NH4 Model 9318 AMMONIUM ELECTRODE INSTRUCTION MANUAL. A Thermo Electron business

Transcription

1 + NH4 Model 9318 AMMONIUM ELECTRODE INSTRUCTION MANUAL A Thermo Electron business

2 AQUAfast, Cahn, EZFlash, Ionalyzer, ionplus, KNIpHE, ORION, perphect, PerpHecT, PerpHecTion, phisa, phix, phuture, Pure Water, Sage, Sensing the Future, SensorLink, Sure-Flow, Titrator PLUS, TURBO2 and Wine Master are registered trademarks of Thermo Orion pHAX-ION, A +, All in One, Aplus, AQUAsnap, AssuredAccuracy, AUTO-BAR, AUTOCAL, AUTO DISPENSER, Auto-ID, AUTO-LOG, AUTO-STIR, AUTO-READ, BOD AutoEZ, Cable-Free, CERTI-CAL, CISA, DataCOLLECT, DataPLUS, digital LogR, DirectCal, DuraProbe, Environmental Product Authority, Extra Easy/Extra Value, FAST QC, Flash GC, GAP, GLPcal, GLPcheck, GLPdoc, KAP, LabConnect, LogR, Minimum Stir Requirement, MSR, NISS, One-Touch, One-Touch Calibration, One-Touch Measurement, Optimum Results, phuture MMS, phuture Pentrode, phuture Quatrode, phuture Triode, QuiKcheK, rf link, ROSS, ROSS Resolution, SAOB, Stacked, Stat Face, The Enhanced Lab, ThermaSense, Triode, TRIUMpH, Unbreakable ph, Universal Access are trademarks of Thermo Orion. Guaranteed Success and The Technical Edge are service marks of Thermo Orion. PerpHecT meters are protected by U.S. patent 4,321,544. ROSS and PerpHecT ROSS are protected by U.S. patent 4,495,050. ORION Series A meters and 900A printer are protected by U.S. patents 5,108,578, 5,198,093, D334,208 and D346,753. ORION 81, 82, 91, and 92 series glass electrodes are protected by U.S. patents 4,661,236 and 4,687,500. Sure-Flow electrodes are protected by European Patent 278,979 and Canadian Patent 1,286,720. ionplus electrodes and Optimum Results solutions have patents pending. Copyright 2001, Thermo Orion. All rights reserved. The specifications, descriptions, drawings, ordering information and part numbers within this document are subject to change without notice. This publication supersedes all previous publications on this subject.

3 TABLE OF CONTENTS GENERAL INFORMATION 1 Introduction 1 Required Equipment 1 Required Solutions 2 USING THE ELECTRODE 3 Setting Up 3 Electrode Assembly 3 Electrode Conditioning Procedure 3 Checking Electrode Operation (Slope) 4 Before Analysis 5 Units of Measurements 5 Selection of Measurement Procedure 5 Sample Requirements 6 Sample Pretreatment 6 Measuring Hints 7 Analytical Techniques 8 Direct Measurement 9 Low Level Measurement 12 Known Addition 14 Electrode Storage 19 TROUBLESHOOTING 20 Troubleshooting Guide 20 Assistance 22 ELECTRODE CHARACTERISTICS 23 Theory of Operation 23 Electrode Response Time 24 Limits of Detection 24 Reproducibility 25 Temperature Effects 25 ph Operating Range 25 Interferences 26 Electrode Life 26 ORDERING INFORMATION 27 WARRANTY 29 SPECIFICATIONS 33

4 GENERAL INFORMATION Introduction The Model Ammonium Ion-Selective Electrode (ISE) measures ammonium in aqueous solutions quickly, simply, accurately, and economically. General analytical procedures, required solutions, electrode characteristics, and electrode theory are discussed in this manual. Operator Instructions for Thermo Orion meters are outlined in the individual meter s instruction manual. Thermo Orion Technical Service Chemists can be consulted for assistance and troubleshooting advice. Please refer to Troubleshooting for information on contacting Thermo Orion. Required Equipment Meter The easiest to use are direct concentration readout ISE meters, such as Thermo Orion Models EA 940, 920A, 720A, 710A, or 290A. If unavailable, a ph/mv meter with readability to 0.1 mv, such as Thermo Orion Models 420A, 520A, or 525A is recommended. Reference Electrode Thermo Orion Model Double-Junction Reference Electrode. Do not use Outer Filling Solution supplied with Reference Electrode. Use supplied with Ammonium Electrode. Magnetic Stirrer, Stir Bars Recommended for laboratory measurements. Graph Paper 4-Cycle semilogarithmic paper for preparing calibration curves (for use with digital ph/mv laboratory meters). 1

5 Required Solutions Distilled or Deionized Water To prepare or dilute all solutions and standards. Standard Solutions Cat. No. 0.1M Ammonium Standard ppm Ammonia as Nitrogen Standard ppm Ammonia as Nitrogen Standard Ionic Strength Adjustment Buffer (ISA) 0.25 M magnesium acetate, 0.5 M acetic acid. Wearing rubber gloves, a lab coat, and safety glasses, prepare the ISA as follows using analytical grade chemicals and distilled or deionized water: Add 500 ml water to a 1000 ml beaker. Provide a means to maintain continuous stirring. Slowly add 28.7 ml of glacial acetic acid followed by 53.6 g of magnesium acetate. When dissolved, dilute to 1000 ml with water. Store in a stoppered glass flask. Model Reference Electrode Filling Solutions: Inner Reference Solution (one 2 oz bottle supplied with Double Junction Reference electrode; five-pack Cat. No ) Outer Reference Solution (five-pack Cat. No ) Do not use Outer Filling Solution supplied with Reference Electrode. Use supplied with Ammonium Electrode. 2

