Page 14 - Grapevine JanFeb 2022

In The Winery

record the amount of solution in that burette. was used to titrate the reaction flask effluent back
to the seawater green color. So 2.3 times 0.01
17. Titrate the solution in the reaction flask from times 32000 divided by twenty yields 36.8 parts per
the bright purple it has become back to the sea- million free sulfur dioxide present in the sample
water green it was before starting the test. tested. Part per million is also equal to milligrams
per liter.
(Note: Make sure to capture this endpoint since
overshooting it will result in an inaccurate calcu- Summary
lation of the sulfur dioxide on the high side)
This test is really very simple once one gets over
18. After the seawater green has been achieved, the intimidation of the unit and chemistry. It is
record the number of milliliters in the burette recommended to follow these directions in the lab
and write this number down on a piece of paper for 8 to 10 test runs and then to run several dozen
or in the lab book. others to commit the process to memory. After
running this process approximately one hundred
19. Subtract the number of milliliters used to titrate times - read this procedure over again and much
the solution in the reaction flask and record this of the process will become crystal clear. When
number to be used in the calculation portion of learning this test, try to find a time when few inter-
the test. Obtain the exact volume of NaOH, to ruptions will be expected so focusing on the task at
the tenth, used to neutralize the reaction flask hand will be easy.
content. Example: 2.3 mils.

Calculation: Other Helpful Tips

Here is the formula to calculate the free sulfur Once the phosphoric acid is added to the wine
dioxide: sample – all of the free SO2 is released – it
is important to capture all of this SO2 in the
Free SO2 = Milliliters of NaOH * Normality of NaOH * 32000 Hydrogen peroxide solution – hence the need for
20 milliliters of wine sample tested connecting the wine sample boiling flask as rapid-
ly as possible to the complete set up. Make sure
The Knowns Above Are: the Pasteur pipette is positioned to extend to the
bottom of the boiling flask, well below the liquids
The milliliters of sodium hydroxide used to titrate surface, for best results.
the reaction flask effluent from purple to green as
calculated from the reading on the burette. Set your aeration oxidation unit up as the manu-
facture has intended for the unit to be assembled.
The normality of the sodium hydroxide used above
since it was standardized recently. While performing the test make sure there is not
an exogenous source of Sulfur Dioxide in the area
The given 32000 number of the testing – such as weighing metabisulfite and
creating a dust or pungent aroma. This may throw
The sample size of 20 milliliters. off the test leading to false readings on the high
side.
From this formula we now have enough informa-
tion to calculate the free sulfur dioxide in the sam- Endpoint: Be sure to catch the end point during
ple of wine. If the Normality of the Sodium hydrox- titration at exactly the precise time the color chang-
ide is exactly 0.01N one may see results as follows: es being sure to swirl the reaction flask during titra-
tion.
Let’s assume 2.3 milliliters of Sodium hydroxide

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