Ion Concentration Controller General Information

Transcription

1 Document 40501, p. 1 of 8 Ion Concentration Controller General Information Common Questions Asked about Ion-Selective Electrodes 1. How often do you need to calibrate your ion-selective electrode (ISE)? You should recalibrate at least once a week. You may need to recalibrate daily if you are in doubt of your results being accurate or reproducible. 2. After how long of being opened or made are standards good? Stock standards will last at least six months before you need to discard them. Whereas diluted standards treated with ISA/pH buffer should be prepared weekly. 3. How do you store an ISE? If you have a gas-sensing electrode, you should empty out the filling solution. If you have a liquidsensing electrode, it is best to store it dry when it is not to be used within the next week or so. For those shorter periods of time, store it in a dilute standard approximating the sample concentration. You may need to add ISA/pH buffer when required. 4. Can your ISE system be temperature compensated? Yes, it is possible, but somewhat difficult. First, you have to know the isopotential point for a given type of ISE 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. When measuring ions in-line continuously do you need to treat the sample? Direct ion measurement is possible in many cases without ISA/pH buffer addition to the sample stream. However, fluoride, sulfide, ammonia, and sodium electrodes do require ph adjustment and must have ISA/pH buffer added to the sample stream. 6. Which standards should be used with an ISE? The most obvious choice will be determined by what concentration units are desired. Also, if an electrode is being used to measure another ion, use a stock standard of the ion to be measured For an example, if you wanted to measure sulfate with a lead electrode, you would use a stock standard for sulfate. 7. Why buy a combination ISE instead of both a reference and a half cell ISE? The advantages of a combination ISE are that you do not need to purchase an external reference electrode, it is more economically priced, and one less electrode to fit in your process.

2 Document 40501, p. 2 of 8 Specifications TYPE AMMONIA AMMONIUM CALCIUM CAT. NO. EX EX EX Concentration 5 x 10-7 to 1 M 5 x 10-6 to 1 M 5 x 10-6 to 1 M range (0.01 to 17,000 ppm NH3) (0.1 to 18,000 ppm) (0.2 to 40,000 ppm) ph range > 11 ph 4 to 10 ph 3 to 10 ph Interferences Volatile amines K + 1 Pb + 2, Hg + 2, Cu + 2, Ni + 2 Complexations Resistance 1000 to 1500 MΩ 100 MΩ 100 MΩ Slope 56 ± 3 mv/decade 55 ± 3 mv/decade 26 ± 2 mv/decade Reproducibility ±2% ±2% ±2% Response time 95% response in 30 seconds 95% response in 30 seconds 95% response in 30 seconds Temperature range 32 to 122 F (0 to 50 C) 32 to 104 F (0 to 40 C) 32 to 104 F (0 to 40 C) Pressure range 0 to 10 psi 0 to 30 psi 0 to 30 psi Temp compensation No Yes Yes Isopotential point Unknown 10 ppm ammonium 1 ppm calcium Treat with a dilute ammonia Treat with distilled water Treat with distilled water Care, maintenance, standard with ph adjusted followed by ammonium followed by calcium and cleaning greater than ph 11. standard. standard. Replace membrane as needed. long-term Dry Dry Dry Storage Store in ammonia standard Store in dilute ammonium Store in dilute calcium short-term with ph buffer added. standard. standard. On-line capability Yes, ph control required Yes Yes High-purity power station Water softening systems; Applications washers; drinking water; Boiler feed water; drinking and mineral waters; steel works effluent; natural waters blood electrolytes/ clinical analysis/dialysis clinical analysis

