Harry Clay Jones.

The absorption spectra of solutions as affected by temperature and by dilution: a quantitative study of absorption spectra by means of the radiomicrometer online

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cium chloride having a concentration of 5.38 normal and a depth of 20 mm.
The corresponding water-curve is marked throughout by the symbol H 2 O.
The solution is the more transparent from 0.9/x to nearly 1/z. The water then
becomes the more transparent over a short region of wave-lengths. From
1.05/z to 1.2ju the solution is the more transparent. In this region the solu-
tion becomes as much as 25 per cent more transparent than the pure water,
as can be seen by comparing the points on the "water" curve with the corre-
sponding points on the curve for the solution which are vertically above the
points on the water-curve. The water becomes appreciably more trans-
parent only at and near the bottom of the "water-band" having a wave-
length of approximately lju. This is the effect that we would expect to get
if the dissolved substance exerted a "damping" effect on the absorption of
light by water.

It will be recalled that the salts which do not form hydrates show, in
aqueous solution, practically the same absorption as the corresponding
amount of water. It would, therefore, seem reasonable to account for the
differences in the case of nonhydrating and strongly hydrating salts as due
to the water of hydration, or the water that, in this case, is combined with
the calcium chloride.

The curves in fig. 13 are for a smaller depth of the same solution of cal-
cium chloride. This figure brings out the same general relations as was
shown in fig. 12. The water-curve in the region 1.25/* is above that of the
solution, showing that water in this region for the shallower depths of solu-
tion is more transparent than the solution. The additional feature brought
out by this figure is the water-band in the region 1.4 to 1.5/*. After the first-
named water-band is passed the solution becomes more transparent than the
water and remains so until the wave-length 1.42 is reached. Here both the
solution and the water are practically opaque, as is shown by both the curves
approaching the abscissas.

The curve for magnesium chloride having a depth of 20 mm. is almost
exactly a duplicate of that for calcium chloride having the same depth.
Practically the only difference worthy of mention is in the region from l.O/i
to 1.1/i. In the case of magnesium chloride the water remains the more
transparent over this region of wave-lengths. In the case of calcium chlo-
ride the solution is the more transparent over this region. The difference



54 ABSORPTION OF LIGHT BY WATER CHANGED

in the transparency of the water and the solution throughout this region is,
however, not very great. From 1.1/j to wards the longer wave-lengths, as
we come down the descending arm of the curve towards the second water-
band, the water in the case of the magnesium chloride (as in the case of cal-
cium chloride) becomes much more opaque than the solution, the differences
here being of the same order of magnitude as those with calcium chloride.

Fig. 15 gives the results for magnesium chloride with a depth of layer of
1 cm., and the same relations hold as in fig. 14, for the relative transparency
of the water and of the solution. The water becomes the more transparent
from 1.22/x to 1.34ju. For the longer wave-lengths the solution becomes
more transparent until the region 1.41ju is passed. For wave-lengths longer
than 1.41/z the transmission of both solution and water is practically zero-
that is, they both become opaque to the longer wave-lengths.

The results in fig. 16 bring out some new features of interest and impor-
tance. These are the results that were obtained with aluminium sulphate.
The new feature shown by the curve for aluminium sulphate, as compared
with those for calcium chloride and magnesium chloride, is that at the
minimum of the curve corresponding to wave-length Iju the solution is
more transparent than the corresponding water. Beyond the wave-length
1.04jLt the water becomes the more transparent with aluminium sulphate as
with magnesium chloride. Beyond the wave-length 1.17/i the solution
becomes more transparent in this case as with magnesium chloride and
calcium chloride.

If we turn to fig. 17 the relations are as follows. In the region of 1.2/z the
water is the more opaque. From 1 .29ju to 1 .36/4 the water becomes the more
transparent. From 1.36// to the longest wave-length studied, the solution
again becomes more transparent than the corresponding layer of water.

An examination of all the results thus far obtained bearing on this prob-
lem leads us to conclude that the greater transparency of the solution as
compared with the water in the solution must be due to some action of the
dissolved substance on the solvent water. The question remains, what is
this action?

EXPLANATION OF THE RESULTS.

