Free Sulphur Dioxide in wine and must, by Dr. Stührk's method:- 

The shelf-life of the dark (titrating) solution is indicated on the bottle label. 
Fill the cylinder with wine or must accurately up to the O mark (10 ccs). When doing this hold the cylinder at eye level and take care that the lowest part of the meniscus just touches the O mark. 
From the dropping bottle add Dr. Stührk's dark (titrating) solution, drop by drop. Initially the blue colouration of the dark solution disappears immediately when the measuring cylinder is shaken briefly. As soon as the colour begins to disappear only slowly, the contents of the cylinder must be thoroughly mixed by applying the thumb to the top of the cylinder and shaking. Eventually, when a faint red-violet colour remains for a short time (1 to 2 seconds) the determination is complete. 
A more marked and persistent violet colouration means a drop too much has been added, giving too high a reading. 
The level of the mixture in the cylinder is accurately read off in the same way as when adjusting the level of wine initially. The scale gives the content of free SO2 in milligrams per litre. 

Total Sulphur Dioxide in wine and must, by Dr. Stührk's method:- 

The supplementary equipment needed comprises a so-called dilution cylinder (with the marks "Getränk - I - II", which in its external dimensions is identical to the titrating cylinder) and two different solutions (solution I and solution II). 
The determination of the total sulphur dioxide is carried out as follows:- 
1. Prior to use, always wash out the dilution cylinder with the drink to be analysed. 
2. Pour the drink into the cylinder up to the mark "Getränk". 
3. Adjust the level accurately with the pipette supplied (the lowest part of the meniscus must just touch the O mark). 
4. Add solution I from the dropping bottle, bringing the level up to the mark "I" on the dilution cylinder. 
5. Allow to stand for 5 to 10 minutes. 
6. Fill up accurately to the mark "II" with solution II (take care - if too much of the solution II is added from the dropping bottle, steps 1 to 6 must be repeated !). 
7. Carefully mix the contents by rotating the cylinder. 
8. Transfer some of the solution obtained to the titrating cylinder (up to the O mark) and titrate in the usual manner with the "titrating solution for the determination of free sulphur dioxide". 
9. The result that is read off the scale (SO2 mg/ltr) must be doubled. For example, reading 120 mg/ltr; total sulphur dioxide in the drink = 240 mg/ltr. 
It is important to point out that the error in the determination is under 10% only if the procedure is carried out with care. In particular, after adding solution II (step 6 in the instructions) the next steps must be carried out without delay. If the solution is allowed to stand for any length of time, loss of SO2 unavoidably occurs and the values obtained are somewhat lower than they actually are. 
The method is suitable for white and red wines. The time required for the determination is about 10 minutes. 
The solutions I and II needed for the examination have unlimited life and can be used for about 30 determinations.

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