TO THE SPECIFIC GRAVITIES OR DENSITIES OF PURE ALCOHOL ( 100 % VOL ) INDICATED IN THE ALCOHOLIC STRENGTH TABLES . THEY ARE LIABLE TO VARY AS A FUNCTION OF THE ATOMIC WEIGHTS OF THE ELEMENTS WHICH ARE REVISED EACH YEAR . THE FIGURES GIVEN ABOVE HAVE BEEN CALCULATED ACCORDING TO THE 1961 TABLE . THESE VARIATIONS ARE OF NO PRACTICAL IMPORTANCE , AS THEY ARE MINIMAL .
( 14 ) UNDER THE CONDITIONS SPECIFIED ABOVE , THE ACETIC ACID FORMED RESISTS THE CHROMIC MIXTURE INDEFINITELY AND THE PROLONGED OXIDATION THUS DOES NOT ALTER THE STOICHIOMETRIC RELATION BETWEEN THE ETHANOL AND THE DICHROMATE .
( 15 ) MORE PRECISELY , THE SUGAR CONTENT S CAN BE ESTIMATED BY THE FOLLOWING FORMULA :
S = 2 590 ( * - 1 + 0.0011A ) - 18.6 ,
WHERE * IS THE DENSITY OF THE WINE ,
A IS THE ALCOHOLIC STRENGTH .
THE VALUES IN THIS FORMULA AND A FORTIORI THE ABOVE TABLE ARE ONLY APPROXIMATE . IT IS SOMETIMES NECESSARY TO REPEAT SUGAR DETERMINATION AFTER FURTHER DILUTION IF THE SUGAR CONTENT EXCEEDS THE THRESHOLD OF 5 G/L .
( 16 ) THIS SOLUTION IS INTENDED TO ENCOURAGE THE OXIDATION OF THE DISTILLED SULPHUR DIOXIDE BY THE ATMOSPHERIC OXYGEN , BECAUSE SULPHUR DIOXIDE EXHIBITS AN APPRECIABLE ABSORPTION AT 256 NM .
( 17 ) PREPARE THIS SOLUTION BY PLACING 20 MG OF SORBIC ACID INTO A LITRE FLASK WITH 900 ML OF HOT WATER , STIR WHILE COOLING AND MAKE UP TO ONE LITRE . ALTERNATIVELY DISSOLVE 26.8 MG OF POTASSIUM SORBATE IN COLD WATER AND MAKE UP TO ONE LITRE WITH COLD WATER .
( 18 ) THE REAGENTS USED MUST BE VERY PURE ; ANALYTICAL REAGENT GRADE CHEMICALS ARE TO BE EMPLOYED . NO PRECIPITATION OR COLORATION SHOULD BE OBSERVED WHEN THE OPERATIONS DESCRIBED BELOW ARE CARRIED OUT IN THE ABSENCE OF CITRIC ACID .
( 19 ) IF THE PRECISE END-POINT OF THE STARCH HAS BEEN PASSED , CAREFULLY BACK TITRATE WITH 0.01N THIOSULPHATE .
( 20 ) N''' IS GENERALLY SMALL : 0.2 TO 0.3 ML 0.05N . IF ASCORBIC ACID HAS BEEN ADDED TO THE WINE , N''' WILL BE MUCH HIGHER AND IT IS POSSIBLE TO MEASURE APPROXIMATELY THE AMOUNT FROM THE VALUE OF N''' ON THE BASIS OF 1 ML OF 0.05N IODINE OXIDIZING 4.4 MG OF ASCORBIC ACID . BY DETERMINING N''' IT IS POSSIBLE TO DETECT THE PRESENCE OF RESIDUAL ASCORBIC ACID IN AMOUNTS GREATER THAN 20 MG/L IN WINES TO WHICH IT HAS BEEN ADDED .
( 21 ) OR 3.55 G OF DISODIUM PHOSPHATE NA2HPO4 DRIED FOR TWO HOURS AT 110 TO 130* C .
( 22 ) FOR THE VOLATILE ACIDITY DETERMINATION IT IS SUFFICIENT TO COLLECT ONLY 250 ML OF DISTILLATE , BUT TO ENSURE THAT ALL OF THE SORBIC ACID HAD DISTILLED OVER 320 TO 330 ML MUST BE COLLECTED . THE DETERMINATION OF SORBIC ACID AS A CORRECTION FOR THE VOLATILE ACIDITY DETERMINATION ( SEE NOTE