Intro to Analytical Chemistry08

 

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Introduction to Analytical Chemistry

Lectu re Date: January 14th , 2008

Whatt is Analytic al Chemistry? Wha  Analytical chemistry seeks ever improved means of measuring the chemical composition of natural and artificial artifici al materials The techniques of this science are used to identify the substances which may be present in a material mater ial 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

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The Role of Analytical Chemistry -Friedrich Wilhelm Ostwald  “Analytical Chemistry, or the art of recognizing different substances and determining determi ning thei t heirr constituents, co nstituents, takes a  prominent  promi nent 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/dy http://www.pace .edu/dyson/acade son/academics/chemistryplv/ mics/chemistryplv/

The Role of Analytical Chemistry  Analytical chemists work work to improve the reliability of existing 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 m aterials or to answer answer new questions about their composition. They carry out research to discover completely completel y new principles of measurements and are at the forefront of the utilization of major discoveries such as lasers and m icrochip devices for practical purposes. Medicine Industry Environmental Food and Agriculture Forensics  Archaeology Space science

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

Classification Cla ssification of Modern Analyti Analyti cal Methods Gravimetric vimetric Method s determine the mass of the analyte or some  Gra compound chemically related to it

 Volum etric Methods measure the volume of a solution containing sufficient reagent to react completely completel y with with the analyte

 Ele Electroanalytical ctroanalytical Me Method thod s invol involve ve the measurement of such such electrical properties as voltage, volt age, current, resistance, and quantity of electrical charge

 Spectroscopi c Me Method thod s are based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes anal ytes Miscellaneous ous Me Method thod s include the measurement of such  Miscellane quantities as mass-to-charge ratio, rate of radioactive radioacti ve decay, decay, heat of reaction, rate of reaction, sample thermal therm al conductivity, conductiv ity, optical activity, and refractive refracti ve index

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 An  Analy alyti ti c al Meth Method odol ol o g y

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

1. Unde Understandin rstandin g and Definin Defining g the Problem • What acc accura uracy cy is requi required red? ? •• • • •

Is there there a tim (orlemo money) lim limit? it? How much mu chtime saemple samp is ney) available? availa ble? How many many samp samples les are to be analy analyzed? zed? What is the the concentr concentration ation range range of of the analyte? analyte? What com compone ponents nts of the syste system m will cause cause an interference? • What are are the physical physical and and chemical chemical properties properties of the sample matrix? (complexity)

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2. History of sample and and background backgro und of the problem

Background info can originate from many sources: sources: • The client, comp competitor’s etitor’s products • Literature searches searches on related system systems s • Sam Sample ple histories histories:: • synt synthetic hetic rout route e • how sample sample was collected, transported, stored • the samp sampling ling process process

3. Plan of Action Acti on Performance Performa nce 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? it ? Accuracy/Bias How small of a difference diff erence can be measured? Sensitivity What concentration/mass/amount/range? Dynamic Range How much interference? Selectivity (univariate vs. multivariate)

S m   

2

 N 

  x    x  s

bias =  - xt

s

s2

 N 

i

i 1

 N  1

 RSD 

Sm =     Sbl+ ksbl s

  x 

  CV  

s

  x 

100%

cm 

S  m  Sbl m

S = mc + Sbl

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

Components of an Analytic Analytic al Method Obtain and store sample Extract data from sample

Pretreat and prepare sample Perform measurement (instrumentation) Compare results with standards

Covert data into information

 Apply require required d statistical technique techniques s Verify results

Transform information into knowledge

 Aft er rev iewin g r esul ts might be n ecessa ecessary ry to modify and repeat repeat procedure

Present information Handbook, Settle

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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

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 Vo ltammetric 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

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Technique Selection Location of sample bulk or surface Physical state of sample gas, liquid, solid, dissolved solid, dissolved gas  Amo unt of Sampl e 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  Anal ys is ty pe Quantitative, Qualitative  Anal yt e co ncen tr atio n major or minor component, trace or ultra trace

 An Exam pl e: HPLC vs. v s. NMR HPL C

NMR

B

B

L, Ds

L, S, Ds

mac ro, mi cro

Ma, Mi

Ma, Mi

pure, simple mix ture, c omplex mixture

S m ,M

P ,S m

N,D

N

total analysis, speciation, isotopic and mass analysis

T,S (ion)

limited

Compounds p prres e en nt , P Po oly a attomic io ionic sp speci es , Functional group, Structural, MW, Physical prop

Cp, Io, St St

Cp,Fn, St

Ql,Qt

Ql,Qt

Location of sample bulk or s urface Physical state of sample  Am oun t o f Sa mp le

gas , li quid, sso olid, d diis solved solid, d diis s olved gas

Estima Estima ted purity of sample Fate of sample des truct ive, non dest ructi ve Elemental information Molecular information