3 Document 40501, p. 3 of 8 TYPE CHLORIDE FLUORIDE LEAD CAT. NO. EX EX EX Concentration 5 x 10-5 to 1 M 1 x 10-6 M to saturated 1 x 10-6 to 0.1 M range (1.8 to 35,500 ppm NH3) (0.02 ppm to saturated) (0.2 to 20,700 ppm) ph range 2 to 12 ph 5 to 8 ph 3 to 8 ph Interferences S -2, I -1, CN -1, Br - 1 OH - 1 Ag + 1, Hg + 2, Cu + 2; high levels of Cd + 2 and Fe + 2 Complexations Heavy metals Al + 3, Fe + 3, H + 1 (fluoride) Resistance 0.1 to 1 MΩ 100 to 200 KΩ 100 to 300 KΩ Slope 56 ± 3 mv/decade 57 ± 2 mv/decade 26 ± 3 mv/decade Reproducibility ±2% ±2% ±2% Response time 95% response in 30 seconds 95% response in 30 seconds 95% response in 30 seconds Temperature range 32 to 176 F (0 to 80 C) 32 to 176 F (0 to 80 C) 32 to 176 F (0 to 80 C) Pressure range 0 to 70 psi 0 to 70 psi 0 to 70 psi Temp compensation Not recommended Yes Yes Isopotential point 50,000 ppm chloride 20 ppm fluoride 1 ppm lead Care, maintenance, and cleaning Treat with a dilute ISA. Polish with polishing strips. Treat with a dilute ISA polish with alumina paste or toothpaste. Treat with a dilute ISA polish with polishing strips. long-term Dry Dry Dry Storage short-term Store in dilute Store in dilute fluoride Store in dilute chloride standard. standard with TISAB. lead standard. On-line capability Yes Yes, ph control required Yes River and tap water; Drinking water; Drinking water; Applications boiler feed water; wastewater; wastewater; blood electrolytes/ natural waters; natural waters; clinical analysis air and stack gases sulphate analysis

4 Document 40501, p. 4 of 8 TYPE NITRATE POTASSIUM SODIUM CAT. NO. EX EX EX Concentration 7 x 10-6 to 1 M 1 x 10-6 to 1 M 1 x 10-6 to 5 M range (0.5 to 62,000 ppm) (0.04 ppm to 39,000 ppm) (0.02 to 100,000 ppm) ph range 2.5 to 11 ph 2 to 12 ph 5 to 12 ph (depending on sodium level) Interferences ClO4-1, I - 1, CN - 1, BF4-1 Cs + 1, NH4 + 1 Ag + 1, Li + 1, H + 1 Resistance 100 MΩ 100 MΩ 300 to 500 MΩ Slope 55 ± 3 mv/decade 56 ± 3 mv/decade 55 ± 3 mv/decade Reproducibility ±2% ±2% ±2% Response time 95% response in 30 seconds 95% response in 30 seconds 95% response in 30 seconds Temperature range 32 to 104 F (0 to 40 C) 32 to 104 F (0 to 40 C) 32 to 176 F (0 to 80 C) Pressure range 0 to 30 psi 0 to 30 psi 0 to 70 psi Temp compensation Yes Yes Yes Isopotential point 10 ppm nitrate 20 ppm potassium 30 ppm sodium Treat with distilled water Treat with distilled water Rinse with dilute Care, maintenance, followed by followed by NH4Cl/NH4OH ISA. and cleaning nitrate standard. potassium standard. Soak with 5 M NaCl. long-term Dry Dry Storage Store in dilute Store in dilute Store in 1 M NaCl. short-term nitrate standard. potassium standard. On-line capability Yes Yes Yes, ph control required Surface waters; Wastewaters; Stream condensates in Applications sewage effluent; river and tap waters; power plants; drinking waters; blood electrolytes/ blood electrolytes/ soil extracts clinical analysis clinical analysis

