GENERAL INSTRUCTIONS Introduction COLE-PARMER INDUSTRIAL AMMONIUM ION ELECTRODE INSTRUCTION MANUAL The Cole-Parmer Industrial Ammonium Ion Electrode is used to quickly, simply, accurately, and economically measure ammonium in in-line batch processing, control processing, or water treatment applications. Required Equipment 1. Cole-Parmer Ion Concentration Controller, Cat. No. 56105-00. Cord with U.S. standard plug, Cat. No. 50001-68 or cord with European plug, Cat. No. 50001-70 is necessary. 2. Cole-Parmer Industrial Ammonium Ion Electrode, Cat. No. 27077-01. It is a sealed, double junction electrode which is easily connected to the Ion Concentration Controller with stripped wire ends. 3. Cole-Parmer Industrial Housings to protect the Ammonium Ion Electrode. See page 1185 of the Cole-Parmer Catalog to choose an Industrial Housing. Required Solutions 1. Deionized or distilled water for solution and standard preparation. 2. Cole-Parmer Ionic Strength Adjuster, 5M NaCl, Cat. No. 27503-53 to keep a constant background ionic strength present in the solution. To prepare the 5M NaCl ISA from your own laboratory stock, add 292 grams of reagent-grade sodium chloride (NaCl) to a 1000 ml volumetric flask about half full of distilled water. Swirl the flask to dissolve the solid and fill to the mark with distilled water. Cap the flask and invert several times to mix the solution. Add 2 ml of ISA to every 100 ml of sample or standard solution for a back-ground ionic strength of 1
0.10M. 3. Cole-Parmer Ammonium Standard, 1000 ppm NH 4 +1, Cat. No. 27503-01. To prepare this standard, add 2.97 grams of reagent-grade ammonium chloride to a 1 liter volumetric flask about half full of distilled water. Swirl the flask to dissolve the solid and fill to the mark with distilled water. Cap the flask and invert several times to mix the solution. ELECTRODE PREPARATION Remove the rubber cap covering the electrode tip. Gently shake the electrode downward in the same manner as a clinical thermometer to remove any air bubbles which may be trapped behind the ammonium membrane. Prior to first usage, or after long-term storage, immerse the ammonium membrane in ammonium standard for thirty minutes. The electrode is now ready for use. CALIBRATION PROCEDURE This quick start procedure is designed to get the unit operational. To set alarms and relays, consult the Operator's manual. Standards must be made in the range of 0.01 to 1000 ppm, as the controller will not accept standards outside this range. 1. Connect the electrode with the stripped wire ends to the controller by inserting the center wire into slot #22 and inserting the outer wire braid into slot #21. 2. Press the [Mode] key 9 times until the display reads [Configuration] 3. Press the [Cal] key. 4. The display should read [KB UNLOCKED]. Press [ENT] 5. The display should now read: [LCD Contrast: 4]. Press [ENT]. 6. The display should now read: [Access Nr.: 0]. Press [ENT]. 7. The display should now read [Valence: #]. Pressing the [/\] and [\/] keys, choose the proper valence of the ion you are testing for. Press [ENT] (If the factory setting is already the correct #, press [ENT]). E.G. For Silver/sulfide, choose -1; For Sodium, choose +1; for Calcium, choose +2. 8. If you chose a valence which is different from the factory setting, the display will read [Reset ppm Cal?] Press [ENT]. 9. The display will now read [Scale: 100.0 ppm]. Press [Mode]. 10.The display will now read [Configuration]. 11.Press [Mode] key 6 times. Make sure the display reads: [Termoc.: OFF]. If so, go to step 14. If the display reads [Termoc.: ON], go to step 12. 2
12.Press the [CAL] key. Press the [\/] key so the display reads [CAL TC: OFF]. The display will flash [UPDATE] and then read: [CAL TC:0.198%/oC]. 13.Press the [Mode] key. The display will now read: [Termoc.: OFF]. 14.Press the [Mode] Key 8 times. The display should read [ppm 5 point cal].(note: It is not necessary to do a 5 point calibration. You can calibrate from 1 to 5 points). Prepare 1 to 5 standard ISE solutions whose concentrations vary by tenfold. Use the 1000 ppm ISE solution as the stock solution. Use the serial dilution method for this preparation and add the ISA when required to each standard 15.To start the calibration sequence, press the [cal] button. The display will now read [Cal Point Nr 1]. 16.Insert the ISE electrode in the standard with the lowest ppm value.(note: for best results, ppm standards should be used in the following order: {0.1, 1, 10, 100, 1000} 17.Press the [cal] button. 18.When a stable millivolt reading is obtained, press the [ENT] button. 