May, 1958
CRYSTALLOGRAPHY OF HYDRATED MONOCALCIUM PHOSPHATES
625
CRYSTALLOGRAPHY OF HYDRATED MONOCALCIUM PHOSPHATES CONTAINING POTASSIUM OR AMMONIUM BY WALTERE. BROWN,JAMES P. SMITH,JAMES R. LEHRAND A. WILLIAMFRAZIER Division of Chemical Development, Tennessee Valley Authority, Wilson Dam, Ala. Received January 27, 1968
The lattice constants of Ca2NH4H7( P04)r.2Hz0and its potassium isomorph were determined. A structural analog between these compounds, monocalcium phosphate monohydrate and dicalcium hosphate dihydrate is shown. These possess a Ca-PO4 sheet-like structure in common. The compound CaClHzP04.H& very probably has the same structural . feature, which also may be present in chlorospodiosite, Ca~P04Cl.
I n their studies of the system Ca++-NH4+POh----HzO, Flatt and co-workers1r2 prepared a "double salt" to which they assigned the formula Ca9(NH4)4H32(P04)18.10Hz0. They also prepared the potassium isomorph. Monocalcium phosphate monohydrate contains parallel Ca-PO4 sheetss4 which are nearly identical structurally to the sheets of dicalcium phosphate dihydrate5 and of gypsum.6 I n moiiocalcium phosphate monohydrate the water molecules and half the phosphate ions lie between the Ca-PO4 sheets. The fact that the sheet-like structure is common to the three compounds indicates that the sheets are very stable. Another feature of the monocalcium phosphate monohydrate structure of importance to this study is the region of relatively low atomic density between the Ca-PO4 sheets, which could accommodate foreign ions.
TABLE I COMPOSITIONS OF FLATT'S "DOUBLESALT" Method of Isomorph prepn.' 4"
NH4
1 I1
Empirical formula
Flatt's data C~~.QO(NH~)~.O~HT.~~(PO~)~.~
C~Z.OZ(NH~)O.~~H~.OZ(PO~)~.OO.~ Present work
NH4
K NHP Kb
K Ref. 1. study.
I1 I1 I1 I1 11 b
Ca~.o~(NH4)0.81H7.08(P01)4.oo.2.24H~O
Ca~.o~Ko.~~H~.o~(PO~)~.oo~2.36HzO Cal.QO(NH~)O.Q~H~.~~(POS~.OO~~.OS C~~.OOKO.~~H?.O~(PO~)~.OO.~.~
C~Z,OOK~.~~HT.OB(PO~)~.M~.~.~OHZO Preparations used for single-crystal X-ray
ning but differ in the orientation of the optical indicatrix. Techniques.-The ammonium and potassium salts were The ammonium salt crystallizes as colorless triprepared by Flatt's' "second method." The crystals were clinic (010) plates that are usually elongated drained, flash washed with distilled water and air-dried. Calcium was determined gravimetrically as the carbonate,' parallel to the c-axis. Polysyiithetic twinning acPhosphorus was determined by a differential spectrophoto- cording to the albite law is common, with (010) as metric method.8 Potassium was determined flame photometrically with a Perkin-Elmer model 52C instrument and the composition plane. The p-angle, (100) A (OOl), with lithium as an internal standard. Ammonia was dis- is 118". The refractive indices ( n " ~ ) are nor = placed by boiling with sodium hydroxide and was absorbed 1.525, np = 1.536 and n, =. 1.553. The crystals in standard sulfuric acid. A single-crystal X-ray Rtudy was made by the equi-inclina- are biaxial (+) with 2V = 78" (calcd.). On (010) $on Weissenberg method with Cu Kcu radiation (A = 1.54 the extinction angle, n, A a, is 6' in acute p. (In A.). Reciprocal lattice lengths were determined from high- monocalcium phosphate monohydrate n, A a = order zero-layer reflections and, where possible, the recipro- 2' 40' in acute p, and n, n,.) The trace of the cal an les were determined by the method of triangulation.9 OAP on (010) approximately bisects obtuse p. The ?-ray data were obtained from a- and c-axis settings The OAP is inclined 40 t o 45" to (OlO), one optic for both crystals. Powder diffraction data were obtained with a cylindrical axis being nearly normal t o (100). camera (143.2 mm. diam.), a wedge-shaped sample and Cu The potassium salt also crystallizes as colorless Kar radiation. Intensities were estimated visually. triclinic (010) plates with the same morphological Results.-Chemical analyses of the present features as the ammonium salt. The p-angle, preparations and those reported by Flatt and co- (100) A (OOl), is 118". The refractive indices are workers' indicate the empirical formulas shown in n, = 1.519 np = 1.530 and n, = 1.545. The crystals are biaxial (+) with 2V = 82" (calcd.). Table I. Petrographic examinations of the crystals showed On (010) the extinction angle, n, A a,is 6 to 7" in that the double salts closely resemble monocalcium acute p. The OAP has approximately the same phosphate monohydrate in habit, form and twin- orientation as in the ammonium salt. The intensities of the lines in the X-ray powder (1) R. Flatt, G. Brunisholz and 9. Chapuis-Gottreux, HeEv. Chim. pattern of monocalcium phosphate monohydrate Acta, 3 4 , 884 (1951). differ radically from those of the double salts, but (2) R. Flatt, G. Brunishols and R. Hots. ibid.. 39. 1406 (1956). (3) J. P.Smith, J. R. Lehr and W. E. Brown, A m : Minerkogiat, 40, the positions of many of the lines for the greater 893 (1955). spacings are nearly the same. X-Ray powder data (4) G. MacLennan and C. A. Beevers, Acta Crust., 9, 187 (1956). for the potassium compound, which can be used for (5) C. A. Beevers and B. Raistrick, Nature (London), 173, 542 the identification of both isomorphs, are given in (1984). (6) W. A. Wooster. 2. k'rist.. 94, 375 (1936). Table 11. The unit-cell constants are given in (7) H.H. Willard and A. W. Boldyreff. J . A m . Chem. Soc., 62, 1888 Table 111, and are compared with a redetermined (1930). set of values for monocalcium phosphate mono(8) A. Gee and V. R. Deitz, AnaE. Chem., 26, 1320 (1953). hydrate and reported values for monocalcium (9) M. J. Buerger, "X-Ray Crystallography," John Wiley and Sons, Ino., New York, N. Y., 1942. chlorophosphate. 10
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W. E. BROWN, J. P. SMITH, J. R. LEHRAND A. W. FRUIER
Vol. 62
can account for the positions of the ammonium and potassium ions. (1) The crystals have symmetry P1. Thus, d I d I d I there are no symmetry restrictions on the position 11.9 S 2.96 M 2.03 VW of these ions and the probable ideal formula is 5.95 W 2.82 VW 1.96 VW CazI).HIO 5 G7 A. 5.76 Asb 5.72 A. 5.79 A. 11.92 17.14 12.13 12.52 6.51 6.41b 6.46 6.50 98' 11' 90" 98" 44' 97" 57' 118" 31' 119O 0' 118" 1' 117' 45' 83" 9' 90 9 5 O 54; 98" 0' 5.89 A. 8.57 A. 5.90 A. 5.99 A.
