Study o f Electrolytic Dissociation by the Raman Effect.

Transcription

1 279 Study o f Electrolytic Dissociation by the Raman Effect. I. Nitric Acid.* B y I. R amakrishna R ao, M.A., Ph.D., Bezwada Research Fellow, Andhra University. (Communicated by 0. W. Richardson, F.R.S. Received January 29, 1930.) [Plate 3.] 1. Introduction. In a recent discovery Ramanf found that when monochromatic light of frequency v is incident upon a substance, the scattered light contains not only light of the original frequency v hut also light of modified frequency v1? and the difference in frequency between the two corresponds to an infra-red characteristic frequency v0 of the molecules constituting the substance. Thus a new field of work has been opened up, in which the infra-red characteristic frequencies of molecules can be determined with as much precision as is possible in the visible and ultra-violet regions of the spectrum. The present investigation has shown a further possibility of the new discovery. On studying the Raman effect in solutions of nitric acid at different concentrations, the author wtas able to trace the progress in the electrolytic dissociation of the acid by measuring the changes in the intensities of the Raman lines due to the undissociated molecules and of the ions. Thus the method not only gives direct evidence of the phenomenon of dissociation, but enables the determination of its amount as accurately as is possible in the measurement of intensities of spectral lines. The only defect of the method lies in the fact that it cannot be extended to those electrolytes that do not show any Raman lines. The purpose of the present communication is to describe the results with nitric 2. Experimental Method. By far the best method of studying the Raman effect is that suggested by Woodyi; for it not only gives a greater amount of scattered light, but the exact collimation, which is important for getting the largest amount of scattered light into the spectrograph, is made much easier. In the present investigation, Wood s arrangement is used with slight modifications in the experimental * A preliminary note of this was given in Nature, vol. 124, p. 762 (1929). t Ind. J. Phys., vol. 2, p. 387 (1928). t Phil. Mag., vol. 6, p. 729 (1928).

2 280 r. R. Rao. tube containing the liquid to be studied. The form of the tube adopted is represented in fig. 1. It consists of a glass tube one end of which is bent into a horn with an opening at the top. At the other end is attached a narrower glass tube whose axis exactly coincides with the axis of the wider tube, and which has a clear Fig. 1. glass bulb blown at its end. The bulb is so made that there are no irregularities in its surface such that a distant light source is seen perfectly undistorted through it. A small glass bead is melted on to the sloping wall of the horn as nearly along the axis of the tube as possible. Leaving the central portion of the tube equal in length to the length of the mercury arc, the front portion of the bulb, and the glass bead, the whole of the apparatus is blackened with black enamel. The tube is surrounded by a water jacket made of glass for circulation of cold water. The mercury arc, specially constructed by the Hewittic Electric Company for work with the Raman effect, is placed above this tube containing the liquid and as near to it as possible. The arc is run on 200 volts at a current of 3 amperes. The horn, drawn at one end of the apparatus, enabled all background illumination to be eliminated. The narrow glass tube between the glass bulb and the wider tube prevented stray light from the walls of the apparatus from entering the spectrograph. The glass bulb not only eliminated the use of any cement for attaching a glass plate to the end of the tube in its place, but it formed a liquid lens to focus the scattered light on to the slit of the spectrograph. Thus the use of an additional lens, which absorbs part of the scattered light, is avoided. To use the spectrograph to the best advantage, the glass bead melted on to the sloping wall of the horn along the axis of the tube is essential. The scattered light being very feeble, the focussing on the slit of the spectrograph is difficult without it. The glass bead is illuminated by a diffuse electric lamp placed near it, and the spectrograph is now adjusted so that the point image of the bead as formed by the lens of the glass bulb is on the slit. The inclination of the spectrograph is so changed that, on placing the eye at the position where the spectrum is formed, the whole field of view of the camera appears illuminated. The best position of the spectrograph is that in which the intensity of the light from the bead as seen through the camera is a maximum. To get the largest amount of light from the mercury arc into the scattering substance, two aluminium reflectors are used, one above the arc and the

3 Electrolytic Dissociation by R am an Effect. 281 other below the tube containing the liquid under investigation. With Hilger s constant deviation glass spectrograph, an exposure of 3 hours with Ilford iso-zenith plates enabled even the faint Raman lines in nitric acid to be recorded clearly. For accurate measurement of the wave-lengths of the lines, a copper arc is used for the comparison spectrum and the measurements are made with a micrometer reading to 0*01 mm. The wave-lengths are calculated by the usual Hartmann s interpolation formula. It is not found necessary to distil the liquids in vacuum so long as there are oo fluorescent impurities and they are comparatively free from dust. The only effect of small traces of dust is to increase the intensity of the unmodified lines in the scattered light which come out very dark in the negative. 3. Results. Table I contains the results of measurements of the Raman hues with concentrated nitric acid (65 per cent. acid). In the first column are given the wave-lengths of the exciting mercury lines. The second column contains the wave-lengths of the Raman lines corresponding to these original lines, and their approximate relative intensities are indicated in brackets. The wavenumbers of these two sets of lines are given in columns 3 and 4. The fifth column represents in wave-numbers the differences between the exciting lines and the corresponding Raman lines, and the reciprocals of these differences giving the infra-red characteristic wave-lengths of nitric acid are given in column 6. Table I. A]ig. Ar. 1 vj{. A v. A u. A.U. A. IT. cm.-1 cm.-1 cm.-1 U (0) (0) (1) (2) (4) (12) (0) (13) (00) (00) (5) (1) to to to to r \

