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Revaluation of the continuous variation method as applied to the uranyl azide complexes
J. lnorg. Nucl. Chem. 1962, Vol.. 24, pp. 179 to 182. Pergamon Press Ltd. Printed in Northern Ireland
REVALUATION OF THE CONTINUOUS VARIATION METHOD AS APPLIED TO THE U R A N Y L AZIDE COMPLEXES F. G. SHERIFand A. M. AWAD Faculty o f Science, Alexandria University, Alexandria, Egypt, U . A . R .
(Received 5 July 1961 ; in revisedform 28 August 1961)
Abstract--The method of continuous variation is discussed critically, particularly when applied to solutions containing more than one complex. The deformations in the curves obtained can be interpreted by following the contours of these curves at different wavelengths and concentrations. The presence of mono-, di- and tri-azido uranyl complex ions is revealed. ONE of the general spectrophotometric methods by means of which complex formation in coloured solutions may be established, is the method of continuous variation of JOBI1~. This method, as it is now applied, requires the plotting of Y values (the difference between the total absorbance measured for each mixture and the absorbance of ions corresponding to no reaction) vs. the mole fraction of the metallic ion. During our investigation of the uranyl azide ~2~ and other metallic azide complexes, ~3~ we found that there is no difference between plotting the values of Y and plotting directly the measured absorbances, when the general contours of the curves in both cases are compared. When VOSBURGHand COOPER(4) modified the method, they stated that if all wavelengths lead to the same result, then a single compound is formed and only one maximum (or minimum) in the curves will be obtained. When more than one compound is formed, the results obtained are dependent on the wavelength and for useful conclusions the wavelengths must be carefully selected. The stepwise formation of many complexes can be represented by A q- nB =- AB,
(1)
AB n q- qB = ABn+ q
(2)
in which A is a metallic ion and B is the ligand; n and q are the numbers of ligand molecules. In order to get a maximum corresponding to a higher complex, such as AB,+q, the absorption contribution made by the lower complex AB, was calculated using known stability constants and subtracted from the total assuming that Beer's law would be obeyed for the lower complex. However, we think that the last assumption would be unlikely in case of highly dissociated complex ions. This might be why the method of continuous variation did not always give full information about complex formation and could only be relied upon for cases such as the formation of relatively stable complex ions whose absorption bands were widely separated from ,1J p. JOB, C.R. Acad. ScL, Paris 180, 928 (1925). ~2~ F. G. SHER-Wand A. M. AWAD, ,L lnorg. Nucl. Chem. 19, 94 (1961). ts~ F. G. Si-mm-F and W. M. ORAB'¢, J. lnorg. NucL Chem. 17, 152 (1961); H. K. EL-SHAMY and F. G. SHEmr, Egypt J. Chem. 2, 217 (1959). t4~ W. C. VOSSURGH and G. R. COOPER, J. Amer. Chem. Soc. 63, 437 (1941). 179
180
F.G. SHERIFand A. M. AWAD
each other and from that of the metallic ion, and that should conform to Beer's law. Unfavourable conditions would lead to deformed curves. However, in our case, by following the contours of the curves at a wide range of wavelength and at different total concentrations, these deformations can be interpreted to show the possible formation of complexes of different molecular ratios. They may appear as one or more maxima, a shoulder or a kink in the same curve. Each maximum, shoulder or kink should correspond to a certain coloured species, or to the interference of absorbance of one ion with the other. The relative intensities of these deformations may indicate the relative stabilities of the different ions at different concentrations of reagents, especially when measurements on a single solution are repeated after a certain time. A shoulder that appears at one concentration can grow up to a maximum by concentrating the solutions, while the already present maximum might fade away to a shoulder or disappear completely. These changes might also appear when the shapes of the curves at different wavelengths and for the same concentration are compared. The appearance of a shoulder can be interpreted as a slight formation of a particular species at a certain concentration or as a weak absorbance at a certain wavelength. Likewise, an accompanying sharp maximum would mean a stable complex or one with a strong absorbance at a particular wavelength. For any concentration, the change in position of one ol more irregularity in the curves would reflect the difference in stabilities of the different complexes. In this investigation, JOB'S continuous variation method was applied to the uranyl azide system at different concentrations. The formation of different complex ions containing different ratios of uranium to azide was suggested. EXPERIMENTAL The preparation and analysis of the different solutions used, together with the experimental procedures applied were given in a previous article (2). A Unicam Spectrophotometer S.P. 500, with glass cells 0-1 cm thick, was used. RESULTS The existence of a 1-uranyl: l-azide coloured