=245. Conditions of Separation.=—In the foregoing paragraphs the
optical methods for determining certain sugars have been described.
Many cases arise, however, in which these processes are inapplicable or
insufficient. In these instances, the analyst, as a rule, will be able
to solve the problem presented by the purely chemical methods which
have been previously described, or by a combination of the chemical
and optical processes. Not only have the different sugars distinctive
relations to polarized light, but also they are oxidized by varying
quantities of metallic salts and these differences are sufficiently
pronounced to secure in nearly every instance, no matter how complex,
data of a high degree of accuracy.
The carbohydrates of chief importance, from an agricultural point of
view, are starch and sucrose; while the alternation products of chief
importance, derived therefrom by chemical and biological means, are
dextrin, maltose, dextrose and invert sugar.
=246. Sucrose, Levulose, and Dextrose.=—The purely chemical methods of
separating these three sugars have been investigated by Wiechmann.[204]
They are based on the data obtained by determining the percentage of
reducing sugars, both before and after the inversion of the sucrose,
and before and after the removal of the levulose. For the destruction
of the levulose, the method of Sieben is employed, and attention is
called to the fact that the complete removal of the levulose by this
process is difficult of accomplishment, and is probably attended with
alterations of the other sugars present.
=247. Sieben’s Method of Determining Levulose.=—The decomposing action
of hot hydrochloric acid on levulose, and its comparative inaction on
dextrose are the basis of Sieben’s process.[205] The hydrochloric acid
employed should contain about 220 grams of the pure gas per liter, that
is, be of twenty-two per cent strength, corresponding to 1.108 specific
gravity. If the substance acted on be invert sugar, its solution should
be approximately of two and a half per cent strength. To 100 cubic
centimeters of such a solution, sixty of the hydrochloric acid are
added, and the mixture immersed in boiling water for three hours.
After quickly cooling, the acid is neutralized with sodium hydrate of
thirty-six times normal strength. Ten cubic centimeters of the hydrate
solution will thus neutralize the sixty of hydrochloric acid which have
been used to destroy the levulose. The work of Wiechmann discloses the
fact, easily prevised, that the method used for destroying levulose is
not always effective and that action of the reagent is not exclusively
confined to the levogyrate constituent of the mixture. Nevertheless,
data of reasonable accuracy may be secured by this process, which is
best carried out as described by Wiechmann. In this connection the
possibility of the polymerization of the dextrose molecules, when
heated with hydrochloric acid, must not be overlooked.
=248. The Analytical Process.=—The total quantity of invert sugar in a
given solution is determined by the methods already given (=136, 141=.)
After this has been accomplished, the levulose is destroyed as
described above, and the dextrose determined by any approved method
(=136, 140=). In the presence of sucrose, the sum of the reducing
sugars is first determined as in =136, 142=. After the inversion of
the sucrose, the invert sugar is again determined, and the increased
quantity found, calculated to sucrose. The levulose is then destroyed
by hydrochloric acid, and the dextrose determined as described above.
The quantity of sucrose may also be determined by an optical method
(=91, 92, 94=.).
=249. Calculation of Results.=—If we represent by a the weight of
metallic copper reduced by the invert sugar present in a solution
containing sucrose, and by _b_ that obtained after the inversion of
the sucrose, the quantity of copper corresponding to the sucrose is _b
- a_ = _c_. After the destruction of the levulose, the copper reduced
by the residual dextrose may be represented by _d_. The weight of
copper equivalent to the levulose is, therefore, _b - d_ = _e_. From
the tables already given, the corresponding quantities of the sugars
equivalent to _c, d_, and _e_ are directly taken. Example:
{ 163.8 milligrams invert sugar.
_a_ = 300 milligrams = { 156.5 ” dextrose.
{ 185.63 ” levulose.
_b_ = 500 ”
_d_ = 275 ” = 142.8 ” dextrose.
_c_ = 200 ” = 106.3 ” invert sugar.
_e_ = 225 ” = 133.89 ” levulose.
The 106.3 milligrams of invert sugar equivalent to _c_, correspond to
101 milligrams of sucrose. The quantity of dextrose equivalent to 275
milligrams of copper is 142.8. Of this amount 53.15 milligrams are
due to the inverted sucrose, leaving 89.65 milligrams arising from
the invert sugar and dextrose originally present. This quantity is
equivalent to 175 milligrams of copper.
Of the 300 milligrams of copper obtained in the first instance, 125
are due to levulose in the original sample, corresponding to 69.73
milligrams which number, multiplied by two, gives the invert sugar
present.
