  The entanglement in a general mixed spin chain with arbitrary spin $S$ and
1/2 is investigated in the thermodynamical limit. The entanglement is witnessed
by the magnetic susceptibility which decides a characteristic temperature for
an entangled thermal state. The characteristic temperature is nearly
proportional to the interaction $J$ and the mixed spin $S$. The bound of
negativity is obtained on the basis of the magnetic susceptibility. It is found
that the macroscopic magnetic properties are affected by the quantum
entanglement in the real solids. Meanwhile, the entanglement can be
quantitatively evaluated by the thermodynamical observable.