We have seen from our earlier work on the absorption spectra of solutions,
which has been in progress in this laboratory continuously for the past eight
years, that the solvent can have a marked effect on the power of the dis-
solved substance to absorb light. This was first shown by Jones and
Anderson, 1 and a large number of examples of this effect have since been
found by Jones and Strong. 2 We interpreted the effect of the solvent on the
power of the dissolved substance to absorb light as due to a combination
between a part of the liquid present and the dissolved substance. This
enabled us to explain a large number of facts which were brought to light for
the first time by our investigations of the absorption spectra of solutions.
Many of the phenomena which were thus explained, it seemed, could not be

1 Cam. Inst. Wash. Pub. 110. 2 Cam. Inst. Wash. Pubs. 130 and 160.



IN THE PRESENCE OF STRONGLY HYDRATED SALTS. 55

explained in terms of any other suggestion that has thus far been made.
In a word, the solvate theory of solution as proposed by Jones about a dozen
years ago, Ho supplement the theory of electrolytic dissociation in order that
we might have a theory of the real solutions which we use in the laboratory,
and not simply a theory of ideal solutions as the theory of electrolytic dis-
sociation alone must be regarded, has served good purpose in explaining the
phenomena that have been previously observed in connection with the
absorption of light by solutions of dissolved substances.

We are inclined to explain the phenomena recorded in this paper by means
of the same theory. For solutions of those substances which have been
shown by entirely different methods not to hydrate to any appreciable
extent, the absorption of light by the solution and by a layer of water equal
in depth to that of the water in the solution, is the same almost to within the
limit of experimental error.

For those substances which have been shown to form complex hydrates,
however, the absorption of light by their solutions and by a layer of water
equal in depth to that of the water in the solution is very different. The
water in these solutions is usually more opaque to light than the solution
or, in other words, a solution is more transparent than the water that is
present in the solution.

The most rational explanation of this phenomenon appears to be that the
part of the water that is combined with the dissolved substance has a smaller
power to absorb light than pure, free, uncombined water. The fact that
we are able to detect the difference between the water in the solution and
pure water, by its action on light, we regard as good evidence that water in
the solution is different from pure, free water. This difference, it seems to
us, can be readily accounted for by the theory that a part of the water
present in the solution is in combination with the dissolved substance.

We have carried out similar investigations with aluminium nitrate, but the
concentration of the strongest solution that could be obtained was not suffi-
ciently great to show the phenomenon in question. We therefore do not
incorporate the results obtained with this substance. That the solutions
must be very concentrated to show clearly the phenomenon with which we
are dealing is seen from the results given in table 10. Here the solutions of
the three salts in question that were used are more dilute than those for
which the results are tabulated in tables 8 and 9. An examination of table
10 will show that the phenomenon in question does not manifest itself to
anything like the same extent as with the more concentrated solutions.
This is exactly what we would expect in terms of the solvate theory of solu-
tions. The more concentrated the solution the larger the total amount of
the water present combined with the dissolved substance. If combination
between water and the dissolved substance explains the facts recorded in this
paper, then the larger the amount of water present that is combined with
the dissolved substance the more pronounced the phenomenon in question.

1 Amer. Chem. Journ., 23, 89 (1900).



56 ABSORPTION OF LIGHT BY WATER CHANGED

The results obtained with aluminium sulphate bring out the same facts
shown by calcium chloride and magnesium chloride, and also that water is
more transparent in the region 1.1/j and more opaque at lju. That the sul-
phate should not agree throughout with the chlorides is really not surprising,
since the sulphates show abnormal results in almost every particular. This
is probably due, in part at least, to the large amount of polymerization
which the sulphate molecules in general undergo in the presence of even
water as a solvent. It should also be remembered in the present connection
that while calcium chloride and magnesium chloride crystallize with only 6
molecules of water, and are therefore only largely hydrated, aluminium sul-
phate crystallizes with 18 molecules of water and is therefore very largely
hydrated.

The results in table 11 are the radiomicrometer deflections for a solution
of aluminium sulphate and those for water having the same depth as the
water in the solution in question, and the corresponding data for potassium
chloride. A comparison of the two columns for potassium chloride and its
corresponding water shows that the two are almost equally transparent to
all the wave-lengths studied.

