Instruction Manual and Experiment Guide for the PASCO scientific Model CI-6734 012-06615A 9/97 SODIUM 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.
Sodium Ion Selective Electrode 012–06615A Always use eye protection and gloves when working with chemicals.
012–06615A Sodium 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 012-06615 A manual is copyrighted and all rights reserved. However, permission is granted to non-profit educational institutions for reproduction of any part of the Sodium Ion Selective Electrode manual providing the reproductions are used only for their laboratories and are not sold for profit.
012–06615A Sodium Ion Selective Electrode Introduction The PASCO scientific Sodium Ion Selective Electrode is used to quickly, simply, accurately, and economically measure sodium ions in aqueous solutions. Theory The Sodium Ion Selective Electrode is composed of a sodium-selective glass membrane bonded to a glass body. When the membrane is in contact with a solution containing sodium ions, an electrode potential develops across the membrane.
Sodium Ion Selective Electrode 012–06615A To adjust the background ionic strength to a high and constant value, ionic strength adjuster is added to samples and standards. The recommended ISA for sodium is an ammonium chloride/ammonium hydroxide buffer. 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 sodium ions.
012–06615A Sodium Ion Selective Electrode ä Note: Electrodes must not be stored in distilled water or air. Do not rinse with distilled water. • Dilute Electrode Rinse Solution To prepare this solution from your own laboratory stock, add 20 ml of ISA to a one liter volumetric flask and fill to the mark with distilled water. Use this solution to rinse the electrode between measurements. • Sodium Standard, 0.
Sodium Ion Selective Electrode 012–06615A General Preparation fill hole Electrode Preparation 1. Remove the rubber cap covering the electrode tip and the rubber insert covering the filling hole of the Sodium Ion Selective Electrode. Fill the electrode with the included filling solution to a level just below the fill hole. No preparation is required with a sealed reference electrode. 2.
012–06615A Sodium Ion Selective Electrode Measurement Measuring Hints • All samples and standards should be at the same temperature for precise measurement, preferably ambient temperature. A difference of l °C in temperature will result in about a 2% measurement error. The electrodes should not be used above 70 °C. • Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can generate sufficient heat to change the solution temperature.
Sodium Ion Selective Electrode 012–06615A Measurement Procedure Direct Measurement Direct measurement is a simple procedure for measuring a large number of samples. A single meter 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 sodium solutions. The temperature of both sample solution and of standard solutions should be the same.
012–06615A Sodium Ion Selective Electrode +140 +100 10-fold change +60 ~59 mV electrode potential (mV) +20 -20 -60 0.1 10 -6 1 10 -5 10 (ppm) 10 - 4 100 10 -3 1,000 10 -2 10 -1 + Na concentration (M) Figure 3 Typical sodium electrode calibration curve 6. To a clean, dry 150 ml beaker, add 100 ml of sample and 2 ml of ISA. Place the beaker on the magnetic stirrer and begin stirring.
Sodium Ion Selective Electrode 012–06615A 6. Add increments of the l.0xl0-3 M or 100 ppm standard as given in Table 2 below. 7. After the reading has stabilized, record the Voltage reading in the Digits display after each addition. TABLE 2: Step-wise Calibration for Low Level Sodium Measurements Step Pipette 1 2 3 4 5 6 7 A A A A A B B Added Volume (ml) 0.1 0.1 0.2 0.2 0.4 2.0 2.0 Concentration ppm M 0.10 0.20 0.40 0.60 0.99 2.91 4.76 l.0 X l0-6 2.0 X l0-6 4.0 X l0-6 6.0 X l0-6 9.9 X 10-6 2.
012–06615A Sodium Ion Selective Electrode Electrode drift and slow response could indicate the presence of high interference from the ions listed. Soak the electrodes in electrode storage solution when this happens to restore proper response. See Measuring Hints section. TABLE 3: Levels of Interfering Ions Resulting in a 10% Error at Specified Levels of Sodium Interference -4 1.0X10 M +1 5X10 M +1 1X10 M Li K +1 Rb NH4 +1 -3 1.0X10 M -4 5X10 M -2 -2 1.
Sodium Ion Selective Electrode 012–06615A Electrode Response Plotting the electrode potential against the sodium concentration on semilogarithmic paper results in a straight line with a slope of about 59 mV per decade. (Refer to Figure 1.) The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from one minute or less for sodium concentrations above 1.0X10-5 M to several minutes near the detection limit. (Refer to Figure 4.
012–06615A Sodium Ion Selective Electrode 10-1 M Na+ +180 +160 +140 electrode potential (mV) 10-2 M Na+ +120 +100 +80 10-3 M Na+ +60 +40 H+ Ion Interference +20 10-4 M Na+ 0 2 4 6 8 10 12 14 pH Figure 5 Electrode potential behavior vs. solution pH in pure NaCl solution at 25 ° The edge of the shaded area (the straight line) shown in Figure 5 indicates a minimum pH at which dilute sodium measurements can be made with less than 10% hydrogen ion interference.
Sodium Ion Selective Electrode 012–06615A Specifications Concentration Range: pH Range: Saturated to 1.0Xl0-6 M (0.
012–06615A Sodium Ion Selective Electrode Standards & Reagents Whenever problems arise with the measuring procedure that has been used successfully in the past, be Sure to check the standard and reagent solutions. If in doubt about the credibility of any of the solutions, prepare them again. Errors may result from contamination of the ISA, incorrect dilution of standards, poor quality distilled/deionized water, or a simple mathematical miscalculation.
Sodium Ion Selective Electrode Symptom Possible Causes Next Step Noisy or Unstable Readings (readings continuously or rapidly changing) electrode exposed to interferences soak overnight in electrode storage solution defective electrode replace electrode ISA not used use recommended ISA stirrer not grounded ground stirrer samples and standards at different temperatures allow solutions to come to room temperature before measurement electrode exposed to interferences soak overnight in electrode
012–06615A Sodium Ion Selective Electrode Symptom Possible Causes Next Step “Incorrect Answer” (but calibration curve is good) incorrect scaling of semilog paper plot voltage potential on the linear axis. On the log axis, be sure concentration numbers within each decade are increasing with increasing concentration. incorrect sign be sure to note sign of millivolt reading correctly incorrect standards prepare fresh standards wrong units used apply correct conversion factor: 1.0X10-3 M = 23.
Sodium Ion Selective Electrode 16 012–06615A
Technical Support Feedback Contacting Technical Support If you have any comments about the product or manual, please let us know. If you have any suggestions on alternate experiments or find a problem in the manual, please tell us. PASCO appreciates any customer feedback. Your input helps us evaluate and improve our product. To Reach PASCO For technical support, call us at 1-800-772-8700 (toll-free within the U.S.) or (916) 786-3800. fax: (916) 786-3292 e-mail: techsupp@pasco.com web: www.pasco.
