SURFACE CONDUCTANCE AT GLASS-SALT SOLUTION INTERFACES * BY
n. L.
WHITE, FRANK URBAN AXD E. A. VAN ATTA
McBain, Peaker and King' reported in I929 on the increase of specific conductance of dilute KC1 solutions in narrow polished glass slits. This was ascribed to surface conductance at or near the glass-solution interface, the excess of conductance becoming greater as the ratio of surface to volume increased and as the dilution became greater. The ratio of surface area in sq. cm. to volume in cc. in their narrowest slits, excluding slit S and slit 2 , rejected because of breakage or cracks, was 1600to I ; their most dilute solution was 0.001N. The found at 25OC. a mean specific surface conductance with 0.001N KC1 of 4.3 X IO+ mhos, with 0.002 N KC1 of 5.3 X IO@ mhos, and with 0.01 N KC1 of 9.3 X IO-^ mhos, specific surface conductance being defined as that of I cm. square of surface. The values obtained with the various slits, from which the means were calculated, varied with 0.001N KC1 from3.0to 5.3 X 1o-~mhos,witho.oozNKClfrom2.8to6.3 X Io-8mhos,and with 0.01N KCl from 1.8to 19 X ~ o - ~ m h o sThe . rapid increase in the experimental error as the solution becomes less dilute is obvious. I n 1930 McBain and Peaker2 reported that the specific surface conductance a t the interface between KC1 solutions and unpolished pyrex glass was 13 X IO@ mhos with 0.001 N KCl and 20.7 X IO+ mhos with 0.01N KCl. The ratio of macroscopic surface area to volume with this apparatus was much lower, 93 to I, than with the slits. The authors suggest that the higher conductances a t the unpolished surfaces could be explained by the assumption that the actual surface area of ordinary pyrex tubing is 2% times greater than that determined by macroscopic measurements. Actually the factor would be about 3 to bring the 2 sets of data on 0.001N KC1 into agreement. We reported a t the 193I Colloid Symposium* on determinations of stream potentials in pyrex capillaries of various sizes which suggested that either McBain and coworkers' values for surface conductance were many times too high or that the stream P D invalid. The argument follows. potential equation E = -is 4 V K
We made our stream potential determinations on pyrex capillaries of from 0.110to 0.005 mm. inside diameter, using 0.0005 N KC1. We can estimate that the extrapolated value for the specific surface conductance with pyrex glass and 0.0005 N KCl according to McBain and Peaker would be not less than I X IO-' mhos. Taking the specific volume resistance of 0.0005 N KC1 a t zs0C. as 1.35 X 104 ohms, we calculate that the normal
* The Department of Physiology and the Department of Biological Chemistry, Waahington University, St. Louis.
I372
H. L. WHITE, FRANK URBAN AND E. A. VAN ATTA
or volume resistance of a column of o.oo05 N KCl in a 0.005 mm. capillary a t zs°C. is 6.88 X I O ~ O ohms per cm. The surface resistance of such a capillary would be, taking I X 10’ ohms as its specific surface resistance, I X 10’/3.1416 X 5 X IO-^ or 6.38 X 1 0 9 ohms per cm. The observed resistance of a 0.005 mm. pyrex capillary filled with o.oo05 N KC1 should then be, if McBain and Peaker’s figures for specific surface conductance are correct, 5.82 X 1 0 9 ohms per cm., since the observed resistance is the resultant of the normal or volume resistance and of the surface resistance in parallel. That is, the observed resistance of this system would be 8.4 per cent of the normal or calculated resistance. This means that the conductance (resultant of volume and surface conductances) in the capillary should be 1 2 times the normal and since the conductance factor, K , in the stream potential equation is in the denominator, the stream potential with a 0.005 mm. capillary should be, other factors being kept constant, only 8.4 per cent of that in a large capillary, say 0.10mm., where surface conductance is relatively insignificant. The facts were, however, that the stream potential of a 0.005 mm. capillary was 7 5 to 8s per cent of that in a large capillary or about I O times as great as would be predicted from McBain and Peaker’s data. If our stream potential figures were correct they could be explained by any one of the following conditions, that McBain and coworkers’ figures for surface conductance were much too high, that our glass was different from theirs, that the stream potential equation does not hold, or that some other factor or factors in the equation were changed in the small capillaries so as almost to compensate for the increased conductance if it exists. The answer depends upon the results of determinations of conductance in our capillaries under the conditions of our stream potential experiments. I n the above-cited paper we reported preliminary attempts to determine the resistance of our capillaries by determining the ratio of the potential drops across the capillary and across a known high resistance, using either a known impressed e.m.f. or the stream potential as the