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Lecture Date: January 14 th , 2008

Introduction to Analytical Chemistry. Lecture Date: January 14 th , 2008. What is Analytical Chemistry?. Analytical chemistry seeks ever improved means of measuring the chemical composition of natural and artificial materials

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Lecture Date: January 14 th , 2008

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  1. Introduction to Analytical Chemistry Lecture Date: January 14th, 2008

  2. What is Analytical Chemistry? Analytical chemistry seeks ever improved means of measuring the chemical composition of natural and artificial materials The techniques of this science are used to identify the substances which may be present in a material and determine the exact amounts of the identified substances Qualitative:provides information about the identity of an atomic, molecular or biomolecular species Quantitative:provides numerical information as to the relative amounts of species Definitions from www.acs.org

  3. The Role of Analytical Chemistry • -Friedrich Wilhelm Ostwald • “Analytical Chemistry, or the art of recognizing different substances and determining their constituents, takes a prominent position among the applications of science, since the questions which it enables us to answer arise wherever chemical processes are employed for scientific or chemical purposes.” http://www.pace.edu/dyson/academics/chemistryplv/

  4. The Role of Analytical Chemistry Analytical chemists work to improve the reliability of existing techniques to meet the demands of for better chemical measurements which arise constantly in our society They adapt proven methodologies to new kinds of materials or to answer new questions about their composition. They carry out research to discover completely new principles of measurements and are at the forefront of the utilization of major discoveries such as lasers and microchip devices for practical purposes. Medicine Industry Environmental Food and Agriculture Forensics Archaeology Space science

  5. History of Analytical Methods Classical methods:early years (separation of analytes) via precipitation, extraction or distillation Qualitative:recognized by color, boiling point, solubility, taste Quantitative:gravimetric or titrimetric measurements Instrumental Methods:newer, faster, more efficient Physical properties of analytes:conductivity, electrode potential, light emission absorption, mass to charge ratio and fluorescence, many more…

  6. Classification of Modern Analytical Methods • Gravimetric Methodsdetermine the mass of the analyte or some compound chemically related to it • Volumetric Methodsmeasure the volume of a solution containing sufficient reagent to react completely with the analyte • Electroanalytical Methodsinvolve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge • Spectroscopic Methodsare based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes • Miscellaneous Methodsinclude the measurement of such quantities as mass-to-charge ratio, rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index

  7. Analytical Methodology 1. Understanding and defining the problem 2. History of the sample and background of the problem 3. Plan of action and execution 4. Analysis and reporting of results

  8. 1. Understanding and Defining the Problem • What accuracy is required? • Is there a time (or money) limit? • How much sample is available? • How many samples are to be analyzed? • What is the concentration range of the analyte? • What components of the system will cause an interference? • What are the physical and chemical properties of the sample matrix? (complexity)

  9. 2. History of sample and background of the problem • Background info can originate from many sources: • The client, competitor’s products • Literature searches on related systems • Sample histories: • synthetic route • how sample was collected, transported, stored • the sampling process

  10. 3. Plan of Action Performance Characteristics: Figures of Merit Which analytical method should I choose? How good is the measurement, information content How reproducible is it? Precision How close to the true value is it? Accuracy/Bias How small of a difference can be measured? Sensitivity What concentration/mass/amount/range? Dynamic Range How much interference? Selectivity (univariate vs. multivariate) bias =  - xt s2 Sm = Sbl+ ksbl S = mc + Sbl

  11. 4. Analyzing and Reporting Results • No work is complete until the “customer” is happy! • Analytical data analysis takes many forms: statistics, chemometrics, simulations, etc… • Analytical work can result in: • peer-reviewed papers, etc… • how sample was collected, transported, stored • technical reports, lab notebook records, etc...

  12. Obtain and store sample Pretreat and prepare sample Perform measurement (instrumentation) Compare results with standards Apply required statistical techniques Present information Verify results Components of an Analytical Method Extract data from sample Covert data into information After reviewing results might be necessary to modify and repeat procedure Transform information into knowledge Handbook, Settle

  13. Techniques Separation Techniques Gas chromatography High performance liquid chromatography Ion chromatography Super critical fluid chromatography Capillary electrophoresis Planar chromatography Spectroscopic techniques Infrared spectrometry (dispersive and fourier transform) Raman spectrometry Nuclear magnetic resonance X-ray spectrometry Atomic absorption spectrometry Inductively coupled plasma atomic emission spectrometry Inductively coupled plasma MS Atomic fluorescence spectrometry Ultraviolet/visible spectrometry (CD) Molecular Fluorescence spectrometry Chemiluminescence spectrometry X-Ray Fluorescence spectrometry

