Chem 28 – analytical chemistry
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Chem-28-Finals-Reviewer
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Reagent Oxidatio n Potential 1° Standar d Stability of Solution Fe (II)
𝐹𝑒 }m → 𝐹𝑒 •m + 𝑒
n -0.77
K 2 Cr 2 O 7 , Fe
Unstable unless protected form O2 Na
S 2 O 3
2𝑆 } 𝑂 • }n → 𝑆
ƒ 𝑂 Y }n + 2𝑒
n 0.08
KIO 3 , I 2
Frequent standardization required As(III) 𝐻 • 𝐴𝑠𝑂 • + 𝐻 } 𝑂 → 𝐻 • 𝐴𝑠𝑂
ƒ + 2𝑒
n -0.56
AsO 3 , I 2
Indefinitely stable if acidic Na 2 S 2 O 3 – moderately strongreducing agent, widely used to determine oxidizing agents by indirect procedure that involves I 2 as an intermediate
Potentiometry: Potentiometry – a simple, easy to use and inexpensive electroanalytical technique based upon potential measurements; makes use of a pH meter; involves measurements of potential difference between a pair of suitable electrodes immersed in a solution to be analyzed Potentiometric Methods: Reference Electrode (E ref ) | Salt bridge (E j ) | Analyte Solution | Indicator Electrode (E ind
) 𝐸 ,+99 = 𝐸 -2/
− 𝐸 *+l
+ 𝐸 ¶ Junction Potential – potential that develops across an interface between 2 solutions that differ in their electrolyte solution; junction potential at 2 ends of a salt bridge tend to cancel each other Reference Electrodes – half-cells which have a constant potential and are completely insensitive or independent of composition of the analyte solution Indicator/Sensing Electrode – electrode having potential that is dependent on analyte solution; should respond rapidly and selectively to anaylte of interest; should have reproducible response; there is no indicator electrode that is absolutely specific in its response Types of Indicator Electrodes: 1. Standard Hydrogen Electrode (SHE) – troublesome to maintain & use 2. Calomel RE – Hg | Ag 2 Cl 2 (satd.), KCl (xM) x = 0.11M and satd. ~ 4.6M E SCE = 0.244V 𝐻𝑔 } 𝐶𝑙 } + 2𝑒 n ↔ 2𝐻𝑔
(9) + 2𝐶𝑙
('f) n 3. Ag/AgCl RE – Ag | AgCl (satd), KCl (satd.) E Ag/AgCl
= 0.199V 𝐴𝑔𝐶𝑙
(() n + 𝑒 n ↔ 𝐴𝑔
(() + 𝐶𝑙
('f) n 4. Metallic Indicator Electrodes of the 1 st Kind – simplest; usually constructed from coils of metal wire or flat metal plates e.g. Ag and Cu wires Pure Metal Electrodes in equilibrium with its cation solution 𝑋 2m + 𝑛𝑒 n ↔ 𝑋 (() During potentiometric measurement, potential of RE remains constant while potential of indicator electrode is proportional to activity of analyte as described by the Nernst Equation (valid only when essentially no current flows through the cell) 1 st Order Metallic Electrode – responds to its cation 2 nd Order Metallic Electrode – responds to its anion that forms a sparingly soluble precipitate or stable complex with their cation 5. Inert Metallic Electrode – inert metallic electrode for redox systems e.g. Pt, Au or C; responds to potential of the redox system with which it is in contact 6. Membrane Electrodes – p Ion electrode employing a selective membrane as a sensing element; most are Ion selective electrodes and exhibit good selectivity or specific ions based on measurement of potential generated across a membrane; selective membrane usually attached to the ed of a tube that contains an internal RE 7. Glass Electrodes – 1 st membrane electrode developed and still the most widely employed glass membrane electrode for pH measurements; pH sensitive part is a thin glass blub at the bottom of the electrode Glass Membrane Composition (Corning 015 Glass): 22% Na 2 O. 6%
