Heat Capacity and Thermodynamic Property of Lithium Pentaborate Pentahydrate

The heat capacity of lithium pentaborate pentahydrate has been measured using an adiabatic calorimeter at the temperature from 297 to 375K. No phase transition and thermal anomalies were observed.Themolar heat capacity of LiB5O8⋅5H2O can be expressed asCp,m (J⋅mol−1⋅K−1) = 396.79376 + 35.87528 [T−(Tmax+Tmin)/2]/[(Tmax−Tmin)/2]+ 0.16494{[T−(Tmax+Tmin)/2]/[(Tmax−Tmin)/2]}2 + 8.3083{[T − (Tmax + Tmin)/2]/[(Tmax − Tmin)/2]}3, where T is the temperature in Kelvin, Tmax = 375K, and Tmin = 297K. The thermodynamic functions of (HT−H298.15), (ST−S298.15), and (GT−G298.15) of LiB5O8⋅5H2O are obtained via themolar heat capacity at the temperature of 5 K intervals.


Introduction
Studies of alkali borates have attracted much interest in recent years because some of these compounds especially the alkali pentaborates show significant physical interest, such as nonlinear optical behavior [1].The [B 5 O 10 ] 2− boron oxycyclic structure of the alkali metal pentaborate crystal contains the BO 3 group, which has a strong macro frequency effect and can be used in the field of nonlinear optical materials [2].In addition, the lithium pentaborate (LiB 5 O 8 ⋅5H 2 O) also can be used in special glass, lubricating oil additives, and production of boron oxide [3,4].
Heat capacity is one of the important parameters of thermodynamic properties to optimize the producing process.And reliable calculation of such thermodynamic functions of entropy, enthalpy, and Gibbs free energy requires data on heat capacity [5].Furthermore, heat capacity is inherent characteristic of crystals, closely related to the specific features of their composition and structure.To better understand the industrial application of LiB 5 O 8 ⋅5H 2 O and perform the corresponding theoretical studies, the thermodynamic properties of this compound are essential.Li et al. [6] reported the standard molar enthalpy of formation of LiB 5 O 8 ⋅5H 2 O. Ge et al. [7] determined the physicochemical properties of LiB 5 O 8 solutions, including the density, viscosity, conductivity, and pH.However, there is no data reported on the heat capacity of LiB 5 O 8 ⋅5H 2 O in the literature.In this paper, the heat capacity of LiB 5 O 8 ⋅5H 2 O has been determined using an adiabatic calorimeter in the temperature range from 297 to 375 K, and the values of the thermodynamic functions, heat capacity, entropy, enthalpy, and Gibbs free energy, were calculated at temperature 5 K intervals.O were dissolved in the quantificational deionized water under stirring for 24 h at 333.15 K.Then, the product was filtered by suction filtration and washed three times with deionized water and absolute ethyl alcohol, respectively.Finally, the samples were dried until the weight was constant and stored in the desiccators for use.diffractometer with Cu-K radiation at 4 ∘ min −1 ) and thermogravimetric (TG) (performed on a SETARAM LABSYS thermal analyzer under argon atmosphere with a heating rate of 10 K min −1 ).The X-ray powder pattern of the synthesized LiB 5 O 8 ⋅5H 2 O in Figure 1 shows that the diffraction peaks on patterns correspond well in position, indicating the phase purity of the synthesized samples.The weight loss of 32.26% from the TG curve in Figure 2 corresponds to the loss of five water molecules and can be compared with the calculated value of 32.26%.

Characterization and Analytical
The B 2 O 3 concentration was analyzed using gravimetric method with sodium hydroxide standard solution in the presence of mixture indicators of methyl red plus phenolphthalein and the excessive mannitol conditions, and the standard uncertainty (BO 2 − ) was 0.0005 in mass fraction [9].The lithium ion content was measured by inductively coupled plasma optical emission spectrometer (Prodigy, Leman Corporation, America) with precision of ±0.5%.The crystallized water content was calculated through subtraction.The chemical analytical result of LiB 5 O 8 ⋅5H 2 O is listed in Table 1.

Adiabatic Calorimetry and Experiment
Method.Heat capacity measurements were carried out in a high-precision SETARAM BT 2.15 adiabatic calorimeter.The BT 2.15 calorimeter comprises a calorimetric chamber, electrical or pneumatic peripherals, and the liquid nitrogen supply.The calorimetric chamber can receive the heat capacity cell, which  is provided with a syringe for sample introduction.The sample mass used for the heat capacity measurement was 8877.74 mg.The performance of this calorimetric apparatus has been verified by measuring the heat capacity of KCl where the average result is 0.6877 J⋅g −1 ⋅K −1 in seven times and the deviation of the results is 0.0007, compared with the reference data of 0.6879 J⋅g −1 ⋅K −1 [10].The heat capacity  , of the sample was measured ranging from 297 to 375 K by an adiabatic continuous heating technique with a heating rate of 0.1 K/min.

Heat Capacity.
According to the experimental method, the heat capacity   of LiB 5 O 8 ⋅5H 2 O was measured using the adiabatic calorimeter from 297 to 375 K with standard uncertainty 0.05 J⋅mol −1 ⋅K −1 , and the result of the molar heat capacity of LiB 5 O 8 ⋅5H 2 O is shown in Table 2 and Figure 3.
In Figure 3, it is shown that the heat capacity of LiB 5 O 8 ⋅5H 2 O sample increases smoothly with the increasing of temperature in the range between 297 and 375 K without any phase transition and thermal anomaly.
On the basis of the Debye Law [11], the molar heat capacity of LiB 5 O 8 ⋅5H 2 O determined in this work has been fitted and shown in (1).On the basis of (1), the molar heat capacity of LiB 5 O 8 ⋅5H 2 O at 298.15 K can be obtained as 354.54J⋅mol −1 ⋅K −1 .
where  is the absolute temperature in Kelvin,  max is the upper temperature (375 K), and  min is the lower temperature (297 K).The deviations between the experimental and the fitted values are within 0.005 and shown in Figure 4.   (

Enthalpy
The results of the molar heat capacity and thermodynamic functions of (  −  298.15 ), (  −  298.15 ), and (  −  298.15 ) are obtained and listed in Table 3 with a temperature of 5 K interval.From the entropy function data in Table 3, it is shown that the molar heat capacity and the changes of enthalpy (  − 298.15 ) and entropy (  − 298.15 ) are increased with the increasing of temperature from 298.15 K to 375 K and the changes of free energy (  −  298.15 ) are just the opposite.

Figure 3 :
Figure 3: Experimental molar heat capacity of LiB 5 O 8 ⋅5H 2 O in the range of 297 to 375 K.

Figure 4 :
Figure 4: Plot of deviations of experimental data from fitted.
The chemical of LiB 5 O 8 ⋅5H 2 O was synthesized in our laboratory according to the phase diagram of system Li 2 O-B 2 O 3 -H 2 O [8].Certain amounts of H 3 BO 3 and LiOH⋅H 2

Table 1 :
Chemical analytical result of lithium pentaborate pentahydrate in mass fraction.