def _replace_special_characters(str_special_chars):
    """
    Replaces special characters in a string with their percent-encoded counterparts
        ':' -> '%3A'
        '/' -> '%2F'
        ',' -> '%2C'
    (e.g. "1/1/9" -> "1%2F1%2F9")

    :param str_special_chars: string in which to substitute characters
    :return: new string with characters replaced by their percent-encoded counterparts
    """
    str_percents = str_special_chars.replace(':', '%3A').replace('/', '%2F'
        ).replace(',', '%2C')
    return str_percents
