Chapter 10

+--------------+------------+---------- Sample. | [eta] | [eta] | Z Water | Uncorrected. | Corrected. | = 100. ---------------------+--------------+------------+---------- Gum arabic.......... | 0.1876 | 0.1856 | 1,233 Cape gum............ | 0.1575 | 0.1555 | 1,029 Indian gum.......... | 0.0540 | 0.0470 | 311 Eastern gum......... | 0.0689 | 0.0639 | 417 Gum arabic.......... | 0.0550 | 0.0480 | 317 Senegal............. | 0.0494 | 0.0410 | 271 Senegal............. | 0.0468 | 0.0380 | 251 Senegal............. | 0.0627 | 0.0557 | 364 Gum arabic.......... | 0.0511 | 0.0430 | 285 Water............... | 0.0149 | 0.0124 | 100 Ghatti.............. | 0.2903 | 0.2880 | 2,322 Ghatti, 5 per cent.. | 0.0903 | 0.0828 | 688 Ghatti, 5 per cent.. | 0.1391 | 0.1350 | 1,089 Ghatti, 5 per cent.. | 0.1795 | 0.1760 | 1,420 Ghatti, 5 per cent.. | 0.1527 | 0.1485 | 1,198 Ghatti, 5 per cent.. | 0.1139 | 0.1083 | 873 Ghatti, 5 per cent.. | 0.1419 | 0.1369 | 1,104 Dextrin............. | 0.0398 | 0.0255 | 169 Dextrin............. | 0.0341 | 0.0196 | 129 Dextrin............. | 0.0455 | 0.0380 | 306 Gum substitute...... | 0.0318 | 0.0224 | 180 Gum substitute...... | 0.0318 | 0.0224 | 180 Amrad............... | 0.0793 | 0.0708 | 570 Australian.......... | 0.0378 | 0.0283 | 228 Australian.......... | 0.0365 | 0.0268 | 216 Brazilian........... | 0.0668 | 0.0627 | 506 Brazilian........... | 0.0516 | 0.0445 | 359 Ghatti.............. | 0.3636 | 0.3621 | 2,920 ---------------------+--------------+------------+----------

In the column for [eta] corrected the differences due to the use of different instruments are of course eliminated. The absolute viscosity of water at 15° C. determined in four different instruments is shown below. Poiseuille's value for water being 0.0122.

+-------------+-------------+-------------+-------------+ Instrument. | 1. | 2. | 3. | 4. | --------------+-------------+-------------+-------------+-------------+ [eta] corrtd. | 0.0109 | 0.01185 | 0.0124 | 0.0120 | of water. | | | | | K_{1} value.. | 0.000000898 | 0.000000863 | 0.000000932 | 0.00000052 | K_{2} value.. | 0.235 | 0.2175 | 0.226 | 0.0204 | --------------+-------------+-------------+-------------+-------------+

The above values for various gums and dextrins were obtained at a constant temperature of 15° C. and are compared with water at that temperature. It is of the utmost importance that the temperature of the water surrounding the bulbs should be adjusted for each series of experiments to the temperature at which the absolute viscosity of the water was determined. As far as we have ascertained, in gum solutions there is a steady diminution in viscosity with increase of temperature until a certain temperature is reached, beyond which increase of heat does not markedly influence the viscosity, and it is possible that above this "critical point," as we may term it, the gum solutions once more begin to increase in viscosity. The temperature at which the viscosity becomes stationary varies somewhat with different gums, but broadly speaking it lies between 60° C. and 90° C., no gums showing any marked decrease in viscosity between 80° C. and 90° C.

The experiments we have made in this direction were conducted as follows. The 300 c.c. bottle containing the gum was placed in a capacious beaker full of hot water, and the viscosity instrument was also surrounded with water at the same temperature. Thermometers were suspended both in the beaker and the outer jar. The viscosity at the highest temperature obtained, about 90° C., was then taken and repeated for every fall of 4° C. till the water reached the temperature of the air.