source of e.m.f. These measurements were not quantitatively satisfactory because our known resistances were not high enough to give a large fraction of the total potential drop. They were introduced merely to show that the “spontaneous” fluctuations in stream potential were not due to variations in conductance. The present paper is a report of conductance measurements on pyrex capillaries of from 0.10to 0.005 mm. bore filled with 0.0005,0.1or 1.0N KCl. Since the ratio of macroscopic surface to volume in our 0.005 mm. capillaries is 8000 to I and our solution is twice as dilute as the most dilute used by McBain, the conditions in our experiments for bringing surface conductance into prominence should be much more favorable. Apparatus and methods. Our first resistance measurements, as noted above, were inaccurate because of lack of suitable resistors and because of our failure to give adequate consideration to the potentials of the electrodes themselves. Considerable time was spent in attempting to make stable rionpolarizable resistances sufficiently high for our purposes. Carbon lines on paper imbedded in paraffin were satisfactory up to about IO* ohms but
CONDUCTANCE AT OLABS-SALT SOLUTION INTERFACES
I373
above that they could not be used, apparently because of polarization. Greater accuracy was attained on using a series of metallic sputtered resistors ( 5 x IO* to I x 1011 ohms) prepared by Dr. L. C . Van Atta4 and lent us by him. These resistors give constant values and do not polarize. We eventually abandoned the potential drop ratio method because of practical difficulties which attended the method even after satisfactory standard high resistances were available. Our next method was to measure the rate of discharge of a condenser through the unknown high resistance, which is given by the equation L
R =
b
2 . 3 0 3 C log Vo/vt
where R is the resistance in ohms, t the time of discharge in seconds, C the capacity of the condenser in farads, V, and V, the voltages on the condenser immediately after charging and after time t, respectively. The condenser was a Leeds & Northrup standard mica, 0.1mf. being used with the dilute solution, I mf. with the concentrated. The ratio V,/Vt was taken as the ratio d,/dt, the ballistic deflections of galvanometer on condenser discharge; the galvanometer had a current sensitivity of 8 X 1 0 - l ~ amp., a period of 1 2 seconds, coil resistance of 575 ohms and C.D.R. of 26,000 ohms; as used it was damped with 2 2 , 0 0 0 ohms. The calibration curve, Le., deflection against impressed voltage, was frequently redetermined. The resistance of the condenser varied between 2 . 5 and 3 X IO'^ ohms for 0.1mf.; a correction was made for condenser resistance, the unknown being determined on the basis that the observed resistance was the resultant of the condenser and unknown resistances in parallel. This method was satisfactory provided the calomel electrodes serving as leads from the capillary were exactly isoelectric. Since this was not always the case and since we could not modify the equation t o allow for this extraneous source of e.m.f. during the discharge we next turned to the other alternative, determining the rate of condenser charge through the unknown resistance. The e.m.f. of the unknown resistance, arising in the calomel electrodes, can be allowed for in this case. The charging e.m.f. can be either the stream potential of the capillary or an impressed e.m.f.; in either case the observed resistance of the capillary is the same. The resistance is expressed by the equation R =
-t 2 . 2 0 3 log ~
(1
--Vt/vf)’
where Vt and Vr are the voltage a t time t and the final or charging voltage, respectively. Here also the observed resistance is the resultant of unknown and condenser resistances in parallel. Since, however, the condenser is charged to only a fraction of the charging voltage, its leak becomes of much less importance than in the condenser discharge method. With the rate of charge method the error due to condenser leak is kept at a minimum because with low unknown resistances, Le., below 5 X 109 ohms, the condenser resistance is so much higher than the unknown that its leak is negligible, while with high unknown the voltage at time t is usually only 20 to 3 5 per cent of the charging voltage so that relatively little opportunity for leak has
I374
H. L. WHITE, FRANK URBAN AND E. A. VAN ATTA
existed. However, it has seemed worth while to calculate the error due to condenser leak; when this has been less than 0 . 2 per cent it has been disre-t has been derived indegarded’ The equation = 2 . 3 0 3 KC log (I -Vt/KVr) r E C ( ~- e - mR f’r) , i n t h e a p p e n pendently and follows from equation 5 , q = R+r dix of a paper by Bish0p.j In our equation K is the correction factor R,/(R+R,) where R, is condenser resistance and R is the unknown resistance as first determined. A second approximation can be made by substituting
FIG.I
