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# Soil pH and EC, P K MANI

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pH, soil pH, working principle of pH meter etc.

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• contains valence electrons from the atoms
• So the individual atomic energy levels interact to form molecular energy levels
• ### Soil pH and EC, P K MANI

1. 1. Soil Fertility and Nutrient Management Experiential Learning, NRM Module-1, ACSS-451 Dr. Pabitra Kumar Mani Assoc. Professor Deptt. Agril. Chemistry and Soil Science Bidhan Chandra Krishi Viswavidyalaya, E-mail: pabitramani@gmail.com, Website: www.bckv.edu.in  : 91-33-25822132, 91-9477465968, August 21, 2013
2. 2. The Danish biochemist Soren Sorensen invented the pH scale in 1909. When applied to the full cell formed from half cells present on inside and outside of glass pH electrode, Nernst equation takes form: pH inside of the electrode has constant value, thus it can be included in the potential part of the equation: This form of the equation describes behavior of the glass electrode used for pH measurements
3. 3. The Ionization of Water Is Expressed by an Equilibrium Constant The degree of ionization of water at equilibrium (Eqn 2–1) is small; at 25 °C only about two of every 109 molecules in pure water are ionized at any instant. The equilibrium constant for the reversible ionization of water (Eqn 2–1) is In pure water at 25°C, the concentration of water is 55.5 M (grams of H2O in 1 L divided by its gram molecular weight: (1,000 g/L)/(18.015 g/mol)) and is essentially constant in relation to the very low concentrations of H and OH, namely, 1X 10-7 M. Accordingly, we can substitute 55.5 M in the equilibrium constant expression (Eqn 2– 3) to yield which, on rearranging, becomes where Kw designates the product (55.5 M)(Keq), the ion product of water at 25 °C. The value for Keq, determined by electrical-conductivity measurements of pure water, is 1.8 X 10 -16 M at 25°C. Substituting this value for Keq in Equation gives the value of the ion product of water:
4. 4. Thus the product [ H+ ][ OH- ] in aqueous solutions at 25°C always equals 1x10 -14 M2. When there are exactly equal concentrations of H+ and OH-, as in pure water, the solution is said to be at neutral pH. At this pH, the concentration of H+ and OH- can be calculated from the ion product of water as follows:
5. 5. How a pH meter works When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility. When a metal is brought in contact with a solution of salts or acids, a similar electric potential is caused, which has led to the invention of batteries. Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart. A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen. To complete the electrical circuit, also a reference electrode is needed. Note that the instrument does not measure a current but only an electrical voltage, yet a small leakage of ions from the reference electrode is needed, forming a conducting bridge to the glass electrode.
6. 6. General Principles Reference electrode | salt bridge | analyte solution | indicator electrode Eref Ej Eind Ecell = Eind – Eref + Ej Reference cell : a half cell having a known electrode potential Indicator electrode: has a potential that varies in a known way with variations in the concentration of an analyte A cell for potentiometric determinations.