A comparison of the aluminium sulphate with its corresponding water
brings out the phenomenon that we are now discussing in a very pronounced
manner.

One other relation of a general character should be pointed out. The
curves (figs. 12 to 17) show that the addition of salt to water shifts the
absorption towards the longer wave-lengths. This is analogous to what had
already been found by Jones and Uhler, 1 Jones and Anderson, 2 Jones and




FIG. 12.



Cam. Inst. Wash. Pub. 60. l Cam. Inst. Wash. Pub. 110,



IN THE PRESENCE OF STRONGLY HYDRATED SALTS. 57

Strong, 1 and Guy and Jones, 2 when the absorption of salts as affected by the
water present was studied. It was found that rise in temperature and
increase in the concentration of the solution both tended to shift the ab-
sorption of the salt towards the longer wave-lengths. The effect of rise in
temperature and the increase in the concentration of the solution tended to
simplify the hydrates in combination with the particles of the salt. The
resonator within this simplified system seems to vibrate so as to shift the
absorption bands towards the red.

The effect of the salt on the absorption of the water is the same as that of
rise of temperature and increase of concentration on the absorption of the
dissolved substance. We would naturally look for a similar explanation of
the two sets of phenomena. It has been suggested by Dr. Guy, that the
effect of the salt on the absorption of light by water may be due to the
breaking down of the associated molecules of water by the dissolved sub-
stance. This would be in keeping with the fact established by Jones and
Murray, 3 that one associated substance when dissolved in another associated
substance diminishes its association.

In terms of this explanation, however, it is a little difficult to see why non-
hydrated salts, such as were used in this work, do not also diminish the asso-
ciation of water and cause a shifting of its absorption bands towards the
longer wave-lengths. It may be that the effect of the dissolved substance
in breaking down the association of the water is pronounced only in the case
of water of hydration or the water that is combined with the dissolved sub-
stance, and that the explanation offered above is fundamentally correct.




CaCI 2 ,5.38N
Depth 1cm.



1.15 1.2 1.3 1.4 1.5

FIG. 13.

1 Cam. Inst. Wash. Pubs. 130 and 160. * Amer. Chem. Journ., 30, 193 (1903),

2 Amer. Chem. Journ., 49, 1 (1913).



58



ABSORPTION OF LIGHT BY WATER CHANGED



70
60

50
40

30

20




MgCI 2 ,4.96N
Depth 2cm.



0,9



1.2



FIG. 14.



80



70



60



50



40



30



20



10



H 2




MgCI 2 ,4.96N
Depth 1cm.



1.15



1.2



1.3
FIG. 15,



1.4



IN THE PRESENCE OF STRONGLY HYDRATED SALTS.



59



70

60

50-

40

30

20



HUO



H 2



AI 2 (S0 4 ) 3 ,I.OI7N.
Depth 2cm.



H ?




09



10



12



FIG. 16.




FIG. 17.



CHAPTER VI.

ABSORPTION SPECTRA OF A NUMBER OF SALTS AS MEASURED
BY MEANS OF THE RADIOMICROMETER.

The results tabulated and discussed in Chapters IV and V, which are con-
cerned with the energy measurements of the absorption spectra of solutions
by means of the radiomicrometer, were made by comparing the intensity of a
given source of light (after passing through the solution) with the intensity
of the same source of light after passing through an equal depth of water.
In a word, the depths of cells in each case were the same. As has already
been stated, a cell whose depth was 1 cm. was filled with the solution and
placed in the path of the beam of light and the deflection of the instrument
noted; then a cell of the same depth was filled with the solvent and interposed
in exactly the same position as the former cell, and the deflection of the
instrument again noted. Denoting the former by I and the latter by /o we
get the ratio I/Io, which represents the percentage transmission of the solu-
tion as compared with water. Such a procedure was repeated at frequent
intervals throughout the spectrum, locating a series of points through
which the transmission curves could be drawn.

