758
Inorg. Chem. 1980, 19, 758-761 Registry No. 1, 72525-56-5; 2, 72525-57-6; 3, 72525-58-7; t-Bu(Me3Si)NSiMe2H,72525-59-8; Me(Me3Si)NSiMe2H,72525-60- 1; ~ - B U N ( S ~ M ~ 72525-61-2; ,H)~, t-Bu(Me3Si)NSiMe2C1,72525-62-3; t-BuN(SiMe,CI),, 72525-63-4; (Me3Si)*NSiMe2H, 17067-57-1; (Me3Si)2NSiMe2C1, 1586-72-7; Me3SiN(SiMe2H)2,16642-71-0; Me3SiN(SiMe2C1),, 18790-09-5; t-BuN(H)SiMe2H, 18182-35-9; Me2SiHC1, 1066-35-9; t-Bu(Me,SiH)NLi, 72525-64-5; Me(Me3Si)NLi, 10568-44-2; t-Bu(Me3Si)NLi, 18270-42-3; t-Bu(Me&)”, 5577-67-3; BCl3, 10294-34-5.
fractional condensation did not completely separate 3 from small amounts (ca. 10% by ‘H NMR) of unidentified impurities. Acknowledgment, The author thanks the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Robert A. Welch Foundation for generous financial support. The l l B N M R spectrum was recorded by Dr. Christopher Foret of the University of Texas at Arlington.
Contribution from the Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, and the Department of Chemistry, Depauw University, Greencastle, Indiana 461 35
Fluorosilicate Equilibria in Sodium Chloride Solutions from 0 to 60
OCt
R. H. BUSEY,la EUGENE SCHWARTZ,lb and R. E. MESMER*la Receiued July 20, 1979 A potentiometric method employing quinhydrone and solid-state fluoride electrodes was used to observe the free hydrogen
ion and the free fluoride ion concentrations in equilibrium experiments in aqueous solutions at 0-60 O C . In dilute silicic acid solutions the predominant reaction can be expressed as Si(OH),(aq) + 4H’ + 6 F + SiF62-+ 4 H 2 0 . The logarithm of the equilibrium quotient has values of 31.610, 29.980, and 28.23 at 0, 25, and 60 OC in 1 m NaCl (molal units). There is evidence for the existence of small amounts of additional species with fewer fluorides under certain conditions. The absence of a regular sequence of species from the array Si(OH)4-xFyX-Y is proven by an analysis of the results. The average number of fluoride ions complexed by millimolal silicic acid approaches 6.0 in 0.01 m fluoride as the pH is reduced to 3 or lower in titration experiments. Simultaneously the average number of hydrogen ions consumed approaches 4.0.
Introduction There is considerable interest in the detailed chemistry of dilute solutions that occur naturally and in practical processes. The fluoride concentrations in geothermal brines are commonly found in the range 10-3-104 m.2a The interactions that occur between fluoride ion and silicic acid in dilute solutions are poorly understood. The only species in aqueous solution whose identity is well-knownzbfrom Raman and N M R spectroscopy is SiF6’-, an octahedral ion. Also, a common means for preparation of salts of hexafluorosilicic acid is by precipitation from aqueous solution. However, there are no definitive equilibrium data involving this species with other dissolved species of silica. There are indeed a large number of conceivable species of the type Si(OH)4-xFyX-Ywhich might occur and must be considered if a complete description of even dilute solutions is to be made. For the analogous ~ y s t e m ~B-F, - ~ a sequence of species has been observed including BF4-, BF30H-, BF2(OH),-, and probably BF(OH)3- in dilute solutions, all of which involve replacement of a hydroxide ion from tetrahedral B(OH),-. There have been several reports6-11describing equilibria of SiF?’ with other fluorosilicates or with Si(OH)4, but none of these studies present conclusive data. Silica can be dissolved in relatively concentrated H2SiF6 solutions to attain SiOz concentrations well beyond the solubility of amorphous silica, indicating the formation of additional species. Several authors’ 1,12 have suggested the presence of species such as SiF5- and SiE,(aq) to account for such observations. In this paper we have employed precision potentiometric methods for the examination of the complexing behavior in dilute (0.001 m) silicic acid solutions in 1 m NaC1.
