Introduction to Ion-Selective Measurement
Introduction to Ion-Selective Measurement
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Date Created: 01/20/15
H 0 Your formula for water analysis Introduction to Ionselective Measurement Dr Axel W Bier HachLange Application Specialist Theory Ionselective electrodes ISE consist of an ionspecific halfcell and a reference halfcell The ionspecific cell gives a potential against the reference cell depending on the specific ion concentration When the specific ion concentration the sample or an ion standard changes the potential changes as well The relationship between the potential measured with the ISE and the ion concentration in the measured solution is expressed using the Nernst equation m EEo 2303R TlogCC0 E measured potential mV between the ion selective and the reference electrode E0 measured potential mV between the ion selective and the reference electrode at a C 1 concentration R Universal gas constant R 8314 J mol391 K391 T Temperature in K Kelvin with T K 27315 t C ift is the temperature of the measured solution 0 F Faraday constant 96485 C molquot n electrical charge of the ion C concentration of ion to be measured C0 detection limit Since R and F are constants they will not change The electrical charge of the ion to be measured is also known As the sample or standard temperature is a variable of the Nernst equation it is essential to continuously record and monitor the temperature while measuring Therefore the equation 1 can be simplified to 2 EEo S3910gCC0 RT where S 2303 39 is called the slope of the ISE 72F Example of ion n electrical charge of the ion S t 25 C Copper Cu2 2 2958 Sodium Na Potassium K 1 5916 Fluoride F39 Chloride Cl39 1 5916 Sulphide 8239 2 2958 As known from pH measurement and calibration the slope S is an indicator of ISE performance If the slope changes over time it may mean that it is necessary to implement a preventative maintenance cycle to clean the ionselective part This may mean refilling the specific inner solution replacing the membrane or in the worst case replacing the entire ISE because the electrode is too old Typically an ISE will last one to two years depending on usage In order to characterize ISE behavior it is necessary to prepare standard solutions of the specific ion in terms of logarithmic concentration values eg 001 01 10 10 100 1000 mgL standards When all standard solutions are measured the ISE characteristic can be displayed by plotting concentration vs potential However it may also be useful to either plot the concentrations on a logarithmic axis or easier to calculate the log C which using the above example is a simple exercise 2 1 0 1 2 3 Be RightT39quot Table 1 The absolute measured mV values of a sodium ISE for 7 Na standard solutions mgL 4565 0001 1564 001 1482 01 1100 1 518 10 64 100 645 1000 Figure 1 Example of sodium ISE calibration curve blue and detection limit red Example Sodium lSE libra on 1000 500 00 gt E 500 4000 4500 2000 0 2 1 0 1 2 3 43909 Cl The slope of the curve decreases with decreasing Na concentrations While the linear section 1 4mgL up to 1000 mgL shows a slope of 582 mVpC at 20 C the lower concentrations shown in the flat left part of the curve show a slope 02 mVpC at 0001 mgL of almost zero That means the ISE calibration curve has a nonlinear section of the curve When measuring low Na concentrations it must be considered that the differentiation of low Na values from other values is more and more difficult to detect A slope difference from one standard to another of less than 30 mVpC for a monovalent ion leads to less reliable and reproducible measurements Design of Ionselective Electrodes Solid state electrodes In this case the ion selective part is a solid substance in direct contact with the solution to be measured The main characteristic of a solid state electrode is that the sensing material is almost insoluble in water meaning a very small solubility product For instance the most commonly used solid crystal is Lanthanium fluoride LaF3 to detect Fluoride ions The equilibrium in aqueous solutions is LOIF 3 ltgt La3 The solid crystal is in contact with the aqueous solution forming a small number of La3 ions and F39 and producing a certain potential If there are P ions present in the sample solution then the equilibrium changes and gives a different potential 80 it is possible to detect specific F39 concentrations PVC membrane electrodes There are many different types of membrane electrodes Usually the PVC material is polymerized with an organic solvent including the specific organic substance to react on the ions being detected by changing the potential against a reference electrode Such membranes have to be maintained regularly and may have a shorter useful life than solid state lSEs depending on the usage A common example is Calcium Ca2 or Nitrate N031 Be Right Gassensing electrodes Although these lSEs are usually not categorized as ISE but as gasdetecting sensors there are some gases which can also measure ions in solution For instance ammonium can be measured by detecting the ammonia in solution since there is an equilibrium NH3H20 lt2 ZWEM OH It is a fact that ammonium dominates in acidic solutions and ammonia gas dominates in alkaline solutions Adjusting the pH of a solution allows detection of the