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The complexing characteristics of insoluble selenides. II. Manganese selenide
Journal
Else&r
ofthe
Less-Common
Metals
281
Sequoia S.A., Lausanne - Printed in The Netherlands
THE COMPLEXING CHARACTERISTICS II. MANGANESE SELENIDE*
11. C. ~IEHIIA**
AND A. 0.
ofChemistry,
Department
(Received
OF
INSOLUBLE
SELENIDES.
GUBELI
Lava1
Ukversity,
Quebec.
P.Q.
(Canada)
May z6th, 1970)
The aqueous
solubility
molar ionic concentration
of manganese
selenide
has revealed the formation
determined
at 25°C and unit
of only free manganese
ions and
a complex aqueous species [Mn(HSe)(OH)] in the supernatent phase in contact with the insoluble compound. The solubility product of MnSe and the stability constant of the complex
species have been determined
from the experimental
data.
INTROL)IJCTIOK
The aqueous chemistry
of insoluble chalcogenides
is now acquiring
a good deal
of importance, but there is very little information available, particularly on that of metal selenides and metal tellurides. In general, nothing is known on the solubility behaviour
and complexing
characteristics
of most
of the selenides
and tellurides,
apart from the suggestion that selenides and tellurides are more insoluble than their corresponding sulphidesr. In the course of systematic studies on the solution chemistry of sparingly-soluble chalcogenides in this laboratory, it has been shown that these compounds often give rise to aqueous complexes of exceptionally high stabilities under usual laboratory conditions 2-5. A mathematical treatment has also been developed to identify such complex species formed in the heterogeneous system constituted by a chalcogenide of interest637. A review of literature showed that the solubility characteristics of manganese selenide have not yet been investigated. Even the physico-chemical constant, KsF, solubility IO-ll.50
product, calculated
is unknown for this compound, except for a reported value of from the thermochemical datas. The solubility characteristics of
manganese selenide have therefore been investigated under uniform conditions with the aim of establishing, experimentally, its solubility product, and the nature of the aqueous species formed in the aqueous phase in contact with the precipitated selenide. The interest in the chemistry of selenides originated from earlier observations on metal sulphides, where the formation of thiocomplexes in solution has been amply * I’art I see ref. 5. ** l’rcsent address:
Department
of Chemistry,
University
ol Moncton,
J. Less-Conznzo~z
bloncton, Metals.
N.B., Canada.
22 (1970) 281s285
282
M. C. MEHRA,
A. 0. GUBELI
demonstrated in our studiesz-4, and the work of othersg~i0. In addition, the formation of selenite4 and selenide5 complexes of silver has recently been proved unambiguously in this laboratory. It was, therefore, envisaged that, like silver, manganese may form seleno complexes characteristic of this system. EXPERIMENTAL
The experimental technique adopted in this study was similar to one employed earlier in the study of silver systems435 and other metal sulphideseyr. The manganese selenide was produced in situ by mixing radioactive manganese perchlorate with an excess of sodium hydrogen selenide at any desired acidity. The radiotracer 54Mn employed here was obtained from the Radiochemical Centre, Amersham, England, and all other chemicals used were of A.R. grade purity. The compound, sodium hydrogen selenide, was synthesised in the laboratory whenever necessary and was stored under an inert atmosphere of nitrogen until required. The precipitations were made in a specially constructed Pyrex glass apparatus under an atmosphere of nitrogen and at a unit molar ionic concentration and 25°C temperature. The system was equilibrated as usual for a period of five days following which the supernatent phase was analysed for total manganese, total selenide, and the acidity of the medium, in each case using the earlier established analytical techniques597. The total manganese concentration was, however, obtained radiometrically in a gamma-ray scintillation spectrometer. The solubility characteristics of manganese selenide The experimentally observedsolubility at a constant ionic strength and temperature is recorded in Fig. I. The apparent features of the solubility curve are, total solubility before a pH value of nearly four, a decreasing solubility till pH 6, and an
40 P”“tot 50
6.0
7
9
11
13
PH Fig.
