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A solid-state ion-selective electrode uses a membrane consisting of either a polycrystalline inorganic salt or a single crystal of an inorganic salt. The magnitude of the junction potential depends upon the concentration of ions on the two sides of the interface, and may be as large as 3040 mV. Despite its importance as the fundamental reference electrode against which we measure all other potentials, the SHE is rarely used because it is difficult to prepare and inconvenient to use. A membrane potential results from a chemical interaction between the analyte and active sites on the membranes surface. Reference electrode For the specific case of solubility equilibria: Qsp < Ksp: the reaction proceeds in the forward direction (solution is not saturated; no precipitation observed), Qsp > Ksp: the reaction proceeds in the reverse direction (solution is supersaturated; precipitation will occur). An instrument using an expanded unit scale or a digital readout system is preferred since it allows more precise measurements. An automatic temperature compensator may be used. In solutions with equal concentrations of target ions, the ion forming the least soluble compound will precipitate first (at the lowest concentration of counter ion), with the other ions subsequently precipitating as their compounds solubilities are reached. [7] The preparation of the QRE electrode is simple, allowing for a fresh reference to be prepared with each set of experiments. A plot of Ecell versus log[A], however, may show curvature at higher concentrations of analyte as a result of a matrix-dependent change in the analytes activity coefficient. (5) Determining pH on samples. ) An example of a typical combination electrode is shown in Figure 11.16. If the concentration of added electrolyte is sufficient, the difference between the samples matrix and that of the standards does not affect the ionic strength and the activity coefficient remains essentially constant. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. The crown ether shown here. As shown in the following example, this is easy to do. First standardize the electrodes using a pH 4.0 buffer at or near 25 deg.C. No effect at all! (b ) Remove the drained solids from the sieve, blend to a uniform paste, adjust the temperature of the paste to 25 deg.C and determine its pH. 4. (Imagine having to draw a picture of each electrochemical cell you are using!) (2) Instruments. (a) AgI, silver iodide, a solid with antiseptic properties, (b) CaCO3, calcium carbonate, the active ingredient in many over-the-counter chewable antacids, (c) Mg(OH)2, magnesium hydroxide, the active ingredient in Milk of Magnesia, (d) Mg(NH4)PO4, magnesium ammonium phosphate, an essentially insoluble substance used in tests for magnesium, (e) Ca5(PO4)3OH, the mineral apatite, a source of phosphate for fertilizers, (a)BaSO4(s)Ba2+(aq)+SO42(aq)Ksp=[Ba2+][SO42];(b)Ag2SO4(s)2Ag+(aq)+SO42(aq)Ksp=[Ag+]2[SO42];(c)Al(OH)3(s)Al3+(aq)+3OH(aq)Ksp=[Al3+][OH]3;(d)Pb(OH)Cl(s)Pb2+(aq)+OH(aq)+Cl(aq)Ksp=[Pb2+][OH][Cl](a)BaSO4(s)Ba2+(aq)+SO42(aq)Ksp=[Ba2+][SO42];(b)Ag2SO4(s)2Ag+(aq)+SO42(aq)Ksp=[Ag+]2[SO42];(c)Al(OH)3(s)Al3+(aq)+3OH(aq)Ksp=[Al3+][OH]3;(d)Pb(OH)Cl(s)Pb2+(aq)+OH(aq)+Cl(aq)Ksp=[Pb2+][OH][Cl]. TripleFunction Electrolyte Regulation toward Advanced Aqueous To review the method of standard additions, see Section 5.3.3. The solution is already saturated, though, so the concentrations of dissolved magnesium and hydroxide ions will remain the same. The indicator is yellow in its acid form (beaker a: 1.0 M HCl) and is blue in its base form (beaker d: H2O). Hg The potential of a calomel electrode is +0.280 V when the concentration of KCl is 1.00 M and +0.336 V when the concentration of KCl is 0.100 M. If the activity of Cl is 1.00, the potential is +0.2682 V. Another common reference electrode is the silver/silver chloride electrode, which is based on the following redox couple between AgCl and Ag. Educ. is very nonselective in its behavior and rapidly reacts with numerous species with the rate constants on the order of 10 8 10 10 M 1 Silver chloride electrode Here, the