AQL-EOL-S SOLUTIOSS O F SODILX SILICATES T-11. SILIC-1TE I O S S . ELCCTROlIETRIC TITKATIOSS DIFFL-SIOS: (’OLORI~IETRI(’ ESTIAIATIOS. B Y R. W. HARRI.137
Introduction From the papers already communicated by the author on the subject of aqueous solutions of varying ratio SadJ:PiO,, it is seen that the existence of definite d i c a t e ions in these solutions rests on three main sources of evidence. viz. ( I ) Conductivity measuren~ents~ resulted in the finding of a much greater conductivity for ratios up t o I :z than could result alone froin such a proportion of S a O H formed by hydrolysis as measured by the E.M.F. method. Above ratio I : z , where the OH ion concentration is very low, the sodium ion accounts for only about, one-half the conductivity found. The only way t o account. for the conductivity is to postulate the existence of silicate ions with mobilities ranging from 40-60 approximately. Moreover. the equivalent conductivity calculated from the concentration of sodium. hydroxyl and silicate ions. as found from F.Pts., OH anti S a ion measurenients, and their respective mobilities, agree well with the experimentally determined conductivity result s. ( 2 ) The most direct and conclusive evidence so far put forward t o show that, the silica in aqueons solutions of these ratios carries an electric charge is obtained from the results of transport nuniher experiments.’ There it’ was shown by the author that 171i9,i, 0.16 for ratio I:I, 0.41 for ratio I : Z . 0.46 for I :3 and 0 . 5 9 for I :4, when calculated on the basis that, the T. S.of thc silicate is given t)y total change in neight of Si02 T.S. silicate = S x ( n t . of $io2 equivalent t o -Ig deposited in couloineter) where K = ratio. Whether this basis of calculation is correct or not these transport nuinher experiinents prove that quite a fair proportion of t h e current, a t least one-half in the higher ratios. is carried by the silica. Since the possibility of adsorbed OH ions on the silica giving the necessary charge t o the silica, has been .shown t o he remote and indeed most improl)al)le3, the only conclusion is that t h r silica mnat exist as ions. The relatively high mobility of these charged silica particles n s tleduced from conductivity and transport number results. is also contrary t o that, expected from colloidal aggregates with 011 ions adsorhed thereon. Harman: J. Phys. Chem.. 29,
* Harman: J. Phys. (‘hem..
I Ijg
!
1925
30, 359 :r9261. a Harman: J. Phys. Chem.. 31, 35j :19z;..
AQCEOCS SOLCTIONS O F SODIChI SILICATES
61;
(3') h third very weighty argument appears when we consider the result of hydrolysis esperiments along with freezing point lowering results. I n the paper on osmotic activit,y' it was shown that the only possible way'to account for the high osmotic activity of these ratios in aqueous solution xas t>oaccept the esistence of silicate ions. The conductivity of the sodium ions and of the hydroxyl ions as found by E.1I.F. esperiments of S a ion and OH ion activities. together account for only a fraction of the total ion concentration as determined by vapour pressure and freezing point, lowering. The nonaccordance is quite beyond the hounds of experimental error. nor could it be accounted for 1)y the assumptions underlying the laws governing ideal solutions. The higher the ratio the wider the divergence, so that in I :3 and I :4 inore than half the crystalloidal content' has t o he accounted for by the silica in t.he most dilute solutions. Whether this is due t o the coniples silica aggregate breaking down into simpler silicate ions. or t o the disintegration of an ionic micelle. or t o crystalloidal H & 0 3 ionising, or t o all these phenomena. can not be tliscwsed fully a t present. The fact remains hoxever, of the existence of a very large proportion of "crystalloidal" silica in solution. The esistence of crystalloidal silica and silicate ions in aqueous solutions seems therefore firnily estalilished, and there now remains the problem of the nature antl composition of the ions and the proportions in which they exist. I n t,he solution of this problem the nature and coniposition of the salts giving rise t o these ions furnish a good guide, and help t o narro\v down the possibilit ies. From the investigation of the ternary system2 S a 2 0 : Si02 : HZO a t z 5 O C we have seen that the ratios I : I and I :z only are definite salts. Ratio I : I is the inetasilicate. SanSiOs,crystallising with 9 , 6, antl 2 . 