057057ne

http://www.chemistrymag.org/cji/2003/057057pe.htm	Jul. 1, 2003 Vol.5 No.7 P.57 Copyright

Determination of constant-volume combustion energy for the compounds of L-threonic acid with Na (I), K (I) and Cu (II)

Yang Xuwu, Chen Sanping, Gao Shengli, Shi Qizhen
(Shaanxi Key Laboratory of Physico-Inorganic Chemistry & Department of Chemistry, Northwest University, Xi'an, 710069, China)

Received Feb. 00 .2003; Supported by the National Natural Science Foundation of China (No.20170136) & the Education Committee of Shaanxi Province (No. 01JK229)

Abstract Three solid compounds of L-threonic acid with Na (I), K (I) and Cu (II) respectively have been prepared. The constant-volume combustion energies, of these 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, Metal, Combustion energy, Standard enthalpy of formation

L-threonic acid makes metal ion combination with amino metallic carriers, as acid or protein for the absorption and application by the life ^[1]. Calcium L-threonate, an excellent calcium additive, has 95% absorption and is 2-3 times than that of the traditional calcium additive.^[2-3] Like calcium, magnesium is an essential life element. Manganese, cobalt and nickel are the essential trace elements of the life, which connect with various metal kinase and enzymes. Thus it is significant to investigate on the compounds of L-threonic acids and these elements. The preparations of the compounds of Fe²⁺, Zn²⁺, K⁺, Na⁺, Mg²⁺, NH₄⁺, Cr³⁺ with L-threonic acid have been reported in the literatures [4-5]. Chen Sanping and coworkers synthesized the compounds of calcium, zinc, magnesium, manganese, cobalt and nickel with L-threonic acid, reported the standard enthalpies of formation of the compounds. ^[6-11] However, study on Na (I), K (I), Cu (II) with L-threonic acid has not been reported.
In this paper, three solid compounds of L-threonic acid with Na (I), K (I) and Cu (II) respectively. have been prepared. The constant-volume combustion energy of compounds, have been determined. The standard enthalpies of combustion, and standard enthalpies of formation， are calculated for these compounds. This work enriches the thermochemical database and provides a theoretical basis for further study on their properties and applications.

1 EXPERIMENTAL
1.1 Preparation and composition of the compounds
For 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 one hour, a large amount of precipitant appeared. The mixed reaction system was filtered and the filtrate was obtained. Excess amount of metallic oxide was added into the filtrate while agitating. The reaction proceeded at 80ºC further for 4h， 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 with a little alcohol for 2 times, the precipitant was dried in vacuum to the constant weight. The compositions of the compounds were listed in Table 1. The content of metal(M) was complexometerically determined. The contents of C, H were analyzed with a 2400-type elemental analyzer of PE Company. The purity of the compounds greater than 99.60% was determined by HPLC.

Table 1 Analytical results related to the composition of the compounds (in %)^a

Compounds	M	C	H
NaC₄H₇O₅·H₂O	13.09(13.06)	27.54(27.28)	5.14(5.15)
KC₄H₇O₅·H₂O	20.48(20.34)	25.21 (24.99)	4.75(4.72)
CuC₄H₆O₅·0.5H₂O	30.87(30.75)	23.40 (23.25)	3.40 (3.41)

^{a The data in brackets are
calculated values.}

1.2 Apparatus and experimental conditions
The constant-volume combustion energy of the compound was determined by a precision rotating-bomb calorimeter (RBC-type II).^[12] The main experimental procedures were described previously.^[13]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.^[13]
The calorimeter was calibrated with benzoic acid of 99.999% purity. It had an isothermal heat of combustion at 25ºC of (-26434±5.8) J/g. The energy equivalent of calorimeter was determined to be (18000.71±8.42) J/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 those in Ref.[12].As a result, neither carbon deposits nor carbon monoxide formed during the combustion reactions, the amount of NOx in the final gas phase was negligible and the amount of CO₂ was within 98.8- 99.0% of the calculated value. The analytical results of the final products showed that the combustion reactions were complete.

