_P_ - _Pʹ_ = 53_d_ + 138_m_
53_R_ = 53_d_ + 32.86_m_
-------------------------------------------
_P_ - _Pʹ_ - 53_R_ = 105.14_m_ (5)
_P_ - _Pʹ_ - 53_R_
Whence _m_ = ------------------ (6)
105.14
_d_ = _R_ - 0.62_m_ (7)
_Pʹ_
_dʹ_ = ---- (8)
193
Sidersky assigns the values [_a_]_{D} = 138.3 and [_a_]_{D} = 194.8 to
maltose and dextrin respectively in the above formulas.[211]
_Illustration_: In the examination of a sample 26.048 grams of midzu
ame in 100 cubic centimeters were polarized in a 200 millimeter tube
and the following data were obtained:
Polarization of sample in angular degrees 69°.06, which is equal to an
apparent gyrodynat of 132.6:
Total reducing sugar as dextrose 33.33 per cent:
Polarization in angular degrees after fermentation 30°.84 = [_a_]_{D} =
59.2.
Substituting these values in the several equations gives the following
numbers:
(1) 0.3333 = _d_ + 0.62_m_
(2) 132.6 = 53_d_ + 138_m_ + 193_dʹ_
(3) 59.2 = 193_dʹ_
(4) 73.4 = 53_d_ + 138_m_
(5) 55.74 = 105.14_m_
(6) _m_ = 0.5301 = 53.01 per cent.
(7) _d_ = 3333 - 3286 = 0.0047 = 00.47 per cent.
(8) _dʹ_ = 59.2 ÷ 193 = 0.3067 = 30.67 ” ”
_Summary_: Sample of midzu ame:
Percentage of dextrin 30.67 per cent.
” ” maltose 53.01 ” ”
” ” dextrose 00.47 ” ”
” ” water 14.61 ” ”
” ” ash 00.31 ” ”
-----
Sum 99.07 ” ”
Undetermined 0.93 ” ”
For polarization the lamplight shadow polariscope employed for sugar
may be used, and the degrees of the sugar (ventzke) scale converted
into angular degrees by multiplying by 0.3467.
The process of fermentation is conducted as described in the paragraph
given further on, relating to the determination of lactose in the
presence of sucrose.
=256. The Oxidation Method.=—The removal of the reducing sugars may
be accomplished by oxidation instead of fermentation. The process
of analysis is in all respects similar to that described in the
foregoing paragraph, substituting oxidation for fermentation.[212]
For the oxidizing agent mercuric cyanid is preferred, and it is
conveniently prepared by dissolving 120 grams of mercuric cyanid and
an equal quantity of sodium hydroxid in water mixing the solutions
and completing the volume to one liter. If a precipitate be formed
in mixing the solutions it should be removed by filtering through
asbestos. For the polarization, ten grams of the sugars in 100 cubic
centimeters is a convenient quantity. Ten cubic centimeters of
this solution are placed in a flask of water marked at fifty cubic
centimeters, a sufficient quantity of the mercuric cyanid added to
remain in slight excess after the oxidation is finished (from twenty to
twenty-five cubic centimeters) and the mixture heated to the boiling
point for three minutes. The alkali, after cooling, is neutralized with
strong hydrochloric acid and the passing from alkalinity to acidity
will be indicated by a discharge of the brown color which is produced
by heating with the alkaline mercuric cyanid. The heating with the
mercury salt should be conducted in a well ventilated fume chamber.
The calculation of the results is conducted by means of the formulas
given in the preceding paragraph. In the original paper describing
this method, it was stated that its accuracy depended on the complete
oxidation of the reducing sugar in a manner leaving no optically active
products, and on the inactivity of the reagents used in respect to the
dextrin present. These two conditions are not rigidly fulfilled, as is
shown by Wilson.[213] According to his data maltose leaves an optically
active residue, which gives a somewhat greater right hand rotation than
is compensated for by the diminished rotation of the dextrin. Wilson,
however, confesses that the dextrin used contained reducing sugars,
which would not be the case had it been prepared by the process of
treating it with alkaline mercuric cyanid as above indicated. Upon the
whole, the oxidation of the reducing sugar by a mercury salt gives
results which, while not strictly accurate, are probably as reliable as
those afforded by fermentation. The author has attempted to supplant
both the oxidation and fermentation methods by removing the reducing
sugars with a precipitating reagent, such as phenylhydrazin, but the
methods are not sufficiently developed for publication.