4 282 I. R. Rao. From spectra taken with NaN03, NH4N03, and KN03 solutions the Raman lines corresponding to the N03 ion are identified as those with wave-number differences of 638, 685 and 1046 cm.-1 corresponding to infra-red absorptions at 15'67, 14'60 and 9'56 (jl respectively. The one at 9*56 [x is the most intense of the three, both in HN03 and in the solutions of the nitrates, and of identically the same wave-length in all of them. The three lines with wavenumber differences of 3202, 3427 and the band 3439 to 3525 belong to water. Thus the rest of the Raman lines with differences 957, 1110, 1299 and 3321 cm.-1 must belong to the undissociated nitric acid molecule, as there is no other molecule present in the liquid. 4. Electrolytic Dissociation Nitric Acid. Plate 3 shows the Raman spectra taken with various concentrations of nitric acid in water. Fig. 1 in the Plate is with pure concentrated nitric acid containing 65 acid, and figs. 2 to 7 correspond to 58 5, 48, 39, 29 2, 19-5 and 9*7 concentrations of the acid respectively. Fig. 8 is the spectrum for pure redistilled water. These spectra are taken with exactly equal exposures of 3 hours each. The amount of light from the mercury arc incident on the liquid is kept constant by keeping the distance between the tube containing the solution and the mercury arc exactly the same for all of them. The aluminium reflectors are also in the same position for all the exposures. By regulating the flow of cold water through the jacket surrounding the experimental tube, the temperature of the solutions was maintained the same for all of them within a variation of 2 C. The average current through the mercury arc was 3 amperes with a variation of not more than 0 2 ampere. The voltage drop between the terminals of the arc was 140 volts with a maximum variation of 3 volts on either side. Thus, on an average the energy consumption of the arc could not have varied from one exposure to another by more than 4 or 5 per cent. Thus all the spectra are taken under identical conditions as far as practicable. Any variations in the intensities of the Raman lines must be due to the dilutions themselves and not to any other circumstances. It is clear from figs. 1 to 7 that the Raman line at 4566 *8 A.U. found in nitric acid and nitrates increases in intensity with increasing dilution of the acid, while the line at A.U., which is absent in the nitrates and which must therefore be due to the excitation of the undissociated HN03 molecule, diminishes in intensity. These changes indicate clearly that, while the number of N03~ ions, giving rise to the Raman line at A.U., increases with increasing dilution, the

5 each curve along which the number of the curve is marked represents the zero line for that curve, which corresponds to complete blackening of the photo Downloaded from on October 17, 2018 Electrolytic Dissociation by R am an Effect. 283 number of undissociated HN03 molecules, giving rise to the line at A.U., diminishes with increasing dilution of the This is clear evidence of the electrolytic dissociation of nitric To get an idea of the intensity variations with acid concentration of these two lines, the intensity records of the different spectra taken with a MolFs microphotometer are represented in fig. 2. The horizontal line at the top of F ig. 2. Microphotometric Intensity Curves of Raman Spectra, taken with diminishing concentration of nitric

6 284 I. R. Kao. graphic plate. The intensity changes indicated above in the actual spectra of the two Raman lines are very clearly demonstrated by these intensity curves. The maxima marked with downward arrows correspond to the undissociated HN03 molecule, and their intensity diminishes so rapidly that they almost disappear in the fourth curve corresponding to a concentration of 39 per cent. The maxima denoted by the upward arrows are for the N03~ lines and they very rapidly increase in intensity up to 39 concentration. They then diminish again showing that, whereas for the first few dilutions the dissociation increases much more rapidly than the diminution in concentration, resulting in a larger number of N 03_ ions, after a certain stage, the increase in dissociation becomes less in proportion to diminution in concentration of the ions. Thus the curves indicate that the intensity of the nitrate lines increases at first and then diminishes again. In the curves 3 to 5 the intensity maxima have reached very nearly the zero line. In these three curves even the continuous spectrum, which is also present in the direct mercury arc, is very prominent. The unmodified lines of the mercury arc spectrum are also very intense in these spectra, showing thereby that the classical scattering also increases very much with the dilution of the Though the author has used the same nitric acid and the same redistilled water, neither of which shows a conspicuous continuous spectrum, in the scattered light, as is seen from figs. 1 and 8 of Plate 3, this continuous spectrum increases in intensity in the same way as the Raman lines corresponding to the nitrate ion. An explanation of this obviously seems to be that the intensity of the classical scattering also increases with increasing ionisation of the But Venkatesw'aran,* working with nitric acid at various concentrations distilled in vacuo and using sunlight, found no such anomalous changes in intensity. He found that the intensity of the scattered light increased with increasing concentration of the acid without any reversal, such as is found in this investigation. With a view to finding the behaviour of other acids in this respect, the author worked with sulphuric and acetic acids also. Even in these cases, the continuous spectrum becomes more prominent with increasing dilution. In fact, in the case of sulphuric acid, the continuous spectrum is so strong that the Raman lines are completely masked by it for intermediate dilutions. Further work is necessary to find the cause of the variation of the intensity of this continuous spectrum. * Venkateswaran, cind. J. Phye., rol, 1? p. 239 (1926).

