1. Introduction

Calcium is an essential element in living organism.It plays an important role in the metabolism of nitrogen in some plants, where a deficiency of calcium leads to poor absorption of nitrogen. Lack of calcium nutrition leads to a reduction in the number and size of chloroplants.Cacium in the most abundant inorganic element in the higher animals, and is located principally in the bones and teeth as apatite, a calcium mineral.Bllod is also a huge reservoir of calcium in animals. Calcium is distributed throughout all tissues where it has special roles in controlling g nerve impulse transmission, muscle action, blood cotting and cell permeability. Calcium deficiency is exhibited by the outset of rickets, failure of blood-clotting, nervous disorder andconvulsive muscular contractions. Vitamin D greatly improves the absorbability of calcium ion. Large intakes of calcium lead to excessive calcification and kidney stones.

Almost all natural waters, including seawater, contain either or both calcium carbonate and calcium sulphate.

Many organisms concentrate calcium in their shells or skeletons. For example calcium carbonate is formed in shells of oysters. Calcium has significant applications in the production of alloys with lead and aluminium and in industrial products of metals from its oxides.It is also used as deoxidizer with iron, steel and its alloys.[1,2]. The main target is to data help for calcium analysis.

The association of salt solutions were studied by Barthel [3] and Deepa[4]. The ion solvent interactions were explained in different electrolytes and polymers using conductivity, relaxation and FTIR measurements¹.

Schori et al.[5,6] used conductivity as an electrical method for studying complexation of Na⁺ with some crown ethers in DMF and in diemthoxyethane solutions.

Different Fuoss theories, Fuoss-Shedlovsky, Fuoss-Kraus and Fuoss-Debye theories[7,8]were used for the estimation of the association constants (K_A) for symmetric electrolytes (1:1 electrolytes).

We apply here simple equation for determining K_A for asymmetric 1:2 salts in solutions based on Fuoss-Shedlovsky and Ostwald law[8]. Evaluation of individual ionic molar volumes[9], preferential solvation of drugs[10] partial molar volumes[11], ligands used for selective metal extraction[12], ionic salvation[13], potntiometric study[14] and specific salvation[15] are some trends of this area of physical inorganic chemistry.

2. Experimental

Kryptofix 221 (Fig.1) was supplied from Fluka Co., whereas Ca(NO₃)₂ was supplied from BDH. The water content of Ca(NO₃)₂ was determined by using (Mettler DL-18) Karl-Fisher titrator and it was found to be less than ±0.01%. methanol (MeOH) and N,N-dimethylformamide (DMF) used were from BDH. All the conductometric titrations were manipulated using 1x10^-3 mol/l Ca(NO₃)₂ and 1x10^-4 molal of Kryptofix 221 as stock solutions. The specific conductivity (K_S) measurements were achieved at different temperatures using a conductometer of the type YSI model-35 connected with an ultra thermostatic water bath of the type Kottermann-4130.

Figure 1. Kryptofix-221 [4,7,13,16,21-pentaoxa-1.10-diazo-bicyclo [8,8, 5] tricosane]

The densities were measured by weighing 1 ml solution in weighing bottle using four digital weighing balance of the type Mettler Toledo DA analytical balance.

3. Results and Discussion

The molar volumes of Ca(NO₃)₂ stork solution (V_M) were calculated by dividing the molecular weight of the salt by the measured densities. From the packing density (P) as reported by Kim[16,17], i.e., the relation between Van der Waals volume (V_W) and the molar volume (V_M) for large molecules (M.W. above 40) the Van der Waals volumes for Ca(NO₃)₂ were evaluated (equation 1)[16,17].

(1)

The electrostriction volume (V_e), which is the volume compressed by the solvent can be calculated by using equation (2) [18] as follows:

(2)

All the evaluated different volumes for calcium nitrate in mixed MeOH-DMF solvents were tabulated in Table (1) at 298.15, 303.15, 308.15 and 313.15 K.

The specific conductance (K_S) of different dilutions of Ca(NO₃)₂ in mixed solvents as well as in the presence of Kryptofix 221 were measured experimentally, from which the molar conductance (Λ) were calculated. The limiting molar conductance (Λ_o) of these solutions were obtained by extrapolating the relation between Λ and square root of concentration to zero concentration. The association constant K_A of different 1:1 (symmetric) electrolytes can be estimated by using Fuoss-Shedlovsky equations [19,20].equation (3)

(3)

Where

ε is the dielectric constant, η_o is the viscosity of the solvent, T absolute temperature and S(Z) is the Fuoss-Shedlovsky factor.

