METALCOMPLEXING BY PHOSPHORUS COMPOUPU'DS
July, 1962
1349
METAL COMPLEXING BY PHOSPHORUS COMYOUNDS. VI. ACIDITY CONSTANTS AND CALCIUM AND MAGNESIUM COMPLEXIXG BY MONO- AND POLY METHYLENE DIPHOSPHONATES^ BY RIYADR. IRANIAND KURTMOEDRITZER Ailonsunto Chemical Company, 1nor.ganic Division, Research Department, St. Louis 66, Alassouri Received February 14, 196.2
0
II
0
I
The acidity constants of methylene and polymethylene diphosphonic acids (H0)2-P-( CH&--P--( OH)* have been measured as a function of n, ionic strength, and temperature in the range 25-50". It is shown that as n increases the effect of the two end phosphonate groups on one another decreases, and becomes negligible for n larger than 3 . Calcium and magnesium complexing by the anions of these acids %rere measured nephelometrically in the presence of oxalate, and by the pH-lowering method as a function of ionic strength and temperature. Complexing drops off sharply as n increases and becomes negligible for n = 10. Comparison with other data suggests that in calcium and magnesium complexing by diphosphonates, the distance between the two phosphonate groups is critically important.
The synthesis and physical properties of monoand polymethylene diphosphonic acids and esters in high purity were described elsewhere.6 There, from the measurement of P31 n.m.r. chemical II II or imidodiphosphoric shifts as well as the infrared spectra, it was con(H0)2-P-O-P--(OH)2, cluded that as n increases, the effect of the two end 0 0 phosphorus atoms on one another decreases and II H /I becomes very small for values of n larger than three. acid, (HO)2-P-N-P-(OH)~, aqueous solutions Experimental containing methylene diphosphonic acid, (H0)sChemicals.-The 99.5 +.% pure methylene- and poly0 0 Introduction In contrast with the salts of pyrophosphoric acid, 0 0
If
I1
P-C!H2-P-(OH)2, or its salts are extremely stable at room temperature against hydrolytic degradation, either in acidic or basic media. This is due to the high stability of P-C bonds relative to other bonds involving phosphorus.2 In view of this, it is of interest to study the ability of the salts of methylene diphosphonic 0 0
I1
II
acid, (HO)2-P--CH2-P-(OH)
2,
and polymethyl-
0
II
0
II
ene diphosphonic acids, (I-IO)Z---P-(CH~)~-P(OH),, to complex metal ions. The results, coupled with the planned determination of phosphorusto-phosphorus distances in these systems, will make it feasible to evaluate optimum separation of phosphorus atoms for the complexing of specific metal ions. Schwarzenbach and Zurca reported acidity constants for methylene-, trimethylene-, and tetramethylenediphosphonic acids. However, their work was done in the presence of alkali metal ions, which form complexes of their own with metalcomplexing anions. Because of their inability to synthesize the member of the series with n = 2, wrong conclusions were drawn regarding the acidity constants of hypophosphoric acid, as will be discussed later. Limited data on calcium and magnesium complexing also have been reported.4 (1) Presented before *the Division of Inorganic Chemistry, 141st Nations1 Meeting of t h e American Chemical Society, Washington, D . C., March, 1962. (2) 12. D. Freedman and G. 0. Doak, Chsm. Reo., 67, 479 (1967). (3) (3. Schwarzenbach and J. Zuro, Monatsh. Chem., 81, 202 (1950). (4) (3. Schwsrzenbaoh, P. Ruckstuhl, and J. Zuro. Hdv. Chim. Acta. 84, 466 (YL961).
methylene diphosphonic acids were prepared and recrystallized as previously described.6 VC hen desired, the corresponding tetramethylammonium salts were prepared through neutralization with the proper amount of pre-standardized Eastman Kodak tetramethylammonium hydroxide. Boiled distilled water was used for solution makeup. All other chemicals were reagent grade. Procedure.-All measurements were made in a nitrogen atmosphere, in the absence of extraneous cations, e.g., alkali metal ions. Ionic strength was made up with tetramethylammonium bromide. The acidity constants were determined using the titration procedure and IBM program previously described.6 The calcium complexing by methylene diphosphonate as a function of ionic strength and temperature was measured using the nephelometric procedure? with oxalate as the precipitating anion. The pH-lowering methods was used for measuring the magnesium complexing constants because of the absence of a well defined magnesium precipitate, as required in the nephelometric rocedure.7 The pH-lowering method ago was used for measuring the complexing of calcium and magnesium by polymethylene diphosphonates. Here, the nephelometric procedure with oxalate as a precipitate could not be used on calcium because of the poor competition for the calcium by the complexing anions.