5 aq., the existence of which has heen long estahlishetl in spite of the difficulty of its crystallisation and the confusion which till now has existed concrrning its hydrates. -4certain amount of evidence that these two ratios I : I and I : z are definite salts is also forthcoming from consideration of the curve3 where conductivity, ion concentration. sodium ion concentration, and the van't Hoff ' from freezing points are severally plot,ted against the ratio, distinct changes of direction at these points being evident. It was also seen froin consideration of freezing point results3 that the existence of salts corresponding t o ratios I :3 and I :4 was w r y unlikely. From the tliagrani of the ternary system S a & : S O ? : H,O (loc. cit.) the fact tJiat t,he solubility of the definite salt I :z extends t o the region 1 :3 antl I :4 seems to indicate also that salts rorrespontling t o I :3 and I :1are unlikely. Ratio I , z may lie either a 0 : . It is not proposcd t o give here all salt of the formula SaHSiOa or Ka salts, except t o state that ( I ) 1Iorej.l the evidence for and against these 0: froin rnelts a t high temperatures over ;ioo"C but not from solutions at ordinary temperatures ( z I the only one anthentic case of a
' Harman:
J . I'hys. 0.1 S . HCY added t o 0.1 N. Sa?SiO,. (I) (2) 1 . 2 1 9 S . HC1 added to 0.2 N. S a 2 S i 0 3 . I n bot,h, when the E.M.F. observed is plotted against the amount of HC1 added as in Fig. I , there is a small unmistakable drop of E.3LZ.F. in the neighbourhood of 0.9250 volts, i.e. a t point, B in the figure, followed by a large and sudden drop from 0.80oo volt,s to 0.40oo volts. The mid-point A of the large drop comes a t 0.6005 volts with 0.1 S . Na,Si03, and at 0.6000 volts wit,h 0 . 2 S . NasSiOs. The neutral point of water is a t 0.7000 vo1t)s. (K.KC1 calomel cell used). The amount of HC1 added, corresponding t o point’ X, is equal t o the calculated amount just sufficient to neutralize all the sodium in solution, while the amount of HC1 added is, in both cases, at the mid-point 13 of the smaller drop, about one-half t,hat corresponding t o point A . This is good evidence of the existence of the acid metasilicate n’aHSiO.3. From these two curves we conclude that H2Si03ionises in t r o stages,H2Si03 H’ H S i 0 3 ’ . . , , . (kl), (I) HSi03’ H’ SiOs” . . . . . . ( k 2 ) . (2)
= e
I
+ +
Zarnbonini: Atti. Accad. Sei. S a p . , 12, S o .
12.
p. 16 ( 1 9 1 2 ’
61.9
AQUEOUS SOLUTIOSS OF SODIUM SILICATES
Point B corresponds to equation (z), and a t that point the following equilibrium exists,Sa2Si03 H C 1 e KaHSi03 SaC1. Point A Corresponds to equation ( I ) and at that point the main equilibrium is SaHSiOa HC1 *-.H&O3 SaC1. If the fall in E.1I.F. at point I3 had been more definite, i.e. larger and sharper, thcn it would hare been possible t o calculate k:, the second dissociation constant of H2SiOatherefrom with accuracy; as it is, it has been considered more prudent to omit, assigning any definite value to k? until further riork has been tlonc with this specific point in x-ierv.
+ +
+
+
4
c o N c m m A T/ON HC/ _.__ 8 1Z I6 PO
\.
24
i
28
FIG. I
First Dissociation Constant kl
+
H2Si03 H' HSiOa' Point A?, E.N.F. = 0.6000 volts (Fig. I ) curve 2 . ) . C'onc. Sa?SiOs = 0.1molar. pH = 5.33. [H'] = 0.47 X 10-j. [HSi03'] = 0.47 X IO-$. [H&Oa] equals 0.1m nearly: assuming, for which there is good reason given later, that the H2Si03is not colloidal, Then,[H'] X [HSiOa'] = k, . [H2Si03]
=
2.2
x
10-10.
1,'or curve ( I ) , Fig. I , conc. S a 2 8 i 0 3equals o.ogm, and E.M.F. = 0.600; volt%.i.e. [H'] = 0.46 X IO-:, hence, kl = 4.2 X ro-lo. attempt was also made to titrate electrometrically S a O H and silicic acid, hut here it is not of much use t o begin with silicic acid or so-called
R. W. HARMAN
620
silicic acid prepared by dialysis, as this must necessarily be almost completely colloidal. Instead, a dilute solution, 0 . 2 S,, of one of the ratios rich in silica, viz. 1:4] was taken and I O 5 . NaOH added, the E.M.F. being measured after successive additions of measured quantities of S a O H , (the NaOH being added in drops.).
1-1 .692___ 4 4
A m u N r N ~ , U HAKJED'
a
I2
I
FIG.2
The result is shown graphically in Fig. 2 where the E.1I.F. observed is plotted against the quantity of XaOH added. This curve is different from that of OH ion concentration against ratio,' because there the sodium was kept constant while here the SiO, is being kept constant. IVe note that the E.M.F. rises very rapidly until just beyond ratio I : 2 , hut only slightly and irregularly after that, until at 2 : 1 a fairly constant value is obtained. It is doubtful however, whether the misture reaches equilibrium in so short a t,ime, and during the titration a break of three hours was made. At the end of this three hours, the E.M.F. had fallen until the value mas almost 'Harman: J. Phys. Chem., 30, 1100 (1926). Fig.
2.
62 I
AQUEOUS SOLUTIONS O F S O D I L X SILICATES
the same as a t I :I. This is indicated a t point X on the figure. On further addition of XaOH the E.M.F. again increased t o a constant value. We cannot conclude much from this one curve alone, except that,( I ) the mixture of silicate and S a O H does not reach equilibrium immediately, a conclusion already arrived a t from conductivity measurements. ( 2 ) the OH ion concentration of ratio z :I is not much greater than that of I :I ; this has already been noticed in the paper on hydrolysis. Calculation of Dissociation Constants of HZSiO3from Hydrolysis Data One would expect metasilicic acid, H2SiO3,the dibasic acid which forms the metasilicate SaSiOs, to ionise in two stages, and indeed there is evidence of its so doing from the electrometric titrations of Ka2SiOaagainst HC1 just described. Considering the hydrolysis of NazSiOs in two stages me can calculate the primary and secondary dissociation constants of HzSi03 in the following manner. Secondary Dissociation Constant kz XazSiO,
+ HzO +NaHSiO3 + NaOH
(1-4 Q1
X
x
Q Z
QJ
(I)
where x = fraction hydrolysed, and C Y ] , a2 and a3 the respective degrees of ionisation. If we put 01. = cy3 = I , i.e. S a O H and SaHSiOs completely dissociated. and take the values given for OH and Ka ion concentrations from the E.M.F. measurements already communicated for z and CYI respectively we get,-, [H‘] X [SiO,”] kz = [HSiO,‘] Hence,E;, x2 CY:. ~
’ki conc. m
s w
X
(I-X)C
C Y ~
a1
Kr/k,
0.I
0.Oj
0.414
0.03
‘95.0
0.05
0.02j
0.440
0.11
125.7
0.02
0 01 0,005
0.465 0.556
0.75
0.01
53.89 139.3
0. IO
ka 0.51 X
0.7
Mean = 0 . 9 7 X Product of Dissociation Constants, k,.kn. Considering the second stage,KaHSiOs H20 XaOH H2Si0)
+
+
+
we getj-
kl
=
[H‘] X (HSiO,’] 1H2SiOa]
10-l~
0.80 1.86
(2)
10-l~.
R . W. HARMAN
622
Combining equations ( I ) and
(21,-
+
+
Ka2SiO3 z H2O H28i03 z KaOH [H'] X [SiO,"] = kl.ks [H2Si03]. Hence
(
2%c)s.
a1
(I
-x)c = kl.kl.xc.
For conc. C = o.ogm, we get kl.k2 = 8.3 X IO-*^. Hence k1 = kl.ks/k, = 1.6
X
Comparison of Dissociation Constants of H 2 S i 0 3 From electrometric titration
ki . k, ki k?
4.2 X
10-l~
From hydrolysis
8.3 X i .6 X 0.51 x
10-l~ 10-16
Silicate Ions Hitherto the silica in solution has usually been regarded as wholly colloidal in nature, whether a solution of an alkali silicat'e or of silicic acid has been the subject of investigation. I n view of the fact that, the present investigations indicate that in some cases the silica is almost wholly crystalloidal, it seemed most advisable to obtain some direct evidence of the presence of silicate ions. This has been done in t'wo ways,( I ) colorimetrically. ( 2 ) by diffusion. Colorimetric Estimation of Silica Oienert and Waldenbulckel describe a colorimetric test for crystalloidal silica depending on the formation of a greenish yellow silicomolybdatc. On adding z cc. of a 1.7~solution of ammonium molybdate and 4 or 5 drops of a 50% solution of H2S04t o a silicate solution. a yellow colour forms immediately in the cold, which deepens for I O minutes or so and then remains constant for several hours. The test is perfectly quantitative in dilute solution.
The mechanism of the reaction involved does not seem t o be very clearly understood as yet. W..4sch2 has shown the existence of the two salts,-zNa?O.~iO?.Iz~\1oOz.aq. and 1.5NasO.o.jH20.fiiO?.rz~Io03.aq., and that in these salts the silicic and molybdic acids form a complex ion. Many compounds of silica besides the simple silicates investigated here, e,g. K.&F6, give the test,, but colloidal silicic acid, i.e. silicic acid prepared by lengthy diffusion does not give any colour, so that at best this colorimetric test can only be said t o be a test for crystalloidal silica and not a test for any particular silicate ion, although probably the ion concerned is the simple silicate Si03 ion from metasilicic acid. ' C o m p t . rend., 176, 1478 (1923). "The Silicates in Chemistry and Commerce", 16 (1923).
623
AQUEOUS SOLUTIONS OF SODIUM SILICATES
The following Table I contains the results of this colorimetric test on ratios 2 : I , I :3 and I :4. Ratio I : I was used as a standard, and the table shows the normality in terms of sodiuni content of ratio I : I required t o give the aame depth of colour as the normality cited for the other ratios, and the ratio of crystalloidal silica in 2 : I , I :3 and I :4 to the crystalloidal silica in I : I .
TABLE I 2:I
Std. Crys. sil. in I :I Crys. sil. in
2:1
1:3
I :I
Std.
9
I :I
I :I
I
:4
Std.
&4
I:I
1:I
o.oooj
0.0002
4.0
4.0 3.3
o.000,;
0.00025
o,j
0.0005
o.001j
0.001
0.oooj
0.5
0.001
0.002j
3.0 2.8
0.001
0.004
o.002
0.001
0.003
0.009
3.0
0.003
0.010
0.003 0.005
o.001j
0.5 0.5
o.ooj
0.014
2.8
0.005
0.016
3.2
0.002
0 .j
0.007
0.018 2.6
0.00;
0.022
3.1
I t is seen that in very dilute solution, 0.005 N, ratio I :4, contains as much crystalloidal silica as 0.002 K, I : I , the normalities in acrordance with the practice adopted in this investigation being expressed in terms of the sodium content. In other words, practically all the silica in ratio 1:4 exists in the crystalloidal state at a dilut,ion of 0.005 S , or more correctly, ratio I :4 contains 4 times as much crystalloidal silica as ratio I :I at this dilution. Similarly a t this dilution, all the silica in ratios 2 : I and I :3 exist in the crystalloidal state. As the solution gets more concentrated we see that ratio I:A+ no longer contains 4 times as much crystalloidal silica as ratio I : I , indicating that in the niore concentrated solutions some of t,he silica in ratio I :3, and still more in ratio 1 :4>passes into the colloidal state. Hence in cstremely dilute solutions of these ratios practically all the silica esists in the crystalloidal state, but with increasing concentrat,ion increasing amounts of colloidal silica are manifested.
Diffusion Experiments The diffusion experiments were carried out with collodion membranes and with parchment paper. 7 h e collodion niembranes were prepared in the form of a flask and were closed by a rubber stopper, through which a glass t,ube serving as a manometer passed, the whole k i n g made airtight by coating the neck of the hag and the stopper with collodion. It was found that all the rat.ios at all concentrations gave evidence of diffusion of silicate ions both with the collodion membrane and with parchment pap'r. Ciffusion was allowed to proceed for from 8-14 days (equilibrium being usually estahlisketl in 6 or 7 days) and then the solutions inside and out'side the membrane were analysed. Results With ratios I : I and I :z equal distribution of both sodium and silica was found to have taken place wit,h a 0.3 K w solution.
624
R. W. HARMAN
I n ratio I :3, with a solution approximately 0.2 N,, after equilibrium had been established in a couple of days, there was an excess of silica within the membrane, thus showing that some of the silica in the original solution was colloidal. More definite evidence of the existence of the silica as partly colloidal, partly crystalloidal, was obtained with a 0.3 S, solution of ratio I :4. The sodium content of the inside and the outside solutions was practically the same after 1 2 days diffusion, but there was 3.j67Cc Si02 outside and 4.96;CG Si01 inside the membrane. Assuming the difference to be colloidal silica, i.e. non-diffusible silica, then in the original solution 1.4 4.2 = 1/3 of its silica, was in the colloidal state. It is not surprising that the general belief exists that silicic acid is almost wholly colloidal in solution when it is remembered, that it is prepared by dialysis, during which the crystalloidal constituent diffuses throuph the membrane and is discarded, leal-ing only the colloidal part. Any investigation int,o the nature of silicic acid should not, as has hitherto been the case, be confined t o this colloidal proportion alone. It was found that when a I S, solution of ratio I : I , to which a small excess of HC1 had been added, was submitted to dialysis, silica rapidly diffused through the membrane, as much as 30:; of the original Si01 content being diffusible. Although this was observed antl pointed out by Graham in his classic researches, its significance does not seem to have been generally recognised. When the outside water is changed continually. as in ordinary dialysis, a solution of silicic acid remained, which gave only the very slightest colour with ammonium molybdate and thus was practically all colloidal.
Conclusion It has thus been shown conclusively that silicate ions exist in all the ratios up to I :4, and in silicic acid it,self. In I : I and I :z the silica is practically all crystalloidal in dilute solution: in ratios I :3 antl I :1 increasing proportions are colloidal. and in extremely dilute solution, practically all the silica in any of these ratios here investigated exists in the crystalloidal state. Summary ( I ) Electrometric titrations of 0 . 1 and 0.2 S, 1 . 2 1 9 S , HCI, and 0.2 ?;, . Sa*SiOa, 1:4! with
. Sa2SiOs\vith 0.1 N and I O S . ?;aOH have been
carried out. The curves obtained with Sa2SiOs and HCI are typical of dibasic (2) acids, thus suggesting HISiOa as dibasic with salts SaHSiOs and ?;a,SiOs. ( 3 ) The primary and secondary dissociation constants have been calculated from hydrolysis results and from these electrometric titrations; kl = 4.2 X 1 0 - l ~and kz = 0.51 X IO-'^. (4) The amount of crystalloidal silica in ratios 2 :I, I :3 and I :4 has been compared with that in ratio I :I in very dilute solution by means of the colorimetric test with H2S0, and ammonium molybdate.
AQUEOUS SOLUTIONS OF SODIUM SILICATES
625
( j ) In very dilute solution most of the silica in these ratios is crystalloidal, but increase of concentration and increase of ratio increase the colloidal content. ( 6 ) Diffusion experiments t'hrough membranes of collodion and parchment paper indicate that most of the silica in ratios I :I and I :z is diffusible: about 2!'3 of the silica in 0.3 N, 1:4> and about 1/3 in 1.0 K, H2Si03was crystalloidal. ( 7 ) H2Si03is much stronger than hitherto supposed, due no doubt to the fact that when prepared by continuous dialysis the ionisable and diffusible portion is lost. I wish to thank the Commissioners of the 1851 Exhibition for a Scholarship which has enabled me t o carry out this investigation, and t o express my gratitude to Professor Donnan a t whose suggestion this work was undertaken, for his constant kindly interest and advice.
The S i r W i l l i a m Ramsay Laboratories of Phvsical and Inorganic Chemistry, Cniversity College, London. October 14, 1926.
