We consider the first-order phase transition of a charged anti-de Sitter black hole, and find that the equation of state with the conditions of the two coexisting phases, leads to the two coupled equations about the thermodynamic volumes of small black hole and large black hole. By solving the equations, it is found that each reduced volume is only a function of the parameter $ω$ . All properties of the coexistence curve can be studied from the two volume functions. In particular, each thermodynamic quantity is described by a piecewise analytic function. The demarcation point is located at $ω_{d}=12(2\sqrt{3}-3)$. The thermodynamic function but not its derivative, is continuous at the point. This property is completely different from that of the ven der Waals fluid. Moreover, the thermodynamic behaviors as $ω→0$ are discussed. From which one can easily obtain some critical exponents and amplitudes for small-large black hole phase transitions.