The values so obtained gradually diminished with the increase of temperature. From the [eta] values obtained the Z values were calculated, using water at 15° C. as a standard. From the Z values thus obtained taken as the ordinate, and the temperature of each experiment as the abscissa, curves were plotted out embodying the results, examples of which are given below. The curves yielded by three gums 2, 7, and 8 changed between 90° C and 100° C., while gum sample 4 has a curve bending between 60° C. and 70° C. Experimentally this increase of viscosity of the latter gum above 60° C. was confirmed, but the critical point of the other solutions tried approaches too nearly to the boiling point of water for experiments to be conducted with accuracy, as the temperature of the bulbs diminishes sensibly while the experiment is being made.

If viscosity values have been determined it is possible to calculate the remaining or intermediate values for Z at any particular temperature from the general equation--

Zt = A + Bt + Ct²

As an example of the mode of calculation we may quote the following. A gum gave the following values for Z at the temperature stated:

Gum. 50° C. Z_{50°} = 228

Gum. 30° C. Z_{30°} = 339

Gum. 20° C. Z_{20°} = 412

from which the constants--

A = 592.99 B = -10.2153 C = 0.0583

can be obtained, and thus the value of Z_{t°} for any required temperature. The numbers calculated for gums all point to a diminution in viscosity up to a certain point, and then a gradual increase. A comparison of some of the figures actually obtained in some of these experiments, compared with the calculated figures for the same temperature, shows their general agreement.

EFFECT OF TEMPERATURE UPON VISCOSITY--GUM VII.

+------+--------+-------------+ Temperature.| [eta]|Z found.|Z calculated.| ------------+------+--------+-------------+ °C | | | | 50 |0.0283| 228 | 228.00 | 45 |0.0305| 246 | 246.55 | 42 |0.0352| 284 | 266.75 | 38 |0.0368| 297 | 289.00 | 34 |0.0410| 330 | 313.06 | 30 |0.0419| 339 | 339.00 | 26 |0.0445| 359 | 367.80 | 22 |0.0492| 398 | 396.47 | 20 |0.0511| 412 | 412.00 | 18 |0.0531| 428 | 428.00 | ------------+------+--------+-------------+

EFFECT OF TEMPERATURE UPON VISCOSITY.--GUM VIII.

+------+--------+-------------| Temperature.| [eta]|Z found.|Z calculated.| ------------+------+--------+-------------| °C. | | | | 50 |0.0430| 347 | 347 | 46 |0.0475| 383 | 371.14 | 42 |0.0502| 405 | 397.09 | 38 |0.0510| 411 | 424.73 | 34 |0.0575| 463 | 454.06 | 30 |0.0602| 485 | 485 | 26 |0.0637| 513 | 517.82 | 22 |0.0667| 538 | 552.25 | 20 |0.0707| 570 | 570 | 18 |0.0755| 609 | 583.07 | ------------+------+--------+-------------+

The constants for the first gum are those given in the preceding column, while for the latter they were--

A = 771.9: B = -11.15: C = 0.053

As will be observed, the effect of heat appears to be the same upon the two typical gum arabics quoted above, an increase of temperature from 18° C. to 50° C. decreasing the viscosity by nearly one half in both cases, and the same seems to be true of most gum arabics. Roughly also the same holds good for Ghattis, as the following numbers show:

+-------------+------------| Gum. | Z at 18° C. | Z at 50° C.| ------------+-------------+------------| Gum arabic. | 1016 | 579 | Gum arabic. | 428 | 228 | Gum arabic. | 609 | 347 | Gum arabic. | 581 | 258 | Ghatti. | 572 | 306 | Ghatti. | 782 | 418 | ---------------------------------------+

The following table shows the effect of heat upon the viscosity of a typical Ghatti:

GHATTI GUM NO. 15.--VISCOSITY.

+------+-----| 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.

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.

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 ------------+--------------+---------

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.