This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
Though rather lengthy, the German official method of estimating the glycerol in wine is the one which, on the whole, appears best to use. The procedure varies somewhat, according as the wine is rich or poor in sugar: this will already have been determined from the amount of fixed residue or " extract," or by estimation of the sugar.
Wines with less than 2 grams of sugar per 100 c.c. - One hundred c.c. of the wine are evaporated in a porcelain basin on the water-bath till the volume is reduced to about 10 c.c. With the residue 1 gram of sand is mixed; and then sufficient milk of lime (40 per cent. strength) is added to give from 15 to 2 c.c. of the "milk" for each gram of extract present. The mixture is evaporated to near dryness, and 5 c.c. of 96 per cent. alcohol added. The portions adherent to the sides of the basin are detached with a spatula, and the whole macerated with a glass " crusher " to a thin cream, small quantities of the strong alcohol being added from time to time. After rinsing off the spatula with alcohol, the basin is warmed on the bath with constant agitation until ebullition commences, and the turbid alcoholic liquid poured through a funnel into a 100 c.c. flask. About 10 to 12 c.c. of the 96 per cent. alcohol are added to the pulverulent residue in the basin, the mixture is again warmed and stirred, and poured off into the flask. This is repeated until the latter contains about 95 c.c. of liquid, after which the funnel is rinsed, the flask cooled, and the volume made up to 100 c.c. at the temperature 156° with more of the strong alcohol. After being well mixed, the contents are filtered through a folded filter, and 90 c.c. of the filtrate evaporated gently on the water-bath. The residue is taken up with small quantities of absolute alcohol and transferred to a stoppered graduated tube, until the volume obtained is exactly 15 c.c. To this, three separate quantities of 7 5 c.c. of dehydrated ether are added, shaking well after each addition. The tube, closed, is then allowed to stand until the liquid is quite clear; the solution is poured off into a tared weighing-flask provided with a stopper, and the tube rinsed out with about 5 c.c. of mixed ether and alcohol (3 ether: 2 alcohol, both "absolute"). After expulsion of the solvents by gentle evaporation on the bath, the syrupy residue is dried in the steam oven for one hour, allowed to cool in the desiccator, and weighed. The result must, of course, be multiplied by 100/90, as only 90 c.c. of filtrate were taken for evaporating.
Wines with 2 grams, or more, of sugar per 100 c.c. - In such cases, 50 c.c. of the wine are taken in a large flask and warmed on the bath, 1 gram of sand is added, and then milk of lime in small quantities until the mixture, at first deep coloured, becomes lighter and gives off an alkaline odour. After being heated and well shaken for a time, on the water-bath, the mixture is cooled, and 100 c.c. of alcohol (96 per cent.) are added. The precipitate is allowed to settle, and is then filtered and washed with alcohol; the filtrate and washings are evaporated, and the residue is treated as in (a), by taking up with absolute alcohol, and so on. A modified form of this process is proposed by M. Canonica.1 Sugars. - To obtain a solution in which the sugars may be determined and the polarisation value observed, place 100 c.c. of the wine in a porcelain basin, and neutralise it exactly with sodium hydroxide solution. Then add a drop of dilute acetic acid, to ensure that there is not even a trace of alkali in excess. The very faint acidity will not affect the sugars, whereas alkali readily acts on them when heated. Evaporate the wine down on the water-bath to about one-third or one-fourth of its bulk in order to expel the alcohol, and then wash the residue into a 200 c.c. flask. Add a slight excess of lead subacetate solution; 5 to 10 c.c. will usually suffice. If the liquid is too turbid to judge when an excess is present, withdraw a drop and test it with red litmus paper; the basic lead acetate has an alkaline reaction. Allow the precipitate to settle, and then add a saturated solution of sodium sulphate, little by little, until the excess of lead is precipitated as sulphate. Make up the bulk with water to 200 c.c, mix well, allow the precipitate to settle, and filter the liquid through a dry filter.
1 See J. Soc. Chem. Ind., 1919, 38, 114a.
The polarimeter reading of the filtrate is taken in a 2-dcm. tube, and multiplied by 2 to correct for the dilution.
A part of the filtrate (50 c.c.) is then inverted as described below under "Saccharose," but using 5 c.c. of hydrochloric acid and making the final bulk up to 100 c.c. The polarimeter reading of this inverted liquid is taken in the 2-dcm. tube as before, but multiplied by 4 to correct for the double dilution. If this result is lower than the first, saccharose is indicated; if the two results are sensibly equal, saccharose can be ruled out, and only the examination for reducing sugars need be proceeded with.
Saccharose is determined in the inverted liquid, and the reducing sugars in the part of the filtrate which has not been inverted. Either a gravimetric or a volumetric estimation by means of Fehling's solution may be employed, the liquids to be tested being suitably diluted, if necessary, to bring their content of sugar down to not more than 1 per cent. if the usual quantities are taken.
The foregoing process, it may be noted, takes no account of the volume of the precipitate produced by the lead acetate in the 200 c.c. flask. Hence some operators prefer to proceed as follows.