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Introduction to Analytical Chemistry

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1 Introduction to Analytical Chemistry

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

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.”

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 All branches of chemistry draw on the ideas and techniques of analytical chemistry

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 Methods determine the mass of the analyte or some compound chemically related to it Volumetric Methods measure the volume of a solution containing sufficient reagent to react completely with the analyte Electroanalytical Methods involve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge Spectroscopic Methods are based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes Miscellaneous Methods include 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 s2
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 Components of an Analytical Method
Obtain and store sample Pretreat and prepare sample Extract data from sample Perform measurement (instrumentation) Compare results with standards 3-Processing the Sample -Preparing Solutions: Physical and Chemical Changes -Most analyses are performed on solutions of the sample made with a suitable solvent. Ideally, the solvent should dissolve the entire sample, including the analyte, rapidly and completely. The conditions of dissolution should be sufficiently mild that loss of the analyte cannot occur or is minimized. -Conversion of the analyte in insoluble materials into a soluble form is often the most difficult and time-consuming task in the analytical process -At this point in the analysis, it may be possible to proceed directly to the measurement step, but more often than not, we must eliminate interferences in the sample before making measurements 4-Eliminating Interferences -Eliminate substances from the sample that may interfere with the measurement step -Species other than the analyte that affect the final measurement are called interferences -A scheme must be devised to isolate the analytes from interferences before the final measurement is made. -Techniques or reactions that work for only one analyte are said to be specific -Techniques or reactions that apply for only a few analytes are selective 5-Calibration and Measurement -All analytical results depend on a final measurement X of a physical or chemical property of the analyte, which must vary in a known and reproducible way with the concentration of the analyte -Ideally, the measurement of the property is directly proportional to the concentration, where k is the proportionality constant -The process of determining k is called calibration 6-Calculating Results -Computing analyte concentrations from experimental data is usually relatively easy -Computations are based on raw experimental data collected in the measurement step, the characteristics of the measurement instruments, and the stoichiometry of the analytical reaction 7-Evaluating Results by Estimating Their Reliability -Analytical results are incomplete without an estimate of their reliability. -The experimenter must provide some measure of the uncertainties associated with computed results if the data are to have any value Apply required statistical techniques Covert data into information Verify results After reviewing results might be necessary to modify and repeat procedure Transform information into knowledge Present information 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 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+ NH3 + H2O  NH4+ + OH- Kb = [NH4+][OH-] / [NH3] BASE HA + H2O  A- + H3O+ Ka = [A- ] [H3O+ ] / [HA] ACID

16 Strength of Acids and Bases
Source:

17 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

18 Strength of Acids and Bases
Source:

19 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 Advantages Disadvantages 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

20 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. Mass Moles Divide by molar mass Multiply by stoichiometric ratio Multiply by molar mass

21 Titration Curves Strong acid - Strong base Strong base - Weak acid

22 Titration Curves Strong base - polyprotic acid

23 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 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.

24 Calculating the pH of Buffered Solutions
Henderson-Hasselbach equation

25 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 reaction NaOH(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 consistency NaOH 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

26 Example 1 (continued) 6.Find moles HCl NaOH: 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

27 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 consistency NaOH 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

28 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

29 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

30 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 Detector response LOQ 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)

31 Concentration in Parts per Million/Billion
ppm: cppm = mass of solute X 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 ppb

32 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).

33 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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