₄ formed.
  
  The amount of H₂SO₄ formed can be calculated from the amount of NaOH required to react with the H₂SO₄ to near neutralise it or reach the end point.
  
  Therefore, the titre of NaOH is used to calculate the initial SO₂ present in whatever form we are determining.
  
  
• The calculation involved when using exactly 0.0100 M NaOH and a 20 ml sample is -
  
  SO₂ ppm = 16 x Titre ml (0.0100 M NaOH) (see calculations).