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The hydrolysis of potassium hexachlororhenate
J. inorg, nucl. Chem., 1974, V o l . 36, p p . 3 8 4 5 - 3 8 4 7 . P e r g a m o n Press, P r i n t e d i n G r e a t B r i t a i n .
NOTES
The hydrolysis of potassium hexachlororhenate (Received 30 May 1973) chloric acid media[2]. Weighed amounts of K2ReCI e were dissolved in a measured volume of doubly distilled water in a specially designed vessel permitting the solution to be kept at constant temperature (25°C) and in an inert atmosphere (N2-99.99 per cent purity). After addition of a portion of the 0.1 N NaOH solution the reaction was followed conductometrically, pH-metrically, potentiometrically (by the amount of the released chloride ions), or spectrophotometrically (by the changes in the light absorbance at 281 nm). The measurements were carried out on a Seibold conductometer, Seibold pH-meter, Type GTB, and a Beckman spectrophotometer DK-2A.
Tr~ BASE hydrolysis of the hexachlororhenate ions was studied by Rulfs and Meyer[l] who claimed the existence of the ionic species Re(OH)3(H20)~ as a stable intermediate which is obtained prior to precipitation of the dioxide: ReCI62 -~3 O H -
Re(OH)3(H20)~ + 6 CI-,
Re(OH)3(H20)~-°~:~ Re(OH)4(H20)2 + H 2 0
ReO2 + 2 H20. The purpose of the present work is to investigate in more detail this hydrolytic process.
RESULTS AND DISCUSSION
Nitrogen was run through a 3.2 x 10 -4 M aqueous solution of K2ReC16 kept at 25"C. No changes in the optical properties of the solution were being observed for about 5 hr. Obviously there is a high energetic barrier to reaction
EXPERIMENTAL
Potassium hexachlororhenate is obtained by the reduction of potassium perrhenate using tin(II) chloride in hydro-
(i)
>9
u
II0
100
9O
30
"o -x
% 70
u
T
O.
6O
I.
I
r
Time,
I
4
__-~
I
5
50
c
40
"~ o
30
~ o "6
20
hr
Fig. 1. Variation of pH of the solution (curve 1) and its molar conductivity (curve 3) as function of time after the addition of 1 equivalent of hydroxide; variation of pH of the solution (curve 2) after the addition of 0.5 equivalents of hydroxide. 3843
3846
Notes ReCI~- + H20 ~-~Re(H20)CI ~ + C1- ' ~t Re(OH)CI~- + H +.
(1) (2)
We have traced the changes in conductivity and pH of the same solution as a function of the time after the addition of one equivalent of sodium hydroxide. The results obtained are presented in Fig. 1 (curve I is for pH and 3 for the molar conductivity). The course of the curves shows that both the conductivity and pH of the solution change continuously until a definite value is reached; this value was found to correspond to the total conversion of the hexachlororhenate ions into rhenium dioxide. At the end of the reaction the pH of the solution was 3.01 ; this is in a good agreement with the calculated result (pH = 3-02) for the hydrolysis proceeding by the following scheme, proposed by us :
ReCI~- + ( 4 - m ) H 2 0 + mOH--* Re(OH)4 + ( 4 - m)H ÷ + 6C1-.
(3)
ReO 2 . 2H20 The same picture was found for the hydrolysis of the hexachlororhenate ions with less than one equivalent of hydroxide (e.g. 0-5 or 0,25 equivalents respectively). The curve obtained by the hydrolysis of a 2.99 x 10 -4 M solution of K2ReCI 6 in the presence of 0-5 equivalents of sodium hydroxide is given in Fig. 1 (curve 2). The experimentally obtained value for the pH of the solution at the end of the hydrolytic reaction (pH = 3.02) in this case is also coincident with the value (pH = 2.97) calculated from the proposed scheme (Eqn 3). Obviously the hexachlororhenate ions are hydrolysed mainly by water molecules and the role of the hydroxyl ions is to initiate the hydrolytic reaction.