5 Document 40501, p. 5 of 8 TYPE SILVER SULFIDE WATER HARDNESS CAT. NO. EX EX Concentration 1 x 10-7 to 1 M 1 x 10-5 to 1 M range (0.01 to 107,900 ppm silver to 32,100 ppm sulfide) (0.4 to 40,000 ppm Ca) ph range 2 to 8 ph (silver); > 11 ph (sulfide) 5 to 10 ph Interferences Hg + 1; Hg + 2 Zn + 2, Fe + 2, Cu + 2, Ni + 2 Complexations Heavy metals Resistance 100 to 300 KΩ 100 MΩ Slope 57 ± 2 mv/decade (silver); 27 ± 2 mv/decade (sulfide) 26 ± 2 mv/decade Reproducibility ±2% ±2% Response time 95% response in 30 seconds 95% response in 30 seconds Temperature range 32 to 176 F (0 to 80 C) 32 to 104 F (0 to 40 C) Pressure range 0 to 70 psi 0 to 30 psi Temp compensation Not recommended Yes Isopotential point 50,000 ppm silver; unknown for sulfide 1 ppm calcium/magnesium Care, maintenance, Treat with distilled water and cleaning Treat with a dilute ISA. Polish with polishing strips. followed by calcium standard. long-term Dry Dry Storage Store in dilute Store in dilute short-term silver or sulfide standard. calcium standard. On-line capability Yes, ph control required for sulfide monitoring Yes Applications Sewage effluent for sulfide; paper industry black pulping liquid for sulfide Water softening systems; drinking and mineral waters

6 Document 40501, p. 6 of 8 Calibration Procedure (Quick Start Guide) 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 [Cal] key. 3. Select [Temp: on/off]. Use [^] and [ v] buttons to turn temperature compensation off. 4. Select electrode type. Use [^] and [ v] buttons to choose ±1 or ±2. 5. 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 ISA/pH buffer when required to each standard. 6. Place ISE in the lowest ppm or M standard, either off-line in a beaker or on-line with a standard pumped through the process line. 7. When a stable mv signal is obtained, press [Ent]. 8. Enter ppm value of standard. Use [^] and [ v] buttons to choose ppm values. 9. Repeat steps 6 through 8 until all calibration standards have been entered into the controller. 10. At the end of the calibration, the message Calibration Complete is displayed. 11. Meter then defaults back to displaying ppm mode (actual concentration value). Electrode Measurement Checklist Check electrode slope weekly if not daily. Always use fresh standards for calibration. Remove solution air bubbles from membrane surface. Measure only aqueous or near-aqueous samples. Operate at a constant pressure. Operate at a constant temperature. Adjust the ph of samples to use ISE operating range if necessary.

7 Document 40501, p. 7 of 8 A Guide for Common Measurement Problems PROBLEMS Not reproducible Slow response (reading slowly changing) Out of range reading Low slope or no slope Noisy response (readings randomly changing) Drifty response (readings continuously changing) Inaccurate (but calibration is okay) CHECK THESE FIRST 1. sample carryover 2. sample interferences or complexing against present 3. contaminated reference electrode junction 1. electrode stored in wrong solution 2. electrode poisoned by sample 1. electrode not plugged into controller properly 2. air bubble on electrode surface 3. not enough fill solution left in reference 4. electrode not in sample solution 1. standards are old, contaminated, or made wrong 2. sample ph has not been adjusted properly to operating range of electrode 3. air bubble on electrode surface 4. controller not calibrated properly 5. electrode not plugged into controller properly 1. controller not grounded 2. air bubble on electrode surface 3. electrode not plugged into controller properly 4. not enough fill solution left in reference 5. electrode not in sample solution 1. excessive leaking at reference electrode junction 2. clogged reference electrode junction 3. sensing membrane poisoned by sample 4. temperature problems 5. sample too concentrated 6. sensing membrane needs conditioning 1. standards are incorrect 2. sample is not ph adjusted properly 3. sample carryover 4. sample interferences or complexing agents present

8 Document 40501, p. 8 of 8 Troubleshooting Difficult Measurement Problems Components of the Electrode System Controller preform checkout procedure in instruction manual Ion-Selective Electrode preform electrode slope check, inspect for physical damage, polish, soak, or rebuild sensing membrane, monitor leak rate on reference side Proper Calibration Standards prepare fresh standards by serial dilution ph Adjustment adjust ph to operational range of electrode Sample Variable Concentration Range see if sample is out of range Pressure check if using wrong sample pressure Temperature verify sample temperature range ph see if correct sample ph Interferences are interferences present in the sample Complexation are complexing agents in the sample