Lattice constants a b C C Y '
P Y dozo
Density, g./cc. 2.17 2.33 2.19 2.28 Calcd. Pycnometric 2.10 2.35 2.22 2.*29 2.22 2.36 2.23 2 30 From refractive indexes" Formula wt. per unit cell 1 1 2 4 See E. S. Larsen and H. Berman, U. S. Geol. Survey Bull. 848 (2nd ed.) 1934. a Gladstone and Dale equation. ported axes of Walter-LBvy, et uE.,'O are interchanged for this comparison.
The apparent high stability of the Ca-PO4 sheets suggests that other calcium phosphates may contain similar sheets. The unit-cell constants for the calcium chloride phosphate, CaC1HzP04.Hz0,in Table III are given to show that this compound has dimensions a, c and 0,that are close to those of the other salts. The plate-like habit of this compound is characteristic of the others and is in accord with the same Ca-PO4 sheet structure being present in this compound. An alternative selection of unit-cell constants in the a-c plane has a = 5.76 A., c = 6.20 .& and 0 = 115' 20'. This may be the correct set for comparison with the other salts, although the fit is not quite as good. The similarities noted between the unit-cell constants apparently were not recognized by Walter-Levy and co-workers.'O It is reasonable to assume that the monohydrated bromide salt CaBrHzP04.Hz0,reported by WalterLBvy and Vincent,ll also possesses the same layer structure. (11) L. Welter-L&vy and J. P. Vincent, Compt. rend., 241, 1207 (1955).
Re-
Chlorospodiosite, Ca2PO4C1,has lattice $mensions12 u = 6.17, b = 6.89 and c = 10.74 A. Of these, a and c are nearly the same as two dimensions in the Ca-P04 sheet o[ dicalcium phosphate dihydrate, 6.239 and 10.84 A. The latter dimensions are for a nearly orthogonal b-centered cell derived from the cell given by Beevers and R a i ~ t r i c k . ~ A slight distortion of the Ca-PO4 sheet along with the extra calcium ions could easily increase the symmetry in the sheet to satisfy the requirements of the orthorhombic space group of chlorospodiosite. The occurrence of the corrugated sheet in chlorospodiosite would be of considerable interest in that it would extend this feature to a thermally stable anhydrous compound wore basic than dicalcium phosphate. The corrugated sheet, therefore, may ocour in many alkali and alkaline earth salts containing the tetrahedral XOrtype anion. Acknowledgment.-Mrs. Inez J. Murphy made the chemical analyses. (12) A. L. Mackay, Mineralog. Mag., SO, (No. 222) 166 (1953).
THE ACTIVITY COEFFICIENT OF HYDROCHLORIC ACID I N POTASSIUM CHLORIDE SOLUTIONS BY HERBERT S. HARNED AND ALANB. GANCY Conti,ibulion No. 1484 f r o m the Department of Chemistry of Yale University, New Haven, Conn. Receiued Februa7y 9, 1958
The activity coefficient of hydrochloric acid in potassium chloride solutions a t constant total molalities at 25" has been determined from suitable cells without liquid junctions. Anal sis of the results confirms the conclusion of McKay that although the logarithm of activity coefficient of the acid varies fnearly with its concentration, the logarithm of the activity coefficient of potassium chloride varies quadratically with ?ts concentration.
Measurements of cells of the type
relationship is valid, the question arises as to whether a similar equa t'ion
H2/HCl(ml),MCl(m2)IAgC1-Ag
indicate that the activity coefficient, y , of the acid varies linearly in a mbture of constant total molality,m. Thus log
YI
= log Y N O )- mm2
(1)
where log -yl(a) is the activity of the pure acid of concentration ml = rn, and m2 is the coilcentration of the salt. For systems for which this linear
log yz = log
7x0)
- a21m1
(2)
holds for the second electrolyte. For systems for which equation 1 is found valid, McKay' has devised an ingenious method of computing olzl without the assumption that equation 2 is valid. For systems coiitaiiiing two uni-univalent electrolytes (1) H. A. C. McRay, Trans. Faradag Soc., 51, 003 (1055).
