Analytical Chemistry I PDF

Analytical Chemistry I OUTLINE ▪ Review of Basic Concepts ▪ Gravimetric Analysis ▪ Quantitative Chemical Analysis Analytical Chemistry 2 DEFINITION Analytical Chemistry ▪ a measurement science consisting of a set of powerful ideas and methods that are...

Analytical Chemistry I OUTLINE ▪ Review of Basic Concepts ▪ Gravimetric Analysis ▪ Quantitative Chemical Analysis Analytical Chemistry 2 DEFINITION Analytical Chemistry ▪ a measurement science consisting of a set of powerful ideas and methods that are useful in all fields of science, engineering, and medicine Qualitative Analysis ▪ reveals the identity of the elements and compounds in a sample Analytical Chemistry 3 DEFINITION Quantitative Analysis ▪ indicates the amount of each substance in a sample Analyte ▪ the components of a sample that are determined Analytical Chemistry 4 APPLICATIONS (Skoog, 2014) Analytical Chemistry 5 CLASSIFICATION OF METHODS According to: ▪ method ▪ extent of analysis ▪ amount of sample available for analysis Analytical Chemistry 6 METHOD Classical Methods ▪ Gravimetric Method: the mass of the analyte or some compound chemically related to it is determined ▪ Volumetric (Titrimetric) Method: the amount of analyte is determined by measuring the volume of a solution of known concentration Classification of Methods 7 METHOD Instrumental Methods ▪ Instrumental Methods: employ instruments other than the analytical balance Electroanalytical methods Spectroscopic methods Chromatographic methods Classification of Methods 8 EXTENT OF ANALYSIS Analysis Description Example Complete or Exact the amount of each Blood analysis gives constituent of the glucose, sodium, sample is determined potassium, bilirubin, alkaline phosphates, etc. Ultimate the amount of each Analysis of gasoline element is determined gives %C, %H, %O, %Pb, etc. Proximate or Partial the amount of a certain partial analysis of aspirin selected constituent in a tablets gives the amount sample is determined of salicylic acid impurity Classification of Methods 9 AMOUNT OF SAMPLE Analysis Mass of Sample Volume of Sample macro > 100 mg > 100 𝜇𝐿 semimicro 10 – 100 mg 50 – 100 𝜇𝐿 micro 1 – 10 mg < 50 𝜇𝐿 ultramicro < 1 mg Classification of Methods 10 CLASSIFICATION OF ANALYTES Major constituent > 1% of the sample Minor constituent 0.01 – 1% of the sample Trace constituent 0.001 – 0.01% of the sample Ultratrace constituent < 0.001% of the sample Analytical Chemistry 11 ANALYTICAL CHEMISTRY Stoichiometry ▪ describes the quantitative relationship among the amounts of reactants and products Analytical Chemistry 12 CONCENTRATIONS Molarity (M) ▪ refers to species concentration 𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 𝑚𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 = 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑚𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 Analytical