Instruction Manual Nitrate Electrode
EUTECH INSTRUMENTS PTE LTD.
INSTRUCTION MANUAL
NITRATE ION ELECTRODE
1
Nitrate Electrode Instruction Manual
TABLE OF CONTENTS
GENERAL INSTRUCTIONS ..........................................................................................................3
Introduction..............................................................................................................................................3
Required Equipment.................................................................................................................................3
Required Solutions...................................................................................................................................3
GENERAL PREPARATION............................................................................................................4
Electrode Preparation...............................................................................................................................4
Electrode Slope Check (with pH/mV meter) ...........................................................................................4
Electrode Slope Check (with ion meter) ..................................................................................................4
MEASUREMENT..............................................................................................................................5
Measuring Hints.......................................................................................................................................5
Sample Requirements...............................................................................................................................5
Units of Measurement..............................................................................................................................6
MEASUREMENT PROCEDURE ...................................................................................................6
Direct Measurement.................................................................................................................................6
Direct Measurement of Nitrate (using a pH/mV meter)...........................................................................6
Direct Measurement of Nitrate (using an ion meter)................................................................................8
Low Level Nitrate Measurements (using a pH/mV meter)......................................................................8
Low Level Nitrate Determination (using an ion meter)...........................................................................9
ELECTRODE CHARACTERISTICS.............................................................................................9
Reproducibility.........................................................................................................................................9
Interferences...........................................................................................................................................10
Temperature Influences..........................................................................................................................11
Electrode Response................................................................................................................................11
Limits of Detection.................................................................................................................................12
pH Effects...............................................................................................................................................12
Electrode Life.........................................................................................................................................12
Electrode Storage ...................................................................................................................................12
ELECTRODE THEORY................................................................................................................12
Electrode Operation ...............................................................................................................................12
TROUBLESHOOTING GUIDE....................................................................................................14
Meter......................................................................................................................................................14
Plastic-ware............................................................................................................................................14
Electrodes...............................................................................................................................................14
Standards and Reagents..........................................................................................................................14
Sample....................................................................................................................................................14
Technique...............................................................................................................................................15
ORDERING INFORMATION.......................................................................................................18
2
Instruction Manual Nitrate Electrode
EUTECH INSTRUMENTS PTE LTD
NITRATE ION ELECTRODE
INSTRUCTION MANUAL
GENERAL INSTRUCTIONS
Introduction
Eutech Instruments Nitrate Ion Electrode is used to measure nitrate in aqueous solutions quickly,
simply, accurately, and economically.
Required Equipment
1. A pH/mV meter or an ion meter, either power line or battery operated.
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 Nitrate Ion Combination Epoxy-body Electrode, Code no. EC-NO3-03.
Required Solutions
1. Deionized or distilled water for solution and standard preparation.
2. Eutech Ionic Strength Adjuster (ISA), 2M (NH4)2SO4, Code no. EC-ISA-NT1-BT. To
prepare this solution from your own laboratory stock, half fill a 1,000 ml volumetric flask
with distilled water and add 264 grams of reagent-grade ammonium sulfate, (NH4)2SO4.
Swirl the flask gentle to dissolve the solid. Fill the flask to the mark with distilled water,
cap, and upend several times to mix the contents. ISA is added at the rate of 2 ml of ISA to
each 100 ml of standard or sample to adjust the ionic strength to about 0.12M.
3. Eutech Nitrate Standard, 0.1M NaNO
from your own laboratory stock, add 8.50 grams of reagent-grade sodium nitrate to a one
liter volumetric flask about half full of distilled water. Swirl the flask gently to dissolve the
solid. Fill to the mark with distilled water, cap and upend several times to mix the solution.
4. Eutech Nitrate Standard, 1,000 ppm NO
solution from your own laboratory stock, add 1.37 grams of reagent-grade sodium nitrate to
a one liter volumetric flask about half full with distilled water. Swirl the flask gently to
dissolve the solid. Fill to the mark with distilled water, cap and upend several times to mix
the solution.
, Code no. EC-SCS-NT1-BT. To prepare this solution
3
-1
, Code no. EC-SCS-NT2-BT. To prepare this
3
3
Nitrate Electrode Instruction Manual
5. Eutech Nitrate Standard, 100 ppm NO
-1
as N, Code No. EC-SCS-NT3-BT. To prepare this
3
solution from your own laboratory stock, add 0.61 grams of reagent-grade sodium nitrate to
a one liter volumetric flask about half full with distilled water. Swirl the flask gently to
dissolve the solid. Fill 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 any rubber inserts covering the filling holes 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. Gently shake the electrode downward in the same manner as a clinical
thermometer to remove any air bubbles which may be trapped behind the nitrate membrane. Prior to
first usage, or after long-term storage, immerse the nitrate membrane in nitrate standard for thirty
minutes. The electrode is now ready for use.
