EN
We investigate the critical behaviour of the chemical potential and average occupation numbers using the extended s -f model with intersite Cooper pairing for systems with fluctuating valence. The model is able to describe phase transitions from normal ferromagnet to normal paramagnet at T=T_{C}, from superconducting paramagnet to normal paramagnet at T=T_{S}, as well as reentrant phase transitions with three critical temperatures T_{S_{1}}, T_{C} and T_{S_{2}} (T_{S_{1}} < T_{C} < T_{S_{2}}). Present investigation, as well as recent results obtained for another models suggest one-to-one correspondence between critical temperatures of the system and kinks appearing in the temperature dependence of the chemical potential and average occupation numbers. This, in turn, indicates a possibility to apply the measurement of the chemical potential vs. temperature as an experimental universal tool when looking for phase transitions in solids.