THE ELECTROLYSIS O F SODIUM C H L O R I D E
BY C. G. I,. WOLF
T h e electrolysis of alkaline chlorides has been examined by numerous observers, of ~ 7 h o mOettel' was the first perhaps to introduce quantitative methods into use, especially those which had to do with the examination of the gas produced at the electrodes. H e also recognized the importance of the close observation of this part of the reaction. T h e usual procedure followed' is to collect the gases and to compare them with the amount furnished by a gas voltameter in series, or to estimate the amount of current with a copper voltameter placed in a similar position in the circuit. This gives a relation between the amount of gas used up in the processes of reduction and oxidation, and the atnount of gas which would be produced did neither of these reactions occur. It has been usual to collect the combined gases, and by an analysis to determine the proportion of the oxygen and hydrogen i n the mixture to one another. No attempt, however, has apparently been made to collect the gases separately. . As the recent work of Foerster* and Miiller3 has shown how iiiiportant and instructive is the electrolysis of chlorides not only from a technical-4 but also from a scientific standpoint, it has been thought worthy of attention that an apparatus should be constructed which should be available for the student. I n the course of some work in this direction, which had as one of its aims the construction of an apparatus for class purposes, the following arrangement has been designed, which permits of the Zeit. Elektrochemie, I , 474 (189j). Zeit. anorg. Chem. 22, I (1899). 'j Ibid. 2 2 , 33 (1899). Kershaw. Elect. Review, 1898, 1096.
EZectrolysis of Sodium ChZoridk
201
variation of conditions as quickly as possible and with a minimum of rearrangement. T h e apparatus can also be used for lecture demonstration, and the effect of alkalies, acids, or neutral salts on electrolysis can be shown without interrupting the current. T h e main piece of apparatus consists of a U-tube with arms I O cm long and 2 cni in width. Between the arms of the tube is sealed a third arm 15 cm in length. At right angles
L
to the plane of the arm and 4 cm from the top are two small tubes for the delivery of the gases evolved. T h e mouth of each arm is closed with a doubly bored rubber stopper, which is provided with a thermometer and a glass tube carrying theelectrode. Contact is made with the usual mercury connection. T h e mouth of the tube may be sealed with mercury to prevent the escape of gas through the rubber stopper. T h e small side tube for the delivery of the gas from each of the arms is joined to a three-way stop-cock by means of a stout piece of pressure tubing. T h e stop-cock is in turn connected with a Schiff's nitrometer. In order to use the apparatus the.measuring tubes of the
202
C. G. L. W o l f
nitrometers are filled with water by shutting off the connection between them and the electrolytic cell. Under these circumstances the cell communicates with the air. T h e reservoir of each eudiometer is lowered till the level of the water contained in it is about 0.5 cm above the lower opening of the small tube of the nitrometer. T h e stopcock is now turned so that the levels in the reservoir and in the small tube are the same. On connecting the apparatus with the eudiometer by tnrning the stop-cock through 90' and starting the current, all the gas formed in the electrolysis is collected in the eudiometer. It will be found that with a rapid evolntion of gas a small amount of back pressme will be exerted in the cell, causing the liquid in the central tube to rise. This is obviated by lowering the levels of the eudiometer reservoirs a fraction of a centimeter, by which the original condition of the liquid in the cell is restored. This might also be done by running the shaft of the stirrer through a mercury seal, thereby completely closing the apparatus.' When it is wished to stop the entrance of the gas into the nitrometer tubes, the stop-cocks are turned so that the cell now communicates with the outside air. T h e volume of the gases in the burettes is measured, the tubes again filled with water and the nitrometers are ready for the reception of gas. T h e central tube of the electrolytic cell was provided with a small but efficient stirrer, run by an Ajax motor, and, as the results of experiments showed, this stirring had a marked effect on the progress of the oxidation. Especially in cases where the stirring was neglected or was performed inefficiently, a formation of bichromate took place around the anode. This was either due to the primary formation of hydrochloric acid in the electrolysis of the sodium chloride itself, which in turn acted on the chromate forming bichromate, or to the direct oxidation of the chromate itself. T h e appearance of the bichromate was much more marked where 1.8 percent of potassium chromate was present than where 1 / 1 0 of that amount was used. Beckmann. Zeit. phys. Chein.
22,
616 (1897).