 Ana ly sis ty pe Quant itative, Qualit at ive

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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

Chemical Che mical Equilibrium Equilibr ium



There is never actually a complete conversion of reactants to product in a chemical reaction, there is only a chemical equilibrium.

chemical equilibrium equilibrium state state occurs occurs when when the ratio of   Aconcentration of reactants and products is constant. An equilibrium-constant expression equilibrium-constant expression is an algebraic equation that describes the concentration relationships that that exist among reactants and products at equilibrium

aA + bB  cC + dD K = [C]c [D]d / [A]a [B]b

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Che Chemical mical Equilibrium Equilibr ium

Typical Equilibri um Constant Expressions  Dissociation of water 2H2O  H3O+ + OH-

K w = [H3O+ ][OH-]

 Acid base  NH3 + H2O  NH4+ + OH-

K  b = [NH4+][OH-] / [NH3]

Solubility PbI2(s)  Pb2+ + 2I-

K sp = [Pb2+ ][I-]2

Oxidation-Reduction IO3- + 5I- + 6H+  3I2(aq) + 3H20

K eq  = [I2]3 / [IO3-][I-]5[H+]6

Cl2(g) + 2AgI(s)  2AgCl(s) + I2 (g)

K eq  = pI2/ pCl2

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 Acti ti vi vitt y Coef Coeffi fi ci en entt s  Ac

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 sh ielding effect becomes significant at ion concentrations of 0.01 M and greater. greater. Doubly or triply charged ions "charge up" a solution more than singly charged ions, so we need a standard stan dard way to talk about charge concentration.

 Acti  Ac ti vi vitt y Coef Coeffi fi ci en entt s Dilute solutions and concentrated solutions have slight differences di fferences and a more precise method of calculating and defining the equilibrium constant is needed:

ax =  x [C]

IDEAL

NON-IDEAL

[ ] < 10-3 in dilute solutions--    = 1

[ ] > 10-3  

  < 1

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Effect ct of Ele Electrol ctrol yte Concentratio Concentratio n Effe

Reason for deviation: The presence of electrolytes results in electrostatic interactions interactions with other ions and the solvent The effect is related to the number and charge of each ion pre prese sent nt - ion ionic ic str streng ength th (  )

  = 0.5 ( [A] Z A2 + [B] ZB2 + [C]ZC2 + …..) where Z = charge (ex. +1, -2, …)

Ionic Strength: Strength: De Definiti finiti ons Dissociation of an electrolyte: MxXm  xMm+ + mXxIonic Strength:

  = 0.5    z 2C i

i

 Activity coefficient: ai =   i [X]I

Debye-Huckel limiting Law relates activity coefficient to ionic strength 2

log  i 

 0.51 z i

 



1  3.28 i  



 

Mean ionic activity: a =   C (mmxx) 1/(m+x)

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Ionic Strength Calculations: Exa Examples mples 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 CaCl 2 for this solution soluti on 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

 Aqueo  Aq ueous us Sol ut utii on Equ Equilil i br ia Equilibria classified by reaction taking place 1) acid-base 2) oxidative-reductive Bronsted-Lowry definitions: Bronsted-Lowry defini tions: acid: anything that donates a [H+] (proton donor) base: anything that accepts a [H+] (proton acceptor)

HNO2 + H2O  NO2- + H3O+  ACID

BASE

HA + H2O  A- + H3O+

 NH3 + H2O  NH4+ + OH-

K a = [A- ] [H3O+ ] / [HA]

K  b = [NH4+][OH-] / [NH3]

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Strength of Acids and Bases Bases

Source: www.aw.com/ma www.aw.com/mathews/ch02/fi2p22.htm thews/ch02/fi2p22.htm

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

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Strength of Acids and Bases Bases

Source: http://cwx.pren http://cwx.prenhall.com/petrucci/med hall.com/petrucci/medialib/media_portfolio/text_images/T ialib/media_portfolio/text_images/TB17_03.JPG B17_03.JPG

 Am ph phip iprr ot ic Com po poun und ds

  Amphiprotic solvents: a solvent that can act as either an acid or base depending on the solute it is interacting i nteracting with  – methanol, methano ethanol, and anhydrous anhy drous acetic acid are all examples exam plesl, of amphiprotic solvents. NH3 + CH3OH  NH4+ + CH3OCH3OH + HNO HNO2   CH3OH2+ + NO2-



Zwitterions: an amphiprotic compound 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)

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Titrations



Definition: an analytical technique technique that measures measures concentration of an analyte by the volumetric addition of a reagent solution (titrant)(titrant)- that reacts quantitatively with the analyte



For titrations to be useful, the reaction must generally be quantitative, fast and well-behaved well -behaved

 Advantages  Advantages great flexibility suit su itab able le fo forr a wid wide e ran range ge of of ana analy lyte tes s manual, sim ple excellent precision an accuracy readily automated

Disadvantages Disadvantages large amount of analyte required lack la cks s spe speci ciat atio ion n (si (simil milar ar st stru ruct ctur ure e) colorimetric -subjective sensitive to skill of analyst reagents unstable

Che Chemical mical Stoichiom etry Stoichiometry: The mass relationships among reacting chemical chem ical species. The stoichiometry stoichiometry of a reaction is the the relationship among the number of moles of reactants and products as shown by a balanced equation.

