Conductometric Studies of Calcium Ions with Kryptofix 221 in Mixed MeOH-DMF Solvents at Different Temperatures

On using conductometric technique, the apparent association constant (KA) of Ca (NO3)2 were measured in mixed MeOH-DMF mixed solvents at 0, 20, 40 , 60 ,80 and 100% MeOH (by volume) and different temperatures (298.15, 303.15, 308.15 and 313.15K) in absence and presence of . Kryptofix221 [4, 7, 13, 16, 21pentaoxa-1.10-diazo-bicyclo (8,8,5) tricosane]. From the experimental results, the molar conductivities (∧) were calculated and limiting molar conductivities (∧0) were calculated by using Shedlovsky and Fouss – Kraus extrapolation methods. The molar solvated (∨), Van der Waals (∨w) and electrostriction (∨e) were evaluated. The association constants KA for Ca(NO3)2 were calculated by using simple equation derived from Ostwald, Arrhenius and Fuoss-Shedlovsky theories. From the association constants the different thermodynamic parameters were evaluated and the results obtained were discussed. The free energy of association ( A G ∆ ), free energy necessary for complexation ( complex G ∆ ) and enthalpy (ΔH ) were determined. It was concluded 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 work give data can help for the analytical determination of calcium in blood and water samples.


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 measurements 1 .
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.

Experimental
Kryptofix 221 (Fig.1) was supplied from Fluka Co., whereas Ca(NO 3 ) 2 was supplied from BDH. The water content of Ca(NO 3 ) 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(NO 3 ) 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. 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.

Results and Discussion
The molar volumes of Ca(NO 3 ) 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(NO 3 ) 2 were evaluated (equation 1) [16,17].
The electrostriction volume (V e ), which is the volume compressed by the solvent can be calculated by using equation (2) [18] as follows: V e = V W -V M (2) All the evaluated different volumes for calcium nitrate in mixed MeOH-DMF solvents were tabulated in Table ( The specific conductance (K S ) of different dilutions of Ca(NO 3 ) 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] (2 ) . 4 .
Thus equation (8) can be applied for Ca(NO 3 ) 2 as dilute solution because this equation depends on Fuoss-Shedlovsky and Ostwald dilution laws.
The different Λ , Λ o for Ca(NO 3 ) 2 solutions were evaluated at concentration equal 8x10 -5 as inflection point and zero concentration, respectively. K A association for Ca(NO 3 ) 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][21][22][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).
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).  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 ∆G C and ∆H values cited in Tables 3 and 4, respectively.