RAMANSPECTRA OF M ~ ( N O B ) ~ - HSYSTEM ~O
1019
A Raman Spectroscopic Investigation of the Magnesium Nitrate-Water System by Mordechai Peleg Department of Inorganic and Analytical Chemistry, T h e Hebrew University of Jerusalem, Jerusalem, Israel (Received August SI, 1971) Publication costs borne completely by T h e Journal of Physical Chemistry
The magnesium nitrate-water system has been examined by Raman spectroscopy covering the entire range from highly dilute solution to the anhydrous molten salt mixture (hlg(N03)2-NaN03). The results indicate that the above system can be divided into five approximate regions. A t high dilutions both ions are completely hydrated, the nitrate ion being perturbed by the water molecules. As the water content is lowered, the polarization power of the magnesium begins to affect the nitrate ion, but no contact ion pairing occurs until the water content is reduced below 6 moles of water per mole of salt. Iselow this region both contact ion pairs and solventseparated species are in equilibrium. A t melt concentrations of Mg(NOa)2.2.4H20 and Mg(N03)2.2.OHzO, a specific rearrangement occurs producing Raman bands that are not present at either of the higher water contents or in the anhydrous melt. A perturbed quasilattice structure is suggested for this region. Evidence is also presented indicating that molten (Mg(NO&- 6H20behaves as a hydrated melt.
Introduction Raman spectroscopy is being increasingly used to investigate ionic association and solvation in solution,’+ especially in aqueous metal nitrate systoms14 since the vibrational modes of the polyatomic ion are sensitive to their environment and (Ian be used as an indicator of interionic interactions. An isolated and unperturbed NOo- ion would have D3hsymmetry and give rise to only four vibrational frequencies, corresponding to the species AI’ (R) A2” (ir) 2E’ (R, ir), where.R and ir indicate Raman and infrared activity, respectively. Lowering of thc NO3- ion symmetry by loss of the equivalence of the thrcc oxygen atoms, to produce Czo or C, symmetry, is expected to lead to loss of degeneracy from both of thc E’ modes whether the nitrate acts as unidentate5 or bidentat@ ligand. Howcver, it has bccn noted that the anticipated splitting of the v4(E’) mode does not occur unless contact ion pairs are present in the solution, while thc v3(E’) band appears as a doublet even in very dilute aqueous solutions. This splitting is attributed to perturbation of the nitrate ion by watcr mole~ules.~~7 Angella has proposed that fused hydratrd salt¶ such as calcium nitrate tctrahydrate and magnesium chloride hexahydrate might be treated as moltcn salts with large cations, with the water bound up to the cation. The proposal of a hydrated cation has been further substantiated by such studies as transport propertiesg~”J and volumetric properties” in Ca(N03)2.4 H 2 0 and in its mixtures with KN08 and by spectrophotometric studies of Ni(I1) in aqueous magnesium chloride solutions.12 Ellis and Hester,I3 however, have shown that the anion in a hydrate melt significantly affects the pmr chemical shift, interpreting this to support displacement equilibria of the type considered in a quasilattice model, with anion displacement of water mole-
+
+
cules from the metal ion inner coordination spheres. Furthar, Raman and infrared spectral studies of concentrated aqueous calcium nitrate solution^^^^'^ and cadmium nitratc solutions16 have been intcrprctcd in terms of the formation of contact ion pairs such as CaKOs+ and Cdx03+. As stated by B r a u n s t c i ~ i , ~ ~ contact ion pairs might not be inconsistent in moltcn tetrahydrates (accompanied by somc distortion of thc coordination shcll) with Angctll’ss proposal of a large hydrated cation, but it is morc difficult to reconcile with hexahydrated cations. The present report prcscnts results on the 11g(N03)2H20 system covering almost thc cwtire concentration range from dilute solutions to ultraconcontrated ayucous solutions (2 mol of water to 1 mol of hIg(K03)2)in order to investigate whethcr a hexahydrate melt also (1) G. J. Janz, J . Electronal. Chem., 29, 107 (1971). (2) It. E. Hester, Annu. Rept. Progr. Chem., 66A,79 (1969). (3) D. E. Irish in “Ionic Interaction: Dilute Solutions to Molten Salts,” S. Petrucci, Ed., Academic Press, New York, N. Y . , 1971, Chapter 9. (4) D. E. Irish, A. It. Davis, and I t . A. Plane, J . Chem. Phys., 50, 2262 (1969). (5) H. Brintziriger and R. E. Ilester, Inorg. Chem., 5 , 980 (1966). (6) R. E. Hester and W. E. L. Grossman, i b i d ., 5, 1308 (1966).