Connect the electrodes to the proper terminals of the meter as recommended by the meter manufacturer.
Electrode Slope Check (with pH/mV meter)
(check electrodes each day)
1. To a 150 ml beaker, add 100 ml of distilled water and two 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 nitrate standard to the beaker. When the reading
has stabilized, record the mV reading.
3. Using a pipet, add 10 ml of the same nitrate standard used above to the beaker. When the
reading has stabilized, record the mV reading.
4. Determine the difference between the two readings. The electrode is operating correctly if the
millivolt potential has changed by 56 ± 2 mV, assuming the solution temperature is between
o
and 25oC. See the TROUBLESHOOTING sections if the potential change is not within this
20
range.
Slope
is defined as the change in potential observed when the concentration changes by a factor of 10.
Electrode Slope Check (with ion meter)
(check electrodes each day)
1. Prepare standard nitrate solutions whose concentrations vary by ten fold. Use either the 0.1M
or 1,000 ppm nitrate 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.
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Instruction Manual Nitrate Electrode
3. Adjust the meter to the concentration of the standard and fix the value in the memory
according to the meter manufacturer's instructions.
4. Rinse the electrodes with distilled water 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 the 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 sections if the
slope is not within this range.
MEASUREMENT
Measuring Hints
All samples and standards should be at the same temperature for precise measurement. A difference of
1oC in temperature will result in a 2% measurement error.
The sensing membrane is normally subject to water uptake and might appear milky. This has no effect
on performance.
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 styrofoam sheet, between the stirrer and beaker.
Always rinse the electrodes with distilled water and blot dry between measurements. Use a clean, dry
tissue to prevent cross-contamination.
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.
A slow responding electrode may be caused by interferences to the electrode. To restore proper
performance, soak the electrode in distilled water for about 5 minutes to clean the membrane, rinse, and
soak in standard solution for about 5 minutes.
Sample Requirements
All samples must be aqueous and not contain organics which can dissolve in the membrane or extract
out the liquid ion exchanger.
The temperature of the standard solutions and of the sample solutions should be the same and below
40oC. About a 2% error will be introduced for a 1oC difference in temperature.
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Nitrate Electrode Instruction Manual
Interferences should be absent. If they are present, use the procedures found in the Interferences
section to remove them.
The pH range for the nitrate ion electrode is 2.5-11. Neutralize samples outside this range with acid or
base to bring them in range.
Units of Measurement
Nitrate concentrations are measured in units of ppm as sodium nitrate, ppm as nitrate, m oles per liter, or
any other convenient concentration unit. Table 1 indicates some of the concentration units.
TABLE 1: Concentration Unit Conversion Factors
ppm NaNO
ppm NO
3
850.0 620.0 1.0 X 10
-1
moles/liter
3
-2
85.0 62.0 1.0 X 10-3
8.5 6.2 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 nitrate solutions. The temperature of both sample
solution and standard solutions should be the same.
Direct Measurement of Nitrate (using a pH/mV meter)
1. By serial dilution, prepare three standard solutions from the 0.1M or 1,000 ppm stock
standard. The resultant concentrations should be 10-2, 10-3, and 10-4M or 1,000, 100 and 10
ppm nitrate standards. Add 2 ml of ISA to each 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 (10-4M or 10 ppm) in a 150 ml beaker 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. After the reading has stabilized, record the mV reading.
3. Place the mid-range solution (10-3M or 100 ppm) in a 150 ml beaker on the magnetic stirrer
and begin stirring. After rinsing the electrodes with distilled water, blot dry and immerse the
electrode tips in the solution. When the reading has stabilized, record the mV value.
4. Place the most concentrated solution (10-2M or 1,000 ppm) in a 150 ml beaker on the magnetic
stirrer and begin stirring. After rinsing the electrodes in distilled water, blot dry, and immerse
the electrode tips in the solution. When the reading has stabilized, record the mV reading.
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Instruction Manual Nitrate Electrode
5. Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the calibration curve down to about 1.0X10-5M (1.0 ppm
-1
NO
). A typical calibration curve can be found in Figure 1.
3
A calibration curve is constructed on semi-logarithmic paper when using the
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 given are for the linear portion of the curve. The nonlinear portion of the curve requires the use of low level procedures.
