COLE-PARMER INDUSTRIAL NITRATE ION ELECTRODE INSTRUCTION MANUAL Cole-Parmer Instrument Company (800)323-4340 Fax:(847)247-2929 625 East Bunker Court, Vernon Hills, Illinois 60061 http://www.coleparmer.com Email: techinfo@coleparmer.com 1
TABLE OF CONTENTS General Instructions...3 introduction...3 required equipment...3 required solutions...3 General Preparation...4 electrode preparation...4 Calibration Procedure...4 Measurement...5 measuring hints...5 sample requirements...6 units of measurement...6 Electrode Characteristics...6 reproducibility...6 interferences...6 temperature influences...7 electrode response...8 limits of detection...9 ph effects...9 electrode life...10 electrode storage...10 Electrode Theory...10 electrode operation...10 Common Measurement Problems, Simple Causes...11 Troubleshooting Difficult Measurement Problems...12 Some Questions Asked About Ion-Selective Electrodes...13 Nitrate Electrode Specifications...15 Ordering Information...15 2
COLE-PARMER INDUSTRIAL NITRATE ION ELECTRODE INSTRUCTION MANUAL GENERAL INSTRUCTIONS Introduction The Cole-Parmer Industrial Nitrate Ion Electrode is used to quickly, simply, accurately, and economically measure nitrate 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 Nitrate Ion Electrode, Cat. No. 270770. 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 Nitrate Ion Electrode. Contact Cole-Parmer to choose an Industrial Housing. Required Solutions 1. Deionized or distilled water for solution and standard preparation. 2. Cole-Parmer Nitrate Standard, 1000 ppm NO 3, Cat. No. 27503-24. To prepare this 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. 3. Cole-Parmer Ionic Strength Adjuster (ISA), 2M (NH 4 ) 2 SO 4, Cat. No. 27503-60 (OPTIONAL). To prepare this solution from your own laboratory stock, half fill a 1000 ml volumetric flask with distilled water and add 264 grams of reagentgrade ammonium sulfate, (NH 4 ) 2 SO 4. 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
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 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. 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. 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. 4
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 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 C. A difference of 1 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. 5
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 nitrate ion electrode is 2.51. 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, or any other convenient concentration unit. Table 1 indicates some of the concentration units. TABLE 1: Concentration Unit Conversion Factors ppm NaNO 3 ppm NO 3 850.0 620.0 85.0 62.0 8.5 6.2 ELECTRODE CHARACTERISTICS Reproducibility Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is independent of concentration within the electrode s operating range. Interferences Certain anions are electrode interferences and will cause electrode malfunction, drift or measurement errors if present in high enough levels. The level of interfering common anions that will cause a 10% error at three levels of nitrate is given in Table 2. 6
TABLE 2: Concentration of Possible Interferences Causing a 10% Error at Various Levels of Nitrate; Background Ionic Strength of 0.12M (NH 4 ) 2 SO 4. Interferences (ppm) 100 ppm N 10 ppm N 1 ppm N Cl 7600 760 76.0 NO 2 230 23.0 2.30 Br 400 40.0 4.00 CN 20.0 2.00 0.20 ClO 3 30.0 3.00 0.30 I 4.00 0.40 0.04 ClO 4 0.07 0.007 0.0007 Interferences such as bromide, iodide and cyanide can be removed by precipitation with 0.5 grams of silver sulfate added to 100 ml of sample. Nitrite interference can be removed by adding 0.3 grams of sulfamic acid to 100 ml of sample. Carbonate and bicarbonate, which are weak interferences, can be removed by acidifying the sample to ph 4.5 with sulfuric acid. Organic (carboxylic) anions hinder the nitrate electrode response and can be removed by adding 10 grams of aluminum sulfate to 100 ml of sample. The above interference removal procedures require similar treatment of standards as well as samples. If the electrode is exposed to high levels of interfering ions which cannot be removed, the electrode reading may drift and the response may become sluggish. Restore performance by soaking in distilled water for 30 minutes followed by soaking in nitrate standard for 30 minutes. Temperature Influences 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 10 ppm level. Because of the solubility equilibria on which the electrode depends, the absolute potential of the reference electrode changes slowly with temperature. The slope of the nitrate electrode, as indicated by the factor S in the Nernst equation, also varies with temperature. Table 3 gives values for the S factor in the Nernst equation for the nitrate ion. The operating range of the nitrate ion electrode is 0-40 C, provided that temperature equilibrium has occurred. If the temperature varies substantially from room temperature, equilibrium times up to one hour are recommended. 7
