=109. Classification.=—Among the volumetric methods will be given those
which are in common use or such as have been approved by the practice
of analysts. Since the use of mercuric salts is now practiced to a
limited extent, only a brief study of that process will be attempted.
With the copper methods a somewhat extended description will be given
of those depending on the use of copper sulfate, and a briefer account
of the copper carbonate process.
In the copper sulfate method two distinct divisions must be noted,
_viz._, first an indirect process depending first upon the reduction
of the copper to a suboxid, the subsequent action of this body on iron
salts, measured finally by titration with potassium permanganate; and
second, a direct process determined either by the disappearance of the
blue color from the copper solution, or by the absence of copper from a
drop of the solution withdrawn and tested with potassium ferrocyanid.
This last mentioned reaction is one which is found in common use. The
volumetric methods are not, as a rule, as accurate as the gravimetric,
depending on weighing the resultant metal, but they are far more rapid
and well suited to technical control determinations.
=110. Reduction of Mercuric Salts.=—The method of determining sugar by
its action on mercuric salts, is due to Knapp.[70] The method is based
on the observation that dextrose and other allied sugars, will reduce
an alkaline solution of mercuric cyanid, and that the mercury will
appear in a metallic state.
The mercuric liquor is prepared by adding to a solution of ten grams of
mercuric cyanid, 100 cubic centimeters of a solution of caustic soda of
1.145 specific gravity, and making the volume to one liter with water.
The solution of sugar to be titrated, should be as nearly as possible
of one per cent strength.
To 100 cubic centimeters of the boiling solution, the sugar solution is
added in small portions from a burette and in such a way as to keep the
whole mass in gentle ebullition.
To determine when all the mercuric salt has been decomposed, a drop of
the clear boiling liquid is removed and brought into contact with a
drop of stannous chlorid solution on a white surface. A brownish black
coloration or precipitate will indicate that the mercury is not all
precipitated. Fresh portions of the sugar must then be added, until no
further indication of the presence of mercury is noted. The approximate
quantity of sugar solution required to precipitate the mercury having
thus been determined, the process is repeated by adding rapidly, nearly
the quantity of sugar solution required, and then only a few drops at a
time, until the reduction is complete.
One hundred cubic centimeters of the mercuric cyanid solution prepared
as directed above, will be completely reduced by
202 milligrams of dextrose,
200 ” ” invert sugar,
198 ” ” levulose,
308 ” ” maltose,
311 ” ” lactose.
By reason of the unpleasant odor of the boiling mercuric cyanid when
in presence of a reducing agent, the process should be conducted in a
well ventilated fume chamber. With a little practice the process is
capable of rapid execution, and gives reasonably accurate results.
=111. Sachsse’s Solution.=—The solution of mercuric salts proposed
by Sachsse, is made by dissolving eighteen grams of mercuric iodid
in twenty-five cubic centimeters of an aqueous solution of potassium
iodid. To this solution are added 200 cubic centimeters of potash lye,
containing eighty grams of caustic potash. After mixing the solution,
the volume is completed to one liter. The sugar solutions used to
reduce this mixture, should be more dilute than those employed with
the mercuric cyanid, and should not be over one-half per cent in
strength. The end of the reduction is determined as already described.
After a preliminary trial, nearly all the sugar necessary to complete
reduction, should be added at once, and the end of the reduction then
determined by the addition of successive small quantities. One hundred
cubic centimeters of the mercuric iodid solution prepared as directed
above, require the following quantities of sugar to effect a complete
reduction:
325 milligrams of dextrose,
269 ” ” invert sugar,
213 ” ” levulose,
491 ” ” maltose,
387 ” ” lactose.
By reason of the great difference between the reducing power of
dextrose and levulose in this solution, it has been used in combination
with the copper reduction method, to be described, to determine the
relative proportion of dextrose and levulose in a mixture.[71]
It is now known that copper solutions require slightly different
quantities of dextrose, levulose, or invert sugar to effect complete
reduction, but the variations are not great and in the calculation
above mentioned, it may be assumed that these differences do not exist.
Instead of using stannous chlorid as an indicator, the end of the
reaction may be determined as follows: A disk of filtering paper is
placed over a small beaker containing some ammonium sulfid. A drop of
the clear hot solution is placed on this disk, and if salts of mercury
be still present a dark stain will be produced; or a drop of the
ammonium sulfid may be brought near the moist spot formed by the drop
of mercury salt. An alkaline solution of zinc oxid may also be used.
The methods depending on the use of mercuric salts have, of late, been
supplanted by better processes, and space will not be given here to
their further discussion.
=112. The Volumetric Copper Methods.=—The general principle on which
these methods depend, is found in the fact that certain sugars,
notably, dextrose, (glucose), levulose, (fructose), maltose and
lactose, have the property of reducing an alkaline solution of copper
to a lower state of combination, in which the copper is separated as
cuprous oxid. The end of the reaction is either determined by the
disappearance of the blue color of the solution, or by the reaction
produced by a drop of the hot filtered solution, when placed in contact
with a drop of potassium ferrocyanid acidified with acetic.
The copper salt which is found to give the most delicate and reliable
reaction, is the tartrate. The number of volumetric processes proposed
and which are in use, is very great, and an attempt even to enumerate
all of these can not be made in this volume. A few of the most reliable
and best attested methods will be given, representing if possible, the
best practice in this and other countries. The rate of reduction of
the copper salt to suboxid, is influenced by the rate of mixing with
the sugar solutions, the temperature, the composition of the copper
solution and the strength of the sugar solution.