6 USING THE ELECTRODE Setting Up Electrode Assembly Take the sealed glass vial from the styrofoam box. Remove the sensing module from the vial. Make sure the rubber washer on the sensing module is in place. See Figure 1. Screw the sensing module into the electrode body until finger tight. To ensure electrical continuity, shake down the electrode like a thermometer. Electrode Conditioning Procedure When sample ammonium levels will be greater than 10-5 M, soak the ammonium electrode for one hour in a standard ammonium solution, e.g., 10-2 M or 100 ppm, to which 10 ml of ISA per 100 ml of standard has been added. For ammonium concentrations of 10-5 M or lower, use distilled or deionized water, to which 10 ml of ISA per 100 ml of standard has been added. Fill the reference electrode according to instructions in the reference electrode instruction manual, using Inner Filling Solution Cat. No and Outer Filling Solution Cat. No Do not use Outer Filling Solution supplied with Reference Electrode. Use supplied with ammonium electrode. NOTE: DO NOT IMMERSE ELECTRODE IN SOLUTION BEYOND RUBBER WASHER. electrode body rubber washer sensing module Figure 1 - Assembling the Electrode 3

7 Checking the Electrode Operation (Slope) These are general instructions which can be used with most meters to check electrode operation. See individual meter instruction manual for more specific information. This procedure measures electrode slope. Slope is defined as the change in millivolts observed with every ten-fold change in concentration. Obtaining the slope value provides the best means for checking electrode operation. 1. If electrodes has been stored dry, perform electrode conditioning procedure. See Electrode Conditioning Procedure. 2. Connect electrodes to the meter by inserting the reference electrode pin-tip connector and the sensing electrode connector into the appropriate jacks on the digital ph/mv meter or specific ion meter. Non-Thermo Orion meters may require special adapters. Consult your meter instruction manual. 3. To a 150 ml beaker, add 100 ml of distilled or deionized water, and 10 ml of ISA. Stir thoroughly. Set the function switch of the meter to the mv mode. 4. Rinse electrodes thoroughly with distilled water, blot dry, and place in the solution prepared in Step Select either 0.1 M ammonium or 1000 ppm ammonia as nitrogen standard. Pipet 1 ml of the standard into the beaker. Stir thoroughly. When a stable reading is displayed, record the electrode potential in millivolts. 6. Pipet 10 ml of the same standard into the same beaker. Stir thoroughly. When a stable reading is displayed, record the electrode potential in millivolts. 7. The difference between the first and second potential reading is defined as the slope of the electrode. The difference should be in the range of 54 to 60 mv for a 10-fold (decade) concentration change when the solution temperature is 25 C. If the potential is not within this range, resoak the electrode as described under Electrode Conditioning Procedure. For other troubleshooting techniques, refer to Troubleshooting. 4

8 Before Analysis Units of Measurement Ammonium can be measured in units of moles per liter, parts per million as ammonium, parts per million as ammonia, parts per million as nitrogen, or any other convenient unit. See Table 1. Table 1 - Concentration Unit Conversion Factors Moles/Liter ppm as NH 4 + ppm as NH 3 ppm as N Selection of Measurement Procedure Successful application of an ISE to an analytical problem requires that the electrode be used as part of a carefully thought-out procedure. Fortunately, in nearly all cases, this procedure is very simple, consisting of: 1. Additional of an ISA solution to samples and standards. 2. Taking care that the temperature is the same for all measurements. 3. Utilization of an appropriate technique, as described under Analytical Techniques. If a procedure other than one described in this manual is selected, then careful attention should be given to all of the information contained under Sample Requirements, Sample Pretreatment, and Electrode Characteristics. 5

9 Sample Requirements Samples must be aqueous; they must not contain organic solvents. Consult Thermo Orion s Technical Service Chemists for using the electrode in specific applications. Samples and standards should be at the same temperature, ph, and ionic strength. At the 10-3 M level, a 1 C difference in temperature will give rise to about 2% error. Temperature must be less than 40 C. ph, ionic strength, and interference levels should be within the limits prescribed under ph Operating Range, Interferences, and Sample Pretreatment. Sample Pretreatment Most Samples will require some form of pretreatment before accurate concentration measurements can be made. In most samples, addition of 10 ml of ISA per 100 ml of sample or standard solution is all the sample pretreatment that will be required. Add to each sample and standard an equal volume of concentrated ionic strength adjustment buffer (ISA). This is a solution with a high ionic strength and ph buffer capacity which, when added to samples and standards, accomplishes ionic strength and ph adjustment in a single step. 6

