SULFUR DIOXIDE IN WINE IPT03 Last Revised: May 2015
1. PURPOSE This exercise determines the sulfur dioxide in free and bound forms in wine. 2. REAGENTS & EQUIPMENT 2.1
SO2 ‐ containing wines – two different samples 2.2
Solid I2 and KI 2.3
Standardized sodium thiosulfate, 0.01M 2.4
Sodium hydroxide pellets 2.5
Concentrated sulfuric acid 2.6
Starch indicator 3. PROCEDURE You are required to analyse two different wines (Wine A & Wine B) in duplicate. The analysis is in two parts for each wine (F & T), performed on separate samples, so there are 8 titrations to perform. The free SO2 is analysed directly, whereas the total SO2 is done 15 minutes after alkaline treatment. Therefore, you need to alternate samples, so that while the 3B sample (total SO2) is sitting for 15 minutes, you move on to 3C sample (free SO2), and then back again. 3A. Reagents 3.1
Prepare 100 mL of 25% H2SO4 (measuring cylinder) 3.2
Prepare 150 mL of 1 M NaOH (beaker) 3.3
Prepare 100 mL of 0.01 M I2, as follows: 3.4

dissolve 0.8 g of KI in 10 mL of water (both approximate) 
weigh accurately about 0.25 g of I2 – record this amount 
dissolve in the KI solution  transfer to a 100 mL volumetric flask and make up to the mark with water Standardise the iodine by titrating a 25 mL aliquot of standardised 0.01 M sodium thiosulfate. For each wine – 3B. Alkaline Treatment for release of fixed SO2 3.5
Pipette 20 mL of sample in duplicate into 250 mL conical flasks. Dilute with about 50mL of water, if darkly coloured. 3.6
Add 25 mL of 1 M NaOH by measuring cylinder to each to release the fixed SOx‐. 3.7
Swirl the flasks, insert a stopper and allow to stand for 15 minutes. Go on to part 3C immediately 1 3C. Analysis of free SO2 3.8
Fill a burette with the iodine solution. 3.9
Pipette 50 mL of sample into a 250 mL conical flask. Dilute with about 50mL of water, if darkly coloured. 3.10 Add some starch indicator to the conical flask. 3.11 Add 5 mL of 25% H2SO4 to force all the free SOx‐ to SO2 gas (dissolved). 3.12 Immediately titrate to the first persistent blue/black colour. 3.13 Repeat from 3.8 with a duplicate sample. 3D. Analysis of total SO2 (continuation from part 3.7, after 15 minutes) 3.14 Fill a burette with the iodine solution. 3.15 Add some starch indicator to the conical flask. 3.16 Add 10 mL of 25% H2SO4 to force all the free SOx‐ to SO2 gas (dissolved). 3.17 Immediately titrate to the first persistent blue/black colour. 3.18 Repeat from 3.14 with a duplicate sample. CALCULATIONS Molarity of I2 titrant = where M is the molarity of the thiosulfate and V is the volume of iodine used in the titration (in mL) SO2 concentration (mg/L) = where M is the molarity of iodine (from above), Vt is the titration volume (in mL) and Vw is the sample (wine) volume 
calculate the average FREE SO2 content for each wine in mg/L from the Part 3C titration 
calculate the average TOTAL SO2 content for each wine in mg/L from the Part 3D titration 
calculate the average FIXED SO2 content (TOTAL – FREE) for each wine in mg/L Report the FREE & FIXED average values for each wine. 2