- Email: [email protected]
On the first equilibrium steps in the acidification of the molybdate ion
J. Inorg. Nuc. Chem., 1959, Vol. 9, pp. 93 to 100. PergamonPressLtd. Printedin NorthernIreland
LETTERS TO THE EDITORS
O n the first equilibrium steps in the acidification of the molybdate ion (Received 6 August 1958)
IT is generally agreed that the molybdate ion in alkaline solution exists in the form of MoOa 2-, and that on acidification, polynuclear complexes are formed. However, there is considerable confusion regarding the formulae of the latter. For instance, JANDER et al. ~I~ claim the existence of Mo3Oll ~-, HMoaOlx a-, HM0602~ 5-, H2M060~l 4-, HaMo~O~I3-, HTM012041 a-, HTMoz4OTs 5- , and H~Mo~4OTaa-. BY~(2~ claims the existence of M07024 ~-, M060204-, M040132-, and HMo~O203-. A survey of w o r k up to 1950 was made in the dissertation of LINDQVlST.(3~ By X-ray crystallographic measurements, LINDQVIST could confirm the existence of the ions MoTO~a6- and Mo80204- in crystal structures ;(a,4) from spectrophotometric measurements he also found evidence for these ions in solution. (Sj Some time ago, Y. SASAKI started a study of molybdate equilibria, applying the experimental technique and the graphical and mathematical methods developed by SILLI!N et al. (6~ in Stockholm. All equilibria were studied at 25°C, with NaC10~ as ionic medium, the sodium ion concentration, [Na+], being kept constant at 3 M. The cells used were of the type RE/solution/glass electrode, or RE/solution, Q,QH2/Pt, where R E = Ag,AgC1/0.01 M AgC104, 2.99 M NaC104/3 M NaC104. By calibrating with solutions of known H + concentration and the same ionic medium, it was possible from the measured e.m.f, to calculate (H +) = h in each solution. From the analytical data one knows B (the total molybdate concentration), and H (the excess of protons over MoO42- and H~O) From the data one may calculate the average number of H + that have been taken up per MoOa 2-, Z = ( H -- h)B -x, and the average charge per Mo, z -- Z -- 2. In Fig. 1, Z and z are plotted as functions of log h, up to Z = 1-4 (z = --0"6). Each series of points corresponds to experiments having the same value of B; nine values of B, between 0.6 m M and 160 raM, were used. The curves are calculated with the reactions and equilibrium constants given below. By straightforward mathematical analysis of the data, using methods previously worked out in this laboratory, (~) it was deduced that the main complex formed is Mo70246-. Strictly speaking, the equilibrium data can only give the number of molybdenum atoms in the complex (which is seven), and the charge ( 6 - ) , but cannot distinguish between complexes with various water contents. Thus the assignment as Mo702~ °-, and not for instance (MoOa)7(OH)66-, is based on the evidence from crystal structures.(3, 4) Deviations at low concentrations and low values of Z could be explained assuming HMoO4- to be the only mononuclear complex besides MOO42-; with only H2MoO4 the agreement would not be so good. However, our data are not incompatible with those of SCHWARZENBACH and MEIER,~v) who found H2MoO4 to be the predominating species at higher Z. ~) G. JANnER, K. F. JAHR and W. HEUKESHOWN,Z. Anorg. Chem. 194, 383 (1930). c2) j. BvL Ann. Chim. (France), 20, 463 (1945). (diss, Paris); C.R. Acad. Sci., Paris238, 239 (1954); Bull. Soc. Chim. Fr. 1023 (1957). (a) I. LINDQVIST, Nova Acta Regiae Soc. Sci. Upsaliensis ser IV, 15, Nr 1 (1950). ~4~ I. LINDQVIST,Arkiv Kemi 2, 325 (1950); Acta Cr)st 3, 159 (1950). 15~ I. LINDQVIST, Aeta Chem. Scand. 5, 568 (1951). (61 S. H~ETANENand L. G. S~LL/~N,Acta Chem. Scand. 6. 747 (1952); G. B~Et)~aMA~Nand L. G. SILL~.~, Arkiv Kemi 5, 425 (1953); L. G. SILL~N, Acta Chem. Scand. 8, 318 (1954); 10, 186, 803 (1956); F. RossoTri and H. Rossox'r~, Aeta Chem. Seand. 10, 957 (1956). (7~ G. SCnWARZESSACHand J. ME,Ere, J. Inorg. Nucl. Chem. 8, 302 (1958). 93
2.C
94
Letters to the editors
1-5
-0.5
TITRATION ,ooZ/o r~~~ / !////i///mM=[ivl°] / / " ,/'
-1.o
//
~'REVERSE
:d
/10 15 /2.5/'1.2.10.6
(2-.=
¢
/
/
-1.5
.....