7. 7. Introduction 1.) Potentiometry  Use of Electrodes to Measure Voltages that Provide Chemical Information - Various electrodes have been designed to respond selectively to specific analytes  Use a Galvanic Cell Unknown solution becomes a ½cell Add Electrode that transfers/accepts electrons from unknown analyte Connect unknown solution by salt bridge to second ½-cell at fixed composition and potential  Indicator Electrode: electrode that responds to analyte and donates/accepts electrons  Reference Electrode: second ½ cell at a constant potential  Cell voltage is difference between the indicator and reference electrode
8. 8. Reference Electrodes 1.) Overview   Potential change only dependent on one ½ cell concentrations Reference electrode is fixed or saturated  doesn’t change! {   [ Fe 2 + ]   0.05916    − 0.222 − 0.05916 log[ Cl − ] E cell = 0.771 − log   [ Fe 3 + ]   1      Potential of the cell only depends on [Fe2+] & [Fe3+] Unknown solution of [Fe2+] & [Fe3+] } Reference electrode, [Cl-] is constant Pt wire is indicator electrode whose potential responds to [Fe2+]/[Fe3+]
9. 9. Reference Electrodes 2.) Silver-Silver Chloride Reference Electrode Eo = +0.222 V Activity of Cl- not 1E(sat,KCl) = +0.197 V  Convenient - Common problem is porous plug becomes clogged Reduction potential
10. 10. Reference Electrodes 3.) Saturated Calomel Reference Electrode (S.C.E) Eo = +0.268 V Activity of Cl- not 1E(sat,KCl) = +0.241 V   Saturated KCl maintains constant [Cl-] even with some evaporation Standard hydrogen electrodes are cumbersome - Requires H2 gas and freshly prepared Pt surface
11. 11. Indicator Electrodes 1.) Two Broad Classes of Indicator Electrodes  Metal Electrodes - Develop an electric potential in response to a redox reaction at the metal surface  Ion-selective Electrodes - Selectively bind one type of ion to a membrane to generate an electric potential Remember an electric potential is generated by a separation of charge
12. 12. Indicator Electrodes 2.) Metal Electrodes Platinum Most common metal indicator electrode Inert: does not participate in many chemical reactions Simply used to transmit electrons Other electrodes include Gold and Carbon Metals (Ag, Cu, Zn, Cd, Hg) can be used to monitor their aqueous ions Most metals are not useable Equilibrium not readily established at the metal surface    Example: E+o = +799 V ½ Reaction at Ag indicator electrode: E(sat,KCl) = +0.241 V ½ Reaction at Calomel reference electrode: Cell Potential from Nernst Equation:   1  0.05916    − { 0.241} E cell = E + − E − = 0.799 − log   [ Ag + ]   1      Cell voltage changes as a function of [Ag +] Potential of Ag indicator electrode
13. 13. Typical electrode system for measuring pH. (a) Glass electrode (indicator) and saturated calomel electrode (reference) immersed in a solution of unknown pH. (b) Combination probe consisting of both an indicator glass electrode and a silver/silver chloride reference. A second silver/silver chloride electrode serves as the internal reference for the glass electrode. The two electrodes are arranged concentrically with the internal reference in the center and the external reference outside. The reference makes contact with the analyte solution through the glass frit or other suitable porous medium. Combination probes are the most common configuration of glass electrode and reference for measuring pH.
14. 14. The majority of pH electrodes available commercially are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode conveniently placed in one housing.
15. 15. When cell is not at standard conditions, use Nernst Equation • In a chemical reaction such as: aA + bB  cC + dD c d aC .aD o o ∆G = ∆G + 2.3RT log a b = ∆G + 2.3RT log Q a A .aB Substitute ∆G = − nFE ∆G o = − nFE o Where: ∆Go : Free energy change when all the reactants and products are in their standard states (unit activity). R : is the gas constant. T : is the temperature in the absolute temperature Q : Reaction Quotient
16. 16. c d aC .aD ∆G = ∆G + 2.3RT log a b a A .aB o [C ]c [ D]d − nFE = −nFE o + 2.3RT log [ A]a [ B]b 2.3RT [C ]c [ D]d E = Eo − log nF [ A]a [ B]b Where concentrations are substituted for activities At 298 K the equation becomes K 0.0591 [C ]c [ D]d E = Eo − log n [ A]a [ B]b …… Nernst Equation  At Equilibrium, ∆G = 0, E = 0. Hence 0.0591 0=E − log K n o 0.0591 E = log K n o
