------------+------+-----|
Temperature.| [eta]| Z. |
------------+------+-----|
°C. | | |
50 |0.0517| 418 |
46 |0.0581| 468 |
42 |0.0628| 506 |
38 |0.0726| 585 |
34 |0.0788| 635 |
30 |0.0857| 691 |
26 |0.0889| 717 |
22 |0.0919| 741 |
20 |0.0946| 763 |
18 |0.0964| 777 |
------------+------+-----+
There is therefore no essential difference in the behavior of a Ghatti
and a gum arabic on heating. Some interesting results, however, were
obtained by heating gums, both Ghattis and arabics, at a fixed
temperature for the same time, cooling, and then after making the
solutions up to the original volume taking their viscosities at the
ordinary temperature. The effect of heating for two hours to 60° C., 80°
C., or 100° C. was a small permanent alteration in viscosity of the
solution, and it would therefore seem desirable that gum solutions
should be made up cold to get the maximum results. The following numbers
illustrate this change, viz.:
------------------------+-----------+-----------------------+
| | After heating to |
Gum Arabic | Without |-------+-------+-------+
10 Per Cent. | heat. | 60°C. | 80°C. | 100°C |
------------------------+-----------+-------+-------+-------+
Z at 18°C | 570 | 468 | 470 | 517 |
Z at 30°C | 485 | 400 | 422 | 439 |
Z at 50°C | 347 | 287 | 258 | 301 |
Ghatti gum No. 15, | | | | |
5 per cent. Z at 18°C. | 1,104 | 780 | 660 | 758 |
------------------------+-----------+-------+-------+-------+
The variation of viscosity with strength of solution was also studied
with one or two typical gums. A 10 per cent. is invariably more than
twice as viscous as a 5 per cent. solution. The following curve was
obtained from one of the Ghattis. Similar results were shown by other
gums.
[Illustration: Variation of Viscosity, with Dilution. Ghatti No. 888.]
It would seem, therefore, that strong solutions, say of 50 per cent.
strength, would be more alike in viscosity than solutions of 5 per cent.
strength of the same gums. In other words, the viscosity of a gum
solution should be taken as nearly as possible to the strength it is
used at, to obtain an exact quantitative idea of its gumming value.
The observation of this fact was one of the circumstances which decided
us to use 5 per cent. solutions for the determination of Ghatti gum
viscosities, the ratio between the 5 per cent. and 10 per cent.
solutions of gum arabics being roughly the same as that between the
respective weights required for gumming solutions of equal value.
From observation of the general nature of the solutions of Ghatti gums,
and from the fact that when allowed to stand portions of the apparently
insoluble matter passed into solution, the hypothesis suggested itself
that metarabin was soluble in arabin, although insoluble in cold water.
If this hypothesis were correct, it would explain the apparent anomaly
of Ghattis giving solutions of higher viscosity than gum arabics,
although they leave insoluble matter behind. The increase in viscosity
would be due to the thickening of the arabic acid by the metarabin.
Moreover, the solutions yielded by various Ghattis leaving insoluble
matter behind would _be all of the same kind_, viz., a saturated
solution of metarabin in arabin more or less diluted by water. Still
further, if the insoluble residue of a Ghatti be the residual metarabin
over and above that required to saturate the arabin, then it will be
possible to dissolve this by the addition of more arabin in the form of
ordinary gum arabic. In order to see if this were the case the following
experiments were performed. Equal parts of a Ghatti and of a gum arabic
were ground up together and dissolved in water. The resulting solution
was _clear_. It was diluted until of 10 per cent. strength, and its
viscosity then taken:
---------------------+-------------+----------------+
| Contains 50 per Cent. Ghatti.|
---------------------+-------------+----------------+
A. Pressure 200 mm | [eta] | Z. |
Temperature 15° C | 0.2517 | 2,030 |
---------------------+-------------+----------------+
The viscosity of this solution therefore was considerably greater than
the mean viscosity of the 10 per cent. solutions of the Ghatti and the
gum arabic, viz., (0.288 + 0.0636)/2 = 0.1758 for the calculated [eta].
