The 500-MW(electric)-capacity Prototype Fast Breeder Reactor (PFBR) using sodium as a coolant is under construction at Indira Gandhi Centre for Atomic Research, Kalpakkam, India. Instrumentation for the measurement of physical and chemical characteristics of liquid sodium in reactor coolant and related applications is described. Various properties of liquid sodium are used for measurement of flow, temperature, level, and leaks. Temperature-dependent solubility of impurities in the liquid sodium is a basis principle of instruments for monitoring the general purity and the specific impurities of oxygen, hydrogen, and carbon. This technical note deals with the properties of sodium that are relevant to successful sodium instrumentation used for PFBR.