Instruction Manual Ammonia Gas-sensing Electrode EUTECH INSTRUMENTS PTE LTD. AMMONIA GAS-SENSING ELECTRODE INSTRUCTION MANUAL 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. 2. A pH/mV meter or an ion meter, either line operated or portable. Semi-logarithmic 4-cycle graph paper for preparing calibration curves when using the meter in the mV mode. A magnetic stirrer. Eutech Ammonia Gas-sensing Electrode, Code No. EC-NH3-01. 3. 4. Required Solutions 1. 2. Deionized or distilled water for solution preparation. Eutech Ammonia Standard, 0.1M NH4Cl, Code No. EC-SCS-AA1-BT. To prepare this 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. Eutech Ammonia Standard, 1,000 ppm NH3 as N, Code No. EC-SCS-AA2-BT. To prepare 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. Eutech Ammonia Standard, 100 ppm NH3 as N, Code No. EC-SCS-AA3-BT. To prepare 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 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. 4. 1 Instruction Manual Ammonia Gas-sensing Electrode 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. 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. 6. 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 20o 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. 2. 3. 4. Connect a newly assembled electrode to a pH/mV meter. Lower the electrode tip in distilled water. Record the reading after stirring the distilled water for about 15 minutes. 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. Unscrew the bottom cap from the outer body. Remove the old membrane from around the threads and electrode tip opening. 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 2 2. 3. Instruction Manual Ammonia Gas-sensing Electrode 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. 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. 5. 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 20o 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. 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. 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. 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 20o and 25oC. See the TROUBLESHOOTING section if the potential change is not within this range. 2. 3. 4. 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 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. 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. 2. 3 Instruction Manual Ammonia Gas-sensing 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. Rinse the electrodes with distilled water and blot dry. 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. Adjust the meter to the concentration of the standard and fix the value in the memory. 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. 4. 5. 6. 7. 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. All samples and standards should be at the same temperature for precise measurement. A difference of 1oC 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 C02 increases with increasing temperature. 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. 4 Instruction Manual Ammonia Gas-sensing Electrode 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 dissolved species below 1M. If the total level is greater than 1M, the sample should be diluted before measurement. For further explanation, see the section entitled Effect of Dissolved Species. Units of Measurement Measurement of ammonia can be expressed in units of moles/liter, ppm as nitrogen, ppm as ammonia, or other convenient concentration unit. Table 1 lists conversion units. TABLE 1: Concentration Unit Conversion Factors moles/liter 10-2 10-3 10-4 ppm N 140.0 14.0 1.4 ppm NH3 170.0 17.0 1.7 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 ammonia solutions. The temperature of both sample solutions and standard solutions should be the same. Direct Measurement of Ammonia (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 10-2M, 10-3M, and 10-4M or 1,000, 100 and 10 ppm ammonia standards. Add 1 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. Place the most dilute solution (10-4M 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 tip into the solution. After the reading has stabilized, record the mV value. Place the mid-range solution (10-3M or 100 ppm) on the magnetic stirrer and begin stirring at a constant rate. After rinsing the electrode with distilled water, blot dry, and immerse the electrode tip in the solution. When the reading has stabilized, record the mV value. Place the most concentrated solution (10-2M or 1,000 ppm) on the magnetic stirrer and begin stirring at a constant rate. After rinsing the electrode with distilled water, blot dry, and immerse the electrode tip in the solution. When the reading has stabilized, record the mV value. 5 2. 3. 4. Instruction Manual Ammonia Gas-sensing 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 1.0 x 10-5M. A typical calibration curve can be found in Figure 1. A calibration curve is constructed on semi-logarithmic paper when using the pH/mV meter in the millivolt mode. The measured ...