6. To a clean, dry 150 ml beaker, add 100 ml of the sample and 2 ml of ISA. Place the beaker
on the magnetic stirrer and begin stirring at a constant rate. Rinse the electrode tips with
distilled water, blot dry, and lower the electrode tips into 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 differing 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.
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Nitrate Electrode Instruction Manual
Direct Measurement of Nitrate (using an ion meter)
1. By serial dilution of the 0.1M or 1,000 ppm nitrate standard, prepare two nitrate standards
whose concentration is near the expected sample concentration. Measure out 100 ml of each
standard into individual 150 ml beakers and add 2 ml of ISA to each.
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. Lower the electrode tips into the solution.
3. Adjust the meter to the concentration of the nitrate 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 distilled water and blot dry.
5. Place the more concentrated solution on the 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 nitrate 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.
9. Immerse 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 nitrate standard. After the reading has
stabilized, compare it to the original reading in Step 3 above. A reading differing by more
than 0.5 mV or a change in ambient temperature will necessitate the repetition of Steps 2-6
above. The meter should be re-calibrated daily.
Low Level Nitrate Measurements (using a pH/mV meter)
This procedure is recommended for solutions with ionic strengths less than 1.0X10-2M. If the solution
is high in ionic strength, but low in nitrate, use the same procedure, but prepare a calibration solution
with a composition similar to the sample.
1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water. This low level ISA [0.4M
(NH4)2SO4] is added at the rate of 1 ml low level ISA to each 100 ml of sample. The
background ionic strength will be 4.0X 10-3M.
2. Dilute 1 ml of 0.1M standard to 100 ml to prepare a 1.0X10-3M NO
measurements in moles per liter. Use the 1,000 ppm standard for preparing a 100 ppm NO
-1
solution for
3
-1
3
standard by diluting 10 ml of the 1,000 ppm standard to 100 ml. Standards should be prepared
fresh daily.
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Instruction Manual Nitrate Electrode
3. Add 1 ml of the low level ISA to a 100 ml volumetric flask and fill to the mark with distilled
water. Pour this solution into a 150 ml beaker and place the beaker on the magnetic stirrer.
Begin stirring at a constant rate.
4. Place the electrode tips in the solution. Assure that the meter is in the mV mode.
5. Add increments of the 1.0X10-3M or 100 ppm standards as given in Table 2 below.
6. After the reading has stabilized, record the mV reading after each addition.
TABLE 2: Step-wise Calibration for Low Level Nitrate Measurements
Added Concentration
Step Pipet Volume (ml) M NO
-1
ppm NO
3
-1
3
1 A 0.1 1.0X10-6 0.1
2 A 0.1 2.0X10-6 0.2
3 A 0.2 4.0X10-6 0.4
4 A 0.2 6.0X10- 0.6
5 A 0.4 9.9X10-6 1.0
6 B 2.0 2.9X10-5 2.9
7 B 2.0 4.8X10-5 4.8
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 ISA as prepared in Step 3
above.
7. On semi-logarithmic graph paper, plot the millivolt reading (linear axis) against the
concentration (log axis) as in Figure 1.
8. Rinse the electrodes and blot dry. Measure out 100 ml of the sample into a 150 ml beaker, add 1
ml of low level ISA. Place the beaker on the magnetic stirrer and begin stirring. 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.
9. Prepare a new low level calibration curve daily. Check the calibration curve every two hours
by repeating Steps 2-7.
Low Level Nitrate Determination (using an ion meter)
Follow the procedure given for normal nitrate determinations using an ion meter and the blank
correction procedure.
ELECTRODE CHARACTERISTICS
Reproducibility
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Nitrate Electrode Instruction Manual
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.
Interferences
Certain anions are electrode interferences and will cause electrode malfunction, drift or measurement
errors if present in high enough levels. The level of interfering common anions that will cause a 10%
error at three levels of nitrate is given in Table 3.
TABLE 3: Concentration of Possible Interferences Causing a 10% Error at
Various Levels of Nitrate; Background Ionic Strength of 0.12M (NH4)2SO4.
