Free, Total and Double SO2
in Wine
DOC316.52.93088
Endpoint potentiometric titration with imposed current
Scope and application: Wines
1.
Introduction
This working procedure refers to the titration of sulfur dioxide in wines. A double platinum electrode is used:
an alternating current is imposed and the resulting potential is measured.
2.
Principle
The protocol rests on the redox reaction between sulfur dioxide and iodine:
SO2 + I2 + 2 H2O  H2SO4 + 2 HI
According to the Ripper method, free SO2 is directly titrated in acidic media (sulfuric acid is added to the
sample). Total SO2 is the sum of free SO2 and combined SO2. The latter is released by hydrolysis in alkaline
media (sodium hydroxide is added to the sample prior to sulfuric acid). Then, iodine solution is added until the
measured potential drops abruptly, which corresponds to the equivalence. The SO2 concentration is expressed
in mg/L.
3.
Electrode and reagents
Electrodes:
Intellical double platinum electrode with integrated temperature sensor, MTC695
Titrant:
Iodine I2 0.0282 eq/L (part number 2333353)
Acid reagent:
Sulfuric acid H2SO4 10 eq/L (part number 93153); install this reagent on PUMP 1
Alkali reagent:
Sodium hydroxide NaOH 5 eq/L (part number 245053); install this reagent on PUMP 2
Solid KI:
With dispenser (part number 2834100)
Standard for titrant calibration: Sodium thiosulfate Na2S2O3 0.00564 eq/L (part number 2408853)
Deionized water
4.
Ranges and settings
4.1. Default parameters
The working procedure is described using the following parameters:
•
•
V sample = 20 mL
Syringe volume = 10 mL
4.2. Working range
For most samples, 1 syringe (10 mL) of titrant should be sufficient to reach the equivalent point. It provides the
following range:
Veq (mL)
SO2 (mg/L)
0.1
4.5
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
9.5
430
Page 1 of 8
04/2015, Edition 2
For low SO2 concentrations below 45 mg/L, (equivalent volumes under 1 mL) the titration typically lasts less
than 1 minute.
For SO2 concentrations up to 110 mg/L (equivalent volume around 2.5 mL), the titration typically lasts then
than 2 minutes.
For SO2 concentrations between 110 and 430 mg/L, it is recommended to add a predose in order to keep
titration duration to around 2-3 minutes. Orders of magnitude are given in the table below:
Expected SO2 concentration (mg/L)
Expected equivalent
volume (mL)
3.3
4.4
5.5
6.6
7.7
8.9
150
200
250
300
350
400
Recommended
predose (mL)
0.8
1.9
3.0
4.1
5.2
6.4
4.3. Settings
Name
Sample
Name
Amount
Amount min
Amount max
Titrant
Name
Titrant concentration
Syringe
Probe
Recommended probe
Leveling
Active
Time
Default parameter
Unit
Wine
20
0
22
[mL]
[mL]
[mL]
I2
0.0282
Syringe 1
[eq/L]
MTC695
No
30
[s]
4.3.1. Free SO2
Name
Step 1: Acid addition
Duration
Stirrer speed
Step 2: Reaction
Duration
Stirrer speed
Step 3: Titration
Stirring speed
Measured parameter
Predose
Max volume stop point
Stop on last EQP
Delay
Addition mode
Increment size
EP Ordinates
Result 1 name
R1 resolution
R1 min
R1 max
R1 QC min
R1 QC max
Default parameter
Unit
1
20
[s]
[%]
15
0
[s]
[%]
10
[%]
[mV]
[mL]
[mL]
0
10
True
0
Monotonic
0.08
160
Free SO2
Integer
5
430
5
430
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
[s]
[mL]
[mV]
[mg/L]
[mg/L]
[mg/L]
[mg/L]
Page 2 of 8
04/2015, Edition 2
4.3.2. Total SO2
Name
Step 1: Alkali addition
Duration
Stirrer speed
Step 2: Reaction
Duration
Stirrer speed
Step 3: Acid addition
Duration
Stirrer speed
Step 4: Reaction
Duration
Stirrer speed
Step 5: Titration
Stirring speed
Measured parameter
Predose
Max volume stop point
Stop on last EQP
Delay
Addition mode
Increment size
EP Ordinates
Result 1 name
R1 resolution
R1 min
R1 max
R1 QC min
R1 QC max
Default parameter
Unit
3
20
[s]
[%]
5
0
[min]
[%]
3
20
[s]
[%]
15
0
[s]
[%]
10
[%]
[mV]
[mL]
[mL]
0
10
True
0
Monotonic
0.08
140
Total SO2
Integer
5
430
5
430
[s]
[mL]
[mV]
[mg/L]
[mg/L]
[mg/L]
[mg/L]
4.3.3. Double SO2
The measurement of double SO2 consists of a first measurement of free SO2 followed directly by a
measurement of combined SO2. The sum of the 2 results gives the total SO2.
