Practical investigations

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I am about to analyse wine and investigate sulphur dioxide content. I have done some research and come across the “ripper” method which analyses for free sulphur dioxide. This is essentially the method that involves a titration with an iodine standard. I have read in numerous places that this method is inaccurate, yet commonly used. Please could you outline the procedure for the analysis of free or total sulphur dioxide content in wines.
When analysing red wines with wet analysis techniques such as titrations, there are apparently problems with recognising the end point. I have read a little about activated charcoal, and how it can be used to decolourise wines. Will the use of activated charcoal inhibit my ability to analyse for SO₂ using the ripper method ?

Igloo writes
Some of the “total” sulphur dioxide in wines is “free”, i.e. present in its molecular form dissolved in the wine, whilst the rest is “bound”, e.g. in the ions HSO₃^-, SO₃^2-, and is attached to certain types of organic molecules, notably aldehydes and sugars.
 
However, much of the attached SO₂ is so loosely bound that the distinction between “free” and “bound” becomes somewhat blurred, which is why the accuracy of titration is not felt to be such an important issue. When carrying out the iodine titration for example, the iodine molecules not only react with the “free” SO₂, but also manage to dislodge a few of the “bound” molecules, so the titration reading is a little higher than it should be, leading to a slight overestimation of [SO₂]_free. It is true that iodine itself reacts with one or other components in the wine, notably aldehydes and ketones present, but for the reasons given the “ripper” method is generally thought to provide meaningful enough results and is certainly a simple and straightforward procedure.
 
As regards procedure, this is straightforward enough. First make a solution of iodine of appropriate concentration (i.e. very dilute, and probably of the order of 0.001 mol dm^-3 or lower). Since iodine isn’t a primary standard, preliminary titrations with a standard solution of sodium thiosulphate – with starch as indicator – will enable the accurate concentration of the iodine to be determined. Then take a known volume of wine and titrate it with this iodine solution.
 
For “total” SO₂ the wine sample needs to be boiled up with sodium hydroxide first (this dislodges “bound” SO₂ molecules from their hosts), the solution allowed to cool, and then very slightly acidified, before the iodine titration is carried out again.
 
I can recommend an excellent web reference on all these issues.
 
The use of activated charcoal with red wines is well documented, and yes, there probably will be some sort of interference with your analysis. The charcoal will almost certainly adsorb some of the SO₂ on its surface, leading to an underestimate of the actual SO₂ in solution.
 
Always carry out a risk assessment and check with your teacher before starting any practical work.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

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updated: 03 March 2006