17. 17. Derivation of the EMF equation For the general rn., Red ⇋ Ox + ne- The chemical potential are given by µRed = µ0Red+ RTlnaRed µOx = µ0Ox+ RTlnaOx µRed - µOx = µ0Red - µ0Ox + RTln(aRed /aOx) -ΔG = (µ0Red - µ0Ox) + RTln(aRed /aOx) but , considering the electrical work associated with transfer of n no. Of electrons, ΔG= -nFE  μ0  − μ0    Red Ox  - RT ln aOx E= nF nF a Red RT a Ox ∴E = E ln nF a Red 0 When aOx =aRed = 1, then μ0  − μ0   Ox  0  Red E = nF a 0.0591 so, E = E log Ox n a Red 0 T=2980K F=96000 Coulomb
18. 18. In principle it should be possible to determine the H + ion activity or concn. of a soln by measuring the potential of a Hydrogen electrode inserted in the given soln. The EMF of a cell, free from liquid junction potential, consisting of a Hydrogen electrode and a reference electrode, should be given by, E = E ref – RT/F ln aH+ E = E ref + 2.303 RT/F pH ∴pH = ( E- Eref )F/2.303 RT R= 8.314 J/mol/°K F= faraday constant ,96,485 T= Kelvin scale So, by measuring the EMF of the Cell E obtained by combining the H electrode with a reference electrode of known potential, Eref , the pH of the soln. may be evaluated. The electric potential at any point is defined as the work done in bringing a unit charge from infinity to the particular point Reduced state ⇋ Oxidised state + n Electron M = Mn+ + nE E(+) = E0 – (RT/F) ln aMn+ Nernst Equation
19. 19. Electrodes and Potentiometry : pH Electrodes 1.) pH Measurement with a Glass Electrode   Glass electrode is most common ion-selective electrode Combination electrode incorporates both glass and reference electrode in one body Ag(s)|AgCl(s)|Cl-(aq)||H+(aq,outside) H+(aq,inside),Cl-(aq)|AgCl(s)|Ag(s) Outer reference electrode [H+] outside (analyte solution) [H+] inside Inner reference electrode Glass membrane Selectively binds H+ Electric potential is generated by [H+] difference across glass membrane
20. 20. Electrodes and Potentiometry pH Electrodes 2.) Glass Membrane  Irregular structure of silicate lattice Cations (Na+) bind oxygen in SiO4 structure
21. 21. pH Electrodes 2.) Glass Membrane  Two surfaces of glass “swell” as they absorb water - Surfaces are in contact with [H+]
22. 22. pH Electrodes 2.) Glass Membrane  H+ diffuse into glass membrane and replace Na+ in hydrated gel region - Ion-exchange equilibrium Selective for H+ because H+ is only ion that binds significantly to the hydrated gel layer Charge is slowly carried by migration of Na+ across glass membrane E = constant − β (0.05916) pH Potential is determined by external [H+] Constant and b are measured when electrode is calibrated with solution of known pH
23. 23. the operation of a glass electrode is related to the situations existing at the inner and outer surfaces of the glass membrane. Glass electrodes require soaking in water for some hours before use and it is concluded that a hydrated layer is formed on the glass surface, where an ion exchange process can take place. If the glass contains sodium, the exchange process can be represented by the equilibrium The concn of the soln within the glass bulb is fixed, and hence on the inner side of the bulb an equilibrium condition leading to a constant potential is established. On the outside of the bulb, the potential developed will be dependent upon the hydrogen ion concentration of the soln in which the bulb is immersed. Within the layer of 'dry' glass which exists between the inner and outer hydrated layers, the conductivity is due to the interstitial migration of sodium ions within the silicate lattice.
24. 24. Most often used pH electrodes are glass electrodes. Typical model is made of glass tube ended with small glass bubble. Inside of the electrode is usually filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed. pH of internal solution varies - for example it can be 1.0 (0.1M HCl) or 7.0 (different buffers used by different producers). Active part of the electrode is the glass bubble. While tube has strong and thick walls, bubble is made to be as thin as possible. Surface of the glass is protonated by both internal and external solution till equilibrium is achieved. Both sides of the glass are charged by the adsorbed protons, this charge is responsible for potential difference. This potential in turn is described by the Nernst equation and is directly proportional to the pH difference between solutions on both sides of the glass.
25. 25. pH Electrodes 3.) Calibration  A pH electrode should be calibrated with two or more standard buffers before use.  pH of the unknown should lie within the range of the standard buffers Measured voltage is correlated with a pH, which is then used to measure an unknown.