solution is used as the gate electrode. [Ag+] = 1.0 1011 M; AgBr precipitates first. A saturated calomel electrode was used as reference electrode and the potentials were converted to the RHE scale (Supplementary Fig. (a) Acid foods means foods that have a natural pH of 4.6 or below. Also known as the mineral calomel[4] (a rare mineral) or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury(I) compound. ISFET {\displaystyle E^{0}(V)=-0.138\ \mathrm {V} } To standardize a pH electrode using two buffers, choose one near a pH of 7 and one that is more acidic or basic depending on your samples expected pH. (1980), sections 50.032-50.035, under "Sodium Hydroxide - Official Final Action" by the "Standard Potassium Hydroxide Phthalate Method," which is also incorporated by reference and available as set forth in paragraph (a)(4)(ii) of this section. For example, a junction potential of 33.09 mV has been measured at the interface between solutions of 0.1 M HCl and 0.1 M NaCl.2 The magnitude of a salt bridges junction potential is minimized by using a salt, such as KCl, for which the mobilities of the cation and anion are approximately equal. Another approach to calibrating a potentiometric electrode is the method of standard additions. In 1906, Cremer discovered that the potential difference across a thin glass membrane is a function of pH when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. Coulometry determines the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity (in coulombs) consumed or produced.It can be used for precision measurements of charge, and the amperes even used to have a coulometric definition. A typical Ag/AgCl electrode is shown in Figure 11.12 and consists of a silver wire, the end of which is coated with a thin film of AgCl, immersed in a solution containing the desired concentration of KCl. The cathodic reaction, which is the right half-cell, is the reduction of Fe3+ to Fe2+. Although equation 11.24 applies to membrane electrodes, we can use if for a metallic electrode by replacing z with n. Precision in potentiometry is limited by variations in temperature and the sensitivity of the potentiometer. The analytes interaction with the membrane generates a membrane potential if there is a difference in its activity on the membranes two sides. Calomel electrode. Figure 11.17 Schematic diagram of a solid-state electrode. For example, the potential of a Ag electrode in a solution of Ag+ is, \[E =E^\circ_\mathrm{Ag^+/Ag} - 0.05916\log\dfrac{1}{a_\mathrm{Ag^+}} = +0.7996\: \ce V - 0.05916\log\dfrac{1}{a_\mathrm{Ag^+}}\tag{11.5}\], If we saturate the indicator electrodes half-cell with AgI, the solubility reaction, \[\ce{AgI}(s) \ce{Ag+}(aq) + \ce{I-}(aq)\], determines the concentration of Ag+; thus, \[a_\mathrm{Ag^+} = \dfrac{K_\mathrm{sp,AgI}}{a_\mathrm{I^-}}\tag{11.6}\], where Ksp, AgI is the solubility product for AgI. The potential of a silver:silver chloride reference electrode with respect to the standard hydrogen electrode depends on the composition of the electrolyte solution and on temperature. Substituting equation 11.8 into equation 11.7, assuming a temperature of 25oC, and rearranging gives, \[E_\ce{cell} = K + \dfrac{0.05916}{z}\log(a_\ce{A})_\ce{samp}\tag{11.9}\]. are licensed under a, Measurement Uncertainty, Accuracy, and Precision, Mathematical Treatment of Measurement Results, Determining Empirical and Molecular Formulas, Electronic Structure and Periodic Properties of Elements, Electronic Structure of Atoms (Electron Configurations), Periodic Variations in Element Properties, Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law, Stoichiometry of Gaseous Substances, Mixtures, and Reactions, Shifting Equilibria: Le Chteliers Principle, The Second and Third Laws of Thermodynamics, Representative Metals, Metalloids, and Nonmetals, Occurrence and Preparation of the Representative Metals, Structure and General Properties of the Metalloids, Structure and General Properties of the Nonmetals, Occurrence, Preparation, and Compounds of Hydrogen, Occurrence, Preparation, and Properties of Carbonates, Occurrence, Preparation, and Properties of Nitrogen, Occurrence, Preparation, and