Thermo Fisher Scientific Ion Selective Electrodes Nitrate User Manual

EUTECH INSTRUMENTS PTE LTD.

INSTRUCTION MANUAL

NITRATE ION ELECTRODE

TABLE OF CONTENTS

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

Introduction.................................................................................................................................................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.................................................................................................................................4

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

Direct Measurement....................................................................................................................................4

Direct Measurement of Nitrate (using a pH/mV meter).............................................................................4

Direct Measurement of Nitrate (using an ion meter)..................................................................................6

Low Level Nitrate Measurements (using a pH/mV meter).........................................................................6

Low Level Nitrate Determination (using an ion meter)..............................................................................7

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

Reproducibility...........................................................................................................................................7

Interferences ...............................................................................................................................................8

Temperature Influences ..............................................................................................................................9

Electrode Response.....................................................................................................................................9

Limits of Detection...................................................................................................................................10

pH Effects.................................................................................................................................................10

Electrode Life ...........................................................................................................................................10

Electrode Storage......................................................................................................................................10

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

Electrode Operation..................................................................................................................................10

TROUBLESHOOTING GUIDE....................................................................................................12

Meter.........................................................................................................................................................12

Plastic-ware...............................................................................................................................................12

Electrodes .................................................................................................................................................12

Standards and Reagents............................................................................................................................12

Sample ......................................................................................................................................................13

Technique .................................................................................................................................................13

ORDERING INFORMATION.......................................................................................................16

Instruction Manual Nitrate Ion 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 NaNO3, Code no. EC-SCS-NT1-BT. To prepare this solution

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.

5. Eutech Nitrate Standard, 100 ppm NO

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

-1

, Code no. EC-SCS-NT2-BT. To prepare this

-1

as N, Code No. EC-SCS-NT3-BT. To prepare this

Instruction Manual Nitrate Ion Electrode

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 20o and 25oC. See the TROUBLESHOOTING sections if the potential change is not within this 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.

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 Nitrate Ion Electrode

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.

Interferences should be absent. If they are present, use the procedures found in the

Interferences

section to remove them.

Instruction Manual Nitrate Ion Electrode

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 NaNO3 ppm NO

850.0 620.0 1.0 X 10

85.0 62.0 1.0 X 10

-1

moles/liter

-2

-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

3. Place the mid-range solution (10

M 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

-2

M 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.

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

). A typical calibration curve can be found in Figure 1.

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