The sample examined, therefore, had the following composition:
Sucrose 101.00 milligrams.
Invert sugar 139.46 ”
Dextrose 19.92 ”
------
Sum 260.38 ”
On the other hand, if the invert sugar be calculated from the quantity
corresponding to the 225 milligrams of copper corresponding to _e_, the
data will be very different from those given above. In this instance of
the levulose found corresponding to 225 milligrams of copper, _viz._,
133.89, 53.15 milligrams are due to the inverted sucrose. Then the
quantity due to the invert sugar at first present is 133.89 - 53.15 =
80.74 milligrams. Since half the weight of invert sugar is levulose,
the total weight of the invert sugar at first present is 161.48,
leaving only 8.91 milligrams due to added dextrose. The difficulties in
these calculations doubtless arise from the imperfect destruction of
the levulose, and from variations in the reducing action of sugars on
copper salts in the presence of such large quantities of sodium chlorid.
=250. Calculation from Data obtained with Copper Carbonate.=—The
wide variations observed in different methods of calculations in the
preceding paragraph, are due in part to the different degrees of
oxidation exerted on alkaline copper tartrate by the dextrose and
levulose. Better results are obtained by conducting the analytical work
with Ost’s modification of Soldaini’s solution (=128=).
The relative quantities of levulose and dextrose oxidized by this
solution are almost identical, and the calculations, therefore, result
in nearly the same data, whether made from the numbers obtained with
the residual dextrose or from the levulose destroyed. The method of
applying this method is illustrated in the following calculation.
_Example._—In a mixture of sucrose, invert sugar, and dextrose, the
quantities of copper obtained by using the copper carbonate solution
were as follows:
Copper obtained before inversion = _a_ = 150 milligrams.
” ” after ” = _b_ = 250 ”
” ” ” destroying lev’e = _d_ = 137.5 ”
” equivalent to inverted sucrose = _b_ - _a_ = _c_ = 100 ”
” ” ” levulose = _b_ - _d_ = _e_ = 112.5 ”
{44.0 milligrams invert sugar.
_a_ = 150 milligrams Cu = {45.3 ” dextrose.
{42.5 ” levulose.
_d_ = 137.5 ” ” = 41.55 ” dextrose.
_c_ = 100 ” ” = 29.5 ” invert sugar = 28.025
sucrose.
_e_ = 112.5 ” ” = 31.9 ” levulose.
14.75 milligrams of dextrose = 48.5 milligrams Cu.
14.75 ” ” levulose = 51.5 ” ”
137.5 - 48.5 = 89.0 milligrams Cu due to dextrose present before
inversion.
89.0 milligrams Cu = 27 milligrams dextrose before inversion.
150.0 - 89.0 ” ” = 61.0 ” Cu due to levulose present
before inversion.
61.0 ” ” = 17.8 ” levulose before inversion.
17.8 × 2 = 35.6 milligrams invert sugar present before inversion.
27.0 - 17.8 = 9.2 ” dextrose ” ” ”
Again:
112.5 - 51.5 = 61.0 milligrams Cu due to levulose present
before inversion.
61.0 milligrams Cu = 17.8 milligrams levulose.
17.8 ” levulose indicate 35.6 milligrams invert sugar.
Dextrose in invert sugar before inversion = 17.8 milligrams.
Total dextrose before inversion = 27.0 milligrams.
Dextrose above amount required for invert sugar = 27.0 - 17.8
= 9.2 milligrams.
The respective quantities of the three sugars in the solution are,
therefore:
Sucrose = 28.025 milligrams.
Invert sugar = 35.6 ”
Dextrose = 9.2 ”
The calculations made from the later data (=234=) give almost the same
results.
