=44. Introduction.=—Carbohydrates, of which sugars and starches are
the chief representatives, form the great mass of the results of
vegetable metabolism. The first functions of the chlorophyll cells of
the young plant are the condensation of carbon dioxid and water. The
simplest form of the condensation is formaldehyd, CH₂O. There is no
convincing evidence, however, that this is the product resulting from
the functional activity of the chlorophyll cells. The first evidence
of the condensation is found in more complex molecules; _viz._, those
having six atoms of carbon. It is not the purpose of this work to
discuss the physiology of this process, but the interested student can
easily find access to the literature of the subject.[23] When a sample
of a vegetable nature reaches the analyst he finds by far the largest
part of its substance composed of these products of condensation of the
carbon dioxid and water. The sugars, starches, pentosans, lignoses, and
celluloses all have this common origin. Of many air-dried plants these
bodies form more than eighty per cent.
In green plants the sugars exist chiefly in the sap. In plants cut
green and quickly dried by artificial means the sugars are found
in a solid state. They also exist in the solid state naturally in
certain sacchariferous seeds. Many sugar-bearing plants when allowed
to dry spontaneously lose all or the greater part of their sugar by
fermentation. This is true of sugar cane, sorghum, maize stalks, and
the like. The starches are found deposited chiefly in tubers, roots or
seeds. In the potato the starch is in the tuber, in cassava the tuber
holding the starch is also a root, in maize, rice and other cereals the
starch is in the seeds. The wood-fibers; _viz._, pentosans, lignose,
cellulose, etc., form the framework and support of the plant structure.
Of all these carbohydrate bodies the most important as foods are the
sugars and starches, but a certain degree of digestibility cannot be
denied to other carbohydrate bodies with the possible exception of
pure cellulose. In the following paragraphs the general principles of
determining the sugars and starches will be given and afterwards the
special processes of extracting these bodies from vegetable substances
preparatory to quantitive determination.
=45. Nomenclature.=—In speaking of sugars it has been thought best
to retain for the present the old nomenclature in order to avoid
confusion. The terms dextrose, levulose, sucrose, etc., will therefore
be given their commonly accepted significations.
A more scientific nomenclature has recently been proposed by Fischer,
in which glucose is used as the equivalent of dextrose and fructose
as the proper name for levulose. All sugars are further classified
by Fischer into groups according to the number of carbon atoms found
in the molecule. We have thus trioses, tetroses, pentoses, hexoses,
etc. Such a sugar as sucrose is called hexobiose by reason of the fact
that it appears to be formed of two molecules of hexose sugars. For a
similar reason raffinose would belong to the hexotriose group.[24]
Again, the two great classes of sugars as determined by the structure
of the molecule are termed aldoses and ketoses according to their
relationship to the aldehyd or ketone bodies.
Since sugars may be optically twinned, that is composed of equal
molecules of right and left-handed polarizing matter it may happen that
apparently the same body may deflect the plane of polarization to the
right, to the left, or show perfect neutrality.
Natural sugars, as a rule, are optically active, but synthetic sugars
being optically twinned are apt to be neutral to polarized light.
To designate the original optical properties of the body therefore the
symbols _d_, _l_, and _i_, meaning dextrogyratory, levogyratory, and
inactive, respectively, are prefixed to the name. Thus we may have _d_,
_l_, or _i_ glucose, _d_, _l_, or _i_ fructose, and so on.
The sugars that are of interest here belong altogether to the pentose
and hexose groups; _viz._, C₅H₁₀O₅ and C₆H₁₂O₆, respectively. Of the
hexobioses, sucrose, maltose, and lactose are the most important,
and of the hexotrioses, raffinose. In this manual, unless otherwise
stated, the term dextrose corresponds to _d_ glucose, and levulose to
_d_ fructose. In this connection, however, it should be noted that the
levulose of nature, or that which is formed by the hydrolysis of inulin
or sucrose is not identical in its optical properties with the _l_
fructose of Fischer.
=46. Preparation of Pure Sugar.=—In using the polariscope or in testing
solutions for the chemical analysis of samples, the analyst will be
required to keep always on hand some pure sugar. Several methods of
preparing pure sugar have been proposed. The finest granulated sugar
of commerce is almost pure. In securing samples for examination those
should be selected which have had a minimum treatment with bluing in
manufacture. The best quality of granulated sugar when pulverized,
washed with ninety-five per cent and then with absolute alcohol and
dried over sulfuric acid at a temperature not exceeding 50° will be
found nearly pure. Such a sugar will, as a rule, not contain more
than one-tenth per cent of impurities, and can be safely used for all
analytical purposes. It is assumed in the above that the granulated
sugar is made from sugar cane.
