T H E ABSORPTION OF ULTRA-VIOLET LIGHT BY INORGANIC HALIDES BY FREDERICK H . GETMa44N
The first systematic investigation of the absorption of ultra-violet light by inorganic compounds was undertaken by Hartleyl in 1902. He studied the behavior of a series of aqueous solutions of metallic nitrates and found, in every case, a characteristic absorption band in the ultra-violet, the position of which was shifted toward the less refrangible end of the spectrum as the atomic weight of the combined metal increased. Subsequently, Drossbach2 showed that ultra-violet light is strongly absorbed by solutions of the heavy metals, and emphasized the fact that atomic weight is a factor of relatively small importance in connection with the absorption of ultra-violet radiation by solutions of metallic salts. Thus, he pointed out that while the atomic weight of thorium is more than four times that of iron, the former is non-absorptive while the latter absorbs strongly. In 1912, Crymble3, in an interesting paper, directed attention to the fact that, although the selective absorption of solutions of certain metallic salts had been made the subject of extensive investigations, comparatively little attention had been paid to the phenomenon of general absorption. As the result of his studies he concluded that the metallic elements may be divided into two groups as follows : (a) those of constant valence whose salts in aqueous solution transmit ultraviolet radiation and, (b) those of variable valence whose salts in aqueous solution absorb ultra-violet radiation. Furthermore, he pointed out that if the metals are arranged in the order of their electrode potentials, the more electropositive metals are found to be of constant valence and non-absorptive. It is of interest to note that Crymble states that “the metals which yield non-absorptive salts are: Li, Na, K, Rb, Cs, G1, Mg, Csh, Ba, Sr, Zn, Cd, Th, A1 and Zr,” and in another place he points out, that he chose the chlorides of the metals for his investigation because of the fact that the chloride radicle is %on-absorptiue. ” A quantitative study of the absorption of ultra-violet radiation by salts was undertaken by Brannigan and Macbeth4 with a view to establishing, if possible, a relation between the extinction coefficients and the atomic weights of the constituent elements of salts of the same family. In this investigation, a Hilger rotating sector photometer was used in conjunction with a quartz spectrogra,ph, in order to detect and measure minute differences between the absorbing power of solution and solvent. The extinction coefficients of very concentrated aqueous solutions of HC1, LiC1, NaC1, RbCl, XaBr, KBr, IYH4Br (
Ilartley: J. Chem. Soc. 81, 556 (1902); 83, 221 (1903). Ber. 35, 91 (1902). Crymble: J. Chem. Soc., 101, 266 (1912). Brannigan and Macbeth: J. Chem. Soc. 109, I 2 7 7 (1916).
* Drossbach:
854
FREDERICK H. GETMAN
LiI, NaI, K I , RbI and CsI were determined and, contrary to the statements hitherto published, that these compounds are diactinic, each was found to exert well-defined absorbing power. It was further shown, that the frequency of the absorption bands decreases with increase of atomic weight of the halogen, and that the extinction coefficient of the alkali chlorides and iodides increases with increase of the atomic weight of the combined metal. In a later communication by Macbeth and Maxwell1, the results of similar studies on the chlorides of arsenic, antimony and bismuth are reported. While the values of the molecular extinction coefficients of the alkali halides were found to lie between 0.05 and 0.5, the corresponding values of the molecular extinction coefficients of the chlorides of arsenic, antimony and bismuth were found to range from 5 to I 2 , 0 0 0 . The optimum concentration for the measurement of the absorption of the alkali halides was 4-molal, while in the case of the chlorides of the arsenic group, the concentration ranged from 0.0001-to 0.01-mola1. Furthermore, bismuth chloride was the only one of the three to exhibit selective absorption, the head of the absorption band corresponding to a wave-length of 323 p p instead of 273 pp as in the case of the alkali halides. The authors point out that the values of the molecular extinction coefficients in the arsenic group increase with increase in the atomic weight of the combined metal. Recent quantitative measurements 0f the absorption of ultra-violet radiation by salts have been made by H. von Halban2and his pupils, employing a photoelectric cell. The method, it is claimed, is capable of a very high degree of accuracy, even for solutions of extremely low absorptive power. The results of these investigations are in accord with the theory that the ionic properties of solutions of strong electrolytes are to be explained in terms of the electrostatic forces between the ions and, since these forces exert a varying influence on the properties of the ions, it follows that there is not, of necessity, a parallelism between light absorption, electrical conductance or ionic activity. The present investigation was undertaken with a view to extending our knowledge of the quantitative absorption of ultra-violet radiation by simple inorganic salts in the hope that, with the accumulation of sufficient reliable data, it may be possible to establish some relationship between absorbing power and other physical properties of the elements.
