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B C D E F G H I J K L M N O P Q R S T U V W X Y Z A measure of the closeness of a result to the true value (cf. precision).Activity : The effective amount of a free ion in a solution (cf. concentration).The chemical effectiveness of an ion is governed by the amount and type of other ions in the solution - i.e. it is dependent on the Ionic Strength of the solution. Varying the composition of a solution makes a fixed concentration of a given ion more or less active. In dilute solutions the ions are relatively far apart and free to move so that the activity and concentration are virtually identical. In more concentrated solutions, and in complex solutions containing other ions, however, the ions are more closely packed and their movement is impeded by inter-ionic interactions. Thus activity may differ from concentration by as much as a factor of five in highly concentrated solutions. It is the ionic activity rather than the concentration which determines both the rate and extent of a chemical reaction. In the case of ISE measurements, it determines the proportion of ions passing through the ion-selective membrane and thus the magnitude of the voltage developed by the electrode. The activity of an ion in solution for any known concentration may be calculated from the formula: Activity coefficient : A factor (f) which relates the activity (a) to the concentration (c) of an ion in solution : f = a/c, where f is always less than 1 The activity coefficient is dependent on the Ionic Strength of the solution. It becomes progressively lower as the ionic strength increases, due to inter-ionic interactions. See Table of Activity Coefficients in our Beginners Guide to Ion Selective Electrode Measurements.
The activity coefficient for any ion in solution can be calculated using the Debye-Huckel equation (as modified by Davies):
Note that it is generally accepted that this formula is only accurate up to about I = 0.1 Molar. At higher ionic strength other factors come into play which make the calculation of activity coefficients virtually impossible. Furthermore it requires a knowledge of the type and concentration of all components in the solution and is thus impossible for many natural samples. Anion : A negatively charged ion. (e.g.: Cl-, S2-, NO3-, CO32-)ATC = Automatic Temperature Compensation : Automatic adjustment of the displayed pH activity or concentration to correct for variations in the electrode slope due to temperature changes during analysis.Average number : The number of instantaneous readings of the electrode potential, taken at one second intervals, used to calculate the average value for the millivolts. An operator-selectable variable in ELIT Ion Analyser computer interface software which helps to reduce noise and increase the precision of ISE measurements.Bridge solution : - see Salt bridge
Buffer solution : BS-1 Buffer :(pH=3.8) Special buffer for measuring the nitrite ion in meat extracts. Consists of 3.26g sodium acetate and 10ml glacial acetic acid dissolved in 1000ml water. BS-2 Buffer :(pH=3.2) Special buffer for measuring the nitrite ion in natural water samples. Made by dissolving 14.32g Disodium Phosphate, and 15.37g Citric acid in 1000ml water. Top Calibration : The process of determining the response of a measuring system to a known amount of the measured component in order to permit the measurement of unknown samples. In ISE measurements, this is achieved by measuring the electrode response in a series of standard solutions and plotting a Calibration Graph of electrode potential (mV) versus activity or concentration in two or more standard solutions. This graph will have mV on a normal y-axis and concentration(activity), in ppm or Moles, on a logarithmic x-axis. Alternative versions use the logarithm of the concentration or pX on a normal x-axis. (see also Direct Potentiometry and Slope) In modern usage with personal computers and microprocessors this is normally done by computer graphics or calculation without graphical display and it is rare for a calibration curve to be plotted manually, except for the initial training of students. Cation : A positively charged ion. (e.g.: Na+, Ca2+, NH4+)Combination electrode : A combination of a sensing electrode and a reference electrode contained in one unit.Complexing agent : Any compound which combines with an ion in solution to form an undissociated molecule so that the resulting complex stays in solution and does not precipitate. The ion is nevertheless effectively removed from solution and cannot be detected by an ISE. Complexing agents are used as titrants and to remove interfering ions.Concentration : The total mass of an ion or molecule in a given volume of solution (c.f. activity). When measuring ionic concentration by ion-selective electrodes, it important to note that the concentration used in the calibration graphs and calculated for the samples is the concentration of the free ion in solution, not the concentration of the compound from which this ion is derived. These will only be the same if Molar concentration units are used and if the detected ion has the highest valency in the compound, or for compounds in which both ions are monovalent. For example, 1 Molar CaCl2 solution contains 1 mol/l of Ca+ but 2 mol/l of Cl-. A distinction must also be made between the concentration of the free unbound ions and the total concentration, which may include any ions bound to complexing agents and any atoms in undissociated molecules.Concentration units : Units of number, volume or weight of a substance dissolved in a given volume of solution. NB: not in a given volume of solvent!grams per litre (g/l). e.g., 10 grams