for CPNA and therefore is less well suited for use as an acid-base indicator. The results of a limited number of experiments in various nonaqueous solvents indicate potential usefulness in this area as well. While all applications of this indicator in aqueous solutions make use of the first ionization only, nonaqueous solvents will allow utilization of either or both of the color changes depending on the exact situation. The results of studies in nonaqueous solvents will be reported in a later paper.
ACKNOWLEDGMENT Thanks are due to Donald G. Luke for assistance with the titrations. Received for review January 26, 1973. Accepted April 5, 1973. Financial assistance from the Faculty Research Committee of Middle Tennessee State University is gratefully acknowledged. A portion of this work was presented a t the 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972.
Conductometric Titrations of Reactions Involving Sodium-Ammonia Solutions at -33.9"C R. L. Jones and R. R. Dewald Department of Chemistry. Tufts University. Medtord. Mass. 02155
Studies of the reactions of metal-ammonia solutions have received considerable attention, mainly because of the reducing properties of these systems (1-5). The reactive nature of these solutions, coupled with their inherent instability owing to reaction with the solvent itself, forces workers to perform all manipulations in closed systems and to take extreme precautions to ensure purity of material and cleanliness of vessels (6). The obvious need for a fast and accurate determination of the stoichiometry of reactions involving these metal solutions prompted us to develop a simple high vacuum, low temperature titration device which is the subject of this report.
EXPERIMENTAL Apparatus. The titration apparatus, constructed of borosilicate glass, is shown in Figure 1. The bright platinum ball electrodes (H) were fabricated in a manner similar to that described elsewhere (7, 8). The titrant-ammonia solution (C) was separated from the Na-NH3 solution (F) by a high vacuum stopcock (E) lubricated with Dow Corning high vacuum silicone lubricant. The apparatus was constructed in such a manner as to not allow the reactive Na-NHa solution to come in contact with stopcock E when the apparatus is rotated and tipped to introduce titrant solution into the calibrated tube (C). No indication of contamination of the titrant solution was observed by its contact and passage through stopcock E. Reagents. Ammonia (Matheson) and sodium (United Mineral and Chemical Co.) were purified and stored utilizing procedures described elsewhere (7, 8). Break-seal tubes containing weighed samples of ammonium bromide (Fisher certified) and imidazole (Eastman) were evacuated and then sealed off under high vacu(1) W. L. Jolly, "Metal Ammonia Solutions," supplement to Pure Appl. Chem.. Butterworths, London, 1970. p 167. (2) G . W . Watt. Chem. Rev.. 46, 289 (1950). (3) G. W. Watt. Chem. Rev.. 46, 317 (1950). (4) J . Jander, "Anorganische and Allgemeine Chemie in flussigen Ammoniak." G . Jander, H. Spandau, and C. C. Addison, E d . , Inter-
um. Dimethyl sulfoxide, dimethyl sulfide, and acetone were obtained from Fisher Scientific. Samples of these compounds were prepared by high vacuum fractional distillation to break-seal tubes after a repeated freezing and evacuation technique described elsewhere (7, 9). Procedure. A break-seal tube (A) containing a weighed amount of reactant is sealed to the titration device. The apparatus is then cleaned using an established procedure (IO). After evacuation through D to less than 5 x 10-6 Torr, sodium is distilled into the sodium make-up vessel (F) through a series of constrictions on a sidearm ( G ) , and the sidearm is subsequently sealed off. Ammonia is then condensed into bulb F and the volume determined by weight difference of a stainless steel storage vessel. Stopcock E is closed, ammonia is condensed into bulb B, and the volume is again determined by weight difference using reported density data (11). The apparatus is next removed from the vacuum line and placed into a convection fluid test chamber (Harris Mfg. Co.) at -33.9 f 0.1 "C. The break-seal tube containing the reactant is broken with a glass encased magnet and the solution is mixed thoroughly. Specific conductance of the Na-NHS solution (F) is then measured and the concentration of Na is determined from reported conductance data (8). The resistance of the metal-ammonia solution (F) is subsequently monitored for approximately 1 hr to ensure the stability of the metal solution. Finally, titrant solution from B is poured into the calibrated tube ( C ) and its volume noted. Small volumes of titrant solution are then added and the volume of titrant solution remaining in C and the resistance of the reacted solution in F are recorded after each addition. This procedure is repeated until sufficient data have been obtained to determine the end point. A number of additions of titrant solution are usually added after the end point is reached in order to determine the effect of reactant upon the resistance of the solution in F.
