James M. Schlegel Rutgers, The State University Newark College of Arts and Sciences Newark, N e w Jersey
Lewis Atid-Base Titration in Fused Salts
For many years the Lewis theory of acids and bases has been taught to freshman and sophomore students, but very few experiments have been designed to illustrate this theory. An experiment has been developed in this laboratory which extends the undergraduate's concept of acids and bases. Dichromate ion, a Lewis acid, is titrated with carbonate ion, a Lewis base, in a fused KNOrNaNOa solvent using equipment and materials which are readily available in the laboratory. The titration is followed potentiometrically using an oxygen electrode as the indicator electrode and a silversilver nitrate electrode enclosed in a thin glass membrane as the reference electrode. The reversibility of the oxygen electrode in fused nitrates has been established.' Also the applicability of the Nernst equation to silver-silver nitrate electrodes in fused nitrates has been demon~trated.~The cell for this electrode pair can be written:
Upon adding a base, oxide ions are produced (COX-CO, 0--) and the potential rises to a constant value. As dichromate ions are added, oxide ions are 0-- = 2Cr04--) and the consumed (CrzOi-potential decreases to a constant value with each addition of dichromate ions. At the equivalence point a large change in potential occurs. This change is large enough to be measured with a pH meter. =
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The Experiment
The indicator electrode was a platinum wire spot welded to a 5 mm square foil of platinum over which oxygen was bubbled. To construct the reference electrode, the end of a Pyrex tube was blown to a thin membrane. Silver nitrate and a mixture of KN03 and NaN03 of the same composition as the solvent were fused in this tube. A silver wire dipped into this solution. Because of the high impedance of this cell, a Beckman 72 pH meter can be used to measure the poModel Pt, OS,0--(KNOs-NaNOz) I Ag+(KNOs-NzNOs),Ag glaaa tential that is developed a t the indicator electrode. Writing the cell reaction, 2Ag+ 0-- = 2Ag '/%02, The shielded cable of a glass electrode was removed from a broken electrode and connected directly to the it follows that the potential for this cell is, silver-silver nitrate electrode. The other lead from the pH meter was connected to the oxygen electrode. Equal weights of NaN03 and KNOa were mixed well If one maintains a constant pressure of oxygen and a in an asbestos-insulated beaker. The beaker was constant concentration of silver ion, then E = E' placed on a hot plate, and the salt mixture was allowed (2.3RT/25) log [O--1. E' includes all those terms to melt. A fused nitrate melt is a very powerful oxiwhich are constant. dizing medium. Consequently, to avoid accidents, a closed glass vessel should be constructed. The top of the vessel should contain two ground-glass openings where the electrodes can be inserted and another KUST. R.. AND DUKE.F.. J . Am. Chem. Soc.. 85.3338 (19631. opening for the evolved gases to escape. The base and FLENDM; S., AND RID& E., PTOC.ROY: s;)c., 2 3 3 ~ .443 acid can be added to the melt through this exit opening. (1956).
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A hot plate with a built in stirring motor is used with a stirring bar enclosed in glass to insure a more uniform temperature distribution and faster mixing of the reactants. The electrodes were lowered into the melt, and oxygen was bubbled in a t a constant rate of three bubbles per second. Once a constant potential was maintained, a known weight of potassium dichromate was added. A drop in potential occurred. The system was allowed to come to a constant potential again before any base was added. The titration was carried out by adding weighed quantities of sodium carbonate. Each time an increment of base was added, the system was allowed to reach a steady potential, and this potential was recorded. Figure 1 is a graph of the data obtained by one of our students. The weight of base iudicated by the inflection point can be shown by calculation to be that for the equivalence point (moles ICZCr207= moles N&CO3).
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0.4 0.6 0.8 Totol Grornr of Bare Added
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This equat,ionmay be written
where g represents the weight of base added and g., is the weight of base needed to neutralize the acid. This relationship is illustrated in Figure 2. The data for this graph were obtained from the points preceding the equivalence point in Figure 1. The solid line represents the theoretical line whose slope equals 0.056 v at 290°C. The other method is the addition of known amounts of sodium carbonate to a fused nitrate melt and the recording of the potential each time.
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Figure 1. A typical plot of the cell poknliol M. the total weight of base odded 01 290' C.
I n addition to the titration experiment, the student can establish the validity of the equation, E = E' (2.3RT/25) log [0--1, by one of two methods. One method utilizes the data already obtained in the titration and is described by Shams El Din and Gerges.3 The potential of the oxygen electrode before the equivalence point is
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SHAMSELDIN,A,, AND GERGES,A,, J. Eleetroanal. Chem., 4, 313 (1962).
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Figure 2. A plot of the equation E = E' (2.3 RT/25) log (0-1 a t 290PC. 0 experirnentol; theoretical line islope = 0.056 voltr).
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I t has been shown that the carbonate ion has a large dissociation constant in fused nitrates.4 Thus, every mole of sodium carbonate represents a mole of oxide ion. I n either case, a plot of potential versus the appropriate logarithm term should yield a straight line whose slope equals 2.3RT/25. -
'K
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~ TR.,,Inorg. Chem., 3,1035 (1964).
Volume 43, Number 7, July 1966
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