26. 26. For soils containing predominantly negatively charged clays, dilution of the soil solution by distilled water increases the absolute value of the surface potential and changes the distribution of H+ ions between the DDL and the bulk solution. The proportion of H+ ions in the DDL relative to the bulk solution increases so that the measured pH, which is the bulk solution pH, is higher than that of the natural soil. This effect is especially noticeable in saline soils. An alternative method is to shake the soil with 0.01 M CaCl2 solution until equilibrium is attained. This solution, containing the most abundant exchangeable cation in many soils, also has an ionic strength I approximating that of the soil solution. In this soil suspension, both H+ and Ca2+ exchange with other cations in the DDL, but the ratio of H+/√Ca2+ in solution remains relatively constant, even when the ratio of soil to equilibrating liquid changes. Thus, pH measured in 0.01 M CaCl2 is a more accurate reflection of the natural soil pH, as is demonstrated by the comparison between soil pH measured in water, in 0.01 M CaCl2 , and in a soln. that has been shaken with successive samples of soil to achieve equilibrium
27. 27. The ionic distribution at a negatively charged clay surface
28. 28. For soils containing predominantly negatively charged clays, dilution of the soil solution by distilled water increases the absolute value of the surface potential and changes the distribution of H+ ions between the DDL and the bulk solution. The proportion of H+ ions in the DDL relative to the bulk solution increases so that the measured pH, which is the bulk solution pH, is higher than that of the natural soil. This effect is especially noticeable in saline soils. An alternative method is to shake the soil with 0.01 M CaCl2 solution until equilibrium is attained. This solution, containing the most abundant exchangeable cation in many soils, also has an ionic strength I approximating that of the soil solution. In this soil suspension, both H+ and Ca2+ exchange with other cations in the DDL, but the ratio of H+/√Ca2+ in solution remains relatively constant, even when the ratio of soil to equilibrating liquid changes. Thus, pH measured in 0.01 M CaCl2 is a more accurate reflection of the natural soil pH, as is demonstrated by the comparison between soil pH measured in water, in 0.01 M CaCl2 , and in a soln. that has been shaken with successive samples of soil to achieve equilibrium
29. 29. I = 0.0127 x Ecw Griffin and Jurinak, 1973 where Ci and zi are the concentration and charge, respectively, of ion i in a mixture of ionic species i = 1 to n. The term ‘concentration’ refers to the mass of an ionic species per unit volume of solution (e.g. mol/L). An individual ion experiences weak forces due to its interaction with water molecules (the formation of a hydration shell), and stronger electrostatic forces due to its interaction with ions of opposite charge. Effectively, this means that the ability of the ion to engage in chemical reactions is decreased, relative to what is expected when it is present at a particular concentration with no interactions. This effect is accounted for by defining the activity ai of ion i, which is related to its concentration by the equation where fi is the activity coefficient of the ion. Values of fi range from 0 to 1. In very dilute solutions where the interaction effects are negligible, fi approaches 1, and ai is approximately equal to Ci. There is extensive theory on the calculation of activity coefficients, but all calculations make use of the ionic strength I, such as in the Debye–Huckel limiting law (Atkins, 1982)
30. 30. The difference in pH(ΔpH) between the sediment and supernatant produced during soil pH determination is termed the 'suspension effect′ (McLean 1982). If the soil suspension is allowed to settle, the pH as measured in the suspension liquid is often higher than in the sediment layer. The suspension effect is influenced by the extent to which electrodes encounter clay and humus particles and the soil CO2 is in equilibrium with atmospheric concentrations, and the magnitude of liquid junction effects (Foth and Ellis 1988). The suspension effect is minimised by measuring soil pH using 0.01 M CaCl2 or 1 M KCl solutions instead of water (Sumner 1994). In addition, the use of CaCl2 or KCl solutions has the advantages of (i) decreasing the effect of the junction potential of the calomel reference electrode, (ii) equalising the salt content of soils, and (iii) Preventing dispersion of the soil (Tan 1995).
