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HISTORY 2 Middle of the 60 s First ISE for fluoride Na+ K+ Ag+ Ca2+ NH + 4 Cu2+ Pb2+ F- Cl- Br- I- CN- S2- NO3- Until the 60 s the only ISE is in fact the ph electrode with a glass sensitive to protons. The fluoride electrode appeared in the middle 60 s. Since then a number of ISE have been developed for cations and anions. 2
WHERE? 3 Environment Food industry and agriculture Medecine, pharmaceutical, cosmetics Power plants Ion measurements can be implemented in a number of applications : for instance nitrate in waster water and brackish water, Chloride and fluoride in mineral waters, potassium in fruit juices, chloride in beer, Chloride in infusion solutions, in soap and shampoo Chloride traces (high sensitivity chloride electrode) for boiler water ( corrosion protection) 3
APPLICATIONS 4 Fluoride Calcium Nitrates Copper water, plants, minerals, drugs water, milk, cheese, beer, wine, serum, blood ground, surface and drinking waters plants, soils, foodstuff plating bathes complex. titration of copper, nickel, cadmium 4
WHY? 5 Economical Time saving Instrumentation simple Automation (sample changer) The measuring chain is very economical compared to other analytical techniques like ion chromatography, AA or ICP... The setup for measurements is fast : programmation of instrument, calibration of electrode and measurements depending on response and stabilization time of the electrode ( depends on electrode type, sample matrix, concentration level and required level of accuracy). The most time consuming part is the preparation of standard solutions, electrolyte and if necessary preparation of sample (wet mineralization etc.) The measuring chain consists of an ionmeter (originally a high accuracy millivoltmeter), an ISE, a reference electrode, an ISB (ion strength buffer) and standard solutions for electrode calibrations. For routine analysis, a sample changer can be connected to the ionmeter. 5
WHY? 6 Wide dynamic range from 10 0 to 10-7 M Measurement not disturbed by colour or turbidity, no interference from solid particles According to the type of electrode and to sample matrix, the detection limit is at 10-6M for most models (high sensitivity chloride electrode) or even at 10-7 M like the fluoride electrode. To the contrary of colorimetric or spectrophotometric techniques for elemental analysis, the measurement is undisturbed by colour, turbidity or particle in suspension in the sample. 6
LIMITATIONS 7 Free ions only Interferences from other ions present in the sample When ions are bound, it is necessary to free them before hand by an adequate handling such as for instance wet mineralisation Other ions present in the sample may interfere with the measurement : either they also respond to the same electrode like for instance iodide, bromide to the chloride electrode or they will bind or precipitate the ion to be measured, like silver ions with chloride ions. The selectivity degree of the electrode is expressed by the selectivity constant K and the activity of the interfering ion. 7
THEORY 8 Based on Nernst equation modified by: Sensitivity factor Selectivity factor E RT S = E 0 + * log nf 100 ( a ) x + Σkxia n ni The response of ISE is nearly Nerstian. R/F can be replaced by 0.198 called Nernst factor. The factor 0.198T/n will be ideally 59 mv for a monovalent ion, this represents 100% sensitivity. The Nernst equation can be modified by the sensitivity of the electrode S/100%. Second the selectivity of an electrode is never 100%, other ion may interfere and the degree of selectivity to the ion to be measured x versus the interfering ion x1 is expressed by the selectivity constant Kxi. The selectivity factor has to be included for all interfering ions. When Kxi = 2.10-2 for instance it means that the electrode is 50 times more sensitive to x than to 1 (1/2. 1/10-2 ). When the value of Kxi is higher than one for a given ion it means that the electrode is more sensitive to the interfering ion than to the ion to be measured. 8
THEORY 9 Parameters of the equation E = electrode potential E0 = standard electrode potential (temperature dependant) T = absolute temperature F = Faraday s constant R= gas constant n= valence of ion x ax= activity of ion x S/100% = sensitivity of electrode Kxi = selectivity constant of primary ion x to interfering ion i 9
WHAT IS NEEDED? 10 Ionmeter Temperature sensor, ISE (with filling solution) reference electrode Standard (addition) solution Ion Strength Adjuster (ISA) Ionmeter ISE with filling solution if it is a ion exchange type electrode Reference electrode preferably with double bridge of type REF251 of with salt bridge extension Temperature sensor : like for ph the electrode behaviour is Nernstian and therefore temperature has to be taken into account Standard solution or addition solution (known concentration) to calibrate the electrode Ionic strength is an important parameter in ion mesurements,it expresses the concentration of all ions present in the solution both as concerns molarity and charge. Ion Strength Adjuster (also called electrolyte ) is needed in order to give the sample and the standard solutions a high and constant ionic strength. It does not interfere on the measurement but compensatse for the difference between activity and concentration. A Total Ion Strength Adjuster Buffer (TISAB) like for the fluoride electrode allows at the same time to adjust the ph range. 10
