OMEGA ISE-8710 (01) PDF DOCUMENT

To operate the OMEGA ISE-8710, you will need the following equipment and solutions:

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. Ammonia-free deionized or distilled water. To prepare, pass distilled or deionized water through an ion-exchange column using a strongly acid exchange resin, such as Dowex 50W-X8.

5. The OMEGA ISE-8710 Ammonia Electrode.

6. Sodium Hydroxide Solution (10M), ISE-8710-R1. This is used to adjust the solution pH to the operating range of the electrode. Add 1 mL of 10M NaOH to each 100 mL of standard or sample.

7. Ammonia Chloride Standard, 0.1M NH4Cl, ISE-8710-S1 or Nitrogen Standard Solution, 1000 ppm, ISE-8710-S2.

8. Ammonia Electrode Filling Solution, PHFS-8710.

Sodium Hydroxide Solution (10M NaOH), ISE-8710-R1:

To prepare a 10M NaOH solution, fill a 100 mL volumetric flask half full of ammonia-free distilled water, add 40 grams of reagent-grade NaOH, swirl the flask to dissolve the pellets, and add distilled water to the mark.

Ammonia Chloride Standard, 0.1M NH4Cl, ISE-8710-S1:

To prepare a 0.1M NH4Cl solution, fill a 100 mL volumetric flask half full of ammonia-free distilled water, add 0.535 grams of reagent-grade NH4Cl, swirl the flask to dissolve the solid, and fill to the mark with distilled water.

Nitrogen Standard Solution, 1000 ppm, ISE-8710-S2:

To prepare a 1000 ppm nitrogen standard, fill a 100 mL volumetric flask half full of ammonia-free distilled water, add 0.382 grams of reagent-grade NH4Cl, swirl the flask to dissolve the solid, and fill to the mark with distilled water.

The OMEGA ISE-8710 electrode is shipped dry. Before using it, follow these assembly steps:

1. Unscrew the big cap at the top of the electrode.

2. Remove the inner glass electrode from the outer body.

3. Soak the inner glass electrode in internal fill solution (PHFS-8710) for at least two hours.

4. Fill the outer body half full with internal filling solution.

5. Place the inner glass electrode back into the outer body.

6. Screw on the big cap until it is fingertight.

7. After assembly, gently feel the membrane with your fingertip to make sure that the inner glass electrode touches the membrane tightly.

A small hole of any size on the membrane or the breakage of the membrane causes total failure of the measurements. It is recommended to check the membrane on every newly assembled electrode. A simple and effective way is available:

1. Connect a newly assembled electrode to a pH/mV meter and place the electrode into DI water.

2. Record the reading after stirring the DI water for about 15 minutes.

3. Add proper buffer solution (10M NaOH) to that DI water.

A drastic change in the reading indicates damage to the membrane.

1. Unscrew the small cap at the bottom from the outer body.

2. Remove the membrane base out from the small cap with a screw-driver.

3. Cut the O-ring that wraps the membrane onto the base and discard the old membrane.

4. Lay a piece of new membrane (20mm x 20mm) onto the base, with the shiny side of the membrane facing the base opening.

5. Put a small O-ring (supplied) on the top of the cone-shaped membrane mounting tool, and slide the O-ring down to the bottom of the cone.

6. Place the cone with the O-ring on it onto the membrane, and gently slide the O-ring into the groove on the base to wrap the membrane. Do not push the cone too hard while sliding down the O-ring into the groove, or the membrane may be damaged.

7. Trim the excessive membrane, screw the base into place, and re-assemble the electrode.

Connect the OMEGA ISE-8710 electrode to the meter in the same manner as any other combination electrode. No external reference electrode is required. To prevent air entrapment, mount the electrode at a 20° angle from the vertical.

Do not be alarmed if white crystals form at the end of the electrode. This is normal with pH electrodes. These crystals are potassium chloride. To resolve this, remove the shipping cap and rinse the electrode with distilled water to dissolve the crystals.

The electrode slope is the change in potential (mV) observed when the concentration changes by a factor of ten. The electrode slope should be checked on a daily basis. To check the slope:

1. Measure out 100 mL of ammonia-free distilled water and 1 mL of 10M NaOH and transfer to a 150 mL beaker. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tip into the solution and make sure the meter is in the mV mode.