Hence it is evident that the increase in viscosity is due to the
solution of the metarabin.
Next a solution was made from a mixture of 70 per cent. Ghatti and 30
per cent. gum arabic. This was also clear and gave a considerably higher
viscosity than the previous solution.
---------------------+------------------------------+
| Contains 70 per Cent. Ghatti.|
---------------------+-------------+----------------+
B. Pressure 200 mm | [eta] | Z. |
Temperature 15° C | 0.3177 | 2,562 |
---------------------+-------------+----------------+
It will be obvious that the increase of viscosity over the previous
solution in this case must be due to the smaller amount of the thin gum
arabic which is present, _i.e._, in the first case there is more gum
arabic than is required to dissolve the whole of the insoluble
metarabin. Further experiments showed that this is also true of the
second mixture, as the viscosities of the following mixtures
illustrate:
-------------------------+--------+-------+
Strength of Solution. | [eta] | Z. |
-------------------------+--------+-------|
C. 80 per cent. Ghatti. |0.3642 | 2,937 |
D. 75 per cent. Ghatti. |0.33095 | 2,669 |
E. 77.5 per cent. Ghatti.|0.4860 | 3,819 |
-------------------------+--------+-------+
This last solution E we called for convenience the "maximum viscosity"
solution, as we believe it to be a 10 per cent. solution containing
arabin very nearly saturated with metarabin. As will be observed, its
viscosity differs widely from those of solutions C and D, between which
it lies in percentage of Ghatti. The first named solution C contains
_too little_ of gum arabic to dissolve the whole of the metarabin.
Consequently there is a residue left undissolved, which of course
diminishes its viscosity. The second solution D is too low in viscosity,
as it still contains too much of the weak gum arabic, and as will be
seen further on, a very slight change in the proportions increases or
decreases the viscosity enormously.
We next tried a series of similar experiments with a Ghatti containing
far less insoluble residue and which consequently would require less gum
arabic to produce a perfect solution. Mixtures were made in the
following proportions, viz.:
----------------------+------------+-----------+
----- | 13.3 per Cent. Ghatti. |
----------------------+------------+-----------+
F. Pressure 200 mm. | [eta] | Z. |
Temperature 15° C. | 0.0976 | 787 |
----------------------+------------+-----------+
----------------------+------------+-----------+
----- | 86.6 per Cent. Ghatti. |
----------------------+------------+-----------+
G. Pressure 200 mm. | [eta] | Z. |
Temperature 15° C. | 0.4336 | 3,497 |
----------------------+------------+-----------+
This latter solution is approaching fairly closely to our "maximum
viscosity" with the previous Ghatti, and probably a very slight decrease
in the amount of gum arabic would bring about the required increase in
viscosity.
When these experiments were first commenced we were still under the
impression, which several months' experience of working with gums had
produced, namely, that the Ghattis were quite distinct in their
properties to ordinary gum arabics. But the new hypothesis, and the
experiments undertaken to confirm it, showed clearly that if the
viscosity of a gum solution depends on the ratio of metarabin to arabin,
then there is no absolute line of demarkation between a Ghatti and a gum
arabic. In other words, there is a constant gradation between gum arabic
and Ghattis, down to such gums as cherry gum, consisting wholly of
metarabin and quite insoluble in water. Therefore those gum arabics
which are low in viscosity consist of nearly pure arabin, while as the
viscosity increases so does the amount of metarabin, until we come to
Ghattis which contain more metarabin than their arabin can hold in
solution, when their viscosity goes down again.
From these observations it would follow, that by taking a gum of less
viscosity than the gum arabic previously used to dissolve the Ghatti,
less of it would be required to do the same work. We confirmed this
suggestion experimentally by taking another gum arabic of viscosity
0.0557 at 15° C. A mixture containing 93.3 per cent. of this Ghatti and
6.7 per cent. of our thinnest gum arabic gave a clear solution which had
the highest viscocity we have yet obtained for a 10 per cent. solution.