  14. More Techniques Mass Spectrometry Electron ionization MS Chemical ionization MS High resolution MS Gas chromatography MS Fast atom bombardment MS HPLC MS Laser MS Electrochemical techniques Amperometric technique Voltammetric techniques Potentiometric techniques Conductiometric techniques Microscopic and surface techniques Atomic force microscopy Scanning tunneling microscopy Auger electron spectrometry X-Ray photon electron spectrometry Secondary ion MS

  15. Technique Selection • Location of sample • bulk or surface • Physical state of sample • gas, liquid, solid, dissolved solid, dissolved gas • Amount of Sample • macro, micro, nano, … • Estimated purity of sample • pure, simple mixture, complex mixture • Fate of sample • destructive, non destructive • Elemental information • total analysis, speciation, isotopic and mass analysis • Molecular information • compounds present, polyatomic ionic species,functional group, structural, molecular weight, physical property • Analysis type • Quantitative, Qualitative • Analyte concentration • major or minor component, trace or ultra trace

  16. An Example: HPLC vs. NMR limited T,S (ion)

  17. Review of Background Material • Chemical equilibrium • Activity coefficients • Ionic strength • Acids and bases • Titrations • Other simple chemical tests (“spot tests”) • Some important figures of merit • Review of a few other helpful concepts

  18. Chemical Equilibrium • There is never actually a complete conversion of reactants to product in a chemical reaction, there is only a chemical equilibrium. • A chemical equilibrium state occurs when the ratio of concentration of reactants and products is constant. An equilibrium-constant expression is an algebraic equation that describes the concentration relationships that exist among reactants and products at equilibrium aA + bB  cC + dD K = [C]c [D]d / [A]a [B]b

  19. Chemical Equilibrium

  20. Typical Equilibrium Constant Expressions Dissociation of water 2H2O  H3O+ + OH- Kw = [H3O+ ][OH-] Acid base NH3 + H2O  NH4+ + OH- Kb = [NH4+][OH-] / [NH3] Solubility PbI2(s)  Pb2+ + 2I- Ksp = [Pb2+ ][I-]2 Oxidation-Reduction IO3- + 5I- + 6H+  3I2(aq) + 3H20 Keq = [I2]3 / [IO3-][I-]5[H+]6 Cl2(g) + 2AgI(s)  2AgCl(s) + I2 (g) Keq = pI2/ pCl2

  21. Activity Coefficients The law of mass action breaks down in electrolytes. Why? Ions in solution have electrostatic interactions with other ions. Neutral solutes do not have such interactions. When the concentrations of ions in a solution are greater than approximately 0.001 M, a shielding effect occurs around ions. Cations tend to be surrounded by nearby anions and anions tend to be surrounded by nearby cations. This shielding effect becomes significant at ion concentrations of 0.01 M and greater. Doubly or triply charged ions "charge up" a solution more than singly charged ions, so we need a standard way to talk about charge concentration.

  22. NON-IDEAL IDEAL [ ] < 10-3 [ ] > 10-3 Activity Coefficients Dilute solutions and concentrated solutions have slight differences and a more precise method of calculating and defining the equilibrium constant is needed: ax = x [C]  < 1 in dilute solutions--  = 1

  23. Effect of Electrolyte Concentration Reason for deviation: The presence of electrolytes results in electrostatic interactions with other ions and the solvent The effect is related to the number and charge of each ion present - ionic strength ( )  = 0.5 ( [A] ZA2 + [B] ZB2 + [C]ZC2 + …..) where Z = charge (ex. +1, -2, …)

  24. Ionic Strength: Definitions Dissociation of an electrolyte: MxXm xMm+ + mXx- Ionic Strength:  = 0.5  zi2Ci Activity coefficient: ai =  i [X]I Debye-Huckel limiting Law relates activity coefficient to ionic strength Mean ionic activity: a =  C (mmxx) 1/(m+x)

  25. Ionic Strength Calculations: Examples What is the ionic strength for a 1.0 M NaCl solution? I = 1/2(1*12 +1*12) I = 1 What is the ionic strength for a solution whose concentrations are 1.0 M La2(SO4)3 plus 1.0 M CaCl2 for this solution the concentrations are: [La 3+] = 2.0 M [SO42-] = 3.0 M [Ca 2+] = 1.0 M [Cl -] = 2.0 M I = 1/2 (2*32 + 3*22 + 1*22 + 2*12) I = 18