31. 31. Junction Potential 1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact    Develops at solution interface (salt bridge) Small potential (few milli volts) Junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements - Don’t know contribution to the measured voltage Different ion mobility results in separation in charge Mobilties of ions in water at 25oC: Na+ : 5.19 × 10 –8 m2/sV K+ : 7.62 × 10 –8 Cl– : 7.91× 10 –8 Again, an electric potential is generated by a separation of charge
32. 32. General relationship between soil pH and availability of plant nutrients in minimally and moderately weathered soils; the wider the bar, the more availability.
33. 33. Nitrogen availability is maximum between pH 6 and 8, because this is the most favorable range for the soil microbes that mineralize the nitrogen in organic matter and those organisms that fix nitrogen symbiotically. High phosphorus availability at high pH-above 8.5-is due to sodium phosphates that have high solubility. In calcareous soil, pH 7.5 to 8.3, phosphorus availability is re-duced by the presence of calcium carbonate that represses the dissolution of calcium phosphates. Maximum phosphorus availablity is in the range 7.5 to 6.5. Below pH 6.5, increasing acidity is associated with increasing iron and aluminum in solution and the formation of relatively insoluble iron and aluminum phosphates. Note that potassium, calcium, and magnesium are widely available in alkaline soils. As soil acidity increases, these nutrients show less availability as a result of the decreasing CEC and decreased amounts of exchangeable nutrient cations. Iron and manganese availability increase with increasing acidity because of their increased solubility.These two nutrients are frequently deficient in plants growing in alkaline soils because of the insolubility of their compounds Boron, copper, and zinc are leachable and can be deficient in leached, acid soils. Conversely, they can become insoluble (fixed) and unavailable in alkaline soils. In acid soils, molybdenum is commonly deficient owing to its reaction with iron to form an insoluble compound. For plant nutrients as a whole, good overall nutrient availability occurs near pH 6.5. In intensively weathered soils, nutrients such as B and Zn may be deficient at lower pH than for minimally and moderately weathered soils. As a consequence, a desirable pH is 5.5 in intensively weathered soils.
34. 34. Ohm's Law States that the current I (amperes) flowing in a conductor is directly proportional to the applied electromotive force E (volts) and inversely proportional to the resistance R (ohms) of the conductor The reciprocal of the resistance is termed the conductance (G): this is measured in reciprocal ohms (or Ω –), for which the name Siemens (S) is used. The resistance of a sample of homogeneous material, length l, and cross-section area a, is given by: where ρ is a characteristic property of the material termed the resistivity (formerly called specific resistance). In SI units, l and a will be measured respectively in metres and square metres, so that ρ refers to a metre cube of the material, and The reciprocal of resistivity is the conductivity, κ (formerly specific conductance), which in SI units is the conductance of a one metre cube of substance and has the units Ω – m - , but if ρ is measured in Ωcm, then κ will be measured in Ω - cm - '.
35. 35. Electrical Conduction • Ohm's Law: V=IR voltage drop (volts = J/C) resistance (Ohms) current (amps = C/s) C = Coulomb • Resistivity, ρ: -- a material property that is independent of sample size and geometry surface area RA ρ= l • Conductivity, σ of current flow current flow path length 1 σ= ρ 45
36. 36. Electrical Properties • Which will have the greater resistance? 2l D l 2D R1 = 2ρl 8ρl = 2 D 2 πD π  2 ρl ρl R1 R2 = = 2 = 2 8 D 2D  π π  2  • Analogous to flow of water in a pipe • Resistance depends on sample geometry and size. 46
37. 37. Electrical Conductivity This is based on the principle that the conductivity, or ease with which an electric current is carried through a solution, is proportional to the quantity of ions (actually, the quantity of ionic charge) in solution. To measure soil conductivity, the soil is mixed with water until it has a paste like consistency. A pair of electrodes, commonly made of platinum metal, is inserted into this "saturated paste." The electrodes are characterized by their surface area, A cm2 (usually about 1 cm2), and their separation distance of L cm (usually about 1 cm), as illustrated in Figure. An alternating current is then applied to the electrode pair, and the conductance of the paste is measured in units of reciprocal ohms (mho),or Siemens Two-electrode cell used to measure electrical conductivity of solutions and saturated soil pastes
