[CONTRIBUTIOX FROM
THE
CHEMISTRY DEPARTMEST, THEO H I O STATE UNIVERSITY]
Spectrophotometric Investigation of the Mixed Complex Formed by Copper Ions with Both Ammonia and Pyrophosphate Ions in Aqueoys Solutions1 BY JAMES I. WATTERS,JOHN ~ I A S O A NN D ARNOLDBARON RECEIVED JUSE 4, 1953 A new mixed complex ion, [Cu(P20i)(NH3)2I2-has been identified and its equilibrium with [ C U ( P Z O ~ )has ~ ] ~been - investigated in solutions containing moderate excesses of pyrophosphate ions and ammonia. The logarithm of its complexity constant is 14.22 f0.15 and its maximum molar extinction coefficient a t 650 mp is 27.0 mole-' cm.-'. In pyrophosphate and ammonia-deficient solutions, the complex dissociates stepwise to form,[Cu(P207)(NH,)]2: and [Cu(P~07)]~-.The neutral complex [CUZ( P,O,)( NH3)kIo was detected in very dilute solutions having the corresponding ratios of its constituents.
m-hen ammonia was added to solutions containing copper(I1) and pyrophosphate ions, the extinctions were observed to be different from either those of the ammine or pyrophosphate complexes or their mixtures. The identity and equilibria of the which is remixed complex, [Cu(P207)("&I2--, sponsible for these deviations is discussed in the present paper, one of a series on mixed complex formation. The extinction curves and complexity constants for pyrophosphato complexes have been obtained by Watters and Aaron2while Bjerrum3b4calculated the stepwise formation constants and extinction curves for the various copper(I1) ammine complexes. Bjerrum and Nielsen5 correlated these results with those obtained for the ethylenediamine complex. Vosburgh and Cooper6included copper ammines in their excellent study of the method of continuous variations. The slope-intercept method of determining complexity constants spectrophotometrically which was used in the present investigation has been applied by Benesi and Hildebrand? to the iodine complexes with aromatic hydrocarbons. A treatment in which the higher (1: 2 ) complex and absorption by the reactants were also considered appears in papers by McConnell and Davidson,8 Landauer and McConnellg and Lawrey and Ir4cConnell.10 In the present paper the method is applied to equilibria among three components. The symbols used by the authors2 and by I I c C ~ n n e l lare ~ ~ retained ~~ when possible.
Experimental The apparatus, techniques, and most of the reagents are described in the previous paper.2 In addition, a stock solution of approximately 0.25 11.1 KHIOH was prepared in a paraffin lined bottle using carbonate-free distilled water and a freshly opened bottle of concentrated ammonium hydroxide. It was standardized just before being used. The buret and reagent bottle were connected by a siphon tube (1) Presented before t h e Division of Physical and Inorganic Chemistry a t t h e convention of t h e American Chemical Society a t Los Angeles, Calif., March 15, 1953. P a r t of t h e d a t a were taken from a thesis submitted b y Arnold Aaron in partial fulfillment of t h e requirements for t h e M.S. degree, June, 1952. ( 2 ) J. I. Watters and .4.Aaron, THISJOVRNAI.,7 6 , 611 (1953). 13) J . Bjerrum, Kgl. Daiiske Videnskob, Selskob M u t h 11, 5 (1931); 11, 10 (1032); 12, 15 (1934). (4) J. Bjerrum, "hletal Ammine Furmation in Aqueous Solution," P . Haase and Son, Copenhagen, Denmark, 1941. (.5) J. Bjerrum and E. N. Nielson, Acto Chcm. Scosd., 2, 297 (1948). (6) W, C. Vosburgh and G. R. Cooper, THISJ O U R N A L , 63, 437 (1941). (7) H. A. Benesi and J. H. Hildebrand, ibid., 7 1 , 2703 (1949). (8) H. McConnell and S. Davidson, i b i d . , 72, 3168 (1950). (9) J. 1.andauer and H. McConneil, i b i d . , 74, 1221 (1952). 'IO) D. >I G. Lawrey and €1. XLIrCunnell, ibici., 74, 017: < 1
and both were protected from atmospheric contamination by absorption tubes filled with soda lime. I n preparing the solutions, various volumes of this reagent were transferred to volumetric flasks by buret and other reagents were added as in the previous study. To obtain the data used in the equilibrium calculations all solutions were purged with nitrogen to remove C02. A stock solution of 10 iM ammonium nitrate was standardized by the classical Kjeldahl distillation into saturated boric acid followed by a titration with standard acid. The spectrophotometric measurements were made a t 10 to 20 m p intervals with the Beckman D E spectrophotometer a t 25'. The exit slit width was kept as close to 0.02 mm. as the sensitivity permitted. Ratio Experiments.-Solutions having various [Cu2+I : [P?OT~-] : ["I] ratios were prepared by transferring the appropriate volumes of 0.1 ill CuSOa, 0.2 M Na4P107, 0.25 M S H s and 10 M NH4N03 to 100-ml. volumetric flasks. After adding some water, the 0.1 M CuSOI was added by pipet with rapid swirling and the solution was diluted to volume. The extinction curves for solutions containing 0.025 Mcopper ion with [Cu++]:[P2O14-] ratios of 1:2 and : ["a]] are shown in Figs. 1 and 2. various ratios of [ C U ~ + Similar families of curves were obtained with ratios of [Cuz+]:[ P Z O ~ ~from - ] 1:l to 1 : 4 . An isosbestic point was obtained a t 763 mp for all solutions having [Cu2+]i [P20i4-]: [ NHa] ratios of 1:2 :0 to 1:4 :6. Practically identical curves were obtained when an increase in the pyrophosphate ion
EFFECT OF (
cu'L
TABLE I VARYISG THE [Cu2+I:[PtOi4-]: [SHd] RATIO
Cp20;4-h
CNHB
pH
E per c m . , 650 mp
0,025 ,025 .02
