EUTECH INSTRUMENTS EC-NA02, EC-SCS-LNA1-BT, EC-SCS-LNA2-BT, EC-SCS-LNA3-BT, EC-ISA-LNA1-BT Instruction Manual

Instruction Manual Sodium Electrode

EUTECH INSTRUMENTS PTE LTD.

INSTRUCTION MANUAL

SODIUM ION ELECTRODE

Sodium Electrode Instruction Manual

2

TABLE OF CONTENTS

GENERAL INSTRUCTIONS ..........................................................................................................1

REQUIRED EQUIPMENT..................................................................................................................................1

REQUIRED SOLUTIONS ..................................................................................................................................1

GENERAL PREPARATION............................................................................................................2

ELECTRODE PREPARATION............................................................................................................................2

ELECTRODE SLOPE CHECK (WITH PH/MV METER) .......................................................................................2

ELECTRODE SLOPE CHECK (WITH ION METER) .............................................................................................2

MEASUREMENT..............................................................................................................................3

MEASURING HINTS........................................................................................................................................3

SAMPLE REQUIREMENTS...............................................................................................................................3

UNITS OF MEASUREMENT .............................................................................................................................3

MEASUREMENT PROCEDURE ................................................................................................... 4

DIRECT MEASUREMENT ................................................................................................................................4

DIRECT MEASUREMENT OF SODIUM (USING A PH/MV METER) ....................................................................4

DIRECT MEASUREMENT OF SODIUM (USING AN ION METER)........................................................................5

LOW LEVEL SODIUM DETERMINATION (USING A PH/MV METER)................................................................6

LOW LEVEL SODIUM DETERMINATION (USING AN ION METER) ...................................................................7

ELECTRODE CHARACTERISTICS............................................................................................. 7

REPRODUCIBILITY .........................................................................................................................................7

INTERFERENCE ..............................................................................................................................................7

TEMPERATURE INFLUENCES..........................................................................................................................8

ELECTRODE RESPONSE..................................................................................................................................8

LIMITS OF DETECTION...................................................................................................................................9

PH EFFECTS ...................................................................................................................................................9

ELECTRODE LIFE .........................................................................................................................................10

ELECTRODE STORAGE.................................................................................................................................10

ELECTRODE THEORY................................................................................................................10

ELECTRODE OPERATION .............................................................................................................................10

TROUBLESHOOTING GUIDE....................................................................................................11

METER.........................................................................................................................................................11

GLASSWARE ................................................................................................................................................11

ELECTRODES ...............................................................................................................................................11

STANDARDS AND REAGENTS.......................................................................................................................12

SAMPLE .......................................................................................................................................................12

TECHNIQUE..................................................................................................................................................12

TROUBLESHOOTING HINTS.....................................................................................................13

SPECIFICATIONS..........................................................................................................................14

ORDERING INFORMATION.......................................................................................................14

Instruction Manual Sodium Electrode

1

EUTECH INSTRUMENTS PTE LTD.

SODIUM ION ELECTRODE

INSTRUCTION MANUAL

GENERAL INSTRUCTIONS

Eutech Instruments Sodium Ion Electrode is used to measure sodium ions in aqueous solutions
quickly, simply, accurately, and economically.

Required Equipment

1. A pH/mV meter or an ion meter, either line operated or portable.

2. Semi-logarithmic 4-cycle graph paper for preparing calibration curves when using the

meter in the mV mode.

3. A magnetic stirrer.

4. Eutech Sodium Ion Combination Epoxy-body Electrode, Code no. EC-LNA-03.

5. Lab-ware made of glass, except for low level measurements where plastic-ware is

required.

Required Solutions

1. Deionized or distilled water for solution and standard preparation.

2. Eutech Ionic Strength Adjuster (ISA), 4M NH4Cl/4M NH4OH, Code no. EC-ISA-LNA1-

BT. To prepare this solution from your own laboratory stock, half fill a 1,000 ml
volumetric flask with distilled water and add 214 grams of reagent-grade ammonium
chloride (NH4Cl). Under a hood, add 270 ml of concentrated ammonium hydroxide
(NH

4

OH), swirl the flask gently to dissolve the solid, and allow to cool. Fill the flask to

the mark with distilled water, cap, and upend several times to mix the solution.

