The Conductance of Electricity through Solutions of Strong an&Weak Electrolytes JOHN G. MILLER and WALTER W. LUCASSE University of Pennsylvania, Philadelphia, Pennsylvania
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N SEVERAL types of experiments in physical chem- solution is well mixed. By careful manipulation, the istry for undergraduates refinement of technique small beaker can be filled with the uniform N / 4 ~ soluhas steadily increased, while often the basic significance tion which has resulted from the dilution. After obof the quantity measured has decreased, or a t least, taining the resistance of this solution, two more diluassumed different emphasis. The conductance of elec- tions are made-with 100 and 200 ml. of water-to tricity through solutions is a case in point and is usually study approximately N / m and N / ~ ~ / l s o solutions, being sure illustrated by conductance titrations and by the deter- to note the temperature of the final solution. mination of the equilibrium constant of one or more The experiment is repeated in the same general fashweak acids, after obtaining the cell constant. Succes- ion starting, now, with exactly 50 ml. of approximately sive editions of laboratory outlines are apt to indicate 0.1 N acetic acid from a stock solution whose exact more accurate temperature control and increasingly concentration is known. By adding 50, 100, and 200 elaborate sources of current. However, with the grow- ml. of distilled water the resulting solution is one-half, ing emphasis in the classroom upon interionic attrac- one-quarter, and, finally, one-eighth its original contion and with the knowledge that the "true dissociation centration. constant" (5, 1) is never obtained by the student, i t In the final repetition, approximately 0.23 g. of monoseems less important than formerly to attempt to re- chloroacetic acid is weighed exactly to the nearest milliproduce the uncorrected literature value of the constant gram directly into the 50-ml. beaker. Solution and of a weak acid as determined by this method. The ob- dilution are repeated in the same manner as with the jective of the experiment might well be altered to stress potassium chloride. the difference in the magnitude and variation of the A plot of the measured resistance of the solutions of conductance of strong and weak electrolytes and the potassium chloride as ordinates against the square root factthat formal substitutionof the conductance ratio into of the normality permits reading the value of the rethe mass action law fails to give a constant in the former sistance a t exactly N/M. By means of this measured case while an approximate constant results for weak resistance ( x ) and the value of the speafic conductance electrolytes. (k) of N/m potassium chloride a t the corresponding Such an objective can be achieved with relatively temperature: simple apparatus and with great saving of time. Conk = P.42 + 0.052 (1 -IS)] X lo-* ducted as indicated below, the experiment makes use of two beakers, a dipping electrode, and a telephone re- the cell constant (c) can be calculated: ceiver with a microphone hummer, together with the c = kx resistance and slide wire of the Wheatstone bridge. The entire experiment is usually completed in a single using the cell constant, the equivalent conductance -*e-hour laboratory period andalways in two two- (A) of each of the three solutions a t thefour known con. hour periods. centrations can be calculated. For the equivalent conThe three parts of the experiment are conducted in ductance of the three solutions a t infinite dilution, the the same general manner. In the first part, approxi- followingequations have proved satisfactory: mately 0.19 g. of potassium chloride is weighed exactly for potassium chloride: A. = 129.4 + 2.80 (1 - 18) to the nearest milligram directly into a 50-ml. beaker. Ao = 350.2 + 5.79 (1 - 18) This is then introduced into a clean, dry 600-ml. beaker acid: = 352 + 5.3 (t and exactly 50 ml. of distilled water added to the salt in the small beaker. When well mixed, this gives an ap- In all cases, the conductance ratio (CR = A/&,) which proximately N/zo solution in sufficient amount to cover results from these values, can be used as a convenient the electrodes which should be previously cleaned znd, quantity for showing the conductance as a function of except for the platinum itself, thoroughly dried. After the concentration. Indeed, the conductance ratio shows three determinations of the resistance have been made, more clearly the difference between strong and weak 50 ml. of distilled water are added to the solution by al- electrolytes than does the equivalent conductance. lowing the water to flow from a pipet into the small For weak electrolytes, since the conductance is due beaker. This is then rolled over on its side so that the primarily to the number of conducting particles, the 565