----------------------+--------+-------+
H. Pressure 200 mm. | [eta] | Z. |
Temperature 15° C. | 0.5525 | 4,456 |
----------------------+--------+-------+
This gum arabic may be regarded as nearly pure arabin (as calcium and
potassium, etc., salt). By diluting the new "maximum viscosity"
solution, therefore, with the 10 per cent. solution of the gum arabic in
fixed proportions we obtain a series of viscosities which are shown in
the following curve.
[Illustration: Curve Showing Influence of Ghatti upon Viscosity.]
Besides obtaining this curve for change in viscosity from maximum amount
of metarabin to no metarabin at all, we also traced the decrease in
viscosity of the "maximum" solution by dilution with water. The
following numbers were thus obtained, and plotted out into a curve.
Having obtained this curve, we are now in a position to follow up the
hypothesis by calculating the surplus amount of insoluble matter in a
Ghatti. For, let it be conceded that the solution of any Ghatti leaving
an insoluble residue is a mixture of arabin and metarabin in the same
ratio as our "maximum" solution, only more diluted with water, then from
the found viscosity we obtain a point on the curve for dilution, which
gives the percentage of dissolved matter.
Now to show the use of this: The Z value for a 10 per cent. solution of
the second Ghatti at 15° C. is 2,940. This corresponds on the curve to
8.4 dissolved matter. 10 - 8.4 = 1.6 grammes in 10 grammes, which is
insoluble.
CHANGE OF VISCOSITY WITH DILUTION--"MAXIMUM" SOLUTION. 15° C.
TEMPERATURE.
------------+--------------+---------
Percentage. | [eta] | Z.
------------+--------------+---------
10 | 0.55250 | 4,456
9 | 0.42850 | 3,456
8 | 0.35120 | 2,832
7 | 0.27660 | 2,230
6 | 0.22290 | 1,797
5 | 0.16810 | 1,355
4 | 0.11842 | 955
3 | 0.08020 | 647
2 | 0.06190 | 499
1 | 0.03610 | 291
------------+--------------+---------
[Illustration: Curve of Variation in Viscosity on Dilution of the
"Maximum" Solution.]
We have already shown that a "maximum" viscosity solution of this gum is
formed when 6.7 per cent, of thin gum arabic is added to it, and
therefore 6.7 parts of a thin gum arabic are required to bring 16 parts
of metarabin into solution. A convenient rule, therefore, in order to
obtain complete solution of a Ghatti gum is to add half the weight in
thin gum of the insoluble metarabin found from the viscosity
determination. But the portion of the gum which dissolved is made up in
a similar manner (being a diluted "maximum" solution).
Therefore the 84 per cent. of soluble matter contains 58 parts of
metarabin, and the total metarabin in this gum is 58 + 16 = 74 per cent,
on the dry gum.
With these solutions of high viscosity some other work was done which
may be of interest. The temperature curves of the mixtures marked E, G,
and F were obtained between 60° C. and 15° C. The two former curves
showed a direction practically parallel to that at the 10 per cent.
solutions, and as they were approaching to the "maximum" solution, this
is what one would expect. Mr. S. Skinner, of Cambridge, was also good
enough to determine the electrical resistances of these solutions and
the Ghattis and gum arabics employed in their preparation. The
electrical resistance of these gum solutions steadily diminishes as the
temperature increases, and the curve is similar to those obtained for
rate of change with temperature. Although the curves run in, roughly,
the same direction, there does not appear to be any exact ratio between
the viscosities of two gums say at 15° C. and their electrical
resistances at the same temperature; hence it would not seem possible to
substitute a determination of the electrical resistance for the
viscosity determination. The results appear to be greatly influenced by
the amount of mineral matter present, gums with the greatest ash giving
lower resistances.
Experiments were conducted with two Ghattis and two gum arabics, besides
the mixtures marked E, F, and H. Comparison of the electrical
resistances with the viscosities at 15° C. shows the absence of any
fixed ratio between them.
-----------+------+-------------+------------
Gum or | °C. | Ohms | Z Viscosity
Mixture. | | Resistance. | at 15° C.