of solid dissolved in solvent and then diluted to 100 ml of solution = 100 g/l (NB: 10 g of solid added to 100 ml of solvent would be slightly less concentrated!) parts per million (ppm). When used in the context of chemical concentration units this is strictly the weight of solute in a million weight units of solution. e.g. 10 micrograms (µg) of solute in 1 gram of solution or 10 milligrams (mg) in 1 kilogram (kg) = 10 ppm. However, since ISE work always uses aqueous solutions and the mass of 1 ml of water is very close to 1 g, it is common to see ppm used as an abbreviation for micrograms per millilitre (µg/ml) or milligrams per litre (mg/l). moles per litre (mol/l or Moles). The number of atomic weights of a monoatomic species or molecular weights of a compound species dissolved in one litre of solution. e.g., 3.55 g of the chloride ion (or 5.85 g of NaCl) in 100 ml = 1 M(olar) or 1 mol/l. Can also be expressed as microMoles (µmol/l) or milliMoles (mmol/l). Equivalents per litre (Eq/l). The number of equivalent weights of a species dissolved in 1 litre of solution. The equivalent weight is that weight of an element or compound which will react with 1g of hydrogen or 8 g of oxygen. It is the atomic weight divided by the valency. For monovalent ions it is the same as the molecular weight. Also expressed as milliequivalents per litre (mEq/l) volume% (%v/v). The percentage of the total volume contributed by the volume of a new component added to the solution. e.g. when adding ISAB to a test solution it is often described as adding 2%v/v. This should strictly mean adding 2 ml ISAB to 98 ml sample but in practice is usually interpreted as adding 2 ml ISAB to 100 ml sample. weight% (%w/w). The percentage of the total weight of a solution contributed by the weight of a dissolved species. Conductivity : A measure of the ability of a solution to conduct electricity. It is the reciprocal of resistivity, which relates the resistance of a conductor (in ohms) to its length and cross sectional area. Units of conductivity are Siemens per centimetre (S/cm). These units are generally quite large so that milliSiemens (mS) or microSiemens (µS) are more commonly used. Conductivity is measured with a conductivity cell. This contains two platinum electrodes of known area rigidly fixed at 1 cm apart. The electrolytic conductivity of the intervening solution is determined by passing an alternating current between the electrodes.Debye - Huckel equation : See Activity Coefficient.Decomplexing agent : A compound which is added to a solution to liberate any ions which may be bound up in complex molecules and permit them to be detected by ISE analysis.Detection Limits: The "Standard Detection Limit" is the concentration (or activity) of the measured ion at the point of intersection between the extrapolated linear segment of the calibration curve representing the normal slope of the electrode and a horizontal line representing the voltage when the concentration is so low that small changes in concentration do not produce any detectable change in the electrode response. The portion of the calibration curve between this point and the beginning of the truly linear section is known as the non-linear range of the electrode. Samples are still measurable within this range provided that several standards are used to define the changing slope of the curve accurately, or where two standards can narrowly span the range of the samples. However, the error in concentration (per millivolt error in measurement) will be progressively greater as the slope reduces.The "Absolute Detection Limit" is less clearly defined but includes a range of values below the Standard Limit where the mV readings are still statistically distinguishable from that for zero concentration. The errors in this range will be high but it is still possible to detect the presence of ions and possibly the order of magnitude of the concentration. Dilution : The effect of changing the concentration of a solution by the addition of more solvent. A dilution of 1:10 means the addition of 9 volumes of the solvent to 1 volume of the original solution. The resulting solution is one tenth as concentrated as the original.Direct potentiometry : This is the simplest method of making ion-selective electrode measurements. The electrodes are immersed in a test solution and the electrode potential is measured directly with a millivolt meter. The concentration is then related directly to this measurement by reading the answer from a calibration graph of log of concentration versus millivolts. (see Slope for details of calculation)Dissociation constant (K) : A number indicating the extent to which a molecule dissociates in solution to form free ions. For a simple two-component system (e.g.: CH3COONa <> CH3COO- + Na+) it is the product of the molar concentration of the two ions divided by the molar concentration of the undissociated molecule.K = ([CH3COO-] x [Na+]) / [CH3COONa] The smaller the value of K, the less dissociation is present. K varies with temperature, ionic strength, and the nature of the solvent. Divalent ion : A doubly charged ion. One in which the original inert atom or molecule has lost or gained two electrons. (e.g. Ca2+, SO42-)Drift : The slow change in the measured potential of an electrode pair. When the electrodes are first immersed in a new solution there is initially a relatively rapid change in the potential which gradually decreases as the ions in solution come into equilibrium with those passing through the ion-selective membrane. Normally after two or three minutes a stable condition is achieved. Thereafter, with prolonged immersion, there may be a second gradual drift due to the development