COPPER SALTS AS LIGHT FILTERS. I B Y W.
T-. BHAGWAT A S D S . R. DHAR
Part I. Perhaps the copper salt,sare the only ones which are most extensively used either individually or in combination with other substances as light filters in photo-chemistry. Although a large number of investigators such as Winther,' I\-eigert-Eiummerer,' Gray,3, Eder.', Lehmannj and others have worked with these salts, the exact knowledge regarding the absorption of radiations by copper salts is still lacking. I t is essential in photo-chemistry to have a monochromatic source to study the effect of various warelengths on any given photochemical reaction. The requirements for a satisfactory light to be used as a source of monochromatic radiation are: ( I ) it' must have high intensity ( 2 ) it must be steady for relatively long periods (3) its spectrum should have a few very strong lines rather than a closely packed large number of lines of moderate intensity. I t is very rare that we can get a monochromatic source of light by itself, such as the flame of the Bunsen burner fed with sodium salt. I t is this that necessitates the study of suitable filters which will just transmit the required wavelength and will absorb the rest. Clearly, a light filter, in order that it may be used with maximum advantage, must satisfy the above-mentioned conditions for a satisfactory nionochromatic light source. The filter must absorb as little as possible of the light to be transmitted and should be completely opaque to other regions. To ascertain the intensity of the transmitted light it is necessary to study the percentage transmission in various regions. To satisfy the second condition for a monochromatic source of light, that t'he light source should be steady for relatively long periods to secure comparable results, the filter should remain unchanged in composition for a very long t'ime. That is, the filter should be photochemically inert. To get the knowledge of the third condition for a monochromatic light source the study of absorption is necessary. In this communication we have recorded our results of the experiments carried out in accordance with the necessity of getting exact knowledge of a filter and ascertaining the degree of accuracy with which it can be used in photochemical work. We have carried out experiments in the visible part of the spectrum and determined the extinction coefficient, light absorption and transmission with a Nutting's photo-spectrometer using various concentrations of the individual copper salts or their combination with other substances placed in a 1
Z physik. Chem., 41, 169 (1902).
* Ber., 46, 3 4
1210
(1913).
J. Phys. Chem , 31, 1732 (1927). Z. wiss. Phot., 26, 373 (1929).
SChem. Ztg., 33, 1167 (1909); Physik. Z.. 11, Io39 (1910).
V. V. BHAGWAT AND
2384
S . R.
DHAR
quartz vessel of one centimeter thickness. We have also determined the absorption in the ultraviolet photographically with a Hilger quartz spectrograph using a copper arc as a source of ultraviolet light. Winther' reported that a solution of 2 j gms. of cupric chloride in I O O c.cs with solution of 0 . 2 j gms. of violet yellow in 2 cm. thickness transmits 5332 A. It is known that a two percent solution of cupric chloride absorbs infra-red. Copper salt solutions are largely used for cutting off heat, rays. The following tables respresent our results regarding absorption, extinction coefficient, and percentage transmission for varying concentrations of copper chloride, copper nitrate, and copper sulphate. The concentrations of the copper salts are expressed on the assumption that molar weight12 = normal solution. The amount of copper in solutions was determined by adding potassium iodide to the solutions and titrating the liberated iodine by a standard thiosulphate solution. Cupric chloride. ( I ) Concentration 7 9.28 jN. A = l17a~elengthin A Units. e = extinction coefficient. % T = percentage transmission.
x
2.1
j600 1.85
0
0.79
1.41
5220
jIj0
1.06 8.7
1.20
j080 1.38
6.3
4.1
7000-j770
e %T
x e
%T
o?
5680
jj20
5440
1.48 3.31
1.20
5020
1.70 1.99
6.3 4960
5360 1.06 8.7 4900 -
2.25
00
0.56
0
5290 1.00 IO
4000
Cupric chloride. (2) Conc. 4.642 j li.
x
7000-6060
e
%T
x
j960
j680
2.