Experimental Section Materials. A stock solution of about 5.2 m NaCl prepared from Fisher Scientific Co. analyzed reagent was purified by acidifying with hydrochloric acid to pH 3.5 and sparging with Hz to remove CO,.
‘Research sponsored by the Division of Engineering, Mathematics, and Geosciences, Office of Basic Energy Sciences, U S . Department of Energy under Contract W-7405-eng-26 with the Union Carbide Corp.
0020-1669/80/1319-0758$01.00/0
After sparging, the solution was again neutralized with a small amount of carbonate-free NaOH solution. A stock solution of 0.25 m N a F and 0.75 m NaCl was prepared from recrystallized NaF (J. T. Baker Chemical Co.) and the above NaCl stock solution. The recrystallized N a F was dried at 150 OC. Stock solutions of 1 rn NaOH, 1 rn HCI, and approximately 0.5 m Si(IV), the same solutions employed and described in our silicic acid equilibrium studies,I3 were employed together with the two above stock solutions to prepare the required cell solutions, reference solutions, and titrant solution. Quinhydrone (Eastman Kodak Co.) was added as a solid to the cell-solution aliquot and reference-solution aliquot when these solutions were introduced into the electrode compartments of the cell, giving a concentration of approximately 0.003 m. The silicic acid concentration used in these experiments was about 0.001 m, which is believed to be below the saturation level for amorphous silica in these experiments. Should the solubility be exceeded by a small amount in the pH range of these experiments, it is well-known14 that polymerization of the Si(OH)4is very slow under these conditions. Potentiometric Cells. The potentiometric-cell assembly was similar to that described previously for the study of fluoride complexes of berylli~m.’~ However, for most of this work quinhydrone electrodes were used in place of hydrogen electrodes. The cell consisted of an (1) (a) Oak Ridge National Laboratory. (b) Depauw University. (2) (a) S. R. Cosner and J. A. Apps, “A Compilation of Data on Fluids from Geothermal Resources in the United States”, Report 5936, Lawrence Berkeley Laboratory, Berkeley, Calif., May 1978. (b) P. A. W. Dean and D. F. Evans, J . Chem. Soc. A, 698 (1967). (3) R. E. Mesmer, K. M. Palen, and C. F. Baes, Jr., Inorg. Chem., 12, 89 (1973). (4) S. L. Grassino and D. N. Hume, J . Inorg. Nucl. Chem., 33,421 (1971). ( 5 ) Y. Moriguchi and I . Hosokawa, Nippon Kagaku Zasshi,92, 56 (1971). (6) L. G. Silldn and A. E. Martell, Chem. Soc., Spec. Publ., No. 17 (1964); Suppl. No. 1 , No. 25 (1971). (7) V. N . Plakhotnik, Russ. J . Phys. Chem. (Engl. Transl.), 48, 1550 (1974). (8) P. M . Borodin and N. K. Zao, Russ.J . Inorg. Chem. (Engl. Transl.), 16, 1720 (1971). (9) P. M. Borodin and N. K. Zao, Russ. J . Inorg. Chem. (Engl. Transl.), 17, 959 (1972). ( I O ) V. N. Plakhotnik and T. K,Kotlyar, Russ. J . Phys. Chem. (Engl. Transl.), 50, 723 (1976). ( 1 1) I. G. Ryss and V. N. Plakhotnik, Rum. J . Inorg. Chem. (Engl. Traml.), 15, 1742 (1970). (12) S. M . Thomsen. J . A m . Chem. Soc., 74, 1690 (1952). (13) R. H. Busey and R. E. Mesmer, Inorg. Chem., 16, 2444 (1977). (14) A. C. Makrides et al., “Study of Silica Scaling from Geothermal Brines”, Rewrt COO-2607-5, U S . Department of Energy, Washington, D.C., Jan i978. ( 1 5) R. E. Mesmer and C. F. Baes, Jr., Inorg. Chem., 8, 61 8 (1 969).
0 1980 American Chemical Society
Inorganic Chemistry, Vol. 19, No. 3, 1980 759
Fluorosilicate Equilibria in N a C l Solutions all-Teflon vessel with several ports for platinum electrodes, the lanthanum fluoride solid-state electrode, gas inlets, and titrant inlets. Quinhydrone (QH) was added to each compartment to make the final concentration about 0.003 m. The assembly was submerged in a constant-temperature(fO.O1 "C) bath, and both solutions were purged with hydrogen gas for 45 min before beginning a titration. The cell representation is shown by (I). Reference compartment B contained soln A soln B
n
I
a solution of 0.005 m HC1 in 1.0 m NaCl, and this composition remained unchanged during the experiments. The concentration of NaF was varied from 0.001 to 0.009 m at 0,25, and 60 O C . A titrant containing HCl and the medium salt was added by means of a syringe microburet to vary the acidity from the nearly neutral condition to pH 2. The reference for the fluoride electrode was the initial fluoride concentrationin the outer compartment (adjusted for any small amount of H F that might be present). Equilibrium was established rapidly, usually within 3 min. Free H+ and F Concentration Measurements. A high concentration of electrolyte was used to control the medium and the ionic activity coefficients. The difference in potential between the two quinhydrone electrodesis given by eq 1 where r refers to the solution in the reference
2
10
i
0 5 nF
nH
A
0
0
I
I O O O l m SI(EI
I
1
1.
I 4
3
I 2
I 6
compartment (B), [i] represents the molality of the ith ionic component, and the coefficients Di are derived from the Henderson equation for liquid junction potentials (see ref 15). The approximation was made that Da- = DF = DHF2-and that DH, = 0. Since the concentration of silicic acid in these studies was always 1/1000th of that of the medium salt, the fluorosilicateswere not included in the liquid junction calculation. In an analogous manner, the fluoride concentration was calculated from the potential of the fluoride electrode vs. the reference hydrogen electrode. In this case the potential between the lanthanum fluoride electrode and the reference electrode was observed, but the difference from the initial value was used to calculate the fluoride concentration for each point, i.e.
where r refers to the initial solution in the titration. Data Analysis. For modeling the data in terms of assumed chemical equilibria, it is convenient to define two quantities, iiF and iiH, which represent the average number of fluoride ions and hydrogen ions involved in the general reaction
5
i
PH
Figure 1. Equilibria emf data on the Si"-F system in 1 m NaCl at 0 OC. iiFand iiH (left ordinate) are shown as a function of pH at three concentrationsof fluoride ion. Ratios of iiH/iiF (filled symbols) are shown on the right ordinate. The experimental uncertainties are shown where they are significantlylarger than the symbols used. The curves are calculated on the basis of the scheme of species giving the best fit: Si(OH)4, SiF62-,and Si(OH)F4- (the minor species).
The summations include all the fluorosilicates assumed to be present. The silicic acid concentration is not determined experimentally but is derived from the material balance expression for silicon, i.e. msl = [Si(OH)4I + CQx,y[Si(OH)~l [H+lx[Fly
(8)
The equilibrium quotients (Q,,) were adjusted by means of a general least-squares computer program to give the best fit to the data.
Results Potentiometry. The results of experiments in which acid
was titrated into an essentially neutral Si(OH),, solution containing varying amounts of fluoride a r e summarized in QXY Table I. Constant signals are obtained in a few minutes after Si(OH)4 + xH+ + yFSi(OH)4,F,X-Y + x H 2 0 (3) each addition. Most of the data were obtained over the pH These quantities are derived for each data point from the relationships range 2-6. The temperature was held a t 0, 25, and 60 O C . A t 60 "C we confirmed the interference of HF2- with the iiH = (mH - [H+] - [HF] - [HF2-] + [OH-l)/ms, (4) response of the lanthanum fluoride electrode observed prev i o ~ s l y . ' For ~ this reason we have only analyzed data a t 0 and iiF = (mF - [ F ] - [HF] - 2[HF2-])/msl (5) 25 OC in detail for the identity of species present (as discussed where mH,mR and ms, are stoichiometric molalities of hydrogen ion, later) . fluoride ion, and silicon, respectively. Previously published values Inspection of Figures 1 and 2 shows that, when sufficient for the equilibrium quotients15for HF and HFF and for the ionization fluoride is present, constant values of fiF and AH are obtained, quotient for water16 were used. Values for the ii quantities were also computed from a least-squares Le., 6 and 4, respectively. Also, plotted on the right ordinate fit to the data for assumed schemes of chemical equilibria: is the ratio AH/& Within the computed error limits this value is equal to 213 over the entire range of the data, showing the ~ X [ S ~ ( O H ) ~ - ~ F , ~ -CXQ,,[S~(OH)~~[H+I~[F-I~ - Y] "(CalCd) = predominance of the SiF2- species. Some increase in this ratio ms1 ms1 is indicated a t the lowest fluoride concentrations. T h e shift (6) of the curves to greater acidities with increasing temperatures reflects the weaker stability of the complex as defined by eq CY [Si(OH)4-xFyX-yl- CYQ,,~ [Si(OHh1 [H+lx[ F l y AF(Ca1Cd) = 3. ms1 ms1 Analysis of Species. Because of the obvious predominance (7) of SiF6Z-in some of these experiments, we attempted to find any one or more species which, included with SiFs2-, would account for the data. In this analysis those species Si(16) R. H.Busey and R. E. Mesmer, J . Solution Chem., 5, 147 (1976).
760 Inorganic Chemistry, Vol. 19, No. 3, 1980 7
I
1
I
Busey, Schwartz, and Mesmer
I
I
fi 0:
2 1 -
0
1
A$
1
i . 0 m NaCl
o l L / L L '
0
'0.2
{ O
(0-4
IF-I
io
05
2 "F
15
3 "F
05 5
6
2
3
4
4
PH
Figure 2. Equilibria emf data on the Si"-F system in 1 m NaCl a t 25 "C. iiF and AH (left ordinate) are shown as a function of pH at three concentrationsof fluoride ion. Ratios of iiH/iiF(filled symbols) are shown on the right ordinate. The experimental uncertainties are shown where they are significantlylarger than the symbols used. The curves are calculated from the sequence of species giving the best fit of the data: SiF?-, and Si(OH)*Fz(aq)(the minor species). I
I
I
I
11. W e are unable from these data alone to uniquely define the composition of the minor species contributing a t the low fluoride concentrations. Because of the p H region of occurrence of these minor complexes (their inherent instability), the high concentrations of F, H F , and HF; contribute to the large uncertainty in ii values which are calculated. Comparison with Other Data. From the p H and freefluoride measurements of Crosby'* on dilute solutions of H2SiF6,we have calculated a value for the logarithm of the equilibrium constant for the formation of SiF2- in eq 3 of 30.8 0.5 in dilute solutions. In 1931, Kubelka and P r i s t ~ u p i l ' ~ carried out a titration experiment in which the following stoichiometry was assumed in order to calculate the freefluoride concentration and the H+ was measured with a hydrogen electrode: yH2SiF, xNaOH ((6y - x)/4)Na2SiF6(s) ((x - 2y)/4)Si02(s) y H 2 0 6((x - 2 y ) / 4 ) / N a F (9)
*
+
2
i
F 4
0-c 2
3
4
5
1
2
Y
3
4
(mi
Figure 4. Distribution of fluorosilicatesas a function of free-fluoride concentration. Results are shown for two values of pH in 1 m NaCl at 25 "C.
by a solid line. T h e best fit a t 25 OC was obtained with the Si(OH),F,(aq) species, and a t 0 OC a better fit was obtained with the Si(OH)F4- species. The stability constants for the formation of these species by equilibrium 3 are given in Table
0
40
(0.'
(0-3
5
Y
Figure 3. Variation of the agreement factor u(A) with the value of y for the minor species, Si(OH)4-xFyX-Y,combined with Si(OH)4and SiF2- in the fit of all the data at 0 and 25 "C. The values of x and y are indicated as x,y below the best value of u(R) obtained for the series of species for a constant coordination number (in parentheses) or 4 - x y . The points for a given series are joined by lines for clarity.
+
(OH),-xF,"-Y, abbreviated as (x,~),were considered in which the silicon was coordinated with 4, 5, or 6 ligands, excluding H 2 0 , Le., all species for which (4 - x ) y (or the number of OH- plus F ligands) equals 4, 5, or 6. The agreement factors" obtained a t 0 and 25 OC from all the data are shown in Figure 3 in which u(A) is plotted as a function of y in the species. The series of species for a given value of (4 - x) y are joined
+
+
(17) The agreement factor u(ii) was computed from a(ii) = [(Cw(ii,iiH(ca1cd))*)+ Cw(ii~ - ilF(CalCd))2/(No- N , ) ] ' / 2where No and N, are the number of observations and the number of variables, respectively. The weights w were assigned by propagation of the errors of the observables.
-+
+
+
These data lead to a value of for the quotient for the formation of SiF62- in eq 3. Considerable uncertainty, possibly 1 log unit, should be assigned to this result. From our value of the equilibrium quotient for the formation of SiF62-from Si(OH), (eq 3), we can demonstrate that the experiments of Plakhotnik and Kotlyar'O were not correctly interpreted. They determined the solubility of K2SiF6in acidic solutions, assuming only SiF62- and SiF,- were present in solution. A t the low concentrations of dissolved silicon, relatively high concentrations of silicic acid would have also occurred. Figure 4 illustrates the free-fluoride concentration needed to produce SiF6,- a t p H 3 and 6 a t 25 "C in 1 m NaCl. A t temperatures near 25 "C, we can calculate that solutions with 0.001 m free fluoride contain a ratio of [siF62-]/[Si(OH)4] less than 0.01 when the p H is greater than 3.5. Likewise, a t higher temperatures the p H a t which this condition is obtained occurs a t a still lower pH. The change in enthalpy for the formation of SiF62- in eq 3 is -23.72 kcal/mol of at 25 "C, on the basis of a constant AC, of 59.4 cal/K. W e cannot draw a close analogy between the silicon(1V) and boron(II1) systems because of the difference in coordination number between the pairs SiF62--Si(OH)4 and BF4--B(OH)4-. There is no regular sequence of fluoro complexes for Si(1V) as was observed for B(II1). This likely can be attributed to the change in coordination number that occurs as fluoride is replaced by hydroxide or oxide ligands on silicon. (18) N. T. Crosby, J . Appl. Chem., 19, 100 (1969). (19) P. Kubelka and V. Pristoupil, Z . Anorg. Allg. Chem., 197, 391 (1931).
Inorganic Chemistry, Vol. 19, No. 3, 1980 761
Fluorosilicate Equilibria in NaCl Solutions Table I. Potentiometric Data on Fluorosilicate Solutions in 1rn NaCl
n uno
0.142 11.349 0.563 0.m I ,068 1.403 1.732 2.IIRR 2.441) 2.836
0 236 0.5117 0.942 1.348 I 8118 2.338 2 MI 3.421 3.9711 4.593 5 174 5.671 6.1131 6.127 6 .I36 6.133
3.222 3.547 3.792 3.868 3.877 3 882 3.9118 3.931 3.974 4.1194 0.m ll.Il8 I1 287 0.449 0.641
0.~25 1.037 1.229 1385
I 50s 1.581 1.641 1.6XI
1.706 1.734 1.146 1.760 1.762 1.792 1.882
6.132 6.131 6.134 6.111h 11.11011 11. I n I
11.44R 0.720 I,112 I 1.307 1.629 I,909 2.1711 2.342 2.452 2.521 2.566 2 594 2.611 2 621 2.619 2.603
2 SRll 2 490
I .nnn 0.999 0.997 0.996 (1.994 0.992 11.990 0.988 11.9~~ n 983 n 9x11 0.977 0.975 0.972 11.969 U.9hh 0.963 0.957 0.951 11.940 0912 11.999 11.998 11.996 0.995 0.994 n 992 0.991) 11.988 0.986 0.984 0.98I n.979 0.976 0.973 0.969 11.965 0.959 0.9511 0.939 0.9113