7 Electrolytic Dissociation by R am an Effect Estimation of the amount of Dissociation from the Intensities and Comparison with Conductivity Data. The relative intensities of the Raman line at A.U. in the spectra with the various concentrations of nitric acid are given in Table II. They are estimated from the deflections in the microphotometer records given in fig. 2. The intensity due to the continuous spectrum is eliminated from each of them. But the intensity of this being a large proportion (in some cases 75 per cent.) of the total intensity, the values are uncertain to a range of as much as 10 to 15 per cent. Thus the table gives the intensities of the Raman lines exclusive of that of the continuous spectrum only approximately. With elimination of the continuous spectrum, whose origin is now unknown, it may be possible to measure the intensities of the Raman lines very accurately. Table II. Relation between Concentration and Degree of Dissociation. Concentration, c. Int. I. I/c Ie5/C65 Concentration c. Molecular conductivities A in cm.-1 ohm-1. Viscosity rj in g. cm.-1 sec. 1. A /iw A y -A-65 7? * The first column in the table gives the concentration of the acid in grams per 100 grams of solution. The second column contains the relative intensity of the Raman fine at 4566* 8 A.U. If Nc is the number of free N 03~ ions present in the solution, and Ic is the intensity of the Raman line at concentration c, Nc ft. Ic, where ft is a constant, since the intensity is proportional to the number of emitters present, the emitters here being the free nitrate ions. The degree of dissociation a is given by the ratio

8 286 I. R. Rao. where Noo is the number of free ions present and I*, is the intensity of the Raman line at concentration ccorresponding to infinite dilution, since here the dissociation is supposed to be complete, the degree of dissociation being unity. But as there is no knowledge of the intensity at large dilutions in the above data, and as only the relative values of the dissociation are aimed at in this communication, the values relative to the dissociation at 65 concentration are given in column 3 of the above table. If the amount of dissociation were the same for all concentrations, the number of free nitrate ions present in the solution should diminish in proportion to dilution, and the intensity of the Raman line due to the N03 ions should also diminish in proportion. Thus the numbers in the third column, giving the ratio of the intensity to concentration, ought to be constant. But the large increase in this value shows how rapidly the dissociation of nitric acid increases with increasing dilution. The results of electrolytic conductivity by Kohlrausch are given in the other columns of Table II. The fourth gives the concentration in grams per 100 grams of solution. The molecular conductivities A* and the viscosities v)f of the solutions at each concentration are given in the next two columns. If Ac is the molecular conductivity at concentration c, and A*, that at infinite dilution, the degree of dissociation according to Kohlrausch s law, that the mobility of ions is independent of concentration, is given by Ac/ A*,. Even at small concentrations this law is supposed not to hold good. So its applicability at higher concentrations such as those used in this work is still less. Another formula for the degree of dissociation at moderate concentrations in terms of the molecular conductivity is a Ac. YU A a,. 7)0 it where yjcand 7)0 are the viscosities of the solutions at concentrations c and 0 respectively. In Table II the degree of dissociation calculated according to the two above formulae are given in columns 7 and 8. Fig. 3 gives the curves indicating the relation between the concentration and the relative amount of dissociation. The steeper curve is from the intensity measurements of the Raman line for the nitrate ion. The middle curve is that according to Kohlrausch s formula and the lowest corresponds to the * Landolt and others, Physikalisch-Chemisch Tabellen, 5th ed., vol. 2, p t Ditto, First Supplement, p. 86. The values given in the table are extrapolated from the actual values.

9 Ramakrishna Roy. Soc. Proc., A, 127, P Fig. 8. Water. F ig Fig *5 Fig F ig Fig Fig *5 Fig co QC co CO CO lo T* (Facing p. 286.)

10 Electrolytic Dissociation by R am an Effect. 287 Downloaded from on October 17, 2018 Concentration % Fig. 3. Relation between Concentration of Nitric Acid and Degree of Dissociation. The lowest is from viscosity-conductivity formula. The middle curve from Kohlrausch s conductivity formula and the uppermost from measurements of Raman lines for the nitrate ion. dissociation calculated from the viscosity formula. the three is very striking. 6. Discussion. The disagreement between To investigate the cause of the discrepancy it is necessary to see how far the various methods give the actual degree of dissociation. Of these methods one is from osmotic pressure measurements. The osmotic pressure can be either directly determined or calculated from the lowering of freezing point of the solutions. The second and more common method is that from conductivity measurements. There are several other methods which are more indirect. All these are known to be inapplicable for determination of the degree of dissociation at high concentrations. Even at low concentrations there is no knowledge as to how far the values calculated represent the socalled true degree of dissociation, which is the ratio of the number of v o l. c x x v n. A. u