TYPICAL PROPERTIES 11 Concentration range ph range Response time Life time The detection limit is different according to type of ion. This is determined by the solubility constant of the crystal (or crystal powder ) used for the sensing membrane. The less soluble the membrane, the lower the detection limit. Let us take the example of chloride : at some point the Chlorides coming from the membrane will be measured, this is the background noise or detection limit of the electrode. The concentration range is an essential spec of the electrode For some electrodes the ph range is essential as hydroxide ions or protons can interfere : for instance OH- interfere on the fluoride crystal and H+ on the measurement of sodium, as the sodium ion selective electrode is based on a modified ph glass. The ph range is given in the specs of the electrode. The response time of the electrode depends on its nature and design, and on the concentration of the ion to be measured. At high concentration the response will be faster than at low concentrations. An approximate response time is given in the users manuals. The life time depends on the working principle of the electrode and on its use: for instance the Ag(CN) - 2 building up when working with cyanide electrode is very soluble and therefore the life time of the electrode is limited. Organic ion exchange membrane electrodes also have a shorter life time than solid electrodes. 11
TYPICAL PROPERTIES 12 Membrane resistance (high) Sensitivity deviation from the 100% (59 mv/decade) Check point : potential for a 10-2 M standard solution The membrane resistance is normally high and therefore the electrode is connected to the high impedance input of the meter. The sensitivity is a priority criterion. The check point is only an approximate indication valid only in the same conditions strictly as the ones of the checking bulletin, in particular using a difference reference electrode may radically modify its value. The check point is an absolute value, whereas the sensitivity is a relative one. Both the sensitivity and the check point can drift with time, therefore regular calibration is necessary. The checking bulletin of a new electrode is always enclosed in the package showing the value of the sensitivity and the check point. 12
CHECKING BULLETIN 13 sensitivity Check point All ISE of Radiometer Analytical are supplied with their checking bulletin showing the calibration curve and the respective potential values for each of the standard used. The procedure used is a measurement in the electrolyte first, after stabilization a 100 µl addition of a concentrated standard solution is made in the electrolyte so as to obtain the lowest concentration i.e. mostly 10-6 M, then successive additions, eventually of more concentrated solutions, are made in the same beaker so as to cover the range of concentration of the electrode. 13
VARIOUS TYPES 14 Glass Solid state Organic membrane (ion exchange) Single or combined Glass electrode is the sodium (glass is of ph type modified) with a strong interference from Potassium Solid state: chloride, fluoride, silver/sulfide, lead, copper, bromide, iodide, cyanide Ion exchange membrane : potassium, nitrate, ammonium, calcium Combined : fluoride 14
H+ and Na+ 15 Hydrated layer around the glass membrane Surface acts as a cation exchanger H+ (ph ) electrode unique 14 decades of concentration Na+ electrode modified ph glass Strong interference from H+ Short response time The hydrated layer is very narrow (order of 10-6 cm), it forms spontaneously when electrode is left in a solution. For H+, the sodium or lithium ions in the glass membrane are exchanged with the protons. The sodium electrode is just a modified ph glass. It is still very sensitive to H+ ions, but this is overcome by adjusting the ph to alkaline values. 15
H+ and Na+ 16 Inner reference element Ag/AgCl wire Inner solution Ion sensitive glass membrane The glass for the sodium electrode is modified compared to the glass used for ph. 16
SOLID STATE F -- LaF3 Cl - AgCl Ag +/ S -- Ag 2 S 17 Solid state Electrodes CN -, -, I -- AgI Pb Pb ++ ++ PbS Br Br -- AgBr Cu ++ ++ Cu 1.8 Se 1.8 Se 17
SOLID STATE 18 Ionically conducting membrane made of one or more inorganic salts Monocrystal (LaF3) or polycrystal (Ag 2 S) Inorganic salt solubility determines detection limit 18