19.The display should now read [P1 Decade: 100.0] 20.Choose the ppm range within which the standard lies using the [/\] or [\/] buttons. - select 10.00 if the standard solution value is < 10.00 ppm - select 100.0 if the standard solution value is > 10.00 ppm - and < 100.0 ppm - select 1000 if the standard solution is > 100.0 ppm 21.Press the [ENT] button. 22.The display should now read [P1 Value: (# selected in step 20)]. 23.Choose the actual ppm value of the standard solution by pressing the [/\] or [\/] buttons until the exact ppm value is displayed. Press the [ENT] key to confirm the ppm value. 24.The screen should read [CAL POINT Nr. 2]. If no further calibration points are required, go to step 25. If you wish to enter more calibration points, go to step 26. 25.If no further calibration points are desired, press the [ENT] button. The display will now read [End Point Cal?]. Press the [ENT] button once more. The screen will now display [Zr Pt updated]. Press [ENT]. The display will now read [ppm 5 point cal]. To display Concentration value, press the [Mode] key 10 times. To display millivolt signal supplied by electrode, press the [Mode] key once. 26.If further calibration points are required, repeat steps 16 through 25. After the 5th calibration point is confirmed at step 11, the meter will flash [chk cal pt] and then read [VALUE UPDATED]. Press the [ENT] key. The display will now 3
read [ppm 5 point cal]. To display Concentration value, press the [Mode] key 10 times. To display millivolt signal supplied by electrode, press the [Mode] key once. MEASUREMENT Measuring Hints Check electrode slope weekly if not daily. Always use fresh standards for calibration. All samples and standards should be at the same temperature for precise measurement and below 40 o C. A difference of 1 o C in temperature will result in a 2% measurement error. All samples and standards should be at the same pressure for precise measurement. The sensing membrane is normally subject to water uptake and might appear milky. This has no effect on performance. 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. Interferences should be absent. If they are present, use the procedures found in the Interferences section to remove them. The ph range for the ammonium ion electrode is 4-10. Units of Measurement Ammonium concentrations are measured in units of parts per million as ammonium, parts per million as N, moles per liter, or any other convenient concentration unit. Table 1 indicates some 4
of the concentration units. TABLE 1: Concentration Unit Conversion Factors ppm NH 4 + ppm N moles/liter NH 4 + 1.80 1.40 1x10-4 18.00 14.00 1x10-3 180.00 140.00 1x10-2 ELECTRODE CHARACTERISTICS Reproducibility Electrode measurements reproducible to ±10% of reading can be obtained if the electrode is calibrated every hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Interferences Table 3 lists some common cations that, if present in high enough levels, will cause electrode interferences and measurement errors or electrode drift when using the ammonium ion electrodes. Electrode drift and slow response could indicate the presence of high interference from the ions listed. Soak the electrode(s) in distilled water for an hour, then for two hours in ammonium standard solution to restore proper response. TABLE 3: Concentration of Possible Interferences Causing a 10% Error at Various Levels of NH 4 Cl; Background Ionic Strength is 0.12M NaCl. Interferences (moles/liter) 10-2 M NH 4 +1 10-3 M NH 4 +1 10-4 M NH 4 +1 Cs +1 3.0X10-3 3.0X10-4 3.0X10-5 K +1 1.0X10-2 1.0X10-3 1.0X10-4 Tl +1 6.0X10-2 6.0X10-3 6.0X10-4 H +1 1.0X10-1 1.0X10-2 1.0X10-3 Ag +1 1.0X10 1 1.0 1.0X10-1 +Tris +1 1.0X10 1 1.0 1.0X10-1 5
Li +1 2.0X10 1 2.0 2.0X10-1 Na +1 2.0X10 1 2.0 2.0X10-1 +Tris +1 is the cation of tris(hydroxymethyl)aminomethane Interferences (ppm) 100 ppm NH 4 + 10 ppm NH 4 + 1 ppm NH 4 + Temperature Influences Cs +1 1.0X10 2 1.0X10 1 1.0 K +1 2.7X10 2 2.7X10 1 2.7 Tl +1 3.1X10 3 3.1X10 2 3.1X10 1 H +1 1.6pH 2.6pH 3.6pH Ag +1 2.7X10 5 2.7X10 4 2.7X10 3 +Tris +1 3.1X10 5 3.1X10 4 3.1X10 3 Li +1 3.5X10 4 3.5X10 3 3.5X10 2 Na +1 1.1X10 5 1.1X10 4 1.1X10 3 Samples and standards should be at the same temperature, since electrode potentials are influenced by changes in temperature. A 1 o C difference in temperature results in a 2% error at the 1.0X10-3 M level. Provided that temperature equilibria has occurred, the ammonium ion electrodes can be used at temperatures from 0 o - 40 o C continuously. Room temperature measurements are recommended, since measurements at temperatures markedly different from room temperature may require equilibrium times up to one hour. 6
Electrode Response Plotting the electrode mv potential against the ammonium concentration on semi-logarithmic paper results in a straight line with a slope of about 53 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 for ammonium concentration above 1.0X10-5 M to several minutes near the detection limit. Refer to Figure 2. 7
