The possibility of long-term storage of standard solutions which do not react chemically with air and with the vessel walls was studied for 3 types of the vessels. In this case, a change in concentration can occur only due to the water evaporation. The amount of evaporated water and thus the concentration change was determined over time on a balance by the decrease of the vessel mass. It was established that, if some precautions were taken, standard solutions may be stored for a long time (many months) in glass volumetric flasks and in PE vessels without significant change of their concentration. Some aspects of the volumetric trilone titration of metals, its precision and accuracy of the standard solutions are discussed. It is shown that when analyzing metal salt hydrates, it is possible to find the water content with acceptable accuracy by determining only one component (metal). But this determination should be carried out with high precision (± 0.1 rel. %). This also requires the preparation of standard metal solutions with the same accuracy. The most convenient method for that is to prepare their salts and salt hydrates with a high degree of stoichiometry. Examples of such salts are given. The method of controlled isothermal drying of the higher salt hydrates can be used to synthesize these salts. The main difference between such drying and conventional drying is the necessity to record the composition of the sample in the former method. Using the results obtained earlier for the processes of thermal decomposition of the lower LaCl₃ hydrates, a clear thermodynamic justification of the method of controlled isothermal drying is given. It was found that long-term storage of standard solutions in PET bottles is impossible due to the permeability of their walls to the water vapour. It is also impossible to store hygroscopic substances in such bottles.