T H E FARADAY-EFFECT O F SOME UNI-UNIVALENT ELECTROLYTES I N AQUEOUS SOLUTIONS. I BY E . BUCH ANDERSEN AND R. W. ASMUSSEN
The magnetic rotation of the plane of polarisation is, in analogy with refractive index, specific heat, etc., to a certain extent an additive property so that a n atom, which enters a molecule, approximately always gives the same contribution to the molecule’s magnetic rotation, independently of the remaining components of the molecule, but often dependent on the kind of link which is formed. As will be known from Perkin’s investigations on organic substances, the rotation is sometimes suitable for the settling of problems of constitution. Whilst the experimental material with respert to organic substances may be considered tolerably complete, this in no way applies to the inorganic compounds. The inorganic substances, whose magnetic rotation has been measured, give the impression of having been chosen by chance or investigated from some special physical point of view.’ A systematic experimental material on inorganic compounds does not exist. From the measurements already published it can be seen that the rotation of salts in solutions is approximately determined additively as the sum of the ions’ rotation. A mixture of two salts in solution can give a rotation, which is the sum of those of the components. But the constitution has a n effect here also, since, if complex salts are formed, the result will be quite different. It would, therefore, seem possible also within the bounds of inorganic chemistry to use this property for the settling of questions of constitution in the chemistry of complex salts. But, furthermore, the measurement of the magnetic rotation gives information (so far in very implicit form), which can be expected to be valuable for the theoretical understanding of molecular structure. The magnetic rotation of the plane of polarisation is a dispersion phenomenon, a kind of Zeeman effect. And just as the investigation of the Zeeman effect on a spectral line from a n atom in a gas gives information about the nature of the stationary states of the atom, between which the transition takes place, so the Faradayeffect in connection with knowledge of the substance’s absorption spectrum and dispersion may be a n important aid in the study of molecular stationary states. We have tried in this work to procure some systematical material for a series of simple substances. The investigation covers different compounds of the cations H, Li, Na, Rb, Cs and NHI with the anions F, C1, Br, I, OH, ClO,, BrOs and 1 0 3 . These ions are all diamagnetic in their normal state (more complicated relations supervene with paramagnetic substances, Ladenburg*) and,
* Only the work of Jahn (Ann. Physik, 43, 280 (1891)isof more surveying character as far as the inorganic material of his paper is considered.‘ Ja% measured a few salts of Li, Na, K, Sr, Cd and Mn, the results being in good general agreement with the correaponding measurements of ours. * Z. Physik, 46, 168 (1927).
2820
E . BUCH ANDERSEN AND R . W. ASMUSSEN
since all measurements are made with light from the visible spectrum, they have been carried out in a part of the spectrum far from any absorption bands. We have not had any precision apparatus at our disposal. Our figures are, therefore, to be considered as a survey material and, in consequence of the whole position, intended more for drawing comparisons between these ions mutually than for a profound investigation of the properties of the individual ion. The rotation of the investigated solutions has been measured for light of wave length 5 4 6 and ~ ~ with water as standard substance. We have followed the common practice and given as results the “Verdet-constant,” (V) of the solutions and the “molecular rotat,ion” (XI) of the dissolved substances. %.Iis given with water’s molecular rotation under the same conditions as unity. The absolute values for T’ have been calculated from Verdet’s constant for mater, which according to Rodger and Watson’ for Xa-light’ is:
V,,uter= ’0.01311 -
0.06
4.t -
0.07
4.t2.
V is here expressed in minutes of arc and t is a temperature between 4’ and As is to be expected Vis for diamagnet,icsubstances only in a slight degree dependent on the temperature. According to our measurements with water a t o°C, J 7 ~ * ~ p=p 1.183.V5s9,,~. I n order to calculate the molecular rot,ation of a dissolved substance from the measured rotation of the solution it is assumed that the dissolved substance and the solvent act independently of each other, and that the measured rot>ation is composed additively from the rotation of these two. This assumption leads to the following expression for the molecular rotation of the dissolved substance (with water’s molecular rotation equal to I ) > when the dissolving medium is water: 98’.
