Determination of constant-volume combustion energy for the compounds of L-threonic acid with Mg (II), Mn (II), Co (II) and Ni(II)

Abstract Four solid compounds of L-threonic acid with Mg(II), Mn(II), Co(II) and Ni(II) have been prepared. The constant-volume combustion energies of the compounds, , have been determined by a precision rotating bomb calorimeter at 298.15K. The standard enthalpies of combustion, , and standard enthalpies of formation, , are calculated for these compounds.
Keywords: L-threonic acid, Transitional metal, Combustion energy, Standard enthalpy of formation

1 EXPERIMENTAL
1.1 Preparation and composition of the compounds            
For the preparation of the compounds, 0.01 mol of calcium L-threonate was dissolved in 200 mL of  water. Keeping agitated, 0.01 mol of oxalic acid was added into the above solution. After 1 hour, a large mount of precipitate appeared. The mixed reaction system was filtered and the filtrate was obtained. Excessamount of metallic oxide was added into the filtrate when agitating. The reaction proceeded further 4h at 80ºC，then cooled to room temperature and kept agitating for 5h, followed by suction filtration, concentrating the filtrate up to about 30 mL and adding 100 mL alcohol, the deposition was formed. After filtration and rinsing for 2 times with a little alcohol, the precipitant was dried in vacuum to the constant weight. The compositions of the compounds were listed in Table 1. The content of M²⁺ was complexometerically determined. The contents of C, H were analyzed with a 2400-type elemental analyzer of PE Company. The purity of the compounds was greater than 99.60% by HPLC.
1.2 Apparatus and experimental conditions               
The constant-volume combustion energy of the compound was determined by a precise rotating bomb calorimeter (RBC-type II).^[6] The main experimental procedures were described previously. ^[5] The initial temperature was regulated to 25.0000 ± 0.0005ºC, and the initial oxygen pressure was 2.5 MPa.
    The correct value of the heat exchange was calculated according to Linio-Pyfengdelel-Wsava formula. ^[7]
     The calorimeter was calibrated with benzoic acid of 99.999 % purity. It had an isothermal heat of combustion at 25ºC of -26434 ± 3 J/g. The energy equivalent of calorimeter was determined to be 17936.01 ± 9.08 kJ/K. The precision of the measurements was in 4.68×10^-4.
    The analytical methods of final products (gas, liquid and solid) were the same as these in Ref. 5. The analytical results of the final products showed that the combustion reactions were complete. As a result, either carbon deposits or carbon monoxide formed during the combustion reactions and the amount of NO_x in the final gas phase were negligible.

2 RESULTS AND DISCUSSION
2.1 Combustion energy of the compounds           
The determination method of combustion energy for the complexes was the same as for the calibration of the calorimeter with benzoic acid. The combustion energies of the samples were calculated by the formula
                                                                           (1)    
where (complex, s)denotes the constant-volume combustion energy of the samples, W is the energy equivalent of the RBC-type II calorimeter (in J/K), the correct value of the temperature rising, a the length of actual Ni-Cr wire consumed (in cm), G the combustion enthalpy of Ni-Cr wire for ignition (0.9 J/cm), 5.983 the formation enthalpy and solution enthalpy of nitric acid corresponding to 1 mL of 0.1000 mol/L solution of NaOH (in J/mL), b the volume in mL of consumed 0.1000 mol/L solution of NaOH and m the mass in g of the sample. The results of the calculations were given in Table 2.
2.2 Standard combustion enthalpies of the compounds                 
The standard combustion enthalpy of the compounds, (compound, s, 298.15 K), referred to the combustion enthalpy change of the following ideal combustion reaction at 298.15 K and 100 kPa.
M(C₄H₇O₅)₂ · H₂O(s) + 7O₂(g) = MO(s) + 8CO₂(g) + 8H₂O(l)                                        (2)
M = Mg, Co    
Mn(C₄H₇O₅)₂ · H₂O(s) +O₂(g) = MnO₂(s) + 8CO₂(g) + 8H₂O(l)                                  (3)
Ni(C₄H₇O₅)₂ · 2H₂O(s) + 7O₂(g) = NiO(s) + 8CO₂(g) + 9H₂O(l)                                      (4)
    The standard combustion enthalpies of the compounds calculated by the following equations:
(compound, s, 298.15K) = (compound, s, 298.15K) +RT                           (5)
= n_g (products) - n_g (reactants)                                                                                     (6)
    where n_g is the total amount in mole of gases present as products or as reactants, R = 8.314 J·K^-1·mol^-1, T = 298.15 K. The results of the calculations were given in Table 3. For comparison, the relevant data of zinc L-threonate and calcium L-threonate were also given in Table 3^[8].

2.3 Standard enthalpies of formation of the compounds               
The standard enthalpies of formation of the compounds, (compound, s, 298.15 K), were calculated by Hess's law according the above thermochemical equation (2) to (4):
(compound, s, 298 K) = [ (MO, s, 298 K) + 8(CO₂, g, 298.15 K)+ 8(H₂O, l, 298.15K)] - (compound, s, 298.15 K)                                                                                                           (7)
M = Mg, Co,
[Mn(C₄H₇O₅)₂ · H₂O, s, 298.15 K] = [(MnO₂, s, 298 K)+ 8(CO₂, g, 298.15 K)+8(H₂O, l, 298.15 K)] - [Mn(C₄H₇O₅)₂ · H₂O]                                                                                          (8)
[Ni(C₄H₇O₅)₂ · 2H₂O, s, 298.15K] = [ (NiO, s, 298 K) + 8(CO₂, g, 298.15 K)+9(H₂O, l, 298.15K)] - [Ni(C₄H₇O₅)₂ · 2H₂O, s, 298.15 K]                                                                            (9)
where(MgO, s, 298.15 K) = (-601.50 ± 0.30) kJ · mol^-1, (MnO₂, s, 298.15 K) = -520.03 kJ · mol^-1, (CoO,s,298.15K) = -237.94 kJ · mol^-1), (NiO, s, 298.15 K) = -239.70 kJ · mol^-1, (CO₂, g, 298.15 K) = (-293.51 ± 0.13) kJ · mol^-1, (H₂O,l,298.15K) = (-285.83 ± 0.04) kJ·mol^-1[9].
The results of the calculations were also shown in Table 3.

REFERENCES              
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