Question: Are you having an issue with your lab Ion selective electrodes(ISE) in obtaining accurate & repeatable results?
Answers : Use of ISE is very commonly used in labs or in fields along with ion meters because those are very easy to use however one must take care these following tips to ensure that he/she is getting accurate and reproducible results in their water or water based samples.
Before we provide details on those tips, let's understand that there are three types of Ion Selective Electrodes are made and used depending upon its construction.
aa) Solid State Ion selective Electrodes: Examples F, Cu, Cl, Br, I, Ag/S, CN, Na etc.
Such electrodes are made with epoxy body , sensing crystal is being installed at one of the ISE
bb) Liquid membrane Ion selective Electrodes: Example, NO3, NO2, CLO4, K, Surfactants etc.
These electrodes are supplied with detachable liquid membrane in the form of module.
cc) Gas sensing Electrodes: Example, CO2, DO, NH3 etc.
These ISES are supplied with gas permeable membranes.
Tip #1 : Use ISE with a meter in its ISE mode:
There are several companies who are making ion meters which have software to provide results of ion directly in concentration mode like ppm, mg/l, ppb. With use of such modern technology, one does not have to plot graph on semi log paper (unlike earlier time when such ion meters were not available by using various standards in their lab. Cumbersome process, manual calculation, possible human error in doing so), one gets direct results on its display in specific unit being selected by chemist in his/her lab.
Tip #2, Use of proper Reference electrode/fill solution:
Now most of the manufacturers are providing ISEs with built in reference electrode(2 in 1) unlike previous time when companies were manufacturing sensor and reference electrodes separately. One should follow instructions provided in the electrode manual(supplied with ISEs) and use that specific reference electrode fill solution only. A lot of work has been done by such companies and they came up with specific reference electrode fill solution to be used for that ISE. One of the main reason is that such fill solution will neither interfere nor react with sample being analyzed.
Tip #3 Prepare Standards carefully:
One must prepare minimum two standards and one can go max up to 7 standards( for wider sample coverage) to check the accuracy of electrode in use before doing sample analysis. One must keep in mind that standards that are being used does cover or bracket sample concentration. Example if your sample is around 5 ppm, then advisable standards should be 1 ppm and 10 ppm. (Pls. note that standards are made with ten fold concentration to follow Nernst equation/principal of operation E=E0+RT/nF log C). Standards should be prepare fresh , be extra careful in serial dilution(use pipette and measuring flask only) alternatively one can buy standards of various concentration from the supplier.
Tip #4: Calibrate and measure at same temp:
Temperature plays a very important role in accuracy and repeatability of results. Ensure that standards(during calibration) and samples(during its analysis) are at same temp. If one looks at above Nernst equation, it explains how temp plays an important role. Calibration with standards should be used in increasing order. Example if you are calibrating with 1 and 10 ppm standards, we should take 1 ppm standard first followed with 10 ppm standard and so on. In between rinse and dry ISEs with soft tissue paper. Pls. do not use any rough blotting paper because it can cause damage to smooth surface of sensing part of ISEs. One must calibrate his/her ISEs at least once a day.
Tip #5 : Use Ionic Strength Adjusting Buffer(ISA)
Companies which are manufacturing ISEs have done extensive works and came out with specific solutions which they termed it as ISA. The purpose of using those ISAs is to ensure samples and standards are having same sample background with its ionic strength and it also masks the influence of interfering ions, allowing ion of interest to be more accurately measured. Therefore use of ISA is must. For more details one must refer to instruction manual being supplied with ISEs. The ISE manual also specifies ratio of use of ISA with volume of sample being taken for its analysis.
Tip #6 Slope check
This paragraph will explain what is slope? with use of minimum two standards of ten fold concentration, the instrument will display change of mv value from standard one and standard two being used. In ideal conditions univalent ions(cation or anion) the difference should be 56-60 mv and in case of divalent ions the mv difference should be 27 to 30 mv. As long as chemist gets this value, he is good to go with the use of that ion selective electrode. Pls. refer to the graph below which is self explanatory.
Tip #7 Spike Recovery:
This is suggested when customer is not satisfied with his sample results. Spike recovery is to use one of the standard as sample. In this exercise, the customer should prepare fresh standards, check the slope and then test of the same standard as sample immediately. If customer is getting accurate value of standard in sample mode, this means that the electrode is working fine. The chemist then needs to do some checks at his samples.
Tips #8 Use of Stirrer:
Very important that one must use gentle stirring of standards and sample during calibration and sample analysis. Pls. ensure that stirrer is without heating in process because sometimes stirrer comes with hot plate and by accident the chemist puts on stirrer along with temp mode of stirrer. Pls. remember our tip # 4 as above temp plays a very important role in analysis.
Tip # 9 Proper storage of ISEs
The life of electrode is greatly dependent upon how one stores these electrodes while not in use during overnight or week end or even for longer period of time. During overnight or week end, rinse the electrode with DI water, place it in mid range standard solution. Close the refilling opening part of reference cell. For long term storage, refill the electrode with fill solution, close the refill hole. If the electrode is liquid sensing type, remove the sensor module, place in dry glass vials ( supplied by the company) and reinstall the blank module on the body of ISE. If the electrode is with solid state crystal, then simply clean it with DI water, dry with soft tissue, install rubber cap( supplied with ISE) and put it back in the original electrode box.
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