of additional potentials at the various liquid junctions in the electrode system. For accurate measurements it is important to take a reading before these secondary processes become significant. An additional phenomenon which is also referred to as drift is the gradual change in reading if the same solution is remeasured several times, with the electrodes removed and washed (and/or immersed in a different solution) in between each reading. This may be due to a number of factors including hysteresis effects, the failure of the reference system to establish the same liquid junction potential at each immersion, and increasing hydration of the ISE membrane.Electrode head : A molded plastic assembly containing one or more sockets for the insertion of appropriate electrodes fitted with plug-in connectors. The electrode head carries the expensive screened cable and plug for connecting to the measuring instruments. Various models are available for use with one electrode (mono head) or an ISE / reference combination (dual head), or a combination of different ISEs and/or pH, redox, or dissolved oxygen electrodes used with a common reference electrode for multi-component analysis (multiple head).Electrolyte : A compound which dissociates into ions in aqueous solution or a solution which contains ions. Weak electrolytes are only partially dissociated (e.g. acetic acid; CH3COOH) whereas strong electrolytes are highly or completely dissociated (e.g. hydrochloric acid; HCL or sodium chloride; NaCl). Strong electrolytes are good conductors of electricity. Conductivity. is a measure of the strength of an electrolyte.Electrical conductor : Any part of the ISE measuring system which allows the passage of electrons through a solid medium. Specifically, the chloridised silver wire immersed in the internal filling solution.Electrolytic conductor : Any solution containing free ions which can carry an electric current. In ISE measurements this includes the electrode filling solutions and the external test solution.Equitransferent solution : An electrolytic solution containing positive and negative ions which have equal ionic mobilities (= speed of diffusion). These can therefore pass through the porous frit of a reference electrode with equal facility and thus inhibit the development of an unequal charge distribution. (See Liquid junction potential).Filling solution : An electrolytic solution inside a sensing or reference electrode which has to be replenished periodically. With modern solid-state technology and / or gel solutions in many electrodes, this is now generally only applicable to the outer filling solution of double junction reference electrodes. This forms a bridge between the internal reference element and the external sample and gradually leaks out through the porous frit during use. The composition of the filling solution is chosen to maximise the stability of the potentials developed at the external and internal liquid junctions. Ideally it should consist of ions which are equitransferent (i.e. both positive and negative ions will cross the boundary in equal numbers), it must not contain the ion being measured or any ion which might interfere with the measurement, and it should have an ionic strength at least ten times that of the sample to be measured.Hysteresis (electrode memory) : The occurrence of a different value in the potential difference after the concentration of the test solution has been changed and then restored to its original value. This systematic error is generally in the direction of the concentration of the previous solution.Incremental methods : Methods of analysis whereby, instead of relating sample measurements to a pre-determined calibration graph, the concentration of the unknown is determined by taking readings before and after mixing known amounts of sample and standard. Advantages of these methods are that they minimise any differences in temperature and ionic strength between the standard and sample and ensure that calibration and sample measurements are closely spaced in time and made under very similar conditions without removing the electrodes from solution; thus minimising any errors due to drift and hysteresis. See Standard Addition and Sample Addition.Interface : An electronic device to connect the output from an ISE / reference cell directly to a desk-top or lap-top computer without the need for an expensive ion meter. Interfaces are provided with sophisticated software which facilitates complex data processing, display and storage. Multiple interfaces can connect several electrode systems to one PC at the same time and permit continuous monitoring of batch processes or simultaneous multi-component analysis.Interference : The effect of any species, other than the ion being measured, who’s presence in the sample solution affects the measured potential difference of the cell. The most common cause of interference is due to the fact that most ion-selective membranes are not 100% selective for the detected ion. Many permit the passage of other ions to some extent. These extra ions generally increase the measured electrode potential and cause spuriously high concentration results. See Selectivity Coefficient Other interferences may be due to chemical reactions with the membrane material which may cause positive or negative anomalies, and reactions in the sample which may cause precipitation, complexing, oxidation or reduction of the ion being measured, and thus spuriously low results.Interfering ion : Any ion in solution, other than that being measured, that reacts with the ISE membrane to change the measured potential.Internal reference electrode (of an ISE) : That part of the electrode which is electrically