def get_transition_chempots(self, element):
        if element not in self.elements:
            raise ValueError("get_transition_chempots can only be called with "
                             "elements in the phase diagram.")
        critical_chempots = []
        for facet in self.facets:
            chempots = self._get_facet_chempots(facet)
            critical_chempots.append(chempots[element])
        clean_pots = []
        for c in sorted(critical_chempots):
            if len(clean_pots) == 0:
                clean_pots.append(c)
            else:
                if abs(c - clean_pots[-1]) > PhaseDiagram.numerical_tol:
                    clean_pots.append(c)
        clean_pots.reverse()
        return tuple(clean_pots)