-----------+------+-------------+-------------
Ghatti, 1 | 10 | 5,667 | 1,490
Ghatti, 2 | 15 | 2,220 | 2,940
Arabic 1 | 15 | 1,350 | 605
Arabic 2 | 10 | 2,021 | 449
Mixture F | 15 | 1,930 | 787
Mixture E | 11.3 | 2,058 | 3,919
-----------+------+-------------+-------------
While performing these experiments, an attempt was made to obtain an
"ash-free" gum, in order to compare its viscosity with that of the same
gum in its natural state. A gum low in ash was dissolved in water, and
the solution poured on to a dialyzer, and sufficient hydrochloric acid
added to convert the salts into chlorides. When the dialyzed gum
solution ceased to contain any trace of chlorides, it was made up to a
10 per cent. solution, and its viscosity determined under 100 mm.
pressure, giving the following results at 15° C.:
-----------------+--------------+-----
-------- | [eta] | Z
-----------------+--------------+-----
Natural gum..... | 0.05570 | 449
"Ash-free" gum.. | 0.05431 | 438
-----------------+--------------+-----
Thus showing that the viscosity of pure arabin is almost identical with
that of its salts in gum.
The yield of furfuraldehyde by the breaking down of arabin and metarabin
was thought possibly to be of some value in differentiating the natural
gums from one another, but we have not succeeded in obtaining results of
much value. 0.2 gramme of a gum were heated with 100 c.c. of 15 per
cent. sulphuric acid for about 2½ hours in an Erlenmeyer flask with a
reflux condenser. After this period of time, further treating did not
increase the amount of furfuraldehyde produced. The acid liquid, which
was generally yellow in color, was then cooled and neutralized with
strong caustic soda. The neutral or very faintly alkaline solution was
then distilled almost to dryness, when practically the whole of the
furfuraldehyde comes over. The color produced by the gum distillate with
aniline acetate can now be compared with that obtained from some
standard substance treated similarly. The body we have taken as a
standard is the distillate from the same weight of cane sugar. The tint
obtained with the standard was then compared with that yielded by the
gum distillate from which the respective ratios of furfuraldehyde are
obtained. The following table shows some of these results:
---------------+--------------------+-----------------+
| Comparative Yield | Amount of |
Substance. | of Furfuraldehyde. |Glucose Produced.|
---------------+--------------------+-----------------+
Cane sugar | 1.00 | .. |
Starch | 0.50 | .. |
Gum arabic | 1.33 | 34.72 |
Gum arabic | 1.20 | 43.65 |
Ghatti, 1 | 1.00 | 26.78 |
Ghatti, 2 | 1.33 | 22.86 |
Metarabin | 1.75 | .. |
---------------+--------------------+-----------------+
The amount of reducing sugar calculated as glucose is also appended.
This was estimated in the residue left in the flask after distillation
by Fehling's solution in the usual way. The yields of furfuraldehyde
would appear to have no definite relation to the other chemical data
about a gum, such as the potash and baryta absorptions or the sugar
produced on inversion.
The action of gum solutions upon polarized light is interesting,
especially in view of the fact that arabin is itself strongly
lævo-rotatory [alpha]_{D} = -99°, while certain gums are distinctly
dextro-rotatory. Hence it is evident that some other body besides arabin
is present in the gum. We have determined the rotatory power of a number
of gum solutions, the results of which are subjoined. On first
commencing the experiments we experienced great difficulty from the
nature of the solutions. Most of them are distinctly yellow in color and
almost opaque to light, even in dilute solutions such as 5 percent. We
found it necessary first to bleach the gums by a special process; 5
grammes of gum are dissolved in about 40 c.c. of lukewarm water, then a
drop of potassium permanganate is added, and the solution is heated on a
water bath with constant stirring until the permanganate is decomposed
and the solution becomes brown. A drop of sodium hydrogen sulphate is
now added to destroy excess of permanganate. At the same time the
solution becomes perfectly colorless.