1. Solubility
of
MnSe as a
function of
M; [Cal Set,t. N 0.0148 M. pMrht.
pH and Setot. [C,] Setot. 2: 0.0851 M; [Cz]Semt. N o.m8g (original) = 3.12.
essentially constant solubility thereafter till late in alkaline condition. Solubility evidently remains unaffected by change in the total selenide concentration in the medium in alkaline condition. A careful examination shows that the decreasing slope conforms closely to a value of unity between a pH value of 4 and 6, thus indicating J. Less-Common
Metals,
22
(1970)
281-285
MANGANESE
SELENIDE
283
that the aqueous species formed here is certainly different from that formed in the alkaline condition where the solubility slope assumes a value of zero. The identity of an individual species is now conveniently established by the method of solubility data slopes referred to in earlier publications 3,597. The analysis of the experimental indicates the existence of only free manganese ions along with the decreasing solubility slope, and the presence of a complex species [Mn(HSe) (OH)] in alkaline medium along with the horizontal solubility slope in the system. The solubility product constant of the compound is thus experimentally obtained from the knowledge of the concentration of the free manganese ions in the medium. TABLE
I
soLu~rLnY
PH
PRODUCT OF MnSe AT 25’c *ND UNIT
p Mn 2+
I
4.22
2 3 4 5 6 87
4.32 4.37 4.42 4.55 4.57 4.77 5.30
3.13 3.31 3.40 3.32 3.85 3.35 4.40 5.10
9
5.40 5.40
4.65 3.82
IO
MOLAR
IONIC
j&+
pKsp
1.23
8.64 8.51
II.77 11.82
I.91 I.21 I.LO
9.14 8.39 8.15
12.59 rr.71
1.46 1.13 1.18 1.18 1.98 Mean
8.49 7.96 7.48 7.38 8.18 + 0.31
p.%“t. I.19
12.10
STRENGTH
12.00
11.84 12.30 12.58 12.03 12.00
product of MnSe The necessary calculations are given in Table I. The [Mnlw. in pH region 4-6 is regarded as due to free Mn2+ ions since no other complex in this region has been identified. The constant K,, is obtained from the following expressions:
Solubility
[Mn2+]. [Sez-] = K,,
(1)
[Se&t. = [H&e] + [HSe-] -t [Se2-1.
(2)
Utilising the two ionization constants, KIKz, of the diprotic acid, HzSe, serving l&and source in this system, and rearrangement of eqn. (2) gives:
as a
‘Se2-1 = [H+]z+ Ki[H+] + K1K2 ’ In the pH region where [Mn”+] ions predominate eqn. (3) reduces to:
@-I =
in the heterogeneous
equilibrium,
KdGISeltot. [H+]z+&[H+]
(4)
since both the terms in the denominator remain significant. eqn. (I) gives the final expression for the solubility product
Putting constant
eqn. (4) into of MnSe as,
1 =12.10
f
(5)
0.31 J. Less-Comnzon
Metals,
22
(1970)
281-285
284
M. C. MEHRA,
The formation
constad
A. 0. GUBELI
of complex [Mn(HSe) (OH)]
The constant solubility of MnSe in alkaline condition should be due to a neutral species and our analysis indicates the formation of species [Mn(HSe)(OH)] in that region. Since this species remains unaffected by the experimental parameters, i.e., pH and [Se]tot., it is presumed that the Mntot. concentration along with the experimental slope of zero corresponds to the concentration of this species only. The stability constant of the complex is obtained from the expression:
WWW (OH)1 ‘11 = [Mn2+][HSe-][OH-]
[Mn]t,t, .K2 = K,,r
where K,, is solubility product, K, is ion ionization constant of hydrogen selenide. medium was found to be IO- 6.48 M, which neutral species. Equation (6) in logarithmic @ii
=pMntot. +p&
-pK,,
(6)
product of water, and KZ is the second An average value of MntOt. in alkaline is attributed to the concentration of this form gives,
-pKw
=6.48+11.60-12.10-14.0 p&l = -8.02
ko.20.
