# Influence of Water Addition on the Latent Heat Degradation of Sodium Acetate Trihydrate
## 3 Aging Study of Sodium Acetate Trihydrate
### The Initial Drop of Latent Heat during Preliminary Cycles  
In order to verify the repeatable apparition of solidification and melting before the aging analyses and compare the measured latent heat with Equation (3), preliminary \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **SAT58\%** & \
\begin{tabular}{c} **Number of** \\ **Experiments** \\ \end{tabular} & **SAT60\%** &
\begin{tabular}{c} **Number of** \\ **Experiments** \\ \end{tabular} \\ \hline Mean \(L\) value (J-g\({}^{-1}\)) & \(214\pm 3\) & \(253\pm 2\) & \(253\pm 2\) & \(3\) \\ Difference with Equation (3) (\%) & 2 & & \(1\) & \(3\) \\ Mean \(C_{P,S}\) value (20; 28 \({}^{\circ}\)C, J-kg\({}^{-1.\circ}\)C\({}^{-1}\)) & & & \(2021\pm 19\) & \(4\) \\ Difference with Equation (1) (\%) & & & \(0.2\) & \(4\) \\ Mean \(C_{P,L}\) value (63; 70 \({}^{\circ}\)C, J-kg\({}^{-1.\circ}\)C\({}^{-1}\)) & & & \(3260\pm 35\) & \(4\) \\ Difference with Equation (1) (\%) & & & \(9.3\) & \(4\) \\ Mean \(C_{P,supercooled}\) value (5; 70 \({}^{\circ}\)C, J-kg\({}^{-1.\circ}\)C\({}^{-1}\)) & \(3199\pm 15\) & \(10\) & & \\ Difference with Equation (1) (\%) & 8 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Initial properties of the sample before aging. Figure 4: Seeding of sodium acetate trihydrate (SAT) with a welding rod. (**a**) Schematic draw of the brick and detailed pictures (after the brick was taken out of the experimental bench) (**b**) initial state; (**c**) insertion throughout the grey tape; (**d**) beginning of crystallization; and (**e**) growth of the crystal. cycles were performed on SAT58%. From Equation (2), the latent heat of melting could be determined and its evolution is given in Figure 5. The uncertainties are given in Table 2.  
It was observed that a phase change (melting and solidification) occurred at every preliminary cycle. In the first three cycles (left blue rectangle), the latent heat determined by Equation (2) differed by a maximum of 2% from Equation (3) from [PERSON] et al. [25]. However, it was suspected that these preliminary thermal cycles had induced the aging of the material. Indeed, a decrease of the latent heat from 220 to 200 \(\cdot\)g\({}^{-1}\) was observed after 12 thermal cycles. In parallel, the bricks containing the different samples were taken out of the experimental bench at the end of the preliminary stage. The segregation of anhydrous salt could be observed at the bottom of the brick. This segregation can either be due to the melting before the filling of the brick (SAT60%, Figure 6a) or the preliminary aging cycles (SAT58%, Figure 6b). According to the phase diagram of sodium acetate--water [23], a peritectic transformation exists during melting of the material for a concentration above \(w_{SA}=0.57\). This transformation is associated with the formation of anhydrous salt and water. Indeed, these two molecules are linked in SAT by a hydrogen bond [27] that can easily be broken. The anhydrous sodium acetate, denser, may fall at the bottom of the brick, whereas water is generally found in the upper part [9]. Thus, the quantities of water and anhydrous salt are  
Figure 5: Preliminary evaluation of latent heat of SAT58%. The arrow symbolizes a reset of the properties by agitation (the two rectangles allow for comparison of the latent heat at the beginning of the experiment and after agitation). Figure 6: Formation of anhydrous salt segregation in (**a**) SAT60% (the colors were modified for contrast accentuation), (**b**) SAT58%, and (**c**) SAT58% after agitation, before insertion in the experimental bench. increased in the global volume, as presented in Figure 7. This phenomenon of aging continues with cycles and reduces the capacity of the global volume to release the latent heat. Thus, it can be considered that the height of the sample also influences aging. As experimented by [PERSON] et al. [9], a sample with a height of 8 cm was more subjected to aging than other samples of 4 or 5 cm. Indeed, the gravity phenomenon and the separation of species would be accentuated for samples of important heights.