Hanna Instruments HI 253 Instruction Manual

Contents

36Typical Electrode LifeAmbient Temperature 1 ñ 3 years90 ∫C Less than 4 months120 ∫C Less than 1 monthAlkaline ErrorHigh concentrations of sodium ions interfere with readings in alkalinesolutions. The pH at which the interference starts to be significant dependsupon the composition of the glass. This interference is called alkaline error andcauses the pH to be underestimated. Hannaís glass formulations have theindicated characteristics.1.0 Mol L-1 Na+0.1 Mol L-1 Na+Sodium Ion Correction for the Glass at 20-25 ∫CConcentration pH Error13.0013.5014.0012.5013.0013.5014.000.100.140.200.100.180.290.40The resistance of glass electrodes partially depends on the temperature. Thelower the temperature, the higher the resistance. It takes more time for thereading to stabilize if the resistance is higher. In addition, the response timewill suffer to a greater degree at temperatures below 25 ∫C.Since the resistance of the pH electrode is in the range of 50 ñ 200 Mohm, thecurrent across the membrane is in the pico Ampere range. Large currents candisturb the calibration of the electrode for many hours.For these reasons high humidity environments, short circuits and staticdischarges are detrimental to a stable pH reading.The pH electrodeís life also depends on the temperature. If constantly used athigh temperatures, the electrode life is drastically reduced.TEMPERATURE CORRELATIONFOR pH SENSITIVE GLASS