Limits of Detection The upper limit of detection is 1M in pure ammonium chloride solutions. The upper limit of detection is above 1.0X10-1 M when other ions are present, but the possibility of a liquid junction potential developing at the reference electrode and the "salt extraction effect" are two limiting factors. Some salts may be extracted into the electrode membrane at high salt concentrations causing deviation from theoretical response. Calibrate the electrode at four or five intermediate points, or dilute the sample, to measure samples between 1.0X10-1 M and 1M. The slight water solubility of the ion exchanger in the sensing module, which causes deviation from theoretical response, determines the lower limit of detection. The theoretical response at low levels of ammonium chloride compared to actual response is shown in Figure 1. ph Effects Hydrogen ion interferes with measurements of low levels of ammonium ion although the electrode can be used over a wide ph range. Table 3 should be used to determine the minimum ph at which low level ammonium measurements can be made without more than a 10% error due to hydrogen ion interference. Electrode Life The ammonium electrode will last six months in normal laboratory use. On-line measurement will shorten operational lifetime to several months. In time, the response time will increase and the calibration slope will decrease to the point calibration is difficult and electrode replacement is required. Electrode Storage The Cole-Parmer Ammonium Ion Electrodes may be stored in-line if the membrane is kept wet. For storage off-line, rinse and dry the ammonium membrane electrode and cover the tip with any protective cap shipped with the electrode(s). ELECTRODE THEORY Electrode Operation The ammonium electrode consists of an electrode body containing an ion exchanger in a sensing membrane. This sensing membrane contains a liquid internal filling solution in contact with a 8
gelled organophilic membrane containing an ammonium selective ion exchanger. An electrode potential develops across the membrane when the membrane is in contact with an ammonium solution. Measurement of this potential against a constant reference potential with a digital ph/mv meter or with a specific ion meter depends on the level of free ammonium ion in solution. The level of ammonium ions, corresponding to the measured potential, is described by the Nernst equation: where: E = E o + S logx E = measured electrode potential E o= reference potential (a constant) S = electrode slope ( ~ 53 mv/decade) X = level of ammonium ions in solution The activity, X, represents the effective concentration of the ions in solution. Total ammonium concentration, C t, includes free ammonium ions, C f, plus bound or complexed ammonium ions, C b. Since the ammonium electrodes only respond to free ion, the free ion concentration is: C f = C t - C b The activity is related to the free ion concentration, C f, by the activity coefficient,γ,by: X = γ C f Activity coefficients vary, depending on total ionic strength, I, defined as: I = ½ ΣC x Z x 2 where: C x = concentration of ion X Z x = charge of ion X Σ = 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, γ, 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 9
standards. The recommended ISA for ammonium is sodium chloride, NaCl. MEASUREMENT PROBLEMS...SIMPLE CAUSES? -NOT REPRODUCIBLE sample carryover? sample interferences or complexing agents present? contaminated reference electrode junction -SLOW RESPONSE (READINGS SLOWLY CHANGING) electrode poisoned by sample? -OUT OF RANGE READING electrode(s) not plugged into controller properly? not enough fill solution left in reference? electrode not in sample solution? air bubble on electrode surface? -LOW SLOPE OR NO SLOPE standards are old? contaminated? made wrong? sample ph has not been adjusted properly to operating range of electrode? air bubble on electrode surface? controller okay? not enough fill solution left in reference? failed electrode? -NOISY RESPONSE (READINGS RANDOMLY CHANGING controller not grounded? air bubble on electrode surface? controller okay? not enough fill solution left in reference? -DRIFTY RESPONSE (READINGS CONTINUOUSLY CHANGING) excessive leaking at reference electrode junction? clogged reference electrode junction? sensing membrane poisoned by sample? temperature problems? sample too concentrated? sensing membrane needs conditioning? -INACCURATE (BUT CALIBRATION IS OK) standards are incorrect? sample is not ph adjusted properly? sample carryover? sample interferences or complexing agents present? 10