38. 38. Conductivity Probe • 2 metals in contact with electrolyte solution • Voltage is applied to electrodes and resulting current that flows btw electrodes is used to determine conductance • Amount of current flowing depends on: – Solution conductivity – Length, surface area, geometry of electrodes
39. 39. Conductivity Probe • Apply an AC Voltage to Two Electrodes of Exact Dimensions • Acids, Bases and Salts (NaCl) Dissolve in Solution and Act as Current Carriers • Current Flow is Directly Proportional to the Total Dissolved Solids in Solution • Physical Dimensions of a Conductivity Electrode are Referred to as the Cell Constant • Cell Constant is Length/Area Relationship – Distance Between Plates = 1.0 cm – Area of Each Plate = 1.0 cm x 1.0 cm – Cell Constant = 1.0 cm-1
40. 40. Cell Constant • Cell constant: – Measure of current response of a sensor conductive solution – Due to sensor’s dimensions and geometry – Units: cm-1 (length divided by area)
41. 41. But this conductance depends on A and L and is therefore a characteristic of the measuring device as well as the solution. It is more useful to define a specific conductance, termed the electrical conductivity or EC, by the equation: EC, unlike conductance, is strictly a property of the solution, whereas L/A is a characteristic of the electrode geometry and is called the cell constant. For a saturated paste, an EC value greater than 4 milli Siemens/cm (mS cm-I) is diagnostic of a saline soil. The current-carrying ability of a mole of cation or anion charge in solution is termed the equivalent conductance. This value is measured in very dilute solutions to ensure that the salts are fully dissociated into individual cations and anions. The measured conductance is then assumed to result from the summation of conductances contributed by each of the ions. As a result, each ion can be assigned an equivalent conductance, listed in Table 8.2, that represents its contribution (per mole of charge) to the measured (total) conductivity.
42. 42. Conduction & Electron Transport • Metals (Conductors): -- for metals empty energy states are adjacent to filled states. -- thermal energy excites electrons into empty higher energy states. -- two types of band structures for metals empty band - partially filled band - empty band that overlaps filled band partly filled band Partially filled band Energy Overlapping bands Energy empty band filled states filled band filled states GAP filled band filled band 52
43. 43. Energy Band Structures: Insulators & Semiconductors • Insulators: • Semiconductors: Energy empty conduction band filled states GAP filled valence band filled band -- narrow band gap (< 2 eV) -- more electrons excited across band gap Energy empty conduction band ? GAP filled states -- wide band gap (> 2 eV) -- few electrons excited across band gap filled valence band filled band 53
44. 44. It is apparent that H+ and OH- have anomalously high conductances, while most cations and anions contribute about equally per mole of ionic charge to the conductivity of solutions. This fact allows simple but approximate empirical relationships to be established between the concentration of electrolyte solutions and the conductivity, irrespective of ionic composition.
45. 45. For strong electrolytes the molar conductivity increases as the dilution is increased, but it appears to approach a limiting value known as the molar conductivity at infinite dilution(Λ∞). The quantity Λ∞ can be determined by graphical extrapolation for dilute solutions of strong electrolytes. For weak electrolytes the extrapolation method cannot be used for the determination of Λ∞ but it may be calculated from the molar conductivities at infinite dilution of the respective ions, use being made of the 'Law of Independent Migration of Ions‘. At infinite dilution the ions are independent of each other, and each contributes its part of the total conductivity, thus:
46. 46. THE BASlS OF CONDUCTIMETRIC TlTRATlONS principle underlying conductimetric titrations, i.e. The substitution of ions of one conductivity by ions of another conductivity.