Results and Discussion Acidity Constants.-Stepwise titration curves with definite breaks for the weakest two hydrogens were obtained with all of the investigated methylene- and polymethylene diphosphonic acids, The remaining two hydrogens are too strongly acidic to show inflection points. The acid-base titration data, obtained in duplicates a t various constant temperatures and constant total ionic strengths, were fit to a least squares program of an IBM 704 computer, as ( 5 ) K. Moedritzer and R, R. Irani, J . Inorg. Nucl. Chsm., 2.2, 297 (1962). (6) R. R. Irani and C. F. Callia, J . Phys. Chem., 66, 934 (1961). (71 R. R. Irani and C. F. Callis, ibdd., 64, 1398 (1960). (8) A. E. Martell and G . Sohwarzenbach, Belo. Chim. Acta, 39, 653 (1956).
1350
RIYADR. IRANI AND KURTMOEDRITZER
previously describedS6 The resultant acid dissociation constants with the statistical 95% confidence limits are listed in Table I. The various pK values refer to the equation
Vol. 66 TABLE
ACID D I s s O C I A T I o N
1
CONSTANTSa
FOR
MONO-AND
POLY-
DIPHOSPHONIC ACIDS, 0 0
METHYLENE
I1
(HO)e-P-( Temp., n
OC.
Ionic strength
I1
CHs)s-P-( pR1
pKo
0H)t pRs
pRc
10.69 7.45 2.87 2.2 10.42 7.33 2.75 1.7 7.28 2.70 1.5 10.31 7.25 2.65 1.5 10.22 2.67 1.7 10.32 7.28 7.62 2.90 1.5 10.51 37 .I 10.24 7.38 2.85 1.5 7.28 2.83 1.5 .2 10.15 7.57 3.08 1.5 50 Ob 10.47 0.1 10.25 7.40 2.88 1.5 7.33 2.80 1.5 10.16 .2 7.38 2.85 1.7 10.07 1.0 7.62 3.18 1.5 2 25-50 Ob 9.28 7.50 2.96 1.5 .1 9.08 2.87 1.5 9.00 7.45 .2 7.44 2.71 1.5 8.96 .3 7.42 2.74 1.5 8.96 1.0 3 25-50 Ob 3.06 1.6 8.63 7.65 7.50 2.81 1.6 0.1 8.43 7.44 2.70 1.6 8.35 .2 2.69 1.7 8.33 7.41 1.0 4 25-50 Ob 8.58 7.78 3.19 1.7 .1 8.38 7.58 2.85 1.7 .2 8.30 7.50 2.71 1.7 7.47 2.70 1.7 8.30 1.0 7.73 3.12 1.8 8.56 6 25-50 Ob 7.65 3.07 1.8 8.34 0.1 3.05 1.8 8.25 7.62 .2 7.59 3.00 1.9 8.27 1.0 7.93 3.27 2.1 8.94 10 25-50 Ob 3.15 2.1 8.83 7.74 0.1 7.67 3.10 2.1 8.79 .2 7.68 3.06 2.0 8.73 1.0 a The 95% confidence limits for pK1, ~ K zor , pK3 average 3~0.07unit and range between 0.05 and 0.09. The values of p& are much more uncertain, with an estimated error of &0.2 unit. b From extrapolation of the experimental data, m discussed in the text. 1
25
O* 0.1 .2 .3 1.0 Ob
where parentheses indicate concentration. Constants in which the activity of the hydrogen ion is used can be obtained by multiplying K, by the appropriate activity coefficients,g which, a t 2 5 O , are: 0.80, 0.78, 0.75, and 0.87 at ionic strengths of 0.1,0.2,0.3, and 1.0,respectively. The dissociation of the weakest hydrogen corresponds to pK1 and the strongest to pKa. This is contrary to the convention of some other workers, who use pK1 to denote the dissociation of the strongest hydrogen. The reason for the change is due to a simplification of the algebra involved, previously presented.6 The thermodynamic acid dissociation constants, pK,, also listed in Table I, were estimated by extrapolating the apparent dissociation constants at low ionic strength values to infinite dilution. Eren though relatively few ionic strengths were investigated, the 95% confidence limits for pK, are &0.1 unit because of the use of previously established relationships for similar acids between apparent acid dissociation constants and ionic st