Properties of Phosphorus, Occurrence, Preparation, and Compounds of Oxygen, Occurrence, Preparation, and Properties of Sulfur, Occurrence, Preparation, and Properties of Halogens, Occurrence, Preparation, and Properties of the Noble Gases, Transition Metals and Coordination Chemistry, Occurrence, Preparation, and Properties of Transition Metals and Their Compounds, Coordination Chemistry of Transition Metals, Spectroscopic and Magnetic Properties of Coordination Compounds, Aldehydes, Ketones, Carboxylic Acids, and Esters, Composition of Commercial Acids and Bases, Standard Thermodynamic Properties for Selected Substances, Standard Electrode (Half-Cell) Potentials, Half-Lives for Several Radioactive Isotopes, Silver chloride is a sparingly soluble ionic solid. Several representative examples are described in Table 11.5, and additional examples can be found in this chapters additional resources. A ion-selective electrode for NO2 has logKA,I values of 3.1 for F, 4.1 for SO42, 1.2 for I, and 3.3 for NO3. The membrane potential for a Ag2S pellet develops as the result of a difference in the extent of the solubility reaction, \[\ce{Ag2S}(s) \ce{2Ag+}(aq) + \ce{S^2-}(aq)\], on the membranes two sides, with charge carried across the membrane by Ag+ ions. Another class of ion-selective electrodes uses a hydrophobic membrane containing a liquid organic complexing agent that reacts selectively with the analyte. 18). A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For example, calcium is present in urine both as free Ca2+ ions and as protein-bound Ca2+ ions. [8], Shortly after yellow fever struck Philadelphia, the disease broke out in Jamaica. Cool the beaker in a water bath and add 5 M NaOH until the pH is between 55.5. The potential of one electrodethe working or indicator electroderesponds to the analytes activity, and the other electrodethe counter or reference electrodehas a known, fixed potential. Ammonia Even in the best circumstances, however, we may observe a difference of approximately 1 mV for samples with equal concentrations of analyte. The ideal slope from the Nernst equation is 59.16 mV per 10-fold change in concentration. Adjust the temperature of the aqueous layer to 25 deg.C and determine its pH. A leak-free saturated calomel electrode was used as the reference electrode, and all potentials in this study were reported on a RHE scale. As noted above, changing the electrolyte concentration changes the electrode potential. Mercury is unique among the group 12 metals for its ability to form the MM bond so readily. 0 The scheduled process shall be established by a qualified person who has expert knowledge acquired through appropriate training and experience in the acidification and processing of acidified foods. [citation needed]. (ii) Temperature. Potentiometry initially was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. The equation that describes the equilibrium between solid calcium carbonate and its solvated ions is: It is important to realize that this equilibrium is established in any aqueous solution containing Ca2+ and CO32 ions, not just in a solution formed by saturating water with calcium carbonate. Mercurous chloride forms by the reaction of elemental mercury and mercuric chloride: It can be prepared via metathesis reaction involving aqueous mercury(I) nitrate using various chloride sources including NaCl or HCl. (h) Water activity (aw) is a measure of the free moisture in a product and is the quotient of the water vapor pressure of the substance divided by the vapor pressure of pure water at the same temperature. Methods that may be used to determine pH or acidity for acidified foods include, but are not limited to, the following: (a) Potentiometric method for the determination of pH - (1) Principles. When it is added to water, it dissolves slightly and produces a mixture consisting of a very dilute solution of Ag, Oil paints contain pigments that are very slightly soluble in water. This section applies previously introduced equilibrium concepts and tools to systems involving dissolution and precipitation. The structure of the cation exchanger, di-(n-decyl) phosphate, is shown in red. \end{align}\]. The manufacturer shall employ appropriate quality control procedures to ensure that finished foods do not present a health hazard. [7], The principle of silver/silver chloride sensors operation is the conversion of ion current at the surface of human tissues to electron current to be delivered through an electrical wire to the measurement instrument. Figure 11.17 shows a typical design. 