47. 47. Definitions Further definitions J = σ ε <= another way to state Ohm’s law J ≡ current density current I = = surface area A like a flux ε ≡ electric field potential = V/ J = σ (V/ ) Electron flux conductivity voltage gradient 57
48. 48. Example: Conductivity Problem What is the minimum diameter (D) of the wire so that V < 1.5 V? = m 100 I = 2.5 A Cu wire - + V 100 m πD 2 4 Solve to get R=  V = Aσ I < 1.5 V 2.5 A 6.07 x 107 (Ohm-m)-1 D > 1.87 mm 58
49. 49. Filling of electronic band structure in various types of material at thermodynamic Equilibrium. In metals and semimetals the Fermi level EF lies inside at least one band. In insulator (electricity)insulators and semiconductors the Fermi level is inside a band gap, however in semiconductors the bands are near enough to the Fermi level to be Fermi-Dirac statistics thermally populated with electrons or Electron holes.
50. 50. The calomel reference electrode consists of a glass tube with a potassium chloride (KCl) electrolyte which is in intimate contact with a mercuric chloride element at the end of a KCL element. It is a fragile construction, joined by a liquid junction tip made of porous ceramic or similar material. This kind of electrode is not easily 'poisoned' by heavy metals and sodium. The glass electrode consists of a sturdy glass tube with a thin glass bulb welded to it. Inside is a known solution of potassium chloride (KCl) buffered at a pH of 7.0. A silver electrode with a silver chloride tip makes contact with the inside solution. To minimise electronic interference, the probe is shielded by a foil shield, often found inside the glass electrode. Most modern pH meters also have a thermistor temperature probe which allows for automatic temperature correction, since pH varies somewhat with temperature
51. 51. The conductance of an electrolytic solution at any temperature depends only on the ions present, and their concentration. When a solution of an electrolyte is diluted, the conductance will decrease, since fewer ions are present per millilitre of solution to carry the current. If all the solution be placed between two electrodes 1 cm apart and large enough to contain the whole of the solution, the conductance will increase as the solution is diluted. This is due largely to a decrease in inter-ionic effects for strong electrolytes and to an increase in the degree of dissociation for weak electrolytes. The molar conductivity (Λ) of an electrolyte is defined as the conductivity due to one mole and is given by:
52. 52. Electrons in solids • In a solid, there are so many electrons with energies very near each other that ‘bands’ of states develop. All we draw is the “band diagram” Solid Isolated atoms
53. 53. Conductivity: Comparison • Room temperature values (Ohm-m)-1 = (Ω - m)-1 METALS Silver Copper Iron conductors 6.8 x 10 7 6.0 x 10 7 1.0 x 10 7 CERAMICS -10 Soda-lime glass 10-10 -11 Concrete 10-9 Aluminum oxide <10-13 SEMICONDUCTORS POLYMERS Polystyrene Silicon 4 x 10 -4 Polyethylene Germanium 2 x 10 0 GaAs 10 -6 semiconductors <10 -14 -15 10 -10-17 insulators Selected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e. 63
54. 54. Electrons in atoms Electrons in an atom have particular energies (quantized energy states) depending on which orbital they are in. Energy H B 1s He Pauli exclusion 1s principle 1p 1s
55. 55. • Bandthere are so manyRepresentation Structure electrons with energies very In a solid, near each other that ‘bands’ of states develop. Adapted from Fig. 18.3, Callister & Rethwisch 8e. 65
56. 56. Electron Energy Band Structures Adapted from Fig. 18.2, Callister & Rethwisch 8e. 66
57. 57. Definition of Conductivity Energy • The free-est electron (the electron with the highest energy) defines the position Band gap of the “Fermi level.” – Above Ef, all available Ef, electron states in the energy bands are empty Fermi level – Below Ef, they are all filled. Band gap • If there is no gap between filled and empty states, the material is conductive. Metal (Cu) • If there is a gap, the material is a semiconductor or insulator. Empty 4p (conduction) partially filled 4s (conduction) Filled (valence)
58. 58. Energy band structures for various metals Energy • Partially filled or empty bands are called ‘conduction bands.’ • Any band that is totally filled is considered to be a “valence band.” – We usually ignore ‘deep’ valence bands. Empty 4p (conduction) Band gap partially filled 4s (conduction) Band gap Filled 3d (valence) filled 3d, 3p, 3s, 2p, 2s, 1p, 1s (valence) Metal (Cu) Deep valence-only an issue for optical properties