TROUBLESHOOTING DIFFICULT MEASUREMENT PROBLEMS -COMPONENTS OF THE ELECTRODE SYSTEM -CONTROLLER -ION-SELECTIVE ELECTRODE -PROPER CALIBRATION -STANDARD(S) -ph ADJUSTMENT -SAMPLE VARIABLES -CONCENTRATION RANGE -PRESSURE -TEMPERATURE -ph -INTERFERENCES -COMPLEXATION perform checkout procedure in instruction manual perform electrode slope check, inspect for physical damage. monitor leak rate on reference side prepare fresh standards by serial dilution adjust ph to operational range of electrode sample out of range? wrong sample pressure? wrong sample temperature? wrong sample ph? interferences in the sample? complexing agents in sample? 11
SOME QUESTIONS ASKED ABOUT ION-SELECTIVE ELECTRODES 1.How often do you need to calibrate? Recalibrate at least once a week, or daily if in doubt of your results being accurate or reproducible. 2.How long after being opened or made are standards good? The stock standard will last at least six months before discarding, whereas diluted standards treated with ISA/pH buffer should be prepared weekly. 3.How do you store the electrodes? It is best to store them dry when they are not to be used in the next week or so. Empty out the filling solution in the gas-sensing electrode. For shorter periods, store in dilute standard approximating the sample concentration, and ISA/pH buffer added when required. 4.Can you do temperature compensation with ISE? Yes, it is possible, but somewhat difficult. First, you have to know the isopotential point for a given electrode system. Second, the concentration of the sample has to be similar in concentration to the isopotential point or else the temperature correction will be very inaccurate. Third, the temperature of the sample can not exceed the operational temperature range of the ISE. Fourth, very little isopotential point data for ISE's is available at this time. It is best to standardize and measure samples at the same temperature without using temperature compensation. 5.Can you do in-line continuous ISE measurement without treating the sample? Yes, direct measurement is possible in many cases without ISA/buffer addition to the sample stream. However, Fluoride, Sulfide, Ammonia, and Sodium Electrodes do require ph adjustment and must have ISA/ buffer added to the sample stream. 6.Which standards should be used with the ISE? The most obvious choice will be determined by what concentration units are desired (e.g. ppm as what?). Also, if an electrode is being used to measure another ion (e.g. sulfate with a lead electrode), use a stock standard of the ion to be measured (e.g. sulfate). 7.Why buy a combination electrode instead of separate ones? Advantages: no external reference electrode needed more economical than price of both one less electrode to fit in the process 12
AMMONIUM ELECTRODE SPECIFICATIONS (MODEL # 27077-01) -SLOPE 53 +/- 3 mv/decade -REPRODUCIBILITY +/- 2% -INTERFERENCES K+1 -TEMPERATURE RANGE 0 to 40 degrees C -PRESSURE RANGE 0 to 30 psi -RESPONSE TIME 95% response in 30 seconds -STORAGE for in-line storage keep the membrane wet, for off-line storage rinse and dry the membrane, and cover with a protective cap -CONCENTRATION RANGE 5 x 10-6 M to 1M 0.1 ppm to 18,000 ppm -ph RANGE 4 to 10 ph -RESISTANCE 100 megaohms -CARE, MAINTENANCE, treat with distilled water & CLEANING followed by ammonium standard -TEMPERATURE COMPENSATION/ Yes -ISOPOTENTIAL POINT 10 ppm ammonium -ON-LINE CAPABILITY Yes -APPLICATIONS: Boiler Feed Water Natural Waters ORDERING INFORMATION P/N DESCRIPTION 56105-00 Ion Concentration Controller 50001-68 Cord with U.S. standard plug 50001-70 Cord with European plug 27077-01 Industrial Ammonium Ion Electrode, sealed, double junction electrode which is easily connected to the Ion Concentration Controller with stripped wire ends. Cole-Parmer Industrial Housings to protect the Ammonium Ion Electrode. See page 1185 of the Cole-Parmer Catalog to choose an Industrial Housing. 27503-01 Ammonium Standard, 1000 ppm NH 4 +1 27503-53 Ammonium ISA (Ionic Strength Adjustor),5 M NaCl 13
INDEX General Instructions...1 introduction...1 required equipment...1 required solutions...1 Electrode Preparation...2 Calibration Procedure...2 Measurement...4 measuring hints...4 sample requirements...4 units of measurement...4 Electrode Characteristics...5 reproducibility...5 interferences...5 temperature influences...6 electrode response...7 limits of detection...8 ph effects...8 electrode life...8 electrode storage...8 Electrode Theory...8 electrode operation...8 Common Measurement Problems, Simple Causes...10 Troubleshooting Difficult Measurement Problems...11 Some Questions Asked About Ion-Selective Electrodes...12 Ammonium Electrode Specifications...13 Ordering Information...13 Index...14 14