11.2: Potentiometric Methods is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. California State Liquid acids are generally more effective than solid or pelleted acids. Bates, R.G. Visit this website for more information on how barium is used in medical diagnoses and which conditions it is used to diagnose. Easym in equation 11.8 is similar to Eo in equation 11.1. saturated For example, the Nernst equation for a metallic electrode of the first kind is, \[E_\ce{cell} = K + \dfrac{0.05916}{n}\log a_{\ce M^{n+}}\tag{11.16}\]. To find the relationship between the activity of H3O+ in the inner solution and the activity CO2 in the inner solution we rearrange the equilibrium constant expression for reaction 11.10; thus, \[a_\mathrm{H_3O^+} = K_\ce{a} \dfrac{a_\mathrm{CO_2}}{a_\mathrm{HCO_3^-}}\tag{11.13}\], where Ka is the equilibrium constant. Anecdotal evidence indicates calomel was more effective than bleeding. As shown in Figure 11.18, the membrane is placed at the end of a non-conducting cylindrical tube, and is in contact with two reservoirs. In 1901 Fritz Haber discovered that there is a change in potential across a glass membrane when its two sides are in solutions of different acidity. The best way to appreciate the theoretical and practical details discussed in this section is to carefully examine a typical analytical method. Other food products may be semisolid in character. V Calomel is used as the interface between metallic mercury and a chloride solution in a saturated calomel electrode, which is used in electrochemistry to measure pH and electrical potentials in solutions. The potential for an Fe3+/Fe2+ half-cell is +0.750 V relative to the standard hydrogen electrode. "Standard potential of the silver-silver chloride electrode". Instructions for Downloading Viewers and Players. The procedure specifies that the standards and the sample should be stored in plastic containers. If we do not know the junction potentials actual valuewhich is the usual situationthen we cannot directly calculate the analytes concentration using the Nernst equation. Redox potential (also known as oxidation / reduction potential, ORP, pe, , or ) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively. Figure 11.14 shows a typical potentiometric electrochemical cell equipped with an ion-selective electrode. The authors found that the ammonia obtained from the electroreduction of nitrate at an optimum potential of 1.3 V versus saturated calomel electrode had a yield of 925.11 g h 1 mg cat 1, a Faradaic efficiency of 96.56%, and a maximum selectivity of 95.31% in a 0.5 M K 2 SO 4 electrolyte solution. The concentration of fluoride in toothpastes containing soluble F may be determined with a F ion-selective electrode using a calibration curve prepared with external standards. In Chapter 6 we noted that the equilibrium position of a chemical reaction is a function of the activities of the reactants and products, not their concentrations. Tomatoes and tomato products having a finished equilibrium pH less than 4.7 are not classed as low-acid foods. The electrode consists of a metallic silver wire (Ag(s)) coated with a thin layer of silver chloride (AgCl), either physically by dipping the wire in molten silver chloride, chemically by electroplating the wire in concentrated hydrochloric acid (HCl)[2] or electrochemically by oxidising the silver at an anode in a chloride solution. If an electrode is poisoned, it can be returned to its original condition by sanding and polishing the crystalline membrane. In a potentiometric electrochemical cell one half-cell provides a known reference potential and the potential of the other half-cell indicates the analytes concentration. See Section 6.4.4 for a review of