Chemistry 13 CONCENTRATIONS Analytical Molarity (MX) or Formality (F) ▪ refers to analytical or total concentration; describes how a solution of a given molarity can be prepared 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 𝑟𝑒𝑔𝑎𝑟𝑑𝑙𝑒𝑠𝑠 𝑜𝑓 𝑖𝑡𝑠 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑠𝑡𝑎𝑡𝑒 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑡𝑜𝑡𝑎𝑙 𝑚𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 𝑚𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 Analytical Chemistry 14 CONCENTRATIONS Mass% (m/m or w/w) Mass/Volume % (m/v or 𝑚𝑎𝑠𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 w/v) = 𝑥100 𝑚𝑎𝑠𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑚𝑎𝑠𝑠 𝑠𝑜𝑙𝑢𝑡𝑒, 𝑔 = 𝑥100 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛, 𝑚𝐿 Volume% (v/v) 𝑣𝑜𝑙𝑢𝑚𝑒 𝑠𝑜𝑙𝑢𝑡𝑒 = 𝑥 𝑣𝑜𝑙𝑢𝑚𝑒 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 Analytical Chemistry 15 CONCENTRATIONS 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐴 6 𝑚𝑔 𝐴 𝑚𝑔 𝐴 𝑝𝑝𝑚 𝐴 = 𝑥 10 = ≅ 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑘𝑔 𝑎𝑞𝑢𝑒𝑠𝑜𝑢𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝐿 𝑎𝑞𝑢𝑒𝑠𝑜𝑢𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐴 9 𝜇𝑔 𝐴 𝜇𝑔 𝐴 𝑝𝑝𝑏 𝐴 = 𝑥 10 = ≅ 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑘𝑔 𝑎𝑞𝑢𝑒𝑠𝑜𝑢𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝐿 𝑎𝑞𝑢𝑒𝑠𝑜𝑢𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 ▪ Density: mass of a substance per unit volume (e.g. g/mL) ▪ Specific gravity: ratio of the density of a substance to the density of water Analytical Chemistry 16 CONCENTRATIONS Normality (N) 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 = 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 Titer (T) 𝑚𝑔 𝐴 = 𝑚𝐿 𝐵 ▪ mass of species A equivalent to 1 mL of solution B Analytical Chemistry 17 CONCENTRATIONS Calculation of Equivalent Mass (EM or EW) 𝐹𝑀 𝐸𝑀 = 𝑛 ▪ the value of n depends on the reaction; unit = eq / mol or meq / mmol How to determine the value of n in the ff reactions: ▪ Acid-base ▪ Redox ▪ Precipitate-Formation and Complex-Formation Analytical Chemistry 18 CONCENTRATIONS Acid-Base Reactions ▪ Acid: n is the number of replaceable or acidic H+ ▪ Base: n is the number of H+ required to neutralize each mole of a base Example: 𝐻3 𝑃𝑂4 + 2𝑁𝑎𝑂𝐻 → 𝑁𝑎2 𝐻𝑃𝑂4 + 2𝐻2 𝑂 EM of H3PO4 = FM/2 EM of NaOH = FM/1 Analytical Chemistry 19 CONCENTRATIONS Redox Reactions ▪ n is the number of electrons gained or lost in the reaction per mole of the species Example: 𝑀𝑛𝑂4− + 𝐶2 𝑂42− → 𝑀𝑛2+ + 𝐶𝑂2 The equation does not have to be balanced, but the atoms which gained or lost electrons have to be balanced 𝑀𝑛𝑂4− + 𝐶2 𝑂42− → 𝑀𝑛2+ + 2𝐶𝑂2 EM of KMnO4 = FM/5 EM of K2C2O4 = FM/2 Analytical Chemistry 20 CONCENTRATIONS Precipitate Formation and Complex Formation ▪ Metal cation: n = the ion charge ▪ For the anion: n = the number of metal ion equivalents that one mole of the anion reacts with Example: 𝐵𝑎2+ + 𝑆𝑂42− → 