Interferences
(moles/liter) 10-2M 10-3M 10-4M NO
-1
3
Cl-1 3.0X10-1 3.0X10-2 3.0X10-3
-1
NO
7.0X10-3 7.0X10-4 7.0X10-5
2
Br-1 7.0X10-3 7.0X10-4 7.0X10-5
CN-1 1.0X10-3 1.0X10-4 1.0X10-5
ClO
-1
5.0X10-4 5.0X10-5 5.0X10-6
3
I-1 5.0X10-5 5.0X10-6 5.0X10-7
ClO
-1
1.0X10-6 1.0X10-7 1.0X10-8
4
Interferences
(ppm) 100 ppm N 10 ppm N 1 ppm N
Cl-1 7,600 760 76.0
-1
NO
Br
CN
ClO
I
ClO
230 23.0 2.30
2
-1
400 40.0 4.00
-1
20.0 2.00 0.20
-1
30.0 3.00 0.30
3
-1
4.00 0.40 0.04
-1
0.07 0.007 0.0007
4
Interferences such as chloride, bromide, iodide and cyanide can be removed by precipitation with 0.5
grams of silver sulfate added to 100 ml of sample. Nitrite interference can be removed by adding 0.3
grams of sulfamic acid to 100 ml of sample. Carbonate and bicarbonate, which are weak interferences,
can be removed by acidifying the sample to pH 4.5 with sulfuric acid. Organic (carboxylic) anions
hinder the nitrate electrode response and can be removed by adding 10 grams of aluminum sulfate to
100 ml of sample. The above interference removal procedures require similar treatment of standards as
well as samples.
If the electrode is exposed to high levels of interfering ions which cannot be removed, the electrode
reading may drift and the response may become sluggish. Restore performance by soaking in distilled
water for 30 minutes followed by soaking in nitrate standard for 30 minutes.
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Instruction Manual Nitrate Electrode
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 the solubility equilibrium on which the electrode depends, the absolute potential of the
reference electrode changes slowly with temperature. The slope of the nitrate electrode, as indicated by
the factor "S" in the Nernstian equation, also varies with temperature. Table 4 gives values for the "S"
factor in the Nernst equation for the nitrate ion.
The operating range of the nitrate ion electrode is 0o-40oC, provided that temperature equilibrium has
occurred. If the temperature varies substantially from room temperature, equilibrium times up to one
hour are recommended.
TABLE 4: Temperature vs. Values for the Electrode Slope
Temp (oC) "S"
0 54.20
10 56.18
20 58.18
25 59.16
30 60.15
40 62.13
50 64.11
Electrode Response
Plotting the mV potential against the nitrate 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 in highly concentrated solutions to several minutes near the detection
limit. (Refer to Figure 2.)
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Nitrate Electrode Instruction Manual
Limits of Detection
The upper limit of detection in pure sodium nitrate solutions is 1M. In the presence of other ions, the
upper limit of detection is above 10-1M nitrate, 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 infuse 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.
The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the
sensing portion of the electrode. Refer to Figure 1 for a comparison of the theoretical response to the
actual response at low levels of nitrate. Nitrate measurements below 10-5M NO
-1
(0.6 ppm as NO
3
-1
)
3
should employ low level procedures.
pH Effects
The operating range of the nitrate electrode is from pH 2.5 to pH 11.
Electrode Life
A nitrate electrode will last six months in normal laboratory use. On-line measurement might shorten
operational lifetime to several months. In time, the response time will increase and the calibration slope
decreases to the point calibration is difficult and electrode replacement is required.
Electrode Storage
The nitrate ion electrode may be stored for short periods of time in 10-2M nitrate solution. For longer
storage (longer than two weeks), rinse and dry the nitrate membrane and cover the tip with any
protective cap shipped with the electrodes. 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 nitrate ion electrode consists of an electrode body containing a liquid internal filling solution in
contact with a gelled organophilic membrane containing a nitrate ion exchanger. When the membrane
is in contact with a solution containing free nitrate ions, an electrode potential develops across the
membrane. This electrode potential is measured against a constant reference potential, using a standard
pH/mV meter or an ion meter. The level of nitrate ions, corresponding to the measured potential, is
described by the Nernst equation:
E = Ep - S log X
where: E = measured electrode potential
Ep= reference potential (a constant)
S = electrode slope (~56 mV/decade)
X = level of nitrate ions in solution
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Instruction Manual Nitrate Electrode
The activity, X, represents the effective concentration of the ions in solution. The total nitrate ion
concentration, Ct, is the sum of free nitrate ion, Cf, and complexed or bound perchlorate ion, Cb. The
electrode is able to respond to only the free ions, whose concentration is :
Cf = Ct - Cb
Since nitrate ions form very few stable complexes, the free ion concentration may be equated to the
total ion concentration.
The activity is related to the free ion concentration, Cf, by the activity coefficient, g, by:
X = g Cf
Activity coefficients vary, depending on total ions strength, I, defined as:
I = ½ SCxZx²
where: Cx = concentration of ion X
Zx = charge of ion X
S = sum of all of the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient, g, 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 (ISA) is
added to samples and standards. The recommended ISA for nitrate is (NH4)2SO4. 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 nitrate ions.
