Sulfur dioxide (molecular method) (Type-IV)
OIV-MA-AS323-04C Sulfur dioxide
Type IV method
- Definitions
Free sulfur dioxide is defined as the sulfur dioxide present in the must or wine in the following forms: H2SO3, HSO3, whose equilibrium as a function of pH and temperature is:
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H2SO3 represents molecular sulfur dioxide.
Total sulfur dioxide is defined as the total of all the various forms of sulfur dioxide present in the wine, either in the free state or combined with their constituents.
- Molecular Sulfur Dioxide
2.1. Principle of the Method
The percentage of molecular sulfur dioxide, H2SO3, in free sulfur dioxide, is calculated as a function of pH, alcoholic strength and temperature.
For a given temperature and the alcoholic strength:
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where
L = [
] + [
]
pkM = pkT 
I = ionic strength
A & B = Coefficients which vary according to temperature and alcoholic strength.
= Thermodynamic dissociation constant; the value of pkT is given in Table 1 for various alcoholic strengths and temperatures.
= Mixed dissociation constant
Taking a mean value 0.038 for the ionic strength I, Table 2 gives the values of pkM for various temperatures and alcoholic strengths.
The molecular sulfur dioxide content calculated by the relationship given in (1) is presented in Table 3 for various values of pH, temperature and alcoholic strength.
2.2. Calculations
Knowing the pH of wine and its alcoholic strength, the percentage of molecular sulfur dioxide is given in Table 3 for a temperature t °C. Let this be X %.
The amount of molecular sulfur dioxide in mg/L is given by:X · C
C = the free sulfur dioxide in mg/L
Table I
Values of the thermodynamic constant pkT
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Alcohol % by volume |
Temperature oC |
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20 |
25 |
30 |
35 |
40 |
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0 |
1.798 |
2.000 |
2.219 |
2.334 |
2.493 |
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5 |
1.897 |
2.098 |
2.299 |
2.397 |
2.527 |
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10 |
1.997 |
2.198 |
2.394 |
2.488 |
2.606 |
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15 |
2.099 |
2.301 |
2.503 |
2.607 |
2.728 |
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20 |
2.203 |
2.406 |
2.628 |
2.754 |
2.895 |
Table II
Values of the Mixed Dissociation Constant pkM (I= 0.038)
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Alcohol % by volume |
Temperature °C |
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20 |
25 |
30 |
35 |
40 |
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0 |
1.723 |
1.925 |
2.143 |
2.257 |
2.416 |
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5 |
1.819 |
2.020 |
2.220 |
2.317 |
2.446 |
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10 |
1.916 |
2.116 |
2.311 |
2.405 |
2.522 |
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15 |
2.014 |
2.216 |
2.417 |
2.520 |
2.640 |
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20 |
2.114 |
2.317 |
2.538 |
2.663 |
2.803 |
Table III
Molecular Sulfur Dioxide as a Percentage of Free Sulfur Dioxide (I=0.038)
|
pH |
T = 20 oC Alcohol % by volume |
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0 |
10 |
15 |
20 |
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2.8 |
7.73 |
9.46 |
11.55 |
14.07 |
17.09 |
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2.9 |
6.24 |
7.66 |
9.40 |
11.51 |
14.07 |
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3.0 |
5.02 |
6.18 |
7.61 |
9.36 |
11.51 |
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3.1 |
4.03 |
4.98 |
6.14 |
7.58 |
9.36 |
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3.2 |
3.22 |
3.99 |
4.94 |
6.12 |
7.58 |
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3.3 |
2.58 |
3.20 |
3.98 |
4.92 |
6.12 |
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3.4 |
2.06 |
2.56 |
3.18 |
3.95 |
4.92 |
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3.5 |
1.64 |
2.04 |
2.54 |
3.16 |
3.95 |
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3.6 |
1.31 |
1.63 |
2.03 |
2.53 |
3.16 |
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3.7 |
1.04 |
1.30 |
1.62 |
2.02 |
2.53 |
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3.8 |
0.83 |
1.03 |
1.29 |
1.61 |
2.02 |
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T = 25 oC |
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2.8 |
11.47 |
14.23 |
17.15 |
20.67 |
24.75 |
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2.9 |
9.58 |
11.65 |
14.12 |
17.15 |
22.71 |
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3.0 |
