**4.2. Hydroxides**

According to research, the 2,497,819 and 4,151,721 U.S. patents proposed an arrangement for absorption of refrigeration systems based on aqueous solutions of hydroxides in order to present alternative mixtures with the aim to avoid the problems presented by the conventional working fluids. The patents propose soluble basic hydroxides such as sodium and potassium [15–17], and cesium hydroxide was considered later [15]. Subsequently, an aqueous mixture of ternary hydroxide was developed [18], and its performance was theoretically compared with the solution of lithium bromide showing promising results [19]. To calculate and optimize absorption processes, accurate data about the different properties of the mixtures are required. Unfortunately, the available information is scarce, and in most cases, it does not cover the full range of concentration and operating temperatures. However, **Table 3** presents different properties for sodium hydroxide and potassium hydroxide according to the analysis of authors.

Based on this lack, we can conclude that there is an area of knowledge yet to be explored.
