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Trouble shooting
H2O2 (hydrogen peroxide) solution fails to change colour (turn purple) during the 15 min. aspiration time.
Possible causes -
1. - No SO2 is present in the juice or wine sample being tested.
2. - You have neglected to add the orthophosphoric acid to your sample in step 8 of the procedure preparation.
3. - You did not adjust your H2O2 soln. according to step 5 of the procedure preparation.
You may have used one drop of 0.1 M NaOH instead of 0.01M NaOH to produce the olive/green colour. This is equivalent to 10 drops (~5ml) of 0.01M NaOH and could well be enough to react with any sulphuric acid (H2SO4) formed.
4. - You may be loosing SO2 through a poor fitting joints of your apparatus (this will occur when applying pressure rather then a vacuum to generate your flow rate).
Solution -
Values of SO2 determined are lower than expected.
Possible causes -
1. - The concentration of your 0.01M NaOH titrant may have been incorrectly standardised and may be more concentrated then ascertained.
2. - You may be loosing SO2 through a poor fitting joints of your apparatus (this will occur when applying pressure rather then a vacuum to generate your flow rate - see recommended aspiration method).
3. - Insufficient volume or strength of H2O2 solution in the pear shaped flask to trap all of the SO2 aspirated (see equation).
4. - The sample used was unrepresentative.
5. - A flow rate less then 1 L/min. was used.
6. - If a flow rate is significantly faster then 1L/min See hints and tips
7. - Aspiration time less then 15 minutes.
8. - Reduced SO2 stock strength (especially if testing for verification of juice/wine SO2 addition levels).
9. - Incorrect calculations or estimation of amounts added (see FSO2).
Solution -
Values of SO2 determined are higher than expected.
Possible causes -
1 -- The concentration of your 0.01M NaOH titrant may have been incorrectly standardised and may be less concentrated then ascertained.
2. - The sample used was unrepresentative.
This can occur after SO2 additions and inadequate mixing. This problem is exacerbated when large tanks, with musts involving grape skins and the use of poorly dissolved powdered forms of sulphur dioxide are used.
3. - A flow rate greater then 1 L/min. was used
4. - Incorrect calculations or estimation of amounts added.
Solution -