for R in the correction factor the corrected R as found on the first approximation, but in any of our experiments the difference between the first and second approximations is hardly appreciable. In most of the determinations the first approximation introduces a correction of considerably less than 1 / 2 per cent; in only one case is it more than 3 per cent. The final arrangement of apparatus is shown in Fig. I ; the potentiometric system has been modified from Bishop’s arrangement6 by substitution of a SPDT quick acting highly insulated switch for the tap key. With this one can determine either stream potential or resistance; the latter can be determined by utilizing either the stream potential or an impressed e.m.f. from the potentiometer. The condenser C, the switch K, and that side of the high resistance source of unknown e.m.f. which is connected with the condenser are thoroughly insulated, the other side of the source of unknown e.m.f. is grounded a t the galvanometer. Adequate insulation of the upper end of the capillary was uncertain because of the possibility of a leak to the flask H through the rubber tubing U; it was, however, a simple matter to insulate the lower end of the capillary, Le., the beaker G and the electrode D. The condition was then fixed that the lead D would be connected with the condenser and since in the stream potential measurements this was always positive the battery current was reversed from the polarity marked on the box and was as indicated in the figure. The condenser C has been placed alternately upon a hard rubber table and upon a grounded sheet of galvanized iron P with no change in extent of charge from a known source of voltage; the
CONDUCTAXCE AT GLASS-SALT SOLUTION INTERFACES a
1375
insulation of the condenser box is apparently adequate. When the potentiometer is set at zero and the electrodes are isoelectric there is no galvanometer deflection on the condenser discharge. The steps of a resistance determination, using an impressed e.m.f., are as follows. The capillary holder I is put into the flask H which contains, in all of the experiments reported in this paper, 0.0005 N, 0.1N or 1.0N KCl. By proper manipulation of stopcocks a positive pressure of a few cm. Hg is applied and the tube F flushed out, cock J closed and tube F dipped into the beaker G, giving a low resistance circuit for the determination of electrode potentials. Calomel electrodes B and D are made up with 0.0005,0.1 or 1.0 N KC1; they are usually o to 4 mv., occasionally as far as I O mv. apart The resistance. of electrodes and connections exclusive of capillary is only I . j to 2 . 5 X 104 ohms with 0.0005 N KC1, Le., Practically zero as compared with the capillary resistances of 1 0 9 to IO'^ ohms. The same proportion of course obtains with the concentrated solutions. This is of great importance as it enables us to disregard the resistance of the connections, which McBain could not do. The heights of the columns of solution in the right and the left arms of the capillary holder are next measured and their difference taken. This difference represents the negative pressure in the glass which will suck solution from the beaker up into the capillary. Since some evaporation from the beaker cannot be avoided, although it is freshly filled for each experiment, we next adjust the positive pressure, as indicated by the mercury manometer &I, so that it just overtops the negative pressure, leaving a net positive pressure of I or 2 mm. Hg in the flask. This adjustment of pressure was occasionally checked as follows. If we have a net positive pressure of a few millimeters of mercury we can calculate the resultant stream potential, with o.ooos N KC1, as about 1.5 mv. per mm. The algebraic sum of this and of the electrode potentials as determined through the low resistance circuit gives the potential which should exist across the capillary. When this is measured it always agrees with the calculated value. Knowing the potential difference of electrodes exclusive of an impressed e.m.f. we next impress from the potentiometer an appropriate voltage, the total e.m.f. being the algebraic sum of the pre-existing and the impressed e.m.f. From 1000to 1500 mv. with 0.1mf. condenser were used with o.oooj N KC1; from I O O to zoo mv. with I mf. with 0.1or 1.0N KC1. With some of the larger capillaries and 1.0 N KC1 the measurements were made with a Wheatstone bridge. The charging time with the switch K to the left was taken with a stop watch, the ballistic throw of the galvanometer being observed when the condenser was discharged by throwing the switch to the right a t time t. In no case was the time short enough that its measurement introduced an appreciable error. From 4 t o 8 consecutive determinations were made on a capillary and their average taken as the resistance for that experiment; the variation of the corrected values of these consecutive determinations was practically always less than I / Z per cent and never more than I per cent.
H. L. WHITE, FRANK URBAN AND E. A. VAN ATTA
I376
The observed values were corrected to a temperature of q 0 C . by the equation & =
Rt
Within the comparatively narrow range (t-25)' of temperatures in these experiments the correction factor 0.02 2 remains, almost constant. The beaker G was a t room temperature, a thermometer was immersed in the beaker at the same depth as and close to the capillary and was read every few minutes. The capillary was completely immersed; the column of fluid in the capillary will almost instantly come to the temperature of the beaker; there is, indeed, no reason why the temperatures should differ to any significant extent. A single figure for the resistance values given in Table I represents the average of a series of consecutive determinations, each determination being corrected for temperature and for condenser leak, when the latter is of significance. Other figures for the same capillary represent averages of similar series made a t different times. After the final arrangement of apparatus had been perfected the results were surprisingly reproducible; no observed data have been rejected in the compilation of these tables except in a few instances where obvious sources of error, as partial obstruction of the capillary or an error in dilution of the solution, had entered. In these cases the source of error was proved by other objective means as by microscopic examination of the capillary or refractometric examination of the solution. 1-0.022
TABLEI Cap. Length Diam. cm. No. (ca1c.f . , micra
60
I
.8
5.IO
Observed resistance with r.oNKCI ohms
Observed resistance with 0.1 NKCI ohms
Observed resistance with 0.0005 N KClohms
x
6 . 8 3 X 108 8 .88 I 0 1 0 6 . 8 2 X 108 8 . 6 5 X 10'" 9.07 IO1'
Calculated Observed resistance Calculated with with 0.0005 N 0.000 N KClohms K d I
. I8 x I011
.752
x
61
64
I
.76
5.34
2 . 6 6 X 108 3 . 6 3 X 10" 4 . 5 9 2.63 X 108 3 . 6 9 X 1 0 ' ~ 2.65 X 108 3 . 6 6 X lo10
.25
5.64
3 . 9 2 x IO8 3.87X108
x IO"
5.60Xro'~ 6.75X I O ~ O 5.43 1O1O 5 . 5 9 x IO1' 5.64X10'~ 5.37 IO"
'798
,819
x x
63
.75
x IO"
5.76
2 . 2 6 X 108 3 . 3 9 x IO" 2 .27X1oS 3.3OxIO'' 3 .33 I O t 0
5.48
3 .o6X1os 4 . 38 X I o10 5 . 36 X I o10 3 . 1 1 XI08 4 . 4 6 X 1ol0 3 . I I X 108 4 . 3 9 x IO"
3.93
.85
x
60a
.94
,823
CONDUCTANCE AT GLASS-SALT SOLUTION INTERFACES
I377
TABLEI (Continued) Cap. Length Diam. No. cm. (ca1c.) micra
56
1.40
8.0
Observed resistance
with
r.oNKC1 ohms
Observed resistance with 0.1 NXCl ohms
z.49X107
Observed resistance with o.ooo5N KClohms
Calculated Observed resistance Calculated with with 0.0005N 0.m N KClohms
Kd
3.2oX1oi0 3.76Xro'O 3 . 2 2 I010 3.33 10'0 3.47x10'0 3 ,3jX10'~
.88
6.70X10' 6 . 7 0 X 109 6 . 7 j X 109 6.59X1o9 6 . 7 6 X 109 6.59X109 6 . 6 4 X 1o9
.933
x x
2.5OxIO7
66
.54
11.4
4.I3XIO7 4 . 12X IO'
65
.89
12.0
6.12X107 9.65X10g 6.14X107 9 . 9 3 X 109 9.97 X I O 9 9 . 7 6 X 108 9.74XIOg
30
1.60
25.6
2.77X106 2.73XI06 2 .76 X I od
57a 1.97
39.0
1.446X10' 1.26X107 2 , 1 9 X 1 0 9 1.43XIO' 1.27XIO' 2.14XIOg 2 . I7 IO9
I . 01
x
7.16X1o8
1 . 0 6 X 1 0 ~ ~,926
1010
2.41X107 4.00X10g 2 . 4 1 x I 0 7 3.94XIO9 3.92 XIO' 3.95xIOs
4.17X109
,948
z.17X1o~
1.00
4.94xIo9
,996
z.72X1o9
.g81
x
57
4.45 39.4
3.27XIO6
3 . 2 7 X 106
4.94X109 j .08 X 1 0 9
4.84 X log 4.8SXiO9 4.85 1 0 9 4.96X1O9
x
57b 2 . 4 8 39.7
1.803X10~1.59X1o~ I . 8 0 5 X 1 o ~ I .60X1o' 1.807X106 I.59X1O7 1.806X10~
z.61X109 Z.66X109 2.66X109 2.73x IO9 2.71 XIO9 2.67X109 2.67 X 1 0 9 2.65 X 1o9
n. L.
I378
WHITE, FRANK URBAN AND E. A. VAN ATTA
TABLEI (Continued) Cap. Length Diam. No. cm. (calc.) micra
59
4.66 4 0 . 5
58a 1 . 9 0 9 6 . 0
Observed resistance with 1.oNKCI ohms
Observed resistance with 0.1N K C l ohms
Observed resistance with 0.0005 N KClohms
Calculated Observed resistance Calculated with with 0.0005 N 0.000 V KCIohms Kd;
3.26X106 z . 8 7 X 1 0 7 3.29x1o6 2.8jX10’ 3.26X106 2 .87X107 3 . 2 3 xI06 3 . 2 3 X106 3 . 2 3 x IO6 3.26X106
4 71Xro’ 4 77x10’ 4 . 7 6 X 10’ 4 . j o X 109
4.9oX1o9
,967
z.38X105 2 . 37 IO’ 2 . 3j X 1 0 6 2 .35 x I O 5
3 . 5 4 X 1 0 8 3 .56X108 3 . 5 3 xI08 3 .55X 1O8 3.50X 1O8 S.6$ 1 0 8 3.55xI08
,992
8.76X108
8.86X108
,986
5 , 1 3X 108
.9...,
x
x
8 . 7 2 X lo8 8 . 6 8 X1 0 8 8 . 8 1 X108
5 , 0 7 X 108 5.06X l o 8 5.02
x I08
5.14X10* 5 . 0 7 X IO* 25
6.15 102.8
6.68X10’ 6 . 6j X IO’
1.014X10g .026X1o9 1.020x109
1 . 0 0 j X 1 0 ~1 . 0 1
I
I . O I O X I O ~
1 .014X109 I I
63a
.2j
5.78
64a
.33
5.22
7.4jXIo’ 7.43XIo’
.OI~XIO~ .ozoX1o9
1 . o g X 1 0 ’ ~ 1 . 2 9 X 1 0 ’ ~ ,814 1.06X10~~ I . 0 4 X IO’O
1 . 2 0 X 1 0 ~ 1.70X1olO ~ 0 x 1 0I~. 71 X 1 0 l O
2 . 0 8 j X 1 0 ~,818 ~
I.
67
1.77
68
.62
18.6
1.82
4 . 6 5 X 1 0 7 7.28X10g 8 . 0 6 X 1 0 9 ,908 4 . 6 4 X IO’ 7 ,42X 10’ 7 .23XIO9 7 . ri4 x I O 9 r.865X10g 2.18X1oI1 3 . 2 4 X 1 0 ~ ~.686 z.26X10’~
CONDUCTANCE AT GLASS-SALT SOLUTION INTERFACES
I379
I n the same way the resistance determinations can be carried out, using stream potential of capillary instead of impressed e.m.f. I n this case a pressure of 60 em. Hg is applied, the stream potential determined and the potentiometer then set at zero. The current from the stream potential then charges the condenser through the capillary resistance from time t, the galvanometer deflection being measured as before. The observed capillary resistance is the same whether potentiometer or stream potential is the charging e.m.f. This fact is of great theoretical interest, as it shows, among other things, that the resistances of a column of liquid in motion and a t rest are the same, that the relatively rapid passage of liquid does not alter those surface conditions which determine surface conductance and that the stream potential can be made to furnish a reasonable amount of current (a great deal more than is required for its own measurement by the null point potentiometer-condenser method) without being affected. The figures in Table I which were obtained by using stream potential as source of e.m.f. are italicized. Table I is a summary of all the results obtained on 2 1 capillaries ranging from 0.00182 to 0.1028 mm. bore. The length is the measured length, the diameter is that calculated from the observed resistance with 0.1 or 1.0 N KCl. This method of calculating diameter is valid only if the observed resistance of capillary plus connections is identical, within the limits of measurements, with the calculated normal resistance of the capillary alone when concentrated solution is used. The practical identity of these two resistances depends upon three conditions, first, that the resistance of connections be negligible, second, that the surface conductance be negligible and, third, that the pinch effect be negligible. The truth of these three conditions has already been established in the preceding paper. Since we are interested in actual conductances we have seen no reason to correct, as McBain has done, for conductance of solvent; in any event the correction is negligible. Since resistance of connections and pinch effect are always negligible and with a concentrated solution surface conductance is also negligible, the diameter of the capillary is calculated from the equation diameter = z
vz
where p is specific volume resistance of solution (8.95 ohms for 1.0 N and 77.8 for 0.1 N KCl), 1 is length of capillary in em. and R is observed resistance in ohms. The value so obtained is the mean diameter of the capillary and has been taken as a better measure of diameter than the microscopic measurements. Having established its dimensions one next calculates the normal volume resistance of the capillary filled with 0.0005 N KCl, the 6th column of Table I. The 7th column compares observed and calculated resistances with dilute solution and shows far lese surface conductance than found by McBain and coworkers. A further analysis of our results with 0.0005 N KC1 is given in Table I1 and Fig. 2 . The last column in Table I1 gives the specific surface conductance, K,, calculated for each capillary.' I n Fig. z the distance of the line OD above the X axis designates C,/C, representing I O O per cent of the
1380
H. L. WHITE, FRANK URBAN AND E. A. V A N A'ITA
normal conductance. For large tubes there is no other significant conductance but as diameter decreases an increasing amount of surface conductance is added. The distance of the line OE above OD represents the ratio C./Cv for the capillary diameter designated on the X axis. The line OE is drawn through the experimental points, the number by each point designates the capillary. The distance of OE above the X axis at any given diameter i
represents 4- cv, the ratio of total or observed conductance to calculated
cv
or volume conductance; the reciprocals of these values coincide with the values in the 7th column of Table I. It will be further noted that the slope of the line OE affords a method of calculating the mean specific surface conductance, K., of all our experiments. ?rd A d2 C. = K* - and C, = K~ -where C. and C, are the surface and volume 1 41 conductances, respectively, in mhos, of a capillary of length 1 and diameter d, and K. and K~ are the specific surface and volume conductances. Then C,/C, = 4 Ka/d K ~ . This is of the form of the straight line equation y = ax, where y = c,/c,, a = 4 K./Kv and x = I/d. Since then we are plotting C./C, against I/d, the slope of the line is 4 K./K,. The figure shows that the slope is K~
.242
or 2 x 108
1.21
X
IO+.
Therefore, 4 K./K,
= 1.21
X IO-^. But
= 7.41 X IO-^, the normal specific conductance of 0.0005 N KCl. There-
fore, K. =
2.24
X IO-^ mhos.
CONDUCTANCE AT GLASS-SALT SOLUTION INTERFACES
1381
1382
H. L. WHITE, FRANK URBAN AND E. A. VAN ATTA
The arithmetic average of specific surface conductances obtained from all the capillaries of 0 . 0 2 5 mm. or less bore is 2.27 X IO-^ mhos. Obviously the results with the smaller capillaries, where the ratio of surface to volume is great, are the most dependable. The single 1.8p capillary, no. 68,is probably an exception to this statement; technical difficulties increase rapidly as one gets below ~ p which , is about the optimum diameter for this work. With the 0.04 and 0.10mm. capillaries the surface conductance is so small a fraction of the whole that its measurement involves a considerable error. Three of our 0.10and 2 of our 0.04 mm. capillaries gave figures close to the average; the surface conductance with the others of these sizes is indeterminate. It must be remembered that with a 0.10mm. capillary an error of 0.1 per cent in resistance determination gives an error of I O per cent in specific surface conductance. Discussion. McBain and Peaker found 13.0 X IO+ mhos as the specific surface conductance with 0.001N KC1 and Pyrex glass; this would have been not less than I X 10-7 for 0.0005 N KC1. As compared with their figure (our extrapolation) of I X IO-’ we find 2.24 X IO-^, Le., their figure for K~ is 45 times as large as ours. We believe that the error of our methods of measurement is less than theirs; our ratio of surface to volume is many times as high, our cell resistance can be neglected and we had no “pinch effect.” As a possible explanation of this great discrepancy in results we offer the suggestion that a large part of their observed increase in conductance was due to traces of chromic acid. The time effects described by McBain, Peaker and King might be explained on this basis. Our capillaries were cleaned by sucking hot water through them for an hour, followed in some cases by steaming for an hour; we found that the steaming process was probably not essential in conductance determinations. We did not use any cleaning solution and we have seen no change in capillary resistance with time.* The increase of conductance with time in McBain’s experiments might also be partly due to a liberation of electrolytes from the glass itself; when the ratio of surface to volume becomes high this factor is exaggerated, particularly with dilute solutions. The possibility of our solutions becoming more concentrated by the leaching of the glass surface was avoided by having a very slow continuous passage of solution through the capillary; in McBain’s work the solution was stationary. The point might be raised that complete lack of relative motion between solution and glass surface is essential to the building up of a conducting layer. It seems unreasonable, however, that the forces of adsorption could be negatived by such a feeble agitation as was exhibited in our capillaries where the measurement was made by impressing an external source of e.m.f. I n our work the resistance of a capillary was the same whether the solution was passing through it relatively rapidly under a pressure of 600 mm. Hg, the stream potential being used as a source of e.m.f., or whether the solution was moving a t the extremely slow rate attained under a pressure of I or 2 mm. mercury. It seems only reasonable that if the movement of the solution past the glass surface disturbed the development of the conducting layer, this disturbance would be measurably greater when
CONDUCTANCE AT GLASS-SALT SOLUTION INTERFACES
I383
the rate of movement was increased three hundred to six hundredfold. We are convinced that the state of a very slow exchange of solution is less open to objection than is no movement of solution. The suggestion that, McBain's finding of a much higher specific surface conductance than ours may be due to the liberation of electrolytes from adsorbed chromic acid or from the glass itself is, of course, not capable of direct proof. The cause of the discrepancy may be some other source of error yet undetected. A discussion of some of the theoretical implications of these data with an extension of the work to include KCl solutions of other concentrations will appear in a later communication. Summary I . An arrangement for determining either stream potential or resistance of pyrex capillaries filled with salt solution is described. Resistances up to 2 . 2 2 x 1ol1 ohms are measured by the rate of condenser charge method. Evidence is present'ed that polarization is avoided. 2 . There is no change of resistance with time. 3. Capillaries of from o.oo18z to 0.10mm. bore were used, macroscopic surface to volume ratios as high as 22,ooo to I being obtained as compared with 1600 to I by McBain, Peaker and King. 4. A specific surface conductance for pyrex glass and 0.0005 N KC1 of 2 . 2 4 x 10-9 mhos was found, as compared with an extrapolated value of I X IO-' by McBain, Peaker and King, their figure being 45 times as great as ours. 5 . The suggestion is made that the conductance figures of McBain and coworkers may be erroneously high because of contamination of solution by chromic acid or by leaching out of electrolytes from the glass. The possibility of this occurrence has been avoided in our work. The work reported in this and in t'he preceding paper has been aided by a grant made by the Rockefeller Foundation to Washington University for research in science. Bibliography and Footnotes McBain, Peaker and King: J. Am. Chem. Soc., 51, 3294 (1929). McBain and Peaker: J. Phys. Chem., 34, 1033 (1930). White, Urban and Krick: J. Phys. Chem., 36, 1 2 0 (1932). Van Atta: Rev. Sci. Instr., 1, 687 (1930). Bishop: Am. J. Physiol., 85, 4 1 7 (1928). Bishop: Proc. Soc. Exp. Biol. Med., 27, 260 (1930). ' Specific surface conductance is calculated by either of the following expressions. K~ = CJ/T d, where K~ is specific surface conductance, C,, 1 and d are the surface conductance in mhos, the length in cm. and the diameter in em. of a capillary. C. is the reciprocal (Rv - R ) KV of the 6th column in Table 11. K* = ratio surface ' where R, is the calculated or
Volume
volume resistance in ohms of a capillary, R the observed resistance and K" the normal or volume specific conductance. The ratio for a capillary is 4 ? , where d is the volume d diameter in micra. Since these data were obtained and this paper written, further work on conductance in the smallest capillaries, both with 5 X 1 0 - 4 molar and with other concentrations of KCI, has not shown exact reproducibilitywhh the same consistencyas obtained in this series. Whether or not this is due to an artefact we are not yet prepared to state.
'urface