M as s

M o l es

M o l es

M as s

Divide by molar mass Multiply by stoichiometric ratio

Multiply by molar mass

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Titration Curve Curves s

Strong Stro ng acid - Stro Strong ng base

Strong Str ong bas base e - Weak aci acid d

Titration Curves

Strong Stron g base - polyp polyprotic rotic acid acid

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Buffer Solutions

 

Buffers contain a weak acid HA and its conjugate base A The buffer resists changes changes in pH by reacting with any added H+ or OH-, preventing their accum accumulation. ulation. How?  – Any added added H+ reacts reacts with the base A-:

 H+ (aq) + A- (aq) -> HA(aq) (since A- has a strong affinity for H+)  – Any added added OH- reacts with the weak acid HA:

 OH- (aq) + HA (aq) -> H 2O + 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 drops from 7 to 3. If the the 0.1 N HCl is added to a 0.01 M solution of 1:1 acetic acid/sodium acetate, acetate, the pH drops only 0.09 units.

Calculating Ca lculating the pH of Buffered Buffered Solutions

Henderson-Hasselbach Henderson-Has selbach equation

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Example 1 30 mL of 0.10M NaOH neutralised 25.0mL of of hydrochloric h ydrochloric acid. Determine the concentration of the acid  1.Write the balanced chemical equation for 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 0.1 0M 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  NaOH:H Cl 1:1

Example 1 (continued)

6.Find moles HCl  NaOH: HCl HCl is 1:1 So n(NaOH) = n(HCl) = 3 x 10-3 moles at the equivalence point Calculate concentration of HCl: 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

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Example 2 50mL of 0.2mol L-1 NaOH neutralised 20mL of of sulfuric acid. Determine the concentration of the acid  1.Write the balanced chemical equation for the th e 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 find the th e mole ratio  NaOH:H2SO4 2:1

Example 2 (continued) 6.Find moles H2SO4  NaOH: H2SO4 is 2:1 So n(H2SO4) = ½ x n(NaOH) 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 1 10 0-3 = 0.25M or 0.25 mol L-1

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Notes Note s on Solutions Solution s and Their Concentratio Concentrations ns Molar Concentration or Molarity – Number of moles moles of solute in one Liter of of solution or millimoles solute per milliliter m illiliter of solution. Analytical Molarity – Total number of of moles of a solute, regardless of chemical state, in one liter of of solution. It specifies specifies a recipe r ecipe for for solution preparation. Equilibrium Molarity – (Species Molarity) Molarity) – The molar concentration concentration of of a  particular species in a solution solution at equilibrium.

Percent Concentration a. percent (w/w (w/w)) = weight solute X 100% weight solution  b.volume percent (v/v) (v/v) = volume solute solute X 100% volume solution c.weight/volume percent (w/v) (w/v) = weight solute, g X 100% volume soln, mL

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   e   s   n   o   p   s   e   r   o    t   c   e    t   e    D

Slope relates to sensitivity

LOQ LOD Dynamic range

Concentration



Limit of quantitation quantitatio n (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)

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Chemical Tests Simpl e Chemical

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While most 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

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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 i mpart a yellowish-green color to a nonluminous flame fl ame that appears blue when viewed through green glass.  – Phosphate: Wit With h 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. hydroxide. With ammonium molybdate TS, acidified solutions of orthophosphates yield a yellow precipitate that is soluble in 6 N ammonium hydroxide. Examples Examp les are from US Pharmacopeia and National Formulary USP/NF

 A Color Col or m etr ic Test fo forr Mer Mercu curr y

  A modern example example of a “spot” test: a test for Hg2+ developed using DNA and relying on the formation of a thymidineHg2+-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: 10.10 02/anie.2007002 02/anie.200700269 69 http://pubs.acs.org/cen/news/85/i19/8519n http://pubs.acs.org/cen/news /85/i19/8519news6.htm ews6.htmll

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Concentration Conce ntration in Parts Parts per Million/Billi on 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

Density De nsity and Specific Specific Gra Gravity vity of Solutions Density: The mass 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).

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Otherr Helpful Information Othe Prefixes for SI Units gigaG m egaM kilok decid centic m illim m icrou nanon picop f em tof attoa

109 106 103 10-1 10-2 10-3 10-6 10-9 10-12 10-15 10-18

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