species at a total concentration of 0-016 M was confirmed previously c2~ by one sharp maximum at a mole fraction of 0.5 for UO22+. In this investigation, the total concentration of uranyl and azide ions was kept constant at 0.08 M and the ionic strength at 0-224 by adding the required amounts of sodium perchlorate. Measurements were taken in the wavelength range of 370-470m#. Representative curves are given in Fig. 1. I f the two maxima at the wavelength 3 8 0 m # are extrapolated, a single maximum that lies at a mole fraction of 0.33 for UO2 z+ is formed and would indicate the formation of a 1-uranyl:2-azide complex. However, the maximum to the right of Fig. 1 falls at a ratio of 1 : 1.5 and can represent a mixture of 1 : I and 1:2 complexes. The other maximum, being at a ratio of 1:2.3, can be the result of a contribution of a slight formation o f a 1:3 complex ion to the 1:2 form. Indications for higher complexes are clearer at shorter than at longer wavelengths as shown by the disappearance of the maximum at 440 #. For the total concentration of 0.16 M, Fig. 2, the curves at 280, 390 and 400 # possess definite maxima substantiating the existence of the 1:3 complex UO2(N3)8-. Shoulders corresponding to a ratio of 1 : 1 are also observed. As we approach longer wavelengths the maxima are reduced gradually. For the total concentration of 0.32 M, Fig. 3, there is sufficient contribution from the 1 : 1 form to produce a definite asymmetry in the curves. As well as the sharp maximum at the 1:3 ratio at 400/~,
R e v a l u a t i o n o f the c o n t i n u o u s variation m e t h o d as applied to the uranyl a z i d e c o m p l e x e s
181
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182
F.G. SHERIFand A. M. AWAD
J
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FXG. 3.--Continuous variation method; total concentration of uranyl and azide ions is 0.32 M. The ionic strength was kept constant at 0'896. lhere is a well defined maximum at 440 m/~ for a 1:2 complex. When the absorbancies of these solutions were measured after 24 hr, a shift in the position of these maxima towards lower azide ratio was observed while the shoulders representing the 1 : 1 form got clearer. This indicates the relative stability of the mono-azido uranyl complex ion, UO~Na-. These data are generally explained by proposing the possible reaction of the UO22+ and N 3- ions in a consecutive order, forming three types of complexes in aqueous solutions, where the ratio of UO22+ to N 3- varies from 1 : 1 to 1:3. The increase in concentration of both reagents leads to the formation and stabilization of higher complexes.
REVALUATION OF THE CONTINUOUS VARIATION METHOD AS APPLIED TO THE U R A N Y L AZIDE COMPLEXES F. G. SHERIFand A. M. AWAD Faculty o f Science, Alexandria University, Alexandria, Egypt, U . A . R .
(Received 5 July 1961 ; in revisedform 28 August 1961)
Abstract--The method of continuous variation is discussed critically, particularly when applied to solutions containing more than one complex. The deformations in the curves obtained can be interpreted by following the contours of these curves at different wavelengths and concentrations. The presence of mono-, di- and tri-azido uranyl complex ions is revealed. ONE of the general spectrophotometric methods by means of which complex formation in coloured solutions may be established, is the method of continuous variation of JOBI1~. This method, as it is now applied, requires the plotting of Y values (the difference between the total absorbance measured for each mixture and the absorbance of ions corresponding to no reaction) vs. the mole fraction of the metallic ion. During our investigation of the uranyl azide ~2~ and other metallic azide complexes, ~3~ we found that there is no difference between plotting the values of Y and plotting directly the measured absorbances, when the general contours of the curves in both cases are compared. When VOSBURGHand COOPER(4) modified the method, they stated that if all wavelengths lead to the same result, then a single compound is formed and only one maximum (or minimum) in the curves will be obtained. When more than one compound is formed, the results obtained are dependent on the wavelength and for useful conclusions the wavelengths must be carefully selected. The stepwise formation of many complexes can be represented by A q- nB =- AB,
(1)
AB n q- qB = ABn+ q
(2)
in which A is a metallic ion and B is the ligand; n and q are the numbers of ligand molecules. In order to get a maximum corresponding to a higher complex, such as AB,+q, the absorption contribution made by the lower complex AB, was calculated using known stability constants and subtracted from the total assuming that Beer's law would be obeyed for the lower complex. However, we think that the last assumption would be unlikely in case of highly dissociated complex ions. This might be why the method of continuous variation did not always give full information about complex formation and could only be relied upon for cases such as the formation of relatively stable complex ions whose absorption bands were widely separated from ,1J p. JOB, C.R. Acad. ScL, Paris 180, 928 (1925). ~2~ F. G. SHER-Wand A. M. AWAD, ,L lnorg. Nucl. Chem. 19, 94 (1961). ts~ F. G. Si-mm-F and W. M. ORAB'¢, J. lnorg. NucL Chem. 17, 152 (1961); H. K. EL-SHAMY and F. G. SHEmr, Egypt J. Chem. 2, 217 (1959). t4~ W. C. VOSSURGH and G. R. COOPER, J. Amer. Chem. Soc. 63, 437 (1941). 179
