def collins_kimball(k_tst, k_diff):
    """Calculate reaction rate constant inclusing diffusion effects.

    This implementation is based on DOI:10.1016/0095-8522(49)90023-9.

    Examples
    --------
    >>> collins_kimball(2.3e7, 3.6e9)
    2.3e7
    """
    return k_tst * k_diff / (k_tst + k_diff)