Apparatus In order to determine extinction coefficients with a high degree of accuracy, a new type of sector photometer developed by Judd Lewis was employed in conjunction with a Hilger quartz spectrograph (Size E 3) fitted with a wavelength scale and giving a spectrum approximately zoo mm. in length, ranging from 800 p p to 210 pp. The outstanding features of this sector photometer, which has already been described in detail by its inventor3,are shown diagram1
Macbeth and Maxwell: J . Chem. SOC.,123, 370 (1923).
H. T T . Halban and Geigel: Z. physik. Chem., 96, 214 (1920); H. v. Halban and Siedentopf: 100, 208 (1922); H. v. Halban and Ebert: 112, 321 (1924). Judd Lewis: J. Chem. Soc., 115, 312 (1919). 2
ABSORPTION O F ULTRA-VIOLET LIGHT BY HALIDES
856
FREDERICK H . GEThlAN
matically in Fig. I . Light from the source Q is rendered parallel by the two quartz lenses L1, Lz,and then enters the two fluorite rhombs P1, Pz. The upper beam is reflected from the face, alcl, to the opposite face, bldl, whence it is reflected as a parallel beam through the sector DI, and the observation cell, W, to the lens Ls, which brings the light to a focus on the slit of the spectroscope, after it has traversed the reflecting rhomb, Ps, and the Albrecht rhomb, R. The lower beam of light traverses an exactly similar path and is brought to a focus in close juxtaposition to the first beam on the slit of the spectroscope. The cell, W, contains the solution and a similar cell, not shown in the diagram, similarly placed in the path of the lower beam contains the solvent. In this manner the effect of the solvent is eliminated and we measure only such differences as may be caused by the solute alone. By adjustment of the lower sector, Dz, the intensity of the light in the path containing the solvent may be diminished by successive steps and the resulting series of spectra compared directly with the absorption spectrum produced by the solute. The general method of procedure in making a quantitative determination of the absorbing power of a solution is briefly as follows: Several pairs of spectra are photographed on the same plate after which the plate is developed and then measured. In each of these pairs of spectra, the intensity of the lower spectrum, measured in terms of the intensity of the incident light, is indicated by the position of the index on the graduated quadrant, Gz. In each pair of spectra, the upper spectrum is that produced by the absorbing medium and, therefore, the aperture remains fixed throughout the entire experiment a t oo on the quadrant, GI. The wave-length a t which the two members of each pair of spectra have equal densities is then noted under a magnifying lens, whence the unknown intensity in the absorption spectrum is measured in terms of the known intensity of the normal spectrum. One of the most distinctive features of the Judd Lewis photometer is the unique construction of the sectors, but for these details reference must be made to the original description given by the inventor. The outstanding advantages of this type of sector photometer are, (a) uniformity of illumination, (b) utilization of the entire aperture, (c) avoidance of the necessity of calibrating photographic plates, and (d) avoidance of the necessity of supplementary rotating mechanism and (e) increase in precision in setting for small apertures. As a source of light, a condensed spark between nickel-steel electrodes was employed, the apparatus being arranged as shown in Fig. 2 . By means of a 0.25 K.W. transformer, T, the primary of which was fed by a I I O volt, 60 cycle alternating current, a secondary voltage of 6000 volts was obtained. The current from the secondary charged the condenser, C, (capacity 0.03 m.€.) which in turn discharged across the spark gap between the nickel-steel electrodes at E. A supplementary spark gap, G, was also included in the circuit. Special attention was given to the form of the terminals of the electrodes. The end of each electrode was first ground to a chisel-shaped edge and then the sharp corners were rounded with a file. The electrodes were mounted in a stand which was so arranged that the position of the spark-gap could be adjusted
ABSORPTION O F CLTRA-VIOLET LIGHT BY HALIDES
85 7
both vertically and horizontally. Special precautions were taken in the final adjustment of the source of light to insure that the prolongation of the optical axis of the photometer should lie in the plane of the two parallel chisel-shaped edges of the electrodes. The time of exposure was determined by means of the formula,
tl Exposure = A -
t
A
where the ratio, tl/t, is the number corresponding to the sector reading, log tl/t, and where A is a constant. In the majority of cases, satisfactory results were obtained with A = I O seconds. All spectrograms were made on special Wratten and Wainwright, I O ” X 4 ” panchromatic plates. Preparation of Solutions
All of the salts used in this investigation were prepared by recrystallization of the puresl samples obtainable from reliable manufacturers, and conductivity water was used as a solvent in making up the solutions. The concentration of each mother solution was determined analytically, and dilution to the desired strength was effected by means of calibrated volumetric apparatus.