180
F.G. SHERIFand A. M. AWAD
each other and from that of the metallic ion, and that should conform to Beer's law. Unfavourable conditions would lead to deformed curves. However, in our case, by following the contours of the curves at a wide range of wavelength and at different total concentrations, these deformations can be interpreted to show the possible formation of complexes of different molecular ratios. They may appear as one or more maxima, a shoulder or a kink in the same curve. Each maximum, shoulder or kink should correspond to a certain coloured species, or to the interference of absorbance of one ion with the other. The relative intensities of these deformations may indicate the relative stabilities of the different ions at different concentrations of reagents, especially when measurements on a single solution are repeated after a certain time. A shoulder that appears at one concentration can grow up to a maximum by concentrating the solutions, while the already present maximum might fade away to a shoulder or disappear completely. These changes might also appear when the shapes of the curves at different wavelengths and for the same concentration are compared. The appearance of a shoulder can be interpreted as a slight formation of a particular species at a certain concentration or as a weak absorbance at a certain wavelength. Likewise, an accompanying sharp maximum would mean a stable complex or one with a strong absorbance at a particular wavelength. For any concentration, the change in position of one ol more irregularity in the curves would reflect the difference in stabilities of the different complexes. In this investigation, JOB'S continuous variation method was applied to the uranyl azide system at different concentrations. The formation of different complex ions containing different ratios of uranium to azide was suggested. EXPERIMENTAL The preparation and analysis of the different solutions used, together with the experimental procedures applied were given in a previous article (2). A Unicam Spectrophotometer S.P. 500, with glass cells 0-1 cm thick, was used. RESULTS The existence of a 1-uranyl: l-azide coloured species at a total concentration of 0-016 M was confirmed previously c2~ by one sharp maximum at a mole fraction of 0.5 for UO22+. In this investigation, the total concentration of uranyl and azide ions was kept constant at 0.08 M and the ionic strength at 0-224 by adding the required amounts of sodium perchlorate. Measurements were taken in the wavelength range of 370-470m#. Representative curves are given in Fig. 1. I f the two maxima at the wavelength 3 8 0 m # are extrapolated, a single maximum that lies at a mole fraction of 0.33 for UO2 z+ is formed and would indicate the formation of a 1-uranyl:2-azide complex. However, the maximum to the right of Fig. 1 falls at a ratio of 1 : 1.5 and can represent a mixture of 1 : I and 1:2 complexes. The other maximum, being at a ratio of 1:2.3, can be the result of a contribution of a slight formation o f a 1:3 complex ion to the 1:2 form. Indications for higher complexes are clearer at shorter than at longer wavelengths as shown by the disappearance of the maximum at 440 #. For the total concentration of 0.16 M, Fig. 2, the curves at 280, 390 and 400 # possess definite maxima substantiating the existence of the 1:3 complex UO2(N3)8-. Shoulders corresponding to a ratio of 1 : 1 are also observed. As we approach longer wavelengths the maxima are reduced gradually. For the total concentration of 0.32 M, Fig. 3, there is sufficient contribution from the 1 : 1 form to produce a definite asymmetry in the curves. As well as the sharp maximum at the 1:3 ratio at 400/~,
R e v a l u a t i o n o f the c o n t i n u o u s variation m e t h o d as applied to the uranyl a z i d e c o m p l e x e s
181
~l
°
~oo
~6 c-
r-_~
o~ ~-
~.~
N~ '~ .
0
~.~
"
I.~
~
e,l.~
6 ~ 0 ":" ,{ 3UOCIJ o s q v
-"
~ 0
0
,-'eq eac'q
t~ e-
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._
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0
I 0
0 ~ nuoqJosq~'
I 0
0
_
¢t~
.~.-
182
F.G. SHERIFand A. M. AWAD
J
2"0
I
-/.
i
g 3
a~ .~ <~
I'C
0.5
0
0 2
0-'4 Mole frechon
06
O'B
U ~
FXG. 3.--Continuous variation method; total concentration of uranyl and azide ions is 0.32 M. The ionic strength was kept constant at 0'896. lhere is a well defined maximum at 440 m/~ for a 1:2 complex. When the absorbancies of these solutions were measured after 24 hr, a shift in the position of these maxima towards lower azide ratio was observed while the shoulders representing the 1 : 1 form got clearer. This indicates the relative stability of the mono-azido uranyl complex ion, UO~Na-. These data are generally explained by proposing the possible reaction of the UO22+ and N 3- ions in a consecutive order, forming three types of complexes in aqueous solutions, where the ratio of UO22+ to N 3- varies from 1 : 1 to 1:3. The increase in concentration of both reagents leads to the formation and stabilization of higher complexes.