2.25
1.22
o
0.56 4580
4700 0.84
e %T
I44
j440 0.61
6.02 24.5 4jjO-4000
1.85
5220
0.31 48.9
5020 0.27
53.7
X
1.4
0
Cupric chloride. (3) Conc. 2.3262 li.
x
7000-6280
e %T
02
o
x
4840
e %T 1
0.13 74.1
5960 1 . j ~ 2.8 4700 0.20
63.1
5680 0.63 23.4
4580 0.26 j4.9
Z. physik. Chem., 41, 169 (1902)
5440
0.33 46.7 4360 0.48
33.1
j220
0.18 66.0 4340 0 . j ~
30.2
5020
0.18 66.0
4840
0.46 34.6
COPPER SALTS AS LIGHT FILTERS
Cupric chloride. (4) Conc 0.928; S . x 7000-6480 6380 e r I Sj %T 0 1.41 x j020 4840 0.I2 0 .I1 e 75.8 77.6 %T Cupric chloride. ( 5 ) Conc. o.ogzS5
6280
5960
1.48
0.72
16.9
3.3 4700
4580
0.08
83. I
0.11
77.6
6600
6280
5960
5680
e
0.25
0.20
0. I 2
0.08
56.2
47.8 4460 0.20
63.I
63.1
85.1
75.8
5440 0.06 89. I
These results can be summarised as follows:Range of Transmission Conc. of Cupric Chloride
s
9.285 4.6425 K 2.3262 ?; 0.928; X o.og28j N
e
%T
0
4580 0.11
77.6
60.2 4340 0.20
63. I
j220-4000
0.04
91.2
%T(Maxinium) IO
53.7 74.1 83. I 91.2
4000-7000
x
%T
5220
0.15 70.7
4900-5770 4 540-6060 4000-62 80 4000-6480
Cupric nitrate. ( I ) Conc. 11.054 X .joo0-5580 j j 1 0 e P 2.5
x
5440 0.22
K.
A
%T
5680 0.32
2385
5220
5020
1.12
0.64 22.9
4840 0.30 50.1
5440 0.90
0.31 4460
7.5
0.I O
0.08
79.4
4340
83.I
Cupric nitrate. (2) Conc. 5 . 5 2 7 X .
x e
%T
x
e %T
7000-5960 2.
0
4840 0.13 74.1
5860
5770
2.4 0.39 4700
2.15
5680 1.6
0.70
2.5
4580
4460
0.10
79.4
0.IO
79.4
0.10
79.4
12.5
4340 0.08
83.1
5220
5020
0.49 32.3
0.25
56.2
2386
W. V. BHAGWAT AND K. R. DHAR
Cupric nitrate. (3) Conc. 2.763 K. A 7000-6280 6160 e XI 2.20
%T
x e
%T
0
0.60
5220
5020
0.22
60.2
0.16 69.1
6060 1.97 1.07 4840
5960 1.67 2.1
5680 0.82 15.1
5440 0.42
38.0
4700-4000 0.08
0.12
75.8
83.1
These results are summed up in the following table:Conc. of CU (N03)2-11.054 N, 5 . 5 2 7 N, 2.763 N. Range of transmission-~o00-~~80 4000-5960 8, 4000-6280
w,
Cupric sulphate. ( I ) Conc. 2.962 N. x 7000-6280 6170 e 00 2.09 %:oT 0 0.81
x e
%T
5220
5020
0.2j
0.16 69.1
56.2
6060 1.89
5960 1.59
1 . 2
2 . 5
5680 0.75 17.0
8.
5440 0.41 38.9
4700-4300 0.13 0.09 74.1 81.2 4840
We have determined the absorption in the ultraviolet region photographically for the various concentrations of the copper salts and the results are tabulated in the following tables .-
Ultraviolet Absorption (I)
Conc. of CuClz 9.782 K 9.285 4.891 4.642 2.445
Region of absorption absorbs all ultraviolet rays
2.321
0.9782 0 . 097a2
0.04642 0.009782
absorbs up to
Region of transmission Xi1
J,
1,
11
9)
11
,,
Jl
,J
3602 Ai 2767 2618 Si1
3602-4000 A 2 767-4000 2618-4000 transmits all
COPPER SALTS AS LIGHT FILTERS (2)
Conc. of C U ( N O ~ )Range ~ of absorption
2387
Range of transmission 3861-4000 -4
11.054
Up to 3861 3861
6.087
3602
5.527
3602
3.043
3426
3426-4000
2'763 1.2174
32i4
3 2 74-4000 3 2 48-4000
12. I 7 4
3248
0 . I217
2618
0.01217
Nil
(3) Conc. of CuS04 2.962 2.716
Range of absorgtion Up to 3 2 0 0 h 3200
11
3602-4000
,.