c
Method of Experiment Having adjusted the position of the source of light so that the upper and lower halves of its spectrum were uniform, and having ascertained that the tilt of the slideholder of the camera of the spectrograph was such as to insure correct registration of the wave-length scale, a series of preliminary exposures with varying concentrations of the solution under examination were made, in order to determine the proper thickness and FIG. 2 concentration to be used in subsequent experiments. When the most suitable concentration and tube-length had been found, a series of exposures was then made. Having set the indices of the two quadrants a t o’, an exposure, known as the (‘test-band,” was first made in order to verify the results of the previous experiment as to the uniformity of the juxtaposed spectra. The two tubes containing the solution and solvent were now placed in their respective troughs, and a second exposure,
FREDERICK H. GETMAN
858
with the indices still at o’, was made. This pair of spectra, known as the “comparison band,” shows the relative absorbing powers of solution and solvent. A series of exposures was now made with the sector in the path of the solvent set a t successively smaller apertures, while the sector in the path of the solution remained fixed a t 0 ’ . In this manner, a series of from twelve to fifteen spectra, 5 mm. apart, were obtained on a single plate. In every case the last exposure was similar to the first, a second test-band being made to detect any slight change in adjustment of the a,pparatus during the experiment. After development, the plate was examined with a magnifying lens and the wavelengths a t which the two members of each pair of spectra were found to have equal densities was noted, and plotted as abscissas against the corresponding values of the molecular extinction coefficients as ordinates. The value of the molecular extinction coefficient is calculated by means of the formula, lm where I and I, denote the intensities of the transmitted and incident radiation respectively, and where 1 and m represent the thickness and concentration of the absorbing medium respectively. As has already been stated, the values of the ratio, log I/Io, are read directly from the graduated quadrant attached to the sectors.
Experimental Data The data of the following tables is compiled from a series of measurements of the absorption coefficients of a number of simple inorganic salts. The wavelength, expressed in millimicrons, is denoted by X and the value of the molecular extinction coefficient by M.
TABLE I Calcium Chloride Concentration 3.613m, I O cm. tube
X
M
TABLE I1 Strontium Chloride Concentration ~ . 4 8 6 m4cm. , tube
0.003
318
0.01
0.006
310
0.02
0.008
305 299 29j
0.03 0.04
0.01 I
0.014 0.017 0.019
246
0.022
292 289 288
254 256
0.025
287
257
0.028
283 279
259 266
0.030
250
x
M
242 420 239 2\36 233 229
0.05
315 300 290 280 278
257
261
235
270
234
ABSORPTION O F ULTRA-VIOLET LIGHT BY HALIDES
859
TABLE I11
TABLE IV
Barium Chloride Concentration 1.171111, Iocm. tube M x
Magnesium Chloride Concentration 2.955m, 4cm. tube
x
M
0.021
313
0.01
0.043 0.064
295 284
0.02
0.085 0.107 0.128
277
0.04
281 279
270
0.05
277
288 283
0.03
256 262 265
233 229 226
268
225
271
223
265
TABLE V
TABLE VI
Zinc Chloride Concentrat'ion 2.254m, zcm, tube M
Cadmium Chloride Concentration 0.421m, 4cm. tube
x
0.04 0.06 0.08 0.09
x
M
-
310 303
0~06 0.12
365 318
298 296
0.18
273
0.24 0.30
264 260
0.36
259
248
238 232 229 228 227
0.10
293
0.11
290
0.12
287
254 258 260
0.13
281
265
TABLE VI1 Mercuric Chloride Concentration o.oIm, rocm. tube M
x
TABLE VI11 Cupric Chloride Concentration 0.145m, 4cm. tube M
x
I
270
0.17
372
2
268
0.35
3
0.52
4
267 266
5 6
265 264
365 360 355 359 350
0.69 0.86 1.04
860
FREDERICK H. GETMAN
TABLE IX Aluminium Chloride Concentration o.9m, 4cm. tube
x
M 0,139 0.167 0.195 0.232
283 280 2 79 278
0.250
2 78
0.278 0.306 0.333 0.361 0.389
277
276 276
241 247 2 48 249 251
275
252
274
253
231
228 227
226 225
The foregoing experimental data, together with that of Brannigan and Macbeth' for HCI, KaCl, LiCl and RbCI, is represented graphically in Figs. 3, 4 and 5 .