SOLID STATE 19 F- electrode LaF 3 monocrystal CN- electrode AgI polycrystal F- CN- Ag+ F- TISAB ph adjuster and Ag(CN) - 2 soluble, limited life decomplexer of AlF 6 and MgF 4 in high CN- concentrations The working principle of the ISE is different according to reaction type. On the most common fluoride electrode the equilibrium (charge carrier) is F- according to the reaction : LaF 3 La 3+ + 3 F - TISAB : total ion strength adjuster buffer. 3 Functions: adjusts the ph, decomplex the bound F- ions and adjust the ion strength so as to make sure that measurements in samples can be referred to calibration conditions. For the cyanide electrode for instance the equilibrium is based on the CNexchanged with Ag+ (from the AgI polycrystal) following the reaction : AgI + 2CN - Ag(CN) - 2 + I - 19
SOLID CONTACT 20 Cyanide, iodide, copper Black epoxy tube Conductive rod Conductive pads Sensing polycrystal 20
LIQUID CONTACT 21 Chloride, Bromide, Fluoride Silver, Lead Black epoxy tube Conductive wire Filling solution Insulating material Sensing element 21
ORGANIC ION EXCHANGER 22 Ammonium, Potassium, Calcium, nitrate Potassium Calcium Nitrate valinomycin antibiotics organic calcium phosphate organic ammonium nitrate Ammonium antibiotic The ion exchange element is incorporated in a PVC membrane. The potassium and ammonium ion exchanger have structure with a cavity where the size of the respective ion fits perfectly. 22
ORGANIC ION EXCHANGER 23 Ammonium, Potassium, Calcium, nitrate Ion exchanger dissolves in water Life time shorter than crystaline electrodes Membrane tube is removable Spare membranes are supplied 23
COMBINED FLUORIDE ELECTRODE 24 Filling aperture for external reference AgAg/Cl wire external reference KCl 3M AgCl solution for ext. ref. AgAg/Cl wire internal reference Internal filling solution ( not refillable) Porous junction of external reference LaF3 Monocrystal The ion exchange element is incorporated in a PVC membrane. The potassium and ammonium ion exchanger have structure with a cavity where the size of the respective ion fits perfectly. 24
REFERENCE ELECTRODE 25 Constant potential No interference with ion in solution, no Hg/HgCl2 or Ag/AgCl with: Sodium Chloride K+ of KCl interferes Cl- of KCl contributes to measurement salt bridge or double junction reference electrode filled with relevant ISA Calomel electrode is very stable but goes only up to 70 C and shows hysteresis when temperature changes, in turn the Ag/AgCl electrode goes up to 100 C fast response to temperature variations, but slightly more instable and sensitive to light. However in some cases, elements from the 2 electrodes may interfere on the measurement : for instance for sodium measurement the potassium from the KCl filling solution will act as an interfering ion, and when measuring chloride, the chloride from the KCl will increase the result and bring an error. Therefore reference electrode with double junction of type REF251 or the use of a removable salt bridge is preferred. The advantage of using a salt bridge is that when measuring different ions, it is easier to fill it with the relevant electrolyte. However in order to avoid contaminations, it is recommended to use one dedicated salt bridge or double junction reference for each different ion. The solution filled in the second junction of in the salt bridge is the electrolyte (Ion Strenght Adjuster) in order to minimize the junction potential. 25
MEASURING METHODS 26 Direct potentiometry (calibration curve) Standard addition Standard subtraction Analate addition Analate subtraction 26
SELECTION OF THE METHOD 27 Sample matrix Concentration level Accuracy requested Time Convenience 27
PROCEDURE 28 Use of 2 or more standards Bracketing procedure Use of Ionic Strength Adjuster ph adjuster and/or decomplexing agent Temperature identical in standards and samples. The sample and standard solutions must be similar in composition in order to avoid matrix effects. This is the reason why ISA is added to both standard solutions and sample. The TISAB used for the fluoride measurements is alltogether an ISA ( NaCl), ph adjuster between 5 and 6 and a decomplexing agent DCTA which builds complexes with Aluminium, iron and magnesium to release fluoride ions. 28
DIRECT POTENTIOMETRY 29 E = E 0 + S 25T / T 25 log( C + Bl) E = measured potential E 0 = standard electrode potential (mv determined by calibration) S25 = sensitivity at 25 C (mv/pc) T = calibration temperature T25 = 298.16 K C = concentration of standard Bl= blank detection limit of electrode Csmp = C meas - Bl The concentration of the sample is the total concentration measured less the value of the Blank, this is specially significant for very low concentrations ( non linear part of the electrode response range). 29
