THE ABSORBANCES OF THESE SOLUTIONS AS A FUNCTION OF THE CITRIC ACID CONTENT IS NOT LINEAR BUT SLIGHTLY CURVED .
13 . SULPHUR DIOXIDE
13.1 . DEFINITIONS
FREE SULPHUR DIOXIDE INCLUDES MOLECULAR SO2 AND THAT COMBINED AS HSO3 , SO3 AND H2SO3 .
COMBINED SULPHUR DIOXIDE IS THE DIFFERENCE BETWEEN TOTAL AND FREE SULPHUR DIOXIDE .
13.2 . PRINCIPLE OF METHODS
( A ) REFERENCE METHOD
THE SULPHUR DIOXIDE IS CARRIED OVER BY A CURRENT OF AIR OR NITROGEN ; IT IS FIXED AND OXIDIZED BY BEING BUBBLED INTO A DILUTED AND NEUTRAL SOLUTION OF HYDROGEN PEROXIDE . THE SULPHURIC ACID FORMED IS DETERMINED BY TITRATION WITH A STANDARD SOLUTION OF SODIUM HYDROXIDE .
ENTRAINMENT AT LOW TEMPERATURE ( 10* C ) ENSURES EXTRACTION AND DETERMINATION OF THE FREE SULPHUR DIOXIDE ONLY . AT A HIGHER TEMPERATURE ( APPROXIMATELY 100* C ) THE TOTAL SULPHUR DIOXIDE IS DETERMINED .
( B ) USUAL METHOD
( AA ) FREE SULPHUR DIOXIDE : DIRECT IODOMETRIC TITRATION WITH A BLANK TITRATION ON THE SAME WINE SAMPLE IN WHICH THE FREE SULPHUR DIOXIDE HAS BEEN COMBINED WITH AN EXCESS OF ACETALDEHYDE OR PROPIONALDEHYDE .
( BB ) COMBINED SULPHUR DIOXIDE : IODOMETRIC TITRATION AFTER DOUBLE ALKALINE HYDROLYSIS OF THE WINE WHERE THE FREE SULPHUR DIOXIDE HAS BEEN OXIDIZED DURING THE IODOMETRIC TITRATION .
EXPRESSION OF RESULTS
THE QUANTITY OF SULPHUR DIOXIDE IS EXPRESSED IN MILLIGRAMS OF SULPHUR DIOXIDE PER LITRE AND DETERMINED TO THE NEAREST 10 MG/L .
13.3 . REFERENCE METHOD
FREE AND COMBINED SULPHUR DIOXIDE
APPARATUS
THE APPARATUS USED SHOULD CONFORM TO THE DIAGRAM SHOWN BELOW WITH PARTICULAR ATTENTION PAID TO THE CONDENSER .
THE GAS FEED TUBE TO THE BUBBLER " B " ENDS IN A SMALL SPHERE OF 1 CM DIAMETER WITH 20 HOLES OF 0.2 MM DIAMETER . ALTERNATIVELY THE TUBE MAY ALSO END IN A GLASS FRIT PLATE WHICH PRODUCES A LARGE NUMBER OF VERY SMALL BUBBLES THUS ENSURING GOOD CONTACT BETWEEN THE GASEOUS AND LIQUID PHASES .
ENSURE THAT THE GAS FLOW THROUGH THE APPARATUS IS APPROXIMATELY 40 LITRES PER HOUR . THE BOTTLE ON THE RIGHT OF THE ILLUSTRATION IS INTENDED TO RESTRICT THE VACUUM EFFECT OF THE WATER VACUUM PUMP TO 20 TO 30 CM WATER . INSTALL A FLOWMETER WITH SEMI-CAPILLARY TUBE BETWEEN THE BUBBLER AND THE BOTTLE TO REGULATE THE VACUUM .
SOLUTIONS
- PHOSPHORIC ACID AT 25 % ( W/V ) .
- HYDROGEN PEROXIDE ( AT 0.3 VOLUME ) .
- INDICATOR REAGENT :
- METHYL RED 100 MG ,
- METHYLENE BLUE 50 MG ,
- ALCOHOL 50 % 100 ML .
- SODIUM HYDROXIDE 0.01 N .
PROCEDURE
MAINTAIN THE WINE AT 20* C IN A FULL AND STOPPERED BOTTLE FOR FOUR DAYS BEFORE THE DETERMINATION OF THE FREE AND COMBINED SULPHUR DIOXIDE . THIS PRECAUTION IS UNNECESSARY IF ONLY TOTAL SULPHUR DIOXIDE IS TO BE MEASURED .
DETERMINATION OF FREE SULPHUR DIOXIDE
INTRODUCE INTO THE 100 ML FLASK " A " OF THE ENTRAINMENT APPARATUS , 10 ML