THE COMBINED SULPHUR DIOXIDE HAS BEEN CARRIED OVER AND OXIDIZED . DETERMINE THE SULPHURIC ACID WHICH HAS FORMED BY TITRATION WITH 0 * 01N 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 AND PASS STEAM THROUGH . DEDUCT THE VOLATILE ACIDITY OBTAINED FROM THE ACIDIMETRIC DETERMINATION OF THE SULPHUR DIOXIDE .
13.4 . USUAL METHOD
FREE AND COMBINED SULPHUR DIOXIDE
TOTAL SULPHUR DIOXIDE
SOLUTIONS
( A ) SODIUM HYDROXIDE 4N ( 160 G / L ) .
( B ) SULPHURIC ACID 1 : 10 BY VOLUME ( 180 G / L ) .
( C ) STARCH SOLUTION 2 * 5 G / L . CRUSH 2 * 5 G OF SOLUBLE STARCH AND 10 MG OF MERCURIC IODIDE IN A MORTAR WITH ENOUGH WATER TO OBTAIN A FLUID PASTE . TRANSFER TO ONE LITRE OF STRONGLY BOILING WATER AND KEEP BOILING FOR 10 MINUTES . THE LIQUID PRODUCED SHOULD BE CLEAR . FILTER IF NECESSARY .
( D ) IODINE 0 * 05N .
( E ) SODIUM THIOSULPHATE 0 * 01N .
PROCEDURE
PUT INTO A 500 ML CONICAL FLASK :
_ 50 ML OF WINE ,
_ 3 ML OF H2SO4 1/10 ,
_ 5 ML OF STARCH PASTE ,
_ 30 MG OF SODIUM EDETATE ( COMPLEXON III ) .
TITRATE WITH 0 * 05N IODINE UNTIL THE BLUE COLOUR , TRANSIENT AT FIRST , PERSISTS CLEARLY FOR 10 TO 15 SECONDS ( 1 ) . LET THE VOLUME OF IODINE USED BE N ML .
ADD 8 ML OF 4N SODIUM HYDROXIDE SOLUTION , SHAKE THE MIXTURE ONCE AND LEAVE IN CONTACT FOR FIVE MINUTES . ADD , WITH VIGOROUS STIRRING , IN ONE OPERATION , 10 ML OF 10 % V / V SULPHURIC ACID . TITRATE