Certain phenomena presented themselves throughout the course of this
investigation, which suggested a more careful study of some of the absorp-
tion bands located in the infra-red portion of the spectrum; and at the same
time it was thought advisable to map the absorption spectra of some of the
more common salts of cobalt, nickel, etc., in terms of Beer's law; since up to
the time of this investigation no satisfactory quantitative study of the
infra-red spectrum of these salts had appeared.

In order to make a careful study of the exact intensity of the various por-
tions of any given bands, it is clear that we are dealing with a much more
complex and intricate problem than simply with the location of the band;
and on this account it was necessary to improve our apparatus and at the
same time to exert more care, if possible, in carrying out any given operation.

It was early found that if we desired to study that region of the infra-red
spectrum in which water had considerable absorption, we must not compare
our solutions with an equal depth of layer of water, as noted above; but with
a depth of layer equal to the water in the solution, which in the most con-
centrated solutions was much less than the actual depth of the cell containing
the solution a part of the cell's depth being occupied by the dissolved sub-
stance. Even when such a correction was made, it was found that for a
given wave-length, in the water absorption bands, the solution gave greater
deflections than did the solvent, i. e., that in such regions the solution was
actually more transparent than an equivalent depth of water.

61



62 ABSORPTION SPECTRA OF A NUMBER OF SALTS

Remembering that the solutions with which we were then working, i. e.,
solutions of salts of neodymium and praseodymium, were strongly hydrated,
it was thought that in view of the fact that at least a part, and in the con-
centrated solutions a considerable part of the water present was there as
water of hydration, it would be advisable to study the effect of colorless
hydrated salts upon the absorption of water.

This chapter of our work has been sufficiently discussed elsewhere in this
monograph, and will be taken up here only to state that these experiments
showed clearly that there were many variables to be considered. We have,
first, the effect of the solvent on the absorption of the solute ; and, secondly,
the effect of the solute upon the absorption of the solvent. In addition to
these, there was, of course, the absorption of the solvent and the solute inde-
pendently. Such being the case, we would not be obtaining comparable
results for various dilutions of any solutions in terms of Beer's law, even if
we did compare each dilution with an equivalent amount of water. It is
clear that by so doing we would not be getting comparable ratios, since the
solvent and the solute were mutually affecting each other's absorption; and
this effect would not be the same for the different dilutions of the same salt.

MODE OF PROCEDURE.

It is, however, possible to get the exact transmission of a given depth of
solution by a method of differentiation. If we placed in cell -411 mm. of a
solution and in cell B 1 mm. of the same solution, the ratio representing the
respective deflections of the instrument, when these cells are alternately
placed in the path of the beam of light, should give the absorption or trans-
mission of (11 1) or 10 mm. of the solution.

Since, if we let A be the percentage absorption of a unit's depth of layer of
the solution, and 7 the initial intensity of the light impinging upon the sur-
face, we get

A/o = amount of light absorbed by first unit layer of the solution.
Then,

IQ IoA =/ (l A) = light incident upon surface of second unit layer.
Denoting this by 7i, we get



M)

Considering again the third unit layer, we get, by similar reasoning,

/i Ii A = amount of light incident upon its surface.
Denoting this by 7 2 , we get

I^Ii-hA =/!(!- A)

but Ji = / (l-4); therefore, 7 2 = / (l-4) 2 ; hence / =/ (l-A) n . We can
then, by this process, obtain transmissions for given depths of solution
and for varying concentrations. This was the method adopted throughout
this chapter of the work.



AS MEASURED BY MEANS OF THE RADIOMICROMETER. 63

DESCRIPTION OF CELLS USED.

In all cases where we were dealing with different depths of layer, it was
necessary to use cells adjustable in length. A very satisfactory form of cell
was devised and used throughout the latter part of this work. It consisted
essentially of two brass cylinders telescoping neatly into each other. The
external diameter of the outside cylinder was about 2| inches, and the thick-
ness of the walls was in every case about 2 mm., which was sufficient to with-
stand handling without danger of changing the shape of the cell. Into the
ends of each cylinder there was sealed, by means of Wood's metal, a glass
plate about 1 mm. thick, made of the very best optical glass. In all cases
the glass plates were so nearly parallel as to show interference fringes ; and
both cells gave the same deflections, either when empty or filled with the
same solution and placed in the path of the light before the radiomicrometer.