3. Dilute Electrode Rinse Solution. To prepare this solution from your own laboratory

stock, add 20 ml of ISA to a one liter volumetric flask and fill to the mark with distilled
water. Use this solution to rinse the electrode between measurements. DO NOT RINSE
WITH DISTILLED WATER.

4. Eutech Sodium Standard, 0.1M NaCl, Code no. EC-SCS-LNA1-BT. To prepare this

solution from your own laboratory stock, half fill a one liter volumetric flask with
distilled water and add 5.84 grams of reagent-grade NaCl. Swirl the flask gently to
dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several
times to mix the solution.

5. Eutech Sodium Standard, 1,000 ppm Na

+

, Code no. EC-SCS-LNA2-BT. To prepare this
solution from your own laboratory stock, half fill a one liter volumetric flask with
distilled water and add 2.542 grams of reagent grade NaCl. Swirl the flask gently to
dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several
times to mix the solution.

Sodium Electrode Instruction Manual

2

6. Eutech Sodium Standard, 100 ppm Na+, Code no. EC-SCS-LNA3-BT. To prepare this

solution from your own laboratory stock, half fill a one liter volumetric flask with
distilled water and add 0.254 grams of reagent grade NaCl. Swirl the flask to dissolve the
solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix
the solution.

GENERAL PREPARATION

Electrode Preparation

Remove the rubber caps covering the electrode tips and the rubber insert covering the filling hole of
the reference electrode. Fill the combination electrode or the reference electrode with the filling
solution shipped with the electrode to a level just below the fill hole. No preparation is required
with a sealed reference electrode. Connect the electrodes to the proper terminals of the meter as
recommended by the meter manufacturer. Soak the sodium electrode tip overnight in the electrode
storage solution.

Electrode Slope Check (with pH/mV meter)

(Check electrodes each day)

1. To a 150 ml glass beaker, add 100 ml of distilled water and 2 ml of ISA. Place the beaker
on a magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is
in the millivolt mode, lower the electrode tips into the solution.

2. Using a pipet, add 1 ml of 0.1M or 1,000 ppm sodium standard to the beaker. When the
reading has stabilized, record the mV reading.

3. Using a pipet, add 10 ml of the same sodium standard used above to the beaker. When
the reading has stabilized, record the mV reading.

4. Determine the difference between the two readings. A difference of 56±2 mV indicates
correct electrode operation assuming the solution temperature is between 20o and 25oC.
See the

TROUBLESHOOTING section if the potential change is not within this range.

Slope is defined as the change in potential observed when the concentration changes be a factor of

10.

Electrode Slope Check (with ion meter)

(Check electrodes each day)

1. Prepare standard sodium solutions whose concentrations vary by tenfold. Use either the

0.1M NaCl or the 1,000 ppm sodium standard. Use the serial dilution method for this
preparation.

2. To a 150 ml beaker, add 100 ml of the lower value standard and 2 ml of ISA. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode
tips into the solution. Assure that the meter is in the concentration mode.

3. Adjust the meter to the concentration of the standard and fix the value in the memory
according to the meter manufacturer's instructions.

Instruction Manual Sodium Electrode

3

4. Rinse the electrodes with the electrode rinse solution and blot dry.

5. To another 150 ml beaker, add 100 ml of the higher value standard and 2 ml of ISA.
Place the beaker on a magnetic stirrer and begin stirring at a constant rate. Lower the
electrode tips into the solution.

6. Adjust the meter to the concentration of the standard and fix the value in the memory.

7. Read the electrode slope according to the meter manufacturer's instructions. Correct
electrode operation is indicated by a slope of 90-100%. See the TROUBLESHOOTING
section if the slope is not within this range.

MEASUREMENT

Measuring Hints

All samples and standards should be at the same temperature for precise measurement, preferably
ambient temperature. A difference of 1oC in temperature will result in about a 2% measurement
error. The electrodes should not be used above 70oC.

Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can
generate sufficient heat to change the solution temperature. To counteract this effect, place a piece
of insulating material, such as a styrofoam sheet, between the stirrer and the beaker.

Always rinse the electrodes with electrode rinse solution from a wash bottle between
measurements. Use a clean, dry tissue to prevent cross contamination. Never use distilled water.

Store the electrodes in electrode storage solution between measurements. Do not store in air or
distilled water. Always soak new electrodes overnight in electrode storage solution prior to first
use. When making low level sodium measurements, use a dilute sodium chloride storage solution.
Add 1 ml of ISA to 100 ml of dilute storage solution.

Plastic lab-ware should be used for low level measurements (<1 ppm). All measurements should be
made in basic solution. All samples and standards should be adjusted to a pH > 9 with ISA.
For samples with high ionic strength, prepare standards whose composition is similar to the sample.

Always check to see that the membrane is free from air bubbles after immersion into standard or
sample.

Sample Requirements

The sample measuring range is pH 9-12. Use the ISA recommended to adjust the pH for best
accuracy. Make sure that the samples and standards are at the same temperature. The glass
electrode sensing bulb will not be attacked by most organic solvents.

Units of Measurement

Sodium concentrations are measured in units of parts per million, equivalents per liter, moles per
liter, or any other convenient concentration unit. Table 1 indicates some of the concentration unit
conversion factors.

Sodium Electrode Instruction Manual

4

TABLE 1: Concentration Unit Conversion Factors

ppm Na+ moles/liter Na+

229.90 1.0 x 10

-

2

22.99 1.0 x 10

-

3

2.30 1.0 x 10

-

4

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 sodium solutions. The temperature of both sample
solution and of standard solutions should be the same.

Direct Measurement of Sodium (using a pH/mV meter)

1. By serial dilution of the 0.1 M or 1,000 ppm standards, prepare 10

-

2

M, 10

-

3

M, and 10

-

4

M
or 100 and 10 ppm sodium standards. Add 2 ml of ISA per 100 ml of standard. Prepare
standards with a composition similar to the samples if the samples have an ionic strength
above 0.1M.

2. Place the most dilute solution (10

-

4

M or 10 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 tips into the solution. When the reading has stabilized, record the mV reading.

3. Place the mid-range solution (10

-

3

M or 100 ppm) on the magnetic stirrer and begin
stirring. After rinsing the electrodes with electrode rinse solution, blot dry and immerse
the electrode tips in the solution. When the reading has stabilized, record the mV reading.

4. Place the most concentrated solution (10

-

2

M or 1,000 ppm) on the magnetic stirrer and
begin stirring. After rinsing the electrodes with electrode rinse solution, blot dry and
immerse the electrode tips 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 5.0X10

-

5

M. For
measurements below this level, follow the instructions for low-level measurement. A
typical calibration curve can be found in Figure 1.

Instruction Manual Sodium Electrode

5

A calibration curve is constructed on semi-logarithmic paper when using a
pH/mV meter in the millivolt mode. The measured electrode potential in mV
(linear axis) is plotted against the standard concentration (log axis). In the
linear region of the curve, only three standards are necessary to determine a
calibration curve. In the non-linear region, additional points must be
measured. The direct measurement procedures are for the linear portion of the
curve. The non-linear portion of the curve requires the use of low level
procedures.

6.

To a clean, dry a 150 ml beaker, add 100 ml of sample and 2 ml of ISA. Place the beaker

on the magnetic stirrer and begin stirring. After rinsing the electrode tips with electrode
rinse solution, blot dry and place the electrode tips in the solution. When the reading has
stabilized, record the mV reading. Using the calibration curve, determine the sample
concentration.

7. The calibration should be checked every two hours. Assuming no change in ambient
temperature, place the electrode tips in the mid-range standard. After the reading has
stabilized, compare it to the original reading recorded in Step 3 above. A reading
difference by more than 0.5 mV or a change in the ambient temperature will necessitate the
repetition of Steps 2-5 above. A new calibration curve should be prepared daily

Direct Measurement of Sodium (using an ion meter)

1.

