def Ttr(x):
    """Equation for the triple point of ammonia-water mixture

    Parameters
    ----------
    x : float
        Mole fraction of ammonia in mixture [mol/mol]

    Returns
    -------
    Ttr : float
        Triple point temperature [K]

    Raises
    ------
    NotImplementedError : If input isn't in limit
        * 0 ≤ x ≤ 1

    References
    ----------
    IAPWS, Guideline on the IAPWS Formulation 2001 for the Thermodynamic
    Properties of Ammonia-Water Mixtures,
    http://www.iapws.org/relguide/nh3h2o.pdf, Eq 9
    """
    if 0 <= x <= 0.33367:
        Ttr = 273.16*(1-0.3439823*x-1.3274271*x**2-274.973*x**3)
    elif 0.33367 < x <= 0.58396:
        Ttr = 193.549*(1-4.987368*(x-0.5)**2)
    elif 0.58396 < x <= 0.81473:
        Ttr = 194.38*(1-4.886151*(x-2/3)**2+10.37298*(x-2/3)**3)
    elif 0.81473 < x <= 1:
        Ttr = 195.495*(1-0.323998*(1-x)-15.87560*(1-x)**4)
    else:
        raise NotImplementedError("Incoming out of bound")
    return Ttr