=251. Winter’s Process.=—Winter has proposed a method of separating
dextrose and levulose in the presence of sucrose based on the
selective precipitation produced on treating mixtures of these sugars
in solution with ammoniacal lead acetate.[206]
The reagent is prepared immediately before use by adding ammonia to
a solution of lead acetate until the opalescence which is at first
produced just disappears. The separation is based on the fact that
the compound of sucrose with the reagent is easily soluble in water,
while the salts formed with levulose and dextrose are insoluble. The
separation of the sugars is accomplished as follows:
The ammoniacal lead acetate is added to the solution of the mixed
sugars until no further precipitate is produced. The precipitated
matters are digested with a large excess of water and finally separated
by filtration. The sucrose is found in the filtrate in the form of a
soluble lead compound, from which it is liberated by treatment with
carbon dioxid. The lead carbonate produced is separated by filtration
and the sucrose is estimated in an aliquot part of the filtrate by
optical or chemical methods. The precipitate containing the lead
compounds of dextrose and levulose is washed free of sucrose, suspended
in water and saturated with carbon dioxid. By this treatment the lead
compound with dextrose is decomposed and, on filtration, the dextrose
will be found in the filtrate, while the lead compound of the levulose
is retained upon the filter with the lead carbonate. After well washing
the precipitate, it is again suspended in water and saturated with
hydrogen sulfid. By this treatment the lead levulosate compound is
broken up and the levulose obtained, on subsequent filtration, in
the filtrate. The dextrose and levulose, after separation as above
described, may be determined in aliquot parts of their respective
filtrates by the usual gravimetric methods. Before determining the
levulose the solution should be heated until all excess of hydrogen
sulfid is expelled.
This method was used especially by Winter in separating the various
sugars obtained in the juices of sugar cane. It has not been largely
adopted as a laboratory method, and on account of the time and trouble
required for its conduct, is not likely to assume any very great
practical importance.
=252. Separation of Sugars by Lead Oxid.=—In addition to the
combination with the earthy bases, sugar forms well defined compounds
with lead oxid. One of these compounds is of such a nature as to have
considerable analytical and technical value. Its composition and the
method of preparing it have been pointed out by Kassner.[207]
Sucrose, under conditions to be described, forms with the lead oxid
a diplumbic saccharate, which separates in spheroidal crystals, and
has the composition corresponding to the formula C₁₂H₁₈O₁₁Pb₂ + 5H₂O.
The precipitation takes place quantitively and should be conducted as
follows:
The substance containing the sucrose, which may be molasses, sirups
or concentrated juices, is diluted with enough water to make a sirup
which is not too viscous. Lead oxid suspended in water is stirred into
the mass in such proportion as to give about two parts of oxid to one
of the sugar. The stirring is continued for some time until the oxid
is thoroughly distributed throughout the mass and until it becomes
thick by the commencement of the formation of the saccharate. As soon
as the mass is sufficiently thickened to prevent the remaining lead
oxid from settling, the stirring may be discontinued and the mixture
is left for twenty-four hours, at the end of which time the sucrose
has all crystallized in the form of lead saccharate. The crystals of
lead saccharate can be separated by a centrifugal machine or by passing
through a filter press, and are thoroughly washed with cold water, in
which they are almost insoluble. The washed crystals are beaten up with
water into a thick paste and the lead separated as basic carbonate by
carbon dioxid. The sucrose is found in solution in the residual liquor
and is concentrated and crystallized in the usual way.
Reducing sugars have a stronger affinity for the lead oxid than the
sucrose, and this fact is made use of to effect a nearly complete
separation when they are mixed together. In order to secure this the
lead oxid is added in the first place only in sufficient quantity to
combine with the reducing sugars present, the process being essentially
that described above. The reducing sugars which are precipitated as
lead dextrosates, lead levulosates, etc., are separated in the usual
way by a centrifugal or a filter press, and the resulting liquor,
which contains still nearly all the sucrose, is subjected to a second
precipitation by the addition of lead oxid. The second precipitation
obtained is almost pure diplumbic saccharate.
In the precipitation of the sugar which is contained in the beet
molasses, where only a trace or very little invert sugar is present,
the sucrose is almost quantitively separated, and by the concentration
of the residual liquor, potash salts are easily obtained. In this case,
after the decomposition of the lead saccharate by carbon dioxid, the
residual sugar solution is found entirely free of lead. Where invert
sugar is present, however, in any considerable proportions, it is found
to exercise a slightly soluble influence on the lead saccharate, and
in this case a trace of lead may pass into solution. For technical
purposes, this is afterwards separated by hydrosulfuric acid or the
introduction of lime sulfid.
Lead oxid is regenerated from the basic lead carbonate obtained by
heating in retorts to a little above 260°, and the carbon dioxid
evolved can also be used again in the technical process.
