Ripper Method

The Ripper Method, developed in 1898, is an analytical chemistry technique used to determine the total amount of sulfur dioxide (SO2) in a solution. This technique uses iodine standard and a starch indicator to titrate the solution and determine the concentration of free SO2. The titration is done again with a new sample of the solution, but the sample is pretreated with sodium hydroxide (NaOH) to release bound SO2. The result of these two titrations can then be used to determine the bound, free, and total amount of SO2 in the solution. Instead of using a starch indicator, an electrode can be used to determine the presence of free iodine. This technique is widely used in wine making.

Chemical Equilibria
The first reaction of iodine with SO2 and water is as follows:

SO2+I2+2H2O→H2SO4+2HI

As the reaction proceeds, all available SO2 will be consumed and the starch indicator added to the solution will bind with the unconsumed iodine, turning the solution black.

The second step of the reaction requires pretreating with solution with NaOH to release bound SO2. The reaction with iodine can then be done.

HSO3−⇌H2SO3⇌SO2

Applications
The Ripper Method is commonly used in wine making applications as SO2 is often added to wine to maintain its freshness and the concentration needs to be determined. The technique is not precise and is prone to systematic error as well. This limits its use, despite being a fast and inexpensive test.