(7) A. R. Davis, J. W. Macklin, and R. A. Plane, J . Chem. Phys., 50, 1478 (1969).
(8) C. A. Angell, J . Electrochem. Soc., 112, 1224 (1965). (9) C. T. Moynihan, J . P h y s . Chem., 70,3399 (1966). (10) J. Braunstein, L. Orr, A. R. Alvarez-Fumes, and H. Braunstein, J . Electroanal. Chem., 15, 337 (1967). (11) J. Braunstein, L. Orr, and W. Macdonald, J . Chem. Eng. Data, 12, 415 (1967). (12) C . A. Angell and D. (1966). (13) (14) (15) (16) (17)
M .Gruen, J . AmeT. Chem. SOC., 8 8 , 5192
V. S.Ellis and R. E. Hester, J . Chem. SOC.A , 607 (1969). R. E. Hester and R. A . Plane, J . Chem. Phys., 40,411 (1964). D. E. Irish and G. E. Walrafen, ibid., 46, 378 (1967). A. R . Davis and It. A. Plane, I n o ~ g Chem., . 7, 2565 (1968). J. Braunstein in ref 3, Chapter 4. T h e Journal of Physical Chemistry, Vol. 76, N o . 7 , 1972
MORDECHAI PELEC)
1020 Table I: Nitrate Frequencies Observed in the Raman Spectra of the Mg(NOa)z-HaO System" RH~O
--
77 42 20 13 9.5 6 5.1 4.2 3.4 2.9 2.4 2.0
Frequenoy, cm---
363 356 354 348 334 344 332 322 322 322 (vw)
717 716 715 712 711 711 719 717 717 717 717 717
719 720 722 725 728 746 751 751 754 755
1048.5 1048.5 1047.5 1047.5 1048.5 1049 1049.5 1053 1053 1052 1034 1038
1060 1066
7
1348 1348 1338 1332 1342 1347 1341 1337 1334 1341 1328 1333
1400 1400 1407 1409 1428 1445 1445 1451 1461 1468 1486 1493
1330
1485
1619 1640 1644 1644 1639 1644 1651 1650
Temp, "C
90 90 90 100 100
1658 1661 1657 1656 1652 1652 1641 1640 1634 1634 1634 1634
120 120 120
1620
200
100
100 100 100
Mg(NOa)z/ NaN03 1/1-35 Assignment
(Cd
713
757
Mg-OH2 (A) Symstr
vs(B1)
va(Ai)
P sh
dP msp
812
1051
va(Bz)
vz(A1)
P msp
P SP
vi(A1)
v4@1)
P b
dP b
' p = polarized, dp = depolarized, sh = shoulder, m = medium, sp = sharp, b = broad, vw shows contact ion pairs, and also to study the structure of concentrated solutions containing R!Ig(N03)z.
Experimental Section Baker Analyzed reagent grade N!g(NO& 6H20 was used as the starting material without any further purification. The water content of the starting material was ascertained to be 6H20 by heating the hexahydrate under vacuum at 120" for several hours until constant weight was attained. Samples containing more than 6 moles of water per mole of salt were prepared by adding water to the hexahydrate, while samples containing less than 6 moles of water were prepared by removing controlled amounts of water from the hydrated salt by careful heating and then weighing tJo measure the amount of water vaporized. All samples were filtered under pressure through a glass frit directly into the Raman sample tube. The solutions mere heated in a simple tube furnace to the desired temperature. The laser Raman apparatus employed in the investigations is that described by Clanssen, et aZ.,l8 and included the following commercial units: a Spex 1400 I1 double monochromator, a Spectra-Physics 125 He-Ne laser, a Spex cryostat fitted with an ITT FW 130-8-20 photomultiplier detector, a Victoreen 1001 dc amplifier, and a Texas Instruments recorder. Further details are incIuded in the cited reference. The positions of the Raman bands were calibrated with reference to the neon lines. The concentrations recorded in this paper are given as mole ratios, R, moles of HzO per mole of Mg(NOs)z. Most of the spectra were run at a temperature of 100" except for the very dilute solutions, where a
-
The Journal of Physical Chemistry, Vol, 7 6 , No. 7 , 19%'
=
b(HOH)
dP m
2vz
P m
very weak.
temperature of 90" was chosen (owing to the high vapor pressure of the water), and for the most concentrated solutions (120"). Experiments have shown that over a limited temperature range the positions of the spectral line do not change.