using the Nernst equation in calculations. The oil layer may be discarded. Recall from the chapter on solutions that the solubility of a substance can vary from essentially zero (insoluble or sparingly soluble) to infinity (miscible). Authority: 21 U.S.C. 3. (6) Preparation of samples. If the activity of Br is sufficiently high, AgCl at the membrane/solution interface is replaced by AgBr and the electrodes response to Cl decreases substantially. standard electrodes are the standard hydrogen electrode (SHE) and the saturated calomel electrode (SCE). Figure 11.23 Ladder diagram for HF/F. Using Table 11.7, this corresponds to an uncertainty of 0.4% for monovalent analytes and 0.8% for divalent analytes. The selectivity coefficient for OH, KNO2/OH, is 630. 19, 1982; 49 FR 5609, Feb. 14, 1984; 54 FR 24892, June 12, 1989; 63 FR 14035, Mar. (4) General procedure for determining pH. Mercurous chloride is toxic, although due to its low solubility in water it is generally less dangerous than its mercuric chloride counterpart. Sawyer, A. Sobkowiak, J.L. First, we transfer a sample with a volume of VA and an analyte concentration of CA into a beaker and measure the potential, (Ecell)A. For a variety of reasonsincluding the slow kinetics of electron transfer at the metalsolution interface, the formation of metal oxides on the electrodes surface, and interfering reactionselectrodes of the first kind are limited to the following metals: Ag, Bi, Cd, Cu, Hg, Pb, Sn, Tl, and Zn. Alternatively, blend the entire contents of the container to a uniform paste, adjust the temperature of the paste to 25 deg.C, and determine the equilibrated pH. If you are redistributing all or part of this book in a print format, While for short periods it may be possible to use such aqueous electrodes as references with nonaqueous solutions the long-term results are not trustworthy. Ammonia is an inorganic compound of nitrogen and hydrogen with the formula NH 3.A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Prepare a total ionic strength adjustment buffer (TISAB) by mixing 500 mL of water, 57 mL of glacial acetic acid, 58 g of NaCl, and 4 g of disodium DCTA (trans-1,2-cyclohexanetetraacetic acid) in a 1-L beaker, stirring until dissolved. A porous wick serves as a salt bridge between the sample and its reference electrode. The development of new membrane electrodes continues to be an active area of research. (c) The added compound does not contain a common ion, and no effect on the magnesium hydroxide solubility equilibrium is expected. What is the maximum pH that we can tolerate if we wish to analyze a solution in which the activity of F is 1105 and if the error is to be less than 1%? As shown in Figure 11.11, in a saturated calomel electrode (SCE) the concentration of Cl is determined by the solubility of KCl. View this simulation to explore various aspects of the common ion effect. When the selectivity coefficient is 1.00 the membrane responds equally to the analyte and the interferent. By convention, the reference electrode is the anode; thus, the short hand notation for a potentiometric electrochemical cell is, \[E_\ce{cell} = E_\ce{ind} E_\ce{ref} + E_\ce{j}\]. What is the potential for the electrochemical cell in Practice Exercise 11.1 if the activity of H+ in the anodic half-cell is 0.100, the fugacity of H2 in the anodic half-cell is 0.500, and the activity of Cu2+ in the cathodic half-cell is 0.0500? (3) Indicator paper. Many of these electrodes, such as Zn, cannot be used in acidic solutions because they are easily oxidized by H+. (2) Elj is the liquid junction potential between the given electrolyte and a reference electrolyte with a molal activity of chloride of 1mol/kg. Measurement of pH. The use of a single electrode greatly simplifies the measurement of pH. \[\ce{NH4+}(aq) + \ce{H2O}(l) \ce{H3O+}(aq) + \ce{NH3}(aq)\], The NH3, in turn, diffuses through the gas permeable membrane where a pH electrode measures the resulting change in pH. Figure 11.12 Schematic diagram showing a Ag/AgCl electrode. The fluoride ion is capable of reacting with glass to form SiF4. Figure 11.20 shows one version of the urea electrode, which modifies