What are the acceptable ranges for the slope and asymmetry potential of a pH electrode

For an ideal electrode the theoretical slope is -59.16 mV/pH and the asymmetry potential at pH 7 is 0 mV. In practice, a range of -55 mV/pH to -60 mV/pH (or 95 – 102%) and an asymmetry potential of +/- 15 mV is acceptable.