acetic
cularly conductance ratios than weak, and that, for the latter, the Ostwald dilution law is approximately valid. The accepted values (3) for acetic acid a t 18' and 25°C. are 1.806 and 1.813 X 10-5 respectively. Dawa'N g = son and Reiman (2) have shown that K for monochloro1-or acetic acid is not quite constant, decreasing a t 25'C. Variation of the conductance of solutions of strong elec- from 1.58 to 1.44 X as the concentration decreases trolytes with the concentration, however, is due for the from 0.05 to 0.005 N. The fact that the constant shows most part to changes in the ionic mobility and to show more drift for this electrolyte than for acetic acid is not the inapplicability of the last equation to solutions of always confirmed by the student, but the reason this such electrolytes, it is of interest to use the equation: should be the case is readily understood. Three obvious disadvantages of the suggested proAsN "K" = cedure should be noted. In general, the cell constant - Adha - A) depends to a slight extent upon the proximity of the Formally these equations are entirely equivalent and electrodes to the walls of the vessel and to the surface of the second, although avoiding the symbol a,embody all the liquid. The former efiect has been practically elimof the concepts of partial dissociation. On the basis of inated by the type of electrode (4) used and the effect his classroom work, the student is not surprised to find of the differing height of the liquid above the electrodes that "K" is not a constant. is well within the experimental error. The conductance Two sets of data, obtained by typical student groups is a marked function of the temperature but by use of a s outlined above, are shown in Table 1. The results distilled water a t room temperature for dilution, this factor was minimized. Finally, except for the initial concentration of acetic acid, the solutions are not on a W y normal basis. However, a t the low concentraSludcnl G r o w "A" Sludcnl C*arp"B" Potassium Chlmide st 2?;:?. ~ ~ t a r s i uChloride m at 18'C. tions studied, the deviation is, again, well within the U r CR '.K" N A CR experimental error. These approximations, together with the lack of a fixed temperature and elaborate electrical equipment, seem to be more than compensated by the advantages Acetic A d d at 18'C. achieved. Indeed, the influence of temperature is more N A CR 10'K likely to be impressed upon the student by calculations 0,1308 4 . 2 9 0.0123 2.00 0.0654 6.06 0.0173 1.99 with the above equations than by the use of a thermo0.0327 8.56 0.0244 2.00 stat. In the simplified procedure, adequate training in 0.0164 11.95 0.0341 1.97 0.0 350.2 1 0 0 0 0 .... technique is attained without loss of focus of attention Monochloroaeetic Acid at 19.5'C. on the major theory of the experiment. The relatively N A C R 10'K 0.0536 59.0 0.164 1.72 few students who go on to advanced experimental work 0.0268 7 9 . 9 0.222 1.70 may better learn the mastery of the latest refinements 1.67 0.0134 106.7 0.296 0.0067 140.9 0.391 1.68 of apparatus and procedure a t that time. The many 0.0 360.0 1.0000 . . . . will profit more by the broader range of exemplified indicate the usual order of accuracy, the magnitude and theory which the saving of time will permit. constancy of K which may be expected, and the sensiLITERATURE CITED tivity of the final values of "K"to errors of experiment- (1) DAV!?, C. W., "The Conductivity of Electrolytes," 2nd ation and plotting. Due to the differences in temperaed~tmn,John Wiley and Sons,Inc., NewYork, 1933, p. 105. ~ . M.. AND C. K. RBIMAN. J. Chen. Sot... 107.. ture a t which the work has been carried out over the 121 D ~ w s o H. 1429(i915)1 ' years, no attempt a t a statistical study has been made. (3) "International Critical Tables," McGraw-Hill Book Cornpany, Inc., NewYork, 1929,Vol. VI. However, it has been clear that all students have been W. W., AND H. J. ABRAHAMS, J. CHBM.EDUC., 7, readily able to illustrate the fundamental objectives (4) Lucnss~, 341 (1930). of the experiment: that a$ finite concentrations strong (5) NOYES, A. A.. AND M. S. SnEnnrLL, J. Am. Chem. Sac., 48, 1861 (1926). electrolytes show higher conductance and more particonductance ratio can immediately be identified with the degree of dissociation and the mass action constant calculated in its usual form:
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