By serial dilution of the 0.1M or 1,000 ppm sodium standard, prepare two sodium
standards whose concentration is near the expected sample concentration. Add 2 ml of ISA
to each 100 ml of standard.

2. Place the more dilute solution on the magnetic stirrer and begin stirring at a constant rate.
Assure that the meter is in the concentration mode.

3. Lower the electrode tips into the solution. Adjust the meter to the concentration of the
sodium standard and fix the value in the memory according to the meter manufacturer's
instructions after stabilization of the reading.

4. Rinse the electrodes with electrode rinse solution and blot dry. Place the more concentrated
solution on the magnetic stirrer and begin stirring at a constant rate.

5. Lower the electrode tips into the solution.

6. Adjust the meter to the concentration of the sodium standard and fix the value in the
memory according to the meter manufacturer's instructions after stabilization of the
reading.

7. For low level measurements, place the rinsed, dried electrodes into a solution containing
100 ml of distilled water and 2 ml of ISA. After stabilization, fix the blank value in the
meter according to the meter manufacturer's instructions.

8. Place 100 ml of the sample and 2 ml of ISA in a 150 ml beaker. Place the beaker on the
magnetic stirrer, and begin stirring.

Sodium Electrode Instruction Manual

6

9. After rinsing the electrodes and blotting dry, place the electrode tips in the solution and
wait for the reading to stabilize. Read the concentration directly from the meter display.

10. The calibration should be checked every two hours. Assuming no change in ambient
temperature, place the electrode tips in the first sodium standard. After the reading has
stabilized, compare it to the original reading recorded in Step 4 above. A reading
difference by more than 0.5 mV or a change in the ambient temperature will necessitate the
repetition of Steps 2-8 (9) above. The meter should be recalibrated daily.

Low Level Sodium Determination (using a pH/mV meter)

This procedure is recommended for solutions with ionic strength less than 1.0X10

-

2

M and a sodium

concentration less than 1X10

-

5

M or 1 ppm. If the solution is high in ionic strength, but low in
sodium, use the same procedure, but prepare a calibration solution with a composition similar to the
sample. Use plastic lab-ware for low sodium measurements.

1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water.

2. Dilute 20 ml of the outer chamber filling solution to 100 ml with distilled water and fill
the reference electrode if using a double junction reference electrode.

3. Dilute 1 ml of the 0.1M standard to 100 ml to prepare a 1.0X10

-

3

M standard solution for
measurements in moles per liter. Dilute 10 ml of the 1,000 ppm standard solution to 100
ml to prepare a 100 ppm standard solution for measurements in ppm. Add 1 ml of low
level ISA to each 100 ml of standard. Standards should be prepared fresh daily.

4. Using a 150 ml plastic beaker, add 100 ml of distilled water and 1 ml of low level ISA.
Add NH4OH, if necessary, to adjust the pH above 9. Place the beaker on the magnetic
stirrer and begin stirring at a constant rate.

5. Place the electrode tips in the solution. Assure that the meter is in the mV mode.

6. Add increments of the 1.0X10

-

3

M or 100 ppm standard as given in Table 2 below.

7. After the reading has stabilized, record the mV reading after each addition.

TABLE 2: Step-wise Calibration for Low Level Sodium Measurements

Added Concentration

Step

Pipet Volume (ml) M ppm

1 A 0.1 1.0X10

-

6

0.10

2 A 0.1 2.0X10

-

6

0.20

3 A 0.2 4.0X10

-

6

0.40

4 A 0.2 6.0X10

-

6

0.60

5 A 0.4 9.9X10

-

6

0.99

6 B 2.0 2.9X10

-

5

2.91

7 B 2.0 4.8X10

-

5

4.76

Pipet A = 1 ml graduated pipet
Pipet B = 2 ml pipet
Solutions: additions of 1.0X10-3M or 100 ppm standard to 100 ml of distilled water and 1 ml
of low level ISA.

Instruction Manual Sodium Electrode

7

8. On semi-logarithmic graph paper, plot the millivolt reading (linear axis) against the
concentration (log axis) as in Figure 1.