MON©NUCLE,&R WALL
-2-0
-~
-6
4 ~og IN+]
-4
-A
FIG. 1.--Equilibria on acidification of molybdate ion (3 M NaCIO 4, 25°C). Horizontal axis : log h. Vertical axis: z (average charge per Mo etom) and Z (average number of protons bound per MoO42-). Points: experimental points for various total concentrations of Mo. For the highest Mo concentration, different symbols refer to titrations in opposite directions. Full curves, calculated with the reactions and equilibrium constants given in the text.
Between z = --1.2 and --0.6, it was deduced from the data that the species HMOTO~4 s- and probably also H~Mo70244-, are formed. The following equilibrium constants were deduced: Reaction MoO~ 2- + H + ~- H M o O 4 7 M o O t 2- 4- 8 H + ~ Mo7024 ~- + 4 H~O Mo70246- + H + ~ HMo702i 5HMo70245- + H + ~ H2MoTO2~ 4-
log K 4.08 ± 0.15 57'7 ± 0-2 4'33 4- 0'02 ~ 3"7
The last species is somewhat uncertain; at one end o f the region where it would be important, there are deviations indicating other complexes as well. The equilibria at z > --0"6 are now being investigated by Y. SASAKI. The agreement obtained over a wide range o f concentrations and p H seems to be quite satisfactory. W e think it is unlikely that as good agreement could be obtained substituting any other mechanism for the three first reactions. Full details o f the work will be published elsewhere. It has been supported by the Swedish Natural Science Research Council. Y. SASAKI Department of lnorffanic Chemistry I. LINDQVIST Royal Institute of Technology L . G . SILL~N
Stockholm. Sweden
LETTERS TO THE EDITORS
O n the first equilibrium steps in the acidification of the molybdate ion (Received 6 August 1958)
IT is generally agreed that the molybdate ion in alkaline solution exists in the form of MoOa 2-, and that on acidification, polynuclear complexes are formed. However, there is considerable confusion regarding the formulae of the latter. For instance, JANDER et al. ~I~ claim the existence of Mo3Oll ~-, HMoaOlx a-, HM0602~ 5-, H2M060~l 4-, HaMo~O~I3-, HTM012041 a-, HTMoz4OTs 5- , and H~Mo~4OTaa-. BY~(2~ claims the existence of M07024 ~-, M060204-, M040132-, and HMo~O203-. A survey of w o r k up to 1950 was made in the dissertation of LINDQVlST.(3~ By X-ray crystallographic measurements, LINDQVIST could confirm the existence of the ions MoTO~a6- and Mo80204- in crystal structures ;(a,4) from spectrophotometric measurements he also found evidence for these ions in solution. (Sj Some time ago, Y. SASAKI started a study of molybdate equilibria, applying the experimental technique and the graphical and mathematical methods developed by SILLI!N et al. (6~ in Stockholm. All equilibria were studied at 25°C, with NaC10~ as ionic medium, the sodium ion concentration, [Na+], being kept constant at 3 M. The cells used were of the type RE/solution/glass electrode, or RE/solution, Q,QH2/Pt, where R E = Ag,AgC1/0.01 M AgC104, 2.99 M NaC104/3 M NaC104. By calibrating with solutions of known H + concentration and the same ionic medium, it was possible from the measured e.m.f, to calculate (H +) = h in each solution. From the analytical data one knows B (the total molybdate concentration), and H (the excess of protons over MoO42- and H~O) From the data one may calculate the average number of H + that have been taken up per MoOa 2-, Z = ( H -- h)B -x, and the average charge per Mo, z -- Z -- 2. In Fig. 1, Z and z are plotted as functions of log h, up to Z = 1-4 (z = --0"6). Each series of points corresponds