In this study, heat-induced transitions and character of the hydration shell of sodium salts of humic and fulvic acids originating from various sources were investigated using ultrasonic velocimetry in the temperature interval from 5 to 90 degrees C. It has been concluded that predominantly the relaxation part of the adiabatic compressibility plays an important role below 1 g L-1 in contrast to both relaxation and intrinsic parts of the compressibility being important at higher concentrations.
Dilution brought several temperature induced transitions which were investigated with respect to composition of all investigated humic substances.