where AI is the molecular rotation of the dissolved substances (Xiatrr = I), with the same layer Di is the measured angle of rotation for water Dz is the measured angle of rotation for the solution thickness, temperatuie and magnetic field ml is the molecular weight of water, m 2 is the molecular weight of the dissolved substance, p is the number of gram molecules of water per gram molecule of dissolved subqtance, d is the densiiy of the solution (water = I) a t the temperature a t which the rotation angle is mcasured It is to be expected t h a t the assumption mentioned above will not hold in epery caw and it also tuins out that the molerular rotations vary with the concentration of the solutions, sometimes even rather greatly
1
1
Z. physik. Chem., 19, 357 (1896).
FARADAY EFFECT OF ELECTROLYTES
282 I
Experimental Technique The apparatus which we used was built up of units procured from a Laurent polarimeter. The polarizing Nicol was fixed in such a way that it could be easily moved and replaced so as to permit the tube to be placed inside the solenoid. When the Nicols were set for complete extinction one half of the field of view transmitted a slight amount of light. The reason for this was that the Laurent polariscope originally had been constructed for the use of sodium light, whereas we used the green mercury line. It was, however, very easy to decide when the two halves of the field of view were of equal colourintensity. The polarisation tube was 40 cm long (volume ca. 35 cm3). It was in nearly the whole of its length (ca. 38 cm) surrounded by a solenoid of copper wire (diameter of wire 1.5 mm). The total copper weight amounted to ca. 2 0 kg, and the resistance of the solenoid a t room temperature was ca. 7 ohm. To obtain fairly large rotation angles for measurement, currents of 20-23 amps. were used, which correspond to effects of 3000-3700 watts. This naturally caused a rapid heating of the solenoid. In view of the rotations-dependence upon temperature special precautions must be adopted to preserve the solutions a t a constant and well-defined temperature during the measurements. The polarisation tube was, for this purpose, surrounded by a jacket with water and chopped ice, since we found, that, under the given conditions, this was the simplest and most effective method of preventing a rise in temperature in the solutions under investigation. All our measurements are therefore taken a t o°C. To prevent condensation of water vapour a stream of dry air was sometimes during the measurements blown over that end of the polarisation tube, which turned towards the observer. As a source of light a quartz mercury lamp was used and the desired line was isolated from the mercury spectrum by means of a Wratten filter No. 77a (special green Hg-line) and a liquid filter consisting of a solution of 18 g didymium chloride in 50 cm3 of water in a thickness of I cm. The solutions under investigation were produced from the purest possible materials (eventually after thorough purifying of these) and their concentration determined by analysis (at least two determinations for each solution). The density a t oo was measured by weighing out in a calibrated pycnometer (ca. 1 5 cm3). The optical measurements were made in the following manner. The polarisation tube with the solution was set aside to cool with ice for ca. I hour, while care was taken that an abundant quantity of ice was continually present. Thereafter, together with an abundant supply of ice, it was placed in the polarisation apparatus. The current (and with it the strength of the magnetic field) altered its value somewhat rapidly during the measurements, and it was found to be difficult to keep the current constant by means of a variable resistance. We therefore decided to let one observer undertake the adjustment of the optical apparatus, while the other at a given signal read the current in the solenoid on a precision amperemeter. Each solution was measured four
2822
E. BUCH ANDERSEN AND R. W. ASMUSSEN
times all together (twice by each observer); similarly the zero position of the apparatus was determined twice by each observer before and after every such set of measurements. The rotation angles were first corrected for the rotation of the empty tube (6')' then reduced to the same current, and finally the four single determinations were brought to an average, which is given as result in the table. The measured rotation angles lay between 18' and 70'. The polarisation apparatus was provided with nonius which allowed the angle to be read with an accuracy of 1'. The alteration of the field strength caused, however, a considerably larger error. For the sake of control, we have repeated the measurements with a number of solutions, and have every time been able to reproduce the results with some few per mille deviations. The error in the measured angles will in general scarcely exceed %Toof the value. An examination of the significance of the remaining sources of error shows that the uncertainty of the final M-values should not exceed 2%.
Results All our measurements are, as mentioned, carried out at 0%. Since the rotation of water has been used as unity, we have measured the effect for this substance several times, evenly distributed over the whole period, Table I. We have therewith aimed partly at getting a good determination of this constant and partly at controlling the working conditions.