to experiments having the same value of B; nine values of B, between 0.6 m M and 160 raM, were used. The curves are calculated with the reactions and equilibrium constants given below. By straightforward mathematical analysis of the data, using methods previously worked out in this laboratory, (~) it was deduced that the main complex formed is Mo70246-. Strictly speaking, the equilibrium data can only give the number of molybdenum atoms in the complex (which is seven), and the charge ( 6 - ) , but cannot distinguish between complexes with various water contents. Thus the assignment as Mo702~ °-, and not for instance (MoOa)7(OH)66-, is based on the evidence from crystal structures.(3, 4) Deviations at low concentrations and low values of Z could be explained assuming HMoO4- to be the only mononuclear complex besides MOO42-; with only H2MoO4 the agreement would not be so good. However, our data are not incompatible with those of SCHWARZENBACH and MEIER,~v) who found H2MoO4 to be the predominating species at higher Z. ~) G. JANnER, K. F. JAHR and W. HEUKESHOWN,Z. Anorg. Chem. 194, 383 (1930). c2) j. BvL Ann. Chim. (France), 20, 463 (1945). (diss, Paris); C.R. Acad. Sci., Paris238, 239 (1954); Bull. Soc. Chim. Fr. 1023 (1957). (a) I. LINDQVIST, Nova Acta Regiae Soc. Sci. Upsaliensis ser IV, 15, Nr 1 (1950). ~4~ I. LINDQVIST,Arkiv Kemi 2, 325 (1950); Acta Cr)st 3, 159 (1950). 15~ I. LINDQVIST, Aeta Chem. Scand. 5, 568 (1951). (61 S. H~ETANENand L. G. S~LL/~N,Acta Chem. Scand. 6. 747 (1952); G. B~Et)~aMA~Nand L. G. SILL~.~, Arkiv Kemi 5, 425 (1953); L. G. SILL~N, Acta Chem. Scand. 8, 318 (1954); 10, 186, 803 (1956); F. RossoTri and H. Rossox'r~, Aeta Chem. Seand. 10, 957 (1956). (7~ G. SCnWARZESSACHand J. ME,Ere, J. Inorg. Nucl. Chem. 8, 302 (1958). 93
2.C
94
Letters to the editors
1-5
-0.5
TITRATION ,ooZ/o r~~~ / !////i///mM=[ivl°] / / " ,/'
-1.o
//
~'REVERSE
:d
/10 15 /2.5/'1.2.10.6
(2-.=
¢
/
/
-1.5
.....
MON©NUCLE,&R WALL
-2-0
-~
-6
4 ~og IN+]
-4
-A
FIG. 1.--Equilibria on acidification of molybdate ion (3 M NaCIO 4, 25°C). Horizontal axis : log h. Vertical axis: z (average charge per Mo etom) and Z (average number of protons bound per MoO42-). Points: experimental points for various total concentrations of Mo. For the highest Mo concentration, different symbols refer to titrations in opposite directions. Full curves, calculated with the reactions and equilibrium constants given in the text.
Between z = --1.2 and --0.6, it was deduced from the data that the species HMOTO~4 s- and probably also H~Mo70244-, are formed. The following equilibrium constants were deduced: Reaction MoO~ 2- + H + ~- H M o O 4 7 M o O t 2- 4- 8 H + ~ Mo7024 ~- + 4 H~O Mo70246- + H + ~ HMo702i 5HMo70245- + H + ~ H2MoTO2~ 4-
log K 4.08 ± 0.15 57'7 ± 0-2 4'33 4- 0'02 ~ 3"7
The last species is somewhat uncertain; at one end o f the region where it would be important, there are deviations indicating other complexes as well. The equilibria at z > --0"6 are now being investigated by Y. SASAKI. The agreement obtained over a wide range o f concentrations and p H seems to be quite satisfactory. W e think it is unlikely that as good agreement could be obtained substituting any other mechanism for the three first reactions. Full details o f the work will be published elsewhere. It has been supported by the Swedish Natural Science Research Council. Y. SASAKI Department of lnorffanic Chemistry I. LINDQVIST Royal Institute of Technology L . G . SILL~N
Stockholm. Sweden