connected to the screened cable which connects to the measuring instrument. Usually a chloridised silver wire immersed in the silver chloride / potassium chloride filling solution or gel. In the case of all solid state electrodes, the material of the reference electode is deposited directly on to the internal surface of the ion-selective membrane and the internal filling solution is not necessary.Internal filling solution (of an ISE) : An aqueous electolyte solution, which may be gelled, containing a fixed concentration of the ion to which the inner reference electrode is reversible; usually the chloride ion in the case of silver / silver chloride electrodes. This forms the electrolytic conductor which transfers the charge from the detected ion in the ion-selective membrane to the electrical conductor (silver wire) which conducts the charge to the measuring system. Not present in all-solid-state electrodes.Ion meter : A sophisticated pH/millivolt meter which contains a microprocessor which allows for the processing of ISE signals to permit electrode calibration and direct reading of sample concentrations.Ionic strength (I): The ionic strength of a solution is a measure of the total effect of all the ions, both positive and negative, present in the solution. It is calculated as half the sum of the concentration (c, in Moles) multiplied by the square of the valency (Z) for all the ions.The ionic strength determines the activity coefficient for each ion. ISE = Ion-selective electrode. An electrode which responds selectively to the ions of a particular species in solution.ISAB = Ionic strength adjustment buffer. A solution of high ionic strength which can be added to both sample and calibration solutions in equal proportions before measurement in order to minimise differences in ionic strength; - thus ensuring that the activity coefficient of the measured ion is the same in all solutions. Under these conditions the calibration graph can be constructed using concentration units rather than activity and the sample concentrations can be calculated directly from the calibration data. In some cases, pH buffers, complexing agents and other components may be included in ISABs in order to minimise the effects of variable pH and interfering ions.See activity coefficient for discussion of the use and effectiveness of ISAB. Ionophore : A complex organic molecule which has an affinity for a particular ionic species in solution. For ion-selective electrodes, these are impregnated into a plastic disc which forms a barrier between the test solution and the electrode. The detected ion is passed from one ionophore molecule to the next by diffusion under a concentration gradient and hence is selectively transferred across the membrane and causes a build up of electrical charge on the inside of the membrane.Isopotential point : The activity of a sample at which the electrode potential does not vary with temperature. For some types of electrode this point is within the range of the calibration graph but for most it lies below this range. The temperature effect on electrode potential increases with increasing divergence of the sample activity from the isopotential point.Leak rate : The rate at which the outer filling solution of the reference electrode enters the sample solution. Low or irregular leak rates may cause variations in the liquid junction potential, resulting in erroneous or unstable readings. This is not a significant factor in modern gel-filled electrodes.Linear range : That range of concentration (or activity) over which the measured potential difference does not deviate from that predicted by the slope of the electrode by more that ± 2 mV.Liquid junction potential : The potential formed at the interface between any two electrolyte solutions of different compositions. In ISE measurements, the most important liquid junction is that between the reference electrode filling solution and the sample solution. Ideally, this potential should be as low and as constant as possible, despite variations in the external solution. Reference electrode filling solutions are chosen to minimise this potential.Liquid junction potential error : An error arising from a breakdown in the assumption that the liquid junction potential remains constant on change of solution.Membrane : A continuous layer covering a structure or separating two electrolytic solutions. The membrane of an ISE is responsible for the potential response and the selectivity of the electrode.Monovalent ion : A singly charged ion. One in which the original inert atom or molecule has lost or gained one electron. (e.g. H+, NH4+, Cl-, NO3-)Multi-component analysis : Analysis using several sensors connected directly to a sophisticated computer interface and software to monitor, process, and display the output from different electrode/reference systems simultaneously. Can include simultaneous measurement of several different ions and/or pH, Redox, dissolved oxygen and temperature.Nernst equation : The fundamental equation which relates the electrode potential to the activity of the measured ions in the test solution.Where E = the total potential (in mV) developed between the sensing and reference electrodes. This is effectively the equation of a straight line: Nernstian response : An electrode is said to have a Nernstian response over a given concentration range if a plot of the potential difference (when measured against a reference electrode) versus the logarithm of the ionic activity of a given species in the test solution, is linear with a slope factor which is given by the Nernst equation.Nikolski equation : An extension of the Nernst equation which relates the electrode potential to