We have found also that the only stable product of this hydrolysis is ReO 2 . nH20. Soon (5-6 min) after the addition of hydroxyl ions, taken in an amount of less than the stoichiometric amount for the reactt~*n: ReCI~- + 4 OH • ~ ReO2.2H20 + 6C1- the appearance of a colloid is visible. No chloride ions were detected in the precipitate, separated by centrifugation, 5 hr after the addition of one equivalent of hydroxide. Additional evidence in support of the claim that the mechanism of base hydrolysis of potassium hexachlororhenate given by Rulfs and Meyer does not reflect the actual processes taking place in the solution is supplied by our spectrophotometric investigations. We have traced the changes in the electronic spectrum of a 2.99 x 10-'*M solution of potassium hexachlororhenate as function of the time of hydrolysis. The electronic spectra of the solution recorded immediately after the addition of one equivalent of hydroxide (curve l), 10 min (curve 2) and 30 min (curve 3) after the addition, are given in Fig. 2. The electronic spectra were recorded after filtration of part ol the solution through a filter-crucible G-5 so that the initially formed colloidal ReO2. nH20 was removed, The absorption spectra are similar to each other which shows th:~t they belong to one and the same complex species and this complex species is the hexachlororhenate ion (see Fig. 3, 2max= 281, S = 11 800 cm 2 mmol- 2). Hence the nature of the absorption curves does not change during the hydrolytic process. The absorbance at 2,,,,, however, is changed, and this change is in agreement with the concept of conversion of the hexachlororhenate ion into unstable intermediate compounds which later give the sparing soluble ReO2. nH20. The other products of this hydrolysis do not absorb in this part of the spectrum (Na +, K +, CI-). Thus, we could not find in the hydrolysed solution any stable rhenium-containing complexes other than the hexachlororhenate ion. This conclusion is further supported by the correlation between
O" 700
O" 6 0 0
O' 5 0 0
=~ 0.400 ~ 0.300 0"200
0"100
.o
2,~o
2~
2;o
2~'o 2~o 2~o
3to
330
35o ~o
nm Fig. 2. The absorption spectrum of a solution of K2ReC16 recorded immediately after the addition of one equivalent of hydroxide (curve I), after 10 min (curve 2), and after 30 rain (curve 3). Wavelength,
3847
Notes the amount of chloride ion released during the course of the hydrolytic process and the amount of the unaltered hexachlororhenate ions remaining in the solution. The results obtained for the concentration of the released chloride ions by the hydrolysis of a 2-99 x 10-¢M solution of hexachlororhenate ions is given in Table 1. Table 1 Method of determination
Amount of chloride ions (g) After 20 min After 75 min
Potentiometrically Calculated
0.0024 0.0023
0.0042 0.0041
The chloride ions were determined directly in the solution using potentiometric techniques and the concentrations were also calculated from the spectrophotometric data on the concentration of the unchanged hexachlororhenate ions assuming that the hydrolytic process proceeds by scheme (3) and the stable product is ReO 2 . nH~O. The good agreement between the experimental and calculated results provides additional support of the proposed scheme. An attempt was made to determine the order of the reaction with respect to the hydroxyl and hexachlororhenate ions. The results obtained, using known expressions for the rate constants, have shown non-integral and high values, evidence that complex reactions are occurring. The data obtained cannot be used to derive the kinetic equation. CONCLUSIONS
It is evident that reaction (1) has a large activation energy and, at room temperature, the reaction mixture remains
0.700
unaltered for hours. At higher temperatures we were able to observe not only this reaction but also the consecutive ones leading to the formation of rhenium dioxide. The rate of this reaction is pH dependent. An increase in the concentration of the hydrogen ions results in a slower rate due to the decreased activity of the water molecules. On the contrary, any increase in the pH value of the solution leads to an increase in the rate of the hydrolytic process due to the formation of Re(OH)CI~- according to reaction (2). The latter compound, unlike ReCI62- is unstable in aqueous solutions and decomposes immediately. The lowered symmetry determines the fast subsequent hydrolysis which gives a single stable product--ReO2, nH20. The conversion of the hexachlororhenate ions into ReO2. nH20 is a complicated process involving catalytic effects. No evidence for the ionic species Re(OH)3(H20)~, postulated by Rulfs and Meyer could be obtained. M. PAVLOVA N. JORDANOV N. POPOVA
