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The absorption spectra of acidified molybdate solutions—II
J. Inorg. Nucl. Chem., 1956, Vol. 3, pp. 24 to 27. Pergamon Press Ltd., London
THE ABSORPTION SPECTRA OF A C I D I F I E D MOLYBDATE SOLUTIONS--II P. J. COOPE a n d W . P. THISTLETHWAITE Department of Chemistry and Applied Chemistry Royal Technical College, Salford, Lancashire (Received 13 March 1956)
Abstract--The absorption spectra of sodium molybdate solutions, acidified with perchloric acid, have been studied. A definite acid concentration is shown to exist for maximum optical absorption by the solutions. This optimum acidityis alinear functionof the molybdate concentration; the implications of this are discussed. IN a previous p a p e r (I) the a b s o r p t i o n spectra o f 0.005 M s o d i u m m o l y b d a t e s o l u t i o n s t h a t were 0.1 N in h y d r o c h l o r i c , perchloric, sulphuric, a n d nitric acids were determ i n e d over the range 270-330 m # a n d f o u n d to be identical. Increase in the conc e n t r a t i o n o f the first three acids to 0.5 N caused no change, b u t a similar increase in nitric acid c o n c e n t r a t i o n c h a n g e d the f o r m o f the curve, suggesting s o m e i n t e r a c t i o n between nitrate a n d m o l y b d a t e . A similar i n t e r a c t i o n h a d b e e n suggested b y CLARENS12) in c o n n e c t i o n with the p r e c i p i t a t i o n o f a m m o n i u m 1 2 - m o l y b d o p h o s p h a t e . A l t h o u g h no specific effect o f h y d r o c h l o r i c a n d sulphuric acids was f o u n d o v e r this p a r t i c u l a r wavelength range, there is o t h e r evidence t h a t b o t h acids c a n i n t e r a c t with m o l y b d a t e in certain circumstances/3) F o r this reason, p e r c h l o r i c acid, for which n o such i n t e r a c t i o n has b e e n r e p o r t e d , was used in this series o f experiments. A t the r a t h e r high a c i d / m o l y b d a t e ratios ( ~ 2 0 ) used in the foregoing work, o p t i c a l density was i n d e p e n d e n t o f acid concentration. If, however, the r a t i o is r e d u c e d to values b e t w e e n 3 a n d 15, extinction b e c o m e s a c i d - d e p e n d e n t . I t is this a c i d - d e p e n d e n c e t h a t has b e e n studied. EXPERIMENTAL All extinction measurements were made with a Unicam SPS00 spectrophotometer, and all pH measurements with a Pye Universal pH meter, using the methods previously described.~1~ However, since wavelengths below 320 m# were not required, a tungsten lamp was used throughout; a blue filter was used below 400 m/~. 1-cm silica cells were used; blank solutions contained acid only, of the appropriate concentration. Fig. 1 shows the optical densities obtained for four different concentrations of molybdate, each with four or five concentrations of acid. Wavelengths were chosen so as to bring the extinction readings within the most accurate range for the instrument. Using the optimum acidities obtained from Fig. 1, the extinction curve for each concentration of molybdate at the optimum acidity was determined, all readings being taken on the same occasion to minimize temperature variations. The results are shown in Table 1. Fig. 2 shows the variation of optimum acidity with molybdate concentration. ~1~ p. j. COOI'Eand W. P. THISTLETHWAITEJ. Inorg. Nucl. Chem. 2, 125 (1956). (2) j. CLARENSBull. Soc. Chim. (4) 23, 147 (1918); 25, 87 (1919). lal K. G. FALKand K. SUGIORAJ. Amer. Chem. Soc. 37, 1507 (1915). R. KUnN Z. physiol. Chem. 129, 64 (1923). I. NELIDOWand R. H. DIAMONDJ. Phys. Chem. 59, 710 (1955). A. TRAYERSand L. MALAPRADEBull. Soc. (?him. (4) 39, 1408, 1543 (1926). 24