specific form ammonium or ammonia Preparation of Sodium Standard Solutions Weigh exactly 2542 g NaCl Transfer it into a 1000 mL glass flask and fill with Deionized DI water up to the mark solution A 1000 mgL Nat Pipette 10 mL of solution A into a 100 mL glass flask Fill with DI water up to the mark solution B 100 mgL Na and shake to mix the solutions Pipette 10 mL of solution B into a 100 mL glass flask Fill with DI water up to the mark solution C 10 mgL Nat Pipette 10 mL of solution G into a 100 mL glass flask Fill with DI water up to the mark solution D 1 mgL Na Pipette 10 mL of solution D into a 100 mL glass flask Fill with DI water up to the mark solution E 01 mgL Na Calibration 2points Out of such a calibration with two standards the slope is calculated for a specific concentration potential area As long as the ion concentration of the sample lies within this concentration range a 2 point calibration is sufficient Figure 2 Linear curve of two standard solutions C1 and C2 and their resulting potential measured as E and E2 2poinlt calibration Concentration log C However the difference of the concentrations of the standards used should be at least a factor of 10 For instance if the sample is expected to have 3050 mgL sodium then a calibration with two standards of 10 mgL and 100 mgL is sufficient Because of the temperature effect on the Nernst equation a 2 point calibration and a sample measurement must be done close to the same temperature A maximum deviation of 1 C is acceptable although it affects the accuracy of the final result Be RightT39quot Calibration Multipoints Lastly the multipoint calibration is used to determine where the ISE leaves the linear region of the curve and where the detection of low concentrations is limited Figure 3 Nonlinear curve of 7 standard solutions Ci and the resulting potential measured as E Example Sodium ISE Iibration curve 10010 q II E a a O F 00 gt M E 500 1 000 51500 2000 3 2 1 0 1 2 3 409 C Especially for low concentrations the use of several standards is required to cover the full nonlinear range By using the nonlinear calibration curve technique the reliability and accuracy of the result is much higher especially if samples are examined with a broad range of sodium ions Detection Limit After recording the calibration curve extraction of the detection limit of that ISE is recommended This can be done graphically by adding one tangent red line to the upper linear part of the curve and one tangent to the lower linear part almost parallel to the Xaxis The detection limit of the ISE is the point at which both tangents cross However this detection limit applies to a concentration of a specific ion detected with a specific ISE It depends on the characteristic of the sample and on the behavior of the ISE The older the ISE the less reliable the measurements of low concentrations will be In addition the temperature plays an important role ISEs in samples at cold hot or room temperature have different slopes Measurement When performing ISE measurements for the first time with a new electrode it is normal for the stabilization time to be longer and the signal may be noisier than the user is accustomed to from previous measurements If this is the case the new ISE electrode should be conditioned in diluted standards for several hours before a fast and stable reading may be achieved Always use a stirrer to ensure consistent mixing of the sample solution and of the standards This also helps the mass transportation of ions to the ionselective surface of the electrode Especially for low concentrations stirring may shorten the stabilization time When using a magnetic stir plate it may be necessary to insulate the sample beaker from the plate to isolate the heat generated from the stirrer As mentioned above changes in temperature can lead to result variations The easiest way to insulate the beaker from the stirrer is to place a piece of cardboard or thin Styrofoam between the beaker and the stir plate Be RightT39quot Selectivity of the Specificion Measurement In the ISE introduction section of this paper an assumption was made that an ISE electrode has a sensitive part specific to just one ion to be detected among other ions in the solution This is not the case for most ISEs Since the ionselective part is sensitive to other ions which are similar in ion radius charge and mobility this sensitivity must be taken into account in the measurement of samples It is possible that an excess of an interfering ion can lead to higher or lower concentrations of the measured ion The sodium ISE electrode is selective for Na but also for potassium K and lithium Li The selectivity constant for K is 0001 and for Li is 001 What does that mean A small selectivity constant of 0001 for example means that the interfering ion is adding 11000 of the potential measured in the solution compared to the main ion Na With sodium potassium in the same solution can cause a 11000 potential value like that of sodium if both ions are present in the same concentration If potassium is 1000 times more highly concentrated than sodium then the potential measured is almost 5050 resulting from both ions Na and W This of course leads to false results where Na is assumed to be of a higher concentration than what is truly there In order to calculate the influence of