Hanna Instruments HI 98160 User Manual

Page 36

The temperature limit for the HI 1230B and HI 1618D is from 0 to 80 °C

with optimum measurements between 20 and 40 °C. The resistance of

glass electrodes partially depends on the temperature. The lower the

temperature, the higher the resistance. It takes more time for the

reading to stabilize if the resistance is higher. In addition, the response

time will suffer to a greater degree at temperatures below 25 ºC (77 ºF).

Since the resistance of the pH electrode is in the range of 50 to 200 Mohm,

the current across the membrane is in the pico Ampere range. Large

currents can disturb the calibration of the electrode for many hours.
For these reasons high humidity environments, short circuits and static

discharges are detrimental to a stable pH reading.
The pH electrode’s life also depends on the temperature. If constantly

used at high temperatures, the electrode life is drastically reduced.
The HI 1618D and HI 1230B are specified for a pH range of 0 to 13 pH.
Alkaline Error
High concentrations of sodium ions interfere with readings in alkaline

solutions. The pH at which the interference starts to be significant

depends upon the composition of the glass. This interference is called

alkaline error and causes the pH to be underestimated. Hanna’s glass

formulations have the indicated characteristics.

TEMPERATURE CORRELATION

FOR

FOR pppppH SENSITIVE GLASS

H SENSITIVE GLASS

1.0 Mol L

-1

0.1 Mol L

-1

Sodium Ion Correction for the Glass at 20-25 ºC (68-77 ºF)

Concentration

Error

13.00

13.50

14.00

12.50

13.00

13.50

14.00

0.10

0.14

0.20

0.10

0.18

0.29

0.40