def check_ph(self,ph_column='pH',thresh=0.4):
        """
        gives the maximal change in pH

        Parameters
        ----------
        ph_column : str
            column with pH-values, default to 'pH'
        threshold : int
            threshold value for warning, default to '0.4'
        """
        dph = self.data[ph_column].max()-self.data[ph_column].min()
        if dph > thresh:
            wn.warn('Strong change in pH during experiment!')
        else:
            self.delta_ph = dph