PASCO Specialty & Mfg. PASCO Scientific SODIUM ION SELECTIVE ELECTRODE CI-6734 Manual De Usuario

Sodium Ion Selective Electrode

012–06615A

Plotting the electrode potential against the sodium concentration on semilogarithmic paper results in a straight line

with a slope of about 59 mV per decade. (Refer to Figure 1.)

The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from

one minute or less for sodium concentrations above 1.0X10

-5 

M to several minutes near the detection limit. (Refer

to Figure 4.)

Figure 4
Typical sodium electrode time response to step changes in NaCl

The upper limit of detection in pure sodium solutions is 1 M. In the presence of other ions, the upper limit of detection

is above l0

-1 

M sodium, but two factors influence this upper limit. Both the possibility of a liquid junction potential

developing at the reference electrode and the salt extraction effect influence this upper limit. Some salts may extract

into the electrode membrane at high salt concentrations, causing deviation from the theoretical response. Either

dilute samples between l M and l0

-1 

M or calibrate the electrode at 4 or 5 intermediate points.

Free sodium ion concentration down to l.0Xl0

-6 

M or 0.1 ppm can be measured in basic solutions. For measurements

below l0

-5 

M or 1 ppm, use plastic lab-ware (and low level procedures) since a significant pickup of sodium may

occur from glassware due to removal from container walls.

The electrode response to sodium ions is greatly influenced by the pH of the solution. Hydrogen ion interferes with

measurements of low level sodium ion measurements, although the electrode can be used over a wide pH range.

(See Figure 5.)

-75

-50

-25

0

+25

+50

+75

+100

+125

1

2

3

4

10

-3 

M to 10

-2 

M NaCl

10

-3 

M to 10

-4 

M NaCl

10

-3 

M to 10

-5 

M NaCl

10

-3 

M to 10

-6 

M NaCl

time (minutes)

electrode
potential
(mV)