COLE-PARMER INDUSTRIAL POTASSIUM ION ELECTRODE INSTRUCTION MANUAL

Transcription

1 COLE-PARMER INDUSTRIAL POTASSIUM ION ELECTRODE INSTRUCTION MANUAL Cole-Parmer Instrument Company (800) Fax:(847) East Bunker Court, Vernon Hills, Illinois Table of Contents

2 TABLE OF CONTENTS General Instructions...3 introduction...3 required equipment...3 required solutions...3 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...8 electrode response...8 limits of detection...9 ph effects...9 electrode life...9 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...12 Potassium Electrode Specifications...14 Ordering Information Table of Contents

3 GENERAL INSTRUCTIONS Introduction COLE-PARMER INDUSTRIAL POTASSIUM ION ELECTRODE INSTRUCTION MANUAL The Cole-Parmer Industrial Potassium Ion Electrode is used to quickly, simply, accurately, and economically measure potassium in in-line batch processing, control processing, or water treatment applications. Required Equipment 1. Cole-Parmer Ion Concentration Controller, Cat. No Cord with U.S. standard plug, Cat. No or cord with European plug, Cat. No is necessary. 2. Cole-Parmer Industrial Potassium Ion Electrode, Cat. No 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 Potassium 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 Ionic Strength Adjuster, 5M NaCl, Cat. No 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 reagentgrade 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 0.10M. 3) Cole-Parmer Potassium Standard, 1000 ppm K +1, Cat. No To prepare this standard, add 1.91 grams of reagent-grade potassium 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. 3 Table of Contents

4 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 potassium membrane. Prior to first usage, or after long-term storage, immerse the potassium membrane in potassium 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 # 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 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: 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 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}. 4 Table of Contents

5 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 if the standard solution value is < ppm. - select if the standard solution value is > ppm. - and < ppm select 1000 if the standard solution is > 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 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 5 Table of Contents

6 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 potassium ion electrode is Units of Measurement Potassium concentrations are measured in units of parts per million as potassium, parts per million as KCl, moles per liter, or any other convenient concentration unit. Table 1 indicates some of the concentration units. TABLE 1: Concentration Unit Conversion Factors ppm K +1 ppm KCl moles/liter K x x x 10-2 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 2 lists some common cations that, if present in high enough levels, will cause electrode interferences and measurement errors or electrode drift when using the potassium 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 potassium standard solution to restore proper response. 6 Table of Contents

7 TABLE 2: Concentration of Possible Interferences Causing a 10% Error at Various Levels of KCl; Background Ionic Strength is 0.12M NaCl. Interferences (moles/liter) 10-2 M K M K M K +1 Cs x x x 10-5 NH x x x 10-4 Tl x x x 10-4 H x x x 10-3 Ag x x Tris x x 10-1 Li x x 10-1 Na x x Tris +1 is the cation of tris(hydroxymethyl)aminomethane Interferences (ppm) 100 ppm K ppm K +1 1 ppm K +1 Cs x x K x x Tl x x x 10 1 H ph 2.6 ph 3.6 ph Ag x x x Tris x x x 10 3 Li x x x 10 2 Na x x x Table of Contents

8 Temperature Influences Samples and standards should be at the same temperature, since electrode potentials are influenced by changes in temperature. A 1 C difference in temperature results in a 2% error at the 1.0 x 10-3 M level. Provided that temperature equilibria has occurred, the potassium ion electrodes can be used at temperatures from 0-40 C continuously. Room temperature measurements are recommended, since measurements at temperatures markedly different from room temperature may require equilibrium times up to one hour. Electrode Response Plotting the electrode mv potential against the potassium concentration on semilogarithmic paper results in a straight line with a slope of about 55 mv per decade. Refer to Figure Figure 1 Typical Potassium Electrode Calibration Curve electrode potential (mv) ~ 55 mv 10-fold change ppm Potassium as K Molarity The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from one minute or less for potassium concentration above 1.0 x 10-5 M to several minutes near the detection limit. Refer to Figure 2. 8 Table of Contents

9 Figure 2 Typical Electrode Time Response to Step Changes in KCl M to 10-2 M KCl electrode potential (mv) M to 10-4 M KCl 10-3 M to 10-5 M KCl Time (minutes) Limits of Detection The upper limit of detection is 1M in pure potassium chloride solutions. The upper limit of detection is above 1.0 x 10-1 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 1.0 x 10-1 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 potassium chloride compared to actual response is shown in Figure 1. ph Effects Hydrogen ion interferes with measurements of low levels of potassium ion although the electrode can be used over a wide ph range. Table 2 should be used to determine the minimum ph at which low level potassium measurements can be made without more than a 10% error due to hydrogen ion interference. Electrode Life The Cole-Parmer potassium electrode will last six months in normal laboratory use. Online 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. 9 Table of Contents

10 Electrode Storage The Cole-Parmer Potassium Ion Electrode may be stored in-line if the membrane is kept wet. For storage off-line, rinse and dry the potassium membrane electrode and cover the tip with any protective cap shipped with the electrode. ELECTRODE THEORY Electrode Operation The Cole-Parmer potassium 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 gelled organophilic membrane containing an potassium selective ion exchanger. An electrode potential develops across the membrane when the membrane is in contact with an potassium 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 potassium ion in solution. The level of potassium ions, corresponding to the measured potential, is described by the Nernst equation: where: E E o S X E = E o + S logx = measured electrode potential = reference potential (a constant) = electrode slope (~ 55 mv/decade) = level of potassium ions in solution The activity, X, represents the effective concentration of the ions in solution. Total potassium concentration, C t, includes free potassium ions, C f, plus bound or complexed potassium ions, C b. Since the potassium 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: where: C x Z x Σ 2 I = ½ Σ C x Z x = concentration of ion X = charge of ion X = sum of all of the types of ions in the solution. 10 Table of Contents

11 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 potassium is sodium chloride, NaCl. COMMON 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? 11 Table of Contents

12 - INACCURATE (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. Monitor leak rate on reference side. - 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 12 Table of Contents

13 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 13 Table of Contents

14 POTASSIUM ELECTRODE SPECIFICATIONS (MODEL # ) SLOPE 55 ± 3 mv/decade REPRODUCIBILITY ± 2% INTERFERENCES Cs +1, NH4 +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 1 x 10-6 M to 1M 0.04 ppm to 39,000 ppm ph RANGE 2 to 12 ph RESISTANCE 100 megaohms CARE, MAINTENANCE, treat with distilled water & CLEANING followed by potassium standard. TEMPERATURE COMPENSATION Yes ISOPOTENTIAL POINT 20 ppm potassium ON LINE CAPABILITY Yes APPLICATIONS Wastewaters, River and Tap Waters Blood Electrolytes/Clinical Analysis ORDERING INFORMATION P/N DESCRIPTION Ion Concentration Controller Cord with U.S. standard plug Cord with European plug Industrial Potassium 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 Potassium Ion Electrode. Contact Cole-Parmer at to choose an Industrial Housing Potassium Standard, 1000 ppm NH Potassium ISA (Ionic Strength Adjustor), 5 M NaCl 14 Table of Contents