  26. ACID BASE HA + H2O  A- + H3O+ NH3 + H2O  NH4+ + OH- Ka = [A- ] [H3O+ ] / [HA] Kb = [NH4+][OH-] / [NH3] Aqueous Solution Equilibria • Equilibria classified by reaction taking place • 1) acid-base • 2) oxidative-reductive • Bronsted-Lowry definitions:acid: anything that donates a [H+] (proton donor)base: anything that accepts a [H+] (proton acceptor) HNO2 + H2O  NO2- + H3O+

  27. Strength of Acids and Bases Source: www.aw.com/mathews/ch02/fi2p22.htm

  28. p-Functions The p- value is the negative base-10 logarithm of the molar concentration of a certain species: pX = -log [X] = log 1/[X] The most well known p-function is pH, the negative logarithm of [H3O+]. pH = - log [H3O+] pKw = pH + pOH = 14 We can also express equilibrium constants for the strength of acids and bases in a log form pKa = - log(Ka) pKb = - log (Kb) Kw = Ka * Kb

  29. Strength of Acids and Bases Source: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB17_03.JPG

  30. Amphiprotic Compounds • Amphiprotic solvents: a solvent that can act as either an acid or base depending on the solute it is interacting with • methanol, ethanol, and anhydrous acetic acid are all examples of amphiprotic solvents. NH3 + CH3OH  NH4+ + CH3O- CH3OH+ HNO2 CH3OH2+ + NO2- • Zwitterions: an amphiprotic compound that is produced by a simple amino acid’s weak acid an weak base functional groups • Zwitterions carry both a positive charge (amino group) and negative charge (carboxyl group)

  31. AdvantagesDisadvantages great flexibility large amount of analyte required suitable for a wide range of analytes lacks speciation (similar structure) manual, simple colorimetric -subjective excellent precision an accuracy sensitive to skill of analyst readily automated reagents unstable Titrations • Definition: an analytical technique that measures concentration of an analyte by the volumetric addition of a reagent solution (titrant)- that reacts quantitatively with the analyte • For titrations to be useful, the reaction must generally be quantitative, fast and well-behaved

  32. Mass Moles Moles Mass Divide by molar mass Multiply by stoichiometric ratio Multiply by molar mass Chemical Stoichiometry Stoichiometry: The mass relationships among reacting chemical species. The stoichiometry of a reaction is the relationship among the number of moles of reactants and products as shown by a balanced equation.

  33. Titration Curves Strong acid - Strong base Strong base - Weak acid

  34. Titration Curves Strong base - polyprotic acid

  35. Buffer Solutions • Buffers contain a weak acid HA and its conjugate base A- • The buffer resists changes in pH by reacting with any added H+ or OH-, preventing their accumulation. How? • Any added H+ reacts with the base A-: • H+ (aq) + A- (aq) -> HA(aq) (since A- has a strong affinity for H+) • Any added OH- reacts with the weak acid HA: • OH- (aq) + HA (aq) -> H2O + A-(aq) (since OH- can steal H+ from A-) • Example: if 1 mL of 0.1 N HCl solution to 100 mL water, the pH drops from 7 to 3. If the 0.1 N HCl is added to a 0.01 M solution of 1:1 acetic acid/sodium acetate, the pH drops only 0.09 units.

  36. Calculating the pH of Buffered Solutions Henderson-Hasselbach equation

  37. Example 1 • 30 mL of 0.10M NaOH neutralised 25.0mL of hydrochloric acid. Determine the concentration of the acid • 1.Write the balanced chemical equation for the reactionNaOH(aq) + HCl(aq) -----> NaCl(aq) + H2O(l) • 2.Extract the relevant information from the question:NaOH V = 30mL , M = 0.10M HCl V = 25.0mL, M = ? • 3.Check the data for consistencyNaOH V = 30 x 10-3L , M = 0.10M HCl V = 25.0 x 10-3L, M = ? • 4.Calculate moles NaOH n(NaOH) = M x V = 0.10 x 30 x 10-3 = 3 x 10-3 moles • 5.From the balanced chemical equation find the mole ratio NaOH:HCl 1:1

  38. Example 1 (continued) 6.Find moles HClNaOH: HCl is 1:1 So n(NaOH) = n(HCl) = 3 x 10-3 moles at the equivalence point Calculate concentration of HCl: M = n ÷ V n = 3 x 10-3 mol, V = 25.0 x 10-3L M(HCl) = 3 x 10-3 ÷ 25.0 x 10-3 = 0.12M or 0.12 mol L-1