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 13 Bulgaria
REFERENCES
1. C. L. Rulfs and R. J. Meyer, J. Am. chem. Soc. 77, 4505 (1955). 2. M. Pavlova, N. Jordanov and D. Staikov, In press.
I
0-600
O. 5 00
0.400
..Q 0 - 3 0 0
0"200
O" I 0 0
1
240
250
260
270
2BO
Wovelength,
290
300
320
340
360
nm
Fig. 3. The absorption spectrum of an aqueous solution of K2ReCI 6 in the absence of hydroxide (curve I); 2 min after the addition of one equivalent of hydroxide (curve II),
NOTES
The hydrolysis of potassium hexachlororhenate (Received 30 May 1973) chloric acid media[2]. Weighed amounts of K2ReCI e were dissolved in a measured volume of doubly distilled water in a specially designed vessel permitting the solution to be kept at constant temperature (25°C) and in an inert atmosphere (N2-99.99 per cent purity). After addition of a portion of the 0.1 N NaOH solution the reaction was followed conductometrically, pH-metrically, potentiometrically (by the amount of the released chloride ions), or spectrophotometrically (by the changes in the light absorbance at 281 nm). The measurements were carried out on a Seibold conductometer, Seibold pH-meter, Type GTB, and a Beckman spectrophotometer DK-2A.
Tr~ BASE hydrolysis of the hexachlororhenate ions was studied by Rulfs and Meyer[l] who claimed the existence of the ionic species Re(OH)3(H20)~ as a stable intermediate which is obtained prior to precipitation of the dioxide: ReCI62 -~3 O H -
Re(OH)3(H20)~ + 6 CI-,
Re(OH)3(H20)~-°~:~ Re(OH)4(H20)2 + H 2 0
ReO2 + 2 H20. The purpose of the present work is to investigate in more detail this hydrolytic process.
RESULTS AND DISCUSSION
Nitrogen was run through a 3.2 x 10 -4 M aqueous solution of K2ReC16 kept at 25"C. No changes in the optical properties of the solution were being observed for about 5 hr. Obviously there is a high energetic barrier to reaction
EXPERIMENTAL
Potassium hexachlororhenate is obtained by the reduction of potassium perrhenate using tin(II) chloride in hydro-
(i)
>9
u
II0
100
9O
30
"o -x
% 70
u
T
O.
6O
I.
I
r
Time,
I
4
__-~
I
5
50
c
40
"~ o
30
~ o "6
20
hr
Fig. 1. Variation of pH of the solution (curve 1) and its molar conductivity (curve 3) as function of time after the addition of 1 equivalent of hydroxide; variation of pH of the solution (curve 2) after the addition of 0.5 equivalents of hydroxide. 3843
3846
Notes ReCI~- + H20 ~-~Re(H20)CI ~ + C1- ' ~t Re(OH)CI~- + H +.
(1) (2)
We have traced the changes in conductivity and pH of the same solution as a function of the time after the addition of one equivalent of sodium hydroxide. The results obtained are presented in Fig. 1 (curve I is for pH and 3 for the molar conductivity). The course of the curves shows that both the conductivity and pH of the solution change continuously until a definite value is reached; this value was found to correspond to the total conversion of the hexachlororhenate ions into rhenium dioxide. At the end of the reaction the pH of the solution was 3.01 ; this is in a good agreement with the calculated result (pH = 3-02) for the hydrolysis proceeding by the following scheme, proposed by us :
ReCI~- + ( 4 - m ) H 2 0 + mOH--* Re(OH)4 + ( 4 - m)H ÷ + 6C1-.
(3)
ReO 2 . 2H20 The same picture was found for the hydrolysis of the hexachlororhenate ions with less than one equivalent of hydroxide (e.g. 0-5 or 0,25 equivalents respectively). The curve obtained by the hydrolysis of a 2.99 x 10 -4 M solution of K2ReCI 6 in the presence of 0-5 equivalents of sodium hydroxide is given in Fig. 1 (curve 2). The experimentally obtained value for the pH of the solution at the end of the hydrolytic reaction (pH = 3.02) in this case is also coincident with the value (pH = 2.97) calculated from the proposed scheme (Eqn 3). Obviously the hexachlororhenate ions are hydrolysed mainly by water molecules and the role of the hydroxyl ions is to initiate the hydrolytic reaction.