9. Rinse the electrodes with electrode rinse solution and blot dry.

10. To a 150 ml plastic beaker add 100 ml of sample and 1 ml of low level ISA. Place the
beaker on the magnetic stirrer and begin stirring. Adjust the pH, if necessary, to above 9.
Lower the electrode tips into the solution. After the reading has stabilized, record the mV
reading and determine the concentration from the low level calibration curve.

11. Prepare a new low level calibration curve daily. Check the calibration curve every two
hours by repeating Steps 3-8.

Low Level Sodium Determination (using an ion meter)

Follow the procedure given for sodium measurements with an ion meter. Always use the blank
correction procedure. Use plastic lab-ware for low level sodium measurement.

ELECTRODE CHARACTERISTICS

Reproducibility

Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility
is independent of concentration within the electrode's operating range.

Interference

Table 3 lists some common cations that, if present in high enough levels, will cause electrode
interference and measurement errors or electrode drift when using the sodium ion electrodes.

Most samples do not contain or contain very little of the cations shown in Table 3. The ammonium
ion (NH

4

+

), found in the recommended ISA, will not result in an error if all samples and standards

have the same level of ISA present.

Electrode drift and slow response could indicate the presence of high interference from the ions
listed. Soak the electrodes in electrode storage solution when this happens to restore proper
response. See

Measuring Hints section.

TABLE 3

: Levels of Interfering Ions Resulting in a 10% Error at Specified Levels of Sodium

Interference 1.0x10

-

4

M 1.0x10

-

3

M 1.0x10

-

2

M

Li+1 5X10

-

4

M 5X10

-

3

M 5X10

-

2

M

K+1 1X10

-

2

M 1X10

-

1

M 1M

Rb+1 3X10

-

1

M 3M --

NH

4

+1

3X10

-

1

M 3M --

Ag

+1

3X10

-

9

M 3X10

-

8

M 3X10

-

7

M

Tl

+1

5X10

-

2

M 5X10

-

1

M --

Interference 1 ppm 10 ppm 100 ppm

Li+1 1.5 ppm 15 ppm 150 ppm
K+1 17 ppm 170 ppm 1,700 ppm

Sodium Electrode Instruction Manual

8

Rb+1 1.1X10

4

ppm 1.1X10

5

ppm --

NH

4

+1

1.8X103 ppm 1.8X10

4

ppm -­Ag+1 0.0001 ppm 0.001 ppm 0.01 ppm
Tl+1 4.5X10

3

ppm 4.5X10

4

ppm --

Temperature Influences

Samples and standards should be at the same temperature, since electrode potentials are influenced
by changes in temperature. A 1oC difference in temperature results in a 2% error at the 10-3M level.
Because of solubility equilibrium on which the electrode depends, the absolute potential of the
reference electrode changes slowly with temperature. The slope of the electrode, as indicated by the
factor "S" in the Nernst equation, also varies with temperature. Table 4 indicates the variation of
theoretical slope with temperature.

Provided that temperature equilibrium has occurred, the sodium ion electrodes can be used at
temperatures from -5o to 70oC. Room temperature measurements are recommended, since
measurements at temperatures markedly different from room temperature may require equilibrium
times up to one hour. The electrode should not be used at temperatures above 70oC, since damage to
the membrane may result.

TABLE 4: Temperature vs. Values for the Electrode Slope

Temperature (oC) "S"
0 54.20
10 56.18
20 58.16
25 59.16
30 60.15
40 62.13
50 64.11

Electrode Response

Plotting the electrode mV potential against the sodium concentration on semi-logarithmic paper
results in a straight line with a slope of about 56 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 2.)

Instruction Manual Sodium Electrode

9

Limits of Detection

The upper limit of detection in pure sodium solutions is 1M. In the presence of other ions, the upper
limit of detection is above 10-1M 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 1M
and 10-1M or calibrate the electrode at 4 or 5 intermediate points.

Free sodium ion concentration down to 1.0X10

-

6

M or 0.1 ppm can be measured in basic solutions.

For measurements below 10

-

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.

pH Effects

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 3.)