Granulated beet sugars may contain raffinose and so may show a
polarization in excess of 100. This sugar may be purified by dissolving
seventy parts by weight in thirty parts of water. The sugar is
precipitated by adding slowly an equal volume of ninety-six per cent
alcohol with constant stirring, the temperature of the mixture being
kept at 60°. While still warm the supernatant liquor is decanted
and the precipitated sugar washed by decantation several times with
strong warm alcohol. The sugar, on a filter, is finally washed with
absolute alcohol and dried in a thin layer over sulfuric acid at from
35° to 40°. By this process any raffinose which the sugar may have
contained is completely removed by the warm alcohol. Since beet sugar
is gradually coming into use in this country it is safer to follow the
above method with all samples.[25] In former times it was customary
to prepare pure sugar from the whitest crystals of rock candy. These
crystals are powdered, dissolved in water, filtered, precipitated with
alcohol, washed and dried in the manner described above.
=47. Classification of Methods.=—In the quantitive determination of
pure sugar the various processes employed may all be grouped into three
classes. In the first class are included all those which deduce the
percentage of sugar present from the specific gravity of its aqueous
solution. The accuracy of this process depends on the purity of the
material, the proper control of the temperature, and the reliability
of the instruments employed. The results are obtained either directly
from the scale of the instruments employed or are calculated from the
arbitrary or specific gravity numbers observed. It is evident that
any impurity in the solution would serve to introduce an error of a
magnitude depending on the percentage of impurity and the deviation of
the density from that of sugar. The different classes of sugars, having
different densities in solution, give also different readings on the
instruments employed. It is evident, therefore, that a series of tables
of percentages corresponding to the specific gravities of the solutions
of different sugars would be necessary for exact work. Practically,
however, the sugar which is most abundant, _viz._, sucrose, may be
taken as a representative of the others and for rapid control work the
densimetric method is highly useful.
In the second class of methods are grouped all those processes which
depend upon the property of sugar solutions to rotate the plane of
polarized light. Natural sugars all have this property and if their
solutions be found neutral to polarized light it is because they
contain sugars of opposite polarizing powers of equal intensity. Some
sugars turn the polarized plane to the right and others to the left,
and the degree of rotation in each case depends, at equal temperatures
and densities of the solutions, on the percentages of sugars present.
In order that the optical examination of a sugar may give correct
results the solution must be of a known density and free of other
bodies capable of affecting the plane of polarized light. In the
following paragraphs an attempt will be made to give in sufficient
detail the methods of practice of these different processes in so
far as they are of interest to the agricultural analyst. The number
of variations, however, in these processes is so great as to make
the attempt to fully discuss them here impracticable. The searcher
for additional details should consult the standard works on sugar
analysis.[26]
In the third class of methods are included those which are of a
chemical nature based either on the reducing power which sugar
solutions exercise on certain metallic salts, upon the formation of
certain crystalline and insoluble compounds with other bodies or
upon fermentation. Under proper conditions solutions of sugar reduce
solutions of certain metallic salts, throwing out either the metal
itself or a low oxid thereof. In alkaline solutions of mercury and
copper, sugars exercise a reducing action, throwing out in the one case
metallic mercury and in the other cuprous oxid. With phenylhydrazin,
sugars form definite crystalline compounds, quite insoluble, which
can be collected, dried and weighed. There is a large number of other
chemical reactions with sugars such as their union with the earthy
bases, color reactions with alkalies, oxidation products with acids,
and so on, which are of great use qualitively and in technological
processes, but these are of little value in quantitive determinations.
THE DETERMINATION OF THE PERCENTAGE OF SUGAR BY THE DENSITY OF ITS
SOLUTION.
=48. Principles of the Method.=—This method of analysis is applied
almost exclusively to the examination of one kind of sugar, _viz._, the
common sugar of commerce. This sugar is derived chiefly from sugar cane
and sugar beets and is known chemically as sucrose or saccharose. The
method is accurate only when applied to solutions of pure sucrose which
contain no other bodies. It is evident however, that other bodies in
solution can be determined by the same process, so that the principle
of the method is broadly applicable to the analyses of any body
whatever in a liquid state or in solution. Gases, liquids and solids,
in solution, can all be determined by densimetric methods.