the activity of all the contributing ions in the sample, including any interfering ions. It is the same as the Nernst equation but with Log (a) replaced byWhere Nikolski-Eisenman coefficient : Another name for Selectivity coefficient.Noise : Abrupt, random changes in the measured electrode potential; usually due to induced electrical charges.This may be caused by: 1) air bubbles at the membrane surface or in the internal solutions of the reference electrode; 2) poor connections in the ISE, reference electrode or measuring system; 3) sample solutions containing organic solvents which are poor conductors; 4) high electrical resistance in the measuring circuit, possibly due to moisture on cables or connectors; 5) high and variable electrostatic fields in the vicinity of the measuring apparatus. Open circuit : A failure of electrical contact in any part of the measuring circuit, including inside the electrodes and the measured solutions. This is characterised by rapid changes in displayed potential followed by an off-scale reading. Frequent large erratic changes in the measured potential indicate an intermittent open circuit.pH & pX: A convenient way of expressing the activity (concentration) of ions in solution. pH is the concentration of hydrogen ions (a measure of the degree of acidity or alkalinity of a solution) and pX represents any other ion. pH is calculated as the negative logarithm of the hydrogen ion concentration (in mol/l): pH= - Log10 [H+]Pure water undergoes a certain amount of dissociation or self-ionisation to form hydrogen (or more strictly, hydroxonium) and hydroxyl ions. 2H2O <> H3O+ + OH- The equilibrium constant for this dissociation (i.e. the concentration of the two ions multiplied together) at 25°C is 1 x 10-14 mol/l. This is known as the ionic product for water. In a neutral solution there are equal quantities of both ions and therefore a hydrogen ion concentration of 1 x 10-7 mol/l. Thus a neutral solution has a pH of 7. If the concentration of H+ is higher than 10-7 (and the OH- concentration proportionately lower to maintain the ionic product at 10-14) then the pH value will be lower than 7 (down to 0) and the solution is more acid. Conversely, if the H+ is less than 10-7 the pH will be higher than 7 (up to a maximum of 14) and the solution will be more alkaline. pX is exactly the same as pH but relates to the concentration (activity) of any other ion in solution. Because of the reduced sensitivity of other ion-selective electrodes when compared with pH electrodes, the pX scale rarely extends below 7 (= 10-7 mol/l).NB: pX = 0 represents a concentration of the detected ion of 1 mol/l. pH Range (of an ISE) : All ion-selective electrodes are also sensitive to pH to some extent, most commonly at the high and low ends of the pH scale. The pH range is that range over which a change in pH will not cause a significant change in the measured voltage. It is the plateau on a graph of pH against mV at constant concentration of the detected ion. Outside this range, a change in pH may cause a significant change in the measured mV. So if the samples have different pH to the standards then the measured ion concentration will not be correct. Thus if samples lie outside the pH range then they must be treated with a buffer solution to bring them within the range. Alternatively, if it is necessary to measure samples at a pH outside the range and all have the same pH, then the calibration standard solutions must be treated so as to make them the same pH as the samples.A second alternative is to use the Standard Addition method whereby a small amount of standard (say 2ml) is added to a larger amount of sample (100ml) so that the calibration and sample measurement are essentially measured at the pH of the sample. This method also compensates for differences in temperature and Ionic Strength between sample and standard. pH electrode : An ion selective electrode with a membrane made of a special glass which exclusively permits the passage of hydrogen ions. pH electrode are normally in the form of combination electrodes with a built in reference system. They function over the activity range 1 mol/l H+ (pH=0) to 10-14 mol/l H+ (pH=14).pH meter : A millivolt meter for measuring the potential developed between a pH electrode and a suitable reference system which is calibrated in the corresponding pH units.Poisoning : The chemical conversion of the surface of an ion selective membrane to a form which is less sensitive to changes in the ionic activity of the detected species. In many cases, the electrode function may be restored by physically removing a thin layer of the sensing element or by reversing the poisoning reaction chemically.Precipitation : The removal of one or more ionic species from solution by adding an agent which causes the formation of insoluble compounds.Precision : A measure of the reproducibility of a method. In any analytical process there will be a certain amount of random variation which ensures that it is very unlikely that any two readings of the same measurement will give exactly the same result. Precision is an expression of the amount of this random variation. If sufficient multiple readings are taken on the same sample then the precision of measurement can be expressed as the standard deviation about the mean value (details of these terms can be found in any standard text book on statistics). It is necessary to know the level of precision in order to decide whether two results are significantly different from one another and also to give some idea of how close the result may be to the true value, i.e. the accuracy (assuming that there are no systematic errors in the measurements). If the mean values for