Sodium Electrode Instruction Manual

10

The edge of the shaded area (the straight line) shown in Figure 3 indicates a minimum pH a t whi ch
dilute sodium measurements can be made with less than 10% hydrogen ion interference. The pH
should be adjusted to a pH greater than 9 by the addition of ISA to all standards and samples for
optimal results over the entire concentration range of sodium. Additional ammonium hydroxide
may be necessary to adjust the pH to the desired level in some cases.

Electrode Life

A sodium ion electrode will last six months in normal laboratory use. On-line measurements might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope will decrease to the point calibration is difficult and electrode replacement is
required.

Electrode Storage

The sodium ion electrode should be stored in the sodium electrode storage solution, never in air or
in distilled water. A more dilute sodium chloride solution (with pH adjusted through the use of ISA)
may be used for storage before low level measurements. For longer storage (longer than two
weeks), rinse and dry the sensing glass and cover the glass tip with any protective cap shipped with
the electrode. The reference portion of the combination electrode (or the outer chamber of the
reference electrode) should be drained of filling solution, if refillable, and the rubber insert placed
over the filling hole.

ELECTRODE THEORY

Electrode Operation

The sodium ion electrode is composed of a sodium-selective glass membrane bonded to a glass
body. When the membrane is in contact with a solution containing sodium ions, an electrode
potential develops across the membrane. This electrode potential is measured against a constant
reference potential, using a pH/mV meter or an ion meter. The level of sodium ions, corresponding
to the measured potential, is described by the Nernst equation.

E = E

o

+ S log X

where:
E = measured electrode potential
E

o

= reference potential (a constant)
S = electrode slope (~56 mV/decade)
X = level of sodium ions in solution
The activity, X, represents the effective concentration of free sodium ions in the solution. The
activity is related to the free ion concentration, Cf, by the activity coefficient, γ , by:
X = γ Cf

Activity coefficients may vary, depending on the total ionic strength, I, determined as:

I = 1/2 Σ CxZ

x

2

where: Cx = concentration of ion X

Z

x

= charge of ion

Σ = sum of all of the types of ions in the solution

Instruction Manual Sodium Electrode

11

In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient, γ , is constant and the activity, X, is directly proportional to the concentration.

To adjust the background ionic strength to a high and constant value, ionic strength adjuster is
added to samples and standards. The recommended ISA for sodium is an ammonium
chloride/ammonium hydroxide buffer. Solutions other than this may be used as ionic strength
adjusters as long as ions that they contain do not interfere with the electrode's response to sodium
ions.

The reference electrode must also be considered. When two solutions of different compositions are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the inter-diffusion of ions into the two solutions. Electrode charge will be carried unequally
across the solution boundary resulting in a potential difference between the two solutions, since
ions diffuse at different rates. When making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode
potential.

The composition of the liquid junction filling solution in the reference electrode is most important.
The speed with which the positive and negative ions in the filling solutions diffuse into the sample
should be as nearly equal as possible, that is, the filling solution should be equitransferent. No
junction potential can result if the rate at which positive and negative charge carried in the sample is
equal.

TROUBLESHOOTING GUIDE

The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the glassware, the electrodes, the standards & reagents, the sample,
and the technique.

Meter

The meter may be checked by following the check-out procedure in the instrument instruction
manual.

Glassware

Clean glassware is essential for good measurement. Be sure to wash the glassware well with a mild
detergent and rinse very well with distilled or deionized water. Clean glassware will drain without
leaving water droplets behind.

Electrodes

The electrodes may be checked by using the procedure found in the sections entitled Electrode

Slope Check.

1. Be sure to use distilled or deionized water when following the procedures given in

Electrode Slope Check.

2.

If the electrode fails to respond as expected, see sections Measuring Hints and Electrode

Response.

Repeat the slope check.

3. If the electrodes still fail to respond as expected, substitute another sodium ion electrode
that is known to be in good working order for the questionable electrode. If the problem

Sodium Electrode Instruction Manual

12

persists and you are using an electrode pair, try the same routine with a working reference
electrode.

4. If the problem persists, the reagent may be of poor quality, interference in the sample may
be present or the technique may be faulty. (See the Standards and Reagents, Sample,
and Technique sections below.)

5.