10 Measuring Hints NOTE: DO NOT IMMERSE ELECTRODE IN SOLUTION BEYOND RUBBER WASHER. Keep all samples, standards, and ISA buffers stoppered. Ammonia is ubiquitous in laboratory and workplace atmospheres and will be absorbed by solutions and converted to ammonium. Stir all standards and samples at a uniform rate during measurement. Magnetic stirrers may generate sufficient heat to change solution temperature. Place a piece of insulating material such as cork, cardboard, or styrofoam between the stirrer and beaker. Verify calibration every two hours by placing electrodes in the first standard solution used for calibration. If the value has changed, recalibrate. Always rinse electrodes thoroughly with distilled water between measurements. Shake after rinsing to prevent solution carry over. Do not wipe or rub the sensing membrane, blot dry. Between measurements, store ammonium electrode as described under Electrode Storage. Allow all standards and samples to come to the same temperature for precise measurement. After immersion in solution, check electrode for any air bubbles on membrane surface and remove by gently tapping or by redipping electrode. If electrode response is slow, the membrane may contain a surface layer of contaminants. Restore performance by repeating the electrode conditioning procedure. For high ionic strength samples, prepare standards with composition similar to that of the sample. Concentrated samples (>0.1M) should be diluted before measurement. 7

11 Analytical Techniques A variety of analytical techniques are available to the analyst. The following is a description of these techniques. Direct Measurement is a simple procedure for measuring a large number of samples. Calibration is performed in a series of standards. The concentration of the samples is determined by comparison to the standards. All samples and standards have similar ionic strength. See Sample Pretreatment. Two direct measurement procedures are described in this manual - one for concentration levels greater than 10-5 M, and one under Low Level Measurements. Incremental Techniques provide a useful method for measuring samples, since calibration is not required. As in direct calibration, any convenient concentration unit can be used. The different incremental techniques are described below. They can be used to measure the total concentration of a specific ion in the presence of a large ( times) excess of complexing agents. Known Addition is an alternate method useful for measuring dilute samples, checking the results of direct calibration (when no complexing agents are present), or measuring the total concentration of an ion in the presence of an excess complexing agent. The electrodes are immersed in the sample solution and an aliquot of a standard is added to the sample. From the change in potential before and after the addition, the original sample concentration is determined. Known Subtraction is useful as a quick version of a titration, or for measuring species for which stable standards do not exist. It is necessary to know the stoichiometric ratio between standard and sample. For known subtraction, an electrode sensing the sample species is used. Stable standards of a species reacting completely with the sample in a reaction of known stoichiometry are necessary. Analate Addition is often used to measure soluble solid samples, viscous samples, small or very concentrated samples, to diminish the effects of complex sample matrices, or to diminish the effects of varying sample temperatures. This method is not suitable for dilute or low concentration samples. Total concentration is measured even in the presence of complexing agents. The electrodes are immersed in a standard solution containing the ion to be measured and an aliquot of the sample is added to the standard. The original sample concentration is determined from the change in potential before and after the addition. Analate Subtraction is used in the measurement of ions of which no ion-selective electrode exists. The electrodes are immersed in a reagent solution which contains a species that the electrode senses, and that reacts with the sample. It is useful when sample size is small, or samples for which a stable standard is difficult to prepare, and for viscous or very concentrated samples. The method is not suited for very dilute samples. It is also necessary to know the stoichiometric ratio between standard and sample. 8

12 Specific instructions for known subtraction, analate addition and analate subtraction, can be found in the Thermo Orion meter instruction manuals. Titrations are quantitative analytical techniques for measuring the concentration of a species by incremental addition of a reagent (titrant) that reacts with the sample species. Sensing electrodes can be used for determination of the titration end point. Ion-selective electrodes are good end point detectors, because they are unaffected by sample color or turbidity. Titrations are approximately 10 times more precise than direct calibration, but are more time consuming. Direct Measurement In direct measurement, the electrode is calibrated in standard ammonium solutions and then placed in samples. Sample concentration is obtained by comparison to a calibration curve or read directly when a concentration meter is used. When sample concentrations are in the linear range of the electrode response, greater than 10-5 M ammonium, two standards are required, and the procedure described below should be followed. When the sample ammonium concentrations are 10-5 M or below, follow the procedure under Low Level Measurement. Figure 2 - Typical Ammonium Electrode Calibration Curve 9

13 Set - Up 1. Perform Electrode Conditioning Procedure. 2. Fill the reference electrode as directed in its accompanying manual, however be sure to use Thermo Orion Cat. No Outer Filling Solution, supplied with the ammonium electrode, instead of the which is supplied with the reference electrode. 3. Connect the electrodes to the meter. 4. Prepare two standards which bracket the expected sample range and differ in concentration by at least a factor of ten. Standards can be prepared in any concentration unit to suit the particular analysis requirement. All standards and samples should have the same temperature. ph and ionic strength of samples and standards should be made equal by the addition of an equal volume of ISA to each. 10 ml of ISA per 100 ml of sample or standard is recommended. Samples of high ionic strength, acidity, or alkalinity may require pre-dilution or the addition of a higher proportion of ISA. For details, refer to Sample Pretreatment and Temperature Effects. Procedure with Direct Concentration Meter NOTE: SEE INDIVIDUAL METER INSTRUCTION MANUALS FOR MORE SPECIFIC INFORMATION. 1. Measure 100 ml of the more dilute standard into a 150 ml beaker, add 10 ml of ISA, and stir thoroughly. 2. Rinse electrodes with distilled water, blot dry, and place into the beaker. Wait for a stable reading, then adjust the meter to display the value of the standard as described in the meter instruction manual. 3. Measure 100 ml of the more concentrated standard into a second 150 ml beaker, add 10 ml of ISA, and stir thoroughly. 4. Rinse the electrodes with distilled water, blot dry, and place into the beaker with more concentrated standard. Wait for a stable reading, then adjust the meter to display the value of the second standard, as described in the meter instruction manual. 5. Measure 100 ml of the sample into a 150 ml beaker, add 10 ml of ISA, stir thoroughly. Rinse electrodes with distilled water, blot dry, and place in sample. The concentration will be displayed on the meter. 10