𝐵𝑎𝑆𝑂4 EM of 𝐵𝑎 𝑁𝑂3 2 = FM/2 EM of 𝑁𝑎2 𝑆𝑂4 = FM/2 Analytical Chemistry 21 CONCENTRATIONS For the general reaction: aA + bB → cC + dD mmol Approach milliequivalent approach 𝑎 #𝑚𝑚𝑜𝑙 𝐴 = #𝑚𝑚𝑜𝑙 𝐵 𝑥 #𝑚𝑒𝑞 𝐴 = #𝑚𝑒𝑞 𝐵 𝑏 𝑎 #𝑚𝑔𝐴 = #𝑚𝑚𝑜𝑙 𝐵 𝑥 𝑥𝐹𝑀𝐴 #𝑚𝑔 𝐴 = #𝑚𝑒𝑞 𝐵 𝑥 𝐸𝑀𝐴 𝑏 𝑏 𝐹𝑀𝐵 𝐸𝑀𝐵 #𝑚𝑔𝐵 = #𝑚𝑔 𝐶 𝑥 𝑥 #𝑚𝑔 𝐵 = #𝑚𝑔 𝐶 𝑥 𝑐 𝐹𝑀𝐶 𝐸𝑀𝐶 Analytical Chemistry 22 GRAVIMETRIC FACTOR #𝑚𝑜𝑙𝑠 𝑎𝑛𝑎𝑙𝑦𝑡𝑒 𝐹𝑀𝑎𝑛𝑎𝑙𝑦𝑡𝑒 𝐺𝐹 = 𝑥 #𝑚𝑜𝑙𝑠 𝑝𝑝𝑡 𝐹𝑀𝑝𝑝𝑡 Analytical Chemistry 23 EXAMPLE What is the mass in grams of Na+ (22.99 g/mol) in 25.0 g of Na2SO4 (142.0 g/mol)? Analytical Chemistry 24 EXAMPLE 1 𝑚𝑜𝑙 𝑁𝑎2 𝑆𝑂4 2 𝑚𝑜𝑙 𝑁𝑎+ 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑁𝑎+ = 𝑛𝑁𝑎+ = 25 𝑔 𝑁𝑎2 𝑆𝑂4 𝑥 𝑥 142 𝑔 𝑁𝑎2 𝑆𝑂4 𝑚𝑜𝑙 𝑁𝑎2 𝑆𝑂4 = 0.3521 𝑚𝑜𝑙 𝑁𝑎+ 22.9 𝑔 𝑁𝑎 + 𝑚𝑎𝑠𝑠 𝑁𝑎+ = 0.3521 𝑚𝑜𝑙 𝑁𝑎+ 𝑥 = 8.06 𝑔 𝑁𝑎 + 1 𝑚𝑜𝑙 𝑁𝑎+ Analytical Chemistry 25 EXAMPLE Describe the preparation of 500 mL of 0.0740 M Cl- solution from solid BaCl2∙2H2O (244.3 g/mol). Analytical Chemistry 26 EXAMPLE 0.0740 𝑚𝑜𝑙 𝐶𝑙 1 𝑚𝑜𝑙 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 𝑚𝑎𝑠𝑠 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 = 𝑥 0.500 𝐿 𝑥 𝐿 2 𝑚𝑜𝑙 𝐶𝑙 244.3 𝑔 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 𝑥 = 4.52 𝑔 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 𝑚𝑜𝑙 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 Dissolve 4.52 g of 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂 in water and dilute to 0.500 L or 500 mL Analytical Chemistry 27 CHEMICAL EQUILIBRIUM ▪ Dynamic equilibrium: 𝑟𝑎𝑡𝑒𝑓 = 𝑟𝑎𝑡𝑒𝑏 ▪ Equilibrium State: the ratio of concentrations of reactants and products is constant ▪ Le Châtelier principle: the position of an equilibrium always shifts in such a direction as to relieve a stress that is applied to the system ▪ Mass-action Effect: an equilibrium shift brought about by changing the amount of one or more participating species Analytical Chemistry 28 CHEMICAL EQUILIBRIUM Activity Effect or Salt Effect on Equilibrium ▪ generally, the presence of diverse salts (not containing ions common to the equilibrium involved) will shift equilibrium towards formation of more ions Analytical Chemistry 29 ACTIVITY ▪ the effective concentration of an ion in the presence of electrolytes 𝛼𝑖 = 𝛾𝑖 𝐶𝑖 𝛼𝑖 activity of ion i 𝛾𝑖 activity coefficient of ion i 𝐶𝑖 concentration of ion i Analytical Chemistry 30 IONIC STRENGTH ▪ is a property of a solution that depends on the total concentration of ions in the solution as well as on the charge carried by each of these ions 1 𝜇 = ෍ 𝐶𝑖 𝑧𝑖2 2 𝑧𝑖 charge of ion i In very dilute concentrations: 𝜇 → 0 𝛾𝑖 → 1 𝛼𝑖 → 𝐶𝑖 Analytical Chemistry 31 DEBYE-HUCKEL EQUATION 2 0.51𝑧𝑖 𝜇 −𝑙𝑜𝑔𝛾𝑖 = 𝑓𝑜𝑟 𝑎𝑞𝑢𝑒𝑜𝑢𝑠 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠 𝑎𝑡 25℃ 1 + 3.3𝛼𝑖 𝜇 𝑧𝑖 charge of ion i 𝛼𝑖 effective diameter of the hydrated ion i in nm 𝛾𝑖 activity coefficient of ion i 𝐶𝑖 concentration of ion i 𝜇 ionic strength of the solution Analytical Chemistry 32 EXAMPLE Calculate the activity