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

where denotes the constant-volume combustion energy of the samples(in J), W is the energy equivalent of the RBC-type II calorimeter (in J/K), DT 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), 5.97 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 are given in Table 2.

Table 2 Experimental results for the combustion energies of the compounds

Sample	No.	Mass of sample m/g	Calibrated heat of combustion wire Q_c/J	Calibrated heat of acid Q_N/J	Calibrated DT/K	Combustion energy of sample -/J·g^-1
NaC₄H₇O₅·H₂O	1	1.235 43	12.60	25.43	0.7058	10 253.01
NaC₄H₇O₅·H₂O	2	1.260 30	12.60	25.94	0.7191	10 240.24
	3	1.185 36	12.60	24.40	0.6754	10 225.32
	4	1.165 27	12.60	23.98	0.6656	10 250.58
	5	1.183 21	11.70	24.35	0.6760	10 253.83
	6	1.205 34	12.60	24.81	0.6870	10 228.71
	Mean					10 241.95±5.14
KC₄H₇O₅·H₂O	1	1.256 80	12.60	40.39	0.638 9	9 108.58
	2	1.324 57	12.60	42.57	0.671 6	9 085.29
	3	1.276 26	10.80	41.02	0.647 8	9 096.14
	4	1.235 30	12.60	39.70	0.628 7	9 119.04
	5	1.263 25	11.70	40.60	0.641 7	9 102.52
	6	1.203 62	12.60	38.68	0.611 8	9 107.16
	Mean					9 103.12±4.71
CuC₄H₆O₅·0.5H₂O	1	1.235 60	11.70	19.40	0.538 8	7 824.28
	2	1.201 83	10.80	18.87	0.523 1	7 810.17
	3	1.245 00	11.70	19.55	0.543 4	7 831.60
	4	1.300 56	9.00	20.42	0.566 3	7 815.39
	5	1.254 34	12.60	19.70	0.546 4	7 815.50
	6	1.206 85	11.70	18.95	0.526 6	7 829.08
	Mean					7 821.00±3.50

2.2 Standard Combustion Enthalpies of Compounds
The standard combustion enthalpy of the compounds, is referred to the combustion enthalpy change of the following ideal combustion reaction at 298.15K and 100kPa.
MC₄H₇O₅·H₂O(s)+3.5O₂(g)=0.5M₂O(s)+4CO₂(g)+4.5H₂O(l)                                        (1)
M=Na, K
CuC₄H₆O₅·0.5H₂O(s)+3.75 O₂(g)= CuO (s)+4CO₂(g)+3.5H₂O(l)                                   (2)
    The standard combustion enthalpies of the compounds was calculated by the following equations:
= +RT                                                                                                      (3)
=n_g (products)-n_g(reactants)                                                                                        (4)
    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=298.15K. The results of the calculations are given in Table 3.

Table 3 Combustion energies, standard combustion enthalpies and standard enthalpies of formation of the compounds (in kJ·mol^-1)

Compounds	-	-	-
NaC₄H₇O₅·H₂O	1 803.62±0.91	1 802.38±0.91	1 265.84±1.06
KC₄H₇O₅·H₂O	1 749.71±0.91	1 748.47±0.91	1 292.56±1.06
CuC₄H₆O₅·0.5H₂O	1 616.15±0.72	1 616.77±0.72	1 114.76±0.81

2.3 Standard Enthalpies of Formation of the compounds
The standard enthalpies of formation of the compounds, were calculated by Hess's law according to the above equations (1) and (2):
(M C₄H₇O₅·H₂O, s)= [0.5 (M₂O, s) +4(CO₂, g) +4.5(H₂O, l)]- ( MC₄H₇O₅·H₂O,s)           (5)
M=Na, K
(Cu C₄H₆O₅·0.5H₂O,s)=[ (CuO,s)+4(CO₂,g)+3.5(H₂O,l)]-(CuC₄H₆O₅·0.5H₂O,s)              (6)
    where (Na₂O,s)= -415.89 kJ·mol^-1, (K₂O,s)= -361.50kJ·mol^-1,(CuO,s)= -155.23kJ·mol^-1.^[14]
The results of the calculations are also shown in Table 3.

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