Precautions for successful measurements in Lab using Ion Selective Electrodes(ISE)

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Are you having issues with use of your lab ISEs please check out these tips for accurate and repeatable analysis of various cations, anions and dissolved gases while working with specific ISEs in the labs.?

All ISEs can be classified in three different types :-

a) Solid State: Sensor is fused with main body of the electrode eg: Fluoride, Chloride, Bromide, Sulphide, Iodide etc.

b) Liquid Membrane: In this case one can call sensor or more precise it is called liquid membrne which needs to be installed on the electrode prior to its operation. Examples are Nitrate, Calcium, Chlorine etc.

c) Gas sensing: These types of electrodes are specifically used to measure dissolved gases in aquoeous sample like Ammonia, Carbon dioxide, Nitrogen Dioxide, Dissloved Oxygen.

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Steps to check for successful use of ISEs:

1) Direct readout ISE meter: Select and use the meter which can display results directly in concentration units ( PPM, mg/l). With this kind of meter, the results can be obtained directly in concentration mode by selecting ppm or mg/l.

2) In case of solid state ISEs, no need to install sensor on it because the sensor is fused to the main body of the electrode but in case of liquid membrane and gas sensing, one has to install membrane before operating such electrodes.

3) Fill the reference chamber of the ISEs with suggeste/supplied reference fill solution( we also call electrolyte). The level of fill solution must be above leve of your measurement solution . Please ensure that while its in use the refill hole is opened

4) Condition electrode before its use: The conditioning period depends somewhere between 30 minutes to an hour or so. The purpose of this is to activate the membrane for faster response. One must refer to the instruction manual of ISE being supplied b y the manufacturer.

5) Standards preperation: Now a days ISEs meters are supplied with facility for seven point calibration means one can use up to seven standards for calibration point of view. Most of the time chemists use 2 or max 3 point calibration. This means that he is using 2 or 3 standards for the calibration of electrode in use. Ideally speaking the standards should be prepared fresh and one must use volumeteric flasks as these are more accurate than measuring cylinders for preparing standards in lab. Standards must be used in increasing order of its concentration.

6) Use of Ionic Strength Adjusting solution(ISA): The purpose of using this ISA is ensure that both standards and samples(becuase you have to use same ISA in case of sample as well) are having same ionic strength and it masks the influence of interfering ions likely present in the sample. This ISA maintains pH ( as ISEs are pH sensitive) of sample conducive for best performance of ISEs.

(Slide showing effect of pH)

(Slope check typical graph)

7) Slope Check: One must check slope of electrode in use at least once a day prior to sample analysis. For this chemist uses his standards( we strongly suggest concentration difference between two standards should be 10 fold, example if standard one is 1 ppm then next standard should be 10 ppm. According to Nernst equation, the slope of ISEs are calculated between 10

fold concentrations of standards. In case of univalent ions( F, Cl, Br, Ammonia, Nitrate etc) the slope should be between 52-62 mv change and in case of divalent ions( Ca, Sulphate) the slope should be between 27-30 mv/decade change in standards concentration. it all depends upon industry standards and its protocol, some times recalibration is required twice a day once in the morning and then in 2nd half of the day. Pls. remember to use ISA in all standards & samples.

8) Spike Recovery Test : Before going to sample analysis, one must ensure that the electrode after checking its slope is working fine or not, one can use any of known standards as sample and check its concentration.

9) Storage of ISEs: It depends upon frequency of its use. If one needs to use the same ISE more often then one can store electrode under short term or for overnight point of view. One must rinse the electrode and keep in mid range standard, close the refilling hole and leave it overnight. For long term( means chemists does not need to repeat the analysis for week or more then he can plug the filling hole, remove module or liquid membrane from the electrode, place it in dry vials with wet sponge of mid standard) and cap the vial. For solid state, clean the surface of crystal with DI water, dry and plug with cap & store back in the electrode box.

10) Stirring: Very impotant that all time while working with ISEs whether in standard solution ( at the time of calibration and its slope check) or samples analysis, gentle stirring is very important. Kindly ensure that stirrer is used without hotplate facility in it.

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