Calcium hydrogen citrate trihydrate

Abstract

CaC6HdO7 .3H20, prepared by slow precipitation from an aqueous mixture of calcium chloride and sodium citrate at a pH of 4.0, is monoclinic, P21/c, with Z = 4 , a=8.7955 (54), b=5.5891 (15), c=23.8176 (175) A, fl= 116.770 (57) °, F.W. 284.2, V= 1045.35 A 3, Dx = 1.806 g cm-3, Dm= 1.79 g cm -3; 2(graphite monochromatized Cu K~1)=1.54051 ,~. The final R, for 1317 reflexions, was 0-0756. Each calcium ion is surrounded by seven oxygen atoms, two of which form bridges to another symmetry-related calcium ion. Introduction. There are three possible calcium citrate compounds with calcium to citrate ratios of 1:2, 1:1 and 3:2. The last type is found naturally in some insects (Parker & Rudall, 1955; Kenchington, 1974) and cell dimensions of the tetrahydrate have been published by Pogainis & Shaw (1957). The crystals of calcium hydrogen citrate (calcium to citrate ratio= 1:1) were obtained as well faceted needles and a crystal with dimensions 0.2 x 0.2 x 0.5 mm was selected for examination. The data were collected on an Enraf-Nonius CAD-4 computer-controlled diffractometer by the movingcrystal moving-counter technique in the range 3°< 0 < 70 °. Of 1810 independent reflexions accessible in principle, 1317 were measured and used in the analysis. The systematic absences, 0k0 for k = 2n + 1 and hOl for l = 2 n + 1, determined the space group P21/c (No. 14). No correction for absorption was made. Lorentz and polarization factors were applied to the intensity data and approximate values of the temperature and scale factors were obtained from a Wilson plot. The position of the calcium ion was established from a three-dimensional Patterson map and the carbon and oxygen atoms from a three-dimensional electron-density map calculated with the phases specified by the structure factors calculated from the coordinates of the calcium ion only. The positional and anisotropic thermal parameters were refined by block-diagonal least-squares calculations on the 1906A computer of Leeds University Computing Laboratory, with modified versions of the programs of Cruickshank, Pilling, Bujosa, Lovell & Truter (1961), to an R value of 0.0796. A difference synthesis was calculated and the coordinates of the hydrogen atoms were found. Refinement of the parameters was carried out, anisotropically for all atoms except the hydrogens, which were treated isotropically, to produce the final residual R=0.0756. The atomic scattering factors used were those in International Tables for X-ray Crystallography (1962) and the weighting scheme used was l/w= 1/{1 +[(IFol-P2)/pl]2} 1/z with px=FMax/lO'5 and P2 = FMax/525.* Discussion. A d iagram showing two symmet ry re l a t ed molecules and the n u m b e r i n g of the a toms is given in Fig. 1. A tomic coord ina tes with thei r es t imated stanTable 1. Atomic coordinates and estimated standard deviations