It can now be cooled down and made up to 100 c.c., yielding a 5 per
cent. solution of which the rotatory power can be taken with ease. Using
a 20 mm. tube and white light the above numbers were obtained.
----------------+----------------+-----------------
Gum or Dextrin. | Solution used. | [alpha]_{D}
----------------+----------------+-----------------
| Per Cent. |
Aden, 1 | 5 | - 33.8
Cape, 2 | 5 | + 28.6
Indian, 3 | 5 | + 66.2
Eastern, 4 | 5 | - 26.0
Eastern, 5 | 5 | - 30.6
Senegal, 6 | 5 | - 17.6
Senegal, 7 | 5 | - 18.4
Senegal, 8 | 2½ | - 19.6
Senegal, 9 | 5 | - 38.2
Senegal, 10 | 5 | - 25.8
Amrad | 2½ | + 57.6
Australian, 1 | 5 | - 28.2
Australian, 2 | 5 | - 26.4
Brazilian, 1 | 2½ | - 36.8
Brazilian, 2 | 2½ | + 21.0
Dextrin, 1 | 5 | +148.0
Dextrin, 2 | 5 | +133.2
Ghatti, 1 | 5 | - 39.2
Ghatti, 2 | 5 | - 80.4
----------------+----------------+-----------------
These numbers do not show any marked connection between the viscosity,
etc., of a gum and its specific rotatory power.
When gum arabic solution is treated with alcohol the gum is precipitated
entirely if a large excess of spirit be used. With a view to seeing if
the precipitate yielded by the partial precipitation of a gum solution
was identical in properties to the original gum, we examined several
such precipitates from various gums to ascertain their rotatory power.
We found in each case that the specific rotatory power of the alcohol
precipitate redissolved in water was not the same as that of the
original gum. In other words these gums contained at least two bodies of
different rotatory powers, of which one is more soluble in alcohol than
the other. O'Sullivan obtained similar results with pure arabin. The
experiments were conducted in the following manner:
(a.) Five grammes of a dextro-rotatory gum (No. 3 in table) were
dissolved in 20 c.c. of water. To the solution was added 90 c.c. of 95
per cent. alcohol. The white precipitate which formed was thrown on to a
tared filter and washed with 30 c.c. more alcohol. The total filtrate
therefore was 140 c.c. The precipitate was dried and weighed = 2.794
grammes or 55.88 per cent. of the total gum. The precipitate was then
redissolved in water, bleached as before and diluted to a 5 per cent.
solution. This was then examined in the polarimeter. Readings gave the
value [alpha]_{D} = +58.4°. The previous rotatory power of the gum was
+66°. Now the alcohol was driven off from the filtrate, which, allowing
for the 11.95 per cent. of water in the gum, should contain 32.17 per
cent. of gum. The alcohol-free liquid was then diluted to a known
volume (for 5 per cent, solution), and [alpha]_{J} found to be +57.7°.
This experiment was then repeated again, using 5 grammes of No. 3, when
3.5805 grammes of precipitate were obtained, using the same volumes of
alcohol and water. The precipitate gave [alpha]_{J} = +57.4°; the
filtrate treated as before, only the percentage of gum dissolved being
directly determined instead of being calculated by difference, gave
[alpha]_{J} = +52.5°.
(b.) Another gum (No. 9) with [alpha]_{J} = -38.2° and containing 13.86
per cent, of moisture, gave 2.3315 grms. of precipitate when similarly
treated. The precipitate gave when redissolved in water [alpha]_{J} =
-20.8°. The filtrate containing 39.5 per cent, real gum gave [alpha]_{J}
= -67.5°, so that the least lævo-rotatory gum. was precipitated by the
alcohol.
The Ghattis apparently are all lævo-rotatory, and give much less
alcoholic precipitates than the gum arabic. The precipitation moreover
was in the opposite direction, that is, the most lævo-rotatory gum was
thrown down by the alcohol. The appended table shows the nature of the
precipitates and the respective amounts from two Ghattis and two gum
arabics. It will be observed that the angle of rotation in three of the
cases is decidedly less both for precipitate and filtrate than for the
original solution: