AQUEOUS SOLUTIONS OF SODIUM SILICATES.* PART VIII. GENERAL SUMMARY AKD THEORY O F CONSTITUTION. SODIUM SILICATES AS COLLOIDAL ELECTROLYTES. BY R. W. HARMAN
Introduction The interest of recent years in the ever-increasing industrial importance of sodium silicates of varying ratios ru’anO:SiOzas evidenced by their many and varied technical appplications and their increased production, seems to have overshadowed the more theoretical aspect of their exact nature and behaviour in solution, from a knowledge of which we can, alone, hope to understand fully and apply more widely still this class of exceedingly puzzling but useful compound. Although silicates generally have been subjected to extensive and prolonged investigations] sodium silicates] in particular, have received only isolated and intermittent attention. Until quite recentely our knowledge of aqueous solutions of the alkali silicates was drawn mainly from the work of Kohlrauschl, and from Kahlenberg and Lincoln12on the electrical conductivity and from the work of Kahlenberg and Lincoln, and Loomi@on freezing point lowering. These investigators concluded that alkali silicates are largely hydrolysed in solution, most of the silica being in the colloidal state. Later Bogue4 showed that the degree of hydrolysis as determined by E.M.F. measurements was unexpectedly low and not in accord with the previous conclusions drawn from measurements of conductivity and lowering of the freezing point. The literature contains many references to several definite sodium silicates, notably T\‘azSi03, NazSizOs, NaHSi03, Na2Si5011,T\’aaSiOa,etc. but the only one whose composition had been definitely established was the metasilicate] Na2Si03, and even then much conflicting data existed concerning the number of its hydrates. Then Morey5, Morey and Brown6 proved that KanSi03 and Na2Si205separated as crystalline salts at temperatures from solution and from fused melts. The existence of between 400°-~0000C the other sodium silicates seems more or less to have been taken for granted, as being probably formed from the great array of of silicic acids postulated *This series of papers, “Aqueous Solutions of Sodium Silicates Parts I-VIII, comprise a Thesis presented for the Degree of Doctor of Science of the University of London. Wied. Ann., 47, 756 (1892); Z. physik. Chem., 1 2 , 773 (1893). * J. Phys. Chem., 2, 77 (1898). 3 Wied. ann., 60,531 (1897). * J. Am. Chem. Soc., 42, 2575 (1920). J. Am. Chem. SOC.,36, 215 (1914). E J. Phys. Chem., 28, 1167 (1924).
AQVEOUS SOLUTIONS OF SODIUM SILICATES
45
from time to time, e. g. Schwarz and Mennerl claim the existence of a t least seven silicic acids, ,or hydrates of Si02. Our knowledge of these is, a t best, very meagre and their existence as acids seems very problematical, even the mostly clearly defined of all, metasilicic acid, H2Si03, on account of its partly colloidal nature, being still wrapped in an atmosphere of mystery.
General Summary of Experimental Work and Results I n the present investigation the system Naz0-Si02-H20at 2 5 O C has been studied along the following lines, each of which has formed the subject of a communication to this journal,-( I ) preparation and conductivity, ( 2 ) transport numben, (3) sodium ion activity, (4) hydrolysis ( 5 ) osmotic activity: lowering of F.Pt. and v.p., (6) heterogeneous equilibria (7) electrometric titration, diffusion, and colorimetric estimation of silicate. A brief summary of the results obtained and the more important conclusions therefrom, follow seriatim. Conductivity. From measurements of aqueous solutions of ratios 2 :I, I :I, etc. up to I :4 we have seen2 that solutions of 2 :I and I :I ratios are excellent conductors in dilute solution but the equivalent conductivity falls quickly with increase of concentration, so that at zN, it is relatively low. The ratios containing more silica, viz., I :2, I :3, and I :4 are also quite good conductors in dilute solution and moreover the values of A, for these three ratios are all nearly equal, being 95, 91 and 88, but in concentrated solution the conductivities are abnormally low. Hydrolysis into KaOH and colloidal silicic acid cannot account for this high conductivity, not even with ratios relatively rich in NaOH, nor in dilute solution where hydrolysis is greatest; and with ratios rich in silica, where hydrolysis is practically negligible, it is apparent that the explanation that the conductivity is due to hydrolysis is totally inadequate. The results obtained are reproduced again in Table I for ready reference. (I)
TABLE I. Equivalent Conductivity Sw
SaOH
Z:I
2.0
142.0
57.32
1:I
1.0
172.50
85.57
57.25 81.25
0.5
200.0
1oj.80
96.80
0.2 0.1
209.0 214.5
0.05
220.0
130.80 143.80
0.02
225.5
136.90 157.50 175.50 190.10
0.01
227.5
193.0
I55,O
0.005
228.0
194.2
159.0 160.0
0.0
112.70
152.70
t = 25OC.
1:x.j
I :2
32.09
25.80
50.23
66.76 86,20 99.20
36.I O 49.05 62.59 72.70
107.04
78.00
114.20 118.10 1 2 0 . I4
84.00 89.50 93 ' 2 0
.oo
95.00
I21
'Ber., 57 B, 1477 (1924); 58 B, 73 (192s). J. Phys. Chem., 29, 1155 (192j).
I :3
20.46 31.42 45.41 57.33 66.48 75.63 81.75 8j.16 89.90 91.00
I :4
16.17 23.24 3 3 ,I 4 48.25 57.80 65.80 75.06
81.50 86.04 88.00
46
R. W. HARMAN
The most reasonable assumption to explain these results is the existence of silicate ions and sodium ions due to ionisation, in addition to the NaOH formed by hydrolysis. In concentrated solution this ionisation falls off considerably or else the silicate ions are now not so active in conveying the current.
FIQ.I Equivalent Conductivity azainst Ratio
From the graph where the equivalent conductivity is plotted against the ratio, as in Fig. I , we have seen that in dilute solution there is a sharp change of direction at I :2, the relationship on either aide of this point being linear. As the solutions become more concentrated this simple relationship disappears until at z N, the most marked changes of direction of the curves are evident a t 2 :I and I :I. It may be that definite salts of ratios 2 :I, I :I and I :2 exist, but in any case, concentrated solutions exhibit a radically different behaviour from very dilute solutions, suggestive of a change of state of one cr more of the constituents.
AQCEOUS SOLUTIONS OF SODIUM SILICATES
47
Transport Numbers. From the results of very careful transport number experiments‘ we have seen that as the proportion of SiOz increases in the ratio Na2O:SiOz so does the proportion of the current carried by the silica constituent increase. If the transport numbers are calculated on the assumption of SiO3” being present in all ratios, then the T.N. of this silicate ion is as shown in Table 11. From this i t is fairly evident that ( I ) silicate ions are present and ( 2 ) that either (a) the anion may be a solvated aggregate of simple Si03 ions, carrying a charge equal to the sum of the total charges on the separate ions, or a solvated aggregate containing more than one mol Si02 per divalent charge, (2)
T.N. Silicate No. Expt.
Ratio I :I
4
9
.o
0.5 0.I
I
:3
8
IO
I .O
I I :2
6
7
Approx. Nw.
0.I
3
5
=
2.36
I 2
TABLE 11 total change in wt. of SiOz content wt. Si02 equiv. to Ag deposited in coulometer.
1.0
0.5
I
:4
I .O
0. I
nsios
noH
0.27 0.36 0.31 Mean 0 . 3 1
0.17 0.13 0 . I8 0.16
0.56
0.42 0.35 0.45 Mean 0 . 4 1
0.88
0.40 0.45 Mean 0.43
1.35 I . 42 I .38
0.53 0.44 Mean 0 . 4 8
2.32 2.44 2.38
nNa
.os1
0.51 0.53
0.70
0.87 0.82
these aggregates splitting up on dilution, or (b) the anions may be definite ions more complex than the simple ion SiO3 ion, i.e. definite salts of the following formulae msyexist,-Naz(Si03.SiOz), Naz(Si03.2 SiOz)etc.,whichionise to give definite divalent silicate ionssuchas,-( Sios.SiOz)”, ( Si03.2 SiOz)”,etc. In this paper on transport numbers it was pointed out that the number of mols of Si02 per divalent charge (2 F) of the silicate anion seemed approximately equal to the molar ratio SiOz : NazO. This may be explained in two ways,-( I) ratio I :4 e.g. may be a definite salt with a true anion of the composition ( S O 3 . 3SiOt) aq.“ i.e. such an anion results a t any and all concentrations from ionisation, or (2) in solutions of ratio 1:4 an aggregate such as (Si03 . nSi0z) aq“. exists in a more loosely bound combination, in such a way J. Phys. Chem., 30, 359 (1926).
48
R. W. HARMAX
that “n” may be large in very concentrated solutions, but in very dilute solutions smaller and even zero, though in the range of concentration measured, as far as experimental conditions allow, it appears on an average equal to four. I n such circumstances it was concluded not to be so misleading to calculate the T.N. of the silicate anion from
T. N. Silicate
=
total change in wt. of SiOz content
K x (wt. of Si02 equiv. to Ag deposited in coul.)
and the results obtained are given again in Table 111.
TABLE I11 Transport Numbers Ratio
nsa
nsli
I :I
0.37
0.16
I :2
0.41
0.41
0.53 0.18
I :3
0.43
0.46
0 .I 1
I :4
0.48
0.59
n OH
(3) Sodium Zon Activity. The measurements of sodium ion activity by means of a sodium amalgam electrodel were carried out in the hope of gaining some idea of the sodium ion concentration in solutions of these various ratios. The results with ratio I : I however, were anomalous in that the curve of y, the activity coefficient, against concentration, passed through a minimum at moderate concentration, about o.IN%.,and so caused some irregularity, but the results obtained with the higher ratios did not show similar behaviour. The activity coefficient, from being comparatively high in dilute solution, changed regularly to a very low value in concentrated solution, being more or less parallel to the conductivity. The fall in y as the proportion of SiO, in the ratio NazO : SiOn increased was also fairly regular. TABLE IT’ Sodium Ion Activity Conc. Sr. I :I I
.o
I :2
activity coefficient I :3
0.265
0.10;
I :4 0.050
0.80
0,475
0.27j
0 . I45
0.060
0.50
0,440
0.310
0 .I85
0.090
0.40
0,425
0.335
0.210
0.I10
0.20
0.41j
0.410
0.295
0.185
0 .I O
0,425
0.460
0.365
0.250
0.oj
0.joo
0 500
0.42j
o.32j
0.02j
0.700
0 j2j
0,480
0.405
0.01
0.975
o 785
0,;oo
0.jjo
J. Phys. Chem., 30,91; (1926).
AQUEOUS SOLUTIONS O F SODIUM SILICATES
49
The general conclusions can be drawn that increase of concentration for any one ratio, and especially increase of silica in the ratio, both have a very marked effect on the activity of the sodium ion, so reducing it that in concentrated solutions of ratios I :3 and I :4i t is remarkably and unexpectedly low. The results obtained are summarised in Table IV.
FIQ. 2 y , against
Sodium Ion Activity,
Concentration
These results are shown graphically in Fig. 2. (4) Hydrolysis. The descript'ion of E. M. F. measurements of hydrogen ion concentrations is given in Part I V of this series' the chief results of which are summarised in Table V. TABLE V Percentage Hydrolysis N w
2:1
I :I
I
:3
:4
1:1.j
I :2
2.0
'7.5
15.25
3.0
I .o
0.101
0.032
I .o
19.0 '9.7
16.5 19.I
5.9
1.35
0.192
0.071
0.5
7.10
1.88
0.14
0.2
24.2
20.0
8.25
2.40
0.I
28.4
21.8
2.85
0.05 0.02
31.6 34.2
22.6 23.25
8.7 9.76
0.36 0.58 0,77
11.0
5.1
I . IO 1.20
0.35 0.57 0.93 I .30
0.01
36.0
27.8
12.0
6.5
1.34
I . jo
J. Phys. Chem., 30,
1100(1926).
3.8
I
R. W. HARMAS
50
These results are shown graphically in Fig. 3.
It was there shown that the degree of hydrolysis and the concentration of OH ion are in a measure proportional to the proportion of NazO in the ratio. The great point of interest however, is that even in very dilute solution ratio I :I shows only ~ 7 . 8 7 hydrolysis, ~ while ratios I :3 and I :4 only about 1.57' hydrolysis. I n concentrated solution the degree of hydrolysis is even more remarkably low, being I j% and 0.1and 0.03% respectively for concentration
FIG.3 Percentage Hydrolysis against Concentration 2KX. We t,hus see that hydrolysis can in no way explain the conductivity of these silicate solutions. If, as some later investigators have assumed, the OH ion concentration, as here calculated from measurements by the hydrogen electrode, does not give the true value of the hydrolysis, then OH ion adsorption by the colloidal silica takes place to a very large extent, especially in the higher ratios. Hence these solutions should show a very low osmotic activity, which they do not, a point to be dealt with more fully later in considering results of measurements of F.Pt. lowering. The only other explanation seems to be that silica exists in the crystalloidal state in solution, some possibly as silicate ions, and is not wholly colloidal as heretofore supposed. From Table 1' (Part IT) giving the values of the liquid-liquid potential difference sodium silicate/sat. KCl, as experimentally determined by the Bjerrum extrapolation method, in ratios z :I and I :I this liquid-liquid P.D. is of the order of 3-4 millivolts and opposes the general E. 31.F. of the cell, as expected from a moderate concentration of OH ions. In ratios I :3 and I :4
J. Phys. Chem., 30,
I IOO
(1926).
AQUEOUS SOLC'TIONS O F SODIUM SILICATES
51
the P. D. is just as large but is in the opposite direction. Here the OH ion concentration is practically negligible, hence the silicate ion is less mobile than the sodium, or there is a greater concentration of sodium ions present. We have seen, too, that when the percentage hydrolysis is plotted against the ratio somewhat similar changes of direction occur as when the conductirity is plotted against the ratio, and the conclusion was drawn that the variation in conductivity noted, in the more concentrated solutions of ratios containing little silica, are mainly due to changes in the OH ion concentration.
t
c6
k $
9 $4
8 E
?$ 2 u
P 0
0
A
.8
1.2
1.6
2.0
FIQ.4 Molecular Depression of Freezing Point against Concentration
Osmotzc Activity. Lowering of vapour pressure and freezing points. I n Part V of this seriesi are described measurements of the lowering of the vapour pressure and of the freezing point, the results summarised below being presented from the point of view of both the earlier ionic theory, Table VI, and from the more recent activity theory, Table VII. These results are shown graphically in Fig. 4. These results show us that for ratio I :I both modes of calculation indicate that at concentration 0.01N, dissociation into 4 ions or active constituents is nearly complete and accounts for the conclusions of early investigators that XaaSiOp was almost completely hydrolysed according to the following equation,(5)
J. Phys. Chem., 31, 355 (1927).
R . W. HARMAN
52
+ 2HOH e 2NaOH + H2Si03 (colloidal)
Na2Si03
aNa’
4t+
20”
TABLE VI Lowering of the Freezing Point KTV
2.00
Van’t Hoff factor “i”.
m
1.00
.oo 0.50
0.50
0.25
0.20
0.IO
2:1
I:I
1.98 2.19 2.45 2.88 3 ’ I3 3.55 3.75 3.87
0. I O
0.05
0.05
0.025
2.30 2.85 3.05 3.45 3.85 4.35
0.02
0.01
5.05
0.01
0.005
5.60
I
I
I
:z
.03
1.22
1.67 2.19 2 ’ 74 3.01 3.22
3.55
I
:3
0.772
1.06 1.46 2.17 2.36 2 ’ 73 2.96 3.22
I :4 0.565
0.855 I .06 1.83 2.13 2.69 2.96 3.01
TABLE VI1 Activity Coefficient, from F. Pt. based on IJ = 4, Le. the molecule gives 4 ions at infinite dilution m
NaOH
0.005
0.950 0.920 0.867
0.010 0.025 0.050
0.820
0 .IO0
0.765
0.500
0.700
1.000
0.680
I:I
0.922 0.888 0.778 0.626 0.497 0.280 0.192
I
:3
0.604 0.484 0.359 0.252 0 .179
I
:4
0.412
0.052
0.342 0.244 0.171 0.109 0.029
0.029
0.015
But this view is no longer tenable, as not being in accord with the percentage hydrolysis as directly measured. I n the paper on hydrolysis (loc. cit) it was shown that ratio I : I , i.e. the metasilicate, Na2Si03,undergoes on solution both ionic and hydrolytic dissociation giving rise to Na, OH and Si03 ions and crystalloidal H2Si03,a t least in dilute solution. It was there shown, too, that, as far as hydrolysis, sodium ion activity, and freezing point lowering results are concerned, it appeared very probable that the acid salt NaHSi03 exists in solution. Ratios I :3 and I :4 on the other hand do not appear to be definite salts and their behaviour in solution is quite remarkable, e. g. the van’t Hoff factor “i” for ratio I :4 ranges from 0.565 for 2 N, to 3.01 for 0.01 N.,,., while the activity coefficient ranges from 0.015 to 0.412 for the same concentrations respectively. To explain this it was there tentatively put forward that the results from ratios I :3 and I :4 seemed to suggest the possibility of complex colloidal aggregates in very concentrated solution, and of ionic micelles in moderately concentrated solutions, such ionic micelles breaking up on dilution somewhat after the manner indicated by the following equations,-
AQUEOUS SOLUTIONS OF SODIUM SILICATES
53
+
(m.Si03.nSi02. aq)"" m SiOj [n SiOz aq.] colloidal [n. SiOz . aq], colloidal H2SiO3crystalloidal
+
( 6 ) Heterogeneous Equilibria. System: NalO : SiOz : HzO a t 2 5 O C . I n this paper' (Part VI) it was shown that only two salts corresponding to ratios I:I and 1:2 exist at z s 0 C . Ratio I:I is undoubtedly the metasilicate, NazSi03, and its hydrates have been shown to contain 9, 6 and 2 . 5 aq respectively. Ratio I:P may be either NaZSi2O5or NaHSiO,, but in any case should crystallise out with the composition XazO . zSiOz . gHzO. No evidence for the existence of any other than salts corresponding to these two ratios was found. Attention was drawn in this paper to the fact that Morey had previously obtained Na2SiOa and NazSi205, and these two only, but his Na2Si205was not readily affected by water. It was also noted that both Niggli and Wallace failed to obtain a definite salt richer in silicate than the metasilicate. ( 7 ) Silicate Ions and Crystalloidal Silica. The problem of obtaining some definite knowledge on this aspect of the problem is treated in Part VI1 of this seriesz but only in a meagre preliminary manner. The electrometric titration curves of a dilute HC1 solution against dilute Na2SiOa solutions are typical of dibasic acids, thus suggesting HzSi03 as a true dibasic acid with salts NaHSi03 and Na2Si03; the dissociation constants being given as kl = 4.2 X 10-l~and k2 = 0.51 x 10-l~. The results from a few diffusion experiments indicate that most of the silica in ratios I :I and I : z is diffusible, about % of the silica in 0.3N I :4 and about % in O.IN H2SiO3being crystalloidal. In this paper, also, a colorimetric method of estimating crystalloidal silica by means of HzS04 and ammonium molybdate showed that the amount of crystallodal silica in any ratio was directly proportional to and expressed by the same figure as the ratio SiOz:Na20in extremely dilute solution.
Theoretical An attempt will now be made to correlate the data obtained and to present an explanation of the constitution and of the behaviour in aqueous solution of the various Na2O:SiOz ratios investigated. Attention will be chiefly focussed on the evidence adduced in proof of the following four main statements,( I ) Silica exists in solutions of these ratios not wholly colloidal as heretofore supposed, but wholly or partly as crystalloidal silica depending upon the ratio NazO : Si02 and upon the concentration. This crystalloidal silica existing in equilibrium with silicate ions, or electrically charged aggregates of silicate ions and silica i.e. ionic micelles, or pure colloidal aggregates, as the case may be, depending upon the ratio and concentration. J. Phys. Chem., 31, 511 (1927). J. Phys. Chem., 31, 616 (1927).
54
R . W. HARMAN
I n aqueous solution at z j o C two and only two simple salts viz., (2) Na8iOa Le. ratio I :I, and NaHSiOa i.e. ratio I :z, appear to exist as such, the behaviour and nature of which are now elucidated. (3) Ratios other than I :I and I : z are not definite salts but are typical examples of colloidal electrolytes. (4) The fundamental nature of silica in solution appears to depend upon the existence, at least in the range here investigated, of only one acid, metasilicic acid, in which the equilibrium between the crystalloidal and the collodial constituents depends upon the concentration, the crystalloidal content a t ordinary concentrations being much greater and the acid, therefore, much stronger, than generally supposed.
Evidence for the Existence of ‘LCrystalloidal”Silica Under the heading “crystalloidal silica” are classed definite silicate ions, aggregates of ions carrying an electric charge with or without some colloidal silica, i.e. ionic micelles, and crystalloidal silicic acid or hydrated silica. The evidence produced herein, for the existence of such crystalloidal silica, rests mainly on the following four sources,(I) Conductivity measurements resulted in the finding of a much greater conductivity for ratios up to I :2 than could result alone from the proportion of NaOH formed by hydrolysis, as measured by the E. M.F. method. Above rat’io 1:2 where the hydroxyl ion concentration is very low, the sodium ion accounts for about only one-half the conductivity found. The only way to account for the conductivity is to postulate the existence of silicate ions with mobilities ranging from 40-60 approximately. Moreover, the equivalent conductivity, as calculated from the concentration of sodium, hydroxyl and silicate ions as found from F. Pts., OH and Na ion measurements, and their respective mobilities, agrees well with the experimentally determined conductivity. (2) The most direct and conclusive evidence so far put forward to show that the silica in aqueous solutions of these ratios carries an electric charge is obtained from the results of transport number experiments. From these it was shown that nail, = 0.16 f o r ratio I:I,0.41for ratio 1 : 2 , 0.46 for I :3, and 0.59 for I :4, when calculated by the second method. Whether this basis of calculation is correct or not, these transport number experiments proved that quite a fair proportion of the current, at least one-half in the higher ratios, is carried by the “silica.” Since t.he possibility of adsorbed hydroxyl ions on the silica giving the necessary charge to the silica, has been shown to be remote and indeed most improbable, the only conclusion is that the silica must exist as ions. The relatively high mobility of these charged silica particles as deduced from conductivity and transport number results, is also contrary to that expected from colloidal aggregates with OH ions adsorbed thereon. (3) A third very weighty argument appears when we consider the result of hydrolysis experiments along with freezing point lowering results. In 1 I n this paper the word “crystalloidnl” is used to denote constituents which are“osmoticslly and ionically active.”
AQUEOUS EOLUTIOSS O F E O D I U M SILICATES
55
the paper on osmotic activity it was shown that the only possible way to account for the high osmotic activity of these ratios in aqueous solution was to accept the existence of silicate ions. The conductivity of the sodium ions and of the hydroxyl ions as found by E. M. F. experiments, together account for only a fraction of the total ion concentration as determined by freezing point lowering. The non-accordance is quite beyond the bounds of experimental error, nor could it be accounted for by the assumptions underlying the laws governing ideal solutions. The higher the ratio the wider is the divergence, so that in I :3 and I :4 more than half the “crystalloidal” content has to be accounted for by the silica in the more dilute solutions. Whether this is due to the complex silica aggregates breaking down into simpIer silicate ions, or to the disintegration of an ionic micelle, or to crystalloidal HzSi03 ionising, or to all these phenomena, will be discussed later. The fact remains, however, of a very large proportion of crystalloidal silica in solution, more especially in dilute solution. (4) Direct evidence of the existence of crystalloidal silica is put forward in Part VI1 of this series, giving the results of diffusion experiments with collodion membranes and parchment paper, and the results of a colorimetric estimation for crystalloidal silica based on the formation of silicomolybdate. The diffusion experiments indicated that most of the silica in ratios I :I and I :z is diffusible; about N of the silica in 0.3 N , I :4 and about % in 1.0 N, HzSi03also being crystalloidal. The silicomolybdate colorimetric test showed that in very dilute solution, O.OOIN,,the amount of crystalloidal silica as compared with that in ratio I : I Le. sodium metasilicate, was directly proportional to the ratio. The existence of crystalloidal silica and silicate ions in aqueous solution seems therefore firmly established, and there now remains the problem of what nature and composition are the ions and charged aggregates, and in what proportions they exist. I n the solution of this problem the nature and composition of the salts giving rise to these ions furnish a good guide, and help to narrow down the possibilities. Ratios 1:l and 1:2 are Definzle Salts. Their behaviour in solution. From the investigation of the ternary system S a 2 0 : Si02 : H 2 0 at 25°C we have seen that only ratios I :I and I :2 occur as definite solid salts. Ratio I : I is the metasilicate KazSi03, crystallising with 9) 6 and 2 . j aq., the existence of which has long been established, in spite of the difficulty of its crystallisation and the confusion which till now has existed concerning its hydrates. A certain amount of evidence that these two ratios correspond to definite salts in solution is also forthcoming from consideration of the curves where conductivity, hydroxyl ion concentration, sodium ion concentration, and the van? Hoff factor “i” (from freezing points) are severally plotted against the ratio, distinct changes of direction at these points being evident. It was also seen from consideration of freezing point results that the existence of salts corresponding to ratios I :3 and I :4 was very unlikely.
56
R . W. HARMAN
From these results of the phase rule investigation we have seen that the following solid salts would be expected to separate from solution,( I ) Na2Si03 . 9H20. This salt has been obtained by the author not only from alcoholic solution, but also in the form of large, well-defined crystals from a 2 N, solution of ratio 2 : I without the addition of alcohol or inoculating crystals. Na2Si03. 6H20. Na2Si03. z.sH20. Na2SiO 3 .
This decides the hitherto conflicting and confusing data concerning the hydrates of Na2Si03. Nan0 . zSiO2 . 9H20. This compound has so far resisted all attempts (2) a t separation in a pure form from aqueous solution at 25OC. I t may be from the phase rule results, either NaHSiOI . 4HzO or Na2Si20a. 9H20.
Ratios 1:s and i:4. We have seen that only the ratios I :I and I : z exist as definite silicates and there now remains an explanation of ratios I :3 and I :4. The fact that there is increasingly less percentage hydrolysis as the proportion of silica increases in the ratio until it is practically negligible in 1:4 favours very much the view that these higher ratios are complexes of the two beforementioned salts with excess of silicic acid, or hydrated silica, the presence of the silicic acid, one of the products of hydrolysis of these two salts, causing the diminution in hydrolysis according to the law of mass action. Again, if either of the ratios 1 : 3 or 1 : 4 were definite salts, then, since, hydrolytic decomposition seems negligible, 100% ionisation into three ions would not be sufficient to account for the large molecular depression of the freezing point in dilute solution. However, sodium ion activity measurements indicate that in dilute solution 70% in ratio 1 : 3 and only 5 5 % in 1 : 4 is the extent to which the sodium exists as active sodium ions. To bring the results from F. Pt. measurements and from sodium ion measurements into agreement would necessitate, since only sodium and silicate ions are present, ionisation into 4 ions, and in I :4 in the case of ratio I :3 approximately 807~ approximately 60YGionisation into 5 ions. It appean quite improbable that any simple salt of composition corresponding to ratio I :3 i.e. (Na20 . 3SiOn . aq.), could ionise into 4 ions or any corresponding to I :4 Le. (Na20 . 4SiO2 . aq.), into 5 ions. Nor would it be a reasonable hypothesis even to assume the existence of such problematical compounds when all the evidence (except for the results of experiments on transport numbers) from the other lines of investigation employed, goes against the existence of these higher ratios as definite salts. Thus an explanation on quite different lines must be sought, and a most rational one follows when we consider these ratios as colloidal electrolytes from the viewpoint of aggregate or micelle formation.
57
AQUEOUS SOLUTIONS OF SODIUM SILICATES
Silicates as Colloidal Electrolytes I n very concentrated solution the silicate probably exists as a very complex aggregate, which not only breaks up into similar simple aggregates on dilution but also gives rise to sodium and si03 ions and ionic micelles, in some such manner as the following, = [Na20. R. Si02 , aqlX Na. Si03” (mSiO3 . nSiOn . aq)”’ [nSiOs. aq] [Na20. r . SiO? . as.],. (1) The following equilibria also existing in the more dilute solution mSi03” [nSiOz . aq.] (mSiO3 . nSiOn . aq)m” (2) H~Si03. [nSi02. as.] (31 where aggregates included in square brackets are colloidal. Dilute Solution 1 4 . Since y = 0.55 at concentration N, = 0.01 from sodium ion activity measurements, and since there must be two sodium ions for each SiO3ion or (mSi03 , nSiO2 . as.)”-- aggregate, the total normality of ion constituents is thus 3 . N w . y = 0.0165N,. But the total crystalloidalcontent is0.03 N from F. Pt. measurements, so there is still 0.013 j N crystalloidal content to account for. Part of this is no doubt due to the unionised (NaZO . 3Si02 . as.) which a t this dilution may be supposed broken down into a more or less simple or crystalloidal condition. If it were all so i.e. 0.0045 N in this simple state there is still 0.009 N crystalloidal matter t o account for. The only rational assumption to account for this 0.009 N crystalloidal matter is to consider it a simple or unionised hydrated silica or silicic acid, probably H2Si03, as postulated in
e
+
+
+
+
a
+
o 009
equation (3) above. This means that -= 0.04
0.225
or
22.5%
of the total
silica is in the form of H2Si03. If none of the unionised (NazO . 3SiOn . aq.) is in a simple crystalloidal condition then there is 0.0135 N silicic acid i.e. 0.34 or 34yo of the total silica present. Thus at concentration 0.01 N, 1:4 ratio, we have approximately 3 4 7 0 of the total silica in the crystalloidal form of H2Si08, and 1 4 % in the crystalloidal form of ions or ionic micelles, i.e. about s o y 0 as crystslloidal silica. This amount of crystalloidal and colloidal silica, 50’%, receives direct support from the colorimetric estimation of crystalloidal silica in the form of silicomolybdate, (paper VII) where it was shown that about 7 5 7 0 of the silica is crystalloidal at concentration 0.007 N, ratio I :4. Dilute Solution 1:S. Similarly for a 0.01N, solution 1:3, we get a total crystalloidal content of 0.032 N from F. pts., and a crystalloidal content of 0.014 for sodium ions from activity measurements, leaving 0.018 N to be accounted for by the silica content of 0.03 N,.
0 012
Thus -or 40% of the total silica is colloidal. 0.030
The colorimetric estimation shows that nearly 2 0 7 ~of the total silica is colloidal at concentration 0.007 N, 80 here also the colorimetric method gives a lower value, due no doubt to the error incurred in attempting to measure these relatively high concentrations by its means.
58
R. W. HARMAX
Jfoderately Concentrated Solutions. It was for concentrations 1.0-0.1 N, that the transport numbers were found and from which we concluded that in ratios 1:3 and 1:4 there were three and four equivalents Si02 respectively, per electric charge. I t is in these solutions that the micelle or aggregate would evidence itself if it existed. From sodium ion measurements 2 5 % of the total sodium is in an active ionic form and hence 2 5 % of the total silica is also in an active ionic form, presumably (mSi03.nSiOz aq)"' from transport experiments: these two giving rise to 0.075 N crystall3idal matter, leaving 0.155 N = crystalloidal matter to be accounted for by the remaining 0.3 N, silica and 0.075?;,, sodium content, which means that about 50% of this remaining silica or 30% of the total silica is in the colloidal form associated probably with sodium. From diffusion experiments we have seen that about M of the total silica is in the colloidal form at concentration o.gN, I :4. Although the results obtained and the calculations and conclusions therefrom given above, donot afford absolute and conclusive proof of the two fundamental suggestions upon which this theory of the behaviour of sodium silicates in solution rests, viz., ( I ) the existence of a micelle or aggregate of the composition (mSiOs . nSiO2 . aq)"" ( 2 ) simple unionised crystalloidal hydrated silica, H2Si03, or other silicic acids, yet considering all the experimental evidence to date one is forced to acknowledge that only along some such lines can the behaviour of these solutions be explained. The reasons for assigning such a constitution as the above to these silicate solutions are briefly recapitulnted again,-( I ) no evidence has been found experimentally to show that definite salts corresponding to ratios I :3 and I :4 in particular, and to the ratios higher than I : z in general, exist in solution at 2 5 O C ( 2 ) the transport numbers indicate that the number of mols of Si02 per divalent charge is approximately numerically equal to the ratio N a 2 0 : Si02. ( 3 ) The conductivity in dilute solutions is very good and as there is no OH ion and the sodium ion accounts for a little less than half of the conductivity, silicate ions or charged aggregates of fairly large mobility must exist. (4) I n concentrated solution the conductivity, sodium ion concentration, crystalloidal content (from freezing-points measurements) are all most abnormally low, yet all surprisingly high in dilute solution, such that ionisation into sodium ions and equivalent silicate ions, as measured by the sodium ion concentration, shows a serious discrepancy with freezing point lowering results, unless we consider that complex silicate ions or micelles exist in the stronger solution and split up into simpler crystalloidal states in dilute solution.