-
RESULTS AND DISCUSSION Since the stoichiometry of the reactions NH4+ + eYzH2 has been well established, an ammonium halide salt (NH4Br) was used to verify the accuracy of the titration
science, New York, N . Y . , 1966.
(5) H. Smith, "Organic Reactions in Liquid Ammonia," G. Jander, H . Sparidau. and C. C. Addison, Ed., Interscience. New York, N . Y . , 1963. ( 6 ) D. F. Burow and J . J . Lagowski, Advan. Chem. Ser.. No. 50. 125 (1965). (7) R. R. Dewald and R . V . Tsina. J . Phys. Chem.. 72, 4520 (1968). (8) R. R . Dewald a n d J. H. Roberts. J. Phys Chem.. 72, 4224 (1968)
(9) R. R. Dewald in "Metal Ammonia Solution." supplement to Pure Appl. Chem.. Butterworths. London, 1970, p 193. (10) L. H. Feldrnan, R. R.,Dewald. and J. L. Dye, Advan. Chem Ser, 50, 193 (1965). (11) C. S. Crogoe and D. R. Harper, Bur. Stand. ( U . S . ) Sci. P a p . , 420,313 (1921).
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H
I
Figure 1. High vacuum conductometric titrator
~
4
( A ) Reactant sample, (B) reactant make up vessel, (C) calibrated tube, (13) high vacuum stopcock, ( E ) high vacuum stopcock. (F) metal make up vessel (G) metal still. ( H ) PI tipped ball electrodes
8
I2
16
20
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Titrant Added MI Figure 2. Plots of resistance ( o h m s ) vs milliliters of titrant solution added for the titrations Na-NH3 C N H d B r ( 0 )and Na-NH3 f (CH3)zS( 0 )at - 3 3 9 "C
Table I. Mole Ratios for Reactions of Sodium with a Variety of Substrates in Liquid Ammonia at -33.9' Substrate
NH4Br
Imidazole Dimethyl sulfoxide Dimethyl sulfide Dimethyl sulfide Dimethyl sulfide Acetone
Substrate concn, 102M
Sodium concn. 1 0 3 ~
mol substrate
0.8791 0.3651 5.523 11.35 11.01 3.126 5.921
0.212 0.088 8.10 13.50 15.2 20.0 15.80
1.01 0.98 0.98 1.98 1.98 2.01 0.97
Mol sodium I
device ( 2 ) . The mole ratio of 1.01 moles of Na to moles of NH4Br found verifies the accuracy of the method. Figure 2 shows a representative plot of resistance (ohms) us. volume of titrant solution added. The solid line clearly shows the end point in a titration with dimethyl sulfide. Further addition of titrant solution [(CH3)2S] results in a typical slight increase of resistance (F) due to dilution of product salts, as would be expected from a neutral, nondissociating compound such as dimethyl sul-
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fide. The dashed line in Figure 2 represents the typical behavior encountered after the end point is attained for acids such as ammonium salts. Table I gives the mole ratios for the reactions studied. All reactions studied yielded mole ratios having integral numbers to within 1 to 3%. The mole ratio of 2.0 for the case of dimethyl sulfide agrees with the reported mole ratio for the general class of alkyl sulfides (R-S-R, where R = alkyl) (12, 13). The above method appears to provide a relatively simple and accurate means of determining molar ratios in highly reactive (moisture and air sensitive) systems. Moreover, the method could easily be extended to solvent systems other than liquid ammonia. Received for review January 29, 1973. Accepted April 12, 1973. This work was supported by the National Science Foundation. (12) R C KrugandS Tocker J Org Chem 20. l ( 1 9 5 5 ) (13) F E Williams and E Gebauer-Fuelnegg, J Amer Chem SOC 53, 352 (1931)
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