Results The results are recorded in Table I for the entire spectrum range covered. Also included for comparison are our preliminary results for the Mg(NO&NaN03 mixture (at a ratio of 1 mol of Mg(NOa)zto 1.35 mol of NaN03). The Raman band positions are believed to be correct to within 1 0 . 5 cm-l for the sharp Raman bands and within i l cm-' for the broader bands. Where overlapping or superimposed Raman bands existed or were believed to exist, a computer technique was used to analyze the bands for the best fit to the Gaussian function. As pointed out by Irish, et uZ,,le and observed in our own present results, discretion and considerable physical knowledge of the system must be applied in such analyses lest nonexistent lines be invoked. As observed from Table I, the frequency of the CU. 1050-cm-1 Raman band is quite concentration dependent (Figure 1). Another interesting fact is the sudden appearance of two bands around 1050 cm-1 for R equal to 2.4 and 2 (Figure 2). A value of 1048.5 f 0.5 cm-l obtained in the most dilute solution ( R = 77) is in good agreement with the value of 1049 f 0.5 em-' reported by Irish and Davis2* for dilute alkali (18) H. H. Claassen, H. Selig, and J. Shamir, A p p l . Spectrosc., 23, 8 (1969). (19) D. E.Irish and H. Chen, ibid., 25, 1 (1971). (20) D. E.Irish and A. R . Davis, Can. J. Chem., 46,943 (1988).
1021
RAMAN SPECTRA OF M~(NOFJ~-H~O SYSTEM
-'E -----
I
-Om-\
10461 I
80
60
I
40
1
,
1
2 0 1 0 8
,
6
RH20
,
4
1
2
1
0 Mg(NO&
Figure 1. The variation of the Raman band position a t ca. 1050 cm-1 with the mole ratio concentration R H ~ofOmagnesium nitrate: (0)Mg(NO&-H,O, ( 0 )M ~ ( N O & N ~ N O I (ratio 1/1.35).
n
~
PO 0'0
750 p'
0 mode
740
--
EO
70
60
50
40
30
20
-ERH206
10
4
2
0 Mg(hO&
Figure 3. The variation of Raman band positions in the 700-750-cm-1 region with the mole ratio concentration of magnesium nitrate: (0, 0, 0 ) Mg(NO&-HtO, (a) Mg(NO&NaNOI (ratio 1/1.35).
717
A713
1
741
Figure 4. Raman spectra of Mg(NO& at various mole ratio concentrations in the 700-750-cm-1 region: (a) R H ~= O 42, (b) R H ~ O = 6, (c) R H ~ O = 2, (d) Mg(NOa)2-NaNO3 (ratio 1/1.35), (e) Ca(NOs)2.4Hz0.
1095
985 cifl
Figure 2. Raman spectra of Mg(NOa)z a t various mole ratio concentrations in the ea. 1 0 5 0 - ~ m -region: ~ (a) R E ~ O = 42, (b) RH,O= 2.9, (c) R E ~ O = 2.4.
nitrate solutions. On decreasing the water content, it appears as if the Raman band frequency slowly decreases to a minimum value of 1047.5 cm-' at R = 20 and 13. Between R = 6 and 4.2, there is a sharp increase to a value of 1053 cm-' and then once more a slight decrease which appears to approach the value for the pure dry salt mixture (cf. alkali metal nitrate solutions) .20 The frequencies of the 700-750-~m-~Raman bands are markedly concentration dependent, as observed iii Figures 3 and 4. It is observed that at high dilu-
tion only one frequency is noted (Figure 4a), and it is only a t R = G that the appearance of a slight shoulder on the high-frequency side of the band is visually noted (Figure 4b). This splitting of the Raman band becomes increasingly apparent as the water content decreases (Figure 4c,d). However, by applying computer techniques it was possible to resolve the 700750-cm-' band into two separate bands even for solutions where R = 20, as shown in Table I. The unresolved two main peaks in the 1300-1500cm-l region are listed in Table I. The variation of the two Raman band positions and their change of band shape with concentration are shown in Figures 5 and 6. As the water content is reduced, it is noted that the spectra become more complex and there appear to be region (Figure three or four lines in the 1300-1500-~m-~ 6). This region was examined by a computer technique, and it was observed that for almost all concentrations it was possible to fit three and/or four bands to the observed Raman spectra. The results obtained therefore had to be treated with great care lest nonexisXhe Journal of Physical Chemistry, Vol. 76, N o . 7 , 1072
1022
MORDECHAI PELEG
150C
P
,o
0
,' 0
RH*O
,' 0
-
1450