67511. 7 5.9971. h 8,3841,- 6 i.11321 5 1.2431.-5 1.4951 5 I.H~HI 5 2 2591 5 ~
2.8381 - S 3.6091 - 5 4.91191.-5 6.99111 5 1.1148I-4 l.RS91-4 3.3981. 4 5.4441 4 7.8551 4 1.2791 3 I 8781 3 3.1921 3 h.8771 3 I 3071 6 3.11261 . 5
4.5051. 5 6.11551 - 5 X.2571 5 1.1191.~-4 1.6421 4 2.4351. 4 3.8691 - 4 5.8441. 4 8.261UI 4
i.i?ni.-3 1.4621. 3 1.8721 - 3 2.3761. 3 3.nix1 3 3.8631 3 5 14n1.-3 h.8ISI - 3 i.2n3i'- 2
9.6131. 3 9.3311. 3 8.9551 - 3 x.snii1. 3 8,1541. 3 7.673L- 3 7.1211-3 6.5781. 3 5 9961: 3 5.4271 - - 3 4.7781.- 3 4.1621 3 3.6141 3 3.1321 3 2.8116I - 3 2.5341, 3 2.282I 3 1.8961 3 I 5721 3 1.1411. 3 h.4211. 4 3.2961 3.0581 2,7681 2.48111 2 16(11 I.RShl. ~
n.niio 0.046 0.ISI 0.270 0.445 0.666 0.924 I 234 1.533 I, 8 5 3 2.272 2.648 3 1121
3.338 3.603
3.806 3.926 3.929 4.1134 4.1167 4.1176 4 127 4.137
3
n.1125 0.078 0.138 0 188 0 219 n 2.15 I1 237 0.234 11.241
n.m 0.239 n 245 11.247 n.26~ 11.27R n 375 11.392
n on0 n 042 11.157 11.266 0.357 0.431 11.478 I1 5119 11.530 n 543 11.549 I1 554 I1 5 5 2 I1 548
n 532 nsix 11.498 0473
I non
11.999 0.997 11.996 n 994 11.992 0 990 11.987 I1 985 11.98I 0978 0.973 0.969 11.963 11.951 0.940 11.923 0.905
72291 6 1.11791 - 4 2.2371 4 3.5501 4 5.2731 4 7.65RI 4 I.ll6SI 3 1.3991 3 1.7931 3 2 . 2 5 1 1 ~3 2 76111 3 3,43111 3 4.1041 3 4.9901 - 3 6.6941 - 3 8.3791.-3 1.0711 2 1.3471.- 2
I .mi 0.999 11.998 0.997 (1 996 0.994 11.992 n 9911 (1.988 (1.985 0.9112 0.979 11.97h 0.973 11.9711 11.966 11.963 0.958 11.953 11.946 11.939 11928 11.9117
Y.8hII 7 i.0951. s I.5661bS 1.8951 5 2.2901.-s 2.7661 5 3.31191-5 4.03ll - 5 4.8671:- 5 5.9581 - 5 7,8301. 5 1.11291 4 1 . 4 m- 4 1.9511 4 2.7751. 4 4.2171 4 6.21191 - 4 9.5151 4 1.4D6l 3 2.119w1-3 2.9191 3 4.3581. 3 7 3h5I 3
9.6081. 3 9.4751 3 9.2781 3 9 . 0 7 ~ 1- 3 8.7761 3 n.41151 3 79851 3 7.4861.-3 6.9791 3 6.4521 - 3 5.7641 - 3 5.1351 3 4.4881 3 3,9181 3 3.3841 . 3 2.8561 3 2.4351 3 2.119ni 3 1.6411.3 i . 2 9 1 ~3 1.11321 3 7,hSlll 4 49861 4
3
l1.11l111
I 1 0011
3 3 3
11.1100
n 1137
ll.ll63
11.122 0,2311
11.137 0.227 11.347 11.465 I1 6113 0.750
~
11.n511 I1 9411
I.1141 1.127 L2lS 1.249 1.289 1.312 1.292
I38lll 4 9 2771 s
1.11111 3 9.1411 4 7.1771 4 6.1911. 4 5.11171~ 4 4 11171 4 3.2941 4 2.77111 4 2.35Il 4 2.l12111. 4 1.7631 4 1.5111 4 1.3361 4 I 1631 - 4 9.49111b 5 X.11531. 5 6.6561. 5 5.6231,- 5
0.111~0 0 0114 -11.1103 lI.l)411 I1 129 0.276 11473 0.724 I .ono 1.286 1.551 I ,798 2 11-25 2 219 2 366 2.469 2.541 2 577 2619 2.728 I1 11110 -11 11112
l1.11119
3 3 9 1091 - 4 705n1. 4 5.h471 4 4.6431 4 3.H9Xl. 4 3.3141. 4 2.8771 4 24011. 4 2.0331 4 I 6781 4 1.5111 I.?OSI