(7)
DISCUSSION
The system MnSe forms only simple species as shown by the experimental data of this system, which implies that manganese as a central ion has feeble coordinating properties with any of the selenium ligands in solution. It has earlier been shown, for ions such as silvers and mercury 7, that seleno complexes of exceptionally high stabilities are, in fact, formed without difficulty under laboratory conditions. Hence, it is the property of the central ion that remains the deciding factor in the formation of the aqueous complexes. This observation is not surprising since manganese is known for its poor coordinating abilities in the first transition metal seriesii. It is quite possible that some seleno species with manganese are formed in the system, but their concentrations necessarily remain so weak that solubility characteristics hardly reflect their formation, or, in other words, their concentrations remain inferior to the concentration of the species identified in this study, i.e., [Mn(HSe)(OH)]. It is therefore reasonable to assume that the only species of importance in this case is the neutral complex mentioned above, existence of which appears justified both from experimental and theoretical considerations. This is perhaps the only report on the formation of seleno complexes by manganese selenide. The experimental data of this system have proved useful in another manner. The fact that free manganese ions exist in the medium in near neutral conditions has been successfully employed in the calculation of the solubility product of the compound, which, incidentally, also remains the first experimental observation for any transition metal selenide. The logarithmic value of 12.10 obtained in this study is not far from the theoretically calculated value of 11.50 reported by BUKETOV et aZ.8; the slight difference may be attributed to the high ionic strength of the medium as employed in this study. However, a comparison with any other system is not possible since no experimental data are available for such systems. Since fairly accurate values of the solubility products have been obtained for some sulphide systems by this J.
Less-Common
Metals,22
(1970) 281-285
MANGANESE
SELENIDE
2%
approach29317, it is presumed that the value of manganese selenide is also reliable. A knowledge of the value of the solubility product is often required in many analytical and industrial studies, and also in dealing with genesis of ore deposits and migration of elements in the earth’s crust 10.12.It is therefore hoped that the present data will prove useful in that respect. ACKNUWLEDGEMEiXT
The authors gratefully acknowledge the financial support provided by the National Research Council of Canada. REFERENCES I J, W. MELLOR, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longman, London, 1930. 2 J, STE-MARIE, A. E. TORMA AND A. 0. GUBELI, Can. J. Chem., 42 (1964) 662. 3 A. 0. GUBELI AND J. STE-MARIE, Can. J. Chem., 45 (1967) 2101. 4 M. C. MEHRA AND A. 0. GUBELI, Radiochem. Radioanal. Letters, 2 (2) (Ig6g) 61. 5 XI. C. MEHRA AND A. 0. GUBELI, Can. J. Chem., to be published. 6 J. STE-MARIE, Doctoral dissertation, Lava1 Univ., QuCbec, Canada, 1966. 7 M. C. MEHRA, Doctoral dissevtation, Lava1 Univ., QuCbec, Canada, 1968. 8 E. A. BUKETOV, M. Z. UCORETS AND A. S. PASHINKIN, Russ. /. Inorg. Chem. (English version), 9 (3) (1964) 292. 9 :I. E. MARTELL AND L. G. SILLEN, Stability Constants of Metal Ions, Chem. Sot. Spec. l’ub. No 17, London, 1964. IO H. L. BARNES, Geochemistry of Hydrothermal Ore Deposits, Halt, Rinehart and Winston, New York, 1967. II H. IRVING AND R. WILLIAMS, Nature, 162 (1948) 746. 12 C;. hl. ANDERSON, Econ. Geol., 57 (1962) 809.