Electrolysis of Sodium Chloride
203
I n any event the efficiency at the anode was lowered by the lack of a suitable stirring apparatus, and the inefficiency rapidly increased during the course of the experiment. At low temperatures chlorine and oxides of chlorine were also given off, Both these phenomena disappeared or were diminished when the stirring was continued. T h e cell itself was immersed in a water-bath kept at a fairly constant temperature by means of a small thermoregulator, the water was also stirred by means of a Witt stirrer. T h e general arrangement of the apparatus for this preliminary work was that an oxyhydrogen voltameter and a Weston ammeter were run in series with the cell. By a suitable arrangement of commutators the current was passed through the cell, voltameter, and ammeter, or through the cell, ammeter, and a small open cell containing dilute sulphuric acid of the same resistance as the voltameter. I n this way the amount of current passing through the electrolytic cell was the same whether the voltameter was in a series or not. This arrangement also had the advantage that the current was in use continuously except for the fraction of a second necessary to commutate the switch. T h e time during which the gas was collected was that necessary to give about 65 to 75 cc of electrolytic gas. T h e temperature of the electrolyte was always some three degrees higher than that of the outer bath, owing to the heating effect of the current, but where the bath and the current were both regulated, a satisfactory degree of constancy could be obtained. T h e current was furnished by a IOO volt dynamo circuit, which was cut down by a suitable arrangement of lamps. Fine regulation was made with a wire resistance. I n these experiments the difference of potential at the two electrodes was not measured. I n a simple form of apparatus used by Mr. Pettit in this laboratory, the gases were delivered through a tube passed through the stopper and caught in inverted burettes standing
C. G. L. Wolf
204
in a pneumatic trough. This arrangement, although simpler, is not so convenient as the Schiff nitrometer, as the burettes must be transferred to a levelling trough in order to obtain the gas under atmospheric pressure. T h e solution used was that of Muller containing 300 grams of sodium chloride and 1.8 grams of potassium chromate in 1000 cc. I n Mr. Pettit's experiments a similar solution was used, but potassium chloride replaced the sodium salt. Under these conditions the difficultly soluble potassium chlorate crystallizes out. This does not happen when sodium chloride is used. T h e following table gives the results of the electrolysis of j j cc of the solution for 7 ampere hours. T h e anode was made from platinum foil, while the cathode consisted of a stout piece of platinum wire I inm in thickness.
TABLEI 300 grams NaC1. pere ; D, = 6.66 Total current
1.8 grams K,CrO, in amp D, = 0.14 amp cm' ' cm2 ~
cc. Current I amTime 7 hrs 23 min.
1000 ,
7.38 ampere hours. Amount of solution 55 cc = 16.5 grams NaCl h
8
.e
$
.2%
6,
E2 ---.. ' 0.85 40.0 42.0 1.00 47 20' 1.02 42.5 79 I .og 43.0 131 1 1.00 43.2 42.8 229 , 1.01 42.0 249 1.00 40.2 313 0. go 40.2 0. go 379 41.2 429 1 0.99 41 .o 443 1.00
1 ,
'
0
$2 -__ 0.0
0.5 1.1 0.0
9.8 14.5 19.4 21.1
0.9 1.4
21.7
0.9
19.3 17.8 19.8
0.2
0.9 1.3 1.0
18.4
19.6
19.4
90.2 85.0
79.5 78.9 77.4 80.2
79.8 82.0 79.3 78. I 79.6
Electrolysis of Sodium Chloride
20.5
As will be seen from the foregoing table, the mean current was about one ampere, and the total amount of current used was about 7.4 ampere hours. T h e amount of oxygen consixnied in the oxidation should have been 15.9 percent of that required for the complete oxidation of the sodium chloride to sodium chlorate. As the average efficiency was about 80 percent, the amount of sodium chloride completely oxidized was about I 2.7 percent. With this amount of oxidation the total efficiency of the apparatus was 79.6 percent at the end of the experiment, This result differs from that of Muller, whose efficiency in a run parallel with this in the matter of current used fell from 83.5 percent at the beginning to 67.5 percent at the end of the run. In this experiment Muller used 500 cc of the electrolyte. T h e solution was electrolyzed for 2 0 hours, 102.05 grams of copper being deposited by the voltameter. This amount of copper is equal to 25.75 grams of oxygen. As 500 cc of the solution contained I 50 grams of sodium chloride which requires 124 grams of oxygen for complete oxidation to chlorate the reaction was 17.1 percent complete. T h e electrode densities used by Muller were : D, = 0.075=>;cm D, = 0.18 -amp-. cmz In the above table where 18.3 grams were electrolyzed the reaction was 1 2 . 7 percent complete. In Mr. Pettit’s experiments where the cathode density and the anode density 0.22 the current denwas 6.66 sity at the fornier was 6.0 times that used by Muller, while the anode density was 28 times as great. In these experiments the total efficiency varied from 95 to 59 percent. This result compares favorably with that of Muller when the great difference in the densities is taken into account. In these experiments also the facilities for keeping the temperature constant were not so great as in later work. In my own experiments with the same density at the cathode and a lower density at the anode, the efficiency was more constant, varying from 90.2 to 77.4 percent. These experiments tend to show that this result is due largely to the efficient stirring and the improved means to secure constant temperature.
s,
’
206
Electrolysis of Sodium Chloride
The apparatus would therefore appear to present several advantages over those which have been suggested, especially for demonstration purposes, viz. I. T h e maintenance of a constant temperature. 11. It allows the introduction of thermometers at both electrodes. 111. T h e use of an efficient stirring apparatus during the electrolysis. IV. T h e measurement of the evolved gases without the aid of a gas analysis. V. T h e introduction of substances into the solution during the electrolysis without disarranging the apparatus. Cornell University.
