Instruction Manual and Experiment Guide for the PASCO scientific Model CI-6735 012-06616A 9/97 NITRATE ION SELECTIVE ELECTRODE © 1997 PASCO scientific $7.50 ® better 10101 Foothills Blvd. • P.O. Box 619011 • Roseville, CA 95678-9011 USA Phone (916) 786-3800 • FAX (916) 786-8905 • web: www.pasco.
Nitrate Ion Selective Electrode 012–06616A Always use eye protection and gloves when working with chemicals.
012–06616A Nitrate Ion Selective Electrode Table of Contents Introduction......................................................................................................................................... 1 Theory .................................................................................................................................................. 1 Equipment ...................................................................................................................................
Copyright, Warranty, and Equipment Return PleaseFeel free to duplicate this manual subject to the copyright restrictions below. Copyright Notice The PASCO scientific 012-06616A manual is copyrighted and all rights reserved. However, permission is granted to non-profit educational institutions for reproduction of any part of the Nitrate Ion Selective Electrode manual providing the reproductions are used only for their laboratories and are not sold for profit.
012–06616A Nitrate Ion Selective Electrode Introduction The PASCO scientific Nitrate Ion Selective Electrode is used to quickly, simply, accurately, and economically measure nitrate in aqueous solutions. Theory The Nitrate Ion Selective Electrode consists of an electrode body containing a liquid internal filling solution in contact with a gelled organophilic membrane containing a nitrate ion exchanger.
Nitrate Ion Selective Electrode 012–06616A where: CX = concentrationof ion X ZX = 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 end constant value, ionic strength adjuster (ISA) is added to samples and standards.
012–06616A Nitrate Ion Selective Electrode Additional Required: Equipment • Nitrate Ion Selective Electrode Required Equipment • Nitrate Ion Selective Electrode fill solution PASCO CI-6738 ISE (Ion Selective Electrode) Amplifier • Pipette for fill solution • ScienceWorkshop 2.2.5 or higher • PASCO ScienceWorkshop Computer Interface Sodium Ion Selective Electrode • Semi-logarithmic 4-cycle graph paper for preparing calibration curves.
Nitrate Ion Selective Electrode 012–06616A General Preparation fill hole Electrode Preparation 1. Remove the rubber cap covering the electrode tip. Slide the rubber sleeve down away from the filling hole of the Nitrate Ion Selective Electrode. Fill the electrode with the included filling solution to a level just below the fill hole. Slide the rubber sleeve back over the filling hole (Figure 2a).
012–06616A Nitrate Ion Selective Electrode Measurement Measuring Hints • All samples and standards should be at the same temperature for precise measurement. A difference of 1 °C in temperature will result in a 2% measurement error. • The sensing membrane is normally subject to water uptake and might appear milky. This has no effect on performance. • Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can generate sufficient heat to change the solution temperature.
Nitrate Ion Selective Electrode 012–06616A Measurement Procedure Direct Measurement Direct measurement is a simple procedure for measuring a large number of samples. A single reading is all that is required for each sample. The ionic strength of samples and standards should be made the same by adjustment with ISA for all nitrate solutions. The temperature of both sample solution and standard solutions should be the same.
012–06616A Nitrate Ion Selective Electrode +30 +70 +110 10-fold change electrode potential +150 (mV) ~56mV +190 (ppm as N) +230 0.1 1 10 100 1,000 (ppm as NO3) 0.1 10 -6 Figure 3 Typical nitrate electrode calibration curve 1 10 -5 10 10 - 4 100 10 -3 1,000 10 -2 10 -1 NO3-1 concentration (M) ä Note: A calibration curve is constructed on semi-logarithmic paper The measured electrode potential in (linear axis) is plotted against the standard concentration (log axis).
Nitrate Ion Selective Electrode 012–06616A 2. Dilute 1 ml of 0.l M standard to 100 ml to prepare a l.0Xl0-3 M NO3-1 solution for measurements in moles per liter. Use the 1,000 ppm standard for preparing a 100 ppm NO3-1 standard by diluting 10 ml of the 1,000 ppm standard to 100 ml. Standards should be prepared fresh daily. 3. Add 1 ml of the low level ISA to a 100 ml volumetric flask and fill to the mark with distilled water.
012–06616A Nitrate Ion Selective Electrode 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.
Nitrate Ion Selective Electrode 012–06616A 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.
012–06616A Nitrate Ion Selective Electrode +50 10-3M to 10-2M NaNO3 +75 +100 electrode +125 potential +150 (mV) 10-3M to 10-4M NaNO3 +175 +200 10-3M to 10-5M NaNO3 +225 10-3M to 10-6M NaNO3 1 2 3 4 time (minutes) Figure 4 Typical electrode time response to step changes in NaNO3 Limits of Detection The upper limit of detection in pure sodium nitrate solutions is 1 M. In the presence of other ions, the upper limit of detection is above l0-1 M nitrate, but two factors influence this upper limit.
Nitrate Ion Selective Electrode 012–06616A Maintenance Electrode Storage The nitrate electrode may be stored for short periods of time in l0-2 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 electrode.
012–06616A Nitrate Ion Selective Electrode 4. If the problem persists, the standards or reagent may be of poor quality, interferences in the sample may be present or the technique may be faulty. (See Standards Reagents, Sample, and Technique sections below.) 5. If another electrode is not available for test purposes, or if the electrode in use is suspect, review the instruction manual and be sure to: - Clean and rinse the electrode thoroughly. - Prepare the electrode properly.
Nitrate Ion Selective Electrode 012–06616A Troubleshooting Hints Symptom Possible Causes Next Step Out of Range Reading defective electrode check electrode operation electrodes not plugged in properly unplug electrodes and reseat electrodes reference electrode not filled be sure reference electrode is filled air bubble on membrane remove bubble by re-dipping electrode electrode not in solution put electrodes in solution electrode exposed to interferences soak electrode in nitrate standard
012–06616A Nitrate Ion Selective Electrode Symptom Possible Causes Next Step Low Slope or No Slope standards contaminated or incorrectly made prepare fresh standards ISA not used use recommended ISA standard used as ISA use ISA electrode exposed to interferences soak electrode in nitrate standard defective electrode check electrode operation air bubble on membrane remove bubble by re-dipping probe incorrect scaling of semilog paper plot voltage potential on the linear axis.
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