,I. Less-Common
Metals, 22 (1970) 281-285
Else&r
ofthe
Less-Common
Metals
281
Sequoia S.A., Lausanne - Printed in The Netherlands
THE COMPLEXING CHARACTERISTICS II. MANGANESE SELENIDE*
11. C. ~IEHIIA**
AND A. 0.
ofChemistry,
Department
(Received
OF
INSOLUBLE
SELENIDES.
GUBELI
Lava1
Ukversity,
Quebec.
P.Q.
(Canada)
May z6th, 1970)
The aqueous
solubility
molar ionic concentration
of manganese
selenide
has revealed the formation
determined
at 25°C and unit
of only free manganese
ions and
a complex aqueous species [Mn(HSe)(OH)] in the supernatent phase in contact with the insoluble compound. The solubility product of MnSe and the stability constant of the complex
species have been determined
from the experimental
data.
INTROL)IJCTIOK
The aqueous chemistry
of insoluble chalcogenides
is now acquiring
a good deal
of importance, but there is very little information available, particularly on that of metal selenides and metal tellurides. In general, nothing is known on the solubility behaviour
and complexing
characteristics
of most
of the selenides
and tellurides,
apart from the suggestion that selenides and tellurides are more insoluble than their corresponding sulphidesr. In the course of systematic studies on the solution chemistry of sparingly-soluble chalcogenides in this laboratory, it has been shown that these compounds often give rise to aqueous complexes of exceptionally high stabilities under usual laboratory conditions 2-5. A mathematical treatment has also been developed to identify such complex species formed in the heterogeneous system constituted by a chalcogenide of interest637. A review of literature showed that the solubility characteristics of manganese selenide have not yet been investigated. Even the physico-chemical constant, KsF, solubility IO-ll.50
product, calculated
is unknown for this compound, except for a reported value of from the thermochemical datas. The solubility characteristics of
manganese selenide have therefore been investigated under uniform conditions with the aim of establishing, experimentally, its solubility product, and the nature of the aqueous species formed in the aqueous phase in contact with the precipitated selenide. The interest in the chemistry of selenides originated from earlier observations on metal sulphides, where the formation of thiocomplexes in solution has been amply * I’art I see ref. 5. ** l’rcsent address:
Department
of Chemistry,
University
ol Moncton,
J. Less-Conznzo~z
bloncton, Metals.
N.B., Canada.
22 (1970) 281s285
282
M. C. MEHRA,
A. 0. GUBELI
demonstrated in our studiesz-4, and the work of othersg~i0. In addition, the formation of selenite4 and selenide5 complexes of silver has recently been proved unambiguously in this laboratory. It was, therefore, envisaged that, like silver, manganese may form seleno complexes characteristic of this system. EXPERIMENTAL
The experimental technique adopted in this study was similar to one employed earlier in the study of silver systems435 and other metal sulphideseyr. The manganese selenide was produced in situ by mixing radioactive manganese perchlorate with an excess of sodium hydrogen selenide at any desired acidity. The radiotracer 54Mn employed here was obtained from the Radiochemical Centre, Amersham, England, and all other chemicals used were of A.R. grade purity. The compound, sodium hydrogen selenide, was synthesised in the laboratory whenever necessary and was stored under an inert atmosphere of nitrogen until required. The precipitations were made in a specially constructed Pyrex glass apparatus under an atmosphere of nitrogen and at a unit molar ionic concentration and 25°C temperature. The system was equilibrated as usual for a period of five days following which the supernatent phase was analysed for total manganese, total selenide, and the acidity of the medium, in each case using the earlier established analytical techniques597. The total manganese concentration was, however, obtained radiometrically in a gamma-ray scintillation spectrometer. The solubility characteristics of manganese selenide The experimentally observedsolubility at a constant ionic strength and temperature is recorded in Fig. I. The apparent features of the solubility curve are, total solubility before a pH value of nearly four, a decreasing solubility till pH 6, and an
40 P”“tot 50
6.0
7
9
11
13
PH Fig.