2. Pipet 1 mL of the 0.1M or 1000 ppm standard into the solution. Adjust the calibration control (offset adjustment) to zero.

3. Pipet 10 mL of the 0.1M or 1000 ppm standard into the solution. If the electrode is operating properly, a reading of 56+/-2mV should appear on the meter, assuming the solution temperature is between 20° and 25°C. If the change in potential is not within this range, see the Troubleshooting section.

To minimize ammonia loss from samples when using the OMEGA ISE-8710:

1. Samples should be measured immediately after preparation or collection.

2. If immediate measurement is impossible, store the samples according to the instructions in the Sample Storage section.

3. The ratio of surface area to volume in the beaker should be minimized.

4. Beakers containing the samples or the standard should be kept covered between measurements.

5. Immediately before measurement, add 1mL 10M NaOH to 100 mL of sample or standard.

6. Samples, standards, and electrode must be at the same temperature.

7. Rinse the electrode with ammonia-free distilled water between measurements.

8. Stir samples and standards using a magnetic stirrer.

9. Use an insulating material, such as gauze or styrofoam, between the beaker and the magnetic stirrer to minimize heat transfer from the magnetic stirrer to the solution in the beaker.

Standards and samples must be at the same temperature, since a 1°C difference in temperature will result in a measurement error of approximately 2%.

10M NaOH must be added to standards and samples before measurement. When the 10M NaOH is added, all standards and samples should be in the range of pH 11 to 14 and have a total level of dissolved species below 1M. If the total level is greater than 1M, see the Effect of Dissolved Species section.

Measurement of ammonia can be expressed in units of moles/liter, ppm as nitrogen, or ppm as ammonia. The following table lists conversion units.

Direct measurement is the simplest, fastest method for measuring over a wide range of concentration. The temperature of the standards and the samples should be the same. Only one meter reading need be taken for each sample. The direct measurement procedure is suitable for the 4×10⁻⁶ to 1M ammonia range.

1. Using serial dilution of the 0.1M or of the 1000 ppm standards, prepare 10⁻² M (100 ppm), 10⁻³ M (10 ppm), and 10⁻⁴ M (1 ppm) standards.

2. Put 100 mL of the 10⁻³M (10 ppm) standard into a 150 mL beaker. Place the beaker on the magnetic stirrer and begin stirring. Place the tip of the electrode in the solution and add 1 mL of 10M NaOH to the standard. Set the meter in the mV mode. Adjust the calibration control (offset adjustment) to zero.

3. Put 100 mL of the 10⁻⁴M (1 ppm) standard into a 150 mL beaker. Place the beaker on the magnetic stirrer and begin stirring. After rinsing the electrode with ammonia-free distilled water, immerse the tip of the electrode in the solution and add 1 mL of 10M NaOH to the standard. Record the value when the reading has stabilized.

4. Put 100 mL of the 10⁻²M (100 ppm) standard into a 150 mL beaker. Place the beaker on the magnetic stirrer and begin stirring. After rinsing the electrode with ammonia-free distilled water, immerse the tip of the electrode in the solution and add 1 mL of 10M NaOH to the standard. Record the value when the reading has stabilized.

5. Using standard semi-logarithmic paper, plot the mV value (linear axis) vs. concentration (log axis) to create a calibration curve.

6. Measure 100 mL of sample, put it into a 150 mL beaker, place the beaker on the stirrer, and begin stirring. After rinsing the electrode with ammonia-free distilled water, immerse the tip of the electrode in the solution, and add 1 mL of 10M NaOH to the sample. Wait for the reading to stabilize and record the value. Using the calibration curve, determine the sample concentration.

The known addition technique is useful for measuring occasional samples, since no calibration is necessary. It requires that the concentration approximately doubles as a result of the addition. The sample concentration must be known within a factor of three.

1. Using the 0.1M or 1000 ppm standard, prepare a dilution about 10 times as concentrated as the sample concentration.

2. Measure out 100 mL of sample solution and add it to a 150 mL beaker. Add 1 mL of 10M NaOH. Place the beaker on the magnetic stirrer and begin stirring. Rinse the electrode and place the electrode tip in the solution.

3. Make sure the meter is in the mV mode. Adjust the calibration control (offset adjustment) to zero.

4. Add 10 mL of standard, by pipet, into the beaker. After the reading has stabilized, record the potential and determine the change in potential (ΔE).

