23
22
Alkaline Error
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.
-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
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
Note:
To prevent damage to the electrode, remove the pH electrode
from the sample before turning the meter off.
If the meter is OFF, detach the electrode from the meter before
immersing the electrode in the storage solution.
PREPARATION
Remove the protective cap.
DO NOT BE ALARMED IF SALT DEPOSITS ARE PRESENT.
This is normal with electrodes and they will disappear when rinsed
with water.
During transport tiny bubbles of air may form inside the glass bulb.
The electrode cannot function properly under these conditions. These
bubbles can be removed by "shaking down" the electrode as you
would do with a glass thermometer.
If the bulb and/or junction are dry, soak the electrode in
HI 70300
Storage Solution for at least one hour.
For refillable electrodes:
If the fill solution (electrolyte) is less than 1 cm (½") below the fill
hole, add
HI 7082 3,5M KCl Electrolyte Solution
for double
junction or
HI 7071 3,5M KCl+AgCl Electrolyte Solution
for single
junction electrodes.
For a faster response unscrew the fill hole screw during measurements.
For AmpHel electrodes:
If the electrode does not respond to pH changes, the battery is run
down and the electrode should be replaced.
MEASUREMENT
Rinse the electrode tip with distilled water.
Immerse the tip (4cm /1½") in the sample and stir gently for
approximately 30 seconds.
For a faster response and to avoid cross contamination of the samples,
rinse the electrode tip with a few drops of the solution to be tested,
before taking measurements.
STORAGE
To minimize clogging and ensure a quick response time, the glass
bulb and the junction should be kept moist and not allowed to dry
out.
ELECTRODE CONDITIONING
AND MAINTENANCE