2 6 I 8-4000
transmits all Range of transmission 3 2 0 0 - 4 0 0 0 -1
,,
,481
3036
3036-4000
1.358
3000
3000-4000
0.679
2961
2961-4000
0.2716
282j
2 8 2 5-4000
0.1481
2767
0.02716
Xi1
I
276;-4ooo
transmits all
It will be observed from the above results that cupric salts in concentrated solutions can cut off both ultraviolet and red regions of the visible spectrum and hence can be advantageously used to eliminate these regions. Moreover, copper salts are known to absorb infra-red radiations. Thus for the transmission of the visible region up to red, copper salts are the best filters. Cupric chloride has the advantage over cupric nitrate and cupric sulphate that it has strong absorption towards violet even in the visible region and hence a definite short range is available with cupric chloride for photo-chemical work. Thus a cupric chloride solution of conr 9.28 j X transmits only 4900-jiio A, the maximum transmission being 10%. Cupric nitrate solution of conc. I 1.054 N is also useful from the point of view that it absorbs a longer region towards red (up to 7 0 0 0 - j 8 8 0 ~ than cupric chloride, and therefore can be used to cut off that region where cupric chloride is unsuitable. Copper sulphate is no better than either cupric chloride or cupric nitrate. This is attributed to the fact that it is less soluble than the other two salts of copper. This will be clear from the concentrations that we have used. Although we could prepare a solution of cupric nitrate of as high a concentration as 11.oj4 N and cupric chloride of concentration 9.28; S , with copper sulphate solution we could not get a concentration higher than 2.962 K. Many glass and gelatine filters of various makes such as Corning, Wallace, Wratten and others, which transmit all the visible radiations, appear to be less suitable for accurate photochemical work. The transmission of various kinds of gelatine and glass filters have been determined by S'oege,' Cobientz,' Illuminating Engineer, 2, 543 (1909). J. Franklin Institute, 180, 255 (1920).
2388
W. V. BHAGWAT AND N. R. DHAR
Gibson and McXicholas,’ Gibson, Tyndall and McXicholas,* and others. Copper chloride solution of concentration 9.285 N as a light filter is comparable to the Wratten filters 53, 54, 58-1, 60, 61, 6 2 , 63, and 74. Of these 53, 588, 60, 61, and 63 have a much longer range of transmission than the solution of cupric chloride. Wratten filter 54 has a transmission of only one percent. Thus copper chloride is equally good as a light filter for the region 4900-5770 as the Wratten filters 62 and 74. Copper nitrate solution of concentration 11.054 K is comparable to the Wratten filters 44 to 50. It has one great advantage over all these that its transmission is much greater although the range available with cupric nitrate is a little longer than that of the Wratten filters mentioned above. Absorption results clearly point out one important fact, that cupric chloride behaves differently in comparison with the other copper salts, especially in concentrated solutions. Copper nitrate and copper sulphate behave similarly even in concentrated solutions. This abnormal behaviour of cupric chloride is not limited to the visible region only but is also seen in the ultra violet. For the same concentrations copper nitrate and copper sulphate have practically the same range of absorption in the ultra violet and similar values of extinction coefficient for the visible. These two salts have only one-sided absorption, that is, towards the red in the visible, while cupric chloride has absorption towards the violet also. With decreasing concentrations, cupric chloride behaves similarly to the other copper salts. It appears that the nature of the anion has some influence on the colour of the cupric salt solutions, and hence the light absorption depends on the anions as well. We believe that it is much more so when the anions are also coloured, and when the undissociated molecules exist in the solutions and in the vapour states. That is why the abnormality is more prominent in concentrated solutions where the salt is less dissociated than in dilute solutions where the salt is less dissociated than in dilute solutions, where there is greater ionisation. Chlorjne in the gaseous state has an absorption maxima in the region 3800-3000 A. The chlorine ion of cupric chloride appears to exert a marked influence on the absorption of cupric chloride. Even for the concentration 2.231 N of cupric chloride there is no ultra violet transmission although a cupric nitrate solution of concentration 12.174 N has the transmission from 3861-400oA in the ultra violet. I n the case of a molecule used as a light absorbent, the light has to pass through the complete structure of the molecule, and hence the absorption is the net result of the light absorbed both by cations and anions of the molecule. If the molecule is ionised, we have to take into account the influence of the individual ions, the net result may be due to the cation, which is the predominating factor in the case of cupric and other coloured salts. It will be observed that the coefficients of absorption are not proportional to their concentration, that is, Beer’s law is not valid with solutions of cupric chloride. This discrepancy is attributed to the formation of complex ions such as CuC14”, CuCL”, etc. It is Bur. of Standards, Terh. Paper 119 (1919). 148 (1920).
