SIMPLE YET WIDELY APPLICABLE TRANSPORT NUMBER APPARATUS EDWARD M. COLLINS Williams College, Williamstown, Massachusetts
The determination of the transport number of the hydrogen ion in a solution of a n aczd i s recommended for a n experiment i n physical chemistry. The construction, operatcon, and advanlag6s of a n apparatus which can be used for chis experiment hnd also for the determznation of the transport numbers of metallic ions are dcscribed.
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ANY laboratory manuals in physical chemistry give directions for determining the transport number of the silver ion in a solution of silver nitrate. Of eight manuals examined only two gave also the directions for determining the transport number of the hydrogen ion in a solution of an acid, viz., Fajans and Wiist (I) and Gray (2). We feel that the experiment with the hydrogen ion is preferable since a standard acid solution is more often on hand than a standard silver solution and also because the
change in the concentration of the acid is more easily ascertained since a standard base is usually available in most laboratories. The types of apparatus used in the study of the silver ion are not as a rule applicable if the ion under investigation forms a gas upon being liberated, which is the case with the hydrogen ion. With the electrodes placed as usual near the bottoms of the respective compartments the evolution of the hydrogen and its rising through the solution would cause an undesired stirring in the cathode compartment. In the two references cited above this diiculty is avoided by simplifying the apparatus to merely two small beakers with a connecting bridge. This apparatus, however, is not suitable for the metallic ions and i t is difficult to obtain any solution from the middle compartment, the bridge, for analysis. Findlay (3) calls for a modification of his apparatus when a gas is liberated a t the cathode. His ordinary apparatus, consisting of two identical cathode and anode
compartments connected by a U-tube and having the electrodes extending to the bottom of the compartments, is altered by the substitution of a cathode compartment constructed along different lines so that the electrode can be located a t the top. This apparatus, including the modified cathode compartment, may be purchased from the supply houses but its price is such that few laboratories can afford a sufficient number of them so that a t least half the class may be workmg on this experiment simultaneously. The apparatus as purchased holds such a large volume of solution that the electrolysis must be continued several hours in order to effect an appreciable change in the concentration. Such a large apparatus, furthermore, requires a large battery for operation, i. e., 110 volts D.c.,a voltage found in but few laboratories. Washburn's apparatus (4) may be used for hydrogen ions as well as for metallic ions with no structural changes but its large-bore stopcock makes it a very expensive and difficult apparatus t o c o n s t r u c t . Although once stocked by A. H. Thomas of Philadelphia, i t must now be blown to order. A study of several other types of apparatus, cf., Taylor (5), n revealed none that was adaptable to both types of ions and yet also suitable for experimental use in physical chemistry. We have devised an apparatus which, like Washbnm's, can be used for both types of ions but which is cheaper and easier to construct; in fact, it may be made by anyone who possesses an ordinary degree of skill a t glassblowing. It has an advantage over most types of apparatus in the ease of the separation of the solutions in the respective compartments from each other a t the end of the electrolysis and the withdrawal of them for analysis. The electrode compartments A and C as shown in the diagram are approximately 200 mm. long and are made from 25-mm. tubmg, the use of tubing of this size or larger being necessary to decrease the resistance of the solution. The middle compartment B is made from 12-mm. tubing and is joined to the other compartments by means of connectors of rubber tubing a t F and G. Smaller tubing is chosen for this part of the apparatus because of the greater ease in the making of the T-joints. Pinch-clamps applied a t Fand G separate the compartments and their solutions from each other a t the end of the electrolysis. The solution in A is withdrawn for analysis through the stopcock. The solution in the middle compartment
may be removed through the opening a t the top by means of a pipet or allowed to run through the stopcock in the A compartment after the latter has been thoroughly drained of the first solution. We have used this type of apparatus for determining the transport number of the hydrogen ion in N/2 HC1 followingthe general directions for transport numbers as given in most manuals. In this experiment A is made the cathode compartment and the cathode D, consisting of a piece of platinum foil sealed into a glass tube filled with mercury, is held in position a t the top of the compartment by means of a stopper. The stopper must have a second hole or be grooved to allow the liberated gas to escape. The anode E is an amalgamated zinc electrode held a t the base of the respective compartment by means of a stopper. Although most directions call for a current of not more than 10 milliamperes we have obtained good results with as many as 100 milliamperes. Approximately 40 volts D.C. are needed to produce this amount of current in this apparatus. With this larger current an appreciable change in the concentration of the solution in the electrode compartments is effected in seventy-five minutes instead of several hours. It should go without saying that if 40 volts D.C. are not available the experiment may be run on less a t the sacrifice of speed. When using this apparatus for determining the transport nomber of a metallic ion one has merely to substitute for the platinum electrode an electrode of the appropriate metal, e. g., a silver electrode as anode when determining the transport number of the silver ion in a solution of silver nitrate. Since transport numbers are generally calculated from the change in the concentration of the solution in the.vi'cinity of either the cathode or the anode, the choice being governed by the experiment, the electrodes are arranged so that compartment A which possesses the stopcock contains the desired solution. The apparatus permits withdrawal of the solution in the C compartment jf an analysis of this is also desired. LITERATURE CITED
(1)
FAJANS, K. AND W ~ S T J.,, trans. by TOPLEY, B., "A textbook
of oractical Dbvsical chemistrv." .. R. P. Dutton & Co.. New York Ckf, 1930, p. 176. GRAY, F. W., "A manual of practical physical chemistry," Macmillan & Co., Lid., London, 1911, p: 132. FINDLAY. A.. "Practical ~hvsicalchem~strv." Lonmans. Green-& do., New ~ o r kcity, 4th ed., 16%, pp..'i81-7: WASHBURN, E. W., "The hydration of ions detennmed by transference experiments in the presence of a non-electrolyte," I. Am. Chem. Soc., 31, 330 (1909). TAYLOR. H. S., "A treatise on physical chemistry," D. Van Nostrand Co., New York City. 1924, p. 544.