=257. Removal of Dextrose by Copper Acetate.=—Maercker first called
attention to the fact that Barfoed’s reagent (one part copper acetate
in fifteen parts of water, and 200 cubic centimeters of this solution
mixed with five cubic centimeters of thirty-eight per cent. acetic
acid) reacts readily with dextrose, while it is indifferent to maltose
and dextrins. Sieben’s method of removing dextrose is based on this
fact.[214] It is found that under certain conditions pure maltose
does not reduce either the acidified or neutral solution of copper
acetate, while dextrose or a mixture of dextrose and maltose does so
readily. It is also shown that the fermentation residue under suitable
conditions acts like maltose. Maltose solutions reduce the reagent
after boiling four minutes while at 40°-45° they have no effect even
after standing four days. The amount of copper deposited by dextrose,
under the latter conditions, is found to depend to a certain extent on
the amount of free acetic acid present, and as the solutions of copper
acetate always contain varying quantities of acetic acid which cannot
be removed without decomposition and precipitation of basic salt, the
use of an absolutely neutral solution is impracticable. The reagent
prepared according to Barfoed’s directions is almost saturated, but a
half normal solution is preferable. Sieben proposes two solutions: I,
containing 15.86 grams copper and 0.56 gram acetic anhydrid per liter;
II, containing 15.86 grams copper and three grams acetic anhydrid per
liter. The reduction of the dextrose is secured by placing 100 cubic
centimeters of the solution in a bottle, adding the sugar solution,
stoppering and keeping in a water-bath at 40°-45° two or three days. An
aliquot portion is then drawn off and the residual copper precipitated
by boiling with forty-five cubic centimeters of the alkali solution
of the fehling reagent and forty cubic centimeters of one per cent
dextrose solution, filtered and weighed as usual. The results show that
either solution can be used, and that standing for two days at 45°
is sufficient. One hundred cubic centimeters of the copper solution
are mixed with ten cubic centimeters of the sugar solution containing
from two-tenths to five-tenths gram of dextrose, as this dilution
gives the best results. No reduction is found to have taken place when
solutions containing five-tenths gram maltose or five-tenths gram
fermentation residue are used. The data can not be compiled in the
form of a table similar to Allihn’s, as it is impossible to obtain a
solution of uniform acidity each time, and the solution will have to be
standardized by means of a known pure dextrose solution and the result
obtained with the unknown sugar solution properly diluted compared with
this. This method of Sieben’s has never been practiced to any extent in
analytical separations and can not, therefore, be strongly recommended
without additional experience.
=258. Removal of Dextrin by Alcohol.=—By reason of its less solubility,
dextrin can be removed from a solution containing also dextrose and
maltose by precipitation with alcohol. It is impracticable, however, to
secure always that degree of alcoholic concentration which will cause
the coagulation of all the dextrins without attacking the concomitant
reducing sugars. In this laboratory it has been found impossible to
prepare a dextrin by alcoholic precipitation, which did not contain
bodies capable of oxidizing alkaline copper solutions.
The solution containing the dextrin is brought to a sirupy consistence
by evaporation and treated with about ten volumes of ninety per cent
alcohol. After thorough mixing, the precipitated dextrin is collected
on a filter and well washed with alcohol of the strength noted. It is
then dried and weighed. If weaker solutions of dextrin are used, the
alcohol must be of correspondingly greater strength. In the filtrate
the residual maltose and dextrose may be separated and determined by
the chemical and optical methods already described.