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Ion Selective Electrodes Ammonia
User Manual
24 pgs376.24 Kb0

Thermo Fisher Scientific Ion Selective Electrodes Ammonia User Manual

EUTECH INSTRUMENTS PTE LTD.
AMMONIA GAS-SENSING ELECTRODE
INSTRUCTION MANUAL
TABLE OF CONTENTS
TABLE OF CONTENTS....................................... ... .... ... .... .... ... .... .... ... ..........................................................1
GENERAL INSTRUCTIONS.........................................................................................................................1
GENERAL PREPARATION..........................................................................................................................2
MEASUREMENT............................................................................................................................................4
MEASUREMENT PROCEDURE......... .... .... ... .... ... .... .... ... .... ... .... .... ... .... ... .... ... .... ........................................5
ELECTRODE CHARACTERISTICS ...........................................................................................................9
ELECTRODE THEORY...............................................................................................................................11
TROUBLESHOOTING GUIDE.................................. .... ... .... ... .... .... ... .... ... .... .... ... .... ..................................13
TROUBLESHOOTING HINTS.............................................. ... .... ........................................... .... ... .............16
SPECIFICATIONS........................................................................................................................................19
ORDERING INFORMATION......................................................................................................................19
GENERAL INSTRUCTIONS
Introduction
Eutech Instruments Ammonia Gas-Sensing Electrode is used to quickly, simply, accurately, and economically measure dissolved ammonia in aqueous solutions. It can also be used to measure the ammonium ion after conversion to ammonia or organic nitrogen from Kjeldahl digestion of the sample. The measurement is not affected by sample color or turbidity and samples do not need to be distilled. Interference from anions, cations, and dissolved species, other than volatile amines, do not occur. With a flow-through cap, the electrode can be used in flow-through applications.
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 Ammonia Gas-sensing Electrode, Code No. EC-NH3-01.
Required Solutions
1. Deionized or distilled water for solution preparation.
2. Eutech Ammonia Standard, 0.1M NH
Cl, Code No. EC-SCS-AA1-BT. To prepare this
4
solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 5.35 grams of reagent-grade NH4Cl. 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.
3. Eutech Ammonia Standard, 1,000 ppm NH
as N, Code No. EC-SCS-AA2-BT. To prepare
3
this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 3.82 grams of reagent-grade NH4Cl. 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.
4. Eutech Ammonia Standard, 100 ppm NH
as N, Code No. EC-SCS-AA3-BT. To prepare
3
this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 0.382 grams of reagent-grade NH
Cl. Swirl the flask gently to
4
dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.
5. Eutech Ionic Strength Adjuster (ISA) Solution, 10 M NaOH, Code No. EC-ISA-AA1-BT. To prepare this solution from your own laboratory stock, half fill a 1,000 ml beaker with distilled water, add 400 grams of reagent-grade NaOH (sodium hydroxide). Swirl the flask gently under a hood
to dissolve the solid. Allow to cool and fill to the mark with distilled
water. Stir the solution and store in a plastic bottle.
6. Eutech Ionic Strength Adjuster (ISA) Solution, 5M NaOH/.05M Disodium EDTA/10% Methanol with Color Indicator, Code No. EC-ISA-AA2-BT. To prepare this solution from your own laboratory stock, half fill a 1,000 ml beaker with distilled water and add 200 grams of reagent-grade NaOH. Stir the solution to dissolve the pellets, add 18.61 grams of disodium EDTA, and stir the solution again until all solids have dissolved. Allow solution to cool. In a separate 150 ml beaker, add a tiny amount (10-20 mg) of thymolphthalein to 100 ml of methanol and stir to dissolve. Pour the methanol solution into the 1,000 ml beaker and stir to blend. The solution should turn a dark blue. Fill to the 1,000 ml mark with distilled water and stir to blend.
GENERAL PREPARATION
Electrode Preparation
This electrode is shipped dry. Before using, unscrew the large cap (See Figure 5), and remove the inner glass electrode from the outer body. Fill the outer body with 2 to 3 ml of internal filling solution. Place the inner glass electrode into the outer body, and screw on the large cap until finger tight. Place the assembled electrode in an electrode holder with a 20
o
angle from the vertical to
avoid trapping air bubbles at the bottom of the electrode.
Checking Membrane
A small hole of any size on the membrane or breakage of the membrane causes failure of the electrode. It is recommended to check the membrane on every newly assembled electrode.
