Instruction Manual
Potassium Electrode
6
All samples and standards must be aqueous. They must not contain organic solvents. Interferences
found in Table 3 should be absent.
Units of Measurement
Potassium concentrations are measured in units of parts per million as potassium, parts per million
as KCl, moles per liter, or any other convenient concentration unit. Table 1 indicates some of the
concentration units.
TABLE 1: Concentration Unit Conversion Factors
ppm K
+1
ppm KCl moles/liter K
+1
3.91
7.46
1x10
-4
39.10
74.60
1x10
-3
391.00
746.00
1x10
-2
Measurement Procedure
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with ISA for all potassium samples. The temperature of both
sample solution and of standard solutions should be the same.
Direct Measurement of Potassium (using a standard pH/mV meter)
1. Prepare
10
-2
, 10
-3
, and 10
-4
M or 100, 10, and 1 ppm standards by serial dilution of the
0.1M or 1,000 ppm standard. Measure out 100 ml of each standard into individual 150 ml
beakers. Add 2 ml of ISA per 100 ml of standard.
2.
Place the most dilute solution (10
-4
M or 1 ppm) on the magnetic stirrer and begin stirring
at a constant rate. After assuring that the meter is in the mV mode, lower the electrode
tip(s) into the solution. When the reading has stabilized, record the mV reading.
3.
Place the midrange solution (10
-3
M or 10 ppm) on the magnetic stirrer and begin stirring.
After rinsing the electrode(s) with distilled water and blotting dry, immerse the electrode
tip(s) in the solution. When the reading has stabilized, record the mV reading.
4.
Place the most concentrated solution (10
-2
M or 100 ppm) on the magnetic stirrer and begin
stirring. After rinsing the electrode(s) with distilled water and blotting dry, immerse the
electrode tip(s) in the solution. When the reading has stabilized, record the mV reading.
5.
Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the curve down to about 1x10
-5
M or 0.2 ppm. A
typical calibration curve can be found in Figure 1.