11 nnn 0.039 0.190 11.357 0.622 n.953 1.333 l.iY7 2 251 2.731 3.356 3917 4.4811 4.9411 5 334 5.648 5.821 5.771 5.9711 5 992 5.999 5.983 5.916
3
~
I1 3611
n.sxi 11.7118 11.905 1.143 I271 1.413 1.553 1.668 1.794 I 833 I x53 I
xss
I291 1.261 1.214
LX43 I X2I1 1.78R I725 IS99
n nnn
I1 111111
11 nnh 0.01 I
ni114 1 ~ 3 9 n n72 I1 I l l I1 151 I1 I R 2 II zni I1 2 1 3 11.21 7 ll.2I8 11.212 II.2118 n 2117 11.19h 11. 1911 11. I 7 9
I.xiin
o.nno
60°C
25°C
0°C
n.oiin
11.1144 0.076 11. I ns n. I 39
n.153 11. I 7 5
11.192 I1 2114 11 mi
(1.229 11.272 I1 263 (1 278 n.292
11.999 11.998 11.997 11.996 I1 995 I1 994 11.993 11.991 (1.989 ll.988 n . 9 ~ 11.984 11.982 11.977 (1.974 11.971 I1 968 11.9h2 11956 11 9511 I1 938 11.9 I 7
non
I I1 999 11 997
11.995 11.993 11.991 I1 98X 11.9~s 0.981 11.976 11.971 11.965 11.956 11.948 11.939 11.928 ll.9118
4.5421 4.4441 711341 9.1911 1.1341 I4491 I.XI2I 231111 3.1491 3 niiii 4.8(151 h 2781 8 3hlI
7 5 5 5 4 4 4
3.2981 3 3 1451 3 2.9991 3 2.~4713 2682I. 3 2.4661 3 3.2481 3
4
2111151
4
"I 17 I 5.121
J
3 3 3
3.3391 3 4 1711 3 S,llll~l 3 6.6851 3 9 . ~ 8 9 13
3 1.1521 3 9.6851 4 72711.4 h.1941 4 5.311111 4 4.7511. 4 3.9115I 4 3.3451 4 29461.-4 2 3951 4 I XIMI 4
3.411 5 14141 4 2.7241 4 43171-4 6.2641 4 92211.4 I2871 3 1.69RI 3 2.1951 3 2.8331. 3 3.6191 3 J.S33l - 3 5.hH4l 3 6.8991. - 3 R ?OR1 3 9 8441. s i . 2 ~ 12
I.11IHI~3 92641-3 8.27111 4 7.2391-4 6.2371 4 5.1541-4 4 2611 4 3 5871 4 3.11221 4 2.5311 4 2.ii2r J 1.7R2I. 4 1.4X4l 4 1.2551 - 4 1.11861 4 9.2271 - 5 7 ~ I E -I5
4 4
4
1.3151 3 1.682I. 3 212SI 3 2 SIlS1 3
1.35111
l1.lll15
O.Il22 I1 063
o. I08 11. I 4 9
11.194 I1 214 11.2411 0.243 11.241 11.2lS
n.ono
m n
5.4601 - 6
-n.n611 -n.1177 -0.1159
11 999 0.997 0.996 0 994 I1 992 n 989 0.987 11.983 0.9811 11.976 11.973 0.969 11.965 11.9611 I1 955 11.948 0.9411 II 928 118113
1.61181 5 2.81191 5 4.1iin1 - 5 5 21151 - 5 6.5541 5 8.1941- 5 1,11391 - 4 1.3181. 4 I6991 4 2 IR71 - 4 28541 4 381.91 4 5.2111 4 7 2731 - 4 ~.llll6l 3 1.5251-3 2.3231 3 3 5221 3 6.5231 3
94971 3 9.3201 - 3 9.11861 - 3 H 8331 - 3 8.4691.--3 R.060l'-3 75 7 3 ~ 3 h 9781 3 h 3541 3 5.6921 3 5 11541~3 4.4271 3 3 7991 3 3.1911l 3 2.h22l 3 2.1411 3 1.6291 3 1.2131 3 8.9341 4 5 5441 4
11.999 0.998 11.996 I1 994 11.993 n.9911 11.988 II 985 11.982 11.979 11.975 n.97 I n 966 11961 11.95h 11.949 11.939 11914
16741. - 5 4.9881. 5 9.6851 - 5 1.4771.-4 2.1011 4 3.11311 4 4.2321 4 5.8961 4 8.n56I-4 1.1071 - 3 1.4721 3 1.9121-3 2.4501 - 3 3,1141-3 3.8221 3 4 8241. 3 6.2251. 3 9.7341 3