If another electrode is not available for test purposes, or if the electrode in use is suspect,
review the instruction manual and be sure to:

- Clean and rinse the electrodes thoroughly.

- Prepare the electrode(s) properly.

- Use the proper filling solution.

- Adjust the pH and the ionic strength of the solution by the use of the proper ISA.

- Measure correctly and accurately.

- Review TROUBLESHOOTING HINTS.

Standards and Reagents

Whenever problems arise with the measuring procedure that has been used successfully in the past,
be sure to check the standard and reagent solutions. If in doubt about the credibility of any of the
solutions, prepare them again. Errors may result from contamination of the ISA, incorrect dilution
of standards, poor quality distilled or deionized water, or a simple mathematical miscalculation.

Sample

Look for possible interference, complexing agents, or substances which could affect the response or
physically damage the sensing electrode (or the reference electrode) if the electrodes work perfectly
in the standard, but not in the sample.

Try to determine the composition of the samples prior to testing to eliminate a problem before it
starts. (See Measuring Hints, Sample Requirements, and Interference.)

Technique

Be sure that the electrode's limit of detection has not been exceeded. Be sure that the analysis
method is clearly understood and is compatible with the sample.

Refer to the instruction manual. Reread sections on General Preparation and Electrode

Characteristics

. If trouble still persists, please call Eutech Instruments Pte Ltd. at (65) 6778-6876

and ask for the Customer Services Department.

Instruction Manual Sodium Electrode

13

TROUBLESHOOTING HINTS

Symptom Possible Causes Next Step
Out of Range Reading

defective meter check meter with shorting strap

(see meter instruction manual)

defective electrode check electrode operation
electrodes not plugged in

properly

unplug electrodes and reset

reference electrode not filled replenish reference filling solution
electrodes not in solution put electrodes in solution
air bubble on redipping

electrode

remove bubble by membrane

"Incorrect Answer" (but
calibration curve is
good)

incorrect scaling of semi-log
paper

plot millivolts on the linear axis. On
the log axis, be sure concentration
numbers within each decade are
increasing with increasing
concentration

incorrect sign be sure to note sign of millivolt

number correctly
incorrect standards prepare fresh standards
wrong units used apply correct conversion factor:

1.0X10

-

3

M = 23.0 ppm as Na+1

sample pH too acidic adjust pH with ISA

Drift (reading changing
in one direction)

samples and standards at
different temperature

allow solutions to come to room

temperature before measurement
incorrect reference filling

solution

use recommended filling solution

electrode exposed to
interference

soak overnight in electrode storage

solution
pH too acidic adjust pH with ISA
ISA not used use recommended ISA

Low Slope or No Slope

standards contaminated or
incorrectly made

prepare fresh standards

standard used as ISA use ISA
pH too acidic adjust pH with ISA
electrode exposed to

interference

soak overnight in electrode storage

solution
air bubble on membrane remove bubble by redipping probe

Noisy or Unstable
Readings (reading
changing randomly)

defective meter check meter with shorting strap

ISA not used use recommended ISA
meter or stirrer not grounded ground meter or stirrer
defective electrode replace electrode
electrode exposed to

interference

soak overnight in electrode storage

solution

Sodium Electrode Instruction Manual

14

SPECIFICATIONS

Concentration Range: Saturated to 1.0 x 10

-

6

M (0.02 ppm)

pH Range: 5 to 12 (depending on Na+ level)

Temperature Range: -5o to 70oC

Resistance: < 200 Mohm

Reproducibility: ±2%

Size: 110 mm length; 12 mm diameter; 1 m cable length

Storage: Store in 5M NaCl with added ISA

ORDERING INFORMATION

Code No. Descri ption

EC-NA02 Sodium Ion Combination Electrode, epoxy body

EC-SCS-LNA1-BT Sodium Standard, 0.1M NaCl

EC-SCS-LNA2-BT Sodium Standard, 1,000 ppm Na+

EC-SCS-LNA3-BT Sodium Standard, 100 ppm Na+

EC-ISA-LNA1-BT Sodium Ionic Strength Adjuster, 4M NH4Cl/4M NH4OH