Slope Problems - Thermo Scientific Orion 2120XP User Manual

Troubleshooting

Slope Problems

Low Slope
Thermo Scientific Orion 2120XP Calcium Hardness Analyzer User Guide
Slope less than 20 mV/decade
• Check the electrode function, clean the electrode, recalibrate the
analyzer, and if the analyzer continues to read low, replace the electrode.
• Check electronics to read mV input correctly. Use electronic test kit
(Cat. No. 180029) procedure for checking electronics function.
Ion-selective electrodes (ISE) can be shown as DC-voltage sources
with 50 to 1000 Megohm; a very high output impedance. ISEs respond
to ion concentration changes with a voltage change of approximately
30 mV/decade for divalent cation such as calcium. Knowing electrode
and analyzers parameters like offset potential, slope, etc allows testing of
various instruments by applying a known mV voltage.
Input Signal
0 mV
- 1000 mV
+ 1000 mV
• Background concentration too high for the standards used.
• Standards contaminated or incorrectly made – verify calibration with
fresh standards.
• Standard 1 and standard 2 interchanged – check the values on the
standards and repeat the calibration. If using custom standards, verify
the values for each standard.
• Standard 1 and standard 2 added in wrong sequence – check the
sequence of the standards and repeat the calibration.
• pH adjustment is not correct – check the level and flow of the reagent.
Replace the reagent if it is contaminated or the level is less than 50 mL.
Refer to Chapter II, Installation of Reagent and Diffusion Tubing.
Expected Reading: ± (0.5 mV + 0.1%)
- 0.5 mV to + 0.5 mV
-1001.5 mV to – 998.5 mV
+ 998.5 mV to + 1001.5 mV
VI-9