Broadly stated the principle of the method consists in determining the
specific gravity of the liquid or solution, and thereafter taking the
percentage of the body in solution from the corresponding specific
gravity in a table. These tables are carefully prepared by gravimetric
determinations of the bodies in solution of known densities, varying by
small amounts and calculation of the percentages for the intervening
increments or decrements of density. This tabulation is accomplished at
definite temperatures and the process of analysis secured thereby is
rapid and accurate, with pure or nearly pure solutions.
=49. Determination of Density.=—While not strictly correct from a
physical point of view, the terms density and specific gravity are here
used synonymously and refer to a direct comparison of the weights of
equal volumes of pure water and of the solution in question, at the
temperature named. When not otherwise stated, the temperature of the
solution is assumed to be 15°.5.
[Illustration: FIGURE 27. COMMON FORMS OF PYKNOMETERS.]
The simplest method of determining the density of a solution is to
get the weight of a definite volume thereof. This is conveniently
accomplished by the use of a pyknometer. A pyknometer is any vessel
capable of holding a definite volume of a liquid in a form suited
to weighing. It may be a simple flask with a narrow neck distinctly
marked, or a flask with a ground perforated stopper, which, when
inserted, secures always the same volume of liquid contents. A very
common form of pyknometer is one in which the central stopper carries a
thermometer and the constancy of volume is secured by a side tubulure
of very small or even capillary dimensions, which is closed by a ground
glass cap.
The apparatus may not even be of flask form, but assume a quite
different shape as in Sprengel’s tube. Pyknometers are often made to
hold an even number of cubic centimeters, but the only advantage of
this is in the ease of calculation which it secures. As a rule, it
will be found necessary to calibrate even these, and then the apparent
advantage will be easily lost. A flask which is graduated to hold fifty
cubic centimeters, may, in a few years, change its volume at least
slightly, due to molecular changes in the glass. Some of the different
forms of pyknometers are shown in the accompanying figures.
In use the pyknometer should be filled with pure water of the desired
temperature and weighed. From the total weight the tare of the flask
and stopper, weighed clean and dry, is to be deducted. The remainder
is the weight of the volume of water of the temperature noted, which
the pyknometer holds. The weight of the solution under examination is
taken in the same way and at the same temperature, and thus a direct
comparison between the two liquids is secured.
_Example._—Let the weight of the pyknometer be 15.2985 grams.
and its weight with pure water at 15°.5 be 26.9327 ”
Then the weight of water is 11.6342 ”
The weight filled with the sugar solution is 28.3263 ”
Then the weight of the sugar solution is 13.0278 ”
The specific gravity of the sugar solution is therefore, 13.0278 ÷
11.6342 = 1.1198.
For strictly accurate results the weight must be corrected for the
volume of air displaced, or in other words, be reduced to weights in
vacuo. This however is unnecessary for the ordinary operations of
agricultural analysis.
If the volume of the pyknometer be desired, it can be calculated from
the weight of pure water which it holds, one cubic centimeter of pure
water weighing one gram at 4°.
The weights of one cubic centimeter of water at each degree of
temperature from 1° to 40°, are given in the following table:
TABLE SHOWING WEIGHTS OF ONE CUBIC CENTIMETER OF
PURE WATER AT TEMPERATURES VARYING FROM 1° TO 40°.
Weight, Weight,
Temperature. Gram. Temperature. Gram.
0° 0.999871 21° 0.998047
1° 0.999928 22° 0.997826
2° 0.999969 23° 0.997601
3° 0.999991 24° 0.997367
4° 1.000000 25° 0.997120
5° 0.999990 26° 0.996866
6° 0.999970 27° 0.996603
7° 0.999933 28° 0.998331
8° 0.999886 29° 0.995051
9° 0.999824 30° 0.995765
10° 0.999747 31° 0.995401
11° 0.999655 32° 0.995087
12° 0.999549 33° 0.994765
13° 0.999430 34° 0.994436
14° 0.999299 35° 0.994098
15° 0.999160 36° 0.993720
16° 0.999002 37° 0.993370
17° 0.998841 38° 0.993030
18° 0.998654 39° 0.992680
19° 0.998460 40° 0.992330
20° 0.998259
From the table and the weight of water found, the volume of the
pyknometer is easily calculated.
_Example._—Let the weight of water found be 11.72892 grams, and the
temperature 20°. Then the volume of the flask is equal to 11.72892 ÷
0.998259, _viz._, 11.95 cubic centimeters.