Name
Step 1: Acid addition
Duration
Stirrer speed
Step 2: Reaction
Duration
Stirrer speed
Step 3 : Free SO2 titration
Stirring speed
Measured parameter
Predose
Max volume stop point
Stop on last EQP
Delay
Addition mode
Increment size
EP Ordinates
Result 1 name
R1 resolution
R1 min
R1 max
R1 QC min
Default parameter
Unit
1
20
[s]
[%]
15
0
[s]
[%]
10
[%]
[mV]
[mL]
[mL]
0
10
True
0
Monotonic
0.08
160
Free SO2
Integer
5
430
5
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
[s]
[mL]
[mV]
[mg/L]
[mg/L]
[mg/L]
Page 3 of 8
04/2015, Edition 2
R1 QC max
Result 2 name
Hide
Step 4: Alkali addition
Duration
Stirrer speed
Step 5: Reaction
Duration
Stirrer speed
Step 6: Acid addition
Duration
Stirrer speed
Step 7: Reaction
Duration
Stirrer speed
Step 8: Combined SO2 titration
Stirring speed
Measured parameter
Predose
Max volume stop point
Stop on last EQP
Delay
Addition mode
Increment size
EP Ordinates
Result 3 name
Hide
Equation result name
Equation result resolution
Equation result min
Equation result max
Equation result QC min
Equation result QC max
430
Veq1
True
[mg/L]
4
20
[s]
[%]
5
0
[min]
[%]
3
20
[s]
[%]
15
0
[s]
[%]
10
[%]
[mV]
[mL]
[mL]
0
10
True
0
Monotonic
0.08
140
Veq2
True
Combined SO2
Integer
5
430
5
430
[s]
[mL]
[mV]
[mg/L]
[mg/L]
[mg/L]
[mg/L]
4.4. Modification of the settings
The parameters are defined in order to have the best compromise between accuracy and titration time.
For higher concentration with a high titrant volume, titration time can be reduced with an addition of titrant
(predose) at the beginning of the titration. Enter the predose volume (in mL) and the stirring time after the
addition in the application edit window.
5.
Titration procedure
5.1. Leveling
To use this method, an external pump is required. All elements (probes, tubes from the titrator and the tube
from the external pump) have to be well installed on the probe holder. The beaker has to contain a level of
sample higher than the position of the tube of the external pump. When the beaker is attached to the probe
holder, this method allows the system to automatically remove the excess sample by a defined pump working
time, and always keep the same sample volume before launching the analysis.
In order to define this volume, autoleveling calibration sequence has to be previously executed (see section 8.2
Autoleveling calibration).
When this option is active, the working time of the external pump must be set (default 30 s). The minimum
working time must allow the pump to be removing air during the last few seconds of the external pump
activation.
Note: Do not forget to re-edit the sample amount with the expected value when deactivating the leveling
method.
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04/2015, Edition 2
5.2. Titration
Rinse the probe with deionized water. If leveling is disabled, use a pipette to collect precisely 20 mL of sample.
Pour the sample into the 50 mL polypropylene beaker, put in a magnetic stir bar, dip the probe and the delivery
tip in the solution and then start the application.
At the end of the titration, a first window displays the result. A second window displays the titration curve and
the equivalent point coordinates.
After the titration, there are two possibilities:
•
•
6.
Replicate the sample. This is used to study the repeatability by analyzing several samples
successively. At the end of each titration, a window displays the average value, the standard
deviation (SD) and the relative standard deviation (RSD in %).
Analyze a new sample. Another titration can be started but no Standard Deviation and RSD
value will be made.
Results
6.1. Result calculation
The generic equation is:
For Free SO2:
For Total SO2:
For Double SO2:
SO2 (mg⁄L) =
ne- titrant Ctitrant (eq⁄L) × Vtitrant (mL)
×
× MSO2 (g/mol) × 1000
ne- titrant × Vsample (mL)
ne- SO2
Free SO2 (mg⁄L) =
Total SO2 (mg⁄L) =
Free SO2 (mg⁄L) =
Combined SO2 (mg⁄L) =
2 0.0282 (eq/L) × Veq (mL)
×
× 64.1 (g/mol) × 1000
2 × 20 (mL)
2
2 0.0282 (eq/L) × Veq (mL)
×
× 64.1 (g/mol) × 1000
2 × 20 (mL)
2
2 0.0282 (eq/L) × Veq1 (mL)
×
× 64.1 (g/mol) × 1000
2 × 20 (mL)
2
2 0.0282 (eq/L) × [Veq2 + V'] (mL)
×
× 64.1 (g/mol) × 1000 × FX
2 × 20 (mL)
2
Where V’ is the volume of excess titrant added after Veq1 during Free SO2 titration.