an interfering ion against the final potential an extended Nernst equation can be used Nikolsky developed a specific equation where all interfering ions can be considered 3 Kis39CIjnS m 39 F a 0 hi electrical charge of the ion to be measured nS electrical charge of the interfering ion ai activity of ion to be measured aj activity of interfering ion Kis selectivity constant ion to be measured interfering ion Ionic Strength All ions in a solution add to the total ionic strength Ions are the only species to transport charge through the solution If there are sufficient ions present the ion transportation to the ionselective surface is continuous In the case of low ionic strength samples like drinking water boiler water etc the charge transportation through the solution is covered by only a few ions and the transportation of the measured ions is rare and random 4 I05 EC Z 2 l ionic strength Ci concentration of the ion i Zi charge of the ion i To avoid such a random measurement where diffusion potentials also create transport barriers an Ionic Strength Adjustment ISA helps to reach sufficient ions in the solution without interfering with the searchedfor ion In the case of the Hach lntelliCALTM Sodium ISE ISENA381 the ISA is an ammonium salt like NH4CI Therefore it is necessary to add an ISA to all standards and samples to create a minimum ionic strength in these solutions In addition some lSAs also contain a pHbuffer to create the optimal pH environment for the specific ion to be measured Be RightT39quot Measurement Mode The most common ISE measuring technique is Direct Measurement Therefore a calibration curve is necessary based on 2 or multi points Once the calibration has been performed and saved the sample can be measured directly and the ion concentration can be calculated by the meter The Incremental Measurement mode must be used instead of Direct Measurement whenever the matrix of a sample changes the analytical sensitivity of the method In other words the slope of the working curve for standards made with distilled or DI water is different from the same working curve made up in a matrix that is close to the sample Here several methods can be used such as 0 Standard addition 0 Double standard addition 0 Standard subtraction 0 Sample addition 0 Sample subtraction Standard Addition A commonly used method the ISE is placed in the sample and the potential is recorded Then a standard solution is added and again the potential is recorded After entering the sample volume standard volume standard concentration into the ion meter it calculates the sample concentration This can also easily be done by spread sheet software such as Microsoft Excel by using the following equation 5 VS Cs Vp Cp E E Vs 2 1 1 10 S 1 Cp concentration of the sample p GS concentration of the standard Vp volume of the sample Vs volume of the standard E1 potential of the sample before addition E2 potential of the sample after standard addition S slope at the sample and standard temperature The optimal addition is to double the sample concentration or a ratio sample standard addition of maximum 110 is possible Therefore it is recommended to work with highly concentrated standard solutions where the standard volume added is rather small compared to the sample volume Sample Addition This method is as easy to perform as the Standard Addition method First take a standard solution and measure the potential Then add a defined volume of your sample and measure the potential again Add the sample volume so that the standard concentration is increased by 2 to 10 times Then the ion meter can calculate the searched concentration Again there is a formula to calculate manually V g 6 Cs1p10 S 1 s Cp V Cp concentration of the sample VS GS concentration of the standard Vp volume of the sample VS volume of the standard E1 potential of the standard before addition E2 potential of the standard after sample addition S slope at the sample and standard temperature Be RightT39quot Maintenance Since the lntelliCAL Sodium ISE has a glasssensing element there is low risk of contamination inside the layer by other ions However the H and Li ions are small enough to enter the glass material and to exchange with Na Therefore when not in use keep the Na ISE in the special storage solution Prior to first use the lntelliCAL Sodium ISE must be conditioned in a 01 molL Na standard for several hours refer to the manufacturer s recommendations in the User Manual The lntelliCAL Sodium ISE arrives delivered in such specialized standard solution Treat the Sodium ISE glass bulb similarly to a pH probe including cleaning Any abrasive polishing or scratches must be avoided because this irreversibly damages the sensing surface For oilfat deposits on the glass bulb soak in a solution with mild detergent or the Hach cleaning solution Afterwards rinse the probe with DI water and recondition Use only soft tissues to wipe the probe or sensing element and never store the ISE probe dry or directly in DI water This will make it necessary to condition before measurement or may even damage the probe Call 8002274224 or visit wwwhachcomSmartProbes Lit No 2071 TM J9 Printed in usA Be Right Hach Company 2009 All rights reserved Trademarks are the property of their respective owners
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