THE INFLUENCE OF SMALL AMOUNTS OF DISSOLVED SILICATES ON THE CONDUCTANCE OF CONDUCTIVITY WATER AND VERY DILUTE SOLUTIONS O F ELECTROLYTES T. IVAN TAYLOR Department of Chemistry, University of Idaho, Moscow, Idaho Received November 7, 1932 INTRODUCTION
The magnitude and sign of the solvent correction to be made in conductivity data is sometimes very uncertain, and its application has been the subject of much discussion and controversy in the past few years. This arises mainly from uncertainties as to the impurities present in conductivity water and their reaction with the electrolyte in solution. Kendall’s (1) “carbonic acid correction” has not been generally accepted, partly because of the maximum observed a t high dilutions in the data for strong acids even after the correction had been made, and partly because some authors have found disturbing factors greater than the carbon dioxide present. Washburn (2) suggests that this ‘‘abnormal behavior’’ is probably due to the presence of some basic or saline impurities, while Whetham and Paine (3) attribute it to a salt, probably ammonium carbonate. Though it is conceded that carbon dioxide is generally the major impurity, Wynne-Jones (4) has shown a deviation between the calculated and theoretical curves and suggests that solution of silica from the cell walls might cause the variation. Kraus and Parker in their work on iodic acid with concentrations as low as .00005 N , using very pure water and cells of quartz, Pyrex, and soda-lime glass have shown that the nature of the cell is of greater importance than the initial conductivity of the water or the carbon dioxide content. The extent of the solubility or decomposition of glass under the conditions ordinarily met with in conductance work has not as yet been studied very thoroughly. Such measurements would be difficult to make and reproduce, because there are so many factors influencing the decomposition and rate of solution. Glasses also vary much in their physical and chemical constitution, even those of the same brand. Glass in contact with moist air and carbon dioxide is continually being acted upon, giving products which are easily soluble in water. Thus solutions put in such a container, which has not been well leached just before use, will be affected abnormally a t first. This phenomenon is easily noticeable by pouring a 765
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T. IVAN TAYLOR
very dilute solution of hydrochloric acid into a bottle which has been standing open to air. The reduction in conductance a t first is comparatively large, changing only gradually thereafter. Forester’s ( 5 ) and Walker’s (6) work on the solubility of glass under laboratory conditions shows that it is by no means negligible, and that both alkali and silica are taken into solution, probably in the form of a sodium silicate. The nature and behavior of the silicates of sodium in solution has been studied quite thoroughly by Haag (7) and by Harman (8). From their work it has been concluded that the only sodium silicates existing in solution as such are the metasilicate, NazSi03,and the disilicate, NaHSiOI; that the hydrolysis of Na2Si03 gives rise to HSiO, ions, which do not separate out colloidally; that only ratios larger than NazO :Si02 = 1:2 give appreciable amounts of colloidal silicic acid. I n addition, Harman has shown that in dilute solutions of any ratio, practically all the silica exists in the crystalloidal state. Under the term “crystalloidal silica,” he classifies definite silicate and bisilicate ions, aggregates of ions carrying an electric charge with or without colloidal silica, i.e., ionic micelles and ,crystalloidal silicic acid or hydrated silica. The present investigation is an attempt to show the effect of small amounts of sodium silicates, such as are dissolved from glasses, on very dilute solutions of electrolytes and on conductivity water. Although there is no definite proof that the material dissolved or decomposed from the glass is in the form of silicates of sodium, it is shown from equilibrium relations between the crystalloidal silica and alkali that effectively it is in that form. Further, the solution of glass and change in the nature of impurities in conductivity water with collection and storage in Pyrex and soft glass containers will be demonstrated. EXPERIMENTAL
The conductivity apparatus was one arranged and set up by Alonzo W. Martin.‘ A telephone head-set tuned to a 1000 cycle E.M.F. from a Vreeland oscillator was used as a detector. To increase its sensitiveness a onestage audio tube amplifier, as suggested by Hall and Adams (9) was placed in series with the telephone. A Kohlrausch drum-wound slidewire bridge divided in one thousand divisions was calibrated for use by the method of Stronhal and Barus. For the resistance, two calibrated Curtiss coils of capacities 10,000 ohms and 100,000 ohms were used. Adjustable air condensers cut out all capacitance and eddy currents, thereby increasing the sensitiveness and clearness of the minimum point. The whole apparatus was well grounded with a grounding arrangement similar to that suggested 1
The writer is indehted t o Professor Martin for the use of the apparatus.