14 Procedure with the mv Meter 1. Adjust the meter to measure mv. 2. Measure 100 ml of the more dilute standard into a 150 ml beaker, add 10 ml of ISA, and stir thoroughly. 3. Rinse electrodes thoroughly with distilled water, blot dry, and place into the beaker. When a stable reading is displayed, record the mv value and corresponding standard concentration. 4. Measure 100 ml of the more concentrated standard into a second beaker, add 10 ml of ISA, and stir thoroughly. 5. Rinse electrodes with distilled water, blot dry, and place into the second beaker. When a stable reading is displayed, record the mv value and corresponding standard concentration. 6. Using semilogarithmic graph paper, prepare a calibration curve by plotting the millivolt values on the linear axis and the standard concentration values on the logarithmic axis. 7. Measure 100 ml of the sample into a 150 ml beaker, add 10 ml of ISA, and stir thoroughly. 8. Rinse electrodes with distilled water, blot dry, and place into the beaker. When a stable reading is displayed, record the mv value. 9. Using the calibration curve prepared in Step 6, determine the unknown concentration. 11

15 Low Level Measurement Determination of low ammonium levels, that is, concentrations which lie below the linear portion of the electrode response curve (see Figure 2), can be accomplished by using more than two standard solutions. The procedure below describes creation of a multi-point calibration curve using a ph/mv meter. With direct-reading concentration meters which have the blank correction and/or multi-point calibration features, the described procedure can be adapted in accordance with the instructions which accompany the meter. With direct-reading meters which do not have these features, it is recommended to use the meter in the mv mode and follow the procedure described below. When samples are low in ammonium but high in total ionic strength, such that the recommended ISA addition cannot swamp-out the sample matrix effects, and yet dilution would make the ammonium level too low, then it is recommended that the procedure be performed with one change: prepare a calibration solution with a composition similar to the sample. The following conditions should be kept in mind when making low-level measurements: Adequate time must be allowed for electrode stabilization. Longer response time will be needed at low levels. Remember to stir all standards and samples at a uniform rate. Be sure to use the electrode conditioning and storage procedures recommended for low-level measurements. Keep solutions covered when not in use. Ammonia is present in laboratory and workplace environments and will contaminate solutions. Set-up 1. Perform the electrode conditioning procedure for low ammonium levels. 2. Fill the reference electrode as instructed in its manual, however, be sure to use Thermo Orion Cat. No Outer Filling Solution, supplied with the ammonium electrode, instead of the which is supplied with the reference electrode. 3. Connect the electrodes to the meter. 4. Select a standard solution. Use either 100 ppm or 10-3 M. 12

16 Measurement 1. Measure 100 ml distilled water into a 150 ml beaker, add 10 ml of ISA, and stir thoroughly. 2. Rinse the electrode thoroughly with distilled water, blot dry, and place into the beaker. Stir thoroughly. 3. Add increments of the 100 ppm or 10-3 M standard to the beaker using steps outlined in Table 2. Record stable millivolt reading after each increment. On semilogarithmic paper, plot the concentration (log axis) against the millivolt potential (linear axis). See Figure 2. Prepare a new calibration curve with fresh standards each day. 4. Measure 100 ml of sample into a beaker, add 10 ml of ISA. Rinse the electrodes thoroughly with distilled water, blot dry, and place into the sample. 5. Stir thoroughly. When a stable reading is displayed, record the mv value. 6. Determine the sample concentration corresponding to the measured potential from the low level calibration curve. Table 2 - Calibration Curves For Low Level Measurements Additions of 100 ppm or 10-3 standards to 100 ml distilled water. Graduated Pipet Added Concentration Step Size Volume ppm / Molarity 1 1 ml 0.1 ml / ml 0.1 ml / ml 0.2 ml / ml 0.2 ml / ml 0.4 ml / ml 2.0 ml 2.91 / ml 2.0 ml 4.76 /

17 Known Addition Known Addition is a convenient technique for measuring samples because no calibration curve is needed. It can be useful to verify the results of a direct calibration or to measure the total concentration of an ion in the presence of a large excess of complexing agent. The sample potential is measured before and after addition of a standard solution. Accurate measurement requires that the following conditions be met: Concentration should approximately double as a result of the addition. Sample concentration should be known to within a factor of three. In general, either no complexing agent nor a large excess of the complexing agent may be present. The ration of the uncomplexed ion to complexed ion must not be changed by addition of the standard. All samples and standards must be at the same temperature, ph, and ionic strength. Set-up 1. Perform the Electrode Conditioning Procedure. 2. Fill the reference electrode as instructed in its manual, however be sure to use Thermo Orion Cat. No Outer Filling Solution, supplied with the ammonium electrode, instead of the , which is supplied with the reference electrode. 3. Connect the electrodes to the meter. 4. Prepare a standard solution, which upon addition to the sample, will cause the concentration of the ammonium electrode to double. It is recommended that a sample concentration be used. For other concentrations, refer to Table 3 as a guideline. 5. Determine the slope of the electrode by performing the procedure under Checking Electrode Operation (Slope). 6. Rinse electrodes between solutions with distilled water. 14

