code: def _OsmoticPressure(T, P, S):
      
      pw = _Region1(T, P)
      gw = pw["h"]-T*pw["s"]
  
      def f(Posm):
          pw2 = _Region1(T, P+Posm)
          gw2 = pw2["h"]-T*pw2["s"]
          ps = SeaWater._saline(T, P+Posm, S)
          return -ps["g"]+S*ps["gs"]-gw+gw2
  
      Posm = fsolve(f, 0)[0]
      return Posm
docs: Procedure to calculate the osmotic pressure of seawater
  
      Parameters
      ----------
      T : float
          Tmperature, [K]
      P : float
          Pressure, [MPa]
      S : float
          Salinity, [kg/kg]
  
      Returns
      -------
      Posm : float
          Osmotic pressure, [MPa]
  
      References
      ----------
      IAPWS,  Advisory Note No. 5: Industrial Calculation of the Thermodynamic
      Properties of Seawater, http://www.iapws.org/relguide/Advise5.html, Eq 15