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]

    Notes
    ------
    Raise :class:`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) - 15.8756 * (1 - x) ** 4)
    else:
        raise NotImplementedError('Incoming out of bound')
    return Ttr