a gas-sensing NH3 electrode by adding a dialysis membrane that traps a pH 7.0 buffered solution of urease between the dialysis membrane and the gas permeable membrane.4 (An NH3 electrode, as shown in Table 11.4, uses a gas-permeable membrane and a glass pH electrode. You will find values for the standard-state reduction potential in Appendix 13. The differences in experimental potentials vary little from literature values. Calculations of this sort are most conveniently performed using a compounds molar solubility, measured as moles of dissolved solute per liter of saturated solution. The electrodes should then be thoroughly rinsed with water and blotted with soft tissue before proceeding with the standardization. Write the shorthand notation for the electrochemical cell. Cyclic Voltammetry Aranzaes, J. R., Daniel, M.-C., Astruc, D. "Metallocenes as references for the determination of redox potentials by cyclic voltammetry. Most of the polycrystalline ion-selective electrodes listed in Table 11.2 can be used over an extended range of pH levels. The equilibrium constant for solubility equilibria such as this one is called the solubility product constant, Ksp, in this case. Fugacity is the equivalent term for the activity of a gas. The indicator electrode consists of a pH-sensitive glass membrane and an internal Ag/AgCl reference electrode in a solution of 0.1 M HCl. Most indicator solutions are prepared as a 0.04 percent solution of the indicator dye in alcohol. Various types of electrodes designed for specific uses are available. 16, 1979, as amended at 61 FR 14245, Apr. Since QREs are made fresh, there is also no concern with improper storage or maintenance of the electrode. (1980), section 22.060, under "Titratable Acidity - Official Final Action," for "Indicator Method," and section 22.061 for "Glass Electrode Method - Official Final Action," which is incorporated by reference. Battery and line-operated instruments are available commercially. Subtracting equation 11.22 from equation 11.21 and solving for pHA gives, \[\mathrm{pH_A = pH_{std}} \dfrac{\left\{(E_\ce{cell})_\ce{A} (E_\ce{cell})_\ce{std}\right\}F}{2.303RT}\tag{11.23}\], which is the operational definition of pH adopted by the International Union of Pure and Applied Chemistry.8, The equations in this section assume that the pH electrode is the cathode in a potentiometric cell. The limiting factor when analyzing a large number of samples is the need to rinse the electrode between samples. Chemistry, 64, 652, 1960. Quantitatively transfer the resulting suspension to a 100-mL volumetric flask along with 50 mL of TISAB and dilute to volume with distilled water. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs.A conventional current describes the direction in which positive charges move. The cell potential for the sample is, \[(E_\ce{cell})_\ce{A} = K + \dfrac{0.05916}{2}\log C_\ce{A}\], and that following the standard addition is, \[(E_\ce{cell})_\ce{std} = K + \dfrac{0.05916}{2}\log\left\{\dfrac{V_\ce{A}}{V_\ce{tot}}C_\ce{A} + \dfrac{V_\ce{std}}{V_\ce{tot}}C_\ce{std}\right\}\], where Vtot is the total volume (VA + Vstd) after the standard addition. Accuracies of 15% for monovalent ions and 210% for divalent ions are typical. A cathode is the electrode from which a conventional current leaves a polarized electrical device. Perhaps the most frequent analytical measurement is the determination of a solutions pH, a measurement we will consider in more detail later in this section. Identify these purposes. (credit modification of work by glitzy queen00/Wikimedia Commons), Anticoagulants can be added to blood that will combine with the Ca, The Role of Precipitation in Wastewater Treatment, Wastewater treatment facilities, such as this one, remove contaminants from wastewater before the water is released back into the natural environment. Some common reference electrodes used in aqueous media include the saturated calomel electrode (SCE), standard hydrogen electrode (SHE), and the AgCl/Ag electrode. The average lifetime for a typical glass electrode is several years. Sodium ions, Na+, serve as counter ions. Some food products may consist of