=50. Use of Pyknometer at High Temperatures.=—It is often found
desirable to determine the density of a liquid at temperatures above
that of the laboratory, _e. g._, at the boiling-point of water. This is
easily accomplished by following the directions given below:
_Weight of Flask._—Use a small pyknometer of from twenty-five to thirty
cubic centimeters capacity. The stopper should be beveled to a fine
edge on top and the lower end should be slightly concave to avoid any
trapping of air. The flask is to be thoroughly washed with hot water,
alcohol and ether, and then dried for some time at 100°. After cooling
in a desiccator the weight of the flask and stopper is accurately
determined.[27]
[Illustration: FIGURE 28. BATH FOR PYKNOMETERS.]
_Weight of Water._—The flask in an appropriate holder, Fig. 28,
conveniently made of galvanized iron, is filled with freshly boiled and
hot distilled water and placed in a bath of pure, very hot distilled
water, in such a way that it is entirely surrounded by the liquid with
the exception of the top.
The water of the bath is kept in brisk ebullition for thirty minutes,
any evaporation from the flask being replaced by the addition of
boiling distilled water. The stopper should be kept for a few minutes
before use in hot distilled water and is then inserted, the flask
removed, wiped dry, and, after it is nearly cooled to room temperature,
placed in the balance and weighed when balance temperature is reached.
A convenient size of holder will enable the analyst to use eight or ten
flasks at once. The temperature at which water boils in each locality
may also be determined; but unless at very high altitudes, or on days
of unusual barometric disturbance the variations will not be great, and
will not appreciably affect the results.
=51. Alternate Method of Estimating the Weight of Water in
Flasks.=—Formulas for calculating the volume _V_, in cubic centimeters,
of a glass vessel from the weight _P_ of water at the temperature _t_
contained therein, and the volume _Vʹ_ at any other temperature _t’_
are given by Landolt and Börnstein.[28] They are as follows:
_p_
_V_ = _P_ ---
_d_
_p_
_Vʹ_ = _P_ ---- [1 + _γ_(_tʹ_- _t_)]
_d_
_p_ = weight (in brass weights) of one cubic centimeter H₂O in vacuo.
This is so nearly one gram that it will not affect the result in the
fifth place of decimals and may therefore be disregarded. Hence the
formula stands:
1
_Vʹ_ = _P_ ---- [1 + _γ_(_tʹ_-_t_)]; in which
_d_
_d_ = density of water at temperature _t_.
_γ_ = 0.000025, the cubical expansion coefficient of glass.
From this volume the weight of the water may be readily obtained by
referring to tables 13, 14 and 15_a_ in Landolt and Börnstein’s book.
=52. Example Showing Determination of Specific Gravity of a Fat.=—The
flask is emptied of its water, rinsed with alcohol and ether, and dried
again for a few minutes at 100°. It is then filled with the dry, hot,
fresh-filtered fat, which should be entirely free from air bubbles.
The stoppered flask is then replaced in the water-bath, kept for thirty
minutes at the temperature of boiling water, removed, and treated as
above. The weight of fat having been determined, the specific gravity
is obtained by dividing it by the weight of water previously found.
_Example._
Grams.
Weight of flask, dry 10.0197
Weight of flask, plus water 37.3412
Weight of water 27.3215
Weight of flask, plus fat 34.6111
Weight of fat 24.5914
Specific gravity = 24.5914 ÷ 27.3215 = 0.90008.
The weight of the flask dry and empty and the weight of water at 99° to
100° contained therein may be used constantly if great care be taken in
handling and cleaning the apparatus.
_Example._
Grams.
Weight of flask, dry and empty 10.0028
Weight of flask after three weeks’ use 10.0030
[Illustration: FIGURE 29. AEREOMETERS, PYKNOMETERS, AND HYDROSTATIC
BALANCE.]
=53. Determination of Density by the Hydrostatic Balance.=—While
the pyknometer is useful in control work and in fixing standards of
comparison, it is not used extensively in practical work. Quicker
methods of determination are desired in such work, and these are
found in the use of other forms of apparatus. A convenient method of
operation consists in determining the weight of a sinker, whose exact
weights in air and in pure water of a definite temperature, have been
previously determined. The instrument devised by Mohr and modified by
Westphal, is based upon that principle, and is extensively used in
practical work. The construction of this apparatus and also that of the
pyknometers and areometers is shown in the illustrations, figures 29
and 30.
[Illustration: FIGURE 30. HYDROSTATIC BALANCE.]
The weight of the sinker is so adjusted that the index of the balance
arm marks zero when the sinker is wholly immersed in pure water at
the standard temperature. The density of a solution of sugar at the
same temperature, is then determined by placing the rider-weights on
the divided arm of the balance, until the index again marks zero. The
density can then be read directly from the position of the weights in
the arm of the balance or calculated therefrom.