  39. Example 2 • 50mL of 0.2mol L-1 NaOH neutralised 20mL of sulfuric acid. Determine the concentration of the acid • 1.Write the balanced chemical equation for the reaction NaOH(aq) + H2SO4(aq) -----> Na2SO4(aq) + 2H2O(l) • 2.Extract the relevant information from the question:NaOH V = 50mL, M = 0.2M H2SO4 V = 20mL, M = ? • 3.Check the data for consistencyNaOH V = 50 x 10-3L, M = 0.2M H2SO4 V = 20 x 10-3L, M = ? • 4.Calculate moles NaOH n(NaOH) = M x V = 0.2 x 50 x 10-3 = 0.01 mol • 5.From the balanced chemical equation find the mole ratio NaOH:H2SO4 2:1

  40. Example 2 (continued) 6.Find moles H2SO4 NaOH: H2SO4 is 2:1 So n(H2SO4) = ½ x n(NaOH) = ½ x 0.01 = 5 x 10-3 moles H2SO4 at the equivalence point 7.Calculate concentration of H2SO4: M = n ÷ V n = 5 x 10-3 mol, V = 20 x 10-3L M(H2SO4) = 5 x 10-3 ÷ 20 x 10-3 = 0.25M or 0.25 mol L-1

  41. Notes on Solutions and Their Concentrations Molar Concentration or Molarity – Number of moles of solute in one Liter of solution or millimoles solute per milliliter of solution. Analytical Molarity – Total number of moles of a solute, regardless of chemical state, in one liter of solution. It specifies a recipe for solution preparation.  Equilibrium Molarity – (Species Molarity) – The molar concentration of a particular species in a solution at equilibrium. Percent Concentration a. percent (w/w) = weight solute X 100% weight solution b.volume percent (v/v) = volume solute X 100% volume solution c.weight/volume percent (w/v) = weight solute, g X 100% volume soln, mL

  42. Some Other Important Concepts • Limit of detection (LOD): the lowest amount (concentration or mass) of an analyte that can be detected at a known confidence level • Linearity: the degree to which a response of an analytical detector to analyte concentration/mass approximates a linear function Limit of linearity Slope relates to sensitivity LOQ Detector response LOD Dynamic range Concentration • Limit of quantitation (LOQ): the range over which quantitative measurements can be made (usually the linear range), often defined by detector dynamic range • Selectivity: the degree to which a detector is free from interferences (including the matrix or other analytes)

  43. Simple Chemical Tests • While most of this class is focused on instrumental methods, a very large number of simple chemical tests have been developed over the past ~300 years • Examples: • Barium: solutions of barium salts yield a white precipitate with 2 N sulfuric acid. This precipitate is insoluble in hydrochloric acid and in nitric acid. Barium salts impart a yellowish-green color to a nonluminous flame that appears blue when viewed through green glass. • Phosphate: With silver nitrate TS, neutral solutions of orthophosphates yield a yellow precipitate that is soluble in 2 N nitric acid and in 6 N ammonium hydroxide. With ammonium molybdate TS, acidified solutions of orthophosphates yield a yellow precipitate that is soluble in 6 N ammonium hydroxide. Examples are from US Pharmacopeia and National Formulary USP/NF

  44. A Colormetric Test for Mercury • A modern example of a “spot” test: a test for Hg2+ developed using DNA and relying on the formation of a thymidine-Hg2+-thymidine complex • LOD = 100 nM (20 ppb) in aqueous solution • Linearity from the high nanomolar to low micromolar range • Selective for Hg2+ and insensitive to Mg2+, Pb2+, Cd2+, Co2+, Zn2+, Ni2+, and other metal ions Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700269 http://pubs.acs.org/cen/news/85/i19/8519news6.html

  45. Concentration in Parts per Million/Billion ppm: cppm = mass of solute X 106 ppm mass of solution For dilute aqueous solutions whose densities are approximately 1.00 g/mL, 1 ppm = 1 mg/L ppb: cppb = mass of solute X 109 ppb mass of solution

  46. Density and Specific Gravity of Solutions Density:The mass of a substance per unit volume. In SI units, density is expressed in units of kg/L or g/mL. Specific Gravity:The ratio of the mass of a substance to the mass of an equal volume of water at 4 degrees Celsius. Dimensionless (not associated with units of measure).

  47. Other Helpful Information Prefixes for SI Units giga- G 109 mega- M 106 kilo- k 103 deci- d 10-1 centi- c 10-2 milli- m 10-3 micro- u 10-6 nano- n 10-9 pico- p 10-12 femto- f 10-15 atto- a 10-18

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