3 21131. 3 311461 3 2.8391-3 2.6391 3 2.4161 3 2 1361 3 1.8571-3 1.5651-3 13101 3 Ill761 3 n.11881 4 7.3811-4 h 1451 . 4 51121 4 4 3251 4 3.5541 4 2 8421 4 1.n71i 4
1.3211-5
3.42nl 4 5 1281 4 76n21 4 1.n731' 3 14741-3 1.9711:-3 2.578~--3 3.1731-3 3.8721 - 3 4.9671 3 6.3831-3 8.11961 3 1.2151. 2
99391. 4 8.9521 - 4 7.7161 4 6 6471 - 4 5 6651. 4 4.6641 4 3 . ~ 1 5 1- 4 3,11871 4 2.4961 4 2 . 1 ~ 1 14 1.7001-4 1.4321 ~4 I 1451 4 90621 5 7.2101 - 5 4.7991 5
2.1721-6 h.5881 6 1.1261 - 5 1.4841~5 I~ L - S 2 SI - - s 27111-5 3.4531.-5 4.5391 - 5
19911.-2 I 9711 2 1.9351-2 1.8941- 2 I 8451 - 2 1.7821 - 2 17011 2 1.5961 - 2 1.4771 2
n inn
I1 321 0.61 I 11.994 1.4116 1.835 2 237 2.6117 2 939 3 225 3423 3 535 3 5511 3453 3 237 2 718 I1 111111 I1 I122 I1 1134 - 0 1134
0.1114 ll.ll33 1I.ORh 11.163 11.212 11.2411 11.246 11 237 11212 11.176
11.2311
n IS?
11.238 I1 263 0.361
11.119
ri.on? 11.1131
11.99X 0.998 n 997 0.995 I1 993 0.991 0.988 0.9R5 0.9RI 0.977 0.973 11.968 11.9611 (1951 n 939 11912
I1 11011
11.014 11.1114 I1 Ill 2 I1 1121
11.1138 11.1149 n.iihn 11.1168 11 1175 I1 1178 0.084 11.1192 0.111 1l.I26 11.226
n.noo -11.014 11.039 ii.1~2 11.392 0.695 I ,089 1.615
2.198 2 R114
i1.m
n.107 -0.054 0.133 11431 n.~49 I ,428 2 196
3.033
1x997 0.996 n 994 11.993 n.990 0.988 n 983 11.979 n.974
xxn21 5 2 11571 4
~
~
3.905 A A M
*II
5 = 1 ~ t n - (
Table 11. Stability Constants for Selected Schemes of
Fluorosilicates Formed by Reaction 3 T,O C SiF,*Si(OH),F,(aq)
Si(OH)F,-
u(E)
31.679 * 0.005 5.68 31.610 * 0.004 21.40 f 0.02 2.56 29.994 f 0.005 2.74 25 29.980 + 0.002 11.09 f 0.02 1.13 60 28.233 f 0.011 1.33 The relative stabilities of SiFb2- and BF; can be seen from the following relationship derived from results in this paper and ref 3: 0 0 25
T h e relationship is independent of the free-fluoride concen-
tration and indicates that at p H 3, when half of the silicon is complexed as SiF62-, the ratio [BF4-]/[B(OH),] has a value of 0.01. Additional information is needed to define the composition of species formed in relatively concentrated HzSiFs solutions saturated with silica. In these solutions the ratio mF/mSi approaches about 5 in 40% H2SiFs, and this increases to about 5.3 in 0.02 m H2SiF6. This result in the more dilute solutions is consistent with our results, considering the difference in medium. W e expect that Raman studies in the more concentrated solutions to which silica is added to H2SiF6could yield helpful information on the nature of the species present. Registry No. SiFs2-, 17084-08-1; Si(OH)2F2,39630-76-7; Si(0H)F4-, 72541-63-0; Si(OH)4, 10193-36-9; F,16984-48-8.