TABLE 3: Temperature vs. Values for the Electrode Slope Electrode Response Temp ( o C) S 0 54.20 10 56.18 20 58.18 25 59.16 30 60.15 40 62.13 50 64.11 Plotting the mv potential against the nitrate concentration on semi-logarithmic paper results in a straight line with a slope of about 56 mv per decade. (Refer to Figure 1.) + 30 Figure 1 Typical Nitrate Electrode Calibration Curve + 70 electrode potential (mv) + 110 + 150 ~ 56 mv 10-fold change + 190 + 230 0.1 1 (ppm as N) 10 100 1000 (ppm as NO 3- ) 0.1 1 10 100 1000 10-6 10-5 10-4 10-3 - NO3 Concentration (M) 10-2 10 8
The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from one minute or less in highly concentrated solutions to several minutes near the detection limit. (Refer to Figure 2.) Figure 2 Typical Electrode Time Response to Step Changes in NaNO 3 + 50 + 75 + 100 10-3 M to 10-2 M NaNO 3 electrode potential (mv) + 125 + 150 + 175 + 200 + 225 10-3 M to 10-4 M NaNO 3 10-3 M to 10-5 M NaNO 3 10-3 M to 10-6 M NaNO 3 1 2 Time (minutes) 3 4 Limits of Detection The upper limit of detection in pure sodium nitrate solutions is 1M. In the presence of other ions, the upper limit of detection is above 10 M nitrate, but two factors influence this upper limit. Both the possibility of a liquid junction potential developing at the reference electrode and the salt extraction effect influence this upper limit. Some salts may infuse into the electrode membrane at high salt concentrations, causing deviation from the theoretical response. Either dilute samples between 1M and 10 M or calibrate the electrode at 4 or 5 intermediate points. The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the sensing portion of the electrode. Refer to Figure 1 for a comparison of the theoretical response to the actual response at low levels of nitrate. Nitrate measurements below 10-5 M NO 3 (0.6 ppm as NO 3 ) should employ low level procedures. ph Effects The operating range of the nitrate electrode is from ph 2.5 to ph 11. 9
Electrode Life The nitrate electrode will last six months in normal laboratory use. On-line measurement might shorten operational lifetime to several months or less. 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 nitrate electrodes may be stored for short periods of time in 10-2 M nitrate solution. For longer storage (longer than two weeks), rinse and dry the nitrate membrane and cover the tip with any protective cap shipped with the electrodes. ELECTRODE THEORY Electrode Operation The nitrate electrode consists of an electrode body containing a liquid internal filling solution in contact with a gelled organophilic membrane containing a nitrate ion exchanger. When the membrane is in contact with a solution containing free nitrate ions, an electrode potential develops across the membrane. This electrode potential is measured against a constant reference potential, using a standard ph/mv meter or an ion meter. The level of nitrate ions, corresponding to the measured potential, is described by the Nernst equation: E = E 0 - S log X where: E E 0 S X = measured electrode potential = reference potential (a constant) = electrode slope (~ 56 mv/decade) = level of nitrate ions in solution The activity, X, represents the effective concentration of the ions in solution. The total nitrate ion concentration, C t, is the sum of free nitrate ion, C f, and complexed or bound perchlorate ion, C b. The electrode is able to respond to only the free ions, whose concentration is : C f = C t - C b Since nitrate ions form very few stable complexes, the free ion concentration may be equated to the total ion concentration. The activity is related to the free ion concentration, C f, by the activity coefficient, γ, by: X = γ C f 10
