GENERAL INSTRUCTIONS Introduction COLE-PARMER INDUSTRIAL SILVER/SULFIDE ION ELECTRODE INSTRUCTION MANUAL The Cole-Parmer Industrial Silver/sulfide Ion Electrode is used to quickly, simply, accurately, and economically measure silver or sulfide ions in in-line batch processing, control processing, or water treatment applications. The two ions are virtually never present in solution together, owing to the extreme insolubility of silver sulfide. 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 Silver/sulfide Ion Electrode, Cat. No. 27077-14. 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 Silver/sulfide Ion Electrode. See page 1185 of the Cole-Parmer Catalog to choose an Industrial Housing. Required Solutions Deionized or distilled water for solution and standard preparation. Water used in the preparation of sulfide standards and of SAOB should also be deaerated. For Silver: 1. Cole-Parmer Ionic Strength Adjuster (ISA), 5M NaNO 3, Cat. No. 27503-51. To prepare this solution from your own stock, fill a 1000 ml volumetric flask about half full of distilled water and add 425 grams of reagent grade sodium nitrate. Swirl the flask to dissolve the solid. Fill the flask to the mark with distilled water. Cap the flask and invert 1
several times to mix the solution. 2. Cole-Parmer Silver Standard Solution, 1000 ppm as Ag +, Cat. No. 27503-32. To prepare this solution from your own laboratory stock, dry reagent grade, pulverized silver nitrate in a laboratory oven for one hour at 150 o C. Quantitatively transfer 1.57 grams of the dried silver nitrate to a 1 liter volumetric flask containing about 500 ml of distilled water. Swirl the flask to dissolve the solid. Fill the flask to the mark with distilled water. Cap the flask and invert several times to mix the solution. Store the solution in a brown bottle, tightly capped, in a dark place. For Sulfide: 1. Sulfide Anti-Oxidant Buffer (SAOB). This buffer must be used fresh and will range in color from clear to yellow-brown. It has become oxidized when it turns dark brown and should then be discarded. Store fresh SAOB in a tightly stoppered bottle. To prepare SAOB from your own stock, fill a 1 liter volumetric flask with about 500 ml of distilled, deaerated water, 200 ml of 10M NaOH, 35 grams of ascorbic acid, and 67 grams of disodium EDTA. Swirl the mixture until the solids dissolve and fill to the mark with distilled, deaerated water. 2. Sulfide Standards. To prepare a stock solution of saturated sodium sulfide, add about 100 grams of reagent grade Na 2 S. 9H 2 O to 100 ml of distilled, deaerated water. Shake well and let stand overnight, storing in a tightly stoppered bottle in a hood. Prepare a weekly sulfide standard by adding 500 ml SAOB to a 1 liter volumetric flask, pipetting 10 ml of the stock solution into the flask, and diluting to the mark with distilled, deaerated water. The exact concentration, C, can be determined 2
by titrating 10 ml of the standard with 0.1M lead perchlorate. Use the silver/sulfide ion electrode (and the reference electrode) to indicate the endpoint. The calculation is as follows: C = 3206(V t /V s ) where: C = concentration as ppm sulfide V t = volume of titrant at endpoint V s = volume of standard used (10 ml) Prepare other standards each day by serial dilution of the weekly standard. To do a ten-fold dilution, accurately measure 10 ml of the standard and add it to a 100 ml volumetric flask. Add 45 ml of SAOB and dilute to the mark with distilled, de-aerated water. ELECTRODE PREPARATION Remove the rubber cap covering the electrode tip. Prior to first usage, or after long-term storage, immerse the silver/sulfide membrane in silver/sulfide 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. 3
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. 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 4
[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 80 o C. A difference of 1 o C in temperature will result in a 2% measurement error for silver and a 4% measurement error for sulfide. All samples and standards should be at the same pressure for precise measurement. All silver samples and silver standards should be stored away from light. 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 point to deposits on the membrane. Use polishing paper to remove such deposits. See the section on Electrode Response. Sample Requirements All samples must be aqueous and not contain organics that can dissolve the epoxy electrode body and/or cement bonding the sensing crystal to the electrode body. Infrequent measurements in solutions containing methanol, benzene, or acetone are permitted. Please check before using the electrode in other organic solvents. 5