=253. Commercial Glucose and Grape Sugar.=—The commercial products
obtained by the hydrolysis of starch are known in the trade as glucose
or grape sugar. The former term is applied to the thick sirup obtained
by concentrating the products of a partial hydrolysis, while the latter
is applied to the solid semi-crystalline mass, secured by continuing
the hydrolyzing action until the intermediate products are almost
completely changed to dextrose. In this country the starch employed
is obtained almost exclusively from maize, and the hydrolyzing agent
used is sulfuric acid.[208] The products of conversion in glucose are
chiefly dextrins and dextrose with some maltose, and in grape sugar
almost entirely dextrose. When diastase is substituted for an acid, as
the hydrolytic agent, maltose is the chief product, the ferment having
no power of producing dextrose. In the glucose of Japan, known as midzu
ame dextrin and maltose are the chief constituents.[209]
Commercial glucose is used chiefly by confectioners for manufacturing
table sirups and for adulterating honey and molasses.
Commercial grape sugar is chiefly employed by brewers as a substitute
for barley and other grains.
In Europe, the starch which is converted into glucose, is derived
principally from potatoes. The method employed in conversion, whether
an acid or diastatic action, is revealed not only by the nature of
the product, but also by the composition of its ash. In the case of
diastatic conversion the ash of the sample will contain only a trace of
sulfates, no chlorin, and be strongly alkaline, while the product of
conversion with sulfuric acid will give an ash rich in sulfates with a
little lime and be less strongly alkaline.
The process of manufacture in this country consists in treating the
starch, beaten to a cream with water, with sulfuric acid, usually under
pressure, until the product shows no blue color with iodin. The excess
of acid is removed with marble dust, the sirup separated by filtration,
whitened by bleaching with sulfurous acid or by passing it through
bone-black and evaporated to the proper consistence in a vacuum. The
solid sugar, consisting mostly of dextrose, is made in the same manner,
save that the heating with the acid is continued until the dextrin and
maltose are changed into maltose. The product is either obtained in
its ordinary hydrated form or by a special method of crystallization
secured as bright anhydrous crystals. Solutions of dextrose, when first
made, show birotation, but attain their normal gyrodynatic state on
standing for twenty-four hours in the cold, or immediately on boiling.
=254. Methods Of Separation.=—The accurate determination of the
quantities of the several optically active bodies formed in commercial
glucose is not possible by any of the methods now known. Approximately
accurate data may be secured by a large number of processes, and these
are based chiefly on the ascertainment of the rotation and reducing
power of the mixed sugars, the subsequent removal of the dextrose and
maltose by fermentation or oxidation and the final polarization of the
residue. The difficulties which attend these processes are alike in
all cases. Fermentation may not entirely remove the reducing sugars or
may act slightly on the dextrin. In like manner the oxidation of these
sugars by metallic salts may not entirely decompose them, may leave an
optically active residue, or may affect the optical activity of the
residual dextrin. The quantitive methods of separating these sugars
by means of phenylhydrazin, lead salts or earthy bases have not been
developed into reliable and applicable laboratory processes. At the
present time the analyst must be contented with processes confessedly
imperfect, but which, with proper precautions, yield data which are
nearly correct. The leading methods depending on fermentation and
oxidation combined with polarimetric observations will be described in
the subjoined paragraphs.
=255. Fermentation Method.=—This process is based on the assumption
that, under certain conditions, dextrose and maltose may be removed
from a solution and the dextrin be left unchanged. In practice,
approximately accurate results are obtained by this method, although
the assumed conditions are not strictly realized. In the prosecution
of this method the polarimetric reading of the mixed sugars is made,
and the maltose and dextrose removed therefrom by fermentation
with compressed yeast. The residual dextrins are determined by the
polariscope on the assumption that their average gyrodynat is 193.
In the calculation of the quantities of dextrose and maltose their
gyrodynats are fixed at 53 and 138 respectively. The total quantity
of reducing sugar is determined by the usual processes. The relative
reducing powers of dextrose and maltose are represented by 100 and 62
respectively. The calculations are made by the following formulas:[210]
_R_ = reducing sugars as dextrose
_d_ = dextrose
_m_ = maltose
_dʹ_ = dextrin
_P_ = total polarization
(calculated as apparent gyrodynat)
_Pʹ_ = rotation after fermentation
(calculated as apparent gyrodynat).
Whence _R_ = _d_ + 0.62_m_ (1)
_P_ = 53_d_ + 138_m_ + 163_dʹ_ (2)
_Pʹ_ = 193_dʹ_ (3)
From these three equations the values of _d_, _m_, and _dʹ_ are readily
calculated:
_Example_: To find _d_ and _m_:
Subtract (3) from (2) _P_ = 53_d_ + 133_m_ + 193_dʹ_
_Pʹ_ = 193_dʹ_
--------------------------------------
_P_ - _Pʹ_ = 53_d_ + 138_m_ (4)