15
14
TEMPERATURE-RESISTANCE CORRELATION FOR
HANNA pH SENSITIVE GLASS
The resistance of glass electrodes partially depends on the temperature.
The lower the temperature, the higher the resistance. It takes longer 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 10°C.
Since the resistance of the pH electrode is in the range of 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.
Typical Electrode Life
Ambient Temperature
1- 3 years
90 °C
Less than 4 months
120°C
Less than 1 month
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 the
alkaline error and causes the pH to be underestimated. Hanna's glass
formulations have the indicated characteristics.
CLEANING PROCEDURE
General
Soak in Hanna
HI 7061
general cleaning solution for
approximately 30 minutes.
Removal of films, dirt or deposits on the membrane/junction:
-
Protein
Soak in Hanna
HI 7073 p
rotein cleaning solution for
15 minutes.
-
Inorganic
Soak in Hanna
HI 7074
inorganic cleaning solution
for 15 minutes.
-
Oil/grease
Rinse with
HI 7077
Oil & Fat cleaning solution.
IMPORTANT:
After performing any of the cleaning procedures rinse
the electrode thoroughly with distilled water, refill the reference cham-
ber with fresh electrolyte (not necessary for gel-filled electrodes) and
soak the electrode in
HI 70300
storage solution for at least 1 hour
before taking measurements.
TROUBLESHOOTING
Evaluate the performance of your electrode based on the following.
•
Noise
(readings fluctuate up and down) could be due to:
– Clogged/Dirty Junction:
refer to the Cleaning Procedure.
– Loss of shielding
due to low electrolyte level (in refillable
electrodes only): refill with fresh
HI 7071
for single junction or
HI 7082
for double junction electrodes.
•
Dry Membrane/Junction:
soak in
HI 70300
storage solution
for
at least 1 hour.
•
Drifting:
soak the electrode tip in warm (approx. 50-60°C)
Hanna
HI 7082
solution for one hour and rinse the tip with
distilled water. Refill with fresh
HI 7071
for single junction
electrodes and
HI 7082
for double junction electrodes (refillable
electrodes only).
•
Low Slope:
refer to the cleaning procedure above.
•
No Slope:
check the electrode for cracks in glass stem or bulb
and replace the electrode.
•
Slow Response/Excessive Drift:
soak the tip in
HI 7061
solution for 30 minutes, rinse thoroughly in distilled water and
then follow the Cleaning Procedure above.
-20 -20
-10
0 +10+20 +30+40 +50+60+70 +80+90
° C
-10
-10
Ω
-10
-10
-10
-10
Ω
2x10
9
1x10
9
2x10
8
1x10
8
2x10
7
1x10
7
-20 -10 0
+
10
+
20
+
30
+
40
+
50
+
60
+
70
+
80
+
90
°C
Alkaline Error
1.0 Mol L
-1
Na
+
0.1 Mol L
-1
Na
+
Sodium Ion Correction for the Glass at 20-25°C
Concentration
pH
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