Samples containing sulfide must be buffered with SAOB above ph 11, to convert HS -1 and H 2 S to S -2. Samples containing silver must be below ph 8 to avoid reaction with OH -1. Silver samples should be acidified with 1M HNO 3, if necessary. Mercury cannot be present in silver samples. Since HgS and Hg S 2 are insoluble, no mercury will be present in sulfide samples. Other interferences should be absent. Units of Measurement Silver and sulfide concentrations are measured in units of parts per million, equivalents per liter, moles per liter, or any other convenient concentration unit. Table 1 indicates some of the concentration units. TABLE 1: Concentration Unit Conversion Factors ppm S -2 ppm Ag +1 N(S -2 ) M(Ag +1 ) 321.0 1079.0 2.00X10-2 1.00X10-2 100.0 --- 6.23X10-3 3.12X10-3 ELECTRODE CHARACTERISTICS Reproducibility 32.1 108.0 2.00X10-3 1.00X10-3 --- 100.0 1.84X10-3 9.27X10-4 3.2 10.8 2.00X10-4 1.00X10-4 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 A surface layer of silver metal may be formed by strongly reducing solutions. A layer of silver salt may be deposited on the membrane if high levels of ions forming very insoluble salts are present in the sample. Performance may be restored by 6
polishing. See the section Electrode Response for proper polishing procedure. All silver samples must be free of mercury. Sulfide samples will not have mercury present due to the extreme insolubility of HgS and Hg 2 S. Biological samples and protein in food interferes with silver measurements, but the protein interference can be removed by acidifying to ph 2-3 with 1M HNO 3. Complexation Total concentration (C ) whether sulfide or silver ions, consists t of free ions (C f ) and complexed or bound ions (C c ) in solution: C t = C f + C c Since the electrode only responds to free ions, any complexing agent in the solution reduces the measured concentration of ions. Silver ions complex with many species, notably cyanide, thiosulfate, ammonia and chelants such as EDTA. Sulfide ions form complexes with hydrogen ions (HS -1 and H 2 S). Sulfide ions also form soluble complexes with elemental sulfur (S -2 2, S -2 3, S -2 4, etc.) and tin, arsenic, and antimony ions. 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 1.0X10-3 M level for silver and a 4% error for sulfide. Provided that temperature equilibria has occurred, the silver/sulfide ion electrodes can be used at temperatures from 0 o - 80 o C continuously. Room temperature measurements are recommended, since measurements at temperatures markedly different from room temperature may require equilibrium times up to one hour. 7
Electrode Response Plotting the electrode mv potential against the silver concentration on semi-logarithmic paper results in a straight line with a slope of about 57 mv per decade. (Refer to Figure 2.) The sulfide ion also gives a straight line when the electrode mv potential is plotted against the sulfide concentration, but the slope is about 26 mv per decade. (See Figure 1.) 8
The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from several seconds in highly concentrated solutions to several minutes near the detection limit. (Refer to Figure 5.) A drifting potential reading or a decrease in electrode slope may mean that the electrode membrane needs polishing. To polish the membrane: 1. If using polishing paper, cut off a 1-2" piece and place it face up on the lab bench. 2. Put a few drops of distilled or deionized water in the center of the paper. 3. Holding the paper (cotton) steady with one hand, bring the membrane of the electrode down perpendicular to the paper and, with a slight swirling motion, gently polish the tip of the electrode against the surface of the polishing paper (cotton) for a few seconds. 4. Rinse the electrode surface with distilled or deionized water and soak the electrode tip in standard solution for about five minutes before use. 5. If using jeweller's rouge, place a cotton ball on the table top and flatten it using the bottom of a beaker. 6. Put 1-2 drops of distilled or deionized water in the 9
center of the cotton pad. 7. Add a small amount of jeweller's rouge to the damp cotton. 8. Continue with Steps 3 and 4 above. Limits of Detection The upper limit of detection in pure silver nitrate solutions is 1M. In the presence of other ions, the upper limit of detection is above 1.0x10-1 M silver, 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 extract into the electrode membrane at high salt concentrations, causing deviation from the theoretical response. Either dilute samples between 1M and 1.0x10-1 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 electrode pellet. Refer to Figure 1 and Figure 2 for a comparison of the theoretical response to the actual response at low levels of sulfide and silver. ph Effects Silver reacts with hydroxide ions to form a precipitate of Ag 2 O, in ammonia-free basic solutions. By keeping all solutions slightly acidic, this can be avoided. Adjust the ph of silver solutions below 8, if necessary, with 1M HNO 3. Bisulfide ion (HS -1 ) and hydrogen sulfide (H 2 S) result when hydrogen ion complexes sulfide ion. Larger amounts of sulfide ion are complexed as the ph is lowered. The use of SAOB in all samples containing sulfide maintains a fixed level of S -2 ions, since the free sulfide ion (S -2 ) exists in only very basic solutions. In the acid range, sulfide is chiefly in the form of H S, while in the ph range 6-12, almost all the sulfide is in the 2 HS -1 form. Electrode Life The silver/sulfide electrode will last six months in normal laboratory use. On-line 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. 10