18 Using an instrument with direct known addition readout capability NOTE: SEE INDIVIDUAL METER INSTRUCTION MANUALS FOR MORE SPECIFIC INFORMATION. 1. Set up meter to measure in the known addition mode. 2. Measure 100 ml of the sample into a beaker. Rinse the electrodes with distilled water, place in sample solution. Add 10 ml ISA. Stir thoroughly. 3. When a stable reading is displayed, set the meter as described in the meter instruction manual. The dilution effect of ISA may need to be taken into account. 4. Pipet the appropriate amount of the standard solution into the beaker. Again, the dilution effect of ISA may need to be taken into account. Stir thoroughly. 5. When a stable reading is displayed, record the sample concentration. Using a meter with mv readout only Use this procedure when no instruction manuals for known addition are available in the meter instruction manual. When the exact sample, ISA, and standard volumes described in this procedure are used and the determined electrode slope is between 56 and 59 mv, then Table 4 can be used to calculate the result; otherwise, use the formula provided below. 1. Set the meter to the relative millivolt mode. 2. Measure 100 ml of the sample into a 150 ml beaker. 3. Rinse the electrode with distilled water, blot dry, and place into beaker. When a stable reading is displayed, record the mv value as E Pipet the 10 ml of the standard solution into the beaker. Stir thoroughly. 5. When a stable reading is displayed, record the mv value as E 2. Subtract the first reading from the second to find E; E = E 2 - E Obtain the value Q that corresponds to the change in potential, E, either from Table 4 (if the recommended sample, ISA, and standard volumes were used and the determined slope is between 56 and 59 mv) or by calculation from formula on the next page. To determine the original sample concentration, multiply Q by the concentration of the added standard: 15

19 C sam = Q C std where: C std = standard concentration C sam = sample concentration Q = reading from Table 4 or is calculated from the formula below If volumes other than those recommended were used, of if the electrode slope is not between 56 and 59 mv, then use the following formula to calculate Q: 1 V Q = std [(V 2 /V 1 ) 10 E/s ]-1 V sam where: V sam = volume of sample V ISA = volume of ISA V std = volume of standard V 2 = V sam + V ISA + V std V 1 = V sam + V ISA E = E 2 - E 1 s = slope of the electrode Table 3 Volume of Addition Concentration of Standard 1mL 100 sample concentration 5 ml 20 sample concentration 10 ml (recommended) 10 sample concentration 16

20 Table 4 - Known Addition Values For Q vs. E, when recommended volumes are used. Slopes are in parentheses at tops of columns. Q Concentration Ratio E (56) (57) (58) (59)

21 Q Concentration Ratio E (56) (57) (58) (59)

22 Q Concentration Ratio E (56) (57) (58) (59) Electrode Storage The sensing module should be kept in the glass vial until it is to be used. When the electrode is to be used for samples with moderate ammonium concentrations, that is, greater than 10-5 M, the assembled electrode should be stored in an ammonium standard (e.g., 10-2 M), to which the appropriate amount of ISA has been added. When the electrode is to be used for samples with ammonium concentrations less than 10-5 M, the assembled electrode should be stored in distilled or deionized water to which the appropriate amount of ISA has been added. For long periods of time (over 2-3 days), disassemble the electrode, rinse with distilled water, blot dry, and store the module in its vial. The reference may be stored in air between sample measurements (up to 1 hour). For short periods of time (up to one week), the reference electrode may be stored in its filling solution. The solutions inside the reference electrode should not be allowed to evaporate and crystallize. For long periods of time (over one week), drain the reference electrode completely, rinse with distilled water, and store dry. 19

23 20

24 Troubleshooting Troubleshooting Guide The most important principle in troubleshooting is to isolate the components of the system and check each in turn. The components of the system are: 1, Meter; 2, Electrodes; 3, Standard; 4, Sample; and 5, Technique. Meter The meter is the easiest component to eliminate as a possible cause of error. Thermo Orion Meters are provided with instrument checkout procedures in the instruction manual and a shorting cap for convenience in troubleshooting. Consult the manual for complete instructions and verify that the instrument operates as indicated and is stable in all steps. Electrodes 1. Rinse electrodes thoroughly with distilled water. 2. Check Electrode Operation (slope). 3. If electrode fails this procedure, resoak ammonium electrode as directed in Electrode Conditioning Procedure. Clean reference electrode as described in reference electrode instruction manual. 4. Repeat step 2, Checking Electrode Operation (slope). 5. If the electrode still does not perform as described, determine whether the ammonium or reference electrode is at fault. To do this, substitute a known working electrode for the electrode in question and repeat the slope check. 6. If the stability and slope check out properly, but measurement problems persist, the sample may contain interferences or complexing agents, or the technique may be in error. See Selection of Measurement Procedure, Sample Requirements, and Sample Pretreatment. 7. Before replacing a faulty electrode, or if another electrode is not available for test purposes, review the instruction manual and be sure to clean the electrode thoroughly, prepare the electrode properly, use proper filling solutions, ISA, and standards, measure correctly, and review the Troubleshooting Checklist. 21