a mixture of liquid and solid components that differ in acidity. As shown in Figure 11.17, the LaF3membrane is sealed into the end of a non-conducting plastic cylinder, which contains a standard solution of F, typically 0.1 M NaF, and a Ag/AgCl reference electrode. The gel contains free chloride ions such that the ion charge can be carried through the electrolyte solution. and MacAskill, J.B. (1978). One advantage of the F ion-selective electrode is its freedom from interference. Figure 11.16 Schematic diagram showing a combination glass electrode for measuring pH. The SHE consists of a Pt electrode immersed in a solution in which the activity of hydrogen ion is 1.00 and in which the fugacity of H2(g) is 1.00 (Figure 11.10). The selectivity of this electrode for Ca2+ is very good, with only Zn2+ showing greater selectivity. Selectivity coefficients for most commercially available ion-selective electrodes are provided by the manufacturer. The colorimetric method for pH involves the use of indicator dyes in solutions that gradually change color over limited pH ranges. is, by definition, 0.00 V at all temperatures. The surface of the electrode is subject to poisoning, as described above for a Cl ISE in contact with an excessive concentration of Br. Sec. Depending upon the pH of the solution, the tape will change color and an approximate pH can be determined by comparison with a standard color chart. Sec. Acidified foods shall be thermally processed to an extent that is sufficient to destroy the vegetative cells of microorganisms of public health significance and those of nonhealth significance capable of reproducing in the food under the conditions in which the food is stored, distributed, retailed and held by the user. See Chapter 3.4.4 for an additional discussion of selectivity. More accurate colorimetric tests can be made using a comparator block fitted with sets of tubes of standard indicator solutions of known pH. Insert a piece of silver wire into concentrated HCl then allow the wire to dry on a lint-free cleaning cloth. We call this difference in potential across the membrane a junction potential, which we represent as Ej. This porous frit is a salt bridge. An insulated electrical wire connects the silver rod with the measuring instrument. where aMn+ is the activity of the metal ion. The functional relationship between current response and time is measured after applying single or double potential step The Nernst equation relates the cell potential to the analytes activity. A stopper in the outer tube provides an opening for adding addition saturated KCl. and 30 deg.C, the optimum being 25 deg.C. Figure 11.8 Potentiometric electrochemical cell for Example 11.1. Poisoning simply means that the surface has been chemically modified, such as AgBr forming on the surface of a AgCl membrane. The microelectrodes tip is made hydrophobic by dipping into a solution of dichlorodimethyl silane, and an inner solution appropriate for the analyte and a Ag/AgCl wire reference electrode are placed within the microelectrode. The difference between this equation and equation 11.23 does not affect the operation of a pH meter. Modern high impedance potentiometers minimize the amount of current passing through the electrochemical cell. 1996, 96, 877. [44 FR 16235, Mar. The Nernst equation below shows the dependence of the potential of the silver-silver(I) chloride electrode on the activity or effective concentration of chloride-ions: The standard electrode potential E0 against standard hydrogen electrode (SHE) is 0.230V 10mV. (ii) Rinse and blot the electrodes. \[\ce{Fe^3+}(aq) + e^ \ce{Fe^2+}(aq)\], \[\ce{Ag}(s) + \ce{Fe^3+}(aq) + \ce{Cl-}(aq) \ce{AgCl}(s) + \ce{Fe^2+}(aq)\], The electrochemical cells shorthand notation is, \[\ce{Ag}(s) \,|\, \ce{HCl}(aq,\, a_\mathrm{Cl^-} = 0.100),\: \ce{AgCl(sat'd)} \,||\, \ce{FeCl2}(aq,\, a_\mathrm{Fe^{2+}} = 0.0100),\: \ce{FeCl3}(aq,\, a_\mathrm{Fe^{3+}} = 0.0500) \,|\, \ce{Pt}(s)\]. 1999-2022, Rice University. The relative error in concentration, therefore, is a function of the measurement error, Ecell, and the analytes charge .
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