1. Solubility
of
MnSe as a
function of
M; [Cal Set,t. N 0.0148 M. pMrht.
pH and Setot. [C,] Setot. 2: 0.0851 M; [Cz]Semt. N o.m8g (original) = 3.12.
essentially constant solubility thereafter till late in alkaline condition. Solubility evidently remains unaffected by change in the total selenide concentration in the medium in alkaline condition. A careful examination shows that the decreasing slope conforms closely to a value of unity between a pH value of 4 and 6, thus indicating J. Less-Common
Metals,
22
(1970)
281-285
MANGANESE
SELENIDE
283
that the aqueous species formed here is certainly different from that formed in the alkaline condition where the solubility slope assumes a value of zero. The identity of an individual species is now conveniently established by the method of solubility data slopes referred to in earlier publications 3,597. The analysis of the experimental indicates the existence of only free manganese ions along with the decreasing solubility slope, and the presence of a complex species [Mn(HSe) (OH)] in alkaline medium along with the horizontal solubility slope in the system. The solubility product constant of the compound is thus experimentally obtained from the knowledge of the concentration of the free manganese ions in the medium. TABLE
I
soLu~rLnY
PH
PRODUCT OF MnSe AT 25’c *ND UNIT
p Mn 2+
I
4.22
2 3 4 5 6 87
4.32 4.37 4.42 4.55 4.57 4.77 5.30
3.13 3.31 3.40 3.32 3.85 3.35 4.40 5.10
9
5.40 5.40
4.65 3.82
IO
MOLAR
IONIC
j&+
pKsp
1.23
8.64 8.51
II.77 11.82
I.91 I.21 I.LO
9.14 8.39 8.15
12.59 rr.71
1.46 1.13 1.18 1.18 1.98 Mean
8.49 7.96 7.48 7.38 8.18 + 0.31
p.%“t. I.19
12.10
STRENGTH
12.00
11.84 12.30 12.58 12.03 12.00
product of MnSe The necessary calculations are given in Table I. The [Mnlw. in pH region 4-6 is regarded as due to free Mn2+ ions since no other complex in this region has been identified. The constant K,, is obtained from the following expressions:
Solubility
[Mn2+]. [Sez-] = K,,
(1)
[Se&t. = [H&e] + [HSe-] -t [Se2-1.
(2)
Utilising the two ionization constants, KIKz, of the diprotic acid, HzSe, serving l&and source in this system, and rearrangement of eqn. (2) gives:
as a
‘Se2-1 = [H+]z+ Ki[H+] + K1K2 ’ In the pH region where [Mn”+] ions predominate eqn. (3) reduces to:
@-I =
in the heterogeneous
equilibrium,
KdGISeltot. [H+]z+&[H+]
(4)
since both the terms in the denominator remain significant. eqn. (I) gives the final expression for the solubility product
Putting constant
eqn. (4) into of MnSe as,
1 =12.10
f
(5)
0.31 J. Less-Comnzon
Metals,
22
(1970)
281-285
284
M. C. MEHRA,
The formation
constad
A. 0. GUBELI
of complex [Mn(HSe) (OH)]
The constant solubility of MnSe in alkaline condition should be due to a neutral species and our analysis indicates the formation of species [Mn(HSe)(OH)] in that region. Since this species remains unaffected by the experimental parameters, i.e., pH and [Se]tot., it is presumed that the Mntot. concentration along with the experimental slope of zero corresponds to the concentration of this species only. The stability constant of the complex is obtained from the expression:
WWW (OH)1 ‘11 = [Mn2+][HSe-][OH-]
[Mn]t,t, .K2 = K,,r
where K,, is solubility product, K, is ion ionization constant of hydrogen selenide. medium was found to be IO- 6.48 M, which neutral species. Equation (6) in logarithmic @ii
=pMntot. +p&
-pK,,
(6)
product of water, and KZ is the second An average value of MntOt. in alkaline is attributed to the concentration of this form gives,
-pKw
=6.48+11.60-12.10-14.0 p&l = -8.02
ko.20.