CALIBRATION CURVE E (mv) E 90 30 Standard potential dc/std C 2 C 1 S T 60 30-0 E 1 E 2 Deviation from fitted curve for each point -30-90 -120 log[c] 10-7 10-6 10-5 Bl 10-4 10-3 10-2 10-1 -150-180 -210 1 Blank = experimental detection limit of the electrode The blank is determined during the calibration. It represents the detection limit of the electrode. It is the intersection of the extrapolated linear response part and horizontal part of the electrode. It allows to have results in the non linear response area of the electrode which have the same accuracy as in the linear part. 30
ACCURACY OF DIRECT POTENTIOMETRY 31 Typically ± 2-4% for monovalent ions, ±4-8 % for divalent ions Accuracy of the meter Precision of the electrode Accuracy on sensitivity (standard solutions) Accuracy on temperature control Sample pretreatment (all ions free or not) 31
STANDARD ADDITION PRINCIPLE 32 ISE = F - Sample = F - Standard addition = F - Sample Addition 1 Addition 2 9 In the standard addition method, the potential of the sample is measured first, then several additions of identical volume of a concentrated standard solution are added and each time the potential of sample + addition recorded. The addition volume must be small with regard to the sample volume so that the total ionic strength remains constant, the degree of complexation of the specific ion remains constant, the liquid junction potential of the reference electrode remains constant. The electrode sensitivity is determined by 2or more additions. If only one addition is made, the slope/sensitivity of the electrode must be known. 32
STANDARD ADDITION WHEN? 33 If sample matrix difficult to reproduce for standards Best accuracy if total amount of ion added is 0.5 to 2 total amount of ions in sample Valid only in linear response range Potential variation due to addition : 10-30 mv for a monovalent ion 5-15 mv for a divalent ion 33
STANDARD SUBTRACTION PRINCIPLE 34 ISE = I - Sample = I - Standard addition = Ag+ - Sample Addition 1 Addition 2 9 The potential measurement is taken first in the sample. A reagent which combines with the specific ion is added ( here silver to iodide), i.e. the concentration of the specification in the sample will decrease at each subtraction. Calculation of sample concentration is same as for addition with a negative sign. 34
STANDARD SUBTRACTION WHEN? 35 When standard of the ion is instable (I - or S 2- ) Addition volume and concentration adjusted so as not to react with all the amount of ion in the sample 35
STANDARD ADDITION/SUBTRACTION C std std smp = ( E1 E ) smp std ( V + V ± C )10 V / S V smp 36 For one addition/subtraction Csmp sample concentration Cstd standard concentration Vsmp sample volume Vstd standard volume E potential before 1st addition/subtraction E1 potential after addition/subtraction S sensitivity 36
ANALATE ADDITION PRINCIPLE 37 ISE = F - Standard = F - Analate (sample) = F- Sample Analate add. 1 Analate add. 2 9 First a potential measurement is taken in the standard solution. The, sample aliquots (same volume, small volumes) are added to the standard and potential taken after each measurements The sample concentration is calculated after the procedure has been completed. 37
ANALATE ADDITION WHEN? 38 High ion concentration in sample, avoids dilution errors When sample volume available is small 38
ANALATE SUBTRACTION PRINCIPLE 39 ISE = Pb2+ Complexing Standard = Pb(NO 3 ) 2 Analate (sample) = SO 4 2- Sample Analate add. 1 Analate add. 2 9 First a potential measurement is taken in the standard solution, which is in this case a complexing reagent of known concentration like lead nitrate (in solution), the electrode is selective to lead. Additions of the sample (here sulfate) are made. The reaction is the formation of lead sulphate which precipitates, i.e. the lead detected by the electrode is decreasing (difference between the solubility product of lead nitrate and lead sulphate). This is a way to measure the sulphate in a sample by direct potentiometry with a lead electrode, as no sulphate electrode is available. 39
ANALATE SUBTRACTION WHEN? 40 Indirect method, no ISE for the ion to determine High concentration in samples, avoids dilution CARE : the sample aliquots have to be well adjusted to avoid reacting with all reagent upon first addition. 40
ANALATE ADDITION/SUBTRACTION 41 C smp = ± C std ( V std + V smp )10 V smp ( E1 E ) / S C std V std For one addition/subtraction Csmp sample concentration Cstd standard concentration Vsmp sample volume Vstd standard volume E potential before 1st addition/subtraction E1 potential after addition/subtraction S sensitivity 41
BENEFITS OF ADDITIONS/ANALATES 42 When matrix is difficult to reproduce From 2 addition or analates: S calculated automatically: No need to calibrate Permanent check of electrode response Avoids dilution errors No temperature variation between measurements: volumes added are very small. 42
LIMITS OF ADDITIONS/ANALATES 43 Only in electrode linear response range Time consuming : procedure repeated at each measurement Ion concentration in sample must be approx. known to adjust volume and concentration s of additions/analates Total volume added must be kept small to keep ionic strength/ph constant. 43