25 Standard The quality of results depends greatly upon the quality of the standards. Always prepare fresh standards when problems arise; it could save hours of frustration troubleshooting. Error may result form contamination of prepared standards, accuracy of dilution, quality of distilled water, or a mathematical error in calculating the concentrations. Keep all solutions well-stoppered when not in use. Ammonia will be absorbed from laboratory or workplace atmospheres. The best method for preparation of standards is by serial dilution. This means that an initial stand is diluted, using volumetric glassware, to prepare a second standard solution. The second is similarly diluted to prepare a third standard, and so on, until the desired range of standards has been prepared. Sample If the electrodes work properly in standards, but not in samples, look for possible interferences, complexing agents, or substance which could affect responses or physically damage the sensing electrode or the reference electrode. If possible, determine the composition of the samples and check for problems. See Sample Requirements, Sample Pretreatment, Interferences, and ph Operating Range. Technique Check the method of analysis for compatibility with your sample. Direct measurement may not always be the method of choice. If large amounts of complexing agents are present, known addition may be best. If the sample is viscous, analate addition may solve the problem. If working at low levels, be sure to follow the low level measurement technique. Also, be sure that the expected concentration of the ion of interest is within the electrode s limits of detection. If problems persist, review operational procedures and instruction manuals to be sure that proper technique has been followed. Reread Measuring Hints and Analytical Procedures. 22

26 Assistance If after checking each component of your measuring system, the source of the trouble remains unknown, call Thermo Orion s Technical Service Chemists. In the United States (except Massachusetts, Alaska, and Hawaii) call Thermo Orion 500 Cummings Center Beverly, MA USA Tel: Dom. Fax: Int l. Fax: Thermo Orion Europe Sedgeway Business Park Witchford, Cambridgeshire England, CB6 2HY Tel: Fax: Thermo Orion Far East Room 904, Federal Building 369 Lockhart Road Wanchai, Hong Kong Tel: Fax: Thermo Orion Customer Support Toll Free: Dom. domcs1@thermoorion.com Int l. intcs1@thermoorion.com 23

27 24

28 ELECTRODE CHARACTERISTICS Theory of Operation Electrode Construction The ammonium electrode consists of an electrode body and a replaceable, pretested sensing module. The sensing module contains an internal reference element, and a liquid internal filling solution in contact with a gelled, organophilic membrane containing an ammonium-selective ionsphore. See Figure 3. The Nernst Equation When the membrane is in contact with ammonium solution, a potential develops across the membrane. This potential depends on the activity of free ammonium ion in solution and is measured against a constant reference electrode potential with a digital ph/mv or concentration meter. The measured potential corresponding to the activity of ammonium ion in solution is described by the Nernst equation. E = E 0 + s Log (a i ) where: E = measured electrode potential E 0 = reference potential (a constant) a i = ammonium ion activity level in solution s = electrode slope (about 58 mv per decade at 25 C) Figure 3 - Construction of Electrode Sensing Module 25

29 Electrode Response Time The electrode potential plotted against concentration on semilogarithmic paper results in a straight line until concentration reaches 10-5 M, with a slope of about mv per decade. See Figure 2. The electrode exhibits good time response (99% response in one minute or less) for ammonium concentrations above 10-5 M. Below this value response times vary from 2-5 minutes. See Figure 4. Limits of Detection In pure ammonium chloride solutions, the upper limit of detection is 1 M, when other ions are present, the upper limit of detection is above 10-1 M ammonium, but is influenced by two factors - the possibility of a liquid junction potential developing at the reference electrode and the salt extraction effect. At high salt concentrations, some salts may be extracted in to the electrode membrane, causing deviation form theoretical response. To measure samples between 10-1 M and 1 M, calibrate the electrode at 4 or 5 intermediate points, or, preferable, dilute the sample. The low limit of detection is determined by the slight water solubility of the membrane components, which causes deviation from theoretical response. Figure 2 shows the theoretical response at low levels of ammonium chloride compared to the actual response. If ammonium measurements are made below 10-5 (0.14 ppm as N), a low level measurement procedure is recommended. The lower limit of detection can also be determined by the presence of interfering ions. In the absence of interferences, detection of concentrations as low as 10-6 can then be achieved. Figure 4 - Typical Electrode Response to Step Changes in NH 4 Cl 26

30 Reproducibility Reproducibility is limited by factors such as temperature fluctuations, drift, and noise. Within the electrode s operation range, reproducibility is independent of concentration. With calibration every hour, direct electrode measurements reproducible to ±2% can be obtained. Temperature Effects Since electrode potentials are affected by changes in temperature, samples and standard solutions should be within ±1 C (±2 F) of each other. At the 10-3 level, a 1 C difference in temperature results in a 2% error. The absolute potential of the reference electrode changes slowly with temperature because of the solubility equilibria on which the electrode depends. The slope also varies with temperature, as indicated by the factor s in the Nernst equation. Values of the Nernst factor for ammonium ion are given in Table 5. If temperature changes occur, meter and electrodes should be recalibrated. The electrode can be used at room temperatures from 0 to 40 C, provided that temperature equilibrium has occurred. For use at temperatures substantially different from room temperature, equilibrium times of up to one hour are recommended. Table 5 - Values of Electrode Slope vs. Temperature T C s T C s T C s ph Operating Range Ammonium ion is converted to ammonia at high ph and is then sensed by the electrode. Since ammonium has a pk of approximately 9.2, 50% is converted to ammonia at ph 9.2, 10% at 8.2, and 1% at 7.2. Operation at too low a ph can adversely affect the limit of detection of ammonium, due to hydrogen ion interference, as discussed under Interferences and again under Limits of Detection. Electrode response is not directly influenced by hydrogen ion between ph 2 and 7, however, optimum analytical results are obtained when the ph of all samples and standards is held to within 0.1 ph unit of a selected value within this range. The recommended magnesium acetate ISA fixes the ph at about