5. Find the concentration ratio, Q, that corresponds to the change in potential from Table 3-3 (see separate Q&A). The original total concentration (Co) is determined by multiplying Q by the concentration of the added standard (Cs): Co = Q * Cs.

Table 3-3 provides the value of ‘Q’ based on the change in potential (ΔE) for a 10% volume change (e.g., adding 10 mL of standard to 100 mL of sample). After performing the known addition steps, find your measured ΔE in the table to get the corresponding Q value, which you then use to calculate the original sample concentration.

Measurements of a sample containing less than 4 x 10⁻⁶ M ammonia (0.07 ppm NH₃ or 0.06 ppm as N) can be speeded up by following these steps. If the internal filling solution is diluted with ammonia-free distilled water (1:10), response at low levels can improve.

1. Measure out 100 mL of a pH 4 buffer solution, add it to a 150 mL beaker, place beaker on magnetic stirrer, and begin stirring. Place the electrode tip in the solution for about 3 minutes.

2. Prepare a serial dilution of the 0.1M or 1000 ppm standard to 10⁻²M or 100 ppm.

3. To a 250 mL erlenmeyer flask, add 100 mL of ammonia-free distilled water and 1 mL of 10M NaOH. Place on magnetic stirrer and begin stirring. After rinsing the electrode, place the tip in this calibration solution. Make sure that the meter is in the mV mode.

4. Using a 1 mL graduated pipet (A) and a 2 mL pipet (B), add increments of the 10⁻²M or 100 ppm dilution to the calibration solution using the steps outlined in Table 3-2. Allow the reading to reach equilibrium and record the electrode potential (linear axis) on semi-logarithmic paper to obtain the calibration curve.

5. After rinsing the electrode, immerse the electrode tip in pH 4 buffer for 3 minutes. Be sure to use the magnetic stirrer.

6. Place 100 mL of the sample into a 150 mL beaker, add 1 mL of 10M NaOH, place the beaker on the magnetic stirrer, and begin stirring. After rinsing the electrode, immerse the tip in the solution. When the reading has stabilized, record the mV potential.

7. Determine the concentration from the calibration curve. (Prepare a new low-level calibration curve on a daily basis using freshly prepared solutions).

Since the membrane of the OMEGA ISE-8710 is hydrophobic, a solution containing a surfactant or a non-aqueous solution will wet the membrane, causing measurement difficulties. For such samples (e.g., sewage, latex paints), the electrode must be suspended above the sample to measure ammonia in the gas phase.

1. Adjust the sample pH to 11-13 with 10M NaOH.

2. Transfer the sample to an erlenmeyer flask large enough to contain approximately 2/3 volume of air after sample addition.

3. Place a magnetic stir bar in the flask.

4. Fit the flask with a one-hole rubber stopper (opening large enough to insert the electrode snugly).

5. Insert the electrode until it is just above the level of liquid in the flask. The closed flask now forms an air-tight closed system with the gas phase saturated with water vapor. The partial pressure of ammonia is in equilibrium with the solution.

For gas phase measurements, calibrate the electrode in a closed flask using standards or use the known addition method in the closed flask. Note that the response time for the electrode will be longer in the gas phase than if it were immersed in a surfactant-free, aqueous solution.

Volatile amines interfere with the operation of the ammonia electrode. Most other gases do not interfere as they are converted to the ionic form in basic solutions.

The level of ions in solution can change the solubility of ammonia, though ionic species cannot cross the gas-permeable membrane and are not considered direct electrode interferences. The level of ions in sample solution and standards do not interfere, given that they are equal. The same holds true for dissolved species.

Low results can occur in direct measurements due to the presence of some metallic ions and their complexation effect on ammonia.

If the electrode slope is found to be slow during operation, you can check the inner body sensing elements with the following procedure:

1. Disassemble the ammonia electrode. If the electrode is dry, soak the glass tip of the inner body in Ammonia Electrode Internal Filling Solution (PHFS-8710) for at least two hours.

2. Rinse the inner body thoroughly with ammonia-free distilled water.

3. Put 100 mL of pH 7 buffer in a 150 mL beaker, place the beaker on the magnetic stirrer, and begin stirring. Immerse the tip of the inner body in the solution so that the reference element is covered. Make sure the meter is in the mV mode. Record the meter reading.