replicate measurements of two samples differ from one another by more than twice the standard deviation then they are said to be statistically different at the 66% confidence level. If the same number of replicate measurements are made again several times more, then the new mean values can be expected to be within one standard deviation of the original in 66 cases out of 100 (or within twice the standard deviation in 95 cases out of 100). Thus if an analytical method is reported as having a one standard deviation precision of 5% then, assuming no systematic errors, it can be expected that, on average, 95% of the analyses should give a result that is within 10% of the true answer.Redox electrode : A metallic electrode, usually made of platinum, that is used to monitor reversible reduction/oxidation reactions. The potential difference developed between the redox electrode and a reference electrode is a Nernstian function of the ratio of two different oxidation states of a species in solution.Redox potential : The potential developed by a metallic electrode when placed in a solution containing a species in two oxidation states in reversible equilibrium. The redox potential is a measure of the tendency for the reduction reaction to occur. Standard redox potentials range from about -3 volts to +3 volts. The more positive the value the more likely it is that reduction will occur. Negative values indicate that oxidation is more likely. Reference electrode : That part of an electrode measuring system which provides a constant stable voltage regardless of the composition of the external solution. This voltage is used as the base from which the sample-induced changes in potential of the ISE are measured. Single junction reference electrodes have a single chamber generally filled with a potassium chloride solution saturated with silver chloride which contacts with the sample solution through a single liquid junction by means of a semi-porous ceramic plug or fritted disc. Double junction reference electrodes have two chambers with the internal reference system having a liquid junction with an intermediary salt bridge and then a second liquid junction to the external sample. The outer filling solution composition is chosen to avoid contamination of the sample with the target ion or any that would interfere with it, and to minimise the effects of the liquid junction potential. Reference internal element : That part of a reference electrode which reacts with the filling solution to produce a constant reference potential. In the case of the silver chloride / potassium chloride filling solution the reactive elements is a silver wire coated with silver chloride (it is chloridised) at its lower end, partially immersed in the filling solution The silver wire then makes contact with the external measuring circuit.Reproducibility : A measure of the closeness of replicate measurements of the same test solution using the same measurement techniques and under the same conditions. See Precision. Reproducibility can be affected by a number of factors including instrument or electrode instability, electrode drift, frequency of calibration, temperature variation, loss of sample, contamination.Response time : The length of time necessary to obtain a stable electrode potential when the electrode is removed from one solution and placed in another of different concentration. Response time is affected by the electrode type, the magnitude and direction of the concentration change, hysteresis, temperature, the presence of interfering ions, if ISAB is added, and if the samples are stirred or measured in static conditions. For ISE specifications it is defined as the time to complete 90% of the change to the new value and is generally quoted as less than ten seconds. In practice however it is often necessary to wait several minutes to complete the last 10% of the stabilization in order to obtain the most precise results. It must also be noted that often the stabilization time is controlled by the time taken for the reference electrode to regain a stable liquid junction potential, but this is not taken into account when quoting ISE specifications.SAOB = Sulphide anti-oxidant buffer: An ISAB for sulphide determinations which is added in equal quantities to standards and samples to raise the pH, free any sulphide bound in a complex with hydrogen, equalise the total ionic strength and retard any oxidation of the sulphide.SAOB can be prepared by adding 200ml 10N NaOH (or 80g reagent grade pellets) to approximately 600ml de-ionised water - slowly to avoid over heating and spitting. Then add 35g ascorbic acid and 67g disodium EDTA and stir until all has dissolved. Transfer to a 1000ml flask, dilute to volume with water and shake well. This should last for at least two weeks in a tightly stoppered bottle - but the colour will gradually darken and the solution should be discarded when dark brown. Salt bridge : The outer filling solution of a double junction reference electrode used to isolate the internal reference electrolyte from the sample solution and prevent contamination of the sample with K, Cl or Ag ions.Screened cable : A coaxial shielded cable used to connect the ISE internal reference electrode system to a plug for input into the external millivolt measuring instrument. Designed to minimise interference effects from external electrostatic fields.Sensor : Any device which responds directly to the amount of a given environmental component. Ion selective electrodes are sensors which respond to the concentration of particular dissolved ions in a solution. pH electrodes respond to hydrogen ions. Other sensors used in measuring the properties of solutions include