After adjusting the glass ends and fixing them securely by means of Wood's
metal, the entire cell was first plated with silver, being taken out of the
plating-bath from time to time and polished to a bright surface with the
finest crocus paper. On top of this silver coating a heavy plating of gold
was deposited. The distance between the glass plates fastened to the ends
of the telescoping cylinders, which determined the depth of layer of solution
used, was in all cases fixed by gold-plated washers, whose thickness had been
accurately measured to 0.001 inch by means of a vernier caliper.

Before any series of readings was made, the positions of the two cells was
so adjusted in the sliding carriage as to give equal deflections, when alter-
nately placed in the same position before the radiomicrometer, in that part
of the spectrum where neither the solute nor solvent had any absorption;
and from time to time throughout the experiment duplicate readings were
made on this point to see that the cells had not changed their relative
positions. In case any change was noted, a duplicate series of readings was
always made. Such readings upon the same cell usually agreed to about
one division of the scale, which corresponded to about 1 to 2 per cent,
depending upon the throw of the instrument. In the midst of the very
intense absorption bands,where the deflections of the instrument were small,
reaching zero at many points, the error resulting from any drift in the instru-
ment or reading of the scale was greater than the mean error given above.

In nearly all cases new solutions were made up and the results duplicated,
so that the tables and curves below represent a mean of several series of
readings. In most cases the agreement was very satisfactory, usually the
difference not being over 3 per cent.

Since any change in the position of the prism was a determining factor in
the portion of the spectrum which fell upon the thermo-j unction, and since
in the very intense, sharp bands of the neodymium salts any slight change in
the position of the prism would make a great difference in the final results,
great care had to be exerted in setting the head reading of the spectroscope.
Such difficulties were not met with in solutions where the absorption bands
were broad and diffuse, as in salts of cobalt, nickel, etc.



64 ABSORPTION SPECTRA OF A NUMBER OF SALTS

In studying the changes which might occur in any band, it is of course
necessary that all conditions be as nearly as possible the same. One of the
most important factors here is that of the width of the slits of the spectro-
scope. With those solutions whose absorption bands are broad and diffuse,
not having such well-defined edges as with the salts of neodymium and
praseodymium, this is not such a determining factor. Should the band be
very narrow say approaching that of the width of the slit necessary to be
used in order to secure reasonable deflections of the instrument it is seen
that any slight change in the slit will make a large difference in the amount
of light falling on the thermal-junction.

Considering a concrete example, let us suppose that the slit-width is just
equal to that of the absorption band, under a given dispersion. If, now, the
band and the slit exactly coincide, it is evident that no light will be falling
upon the junction, this being indicated by zero deflection of the instrument.
If, on the other hand, the slit is slightly wider than the band, some light will
enter around the edges of the band; and, though the narrow band may act-
ually have complete absorption at a given point, it would not be indicated by
the instrument, since some light is entering around the edges of the band.

Denoting the deflection of the instrument for a cell of 2 mm. depth of a
solution of x concentration by A, and the same for 1 mm. of the same solution
by Bj we get, by the differential method discussed above, the ratio A/B for
the intensity of the light transmitted by (2 1) or 1 mm. of the solution in
question.

By a similar reasoning we get the ratio A' /B r for the value of the trans-
mission of a solution of concentration , using absorbing layers 21 mm. and

1 mm., respectively. While such a method is theoretically and mathe-
matically correct for infinitely narrow slit-widths, and practically so for
bands which are wide in comparison with the necessary slit-widths, yet in
the case of very sharp, narrow neodymium bands it has been found not to
give comparable results. The reason for this is clearly seen in the light of
the facts discussed above.

Let us consider the ratios A/B and A'/B'. In the first case we are deal-
ing with concentrated solutions, where the absorption bands are broad;
hence B is small, and, in case the slit-width is comparable with the width of
the absorption band, B will be very much smaller than B', since B' is only

x
1 mm. of an concentration solution. In a word, B, which is 1 mm. of the

more concentrated solution, has 20 times the number of absorbers as has an
equal depth represented by B' } and a decrease in the denominator of the
fraction means an increase in its value.


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