The absorption spectra of acidified molybdate solutions--II DISCUSSION
25
OF RESULTS
The results in Fig. 1 show clearly that for each molybdate concentration there is a definite acidity for the development of maximum absorption at that wavelength. This is referred to as the "optimum acidity." It is assumed that the maximum absorption for a given concentration ofmolybdate occurs at the point of maximum formation of the largest complex. The decrease in the absorption, at acidities above the optimum, is attributed to decomposition of the complex with the formation of mononuclear Mo w cations. It is noteworthy that CHAUVEAU,SCHAAL,and SOUCHAY(4) have suggested, from variations in the absorption spectra below 230 m#, that, for 0"01 M molybdate, the formation of Movt cations occurs at acid concentrations above 0.05 N; this may be compared with the optimum acidity 0.06 M in the present paper. The optimum acid concentration should, therefore, correspond to virtual completion of the reaction: nMoO4 2- + 2nil + = (MoO3),~-+ nHzO which would imply an equation of the type: opt. acid =: 2[MoO4 z-] -< [H+]ob~,.vo,l
(I)
from which the pH of the solution at the optimum acidity should correspond to the concentration of added acid minus twice the molybdate concentration. Good agreement between the experimental pH values and those calculated on this basis is evident from Table 1. TABLE 1
Series Na2MoO~ concn. (M) HC104 concn. (M) Wavelength (m/0 440 43O 420 410 4O0 390 380 370 360 350 34O 33O 320 p H (observed) [H+I (calculated) p H (calculated)
1 0.1 0.44
I
2
3
4
0.05 0.23
0-025 0.12
0.01 0.06
5 l i
0-005 0.04*
Extinction <0.01 0.01 0.02 0.03 0.06 0.12 0.30 0-81 2.07
I ! <0.01 0.01 0.02 0.03 0-05 0-14 0.37 0.99
+
+
O.24 0-62
0"13 0.89
* Optimum acidity calculated from equation (2).
<0'01 0"01 0.02 0-03 0"07 0.17 0.46 1"16
1.15 0.07 1.15
<0.01 0.01 0.02 0-06 0.13 0.32 0'74 1 "53 1.35 0.04 1.40
<0.01 0.01 0-02 0.05 0-11 0.24 0.52 1.04 1.55
0.03 1.52
~+pH values below 1 are unreliable.
(4) F. CHAUVEAU,R. SCHAAL, and P. SOUCHAYCompt. Rend. 240, 194 (1955).
26
P, J, COOPE a n d W. P. THISTLETHWAITE
1"6
1.4 4
1.2
•
"0
1"0
0
(~ 0 . 8
0"6
0"4 ~ 0
0-4
0"2
0.6 O.B Normality of acid
1.2
1.0
FIG. 1.--1. 0"1 M. Na2MoOa at 370 m # 2. 0.05 M. Na2MoO~ at 360 m/~ 3.0.025 M. Na~MoO~ at 350 mbt 4. 0.01 M. Na~MoO 4 at 330 mlt
04
0.~
E O-3
0
O 0
0.2
/
E E 0
0.1
0
/
/
/
/
f 0"02 0.04 0'06 0"08 0'10 Molybdate (molar concentration) FIG. 2.
0"12
1.4
The absorption spectra of acidified molybdate solutions--ll
27
The equation for the straight line shown in Fig. 2 was found to be: opt. acid = 4.2[MOO42-] + 0.02
(2)
From this would follow, according to the above argument, that: -- 2"2[MoO~ 2-] + 0.02 [H+]observed-
(3)
This implies that the residual hydrogen-ion concentration observed in the solution is also a linear function of the original molybdate concentration. It is not clear why such a linear variation should apply, but it is thought that it may be an empirical resultant of the various equilibria existing in the solution. The equilibria envisaged are of the general type: OH-
1f+
Mo v1 anions ~ (Mo03)~, ~ Mo v~ cations. It is conceivable that anions and cations involving different states of aggregation may be involved.