For Combined SO2, an equation is used since the result cannot be calculated directly from the equivalent
volume. Therefore, the factor FX (1 by default) has to be changed if the titrant unit is changed, for example:
Titrant titer unit
Factor FX
eq/L
1
meq/L
0.001
6.2. Experimental results
These results are indicative and have been obtained for given samples, for 8 to 10 successive determinations.
6.2.1 Free SO2
For 10 determinations on red wine:
Measurement
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
Parameter
25
0.4
1.7
45
Unit
[mg/L]
[mg/L]
[%]
[s]
Page 5 of 8
04/2015, Edition 2
For 10 determinations on white wine:
Measurement
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
Parameter
4
0.1
1.8
19
Unit
[mg/L]
[mg/L]
[%]
[s]
Parameter
80
2.8
3.5
108
Unit
[mg/L]
[mg/L]
[%]
[s]
Parameter
72
1.2
1.7
99
Unit
[mg/L]
[mg/L]
[%]
[s]
Parameter
Unit
6.2.2. Total SO2
For 10 determinations on red wine:
Measurement
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
For 10 determinations on white wine:
Measurement
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
6.2.3. Double SO2
For 8 determinations on red wine:
Measurement
Free SO2
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
Combined SO2
Mean concentration
Standard deviation
Relative standard deviation
Mean titration duration
9
0.1
0.9
14
[mg/L]
[mg/L]
[%]
[s]
48
0.9
1.8
48
[mg/L]
[mg/L]
[%]
[s]
6.3. Example of a titration curve
7.
Recommendations
Always rinse the pH probe and the delivery tip between measurements.
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
Page 6 of 8
04/2015, Edition 2
8.
Appendices
8.1. Titrant calibration
The iodine solution can be calibrated: its exact concentration can be determined from a titration using a
sodium thiosulfate standard solution.
Use a pipette to collect precisely 20 mL of 0.00564 N sodium thiosulfate solution. Pour the standard in the 50
mL propylene beaker and add a stir bar. Dip the probe and the delivery tip into the solution and launch the
titrant calibration sequence.
At the end of the titrant calibration, titer (eq/L) is displayed and the user can reject, replicate, or save the
result.
Default settings for titrant calibration
Name
Titrant
Name
Titrant concentration
Syringe
Standard
Name
Amount
EP titration
Stirring speed
Measured parameter
Predose
Max volume stop point
Stop on last EQP
Delay
Increment size
EP Ordinates
Result name
Result resolution
Result min
Result max
Default parameter
I2
0.0282
Syringe 1
Na2S2O3
20
15
2
8
True
0
0.04
400
Titer
5 decimals
0.02538
0.03102
Unit
[eq/L]
[mL]
[%]
[mV]
[mL]
[mL]
[s]
[mL]
[mV]
[eq/L]
[eq/L]
8.2. Autoleveling calibration
The aim of this method is to calibrate the volume of sample by leveling. The result of this calibration will be
used as sample volume for the following titrations.
This option is ONLY available from the calibration menu if Method Leveling is set to Active (Yes). Refer to the
documentation delivered with the external pump for a correct installation, paying particular attention to the
suction tube from the pump.
Use a 0.00564 eq/L sodium thiosulfate standard solution and pour a sufficient amount of the solution into a
beaker allowing the external pump tube to be immersed in the liquid. In the calibration menu select
Autoleveling calibration and then Free SO2, Total SO2 or Double SO2.
The result in mL is compared to minimum and maximum amounts defined for the sample volume. The
calculation used is:
Vsample =
Vtitrant (mL) × ne- titrant × Ctitrant (eq/L)
ne- sodium thiosulfate × Csodium thiosulfate solution (eq/L)
=
Vtitrant (mL) × 2 × 0.0282 (eq/L)
2 × 0.00564 (eq/L)
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
Page 7 of 8
04/2015, Edition 2
Autoleveling calibration uses the same settings as for a titrant calibration (see section 8.1 Titrant calibration).
Default settings for autoleveling calibration
Name
Sample
Amount min
Amount max
Autoleveling calibration
Solution name
Concentration
Resolution
Default parameter
Unit
0
22
[mL]
[mL]
Na2S2O3
0.00564
3 decimals
© Hach Company/Hach Lange GmbH, 2014-2015. All rights reserved.
[eq/L]
Page 8 of 8
04/2015, Edition 2