AQUEOUS SOLUTIONS O F SODIUM SILICATES. PART 111. SODIUM IOK ACTIVITY BY R . W. HARMAN
Introductory The problem of the nature of the ions and their respective concentrations in aqueous solutions of sodium solicates cannot be determined solely by measurements of the total number of ions, say by means of vapour pressure or freezing point measurements and by measurement of the extent of the hydrolysis of these solutions. The number of ions deduced, assuming the laws of ideal solution to apply, from freezing point measurements (as will be seen from the discussion of those measurements in a paper to be communicated soon) is not in agreement with that which would correspond to the percentage hydrolysis if hydrolysis only took place, Hitherto, the silica in solution derived from the dissociation of silicate solutions has been considered colloidal, and the suggestion has been put forward that complete hydrolysis takes place, the low value found for the degree of hydrolysis being only apparent, since such degree of hydrolysis has been computed from the ‘free’ hydroxyl ion concentration (as measured by the hydrogen electrode), whereas the silica may be colloidal and may adsorb hydroxyl ions. If such were the case, then the hydroxyl ion concentration plus the sodium ion concentration should equal the total ion concentration as found by freezing point measurements. However, both conductivity and transport number experiments1 give evidence of silicate ions or complex ionic aggregates being present in solution. If this is so, and the sodium ions are equivalent to the sum of the silicate and hydroxyl ions, and the sum of these three ions is equal to the total ion concentration, then correct conclusions c;tn be drawn as to what really takes place in solution, the extent of both hydrolytic and ionic dissociation can be determined, and the question of the existence of colloidal or crystalloidal silica may be definitely settled. Hence the importance of determining the concentration of either the sodium or the silicate ions. The concentration of the silicate ions is not capable of direct measurement, although a colorimetric method is useful whereby the amount of crystalloidal silica in solution may be deduced in very dilute solution. Experiments relating to the concentration and diffusion of the silicate ions are in progress and will be communicated in a later paper. The sodium ion activity can be measured directly by means of the sodium amalgam electrode devised by G. N. Lewis, and from this we can get a fair conception of the sodium ion concentration, in fact, a correct measure of the concentration in very dilute solution; and if the assumption be made that the 1
“Aqueous Solutions of Sodium Silicate,” Parts I and 11: J. Phys. Chem., 29,
(192.5); 30, 3.59 (1926).
1 1j 5
91%
R. W . HARMAN
laws of ideal solution apply, as is usually done with the hydrogen electrode, i. e. that the ion activity gives a measure of the ion concentration, we can determine the sodium ion concentration in the more concentrated solutions.
Experimental The technique of measurements with the alkali amalgam electrode first used by Lewis and Krausl has since been developed and perfected by Lewis and by MacInnes and their co-workers, until now, the E. M. 5'. between a sodium amalgam electrode and a solution containing sodium ions is capable of very accurate and reproducible measurement, providing care is taken with the preparation of the amalgam, with the preparation of the solution to be experimented upon, and with the design and operation of the cell. Preparation of Amalgam:-The mercury was purified according to Hulett's method by running it several times in a very fine spiral spray through 5 % mercurous nitrate solution acidified with HKOs, and then twice distilling it in a current of air at a pressure of 2 5 mm., the resulting scum of oxides (very little in the second case) being separated by filtration. The sodium amalgam was prepared in vacuo by a method similar to that employed by MacInnes and Parker.2 As pointed out by these authors, the method is quite simple and very convenient but care must be taken in melting the sodium, as the capillary on the side tube through which the molten metal filters from the oxide is apt to become blocked and may cause an explosion. The rlmalgam ElecLrode:-The cell used was somewhat similar in design to that used by Lewis and Kraus.l It was found necessary to have the platinum contact with the amalgam sealed in the capillary tube without distorting the capillary and to have the platinum bent down into the capillary about 5 mm. to prevent the amalgam thread breaking when it was allowed to run out during an experiment. The electrode vessel was filled by sealing it to the amalgam preparation vessel, evacuating with the vacuum pump and thep allowing the amalgam to run down into the electrode vessel. The amalgam preparation vessel was then replaced by a P206 tube. By this means dry air, which has no effect o n t h e amalgam, replaces the amalgam as it is used during the course of an experiment. Design and Operation of the Cell:-Two side reactions interfere with the complete reversibility of these amalgam electrodes, but both have been eliminated. The two difficulties are:( I ) the side reaction 2 l\'a+2H20 = zNaOH +H2. This causes decrease of potential difference between electrode and solution owing to the concentration of the amalgam being lowered, and the ion concentration in its vicinity being increased. J. Am. Chem. SOC.,32, 1459 (1910). J. Am. Chem. SOC.,37, 1445 (1915). J. Am. Chem. SCC.,32, 1459 (1910).