The degree of reduction is also modified by the rate at which the
sugar solution is added, and by the degree and duration of heating,
and all these variables together, make the volumetric methods somewhat
difficult and their data, to a certain extent, discordant. By reason,
however, of the ease with which they are applied and the speed of their
execution, they are invaluable for approximately correct work and for
use in technical control.
=113. Historical.=—It is not the purpose in this paragraph to trace
the development of the copper reduction method for the determination
of reducing sugars, but only to refer to the beginning of the exact
analytical application of it.
Peligot, as early as 1844, made a report to the Society for the
Encouragement of National Industry on methods proposed by Barreswil
and Fromherz for the quantitive estimation of sugar by means of copper
solution.[72] These methods were based on the property of certain
sugars to reduce alkaline copper solution to a state of cuprous oxid
first announced by Trommer.[73] This was followed by a paper by Falck
on the quantitive determination of sugar in urine.[74]
In 1848 the methods, which have been proposed, were critically examined
by Fehling, and from the date of his paper the determination of sugar
by the copper method may be regarded as resting on a scientific
basis.[75]
Since the date mentioned the principal improvements in the process
have been in changing the composition of the copper solution in order
to render it more stable, which has been accomplished by varying the
proportions of copper sulfate, alkali and tartaric acid. For the better
keeping of the solution the method of preserving the copper sulfate
and the alkaline tartrates in separate flasks and only mixing them
at the time of use has been found very efficacious.[76] For testing
for the end of the reaction by means of an acetic acid solution of
potassium ferrocyanid the filtering tube suggested by the author, the
use of which will be described further on, has proved quite useful.
Pavy has suggested that by the addition of ammonia to the copper
solution the precipitated suboxid may be kept in solution and the end
of the reaction thus easily distinguished by the disappearance of the
blue color.[77] Allen has improved on this method by covering the hot
mixture with a layer of paraffin oil whereby any oxidation of the
suboxid is prevented.[78]
The introduction and development of the gravimetric process depending
on securing the reduced copper oxid in a metallic state as developed by
Allihn, Soxhlet, and others, completes the resumé of this brief sketch
of the rise and development of the process.
=114. Action of Alkaline Copper Solution on Dextrose.=—The action to
which dextrose and other reducing sugars are subjected in the presence
of a hot alkaline copper solution is two-fold in its nature. In the
first place there is an oxidation of the sugar which is transformed
into tartronic, formic and oxalic acids, the two latter in very small
quantities. At the same time another part of the sugar is attacked
directly by the alkali and changed to complex products among which
have been detected lactic, oxyphenic and oxalic acids, also two bodies
isomeric with dioxyphenolpropionic acid. When the sugar is in large
excess melassic and glucic acids have also been detected. The glucic
acid may be regarded as being formed by simple dehydration but becomes
at once resolved into pyrocatechin and gluconic acid according to
the reaction C₁₂H₁₈O₉ = C₆H₆O₂ + C₅H₁₂O₇. The gluconic acid also is
decomposed and gives birth to lactic and glyceric acids according to
the formula C₆H₁₂O₇ = C₃H₆O₃ + C₃H₆O₄. The glyceric acid also in the
presence of a base is changed into lactic and oxalic acids. Between
lactic acid and pyrocatechin, existing in a free state, there is
produced a double reciprocal etherification in virtue of which there
arise two ethers isomeric with hydrocaffeic acid, C₉H₁₀O₄. One of these
bodies is an acid and corresponds to the constitution
CH₃
/
O ── CH
/ \
C₆H₄ CO₂H (2)
\
OH (2)
and the other is of an alcoholic nature corresponding to the formula
CO₂ ── CHOH ── CH₃ (1)
/
C₆H₄
\
OH₂ (2)
Of all these products only oxyphenic and lactic acids and their ethers
and oxalic acid remain unchanged and they can be isolated. All the
others are transformed in an acid state and they can only be detected
by operating in the presence of metallic oxids capable of precipitating
them at the time of their formation.[79]
=115. Fehling’s Solution.=—The copper solution which has been most used
in the determination of reducing sugars is the one proposed by Fehling
as a working modification of the original reagent used by Trommer.[80]
Following is the formula for the preparation of the fehling solution:
Pure crystallized copper sulfate CuSO₄.5H₂O, 34.64 grams:
Potassium tartrate, 150.00 ”
Sodium hydroxid, 90.00 ”
The copper sulfate is dissolved in water and the potassium tartrate in
the aqueous solution of the sodium hydroxid which should have a volume
of about 700 cubic centimeters. The two solutions are mixed and the
volume completed to a liter. Each cubic centimeter of this solution
will be reduced by five milligrams of dextrose, equivalent to four and
a half milligrams of sucrose.
The reaction which takes place is represented by the following
molecular proportions:
C₆H₁₂O₆ = 10CuSO₄.5H₂O
Dextrose. Copper sulfate.
180 2494
Fehling’s solution is delicate in its reactions but does not keep well,
depositing cuprous oxid on standing especially in a warm place exposed
to light. The fehling liquor was soon modified in its constitution by
substituting 173 grams of the double sodium and potassium tartrate for
the neutral potassium tartrate first used, and, in fact, the original
fehling reagent contained forty grams of copper sulfate instead of the
quantity mentioned above. Other proportions of the ingredients are also
given by many authors as fehling solution.
=116. Comparison of Copper Solutions for Oxidizing Sugars.=—For the
convenience of analysts there is given below a tabular comparison
of the different forms of fehling liquor which have been proposed
for oxidizing sugars. The table is based on a similar one prepared
by Tollens and Rodewald, amended and completed by Horton.[81] The
solutions are arranged alphabetically according to authors’ names: