h METHOD OF MEASURIKG T H E RELATIVE SURFACE CHARGES ON ELECTROLYTES BY ALLEX GARRISOX
From the fact that bubbles of air when suspended in an electrolyte move under the influence of an electric field, it is known that the film forming the interface between the air and the electrolyte contains an electric charge, an excess of either positive or negative electricity depending on the nature of the electrolyte. From the direction of the cataphoresis of the bubble the sign of the charge can be determined, but due to the many disturbing factors the relative magnitude of the electric charge can not be accurately estimated. In fact the sign of the charge is sometimes doubtful. Experiments have also been performed in which air bubbles were driven through the liquid by the force of gravity and the sign of the charge estimated from the difference in potential set up along their path. ( I ) This is similar in principle to the cataphoresis method and in spite of certain advantages it has not yet yielded quantitative results. In the experiments to be described the method of measuring the surface charge was independent of the cataphoresis inethod and has been found to be capable of giving a quantitative measure of the relative surface charge to within . o r of a volt, The Experimental Method
If an electric condenser is constructed of two inetallic plates separated by a dielectric such as air, it is possible to determine when the surfaces of the plates have the same potential. This can be done by moving one of the plates nearer the other thus increasing the capacity of the condenser. It the surface of the plate moved is positive relative to the surface of the stationary plate a positive charge will tend to flow off of the stationary plate and may be detected by insulating the plate from its surroundings and connecting it by a wire to an electrometer. If no charge flows off the stationary plate when the capacity of the condenser is changed, there is no electric field between the plates and their surfaces have the same potential. This principle has been used to measure the potentials of the surfaces of a number of electrolytes relative to the surface of water. The apparatus employed is illustrated in Figure I . The electric condenser consisted of the electrolyte A and the horizontal metal plate F. The electrolyte was contained in a round glass vessel I O em. in dimeter and 2. j cm. deep. The vessel was supported on and insulated from the base D by sulphur plugs 2 cm. in height. The base D was equipped with leveling screws so that the surface of the electrolyte in A could be made flush with the rim of the vessel. This surface formed one plate of the electric condenser. lBillitzer: Z. Elektrochem. 8, 638; -Inn. Physik, (4) 2, 902 (1903’1,Rlceman: J . Phys. Chem. 2 9 , 508 (1925).
The electrolyte was connected through the glass tube B to a 0.I N calomel electrode C. The tip of the calomel electrode contained a salt bridge of saturated potassium chloride which eliminated the electrolytic potential difference to a magnitude which was less than the experimental error. Thus the inside of the electrolyte at A may be taken as the same potential as the electrolyte in C which is maintained at a very constant potential relative to the mercury. The mercury of the calomel electrode was connected by a copper wire to the quadrant electrometer E and could be connected to a ground wire through the key K. The calomel electrode was supported and insulated from the table by the sulphur blocks S. The metallic plate F formed the reference plate of the condenser and could be raised to the position indicated by the dotted line by drawing the cord which passed over the pulley G. The plate was connected to the ground wire through the potentiometer P which was of standard make and requires no description. By setting the potentiometer the potential of the plate could be set at any desired value FIG. I relative to the mercury of the calomel electrode. The vessel containing the electrolyte was surrounded by a circular ring made of sheet copper H, H. This was separated from the vessel A by a 2 mm air space and was connected to the ground wire. It thus acted as a guard ring. The table top was covered with a sheet of tin foil also connected with the ground wire. This prevented the induction of any charge by the sudden change in potential of the table top or supports. In performing the experiment the electrometer was used as a null instrument. The electrolyte whose surface charge was sought was placed in A and connected to earth through the key E(. The key was then opened and the plate F moved thus changing the capacity of the condenser. If a charge was induced in the electrometer the potential of the plate was changed by adjusting the potentiometer. Thus the plate was made more negative if a positive charge was induced and more positive if a negative charge was induced. The experiment was then repeated until no charge was induced when the capacity of the condenser was changed, The reading of the potentiometer would then give the potential difference in volts between the electrolyte surface and the reference plate if they had both been earthed. If now a second electrolyte is placed in h its inside potential will be the same as the electrolyte in C and will thus be the same as the inside potential of the first electrolyte. When there exists a surface charge on the .first electrolyte having a different magnitude from the surface charge on the second
RELATIVE SURFACE CHARGES OK ELECTROLYTES
1.519
electrolyte, the plate F must be given a different potential by the potentiometer in order to get its potential to the same value as electrolyte number two. Therefore the difference between the readings of the potentiometer for the two electrolytes is the difference in their surface charge. I n this way a number of eledrolytes have been compared with pure water taken as a standard. Since the metallic plate is used as a reference it is necessary that the potential of its surface shall remain constant during the experiments. The plate was first constructed of copper but it was found that the oxidation of the surface of the copper in air rapidly changed its contact potential. To avoid this the copper plate mas covered with a sheet of palladium foil. The resistance of palladium to chemical action made it a verysatisfactorystandard. Its polished surface would retain approximately the same contact potential for several days.
The Experimental Results
It was found that the surface charge on water is an exceedingly variable quantity. Distilled water taken from the same container varied several hundredths of a volt the same day and even more on different days. Thus seven determinations with distilled water on different days gave the following values: + . 0 0 5 , +.035, + . 0 6 0 , +.040, +.080, +.OIO and +.so5 with an average of +.040 volts. The experimental error in making the measurements was less than .oo5 of a volt. The larger part of the variation occurred on the surface of the water and not on the surface of the palladium for a salt solution taken from the same bottle on successive days shows far less variation tham distilled water. For example a 0.112: KCl solution measured +.IIO, +.12o, +.130, +.13o, and +.IZ j volts the readings being taken on five succeeding days. TQhen the copper plate was used in the place of palladium the variations mere larger and in a direction to indicate that the surface of the copper ivs becoming positive. With the freshly polished copper plate the 0.1K KCl had a potential of - .030 and on succeeding days -.035, -.ass, and -.090 volts. The Influence qf Varaous Substances on the Surface Charge of W a t e r . In order to compare the effects of various salts on the surface charge of water a number of 0.1molar solutions were measured. Table I gives the results of the measurements. The first column gives the substance in solution and the second column the potential to which the palladium plate must be raised in order to be at the same potential as 'he electrolyte surface. The average value for piire water (+ 040 volts) was subtracted from these measurements to give the voltages relative to the water surface in column three. It was found that the surface charge was shifted appreciably by even the smallest traces of substances of oily nature. X sample of pure water measuring f.045 volts was changed to +.IIO by touching with a glass rod on which there was a trace of oil of cloves and in another experiment pure water
ALLEN GSRRISON
1520
TABLE I The Surface Charge on Various Electrolytes. ,I
Salt Molar
Volts Relative to Pur? H20
Potentiometcr Voltage
+
+. + ,085 + ,080 + +
KC1 KRr KI KSCN
IT0
055 + ,.040 -
,015
3. ,010
,050
K3FeCNs K4FeCN6
+
IO0
- ,030
.060
- ,070
+. +. + +.
KC1 NaCl CaCL BaClz CUClZ A1C13 FeC&
I10 720 I35
'
160
- ,025 4.215
+
HN03' HC1 .I% Picric Acid .I y6Benzoic Acid
+
.070
+ ,045
,130
IO0
+ +. + ,060
I20
-t- ,080
,210
+.250
+ +. +. was changed t o +
,210
170
measuring . OIO I I j by contact with the tip of the finger. This explains in a large measure the fact that it is difficult t o get, consistent results with water. The Variation of the Surface Charge with the Salt Concentration. Table 2 gives the results of the measurement of several salts and acids a t different concent,rations. ,
TABLE I1 Surface Charge and Concentration. Solmality 3.000
HSO,
HC1
IC1
CU(N@Q)*
Cuhca
,030
I