TABLE I Water's rotation at O'C. X
=
5 4 6 ~ ~
20.76'
20.72'
20.68'
20.71'
20.69'
20.76' 20.74'
20.63' 20.76'
20.57' 20.70~
o.020'
All these rotation angles are reduced to an arbitrarily chosen but fixed current (i = IOO scale divisions on the amperemeter, corresponding to ca. 19 amps.). The current was as a rule greater during the measurements. From the measurements with water it can be calculated that the reduced rotation angles correspond to a magnetic potential difference of ca. 80,000 Gauss. cm between the ends of the tube or an average field strength of ca. 2 , 0 0 0 Gauss along the tube. Table I1 gives the results of our measurements. The letters used have the same meaning as mentioned before. IT is the Verdet-constant in minutes of arc. The column "To" gives the solution's percentage contents of the stated substance. The angles are reduced to the same current as for water.
FARADAY E F F E C T O F ELECTROLYTES
2823
TABLE I1 Magnetic Rotation of the Polarisation Plane. o°C. X = 546pp M V D?/DI m Substance P d % 1.44 58.I O KF 45.96 3,791 1.3815 I.0304 o ,0160 21.46 11.80 1.2034 I ,0242 0.0159 0.99 KF 4.51 HC1 36.47 16.22 10.46 1.0826 I . 2976 0 . 0 2 0 1 7.612 24.57 1.0402 I . 1386 0.0177 4.55 HC1 LiCl 42.40 29.31 5.675 1.2403 1.5425 0.0239 4.31 15.69 1.0785 1.2198 o ,0189 4.72 LiCl '3 .os ~.IG 9.526 1.2040 I ,3845 0.0215 NaCl 58.46 25.41 12.80 22.10 1.0992 I . 1845 0.0184 5.22 NaCl KCl 74.56 21.24 15.35 1.1480 I .2227 o ,0190 5.41 KC1 15.34 22.84 1.1059 I. 1643 o ,0181 5 . 5 7 6.09 120.96 30.17 15.54 1.2833 I.2469 0.0193 RbCl 24.13 1.1846 I . I527 0.0179 5.89 RbCl 21.77 168.26 59.62 6.325 1.7425 1.5324 0.0238 7.45 CSCl CSCl 36.27 16.41 1.3779 I .2541 0.0195 7.03 14.06 57.01 1.1206 I ,0816 o ,0168 6.51 CSCl 6.31 NHiCl 53.50 19.54 12.25 1.0596 I.2918 0.0200 NHiCl 5.79 12.54 20.71 1.0407 I . 1807 0.0183 8.63 HBr 80.93 43.41 5.855 1.4327 2 ,0053 0.0311 8.85 20.60 17.31 1.1629 1.3952 0.0216 HBr 86.86 45.27 5.828 1.4663 2 ,0043 0.0311 LiBr 8.73 LiBr 23.70 15.52 1.2029 1.4435 0.0224 8.89 NaBr 102.92 40.06 8.543 1.4278 1 7773 0.0276 9.21 NaBr 9.I4 25.58 16.62 1.2407 I ,4309 0,0222 119.02 33.32 13.23 1.3033 I ,4961 0.0232 KBr 9.54 1.2058 1 ,3304 0,0206 9.42 KBr 24.30 20.58 165.42 45.92 10.81 1.4907 1.5599 0.0242 1 0 . 1 1 RbBr RbBr 30.94 20.48 1.2882 1.3415 0.0208 10.41 NH4Br 97.96 26.79 14.86 1.1749 1 ,4546 0.0226 I O .27 21.68 1.1244 1,3285 o ,0206 10.37 20.05 NH4Br LiI * 133.86 23.04 24.82 1.2009 I .6092 0.0220 18.40 NaI 19.41 5.325 1.8532 3.3585 0.0521 149.92 60.98 NaI 34.88 15.53 1.3624 I ,9691 0.0305 18.95 166.02 5 1 . 5 1 KI 8.674 1.5799 2.4855 0.0386 '9.48 KI 39.75 13.96 1.3987 1.9797 0.0307 19.85 6.555 1.5166 2.8145 0.0437 2 0 . 5 5 "41 144.96 55.10 "41 35.13 14.86 1.2793 I ,9488 0.0302 20.04 40.01 12.98 14.88 1.1613 1 . I754 0.0182 2.43 NaOH 56.11 14.78 17.96 1.1474 I . 1275 0.0175 KOH 2.75 NHaOH 35 .os IO.25 17.03 0.9790 1.0333 0.0160 3 .oo 106.46 43.26 NaClOa 7 . 7 5 7 7.3771 I. 1188 0.0174 3.35 NaClOa 23.91 18.80 1.1941 I ,0580 0.0164 3 .IO 6.21 150.92 19.86 33.78 1.1918 I . I304 0.0175 NaBrOa 181.86 28.03 25.90 1.3034 1.3077 0.0203 10.22 LiIOa * As it was im oseible to obtain a colourlesa solution of LiI this substance has been measured with &ow light (A = 578 w, DI = 18.027~f o . o I ~ " ) .