redox, dissolved oxygen and conductivity electrodes and temperature sensors. There are many other sensors used for gaseous and dissolved organic molecules which are beyond the scope of this work.Selectivity : The ability of an ISE to distinguish between the different ions in the same solution. Ion selective electrodes are not 100% ion-specific. Most are sensitive to some other ions to some extent. Some ISEs cannot be used in the presence of certain other interfering ions or can only tolerate very low contributions from these ions. In some cases interfering ions can be removed by adding reagents to cause precipitation or complexing or the magnitude of the effect can be assessed by measuring the Selectivity Coefficient.Selectivity Coefficient (SC): An expression of the extent to which an ion selective electrode reacts with an interfering ion in proportion to the measured ion. A selectivity coefficient of 0.1 implies that the electrode is ten times more responsive to the primary ion than to the interfering ion; i.e. if a solution contains equal concentrations of both ions, the interfering ion will cause the apparent measured concentration of the primary ion to be about 10% too high. Unfortunately, the selectivity coefficient is not constant but depends on a number of factors including the total ionic strength of the solution, the absolute and relative concentration of both ions, and the temperature. ISE specifications usually quote a value for the SC for the main interfering ions, but they seldom state under what conditions they were measured - thus they cannot be used for precise corrections of interference. The selectivity coefficient is expressed as: kA,B. Where A is the primary ion and B is the interfering ion. The higher the kA,B the greater the effect of the interference from B. At kA,B = 1 there is an equal response to both ions. The total potential developed is given by a modified form of the Nernst equation (see Nikolsky equation) One standard method for determining the selectivity coefficient is as follows. However it must be noted that this is measured in pure standard solutions and does not take into account any effects from sample matrix. Where ZA and ZB are the valencies of the ions A and B. Alternatively, it may be easier and quicker, and more useful, to measure the SC directly in a typical sample containing possible interfering ions. First measure the concentrations of the primary ion and the interferent in the sample, then add more interferent - sufficient to ensure a significant increase in the signal for the primary ion. The amount to add can be calculated from the initial concentration measurements and the indicative SC quoted in the electrode specifications. Then measure the apparent concentration of the primary ion again and calculate the SC for the interferent from the increase caused by the known amount added. e.g. if the added interferent increased it's concentration by 10ppm and this produced a 1 ppm increase in the measured value of the primary ion then the SC would be 0.1. Subsequent sample measurements can then be made by measuring both the primary ion and interferent and using the measured SC to make a more reasonable correction for the interference. Click here for practical example. These measurement can be made, with or without ISAB, by Direct Potentiometry or Standard Addition depending on the Ionic Strength and nature of the samples. However, it must be noted that the accuracy and precision of this correction may be quite variable and will need to be tested for validity and reliability for each particular application before being used on a routine basis. Possible Alternative to using Selectivity Coefficient If the interfering ion is fairly constant in the samples and is not too concentrated (ie will not cause too much of an error in the measured ion concentration - say not more than 100%) Then it may help to minimize this error by spiking the standards with the same concentration of interfering ion as in the samples. But this will inevitably lead to a raising of the lower limit of concentration for reasonable routine measurements. Standard (or Known) addition : An incremental method of analysis where an accurately measured small volume of concentrated standard is added to a much larger volume of sample. See Advantages of Standard Addition Method for more details.The concentration of the unknown sample may be calculated as follows: Where, Sample addition : Similar to Standard Addition but here an accurately measured small volume of sample is added to a much larger volume of standard. Mostly used when there are only small amounts of sample available or sample has high concentration. The concentration of the unknown sample may be calculated as follows:Slope : The gradient of the line formed by plotting the electrode response in millivolts against the logarithm of the activity (or concentration) of the measured ion. The theoretical Nernstian slope at 25°C is 59.16 mV per decade change in activity for monovalent ions and 29.58 mV/dec. for divalent ions. In practice the slope is generally lower than this due to inefficiency of the ion selective membranes and failure to meet ideal conditions. Measured slopes generally lie in the range 54±5 and 26±3 respectively. Measured slopes lower than this, or a gradual reduction of slope during use, are indicative of contamination of the ISE membrane. The slope of an electrode can be determined by measuring the mV response in two standard solutions with concentrations (activities) of a1 and a2, (effectively creating a calibration graph of mV (E) against the log of the concentration).The slope of the line is calculated from: And the intercept from: Thus if an unknown