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur from
the inter-diffusion of ions in 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 samples 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 into the sample is equal.
Strongly acidic (pH = 0-2) and strongly basic (pH = 12-14) solutions are particularly troublesome to
measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask
their effect on the junction potential with any concentration of equitransferent salt. One must either
calibrate the electrodes in the same pH range as the sample or use a known incremental method for ion
measurement.
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Nitrate Electrode Instruction Manual
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the plastic-ware, 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.
Plastic-ware
Clean plastic-ware is essential for good measurement. Be sure to wash the plastic-ware well with a
mild detergent and rinse very well with distilled or deionized water.
Electrodes
The electrode may be checked by using the procedure found in the sections entitled Electrode Slope
Check.
Be sure to use distilled or deionized water when following the procedures given in Electrode
1.
Slope Check.
2. If the electrode fails to respond as expected, see the section Measuring Hints. Repeat the
slope check.
3. If the electrodes still fail to respond as expected, substitute another nitrate ion electrode that is
known to be in good working order for the questionable electrode. If the problem persists and
you are using an electrode pair, try the same routine with a working reference electrode.
4. If the problem persists, the standards or reagent may be of poor quality, interferences in the
sample may be present or the technique may be faulty. (See 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 electrodes 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/deionized water, or a simple mathematical miscalculation.
Sample
Look for possible interferences, 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.
14
Instruction Manual Nitrate Electrode
Try to determine the composition of the samples prior to testing to eliminate a problem before it starts.
(See Measuring Hints, Sample Requirements, and Interferences.)
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 again. Reread
GENERAL PREPARATION and ELECTRODE CHARACTERISTICS.
If trouble still persists, call Eutech Instruments Pte Ltd. at (65) 6778-6876 and ask for the Customer
Services Department.
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Nitrate Electrode Instruction Manual
TROUBLESHOOTING HINTS
Symptom Possible Causes Next Step
Out of Range defective meter check meter with shorting strap
Reading (see meter instruction manual)
defective electrode check electrode operation
electrodes not unplug electrodes and reset
plugged in properly
reference electrode be sure reference
not filled electrode is f illed
air bubble on remove bubble by
membrane re-dipping electrode
electrodes not in put electrodes in
solution solution
Noisy or Unstable defective meter check meter with
Readings (readings shorting strap
continuously or
rapidly changing) air bubble on remove bubble by
membrane re-dipping electrode
defective electrode replace electrode
ISA not used use recom mended ISA
meter or stirrer ground meter or stirrer
not grounded
electrode exposed soak electrode in
to interferences nitrate standard
outer filling fill electrode to
solution level level just below
too low the fill hole
Drift (reading samples and standards allow solutions to come to room
slowly changing at different temperatures temperature in one direction) before
measurement
electrode exposed soak electrode in
to interferences nitrate standard
incorrect reference use recommended
filling solution filling solution
16
Instruction Manual Nitrate Electrode
Low Slope or standards contaminated prepare fresh standards
No Slope or incorrectly made
ISA not used use recom mended ISA
standard used as ISA use ISA
electrode exposed soak electrode in
to interferences nitrate standard
defective electrode check electrode operation
air bubble on remove bubble by
membrane re-dipping probe
"Incorrect Answer" incorrect scaling plot millivolts on the linear axis. On the
(but calibration of semi-log paper log axis be sure concentration numbers
curve is good) 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:
10-3M = 62.0 ppm as
NO
= 14.0 ppm as N
3
sample carryover rinse electrodes thoroughly between
samples
17
Nitrate Electrode Instruction Manual
SPECIFICATIONS
Concentration Range: 1M to 7 x 10-6M
(6.2 x 104 to 0.5 ppm as NO
-1
)
3
pH Range: 2.5 to 11
Temperature Range: 0o to 40oC
Resistance: 100 Mohms
Reproducibility: ±2%
Samples: aqueous solutions only
no organic solvents
Size: 110 mm in length; 12 mm in diam eter; 1 m cable length
Storage: Store in dilute nitrate solution
ORDERING INFORMATION
CODE NO. DESCRIPTION
EC-NO3-03 Nitrate Ion Combination Electrode, epoxy body
EC-SCS-NT1-BT Nitrate Standard, 0.1M NaNO3
EC-SCS-NT2-BT Nitrate Standard. 1,000 ppm NO
-1
3
EC-SCS-NT3-BT Nitrate Standard. 100 ppm NO
-1
3
EC-ISA-NT1-BT Nitrate ISA (Ionic Strength Adjuster), 2 M (NH
4)2SO4
18
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