THE ABSORPTION SPECTRA OF A C I D I F I E D MOLYBDATE SOLUTIONS--II P. J. COOPE a n d W . P. THISTLETHWAITE Department of Chemistry and Applied Chemistry Royal Technical College, Salford, Lancashire (Received 13 March 1956)
Abstract--The absorption spectra of sodium molybdate solutions, acidified with perchloric acid, have been studied. A definite acid concentration is shown to exist for maximum optical absorption by the solutions. This optimum acidityis alinear functionof the molybdate concentration; the implications of this are discussed. IN a previous p a p e r (I) the a b s o r p t i o n spectra o f 0.005 M s o d i u m m o l y b d a t e s o l u t i o n s t h a t were 0.1 N in h y d r o c h l o r i c , perchloric, sulphuric, a n d nitric acids were determ i n e d over the range 270-330 m # a n d f o u n d to be identical. Increase in the conc e n t r a t i o n o f the first three acids to 0.5 N caused no change, b u t a similar increase in nitric acid c o n c e n t r a t i o n c h a n g e d the f o r m o f the curve, suggesting s o m e i n t e r a c t i o n between nitrate a n d m o l y b d a t e . A similar i n t e r a c t i o n h a d b e e n suggested b y CLARENS12) in c o n n e c t i o n with the p r e c i p i t a t i o n o f a m m o n i u m 1 2 - m o l y b d o p h o s p h a t e . A l t h o u g h no specific effect o f h y d r o c h l o r i c a n d sulphuric acids was f o u n d o v e r this p a r t i c u l a r wavelength range, there is o t h e r evidence t h a t b o t h acids c a n i n t e r a c t with m o l y b d a t e in certain circumstances/3) F o r this reason, p e r c h l o r i c acid, for which n o such i n t e r a c t i o n has b e e n r e p o r t e d , was used in this series o f experiments. A t the r a t h e r high a c i d / m o l y b d a t e ratios ( ~ 2 0 ) used in the foregoing work, o p t i c a l density was i n d e p e n d e n t o f acid concentration. If, however, the r a t i o is r e d u c e d to values b e t w e e n 3 a n d 15, extinction b e c o m e s a c i d - d e p e n d e n t . I t is this a c i d - d e p e n d e n c e t h a t has b e e n studied. EXPERIMENTAL All extinction measurements were made with a Unicam SPS00 spectrophotometer, and all pH measurements with a Pye Universal pH meter, using the methods previously described.~1~ However, since wavelengths below 320 m# were not required, a tungsten lamp was used throughout; a blue filter was used below 400 m/~. 1-cm silica cells were used; blank solutions contained acid only, of the appropriate concentration. Fig. 1 shows the optical densities obtained for four different concentrations of molybdate, each with four or five concentrations of acid. Wavelengths were chosen so as to bring the extinction readings within the most accurate range for the instrument. Using the optimum acidities obtained from Fig. 1, the extinction curve for each concentration of molybdate at the optimum acidity was determined, all readings being taken on the same occasion to minimize temperature variations. The results are shown in Table 1. Fig. 2 shows the variation of optimum acidity with molybdate concentration. ~1~ p. j. COOI'Eand W. P. THISTLETHWAITEJ. Inorg. Nucl. Chem. 2, 125 (1956). (2) j. CLARENSBull. Soc. Chim. (4) 23, 147 (1918); 25, 87 (1919). lal K. G. FALKand K. SUGIORAJ. Amer. Chem. Soc. 37, 1507 (1915). R. KUnN Z. physiol. Chem. 129, 64 (1923). I. NELIDOWand R. H. DIAMONDJ. Phys. Chem. 59, 710 (1955). A. TRAYERSand L. MALAPRADEBull. Soc. (?him. (4) 39, 1408, 1543 (1926). 24