We have found also that the only stable product of this hydrolysis is ReO 2 . nH20. Soon (5-6 min) after the addition of hydroxyl ions, taken in an amount of less than the stoichiometric amount for the reactt~*n: ReCI~- + 4 OH • ~ ReO2.2H20 + 6C1- the appearance of a colloid is visible. No chloride ions were detected in the precipitate, separated by centrifugation, 5 hr after the addition of one equivalent of hydroxide. Additional evidence in support of the claim that the mechanism of base hydrolysis of potassium hexachlororhenate given by Rulfs and Meyer does not reflect the actual processes taking place in the solution is supplied by our spectrophotometric investigations. We have traced the changes in the electronic spectrum of a 2.99 x 10-'*M solution of potassium hexachlororhenate as function of the time of hydrolysis. The electronic spectra of the solution recorded immediately after the addition of one equivalent of hydroxide (curve l), 10 min (curve 2) and 30 min (curve 3) after the addition, are given in Fig. 2. The electronic spectra were recorded after filtration of part ol the solution through a filter-crucible G-5 so that the initially formed colloidal ReO2. nH20 was removed, The absorption spectra are similar to each other which shows th:~t they belong to one and the same complex species and this complex species is the hexachlororhenate ion (see Fig. 3, 2max= 281, S = 11 800 cm 2 mmol- 2). Hence the nature of the absorption curves does not change during the hydrolytic process. The absorbance at 2,,,,, however, is changed, and this change is in agreement with the concept of conversion of the hexachlororhenate ion into unstable intermediate compounds which later give the sparing soluble ReO2. nH20. The other products of this hydrolysis do not absorb in this part of the spectrum (Na +, K +, CI-). Thus, we could not find in the hydrolysed solution any stable rhenium-containing complexes other than the hexachlororhenate ion. This conclusion is further supported by the correlation between
O" 700
O" 6 0 0
O' 5 0 0
=~ 0.400 ~ 0.300 0"200
0"100
.o
2,~o
2~
2;o
2~'o 2~o 2~o
3to
330
35o ~o
nm Fig. 2. The absorption spectrum of a solution of K2ReC16 recorded immediately after the addition of one equivalent of hydroxide (curve I), after 10 min (curve 2), and after 30 rain (curve 3). Wavelength,
3847
Notes the amount of chloride ion released during the course of the hydrolytic process and the amount of the unaltered hexachlororhenate ions remaining in the solution. The results obtained for the concentration of the released chloride ions by the hydrolysis of a 2-99 x 10-¢M solution of hexachlororhenate ions is given in Table 1. Table 1 Method of determination
Amount of chloride ions (g) After 20 min After 75 min
Potentiometrically Calculated
0.0024 0.0023
0.0042 0.0041
The chloride ions were determined directly in the solution using potentiometric techniques and the concentrations were also calculated from the spectrophotometric data on the concentration of the unchanged hexachlororhenate ions assuming that the hydrolytic process proceeds by scheme (3) and the stable product is ReO 2 . nH~O. The good agreement between the experimental and calculated results provides additional support of the proposed scheme. An attempt was made to determine the order of the reaction with respect to the hydroxyl and hexachlororhenate ions. The results obtained, using known expressions for the rate constants, have shown non-integral and high values, evidence that complex reactions are occurring. The data obtained cannot be used to derive the kinetic equation. CONCLUSIONS
It is evident that reaction (1) has a large activation energy and, at room temperature, the reaction mixture remains
0.700
unaltered for hours. At higher temperatures we were able to observe not only this reaction but also the consecutive ones leading to the formation of rhenium dioxide. The rate of this reaction is pH dependent. An increase in the concentration of the hydrogen ions results in a slower rate due to the decreased activity of the water molecules. On the contrary, any increase in the pH value of the solution leads to an increase in the rate of the hydrolytic process due to the formation of Re(OH)CI~- according to reaction (2). The latter compound, unlike ReCI62- is unstable in aqueous solutions and decomposes immediately. The lowered symmetry determines the fast subsequent hydrolysis which gives a single stable product--ReO2, nH20. The conversion of the hexachlororhenate ions into ReO2. nH20 is a complicated process involving catalytic effects. No evidence for the ionic species Re(OH)3(H20)~, postulated by Rulfs and Meyer could be obtained. M. PAVLOVA N. JORDANOV N. POPOVA
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 13 Bulgaria
REFERENCES
1. C. L. Rulfs and R. J. Meyer, J. Am. chem. Soc. 77, 4505 (1955). 2. M. Pavlova, N. Jordanov and D. Staikov, In press.
I
0-600
O. 5 00
0.400
..Q 0 - 3 0 0
0"200
O" I 0 0
1
240
250
260
270
2BO
Wovelength,
290
300
320
340
360
nm
Fig. 3. The absorption spectrum of an aqueous solution of K2ReCI 6 in the absence of hydroxide (curve I); 2 min after the addition of one equivalent of hydroxide (curve II),