31 Interferences Hydrogen ion and other cations can interfere with the measurement of ammonium when they are present at high enough concentration (activity) levels. If the electrode is exposed to high levels of interfering ions, it may become drifty and sluggish in response. When this happens, restore normal performance by repeating the electrode conditioning procedure. Table 6 lists the levels of interfering ion that will result in a 10% error at various concentrations of ammonium ion. Concentrations are given in molarity, except for hydrogen which is expressed as ph. Table 6 - The 10% Error Thresholds for Interfering Ions at Various Ammonium Concentrations Expressed as Molarity Interfering Ion + NH 4 + NH 4 + NH 4 H + (ph) <2 <1 <1 Li Na K Cs Mg +++ >0.5 >1 >1 Ca ++ >0.2 >1 >1 Sr ++ >0.2 >1 >1 Ba ++ >0.1 >0.5 >0.5 Zn N 2 H 5 >0.1 >0.1 >0.1 Bu 4 N When the level of interferences in samples is constant, it is sometimes possible to measure ammonium accurately when interference levels are higher than those in Table 6. Call or write Thermo Orion s Technical Services Chemists for more information. See Assistance. Electrode Life Each sensing module should last at least six months in normal laboratory use. In time, electrode slope will decrease and readings will start to drift, indicating that the module should be changed. Before replacement, refer to Troubleshooting checklist, to make sure that the difficulties are caused by the sensing module. 28

32 ORDERING INFORMATION Cat. No. Description Electrode Handle with US Standard Connector 9300BN Electrode Handle with BNC Connector Replacement Ammonium electrode Module Ammonia Standard Solution (0.122M), 475 ml Ammonium Standard Solution (1000 ppm as N), 475 ml Ammonium Standard Solution (100 ppm as N), 475 ml Double-junction Reference Electrode Inner Reference Filling Solution, five-pack of 2 oz bottles Outer Reference Filling Solution, five-pack of 2 oz bottles 29

33 WARRANTY The Thermo Orion warranty covers failures due to manufacturer s workmanship or material defects from the date of purchase by the user. User should return the warranty card to Thermo Orion and retain proof of purchase. Warranty is void if product has been abused, misused, or repairs attempted by unauthorized persons. Warranties herein are for product sold/installed by Thermo Orion or its authorized dealers. Any product sold by a U.S. or Canadian distributor must be returned to Thermo Orion for any warranty work. A Return Authorization Number must be obtained from Thermo Orion Laboratory Technical Service before returning any product for in-warranty repair or replacement. In the event of failure within the warranty period, Thermo Orion will at Thermo Orion s option, repair or replace product not conforming to this warranty. There may be additional charges, including freight, for warranty service performed in some countries. For service, call Thermo Orion (or its authorized dealer outside the United States and Canada). Thermo Orion reserves the right to ask for proof of purchase, such as the original invoice or packing slip. Laboratory ph Meters (Models 611 and 301), SensorLink, PerpHecT ph/ise Meters, phuture ph Meters (Models 610 and 620), Sage Pumps, Cahn Balances, 930 Ionalyzer, 950 ROSS FAST QC Titrator, 960 Titrator PLUS, Karl Fischer Titrators, Autosamplers, Liquid Handling Devices, phuture Conversion Box, Wine Master, 607 Switchbox, rf link, AQUAfast II Colorimeters, Vacuum Degasser, Flowmeter are warranted to be free from defects in material and workmanship for a period of twelve (12) months from the date of purchase by the user or eighteen (18) months from date of shipment from Thermo Orion, whichever is earlier, provided use is in accordance with the operating limitations and maintenance procedures in the instruction manual and when not having been subjected to accident, alteration, misuse, or abuse. The warranty period for 960 Titrator PLUS, 950 Fast QC Titrator, Wine Master and 930 Ionalyzer pumps is three (3) months from date of purchase. 30