With the use of equation (3) for 1: 2 electrolytes, small values are obtained for K_A . Therefore another simple equation can be applied by following equations (4-8)

(4)

(5)

(6)

(7)

(8)

Thus equation (8) can be applied for Ca(NO₃)₂ as dilute solution because this equation depends on Fuoss-Shedlovsky and Ostwald dilution laws.

The different, _o for Ca(NO₃)₂ solutions were evaluated at concentration equal 8x10^-5 as inflection point and zero concentration, respectively.

K_A association for Ca(NO₃)₂ in absence and presence of Kryptofix 221 were evaluated and their values are given in Tables 2 and 3, knowing that S(Z) factor for all solutions were found to be approximately one.

The Gibbs free energies of association were calculated by applying equation (9) [20-23] in absences and presences of Kryptofix 221 in mixed MeOH-DMF solvents at different temperatures and their values are shown also in Tables (2) and (3).

(9)

From the difference between association Gibbs free energies in presence and absence of the ligand (Kryptofix 221), the Gibbs free energies for complexations were evaluated and listed in Table (3).

Table 1. Molar volumes (VM), Van der Waals volumes (VM), electrostriction volumes (Ve) and solvated radii Rs for Ca(NO3)2 in mixed MeOH-DMF solvents at different temperatures

Table 2. Molar conductance (Λ), limiting molar conductance (Λo), activity coefficient (γ±), association constant (KA) and Gibbs free energies of association (Δ GA) for Ca(NO3)2 in absence of Kryptofix in mixed MeOH-DMF solvent at different temperatures Vol.% () MeOH T Λ Λo γ± KA(x10-6) ΔGA (k.J/mole) 0 298.15 130 140 0.9423 981 -34.204 303.15 170 200 0.944 2676.643 -37.308 308.15 205 255 0.944 4135.660 -39.038 313.15 170 205 0.9497 3241.575 -39.038 20 298.15 124 143 0.9380 1045.895 -34.364 303.15 188 230 0.991 4496.876 -38.616 308.15 197 250 0.940 4788.522 -39.414 313.15 160 200 0.9370 4344.911 -39.800 40 298.15 156 168 0.9342 998.266 -34.208 303.15 232 280 0.937 3352.078 -37.875 308.15 266 335 0.936 4586.085 -39.303 313.15 224 280 0.9331 4381.307 -39.822 60 298.15 145 156 0.9312 988.899 -34.228 303.15 222 270 0.934 3580.272 -38.041 308.15 242 300 0.933 4132.008 -39.036 313.15 216 272 0.9300 4641.931 -39.937 80 298.15 126 141 0.9300 1683.263 -35.543 303.15 266 276 0.932 3709.634 -38.131 308.15 252 315 0.930 4410.564 -39.203 313.15 220 257 0.9271 4448.200 -39.855 100 298.15 115 130 0.9282 1889.301 -35.829 303.15 180 212 0.921 2839.127 -37.457 308.15 212 265 0.928 4006.364 -38.957 313.15 202 255 0.9250 3715.38 -39.393

Table 3. The values of limiting molar conductance (Λo), molar conductance (Λ), activity coefficient (γ±), association constant (KA) Gibbs free energy of association (ΔGA) and compelxation free energies (ΔGc) for Ca(NO3)2 in (MeOH-DMF) in presence of Kryptofix-221

Table 4. Solvation enthalpies of Ca(NO3)2 in presence and absence of Kryptofix 221 (in k.J/mole) Vol.% () MeOH ΔH (without ligand) ΔH in presence of (Kryptofix-221) 0% -54.173 -53.263 20% -50.395 -26.546 40% -102.926 -52.741 60% -107.763 -43.929 80% -52.685 -54.407 100% -27.977 -73.905

The enthalpies of solvation for Ca (NO₃)₂ in absence and presence of Kryptofix 221 were obtained graphically from the relation between log K_A against with slope equal. The evaluated data were listed in Table (4) in absence and presence of ligand.

It was concluded from the given tables that the maximum complexation between calcium ions and Kryptofix-221 were by using 80% and 100%. MeOH in the mixtures. Where as the least complexing ability at 40% and 60% by volume of MeOH. This behaviour was indicated from ΔG_C and ΔH values cited in Tables 3 and 4, respectively.

4. Conclusions

We tried in this work to give data that can help for biological and analytical determination of calcium ions in blood,tissues,water and industrial samples.The free energy values for calcium ions are great in 80% MeOH-DMF and pure MeOH, whereas the conductance values are great in 20 % MeOH-DMF and pure DMF.These data explains the ion solvent interaction trend in the same order. All the thermodynamic parameters for calcium ions are greater in presence of Krytofix 221 than in absence of it, favouring more complexing character and ease of detection on using Kryptofix221. The ligand used can be extracted by chloroform and by evaporating the solvent the solid can be obtained for further use.