The absorption spectra of acidified molybdate solutions--II DISCUSSION
25
OF RESULTS
The results in Fig. 1 show clearly that for each molybdate concentration there is a definite acidity for the development of maximum absorption at that wavelength. This is referred to as the "optimum acidity." It is assumed that the maximum absorption for a given concentration ofmolybdate occurs at the point of maximum formation of the largest complex. The decrease in the absorption, at acidities above the optimum, is attributed to decomposition of the complex with the formation of mononuclear Mo w cations. It is noteworthy that CHAUVEAU,SCHAAL,and SOUCHAY(4) have suggested, from variations in the absorption spectra below 230 m#, that, for 0"01 M molybdate, the formation of Movt cations occurs at acid concentrations above 0.05 N; this may be compared with the optimum acidity 0.06 M in the present paper. The optimum acid concentration should, therefore, correspond to virtual completion of the reaction: nMoO4 2- + 2nil + = (MoO3),~-+ nHzO which would imply an equation of the type: opt. acid =: 2[MoO4 z-] -< [H+]ob~,.vo,l
(I)
from which the pH of the solution at the optimum acidity should correspond to the concentration of added acid minus twice the molybdate concentration. Good agreement between the experimental pH values and those calculated on this basis is evident from Table 1. TABLE 1
Series Na2MoO~ concn. (M) HC104 concn. (M) Wavelength (m/0 440 43O 420 410 4O0 390 380 370 360 350 34O 33O 320 p H (observed) [H+I (calculated) p H (calculated)
1 0.1 0.44
I
2
3
4
0.05 0.23
0-025 0.12
0.01 0.06
5 l i
0-005 0.04*
Extinction <0.01 0.01 0.02 0.03 0.06 0.12 0.30 0-81 2.07
I ! <0.01 0.01 0.02 0.03 0-05 0-14 0.37 0.99
+
+
O.24 0-62
0"13 0.89
* Optimum acidity calculated from equation (2).
<0'01 0"01 0.02 0-03 0"07 0.17 0.46 1"16
1.15 0.07 1.15
<0.01 0.01 0.02 0-06 0.13 0.32 0'74 1 "53 1.35 0.04 1.40
<0.01 0.01 0-02 0.05 0-11 0.24 0.52 1.04 1.55
0.03 1.52
~+pH values below 1 are unreliable.
(4) F. CHAUVEAU,R. SCHAAL, and P. SOUCHAYCompt. Rend. 240, 194 (1955).
26
P, J, COOPE a n d W. P. THISTLETHWAITE
1"6
1.4 4
1.2
•
"0
1"0
0
(~ 0 . 8
0"6
0"4 ~ 0
0-4
0"2
0.6 O.B Normality of acid
1.2
1.0
FIG. 1.--1. 0"1 M. Na2MoOa at 370 m # 2. 0.05 M. Na2MoO~ at 360 m/~ 3.0.025 M. Na~MoO~ at 350 mbt 4. 0.01 M. Na~MoO 4 at 330 mlt
04
0.~
E O-3
0
O 0
0.2
/
E E 0
0.1
0
/
/
/
/
f 0"02 0.04 0'06 0"08 0'10 Molybdate (molar concentration) FIG. 2.
0"12
1.4
The absorption spectra of acidified molybdate solutions--ll
27
The equation for the straight line shown in Fig. 2 was found to be: opt. acid = 4.2[MOO42-] + 0.02
(2)
From this would follow, according to the above argument, that: -- 2"2[MoO~ 2-] + 0.02 [H+]observed-
(3)
This implies that the residual hydrogen-ion concentration observed in the solution is also a linear function of the original molybdate concentration. It is not clear why such a linear variation should apply, but it is thought that it may be an empirical resultant of the various equilibria existing in the solution. The equilibria envisaged are of the general type: OH-
1f+
Mo v1 anions ~ (Mo03)~, ~ Mo v~ cations. It is conceivable that anions and cations involving different states of aggregation may be involved.