4. Rinse the inner body thoroughly in ammonia-free distilled water. Put 100 mL of pH 4 buffer in a 150 mL beaker, place the beaker on the magnetic stirrer, and begin stirring.

5. Immerse the tip of the inner body in the solution so that the reference element is covered. Observe the change in the meter reading carefully.

In less than 30 seconds after immersion, the reading should change 100 mV. The meter reading should stabilize in 3-4 minutes, with a difference greater than 150 mV, if the inner body sensing elements are operating properly.

Proper storage of the OMEGA ISE-8710 is crucial for its performance.

Correcting Erratic Results: If erratic results are obtained from accidentally leaving the electrode in air, the space between the sensing element and the inside of the membrane may be dry. To remedy this, withdraw the glass electrode from the membrane by pulling the cable slightly to allow new filling solution to flow into the space.

For Low-Level Measurements: Between measurements, immerse the tip of the electrode in pH 4 buffer.

For Normal-Range Measurements: Keep the electrode tip immersed in a 0.001M or 10 ppm standard with added NaOH.

Overnight or Weekend Storage: Immerse the tip in the 0.1M standard without added NaOH. Do not store overnight in the pH 4 buffer.

Long-Term Storage: For longer periods of time, completely disassemble the electrode. Rinse the inner body, the outer body, and the cap with distilled water. After drying, reassemble the electrode without filling solution.

If samples have osmotic strengths greater than 1M, they should be diluted before measurement. However, this dilution should not reduce the ammonia level below 10⁻⁵ M. If dilution is not possible, the sample can be measured by adjusting the osmotic strength of the internal filling solution. The total level of dissolved species in the internal filling solution may be adjusted by adding 0.425 grams of reagent-grade sodium nitrate (NaNO₃) to 10 mL of the internal filling solution.

The components of the OMEGA ISE-8710 are as follows:

A. Big cap

B. O-ring

C. Outer body

D. Membrane base

E. Membrane

F. O-ring

G. Small cap

H. Inner glass electrode

I. Gas-sensing electrode (assembled)

Temperature affects both the shift and the slope of the OMEGA ISE-8710 electrode response. A 2% error results with a 1°C temperature change for a 10⁻³M solution. Standards and samples should always be at the same temperature. The theoretical slope changes with temperature as shown in the table below.

If you are getting an out of range reading, check the following:

Possible Cause: Defective meter

Next Step: Check meter with shorting strap.

Possible Cause: Electrode not plugged in properly

Next Step: Reseat electrode.

Possible Cause: Reference electrode junction is dry

Next Step: Hold cap and lift outer sleeve to expel a few drops of filling solution.

Possible Cause: Reference electrode not filled

Next Step: Be sure reference electrode is filled.

Possible Cause: Calibration control not turned far enough

Next Step: Continue turning the calibration control.

For noisy or unstable readings, consider these causes and solutions:

Possible Cause: Defective meter

Next Step: Check meter with shorting strap.

Possible Cause: Wrong reference electrode

Next Step: Do not use calomel or Ag/AgCl (frit or fiber type) reference electrode.

Possible Cause: TISAB not used

Next Step: Use recommended TISAB.

Possible Cause: Meter or stirrer not grounded

Next Step: Ground meter or stirrer.

If the reading is changing or drifting in one direction, check the following:

Possible Cause: Samples and standards at different temperatures

Next Step: Allow solutions to come to room temperature before measurement.

Possible Cause: Incorrect reference

Next Step: Use recommended fill solution.

Possible Cause: Membrane dirty

Next Step: Remove organic deposits.

Possible Cause: Glassware used

Next Step: Use plastic labware.

If you are experiencing a low slope or no slope during calibration:

Possible Cause: Standards contaminated

Next Step: Prepare fresh standards.

Possible Cause: Standard used as TISAB

Next Step: Use TISAB.

If your results are incorrect but the calibration curve appears to be good, check for these issues:

Possible Cause: Incorrect scaling of semilog paper

Next Step: Plot millivolts on the linear axis. On the log axis, be sure concentration numbers within each decade are increasing with increasing concentration.

Possible Cause: Incorrect sign

Next Step: Be sure to note the sign of the millivolt reading correctly.

Possible Cause: Incorrect standards

Next Step: Prepare fresh standards.