=54. The Areometric Method.=—The most rapid method of determining
the density of a solution and the one in most common use, is based
on the distance to which a heavy bulb with a slender graduated stem
will sink therein. An instrument of this kind is called an areometer.
Many forms of this instrument are employed but they all depend on the
same principle and differ only in the manner of graduation. The one of
widest application has the stem graduated in such a manner as to give
directly the specific gravity of the solution in which it is placed.
Others are made with a special graduation giving directly the
percentage of solid matter in the solution. These instruments can be
used only for the special purposes for which they are constructed.
Other forms are provided with an arbitrary graduation, the numbers
of which by appropriate tables can be converted into expressions of
specific gravity or of per cents of dissolved matters. It is not
practicable to give here, a discussion of the principles of the
construction of areometers.[29] The two which are commonly used, are the
baumé hydrometer and the balling or brix spindle.
In the baumé instrument the zero of the scale is fixed at the point
marked by the surface of distilled water at 15°, and the point to which
it sinks in pure monohydrated sulfuric acid at the same temperature is
marked 66, corresponding to a specific gravity of 1.8427.
The specific gravity corresponding to any degree of the scale, may be
calculated in the absence of a table giving it, by the following formula
144.3
_P_ = -----------.
144.3 - _d_
In this formula _P_ is the density and _d_ the degree of the scale.[30]
In former times the baumé instruments were graduated with a solution
of common salt and a different formula was employed for calculating
specific gravity, but these older instruments are no longer in common
use.
The following table shows the specific gravities of solutions
corresponding to baumé degrees from 1° to 75° consecutively[31]:
Degree Specific Degree Specific Degree Specific Degree Specific
baumé gravity baumé gravity baumé gravity baumé gravity
0 1.0000 19 1.1516 38 1.3574 57 1.6527
1 1.0069 20 1.1608 39 1.3703 58 1.6719
2 1.0140 21 1.1702 40 1.3834 59 1.6915
3 1.0212 22 1.1798 41 1.3968 60 1.7115
4 1.0285 23 1.1895 42 1.4104 61 1.7321
5 1.0358 24 1.1994 43 1.4244 62 1.7531
6 1.0433 25 1.2095 44 1.4386 63 1.7748
7 1.0509 26 1.2197 45 1.4530 64 1.7968
8 1.0586 27 1.2301 46 1.4678 65 1.8194
9 1.0665 28 1.2407 47 1.4829 66 1.8427
10 1.0744 29 1.2514 48 1.4983 67 1.8665
11 1.0825 30 1.2624 49 1.5140 68 1.8909
12 1.0906 31 1.2735 50 1.5301 69 1.9161
13 1.0989 32 1.2849 51 1.5465 70 1.9418
14 1.1074 33 1.2964 52 1.5632 71 1.9683
15 1.1159 34 1.3081 53 1.5802 72 1.9955
16 1.1246 35 1.3201 54 1.5978 73 2.0235
17 1.1335 36 1.3323 55 1.6157 74 2.0523
18 1.1424 37 1.3447 56 1.6340 75 2.0819
=55. Correction for Temperature.=—The baumé hydrometer should be used
at the temperature for which it is graduated, usually 15°. In this
country the mean temperature of our working rooms is above 15°. The
liquid in the hydrometer flask should therefore be cooled to a trifle
below 15°, or kept in a bath exactly at 15° while the observation is
made. When this is not convenient, the observation may be made at any
temperature, and the reading corrected as follows: When the temperature
is above 15° multiply the difference between the observed temperature
and fifteen, by 0.0471 and add the product to the observed reading of
the baumé hydrometer; when the temperature on the other hand, is below
fifteen, the corresponding product is subtracted.[32]
=56. The Balling or Brix Hydrometer.=—The object of the balling or
brix instrument is to give in direct percentages the solid matter in
solution. It is evident that for this purpose the instrument must be
graduated for a particular kind of material, since ten per cent of
sugar in solution, might have a very different specific gravity from a
similar quantity of another body. Instruments of this kind graduated
for pure sugar, find a large use in technical sugar analysis. To
attain a greater accuracy and avoid an instrument with too long a
stem, the brix hydrometers are made in sets. A convenient arrangement
is to have a set of three graduated as follows; one from 0° to 30°, one
from 25° to 50°, and one from 45° to 85°. When the percentage of solid
matter dissolved is over seventy the readings of the scale are not very
reliable.
=57. Correction for Temperature.=—The brix as the baumé scale is
graduated at a fixed temperature. This temperature is usually 17°.5.
The following table shows the corrections to be applied to the scale
reading when made at any other temperature:[33]