Electrode Storage The Cole-Parmer Silver/Sulfide Electrode may be stored in-line if the membrane is kept wet. For storage off-line, rinse and dry the silver/sulfide electrode and cover the tip with any protective cap shipped with the electrode. ELECTRODE THEORY Electrode Operation The Cole-Parmer Silver/Sulfide Ion Electrode is composed of a silver sulfide crystal membrane bonded into an epoxy or glass body. When an electrode potential develops across the membrane, the membrane is in contact with a solution containing sulfide or silver ions and is capable of measuring free sulfide or silver ions. This electrode potential is measured against a constant reference potential, using a ph/mv meter or an ion meter. The level of sulfide or silver ions, corresponding to the measured potential, is described by the Nernst equation: where E = E o + S logx E = measured electrode potential E o = reference potential (a constant) S = electrode slope (-26 mv for sulfide; +57 mv for silver) X = level of sulfide or silver in solution The activity, X, represents the effective concentration of the ions in solution. The activity is related to the free ion concentration, C f, by the activity coefficient, γ, by: X = γc f Activity coefficients vary, depending on total ionic strength, I, defined as: I = ½ Σ C x Z x 2 where C x = concentration of ion X Z x = 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 11
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 solution for sulfide is SAOB, used to prevent oxidation and free sulfide ion from hydrogen ion, in addition to adjusting the ionic strength. The recommended ISA for silver is NaNO 3. Solutions other than these may be used as ionic strength adjusters as long as ions that they contain do not interfere with the electrode's response to sulfide ions or to silver ions. Samples with high ionic strength (greater than 0.1M) do not need ISA added and standards for these solutions should be prepared with a composition similar to the samples. 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? 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 -ION-SELECTIVE ELECTRODE -PROPER CALIBRATION -STANDARD(S) -ph ADJUSTMENT -SAMPLE VARIABLES -CONCENTRATION RANGE -PRESSURE -TEMPERATURE -ph wrong sample ph? -INTERFERENCES -COMPLEXATION perform checkout procedure in instruction manual perform electrode slope check, inspect for physical damage. monitor leak rate on reference side prepare fresh standards by serial dilution adjust ph to operational range of electrode sample out of range? wrong sample pressure? wrong sample temperature? interferences in the sample? complexing agents in sample? 13
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 continuous ISE measurement without treating the sample? Yes, direct measurement is possible in many cases without ISA/buffer addition to the sample stream. However, Silver/sulfide, 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 14
SILVER/SULFIDE ELECTRODE SPECIFICATIONS (MODEL # 27077-14) -SLOPE 57 +/- 2 mv/decade (Silver) 26 +/- 2 mv/decade (Sulfide) -REPRODUCIBILITY +/- 2% -INTERFERENCES Hg+1, Hg+2 -COMPLEXATION heavy metals -TEMPERATURE RANGE 0 to 80 degrees C -PRESSURE RANGE 0 to 70 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 10-7 M to 1M 0.01 ppm to 107,900 ppm (Silver) 0.003 ppm to 32,100 ppm (Sulfide) -ph RANGE 2 to 8 ph (Silver) above 11 ph (Sulfide) -RESISTANCE 100 to 300 kiloohms -CARE, MAINTENANCE, treat with a dilute ISA & CLEANING polish with polishing strips -TEMPERATURE COMPENSATION/ Not Recommended -ISOPOTENTIAL POINT 50,000 ppm silver, sulfide is unknown -ON-LINE CAPABILITY Yes, ph control required for sulfide monitoring -APPLICATIONS: Sewage Effluent for Sulfide Paper Industry Black Pulping Liquid for Sulfide ORDERING INFORMATION P/N DESCRIPTION 56105-00 Ion Concentration Controller 50001-68 Cord with U.S. standard plug 50001-70 Cord with European plug 27077-14 Industrial Silver/sulfide Ion Electrode, sealed, double junction electrode which is easily connected to the Ion Concentration Controller with stripped wire ends. 15
Cole-Parmer Industrial Housings to protect the Silver/sulfide Ion Electrode. See page 1185 of the Cole-Parmer Catalog to choose an Industrial Housing. 27503-32 Silver Standard, 1000 ppm AgNO 3 27503-51 Silver Ionic Strength Adjuster (ISA), 5 M NaNO 3 16
INDEX General Instructions...1 introduction...1 required equipment...1 required solutions...1 Electrode Preparation...3 Calibration Procedure...3 Measurement...5 measuring hints...5 sample requirements...5 units of measurement...6 Electrode Characteristics...6 reproducibility...6 interferences...6 complexation...7 temperature influences...7 electrode response...8 limits of detection...10 ph effects...10 electrode life...10 electrode storage...11 Electrode Theory...11 electrode operation...11 Common Measurement Problems, Simple Causes...12 Troubleshooting Difficult Measurement Problems...13 Some Questions Asked About Ion-Selective Electrodes...14 Silver/sulfide Electrode Specifications...15 Ordering Information...15 Index...17 17