Activity coefficients vary, depending on total ions strength, I, defined as: I = ½ ΣC x Z x ² where: C x Z x Σ = concentration of ion X = charge of ion X = 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 standards. The recommended ISA for nitrate is (NH 4 ) 2 SO 4. Solutions other than this may be used as ionic strength adjusters as long as ions that they contain do not interfere with the electrode s response to nitrate ions. The reference electrode must also be considered. When two solutions of different composition are brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur from the inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally across the solution boundary resulting in a potential difference between the two solutions, since ions diffuse at different rates. When making measurements, it is important to remember that this potential be the same when the reference is in the standardizing solution as well as in the sample solution or the change in liquid junction potential will appear as an error in the measured electrode potential. The composition of the liquid junction filling solution in the reference electrode is most important. The speed with which the positive and negative ions in the filling solutions diffuse into the samples should be as nearly equal as possible, that is, the filling solution should be equitransferent. No junction potential can result if the rate at which positive and negative charge carried into the sample is equal. Strongly acidic (ph = 0-2) and strongly basic (ph = 124) solutions are particularly troublesome to measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask their effect on the junction potential with any concentration of equitransferent salt. One must either calibrate the electrodes in the same ph range as the sample or use a known incremental method for ion measurement. COMMON MEASUREMENT PROBLEMS...SIMPLE CAUSES? - NOT REPRODUCIBLE sample carryover? sample interferences or complexing agents present? contaminated reference electrode junction? 11
- SLOW RESPONSE (READINGS SLOWLY CHANGING) electrode stored in wrong solution? electrode poisoned by sample? - OUT OF RANGE READING electrode(s) not plugged into controller properly? no reference electrode? not enough fill solution left in reference? electrode not in sample solution? - 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? - 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 CONTINOUSLY 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? - INNACURATE (BUT CALIBRATION IS OK) standards are incorrect? sample is not ph adjusted properly? sample carryover? sample interferences or complexing agents present? TROUBLESHOOTING DIFFICULT MEASUREMENT PROBLEMS - COMPONENTS OF THE ELECTRODE SYSTEM - CONTROLLER perform checkout procedure in instruction manual. - ION-SELECTIVE ELECTRODE perform electrode slope check, inspect for physical damage. Polish, soak, or rebuild sensing membrane. Monitor leak rate on reference side. 12
- PROPER CALIBRATION - STANDARD(S) prepare fresh standards by serial dilution. - ph ADJUSTMENT adjust ph to operational range of electrode. - SAMPLE VARIABLES - CONCENTRATION RANGE sample out of range? - PRESSURE wrong sample pressure? - TEMPERATURE wrong sample temperature? - ph wrong sample ph? - INTERFERENCES interferences in the sample? - COMPLEXATION complexing agents in sample? 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 continous 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. 13
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 seperate ones? Advantages: no external reference electrode needed more economical than price of both one less electrode to fit in the process 14
NITRATE ELECTRODE SPECIFICATIONS (MODEL # 270776) SLOPE 55 ± 3 mv/decade REPRODUCIBILITY ± 2% INTERFERENCES ClO 4, I, CN, BF 4 TEMPERATURE RANGE 0 to 40 degrees C PRESSURE RANGE 0 to 30 PSI RESPONSE TIME 95% response in 30 seconds STORAGE store in dilute nitrate standard for short-term, and dry long-term CONCENTRATION RANGE 7 x 10-6 M to 1M 0.5 ppm to 62,000 ppm ph RANGE 2.5 to 11 ph RESISTANCE 100 megaohms CARE, MAINTENANCE treat with distilled water followed by nitrate standard TEMPERATURE COMPENSATION Yes ISOPOTENTIAL POINT 10 ppm nitrate ON-LINE CAPABILITY Yes APPLICATIONS Surface Waters and Sewage Effluent, Drinking Waters and Soil Extracts ORDERING INFORMATION P/N DESCRIPTION 270770 Nitrate Industrial Electrode, combination, sealed, double junction, epoxy body with stripped wire ends 27503-24 Nitrate Standard,1000 ppm NO 3 27503-60 Nitrate ISA (Ionic Strength Adjustor), 2 M(NH 4 ) 2 SO 4 56105-00 Ion Concentration Controller 50001-68 Cord with U.S.standard plug 50001-70 Cord with European plug Cole-Parmer Industrial Housings to protect the Nitrate Ion Electrode. Contact Cole-Parmer at 1-800-323-4340 to choose an Industrial Housing. 15