sample is measured and found to have a potential of E3 mV the concentration can be calculated from: Log(a) = (E3-c)/m. This is essentially the calculation used in the direct potentiometry method of sample measurement. Solubility : A measure of the degree to which a given species will dissolve in a solvent. That concentration at which the solution is saturated with respect to that species i.e. solid undissolved material is in equilibrium with the solution. Depends on the nature of the solute and the solvent and on the temperature. Generally expressed as grams per litre (g/l) at 25°C.Spiking : The process of adding a known amount of the ion being measured to a sample in order to determine the original sample concentration. See Standard Addition.Temperature effect : The change in potential developed between a sensing and reference electrode which is due solely to a change in temperature.Unfortunately this is a complicated relationship which cannot be simply quantified in terms of mV change per °C since the effect is different at different concentrations and actual value of the mV. Moreover, the electrode slope, the liquid junction potential, and solubility of the salts in the reference system all vary with temperature. However, the magnitude of the effect of temperature change on the slope can be calculated from a modified form of the Nernst equation (Slope = 2.303RT/nF = T x Constant) to be about 3.4% per 10°C - ie: if the slope is about 55mV/dec then a 10°C rise will increase this to about 57mV/dec. Thus, in order to avoid any errors due to temperature change it is advisable to recalibrate with standards at the same temperature as the sample solutions if the sample temperature deviates by more than about 2°C from the original calibration temperature. Temperature Range for an ISE : The maximum temperature at which an ISE will work reliably is generally quoted as 50°C for a PVC membrane and 80°C for crystal membranes. The minimum temperature is generally quoted as 0°C - but it must be noted that there is no particular change in electro-chemical processes at 0°C other than it is the conventional freezing point for water.The important factor is that the sample and standard solutions must remain liquid and homogeneous and at the same temperature throughout the calibration and measurement process. If samples have sufficient salt content to remain liquid below 0°C and measurements are required at these temperatures then the standards will have to be made up in a similar matrix (but without the target ion) to prevent freezing. Furthermore, such samples will presumably have a relatively high ionic strength and so an appropriate method must be adopted in order to avoid problems with variable activity coefficients ( see: www.nico2000.net/Book/Guide7.html#ac ). A further potential hazard to be aware of when measuring at low temperatures is the possibility of condensation causing short circuits and poor contacts in the electrical connections. All-solid-state ISEs as sold by Nico2000 Ltd. should function reliably below 0°C but a potential user would need to make his own tests to establish the lowest temperature at which liquid or gel-filled reference electrodes will perform. As a rough guide, according to the Vant Hoff relationship (Delta T = 1.86 * molar concentration of salt * number of ions per molecule), freezing point depression can be calculated for pure solutions. Thus a solution of 4M KCl, similar to that used in the Single-Junction KCl/AgCl Reference Electrode should not freeze until about -15°C but Double Junction RE's, with relatively dilute outer filling solutions (0.1M KNO3 or 0.1M Li acetate) cannot be expected to function properly below about 0.3 degrees below zero. TISAB= Total Ionic Strength Adjustment Buffer: A reagent which is added to standards and samples in Fluoride determinations to optimise the pH value at 5.5, liberate any fluoride which may be complexed with hydrogen, aluminium, iron or other cations, and equalise the activity coefficient between sample and standards.There are several different recipes for TISAB but the two most common are: 1) Dissolve 57ml acetic acid + 45g Sodium Chloride + 4g CDTA (1,2-diamino cyclohexan N,N,N,N-tetra acetic acid) in 500ml distilled water. Adjust pH to 5.5 by adding drops of 5M NaOH, then make up to 1L with water. 2) An alternative TISAB solution is a mixture consisting of: 1Molar NaCl + 0.75M sodium acetate + 0.25M acetic acid + 0.001M sodium citrate. Titrant : Any solution, normally contained in a burette, which is incrementally added to a known volume of a sample until an end point is reached during a titration experiment. In ISE work, normally a reagent which is added in order to complex, or liberate from complex, the species being determined.Titration : A quantitative analytical technique for measuring the concentration of a species by incremental addition of a reagent (titrant) that reacts with the sample until a distinct end point is reached where all the sample has reacted. In ISE work, this end point is detected by a sudden rapid change in electrode potential.Water hardness : A measure of the ability of water to form insoluble precipitates when boiled or when soap is added. Hardness is caused by the presence of Ca++, Mg++, Fe++ and to a lesser extent, other divalent cations. It is expressed as ppm of divalent cations. It can be measured with a water hardness electrode which exhibits almost equal response to all divalent cations.Zero potential point : The concentration of a species in solution at which the potential developed between the sensing and reference electrodes is 0 mV. In many instances this is the same as isopotential point.CCR / Nico2000Ltd / www.nico2000.net |