OF WINE AND 5 ML OF 25 % PHOSPHORIC ACID . ASSEMBLE THE APPARATUS IMMEDIATELY . IF A LARGER VOLUME ( 20 OR 50 ML ) OF A WINE OF VERY LOW SULPHUR DIOXIDE CONCENTRATION IS TO BE ANALYZED , USE FLASK OF 250 ML CAPACITY FOR A .
MAINTAIN FLASK A AT A TEMPERATURE OF 10* C .
PLACE 2 TO 3 ML OF HYDROGEN PEROXIDE AT 0.3 VOLUME ( PREVIOUSLY NEUTRALIZED WITH 0.01 N SODIUM HYDROXIDE ) AND TWO DROPS OF INDICATOR REAGENT INTO THE BUBBLER B .
CONNECT THE BUBBLER TO THE APPARATUS AND BUBBLE AIR ( OR NITROGEN ) THROUGH FOR 12 TO 15 MINUTES . THE FREE SULPHUR DIOXIDE CARRIED OVER IS OXIDIZED TO SULPHURIC ACID . REMOVE THE BUBBLER FROM THE APPARATUS AND TITRATE THE ACID WHICH HAS FORMED AGAINST THE 0.01 N SODIUM HYDROXIDE SOLUTION .
LET THE NUMBER OF MILLILITRES USED BE N .
DETERMINATION OF COMBINED SULPHUR DIOXIDE
WHEN THE DETERMINATION OF THE FREE SULPHUR DIOXIDE HAS BEEN COMPLETED , AGAIN PLACE 2 TO 3 ML OF NEUTRALIZED HYDROGEN PEROXIDE IN THE BUBBLER B . HEAT THE WINE CONTAINED IN FLASK A TO BOILING POINT WITH A SMALL FLAME OF 4 TO 5 CM HEIGHT WHICH SHOULD DIRECTLY TOUCH THE BASE OF THE FLASK . DO NOT PUT THE FLASK ON A METAL PLATE , BUT ON A DISC PROVIDED WITH A HOLE OF APPROXIMATELY 300 MM DIAMETER TO PREVENT ANY OVERHEATING OF THE EXTRACTIVE MATTER OF THE WINE ON THE WALLS OF THE FLASK .
SIMMER WHILST PASSING A CURRENT OF AIR ( OR NITROGEN ) . WITHIN 12 TO 15 MINUTES THE COMBINED SULPHUR DIOXIDE HAS BEEN CARRIED OVER AND OXIDIZED . DETERMINE THE SULPHURIC ACID WHICH HAS FORMED BY TITRATION WITH 0.01 N SODIUM HYDROXIDE SOLUTION .
LET THE NUMBER OF ML USED BE N' .
DETERMINATION OF TOTAL SULPHUR DIOXIDE
OBTAIN THE TOTAL SULPHUR DIOXIDE FROM THE SUM OF THE FREE AND COMBINED SULPHUR DIOXIDE .
THE TOTAL SULPHUR DIOXIDE MAY ALSO BE DETERMINED DIRECTLY BY HEATING THE TEST SAMPLE OF 10 ML OF WINE TO BOILING POINT WHILST A CURRENT OF AIR OR NITROGEN IS PASSED .
THE FREE AND COMBINED SULPHUR DIOXIDE ARE CARRIED OVER SIMULTANEOUSLY ; THE SULPHURIC ACID FORMED CORRESPONDS TO THE TOTAL SULPHUR DIOXIDE .
THE NUMBER OF MILLILITRES OF 0.01N SODIUM HYDROXIDE USED IS DESIGNATED N'' .
CALCULATION
FREE SULPHUR DIOXIDE IN MILLIGRAMS PER LITRE : 32 N .
COMBINED SULPHUR DIOXIDE IN MILLIGRAMS PER LITRE : 32 N' .
TOTAL SULPHUR DIOXIDE IN MILLIGRAMS PER LITRE : 32 ( N + N' ) OR 32 N'' .
NOTE :
IF THE WINE CONTAINS MORE THAN 2 G/L OF ACETIC ACID ( ESPECIALLY IN VINEGAR ) DETERMINE THE ACETIC ACID THAT MAY DISTIL INTO THE BUBBLER . THIS MAY OCCUR IF THE GAS FLOW IS TOO GREAT . AFTER TITRATING THE CONTENTS OF THE BUBBLER ADD A CRYSTAL OF TARTARIC ACID TO IT , PUT IN THE VOLATILE ACIDITY DISTILLATION APPARATUS