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by Wagner (10). Measurements were made in a thermostat regulated to 25°C. The dip-type conductance cell with bright platinum electrodes was used so it could be dipped into the solutions through the neck of a well leached liter Pyrex flask. The cell constant was obtained by inter-comparison on a 0.001 N solution of potassium chloride, which value checked very closely with that obtained by using 0.0014695 mhos for the specific conductance of the potassium chloride (11). Its value was taken as 0.081402. Stock solutions of hydrochloric acid, potassium chloride, and sodium hydroxide were carefully made up from the C.P. materials. These were measured out in standard calibrated pipettes and diluted to the desired concentration. One liter of the solution was transferred to the conductance arrangement; the whole was well but gently stirred after the addition of each increasing amount of 0.00046 N sodium silicate; the electrodes were raised and lowered several times in the solution and a short time was allowed for equilibrium to become closely established, after which all readings were immediately taken. Care had to be used throughout to prevent as far as possible the adsorption of carbon dioxide from the air and t o prevent any vigorous motion, since it increases the amount of glass dissolved from the apparatus. Extreme accuracy was not aimed at, since a t such high dilutions accurate values of conductance would be of little value, because the exact nature of all the impurities in the water is not known, and the exact behavior of electrolytes a t these extreme dilutions is not fully understood. The sodium silicate used was made up from Baker’s 40 per cent solution having, according to their analysis, a NanO:SiOz ratio of 1 : 3.22. The normality with respect to sodium was 0.00046. The conductivity water was obtained from a specially constructed Kraus type still charged with alkali-permanganate. Water ranging in conducto 1.4 X tance from 0.4 X was prepared, and the effect of sodium silicate on conductance of water with different initial conductivities noted. The results are given in graphs (figure 1) rather than in tables, since the nature of the changes is more easily seen and compared. The results showing the effect of small amounts of sodium silicate on the conductance of very dilute solutions of hydrochloric acid, sodium hydroxide, and potassium chloride are given in figures 2, 3, and 4, respectively. I n order to demonstrate the rate and extent to which silicates dissolve from Pyrex and soft glass containers, water was left standing in 10-liter bottles, both sealed and open. At various times l-liter samples were drawn and conductometric titrations run with sodium silicate, as above, to show how much carbon dioxide had been removed by action of dissolved silicates. Many such runs under varying conditions were made, and some representative results are given in figures 5 and 6.
768
T. IVAN TAYLOR DISCUSSION
From the curves of figure 1, it is seen that the manner in which the conductance changes with increasing amounts of sodium silicate varies widely with the initial conductivity of the water. The higher the initial conductivity, the greater is the total lowering of the conductance and the more
FIG.1. EFFECTOF SODIUM SILICATE ON THE CONDUCTANCE OF CONDUCTIVITY WATER OF
VARYINQ INITIALSPECIFIC CONDUCTIVITIES
sodium silicate required to cause the maximum depression. A comparison of these curves with those of hydrochloric acid (figure 2) and sodium hydroxide (figure 3) shows quite definitely that the main impurity in the water is acidic in nature and without doubt is carbonic acid. The conductance of water of lower conductivity than 0.5 X is seen to rise continuously, thus showing that either there is no carbon dioxide present below this
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769
FIQ.2. EFFECTOF SODIUMSILICATE ON CONDUCTANCE OF HYDROCHLORIC ACID SOLUTIONS
770
T. IVAN TAYLOR
FIQ. 3. EFFECTOF SODIUM SILICATEON CONDUCTANCE OF SODIUM HYDROXIDE SOLUTIONS
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77 1
value, or that all the carbon dioxide has been removed by reaction with sodium silicate dissolved from the Pyrex container while the water was being collected. The latter is probably the case. It is seen from these curves that a “carbonic acid” solvent correction cannot be applied to water below
FIQ.4. EFFECTOF SODIUM SILICATEON CONDUCTANCE OF POTASSIUM CHLORIDE SOLUTIONS
if it has been collected hot in Pyrex containers. The correction 0.5 X will rather be one for the “solution of glass,” which involves a consideration of the products formed in the reaction of carbonic acid with silicates of sodium. For water of higher initial conductivity it is seen that the
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T. IVAN TAYLOR
CONDUCTANCE O F CONDUCTIVITY WATER
773
correction will have to take into account both the carbon dioxide and the dissolved glass. After the water has been collected, the relative amounts of carbon dioxide and dissolved silicates will vary with the time of storage. This is clearly shown by the curves of figures 5 and 6. The conductance of water in equilibrium with carbon dioxide of the air is 0.8 X The conductance
"t
0.60
i
FIG. 6. CHANGEIN CARBONDIOXIDE CONTENTOF CONDUCTIVITY WATERWITH TIMEOF STORAGE IN OPEN SOFTGLASSBOTTLES AT 25°C.
of the water should tend toward this value, and the amount of sodium silicate required for maximum depression should be approximately 5 cc. (see figure 1) if no foreign substances interfered. From the curves it can be seen that the carbon dioxide is continually being removed, while the amount of dissolved glass increases. Thus with a Pyrex bottle, practically all the carbon dioxide has reacted with dissolved glass a t the end of thirty
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T. IVAN TAYLOR
days and the conductance is now due to the products of the reaction. With soft glass the action is much faster. There is no carbonic acid present a t the end of ninety-six hours. Water collected hot directly in a soft glass bottle showed no depression with sodium silicate solution, indicating the presence of no carbonic acid. Parallel with these a bottle of water was stirred vigorously with a Pyrex glass stirrer. The time required for complete removal of carbon dioxide was forty-eight hours. Further experiments were run by bubbling hydrogen and nitrogen through the water. This had the effect of reducing the partial pressure of the carbon dioxide above the water, thereby decreasing the solubility of the carbon dioxide in the water; also it had the effect of increasing the rate of solution of the glass. Merely shaking the water in the flask gave a corresponding decrease in conductance. A summary of the results is given in table 1. TABLE 1
1
Hydrogen.. ............................ Nitrogen.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shaking. ...............................
TIME
1
~
INITIAL CONDUCTIVITY
FINAL CONDUCTIVITY
hours
4 4
1
0.85 X 0.96 X 10-6 1.17 X
0 . 4 7 X 10-6 0.45 X 10-6 0.93 X 1 V 6
A direct measure of the solubility of Pyrex glass was attempted by evaporating in a platinum dish 10 liters of conductivity water which had been standing in a Pyrex bottle for fifteen months. The initial conductance was 1.0 X and the final 1.23 X Conductometric titration with very dilute alkali showed that no carbon dioxide was present. The total solids obtained was 10.9 mg., of which 4.8 mg. was silica. The need and application of a correction for this “solution of glass” in measuring the conductivity of very dilute solutions of electrolytes or even hydrogen-ion concentrations can clearly be seen from figures 2, 3, and 4. For strong acids a positive correction is needed, rather than the “normal” water correction or the zero correction assumed in the theory of the carbonic acid correction; for a basic solution a slight negative one is needed. The conductance of neutral salts is not affected abnormally by the presence of such small amounts of sodium silicate. The magnitude of the ideal correction could be determined by the solubility of the glass, the equilibrium relations with sodium carbonate, silicate, and bisilicate ions, and the mobilities of the ions involved. The uncertainty of the values and complexity of such calculations is a t once evident when it is remembered they can be carried out only by a series of approximations, However, if the condition of the water is known, estimation of the approximate correction can be made with less difficulty.
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From the laws of diffusion, it is known that if fresh conductivity water is put in a glass cell, the rate of solution or decomposition of glass will a t first be comparatively large, owing to the high concentration gradient from the surface of the glass to the interior of the liquid. This gradient is maintained for a while by the effectual removal of the sodium silicate in the solution by reaction with the comparatively high concentration of carbonic acid. The complications and errors arising from the use of glass apparatus can easily be seen and one can see that water of 0.4 X or better, should be used if carbon dioxide can be excluded. If higher conducting water is to be stored some time before use, it might even be advisable to allow i t to come to a carbonic acid-free condition (this can be determined by conductometric titration with dilute alkali), otherwise, worse complications set in because the relative amounts of carbon dioxide and silicates are not known exactly. The best way to eliminate such errors and complications if platinum is not available, is to use quartz, which is acted on only slightly by electrolytes and water, or wax-coated vessels for collection and storage of the water and solutions. Some of the newer types of bakelite preparations show some promise. A carbonic acid-free condition can nearly be obtained by bubbling nitrogen or hydrogen through until no further decrease in conductance is observed. If, as was supposed by the earlier investigators, the sodium silicate is highly hydrolyzed in dilute solution to sodium hydroxide and colloidal silicic acid, one would expect the slope of the curves for sodium hydroxide to be less than the one for water, owing to depression of hydrolysis by the OH - ion and to adsorption by the colloidal silicic acid. If, as suggested by Haag ( 7 ) the hydrolysis of the sodium metasilicate is represented as follows: NalSiOa
2Na+
TI, . --
+ SiOB
+HzO
NaOH Na+
lt
+ OH-
+
NaHSiOa Na+
T +I HSi03-
then the limiting value of the hydrolysis will be 0.5. This can be assumed to be very nearly true, for it has been quite definitely demonstrated by Haag that practically all the silica in very dilute solutions of that ratio exists in the form of HSi0; ions and does not readily form colloidal particles. Since NaHSiOa is a highly ionized salt, the effect of such small OH- ion concentrations in decreasing the hydrolysis is small, while its effect on the secondary ionization of silicic acid existing as hydrosilicate ions or crystalloidally is comparatively greater and the conductance will rise abnormally. I n the conductivity water, which can be considered a solution of carbonic acid, and in the hydrochloric acid solutions effectually what we have happening is an exchange of the slow-moving sodium ion for a fast-moving hydrogen ion, which results in a lowering of the conductance. This can be offered as an explanation of the maximum found in the data for very dilute solutions of strong acids.
776
’
T. IVAN TAYLOR
If glass, as previously suggested, should be dissolved in the form of silicates of sodium, rich in silica, and practically all the silica is in the crystalloidal state as shown by Harman, then one can rightfully assume there is an equilibrium established between the crystalloidal silica, Si03 - ions, HSiOy ions, and the alkali. As an acid is added, an amount of Si0,- - ions equivalent to the sodium hydroxide formed on hydrolysis plus half the sodium in the unhydrolyzed silicate is converted to HSiOc ions. On further addition these ions are converted to silicic acid which first separates out colloidally and then changes largely to the crystalloidal state. Thus it is seen that in making an ideal correction for the “solution of glass,” one must take into account both the alkali and the silica, for even though, as Morey (12) has concluded from his isothermal saturation curves, there can be no true solubility of the silicate at ordinary temperatures, but rather a decomposition by the water, forming products which contain vanishingly small amounts of silica, the fact still remains that a large part of the substance dissolving is silica which furnishes silicate ions, bisilicate ions, and colloidal electrolyte. The excess silica may give rise to further complications owing to the formation of these colloidal electrolytes originally suggested by McBain (13) and more specifically applied to silicate solutions by Harman (8). Other substances undoubtedly dissolve which have their influence, but under conditions ordinarily met with in conductance or hydrogen-ion work, the solution of alkali and silica will be the greatest disturbing factor. Any reliable work carried out in glass apparatus on very dilute solutions, whether it be conductance, electrode potentials, or hydrogen-ion concentration, should take into consideration a correction for the “solution of glass.” SUMMARY
1. Measurements of the effect of small amounts of sodium silicate, such as are dissolved from glass, on the conductance of conductivity water of different initial conductivities and on very dilute solutions of hydrochloric acid, sodium hydroxide, and potassium chloride have been made. 2. The change in the nature of impurities in conductivity water with different methods of collection and time of storage in Pyrex and soft glass containers has been demonstrated. 3. It has been shown that in addition to a “normal” water correction, taking into consideration the carbonic acid present, there must also be applied a correction for the “solution of glass.” 4. The correction for the “solution of glass” will depend upon the kind of glass used in storage and measurements of the conductivity water and the solutions, on the time of such storage and measurements, and on thenature of the electrolyte worked with. The magnitude of the ideal correction could be calculated from the ionic mobilities and equilibrium relations of
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the substances formed by the interaction of the electrolyte with the carbonic acid, sodium carbonate, bisilicate, and crystalloidal silica found to be present. The approximate correction can best be obtained by taking into consideration the alkali, carbonates, and carbonic acid, the proportion of which can be approximately determined by conductometric titrations. In conclusion I wish to express my thanks to Professors C. L. von Ende and J. A. Kostalek for the helpful suggestions offered and for placing the facilities of the Department a t my disposal. The assistance and suggestions from the other members of the Department is also appreciated. REFERENCES
KENDALL:J. Am. Chem. SOC.38, 1480, 2460 (1916); 39, 7 (1917). WASHBURN: J. Am. Chem. SOC.40, 106 (1918). WHETHAM AND PAINE:Proc. Roy. SOC.London A81, 58 (1908). WYNNE-JONES: J. Phys. Chem. 31, 1651 (1927). FORESTER: Z. anal. Chem. 33, 381 (1894). WALKER:J. Am. Chem. SOC.27, 865 (1905). WALKERAND SMITHER: Bur. Standards, Technol. Paper 107 (1918). (7) HAAG:Z. anorg. allgem. Chem. 166, 21 (1926). (8) HARMAN: J. Phys. Chem. 32, 44 (1928). (9) HALLAND ADAMS:J. Am. Chem. SOC.41, 1515 (1919). (10) WAGNER:Elektrotech. Z. 32, 1001 (1911). (11) SHEDLOVSKY: J. Am. Chem. SOC.64, 1411 (1932). (12) MOREY:J. Glass Tech. 6, 20 (1922). J. Am. Chem. SOC.42, 426 (1920). (13) MCBAINAND SOLMON: (1) (2) (3) (4) (5) (6)