-
I
-
-
-
E. BUCH ASDERSEN AKD R.
2824
a.ASMUSSES
The solutions of sodium and potassium hydroxide were prepared from Merck’s reagents in tablets, which should have a specially small content of carbonate. The solutions were analysed immediately after the optical measurements and the stated results give the total amount of alkali calculated as hydroxide. Special investigations showed that the carbonate content in the two solutions was less than 2y0 of the amount of hydroxide. We have not made any correction for this, since the carbonate-ion and thc hydroxyl-ion have almost the same rotation.
20
/4 15
/U
5
, i L L _ 1 1 _ _ _ 1 _ 1 *
4
14 /-
2u
c/
34
4u
8P
54
6u
2
FIG. I
The LiIOs-solution gave after preparation a slight diment (probably a basic salt). For this reason w e determined the content of both Li and IOa in the solution. The Li-content was found to be a little too low in proportion to the amount of iodate ( 2 % of the value). The concentration given in the table refers itself to the amount of iodate, and in the calculation of 31 we havc also followed this, since the rotation of the Li-ion compared with that of the IOa-ion is very slight. All substances, investigated in solutions of different concentration, show that 31 varies with the concentration (in many cases, however, within the error of measurement). The variation is specially large for KF. In Fig. I t,heresults are plotted graphically. Ordinates are the compound’s molecular magnetic rotation for green light in solutions lr-ith p equal t o I j - 2 j , abscissa the anion’s atomic number. It is evident from the figure that the rotation of these compounds can approximately be composed additively by the rotation of the two ions. Further it may be seen that a change of the anion with the same cation gives a large change in the rotation, while change of the cation with the same anion gives only a slight variation in AI. The molecular magnetic rotation of the individual ions can only be computed if the value for one of them is arbitrarily fixed. We can regard the rotation of the hydrogen ion equal to 0 . (This would be strictly true if the
FARADAY EFFECT OF ELECTROLYTES
2825
hydrogen ion in solutions was identical with a hydrogen nucleus). On this assumption the following “ionic rotations” may be computed from our measurements.
TABLE I11 M
Cation
M
Anion
H
0 .oo
F
0.39
Li Na
0.11
c1 Br
4.43 8.97
K
0.60
I
Rb
1.45
cs
OH
2.08
”4
I
c103 BrOs
0.13
.oo
19.04 2.15
I03
“0
2u
JU
1.76 4.77 10.12
M
50
l5U
FIG.2 This table is shown graphically in Fig. 2. The curves correspond completely in form and position to the analogous curves for ionic volumina and refraction. The additive relations are, however, only approximately valid. A test shows that deviations of systematic character appear between the observed values for M and those calculated by means of the table. It must be expected that exact additive relations of this kind only exist for very dilute solutions. Our figures give some good examples of how the rotation of an individual atom depends upon the kind of chemical linkage. In Table IV is stated how much the molecular rotation of halogen compounds is increased when an atom of chlorine is substituted with bromine or an atom of bromine with iodine. The figures for halides and oxyacids are taken from our own measurements, the figures for organic compounds from earlier determinations of Perkin and others.
2826
E. BUCH ANDERSEN AND R . W. ASMUSSEN
TABLE IV Br - C1 J - Br
MX 1.53 10.07
IVlXOI 3 .OI 5.35
Org. Comp. I
.83
4.20
We shall not here attempt to discuss these relations any closer. In conclusion we shall only call attention to a few remarkable facts which have been made evident by the investigation, namely that the rotation of water is nearly one half of the sum of that of hydrogen ion and hydroxyl ion, while a solution of ammonia in water has a rotation equal to the sum of rotations for ammoniumion and hydroxyl ion. R e thank the director of the laboratory, Prof. Dr. Julius Petersen, for his kind permission to use the means of the laboratory for this work. Chemaeal Laboratory A. Royal Technzeal College, Copenhagen.