1. Connect a newly assembled electrode to a pH/mV meter.
2. Lower the electrode tip in distilled water.
3. Record the reading after stirring the distilled water for about 15 minutes.
4. Add proper ISA solution (see
Required Solutions) to the distilled water. A drastic change
in the reading in a negative direction indicates damage of the membrane.
Changing Membrane (see Figure 4)
1. Unscrew the top cap from the outer body and remove the inner glass body from the epoxy
outer body. Carefully place the glass body aside.
2. Unscrew the bottom cap from the outer body. Remove the old membrane from around the
threads and electrode tip opening.
3. Using the tweezers provided, grab a new piece of white membrane material by the edge
and remove from the separator paper. Then, with the hand not holding the tweezers, hold the electrode body up at the threads with thumb and forefinger. Place one edge of the membrane against the threads and hold in place with your thumb. Stretch new membrane lengthwise across the electrode opening so that it smoothly covers the opening. Place the other edge of the membrane against the threads and hold in place with your forefinger.
4. Place the bottom cap gently over the membrane onto the threads and screw the bottom cap
on until finger-tight. Check that the membrane is free of wrinkles and holes or else repeat the above steps.
5. Using the syringe provided, fill the outer body with approximately 2 ml of inner filling
solution. Place glass inner body into epoxy outer body containing the internal filling solution and screw on the upper cap until finger-tight.
Connecting the Electrode to the Meter
Connect the electrode to the meter according to meter manufacturer's instructions. No external reference electrode is required. To prevent air entrapment, mount the electrode at a 20
o
angle from
the vertical.
Electrode Slope Check (with pH/mV meter)
(check electrodes each day)
1. To a clean, dry, 150 ml beaker, add 100 ml of distilled water and 1 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 tip into the solution. Remove air bubbles by redipping probe.
2. Using a pipet, add 1 ml of 0.1M or 1,000 ppm standard into the solution. When the reading
has stabilized, record the mV value.
3. Using a pipet, add 10 ml of the same ammonia standard used above to the beaker. When
the reading has stabilized, record the mV value.
4. Determine the difference between the two readings. The electrode is operating correctly if
a difference of 56±3 mV is found, assuming the solution temperature is between 20
o
C. See the TROUBLESHOOTING section if the potential change is not within this range.
25
o
and
is defined as the change in potential observed when the concentration changes by a factor of
Slope
10.
Electrode Slope Check (with ion meter)
(check electrodes each day)
1. Prepare standard ammonia solutions whose concentrations vary by tenfold. Use either the
0.1M or 1,000 ppm ammonia standard. Use the serial dilution method for this preparation.
2. To a 150 ml beaker, add 100 ml of the lower value standard and 1 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.
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 TROUBLESHOOTING section if the slope is not within this range.
MEASUREMENT
Measuring Hints
Samples should be measured immediately after collection. Samples should be stored according to the directions given in
Sample Storage if immediate measurement is not possible.
The ratio of a surface area to volume in the beaker should be minimized. Beakers containing the samples or the standard should be kept covered between measurements.
The ammonia ISA, 10M NaOH, should be added just before measurement standards should be at the same temperature for precise measurement. A difference of 1
. All samples and
o
C in
temperature will result in approximately a 2% error. All samples must be aqueous.
Always rinse the electrode with distilled water and blot dry between measurements. Use a clean, dry tissue to prevent cross-contamination.
Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can generate sufficient heat to change solution temperature. To counteract this effect, place a piece of insulating material, such as gauze or styrofoam, between the beaker and the magnetic stirrer.
Always check to see that the membrane is free from air bubbles after immersion into standard or sample.
Sample Storage
Samples should be measured immediately after preparation or collection, if possible. Wait only long enough for temperature equilibration between the sample and the electrode. If stirring a 100 ml basic solution in a 150 ml beaker, the rate of ammonia loss at room temperature is about 50% in six hours. The loss of C0
increases with increasing temperature.
2
If the samples cannot be measured immediately, add 0.5 ml of 1M HCl to each liter of sample to make them slightly acidic (pH 6) and store in tightly capped vessels. Prior to measurement, add 10M NaOH to make the samples slightly basic.
Sample Requirements
Sodium hydroxide buffer must be added to standards and samples before measurement. When 10M NaOH is added, all standards and samples should be in the range of pH 11 to 14. In this range, all ammonium species are converted to ammonia. Adding the 10M NaOH adjusts the total level of
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