The Apparent Degree of Ionization of Hydrochloric, Sulfuric, and Acetic Acids An Electrolytic Conductivity Experiment for General Chemistry LLOYD E. WEST and ARNOLD GAHLER Oregon State College, Corvallis, Oregon
I
N PRESENTING the theories of ionization in
general chemistry, the qualitative demonstration of conductivity using a light bulb leaves much to be desired in a comparison of electrolytes generally classed as "good" or "fair" conductors. The need for a quantitative experiment is met in this paper, which describes an adaptation of the conductivity bridge experiments performed in physical chemistry, and measures the conductivities of various electrolytes over wide ranges of concentration. A simple condnctivity cell with adjustable electrodes allows the current, in amperes, to be numerically equal to the "effective" ionic concentration of the electrolyte. With this simple conversion factor of unity the effective ionic concentration is then expressed in percentage as the apparent degree of ionization. The apparent degrees of ionization of hydrochloric, of sulfuric, and of acetic acid are determined a t various concentrations up to 10 molar. In the case of acetic acid a factor of ten is introduced for converting conductivity, expressed in amperes, into the effective ionic concentration.
by 2.25 cm., were sealed in glass tubing and held in place by sections of cork. The electrodes were blackened by electrolyzing a dilute solution of chloroplatinic acid. Shiny platinum electrodes are less satisfactory.
The condnctivity cell, a current meter, and a tapping key were connected in series with from two to six cells of a storage battery, the number of cells depending upon the acid employed. Three milliammeters with scales 0-5, &30, 0-250, and an ammeter with scales 0-0.5 and 0-5 amperes were employed. In the three electrolytes described in this paper the rnilliammeter of 0-5 scale, each scale division equal to 0.1 milliampere, was satisfactory for measuring the conductivity of the most dilute solutions. The results are more consistent when a minimum number of current meters is used for a series of measurements, since the internal resistances of the different meters may vary. PROCEDURE
Hydrochloric Acid
APPARATUS
A 2 em. by 7.5 cm. by 7.5 cm. conductivity cell (Figure 1) was constructed by cementing together ordinary glass plates. The platinum electrodes, 1 cm.
Solutions of various concentrations (Table 1) from 10 M to 0.001 Mare prepared by successive dilutions of 12 M hydrochloric acid. Fifty milliliters of 0.01 M acid are placed in the cell. The same volume of solution is used for later tests. The cell is connected in series with two cells of a storage battery, a milliammeter which covers the 10 milliampere range, and a tapping key. The key is depressed for a few minutes or until the milliammeter gives a constant reading. The electrodes are then adjusted so that the current is 9.7 milliamperes. This value is chosen since it, 0.0097 ampere, is nnmerically equal to the effective ionic concentration of 0.01 M acid (obtained from data' showing -
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ENGELDER, "Elementary qualitative analysis," John Wiley and Sons, Inc.,New York City, 1933, p. 7.
that 0.01 M hydrochloric acid has an apparent degree of ionization of 97 per cent). It is desirable that this initial adjustment of the electrodes for the entire series of solutions be based upon a solution which approaches 100 per cent apparent degree of ionization. Solutions more dilute than 0.01 M are less satisfactory because too much time is required for constant readings.
The apparent degree of ionization is calculated from the corresponding molar concentrations and conductivity data. For example, the conductivity of 0.5 M acid is 0.42 ampere. The apparent degree of ionization is (0.42 X 100) 0.5
=
84%
In the more concentrated solutions the tapping key is depressed only long enough to observe the current reading. Continued passage of large currentsremoves the black coating on the electrodes. The data from Table 1 are plotted in Figures 2 and 3.
Sulfuric Acid
With the electrodes in this fixed position the cell is emptied and rinsed with the next solution of hydrochloric acid to be tested and then filled with the same volume as before. Solutions of the various concentrations are made up far enough in advance so they are all a t the same temperature when tested. A milliam-
The conductivity of sulfuric acid a t various concentrations is measured in a manner similar to that used for hydrochloric acid. Using 4 volts potential and 36 ml. of 0.05 M solution the electrodes are adjusted so that 32 milliamperes pass a t 2 5 T . In the literature2 the apparent ionization of 0.1 N HzSOpis 59 per cent a t lS°C. Since the data are not given for 25"C., the following method was used to obtain this value. The electrodes were adjusted so that 0.0295 ampere passed through the cell a t lS°C. The temper-
Concentration (Molarity)
FIGURE 3.-APPARENTDEGREEOP IONIZAT~ON OP HYDROCHWRIC. SULPURIC, AND ACETICACIDSAT 2 5 T . OP HYDROCHLORIC. SULFIGURE 2.-~ONDUCTMTY PURIC, AND ACETIC Acms AT 25°C.
meter of appropriate range is placed in the circuit and the current measurements in the series, as in Table 1, are numerically equal to the effective ionic concentration of the acid.
ature was raised to 25"C., and the current reading was observed to be 0.032 ampere. This corresponds to an ionization value of 64 per cent. Other concentrations of the acid are tested with the 2 A N n ~ ~AND s o ~HAZLEHURST, "Qualitative analysis," hentice-Hall. Inc., New York City, 1941, p. 251.
same setting of electrodes. See Table 2 and Figures 2 and 3.
The substitution of this quantitative experiment as a lecture demonstration in the authors' classes has
Acetic Acid Since acetic acid is a weak electrolyte it is not feasible to move the electrodes close enough together or to use enough voltage to make the conductivity when expressed in amperes to be numerically equal to the effective ionic concentration. However, it is possible to introduce a factor of ten to relate the two quantities. A 0.1 M solution is chosen as the reference solution. This solution has an apparent degree of ionization of 1.35 per cent a t 25'C.% Using the technic described above, it is found that by using 12 volts potential the electrodes can be adjusted so that the current is 13.5 aroused more interest than the student performance of milliamperes. See Table 3 and Figures 2 and 3. the usual qualitative, light-bulb conductivity experiWACINNES AND SHEDLOVSKY, J. Am. Chem. Soc., 54, 1429 ment. After the data are collected, the students plot the curves and interpret them. (1932).