CaO, 72% SiO 2 (specific for H + up to pH 9); can respond to cations like Na + and K + at higher pH Boundary Potential – basis for pH Measurement 8. Solid State Membrane Electrode – consists of an insoluble salt/crystal; difference is membrane at the bottom; e.g. Fluoride Electrode – the sensing element is a single crystal of lanthanum fluoride, LaF 3 Fluoride, EuF 2 to create lattice vacancies. Such a crystal is an ionic conductor by virtue of the nobility of fluoride 9. Liquid Membrane Electrode 10. Hydrophobic Polymer membrane – saturated liquid ion exchanger Ionophores – metal macrocyclic ion carrier dissolved in a viscous organic membrane; liquid ion exchanger; interacts with a pH meter via its internal reference cell; enables the ion of interest to penetrate the membrane Direct indicator – species selective Indirect indicator – ion specific electrodes that react with other compounds Molecular Selective Electrodes – 2 membranes are used to generate response ad each membrane and each has its own selectivity characteristics 2 Types of MSE: 1. Gas Sensing Probe – responds to specific gases dissolved in solutions; P inside the probe, inert sensing element glass electrode; other ion selective electrode surrounded by an electrolyte solution surrounded by a gas-permeable membrane 2. Enzyme Electrodes – constructed by covering a surface of MSE with the enzyme immobilized in some matrix. The immobilized enzyme catalyzes the production of an ion that is detected by the electrode response is proportional to the substrate; used for diabetes; analyzes glucose using glucose oxidase Most popular is the Urea Electrode – urease hydrolyses urea and forms ammonia and changes pH; Urea is the first compound synthesized; cellophane wrapped around the pH electrode Combination Electrode – both indicator and reference in the same body Instruments for Measuring Cell Potentials with High Resistance (R Internal = 10 11 to 10
12
Ù) Direct reading digital Voltmeter – pH meters/ p Ion meter; ion meters
Direct Potentiometric Measurements – rapid and convenient method for determining activity of diferrent cations and anions; need standard solutions of known analyte ; needs calibration curve
Conversion in Potentiometry – indicator electrode always treated as cathode; RE as anode 𝐸 ,+99
= 𝐸 -2/
− 𝐸 *+l
+ 𝐸 ¶ For Cations: 𝐸 ,+99
= 𝑘 − 0.0592
𝑛 𝑝𝑋
𝑝𝑋 = − 𝐸 ,+99 − 𝑘 0.0592/𝑛
For Anions: 𝐸 ,+99 = 𝑘 + 0.0592
𝑛 𝑝𝐴
𝑝𝐴 = 𝐸 ,+99 − 𝑘 0.0592/𝑛
Nikolsky Equation: 𝐸 = 𝐸° − 𝑅𝑇 𝑧
𝐹 𝑙𝑛 𝑙𝑛 [𝑎
- + ‡
(𝑘 - 𝑗𝑎 ¶ ) z º z » ]
Nerntian Slope and Selectivity – test for electrodes Potentiometric measurements in terms of concentration; results differ if the activity used high ionic strength in the unknown sample Total Ionic Strength Adjustment Buffer (TISAB) – used to watch the ionic strength of the analyte solution and that of the standards (inert electrolyte) Operational Definition of pH (by NIST and IUPAC) – based on direct calibration of pH meter with standard buffer followed by determination of pH of unknown solution 𝑝𝐻 𝑠 =
𝐸 ( − 𝑘 0.0592 𝑝𝐻 𝑢 = −
𝐸 : − 𝑘 0.0592 𝑝𝐻 𝑢 = 𝑝𝐻 𝑠 − (𝐸 :
( ) 0.0592 Exact pH: 𝑝𝐻 = 𝑙𝑜𝑔 𝑙𝑜𝑔 (𝛾 • •
m ] )
NOISE reacts to one specific compound Selectivity Coefficient, K ij – measure of extent of interference of a particular ion of a given electrode response; ideally 0 and the more selective it is, the closer it is to 0 a j
z j = charge of interfering ion Potentiometric Titration – titrations wherein the potential of a suitable electrode is measured as a function of the titrant solution; readily automated by autotitrators; usual potentiometric set-up but add burette with titrant; E cell
