# Influence of Water Addition on the Latent Heat Degradation of Sodium Acetate Trihydrate
## 3 Aging Study of Sodium Acetate Trihydrate
### Resistance to the Aging of a Sample Having Additional Water  
To evaluate the influence of water on the aging of SAT, similar cycles were applied to the SAT58% sample. For better clarity, the temperature evolution of this experiment is given in Figure 13.  
As before, the 3 rd, 4 th, and 36 th cycles, whose heating and cooling rate were about 2.5 \({}^{\circ}\)C\(\cdot\)h\({}^{-1}\), were dedicated to the study of latent heat evolution under different conditions [19]. Because of their different thermal history, the data could not be included in the global analysis and will be later plotted as stars. This also concerns the 5 th and 37 th cycles, whose thermal history was impacted by the previous heating and cooling steps. These five cycles were not considered for the probability distribution of the crystallization temperatures that are presented in Figure 14 with a 1 \({}^{\circ}\)C step.  
Better repeatability of the crystallization temperature was found in these 35 cycles (after exclusion of the 1 st, 3 rd, 4 th, 5 th, 36 th, and 37 th) with values between 45 and 47 \({}^{\circ}\)C. It can be noted that the temperature distribution is more homogeneous. As shown in Figure 4, a copper welding rod was previously inserted in the liquid SAT58%. Copper may have been corroded by SAT and particles may be present in the SAT's solution, promoting the solidification of the material.  
The mean temperature obtained was \(T_{C}=46.1\,^{\circ}\)C, with a standard deviation \(\sigma~{}=~{}0.5\,^{\circ}\)C. The whole crystallization temperatures are included in the distribution range and the consideration of singular points would not be needed for this SAT58% sample. The evolution of the latent heat throughout cycles was then determined, and the results are shown in Figure 15.  
Figure 13: Evolution of temperature throughout aging cycles.  
This sample presents an initial latent heat between 220 and 230 J\(\cdot\)g\({}^{-1}\) while the first cycles are difficult to consider because of a different thermal history. The trend is more obvious for this sample containing an addition of water, with a coefficient of determination \(R^{2}=0.99\). The decrease of latent heat is also more important, up to 25% of its initial value throughout 35 cycles, and continues for the 5 remaining cycles. Besides, this trend was verified for a slower cooling rate (2.5 \({}^{\circ}\)C\(\cdot\)h\({}^{-1}\)) for the 3 rd, 4 th, and 36 th cycles, plotted as stars. Their crystallization temperatures were close to each other (respectively 49.7 \({}^{\circ}\)C, 51.1 \({}^{\circ}\)C and 50.9 \({}^{\circ}\)C). This behavior was similar to the observations of [PERSON] et al. [14] where a decrease of 8% (from 194 to 179 J\(\cdot\)g\({}^{-1}\)) was found after 20 cycles for a concentration \(w_{SA}\)=56.5 wt.%.  
Figure 14: Probability distribution of crystallization temperatures for SAT58%. In the inset, the crystallization temperature has a 0.1 \({}^{\circ}\)C temperature step.  
Figure 15: Evolution of (**a**) the latent heat and (**b**) the supercooling degree over 40 cycles of a SAT58% sample (\(w_{SA}=0.577\)). Values for 3 rd, 4 th, 5 th, 36 th, and 37 th are plotted as stars. As the open symbols (following a low-temperature crystallization), they will not be considered for exponential fit and aging analysis. \(T_{M}\) and \(T_{C}\) respectively represent melting and crystallization temperatures.  
The water addition, rather than increasing the aging stability of the SAT as proposed by [PERSON] et al. (2019), tends to diminish it. Then, to explain the behavior of the different samples, visual observations of the aged samples were performed.