(5) A fair agreement exists between the relative amounts of colloidal and crystalloidal silica calculated from the osmotic activity measurements on the two assumptions of the existence of a micelle and crystalloidal silica, and those found directly by coloroimetric estimation and by diffusion.
AQUEOUS SOLUTIONS O F SODIUM SILICATES
59
( 6 ) Simple unionised crystalloidal silicic acid has been shown to exist a t much higher concentration than usually granted; it is also a much stronger acid than hitherto supposed. ( 7 ) The widely recognised property of silicon or silica and its hydrates to form complexes, molecular compounds and aggregates, colloidal solutions and gels, would seem to be in agreement with the conclusions here set forth.
Concentrated Solutions! The concentrated solutions of these ratios are characterised by a very low conductivity, practicable negligible hydrolysis, very low sodium ion concentration, abnormally low molecular depression of the freezing-point, very high viscosity, and non-diffusibility, facility to form the gel condition etc., in fact getting more and more typically colloid either with increase of concentration or with increase of SiOz in the ratio. The theory here outlined affords a good explanation of this, in fact it is necessary only to recall that at the lower concentrations we have seen that the equilibria represented by equations ( I ) , ( 2 ) and (3) at the head of this section all show a tendency to go from right to left with increasing concentration in any one ratio, and with increasing ratio SiOs : NazO a t any one concentration. Such being the case it is a natural consequence to expect solutions of any ratio to become more and more colloidal as the concentration increases and to expect the colloidal properties to be evidenced earlier and in more marked respect the higher the ratio. The transition from a typical colloid, through the colloidal electrolyte stage to a good electrolyte is very aTell shown by ratio I :4 on diminishing the Concentration. Ratio 1 : 3 also shows the transition well and it seems permissible to carry the analogy to the lower ratios, where it is seen to offer the best explanation of the very viscous solutions of ratio I:I and its low conductivit,y and osmotic activity. Summary A summary is given of the results and conclusions of seven preceding (I) papers on the aqueous solutions of sodium silicates of ratios 2 : 1 to 1 : 4 a t concentrations ranging from 0.001?;, to 2 . 0 Xxj from the points of view of conductivity, transport numbers, sodium ion activit,y, hydrolysis, osmotic activity, phase rule, and crystalloidal silica. (2) The various data have been correlated and an explanation given of the constitution and behaviour in aqueous solution of these silicates. (31 The evidence for crystalloidal silica and silicate ions has been recapitulated. (4) Ratios I : I and I : z only are definite salts viz., ?;azSi03 and NaHSiOa and their percentage hydrolysis and ionisation have been found and shown to agree with the various measurements, thus affording a complete explanation of their behaviour in solution. ( 5 ) Above ratios 1 : 2 colloidal silica is in evidence, the proportion of colloidal silica increasing with the concentration and the rat,io SiOz: K a 2 0 .
R. W. HARMAE;
60
( 6 ) In dilute solutions of ratios above I :2 crystalloidal uncharged silica probably H&3iOa or simple hydrated silica occurs. ( 7 ) These ratios higher than I:Z exhibit properties characteristic of colloidal electrolytes with a micelle of the composition (mSiOs . nSiOn . as.)””
m+n-
where -- ratio SiOz/NarO. m (8) In concentrateU kolutions of the higher ratios a large colloidal aggregate exists, containing both sodium and silica. ( 9 ) Silicic acid haa been shown to be stronger than usually supposed, its dissociation constants being of the order 10-l~and 10-l~. I wish to express my thanks to the Commissioners of the 1851Exhibition for a scholarship which has enabled me to carry out this work, and to Professor Donnan for his constant, kindly interest and advice. The Ramaay Laboratories of Physical and I noiganic Chemistry, University CoUege, London. M a y 30,1987.