End Point Detection: Direct plot of E vs. V titrant – sigmoidal Plot of ÄE/ÄV vs. V Titrant - Plot of Ä 2 E/ ÄV 2 vs. V Titrant - Graphical determination of endpoint – valid if titration curve symmetrical about equivalence point and inflection curve corresponds to this point; analyte and titrant react in equimolar ratio
Spectrophotometry: Absorption Spectroscopy:
Attenuation of a Beam of Radiation by an Absorbing Solution Power of the beam decreases from P 0 to P (decrease of radiant power) P 0 – incident radiant power P – transmitted radiant power Transmittance, T of solution - fraction of incident radiation transmitted by the solution (often expressed as %T) Absorbance, A of a solution: 𝐴 = − 𝑙𝑜𝑔 𝑙𝑜𝑔 𝑇 = 𝑙𝑜𝑔 𝑙𝑜𝑔 𝑃
= 𝑙𝑜𝑔 𝑙𝑜𝑔 𝑃 (19.+2; 𝑃 (19:;-12
Bouger’s or Lambert’s Law – transmitted radiant power decreases exponentially as path length increases 𝑑𝑃 𝑑𝑏 = 𝐾 ‚ 𝑃 − 𝑑𝑃 𝑃 = 𝐾 ‚ 𝑑𝑏
− ¼ ½ ½ ¾ 𝑑𝑃 𝑃 = 𝐾 ‚ ¼ W
𝑑𝑏 𝑙𝑛 𝑙𝑛 𝑃
𝑃 = 𝐾
‚ 𝑏 𝑙𝑜𝑔 𝑙𝑜𝑔 𝑃
= 𝐾 } 𝑏 𝐾 } = 𝑒𝐾
‚ Beer’s Law – the transmitted radiant power decreases exponentially as concentration increases (strictly applicable only for monochromatic radiation) Combined Bouger-Beer’s Law or simply Beer’s Law: 𝑙𝑜𝑔 𝑙𝑜𝑔
𝑃
𝑃 = 𝐾 𝑏𝐶
𝐴 = 𝛼𝑏𝐶 𝐴 = 𝜀𝑏𝐶 𝜀 = 𝛼 𝑥 𝑀𝑊 𝑇 =
𝑃 𝑃
%𝑇 = 𝑃 𝑃 𝑥 100%
Application of Beer’s Law to Mixtures: Beer’s Law is applicable also to solutions containing more than one kind of absorbing substances provided there is no interaction that occurs among the various species; total A for the multicomponent system is the sum of individual As 𝐴 ž1;'9 = 𝐴 ‚ + 𝐴 } + 𝐴
• + ⋯ + 𝐴
2 𝐴 ž1;'9 = 𝜀 ‚ 𝑏𝐶 ‚ + 𝜀
} 𝑏𝐶 } + 𝜀 • 𝑏𝐶 • + ⋯ + 𝜀
2 𝑏𝐶 2 Spectroscopic Method for Chemical Analysis – non- stoichiometric; need a calibration curve
Instrumentation: Spectrophotometer – instrument for measuring transmittance or absorbance of a sample as a function of λ or at a single λ; may be manual (single beam) or recording (double beam) Basic Components: 1. Light Source 2. Wavelength Selector 3. Sample 4. Detector 5. Signal Processor and Readout Region λ
180-380 nm Visible
380-780 nm Near – IR 0.78 – 2.5 ìm
Mid – IR 2.5 -50 ìm Best Light Source for UV – H 2 & D
2 Lamps
Best Light Source for Visible Light – Tungsten Lamps Limitations of Beer’s Law: 1. only monochromatic light at a single wavelength 2. accounted only in relatively concentrated solutions (usually >0.01M) 3. apparent chemical derivations – when analyte dissociates, associates or reacts with a solvent to generate a product with a different absorption Most common monochromators – gratings (prisms can also be used but not often) Sample Container – fused silica or quartz Detector – transducer (converts one form of signal into another) that converts radiant energy into electrical signal; photon detectors like phototubes Read-Out Device – meter or recorder Monochromatic light – same density – equitransparent cuvettes Master grate – diamond cutter; evenly distributes grooves for grating Grating rotates – angle changes the λ passing through Photosensitive surface turns into photon detectors – electrons will be emitted. If the electron flows, current is developed. The current is converted into voltage since voltage is easily increased 𝑖 = 𝑘𝑉
Standard Addition Method - to minimize effects of sample matrix; add one or more increments of standard solution to sample aliquots of same size with the same final volume Analyte/Sample (V x = aliquot, V T = total volume) 𝐴 ‚
𝜀 W 𝑉 3 𝐶 3 𝑉 ž Analyte + Standard: 𝐴 } = 𝜀 W 𝑉 3 𝐶 3 𝑉 ž + 𝜀 W 𝑉 ( 𝐶 ( 𝑉 ž Concentration of Sample:
P 0
P b
Absorbing Sol’n with Conc. C
𝐶 L = 𝐴 ‚ 𝐶 ( 𝑉 ( (𝐴 } − 𝐴 ‚ )𝑉 3 If weak acid, control pH with a buffer in order to make one form predominate by controlling pH Download 383.93 Kb. Do'stlaringiz bilan baham: 
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