(7)
DISCUSSION
The system MnSe forms only simple species as shown by the experimental data of this system, which implies that manganese as a central ion has feeble coordinating properties with any of the selenium ligands in solution. It has earlier been shown, for ions such as silvers and mercury 7, that seleno complexes of exceptionally high stabilities are, in fact, formed without difficulty under laboratory conditions. Hence, it is the property of the central ion that remains the deciding factor in the formation of the aqueous complexes. This observation is not surprising since manganese is known for its poor coordinating abilities in the first transition metal seriesii. It is quite possible that some seleno species with manganese are formed in the system, but their concentrations necessarily remain so weak that solubility characteristics hardly reflect their formation, or, in other words, their concentrations remain inferior to the concentration of the species identified in this study, i.e., [Mn(HSe)(OH)]. It is therefore reasonable to assume that the only species of importance in this case is the neutral complex mentioned above, existence of which appears justified both from experimental and theoretical considerations. This is perhaps the only report on the formation of seleno complexes by manganese selenide. The experimental data of this system have proved useful in another manner. The fact that free manganese ions exist in the medium in near neutral conditions has been successfully employed in the calculation of the solubility product of the compound, which, incidentally, also remains the first experimental observation for any transition metal selenide. The logarithmic value of 12.10 obtained in this study is not far from the theoretically calculated value of 11.50 reported by BUKETOV et aZ.8; the slight difference may be attributed to the high ionic strength of the medium as employed in this study. However, a comparison with any other system is not possible since no experimental data are available for such systems. Since fairly accurate values of the solubility products have been obtained for some sulphide systems by this J.
Less-Common
Metals,22
(1970) 281-285
MANGANESE
SELENIDE
2%
approach29317, it is presumed that the value of manganese selenide is also reliable. A knowledge of the value of the solubility product is often required in many analytical and industrial studies, and also in dealing with genesis of ore deposits and migration of elements in the earth’s crust 10.12.It is therefore hoped that the present data will prove useful in that respect. ACKNUWLEDGEMEiXT
The authors gratefully acknowledge the financial support provided by the National Research Council of Canada. REFERENCES I J, W. MELLOR, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longman, London, 1930. 2 J, STE-MARIE, A. E. TORMA AND A. 0. GUBELI, Can. J. Chem., 42 (1964) 662. 3 A. 0. GUBELI AND J. STE-MARIE, Can. J. Chem., 45 (1967) 2101. 4 M. C. MEHRA AND A. 0. GUBELI, Radiochem. Radioanal. Letters, 2 (2) (Ig6g) 61. 5 XI. C. MEHRA AND A. 0. GUBELI, Can. J. Chem., to be published. 6 J. STE-MARIE, Doctoral dissertation, Lava1 Univ., QuCbec, Canada, 1966. 7 M. C. MEHRA, Doctoral dissevtation, Lava1 Univ., QuCbec, Canada, 1968. 8 E. A. BUKETOV, M. Z. UCORETS AND A. S. PASHINKIN, Russ. /. Inorg. Chem. (English version), 9 (3) (1964) 292. 9 :I. E. MARTELL AND L. G. SILLEN, Stability Constants of Metal Ions, Chem. Sot. Spec. l’ub. No 17, London, 1964. IO H. L. BARNES, Geochemistry of Hydrothermal Ore Deposits, Halt, Rinehart and Winston, New York, 1967. II H. IRVING AND R. WILLIAMS, Nature, 162 (1948) 746. 12 C;. hl. ANDERSON, Econ. Geol., 57 (1962) 809.
,I. Less-Common
Metals, 22 (1970) 281-285