34 Economy Line Electrodes, Models 91-05, 91-06, 91-15, 91-16, 91-25, 91-26, 91-35, 91-36, 92-06, are warranted to be free from defects in material and workmanship for a period of three (3) months from date of purchase by customer or six (6) months from date of shipment from Thermo Orion, whichever is earlier. Warranty also includes failure for any reason (excluding breakage), except abuse, provided the electrode is not used in solutions containing silver, sulfide, perchlorate, or hydrofluoric acid; or in solutions more than one (1) Molar in strong acid or base at temperatures above 50 C. Ion Selective Electrodes, ionplus Electrodes, ROSS Electrodes, Sure-Flow Electrodes, PerpHecT Electrodes, AquaPro Professional Electrodes, Standard Line ph Electrodes, Tris ph Electrodes, ORP Triode (Cat. No. 9180BN), phuture ph Probes (Cat. No ) and phuture MMS Quatrode and Triode (Cat. Nos and ), Model DO Probe, Series 100 Conventional Conductivity Cells, temperature probes and compensators (except those models noted) are warranted to be free from defects in material and workmanship for a period of twelve (12) months from the date of purchase by the customer or eighteen (18) months from date of shipment from Thermo Orion, whichever is earlier, except for abuse or breakage of electrodes. 93 and 97 ionplus Series sensing modules are warranted to give six (6) months of operation if placed in service before the date indicated on the package, except and Nitrate modules are warranted to give ninety (90) days of operation if placed in service before the date indicated on the package. Thermo Orion phuture (Cat. No ) and phuture MMS Pentrode (Cat. No ), Quatrode (Cat. No ) and Triode (Cat. No ), Low Maintenance Triode (Cat. No. 9107BN), ORP Low Maintenance Triode (Cat. No. 9179BN), and PerpHecT Low Maintenance Triode (Cat. No. 9207BN), Waterproof Triode (Cat. No. 9107WP, 9107WL, 9109WL, 9109WP), QuiKcheK Meters, Micro Electrodes are warranted to be free from defects in material and workmanship for a period of six (6) months from date of purchase by the customer or twelve (12) months from date of shipment from Thermo Orion, whichever is earlier when used in accordance with the operating limitations and maintenance procedure in the instruction manual and when not having been subjected to accident, alteration, misuse or abuse. 31

35 AQUAfast IV colorimeters, Series 100 DuraProbe Conductivity Cells and Series 800 Dissolved Oxygen Probes are warranted to be free from defects in material and workmanship for a period of twenty-four (24) months from the date of purchase by the user or thirty (30) months from the date of shipment from Thermo Orion, whichever is earlier, provided use is in accordance with the operating limitations and maintenance procedures in the instruction manual and when not having been subjected to accident, alteration, misuse, or abuse. Waterproof meters (Models 630, 635, 830A, 835A, 260A, 261S, 265A, 266S, 130A, 131S, 135A, 136S, 1230, 142 and 842), Conductivity meters (Models 105Aplus, 115Aplus, 125Aplus, 145Aplus, 150Aplus and 162A), phuture MMS meters (Models 535A, 555A), ph/conductivity meter (Model 550A), Dissolved Oxygen meters (Models 810Aplus, 850Aplus and 862A), are warranted to be free from defects in material and workmanship for a period of thirty-six (36) months from the date of purchase by the user or forty-two (42) months from date of shipment from Thermo Orion, whichever is earlier, provided use is in accordance with the operating limitations and maintenance procedures in the instruction manual and when not having been subjected to accident, alteration, misuse or abuse. LabConnect and DataCOLLECT carry an out-of-box warranty. Should they fail to work when first used, contact Thermo Orion immediately for replacement. Thermo Orion Meter, Electrode, Analytical System Accessories, Solutions, AQUAfast Test Strips, AQUAfast II and IV Reagent Kits and accessories, Series 800 Dissolved Oxygen Probe Membranes and Cahn Balance Accessories such as cables, printers, and line adapters carry an out-of-box warranty. Should they fail to work when first used, contact Thermo Orion immediately for replacement. Should Thermo Orion Solutions or Buffers be unusable when first out-of-box, contact Thermo Orion immediately for replacement. For products not listed in this warranty statement, please visit our website: thermoorion.com. 32

36 THE WARRANTIES DESCRIBED ABOVE ARE EXCLUSIVE AND IN LIEU OF ALL OTHER WARRANTIES WHETHER STATUTORY, EXPRESS OR IMPLIED INCLUDING, BUT NO LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE AND ALL WARRANTIES ARISING FROM THE COURSE OF DEALING OR USAGE OF TRADE. THE BUYER S SOLE AND EXCLUSIVE REMEDY IS FOR REPAIR OR REPLACEMENT OF THE NON- CONFORMING PRODUCT OR PART THEREOF, OR REFUND OF THE PURCHASE PRICE, BUT IN NO EVENT SHALL THERMO ORION (ITS CONTRACTORS AND SUPPLIERS OF ANY TIER) BE LIABLE TO THE BUYER OR ANY PERSON FOR ANY SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES WHETHER THE CLAIMS ARE BASED IN CONTRACT, IN TORT (INCLUDING NEGLIGENCE), OR OTHERWISE WITH RESPECT TO OR ARISING OUT OF THE PRODUCT FURNISHED HEREUNDER. REPRESENTATION AND WARRANTIES MADE BY ANY PERSON, INCLUDING ITS AUTHORIZED DEALERS, REPRESENTATIVES AND EMPLOYEES OF THERMO ORION WHICH ALTER OR ARE IN ADDITION TO THE TERMS OF THIS WARRANTY SHALL NOT BE BINDING UPON THERMO ORION UNLESS IN WRITING AND SIGNED BY ONE OF ITS OFFICERS. Note: For in- or out-of-warranty repair or service, contact Thermo Orion Technical Service (or its authorized dealer outside the United States and Canada). Technical Service will issue a Return Authorization (RA) for all repair services. You must have an Thermo Orion RA prior to returning/forwarding any product to Thermo Orion. 33