Discussion of Results An examination of the curves reveals well-defined selective absorption in solutions of the chlorides of hydrogen, sodium, lithium, rubidium, calcium, strontium, magnesium, zinc and aluminium, while solutions of the chlorides of barium, cadmium, copper and mercury show marked general absorption. The wave-length corresponding to the head of the absorption band of each salt exhibiting selective action is found to be approximately 2 7 3 1 1 . The persistence of this characteristic band at approximately the same wave-length throughout the series suggests that it may be attributed to the one constituent which is common to all of the salts, viz. the chlorine ion. If the chlorine ion is responsible for this characteristic band, it is apparent that the depth of the band should increase as the number of chlorine ions is increased In order to decide this question, a nearly saturated solution of calcium chloride was prepared, and a portion diluted to one-third the concentration of the original solution ; the concentrations of these solutions were determined and found to be respectively, 5.392m and 1.864m. Spectrograms of these two solutions were then made, a 2 em. tube being filled with the concentrated and a 6 cni. tube with the dilute solution. In this manner, the same total mass of absorbent was interposed in the path of the beam of light, the only difference in the condition of the solute in the two cases being a difference in degree of ionization. If absorption is due to the chlorine ion, the effect should be more pronounced in the more dilute solution. An examination of the results obtained, as shown by the experimental data of Table X and the curves of Fig. 6, proves that the more concentrated solution exerts the greater absorbing action. l!oc.
cit.
ABSORPTION O F ULTRA-VIOLET LIGHT BY HALIDES
861
N
0
0
862
FREDERICK €1. GETMAN
TABLE X Concentration 5.592m,
2
M 0.018
305
0.022
291 287 283 280
263
279 277
0.027
0.031 0.036 0.040 0.045
Calcium Chloride cm. tube Concentration 1.864m, 6 cm. tube
x
x
M 0.009 0.013
300
0.018
284
0.022
281 278 275 273
7
242
0.02
26.5
232
275
225
0.031 0.036
0
WL
250 I
FIG.5 A Rubidium chloride B Zinc chloride
C D E F
Aluminium chloride Cadmium chloride Mercuric chloride Cupric chloride
291
200
260
265 273
235 233 230
ABSORPTION O F ULTRA-VIOLET LIGHT BY HALIDES
863
Not only does this experiment make it appear unlikely that the absorption bands are to be ascribed to free chlorine ions, but also it shows conclusively that calcium chloride does not conform to Beer’s law. According to this law, the absorption exerted by a solute is independent of theconcentration, provided that the tube-length be altered in such a manner as to maintain the product of the two factors, concentration and tube-length, invariant.
After having determined the absorption of ultra-violet radiation by azobenzene over a wide range of concentration in tubes of different lengths, Macbeth and Maxwell’ concluded that “the molecular extinction coefficient remains the same for a particular wave-length, whether a concentrated or a dilute solution is employed.” Having found that solutions of azobenzene conform to Beer’s law, they then assume that the law is equally valid for solutions of metallic chlorides. The foregoing experiment with calcium chloride, however, proves this assumption to be wholly unwarranted and leads us to suspect that Beer’s law does not apply to solutions of strongly polar compounds. J. Chem. SOC.123, 370 (1923).
864
FREDERICK H.GETMAN
The molecular extinction coefficient of the metallic chlorides apparently increases with increase in the atomic weight of the metal. Those chlorides which do not absorb ultra-violet radiation selectively nevertheless exhibit marked general absorption. While barium chloride failed to reveal any tendency toward selective absorption within the range of concentration studied, it is possible that in more dilute solutions it may be found to conform to the behavior of its congeners, calcium and strontium. From the foregoing statements it is appasent that the generalization put forward by Crymblel, that only the salts of metals of variable valence absorb ultra-violet light, has been disproven. Without exception, every metallic chloride thus far examined exhibits either selective or general absorption. The degree of absorbing power varies greatly among the chlorides investigated; for example it is difficult to understand why 4m NaCl should absorb nearly four times more strongly than I zm HCI. At this time it would be premature to venture any explanation of this and other anomalies presented by the experimental data thus far accumulated. It is hoped, however, that ultimately it may be possible, after other groups of salts have been examined, to establish some connection between absorption and atomic or molecular constitution.
Summary The results of this investigation may be briefly summarized as follows: (I) The chlorides of calcium, strontium, magnesium, zinc and aluminium have been found to absorb ultra-violet radiation selectively, the wave-length corresponding to the head of the absorption band of each salt being approximately z73pp. This is in conformity with the results previously obtained by Brannigan and Macbeth in their investigation of the absorption of ultra-violet radiation by the chlorides of hydrogen, sodium, lithium and rubidium. (2) It has been shown that the persistence of this characteristic absorption band throughout a series of chlorides cannot be attributed to the presence of the common ion, chlorine. (3) It has been proven that solutions of calcium chloride do not follow Beer’s law. From this fact it appears doubtful whether the law applies strictly to any strongly polar compounds. (4) The molecular extinction coefficients of the metallic chlorides increase with the atomic weight of the combined metal. (5) While certain metallic chlorides do not exert selective absorption, they, nevertheless, exert marked general absorption. Hillside Laboratory, Stamford, Conn.
J. Chem. SOC.101, 266 ( ~ g r z ) .