* Bur. of Standards, Tech. Paper
COPPER SALTS AS LIGHT FILTERS
2389
assumed t’hat electrolytic dissociation unaccompanied by change of structure does not change the optical absorption. The case of potassium permanganate is always cited as an example, for it follows Beer’s law. Although the degree of dissociation increases with dilution, the absorption of these solutions is not changed by the addition of sulphuric acid, which decreases the permanganate ion concentration. The hypothesis of the formation of hydrates in place of complexes can as well explain the discrepancies. I t is difficult to understand why light absorption should not change with ionisation of a salt., It should change provided a molecule of the salt and the ion have different absorptions. I t is only when both the molecule and ion hare the same absorption that ionisation should not have any effect on absorption, Perhaps this might be the case with potassium permanganate, and that is, why the change in ionisation does not change the absorption. Concentrated solations of cupric chloride differ in colour from the solutions of cupric nitrate and sulphate in that they are green and not blue like the solutions of the copper salts. We attribute the green colour of cupric chloride solution to the undissociated cupric chloride molecules. In the cases of copper sulphate and copper nitrate, the molecules and the ions have practically t8he same absorption, as the anions from these salts appear to be colourless. Such is not the case with cupric chloride. Here both the ions appear to be coloured and thercfore the molecule and the ion have different absorptions. Khen, however, the solutions are diluted, cupric ions, as in case of copper sulphate and copper nitrate, become responsible for the absorption. This view appears justified from the fact that for the same concentrations, the limit of complete absorption on the longer wavelength side is practically the same for all the three copper salts. We have chosen the longer wavelength side because the anion C,l’ of cupric chloride appear to exert a great influence on the shorter wavelength side and chlorine as a gas shows absorption towards violet and ultra violet. Ostwaldl found that 0 . 0 0 2 S solutions of different permanganates showed the same absorption bands. The spectrum was independent of the nature of the cation, except in few cases where deviations occurred and could be satisfactorily explained. Merton,? who photographed the absorption spectra of various permanganates in various solvents reported that the position of the bands in any single solvent is the same for permanganate of potassium, sodium, calcium, barium, zinc etc. Since some of t,he solvents employed have very slight ionising power, the light absorption was ascribed to the M n 0 4 group and was not much influenced by ionisation. Thus Merton contradicts Ostwald’a view on the subject. Unfortunately Ostwald and Rlerton have chosen salts with colourless cations and hence the molecules and ions may have the same absorption as we have stated above in the case of CuSO, and C U ( S O ~ ) ~ . The cause of absorption of light by cations which show selective absorption has been discussed by Miss kat^.^ The selective absorption of the salt ’2. physik. Chem., 9, 579 (1892).
* J. Chem.
SOC.,99, 637 (1911). Sci. Papers Inst. Phys. Chem. Res. Tokyo, 12, 2 3 0 ; 13, 7 (1930).
W. V. BHAGWAT AND S . R. DHAR
2390
solutions are attributed to the electrons in the incomplete d-shells of the cations, most of the transmission being traced to the intercombination between different multiples of the ionized atoms. This conclusion was arrived a t by the comparison of her absorption frequencies with the spectral terms in gases investigated by Gibbs’ and TThite.2 She has studied the selective absorption of copper saolts, which is found to be at 8500.&, the general absorption extending to 2450~4. The electronic configuration for the copper atom is 3
S 2
4
p. 6
S
d.
p . d
f
IO
and hence the outermost electron of this atom is in the state 4 S’. Copper shows its maximum valency as two, but for solutions of cuprous salts no selective absorption was observed, because all S-electrons are removed and all the remaining shells are completed. From these observations she has come to the above conclusion as regards the cause of selective absorption by cations. This view is in agreement with the theory given out by Ill. N. SahaJ3regarding the colour of inorganic salts.
Summary ( I ) The absorption, extinction coefficient and percent,age transmission of chpric chloride, cupric nitrate and cupric sulphate have been determined in the visible region. Ultra violet light transmission for the same salts solutions obtained (2) photographically has been recorded. ( 3 ) The use of copper salts as light filters has been discussed. I t is shown that cupric chloride is a better filter than cupric nitrate or sulphate and is equally good as Kratten filters 6 z and 74. (4) Solutions of cupric sulphate are less suitable as light filters than solutions of cupric nitrate and chloride, because cupric sulphate is less soluble than cupric nitrate and chloride. (5) The molecular and ionic absorptions have been distinguished. I t is shown that the anion may have some effect on light absorption especially when it is coloured and when undissociated molecules exist. The green colour of concentrated cupric chloride solutions and the conflicting views of Ostwalcl and Merton regarding the absorption of permanganates can be explained on the above basis.
I(
Phys. Rev., 29, 426, 6 j 5 (1927). Phys. Rev., 33, 538, 672 (1929). Sature, 125, 163 (1930).