7.76 |
9.48 |
11.55 |
14.12 |
17.18 |
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3.1 |
6.27 |
7.68 |
9.40 |
11.55 |
14.15 |
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3.2 |
5.04 |
6.20 |
7.61 |
9.40 |
11.58 |
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3.3 |
4.05 |
4.99 |
6.14 |
7.61 |
9.42 |
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3.4 |
3.24 |
4.00 |
4.94 |
6.14 |
7.63 |
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3.5 |
2.60 |
3.20 |
3.97 |
4.94 |
6.16 |
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3.6 |
2.07 |
2.56 |
3.18 |
3.97 |
4.55 |
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3.7 |
1.65 |
2.05 |
2.54 |
3.18 |
3.98 |
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3.8 |
1.32 |
1.63 |
2.03 |
2.54 |
3.18 |
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T = 30 oC |
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2.8 |
18.05 |
20.83 |
24.49 |
29.28 |
35.36 |
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2.9 |
14.89 |
17.28 |
20.48 |
24.75 |
30.29 |
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3.0 |
12.20 |
14.23 |
16.98 |
20.71 |
25.66 |
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3.1 |
9.94 |
11.65 |
13.98 |
17.18 |
21.52 |
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3.2 |
8.06 |
9.48 |
11.44 |
14.15 |
17.88 |
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3.3 |
6.51 |
7.68 |
9.30 |
11.58 |
14.75 |
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3.4 |
5.24 |
6.20 |
7.53 |
9.42 |
12.08 |
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3.5 |
4.21 |
4.99 |
6.08 |
7.63 |
9.84 |
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3.6 |
3.37 |
4.00 |
4.89 |
6.16 |
7.98 |
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3.7 |
2.69 |
3.21 |
3.92 |
4.95 |
6.44 |
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3.8 |
2.16 |
2.56 |
3.14 |
3.98 |
5.19 |
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Table III (continued)
Molecular Sulfur Dioxide as a Percentage of Free Sulfur Dioxide (I=0.038)
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pH |
T=35 oC Alcohol % by volume |
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0 |
5 |
10 |
15 |
20 |
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2.8 |
22.27 |
24.75 |
28.71 |
34.42 |
42.18 |
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2.9 |
18.53 |
20.71 |
24.24 |
29.42 |
36.69 |
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3.0 |
15.31 |
17.18 |
20.26 |
24.88 |
31.52 |
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3.1 |
12.55 |
14.15 |
16.79 |
20.83 |
26.77 |
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3.2 |
10.24 |
11.58 |
13. 82 |
17.28 |
22.51 |
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3.3 |
8.31 |
9.42 |
11.30 |
14.23 |
18.74 |
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3.4 |
6.71 |
7.63 |
9.19 |
11.65 |
15.49 |
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3.5 |
5.44 |
6.16 |
7.44 |
9.48 |
12.71 |
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3.6 |
4.34 |
4.95 |
6.00 |
7.68 |
10.36 |
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3.7 |
3.48 |
3.98 |
4.88 |
6.20 |
8.41 |
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3.8 |
2.78 |
3.18 |
3.87 |
4.99 |
6.80 |
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T = 40 oC |
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2.8 |
29.23 |
30.68 |
34.52 |
40.89 |
50.14 |
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2.9 |
24.70 |
26.01 |
29.52 |
35.47 |
44.74 |
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3.0 |
20.67 |
21.83 |
24.96 |
30.39 |
38.85 |
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3.1 |
17.15 |
18.16 |
20.90 |
25.75 |
33.54 |
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3.2 |
14.12 |
14.98 |
17.35 |
21.60 |
28.62 |
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3.3 |
11.55 |
12.28 |
14.29 |
17.96 |
24.15 |
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3.4 |
9.40 |
10.00 |
11.70 |
14.81 |
20.19 |
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3.5 |
7.61 |
8.11 |
9.52 |
12.13 |
16.73 |
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3.6 |
6.14 |
6.56 |
7.71 |
9.88 |
13.77 |
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3.7 |
4.94 |
5.28 |
6.22 |
8.01 |
11.25 |
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3.8 |
3.97 |
4.24 |
5.01 |
6.47 |
9.15 |
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Bibliography
Molecular sulfur dioxide:
- BEECH F.W. & TOMAS Mme S., Bull. O.I.V., 1985, 58, 564-581.
- USSEGLIO-TOMASSET L. & BOSIA P.D., F.V., O.I.V., 1984, n° 784.