AQUEOUS SOLUTIONS O F SODIUM SILICATES
919
The cell was designed so that, as the sodium amalgam flowed out of the capillary, it dropped through the silicate solution and out of contact with it, by passing through a tap through which, of course, some silicate solution also passed. In this manner fresh silicate solution is always continually passing the fresh amalgam surface. By this means very reproducible results were obtained, the €'. D. remaining constant to within 0.1millivolt for 20-30 readings. ( 2 ) the side reaction:4Na+O2 = 2T\Taz0. S a 2 0 +HzO = zNaOH. due to dissolved oxygen in the solution. This also lowers the P. D. This difficulty was eliminated by preparing the solutions free from oxygen in the following way. A 300 to 400 cc. portion of concentrated solution of the desired ratio, made up from freshly distilled air-free water, had pure nitrogen bubbled through it for a couple of hours, and was then left in a flask tightly stoppered with a cork carrying an inlet and an outlet tube. This stock solution was accurately analysed and hence its concentration found. From this stock solution more dilute solutions were prepared by forcing a little of it out into a weighed flask by applying a pressure of nitrogen, weighing, and then adding freshly distilled water, prepared air-free b'y the same means, and weighing again. Any air in the flask above the solution was replaced by nitrogen. The solution so prepared was allowed to stand overnight, and when required for use in an experiment the requisite amount was driven over into the electrode vessel by applying a pressure of nitrogen. Connection was made with the normal calomel electrode used to complete the cell by having some of the same silicate solution that was being investigated in a connecting tube dipping into a small beaker of the same silicate solution. The side limb of the calomel cell also dipped into this beaker. The normal calomel electrodes used were very carefully made up from purified materials and they agreed with one another and with other calomel electrodes made up by different investigators in this laboratory to within 0.1 millivolt. The tap on the side limb of the calomel cell was kept closed during a measurement but some KC1 was run out through this limb just before and after any series of measurements were made. The whole apparatus was kept in an air thermostat electrically heated and regulated to 25OC = ~ o . o I . The measurements were made on a Cambridge and Paul potentiometer with two standardised Weston cells in series balanced against two accumulators, so that the readings in volts on the scale had merely to be doubled to get the observed E, M. F. Concentration of Amalgam:-A weighed quantity, a t 20 gms., of the sodium amalgam was run out of the electrode vessel int orcelain dish and was left overnight, with occasional stirring by means of a platinum wire, in contact with a considerable excess of standard HCl. When complete decomposition had taken place the remaining HC1 was titrated with NaOH. Two quantities of amalgam were prepared at different times, and both were
R. W. HSRMAN
920
analysed twice, once when the electrode vessel was nearly full and once when nearly empty, The concentration of the amalgam was found to be,I.
0.06057. 0.06055~
2.
0.07090
0.07091%
Calculation of Results
If we know the electrode potential between a sodium amalgam electrode and a, solution of known sodium ion activity then it is easy to calculate the sodium ion activity of any other solution containing sodium ions, from the E. M. F. observed with the same sodium amalgam electrode in this particular solution. So measurements were first made of the E. M. F. due to the sodium amalgam in a standard solution of NaOH, i. e. of the following combination,Na amal. I NaOH 1 N.KC1. Hg2C12I Hg. and then the following was investigated,(I)
(2)
Na amal. I Na si]. I N.KC1. Hg2C12I Hg.
varying the concentration and ratio of the sodium silicate in ( 2 ) . NOW,1 x a o= ~E, - RT/nF In axa (in WaOH) ( I) Eamal. (2)
Eamal 1 N~ s,l.
= E,
-RT/nF In aNa (in Na sil.)
Hence, aNa (in Na sil)
Eamal. I Na ail.
- E amal. 1 KaOH = RT/nF In aNa (in NaOH
The concentration of the first preparation of sodium amalgam was E. M. F. observed between this amalgam and a 1.ON.NaOH, corrected for the liquid potential difference 1.0 N.NaOH 1 IN.KC1, calculated by means of the Henderson formula, was 2.103 j volts. The activity coefficient1 y of 1.0N.NaOH was taken as 0.75 whence aNa=o.75. This y( = 0 . 7 5 ) is the mean activity coefficient of the ions; so, to get the activity of the sodium ion the assumption is made that the sodium and hydroxyl ion coefficients are equal. Hence, for the series of measurements made with the sodium amalgam whose concentration was 0.060 j6$& we get the working formula, Eamal. 1 N a s i l . - 2 * I 0 3 5 = 0 * 5 9 I I Log aXa/o-75. 0 . 0 6 0 5 6 7 ~and , the
For the second preparation of amalgam used in the latter part of the measurements, the concentration of the amalgam was o . o ~ o ~ % the , NaOH was 0.6708N, whence aNa in NaOH = o . 5031, and the E. M. F. (corrected for liquid potential difference) was found to be 2 . I 142 volts, 1
See Harned: J. Am. Chem. Soc., 47, 676 (1925).
AQUEOUS SOLUTIONS O F SODIUM SILICATES
92 1
Values for a N a for the same silicate solution, obtained by the amalgams of the first and second preparations, agreed within 0.17~. Liquid-Liquid Potential Difference:-The values of E. M. F. given in Tables 1-11have been corrected for the P. D. between the sodium silicate solution used and N.KC1, by making use of the P. D. found between these silicate solutions and saturated KC1 by the Bjerrum extrapolation method, and of the P. D. between saturated KC1 and N.KC1 calculated by means of the Henderson formula.
Results
TABLE I K W
E. M. F.
ANa
Ratio 0.832 0.373
I
: 0.4006 0 . 1573 0.04629 0 . 0 2 166 0.01485 0.00802 I
0.481 0.422 0,417 0. j04 0 739 0 974
0.2331 0.1623 0 ' 08433 0,93992 0.02043 0,01233 0.0093 74 0.002493
0.275
0.105 0.213 0.356 0.411
0.00823
2.1196 2.1436 2.1750 2.1956 2.2042 2.2200
0.8486 0.5408 0.2056 0.084222 0.0403 0.02296 0.01349 0 . 0 0 2 j 19
2.1330 2.1428 2.1596 2.1788 2.1960 2.2090 2.2160 2.2500
I.073 0.3829
2.1520
0 . I122
2.1604 2.1788 2 . I942 2 . 2 1j o 2.2320
0.08175 0.04062 0.02239 0.0099 I 7 0,005135
0 . I11
0.043 0.020
Ratio
Ratio
0.1142
0.05454 0.01671 0 . 0 0 6 594
Ratio 0.7039 0.3215 0.09517 0.04479 0.01685
2.1766 2.1770 2.1916 2.2080 2.2220
I
I
I
e
:2 0,300 0,410 0.474 0.507 0437 0.695 0,980
:3
0.594 0~779
:4
0.04426
0.063
0.04358 0.02468 0.01582 0.007492
0.135 0.259 0.353 0,445
R. W. HARMAN
922
TABLE I1 y(Activity Coefficient) for Ratios Conc. (N,)
I : I
I.o
0.80
I :2
I
:3
I
: 4.
0.265
0 . 105
0.050
0.275
0.060
0.310
0.145 0.185
0.40
0.475 0.440 0.425
0.335
0.210
0 . I10
0.20
0.415
0.410
0.295
0.185
0.IO
0.42j
0.460
0.250
0.05
0.500
0 . 500
0.025
0.700
0.j 2 j
0.365 0.42j 0.480
0.325 0.405
0.01
0.975
0.850
0.700
0.550
0 . 50
These results are shown graphically in Fig. the concentration.
I,
0.090
where y is plotted against
FIG. Sodium Ion Activity y against Concentration
AQUEOVS SOLUTIONS O F SODIUM SILICATES
923
Discussion of Results The values for y, the activity coefficient of KazSiOa (i. e. ratio I:I) show that in concentrated solution as much as 40% of the total sodium exists in the active ionic state, while in dilute solution practically all the sodium exists so. The most remarkable feature is that y for ratio 1 : s passes through a minimum at concentration o.2?rTw,while none of the other ratios exhibit such a minimum, It is not unusual to find this minimum in y for strong electrolytes in concentrated solution, and this has been shown by Debye and Huckell to have a theoretical basis when the inter-ionic forces are taken into account.
FIG.2 Ratio
y against
In ratio I :3 and I :4, the values of y are very low in concentrated solution and even in dilute solution are still comparatively low, indicating that all the sodium in solution does not exist as sodium ion, or if so, the silica present has considerably affected and reduced its activity. When y is plotted against the ratio, as in figure 2 , then for o.IN, solution, the decrease in y is quite regular as the proportion of Si02 increases. The decrease in y is also regular for the higher concentrations beyond ratio 1 : 2 , but owing to the fact that y for I:I passes through a minimum, and is even lower than y for s : 2 at concentrations near o.1-0.2S,, the changes of direction in the curves above I :I are varied and probably have no special significance, Physik. Z., 24, 185, 334 (1923)
924
R. W. HARMAN
The general conclusion may be drawn however, that both increase of concentration for any given ratio, and increase of ratio for any given concentration, 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 :4 it is remarkably low.
Summary ( I ) Measurements of sodium ion activity by means of a sodium amalgam electrode have been made for ratios I :I, I : z , I :3, and I :4, at concentrations ranging from ~.o-o.orN. The activity coefficient y has been plotted against the weight normal(2) ity, E,, and against the ratio Na2:SiOz. (3) The curve of y against N, for ratio I :I passes through a minimum at a concentration lying between o.1-0.2NW. The other curves show no minimum. (4) In very dilute solution y is high, but not so high as in corresponding concentrations of NaOH, whereas in concentrated solutions of higher ratios y is abnormally low. I wish to thank the Commissioners of the 1851 Exhibition for a research scholarship which has enabled me to carry out this work, and to express my gratitude to Prof. Donnan at whose suggestion this investigation was undertaken, for his constant kindly interest and advice. TheSir Walliam Ramsay Laboratorzes of Physical and Inorganic Chemistry, Unwersity College, London, April r, 1926.
