A SIMPLE AND CONVENIENT SET-UP FOR POTENTIOMETRIC TITRATIONS NBLSON ALLEN* FMCK CHEMICAL L~BOXATORY, PRINCETON U~RSITY PRINCETON. , NEWJERSEY
Smeral wdl-known fi'eces of afi@ratus are combined to produce a set-up for potentiometric titrations offering the advantages of simfilicity, convenience, and portability. Defails and diagrams of the afi@ratus are given.
. . . . . .
During the course of some work on the potentiometric titration of fluorine several well-known pieces of apparatus were combined in such a manner as to produce a set-up offering the advantages of simplicity, compactness, and portability. The diagrams show the details of the apparatus, the upper being an elevation and the lower a plan with the salt bridges omitted for the sake of clarity. The base is a piece of hardwood 22 X 7 X 2 cm., into which (B), an ordinary 250-ml. bottle, and (C), a 50-ml. beaker, are set to a depth of 1.5 cm. and sealed in place with de Khotinsky cement. The bottle (B) contains the calomel half-cell and is the type recommended by Miiller' and by Kolthoff and F ~ r m a n . Two ~ changes have been made in this cell: the salt bridge is constructed from an H-tube instead of two T-tubes and the platinum wire in contact with the mercury is sealed to a copper wire inside the tube (M). This wire is soldered to a binding post which is cemented into the top of the tube (M). These two changes add greatly to the ruggedness of the cell. W i n g connections are made through (A), a double-pole, double-throw switch which has bee-nverted into a reversing switch. The clamp (D) is one of the type designed to be clamped to bottles and to hold burets at the end (E), but the buret clamp has been replaced by a rectangular-headed screw from an ordinary clamp. The neck of the bottle (B)is wound with several turns of friction tape and the clamp (D) firmly fastened on. A piece of bakelite (G) 5 X 2 X 0.5 cm. is inserted in a slot cut in the clamp (D) and sealed in place with cement. The piece of bakelite (F) is 5 X 3.5 X 0.5 cm.and is bolted to the screw head. A jamb nut on (E) holds (F) rigidly in place and enables it to be extended to various lengths. Electrolytic contact is made with the solution to be titrated by another salt bridge held by (F) and (G). This bridge is held at any desired height by small rubber stoppers about 0.5 cm. thick slipped over the tubes of the bridge; one such stopper is shown at (I). Salt bridges containing saturated solutions of potassium chloride, nitrate, or sulfate are quickly inter* J. T. Baker Chemical Co. Fellow in Analytical Chemistry.
' M ~ L L E R"Die , elektrometrische Massanalyse," 4th ed., Theodor Steinkopf, Dresden and Leipzig. 1926,246 pp. ' KoLTHoaa AND F ~ M A N"Potentiometric , Titratious," 2nd ed., John Wiley and Sons. Inc., New York City. 1931, pp. 79-81. 1973
1974
JOURNAL OF CHEMICAL EDUCATION
NOVEMBER, 1932
changed on the apparatus. The beaker (C) should contain saturated potassium chloride solution. The indicator electrode for any particular titration is made up like the platinum electrode of the calomel cell and is supported by (F) in the same way as the salt bridge. The binding post of this electrode is connected to one of the binding posts (K). If a bi-metallic electrode system is being used the salt bridge is removed and the second
electrode is wired to the other binding post (K). Wires from the potentiometer are fastened to the binding posts (L) and the switch (A) enables one to reverse these connections if necessary. An indicator electrode of platinum permits oxidation-reduction titrations to he made and in connection with quinhydrone can he used to carry out neutralizations. An assortment of other electrodes such as antimony, tungsten, silver, nickel, etc., made up like the platinum one, but with a long
VOL. 9, NO. 11
MELTING POINT AND TEMPERATURE
1975
piece of the metal projecting from the constricted end of the glass tube instead of being sealed in, furnish permanent means of performing any kind of potentiometric titration. If the solution to be titrated must be heated the apparatus is placed on a tripod or other support to bring it on a level with the beaker containing the solution. When not in use the ends of the salt bridges should be kept dipped into potassium chloride solution to prevent evaporation of the liquid in the bridges and consequent drying out of the filter paper plugs.
