Ion Concentration Controllers

Ion Concentration Controllers

General Information

Commonly Asked Questions
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 liquid-sensing 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.

Calibration Procedures

1. Connect the electrode with the stripped wire ends to the controller by inserting the center wire into
2. slot #22 and inserting the outer wire braid into slot #21.
3. Press the [Cal] key.
4. Select [Temp: on/off]. Use [^] and [v] buttons to turn temperature compensation off.
5. Select electrode type. Use [^] and [v] buttons to choose ±1 or ±2. 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.

A Guide to Common Measurement Problems

Problems	Check these first
Not reproducible	1. sample carryover 2. sample interferences or complexing against present 3. contaminated reference electrode junction
Slow response (reading slowly changing)	1. electrode stored in wrong solution 2. electrode poisoned by sample
Out of range reading	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
Low slope or no slope	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
Noisy response (readings randomly changing)	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
Drifty response (readings continuously changing)	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
Inaccurate (but calibration is okay)	1. standards are incorrect 2. sample is not pH adjusted properly 3. sample carryover 4. sample interferences or complexing agents present

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