coefficient for Hg2+ in a solution that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of the ion. Analytical Chemistry 33 EXAMPLE 0.51 22 0.085 −𝑙𝑜𝑔𝛾𝐻𝑔2+ = ≈ 0.4016 1 + 3.3 0.5 0.085 𝛾𝐻𝑔2+ = 10−0.416 = 0.397 Analytical Chemistry 34 EQUILIBRIUM CONSTANT For the general reaction: aA + bB → cC + dD 𝑎𝐶𝑐 𝑎𝐷𝑑 𝐶 𝑐 𝐷 𝑑 𝐾 = 𝑎 𝑏 𝑎𝑛𝑑 𝐾′ = 𝑎 𝑏 𝑎𝐴 𝑎𝐵 𝐴 𝐵 𝛾𝐶𝑐 𝐶 𝑐 𝛾𝐷𝑑 𝐷 𝑑 𝑠𝑖𝑛𝑐𝑒 𝛼 = 𝛾𝐶 𝑡ℎ𝑒𝑛 𝐾 = 𝛾𝐴𝑎 𝐴 𝑎 𝛾𝐵𝑏 𝐵 𝑏 𝛾𝐶𝑐 𝛾𝐷𝑑 𝐶 𝑐 𝐷 𝑑 𝛾𝐶𝑐 𝛾𝐷𝑑 𝑅𝑒𝑎𝑟𝑟𝑎𝑛𝑔𝑖𝑛𝑔 𝑔𝑖𝑣𝑒𝑠: 𝐾 = 𝑎 𝑏 𝑎 𝑏 = 𝑎 𝑏 𝐾′ 𝛾𝐴 𝛾𝐵 𝐴 𝐵 𝛾𝐴 𝛾𝐵 Analytical Chemistry 35 MASS BALANCE EQUATION ▪ relate the equilibrium concentrations of various species in a solution to one another and to the analytical concentrations of the various solutes ▪ derived from information about how the solution was prepared and from knowledge of the kinds of equilibria established in the solution Analytical Chemistry 36 CHARGE BALANCE EQUATION ෍ + = ෍[−] + concentration of positive species x magnitude of charge − concentrtion of negative species x magnitude of charge Analytical Chemistry 37 SYSTEMATIC METHOD (Skoog, 2014) Analytical Chemistry 38 BALANCING REDOX REACTIONS EXAMPLE: 𝑀𝑛𝑂4− + 𝑁𝑂2− ↔ 𝑀𝑛2+ + 𝑁𝑂3− Write and balance the two half-reactions, for 𝑀𝑛𝑂4− 𝑀𝑛𝑂4− ↔ 𝑀𝑛2+ To account for the 4 oxygen atoms on the left-hand side of the equation, we add 4H2O on the right-hand side. Then, to balance the hydrogen atoms, we must provide 8H+ on the left: 𝑀𝑛𝑂4− + 8𝐻 + ↔ 𝑀𝑛2+ + 4𝐻2 𝑂 Analytical Chemistry 39 BALANCING REDOX REACTIONS To balance the charge, we need to add 5 electrons to the left side of the equation. Thus, 𝑀𝑛𝑂4− + 8𝐻 + + 5𝑒 − ↔ 𝑀𝑛2+ + 4𝐻2 𝑂 For the other half-reaction, 𝑁𝑂2− ↔ 𝑁𝑂3− we add one H2O to the left side of the equation to supply the needed oxygen and 2H+ on the right to balance hydrogen: 𝑁𝑂2− + 𝐻2 𝑂 ↔ 𝑁𝑂3− + 2𝐻 + Analytical Chemistry 40 BALANCING REDOX REACTIONS Then, we add two electrons to the right-hand side to balance the charge: 𝑁𝑂2− + 𝐻2 𝑂 ↔ 𝑁𝑂3− + 2𝐻 + + 2𝑒 − Before combining the two equations, we must multiply the first by 2 and the second by 5 so that the number of electrons lost will be equal to the number of electrons gained. We then add the two half reactions to obtain 2𝑀𝑛𝑂4− + 16𝐻 + + 10𝑒 − + 5𝑁𝑂2− + 5𝐻2 𝑂 ↔ 2𝑀𝑛2+ + 5𝑁𝑂3− + 8𝐻2 𝑂 + 10𝐻 + + 10𝑒 − 2𝑀𝑛𝑂4− + 6𝐻 + + 5𝑁𝑂2− ↔ 2𝑀𝑛2+ + 5𝑁𝑂3− + 3𝐻2 𝑂 Analytical Chemistry 41 GRAVIMETRIC ANALYSIS ▪ based upon determining the mass of pure compound which the analyte is chemically related to Methods of Analysis 42 TYPES Precipitation Methods ▪ Formation of ppt: 𝐶𝑎2+ + 𝐶2 𝑂42− → 𝐶𝑎𝐶2 𝑂4(𝑠) ▪ Conversion of ppt to a form suitable for weighing: 𝐶𝑎𝐶2 𝑂4(𝑠) → 𝐶𝑎𝑂(𝑠) + 𝐶𝑂(𝑔) + 𝐶𝑂2(𝑔) Gravimetric Analysis 43 TYPES Volatilization Methods ▪ the analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition 𝑁𝑎𝐻𝐶𝑂3(𝑎𝑞) + 𝐻2 𝑆𝑂4(𝑎𝑞) → 𝐶𝑂2(𝑔) + 𝐻2 𝑂(𝑙) + 𝑁𝑎𝐻𝑆𝑂4(𝑎𝑞) The mass of CO2 is determined by connecting the weighed tube with adsorbent to a reaction vessel Gravimetric Analysis 44 FORMATION OF PRECIPITATES ▪ a precipitate is formed when the diameter of an aggregate of atoms, ions, or molecules is greater than 10-4 cm 𝑖𝑜𝑛𝑠 𝑖𝑛 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 → 𝑐𝑜𝑙𝑙𝑜𝑖𝑑𝑎𝑙 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 → 𝑝𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑒 10−8 𝑐𝑚 10−7 𝑡𝑜10−4 𝑐𝑚 > 10−4 𝑐𝑚 ▪ colloidal particles are electrically charged and resist combining to form larger particles Gravimetric Analysis 45 FORMATION OF PRECIPITATES Mechanism ▪ Nucleation: a minimum number of atoms, ions, or molecules join together to give a stable solid Homogeneous: nuclei formed due to fluctuation of ion concentration Heterogeneous: particles form on foreign solids ▪ Particle growth: growth on the existing nuclei to form particles sufficiently large enough to precipitate Gravimetric Analysis 46 PARTICLE SIZE ▪ If nucleation rate < particle growth rate, fewer particles are produced which are relatively large and purer ▪ If nucleation rate > particle growth rate, many particles are produced Gravimetric Analysis 47 FORMATION OF PRECIPITATES Large particles are desired because: ▪ easily filtered ▪ large particles have smaller surface area exposed to contamination than smaller particles Gravimetric Analysis 48 VON WEIMARN ▪ proposed that the initial rate of precipitation is proportional to the relative supersaturation 𝑄−𝑆 𝑉𝑊𝑅 = (𝑉𝑜𝑛 𝑊𝑒𝑖𝑟𝑚𝑎𝑛 𝑅𝑎𝑡𝑖𝑜) 𝑆 Where Q concentration of a species at any instant S equilibrium solubility Gravimetric Analysis 49 VON WEIMARN ▪ a low VWR (Q is low and S is high) results in large crystals ▪ colloidal solids have low solubility, hence have large VWR Gravimetric Analysis 50 ANALYTICAL PRECIPITATION ▪ use dilute solutions (low Q) ▪ add reagents slowly with effective stirring (low Q) ▪ precipitate from hot solution (high S) ▪ use complexing agents (high S) Gravimetric Analysis 51 COLLOID Primary Adsorption Layer ▪ attached directly to the solid surface Counter-ion Layer ▪ surrounding the charged particle is a layer of (Skoog, 2014) solution Gravimetric Analysis 52 COLLOID Paneth-Fajans-Hahn Rule ▪ ions that are preferentially adsorbed on the surface of the crystal lattice are those which are common to the lattice and are in excess Gravimetric Analysis 53 PEPTIZATION ▪ the process of dispersing an insoluble material into liquid as a colloid ▪ may occur if a colloid which is coagulated by increasing electrolyte concentration, is washed with water ▪ a volatile electrolyte is used for washing coagulated colloids to prevent peptization Gravimetric Analysis 54 TYPES OF COLLOIDS Emulsoid ▪ Lyophilic: strong affinity for solvent (hydrophilic if the solvent is water) ▪ also called gels (hydrogels if the solvent is water) ▪ requires high temp for dehydration Counter-ion Layer ▪ surrounding the charged particle is a layer of solution Gravimetric Analysis 55 TYPES OF COLLOIDS Suspensoid ▪ Lyophobic: small affinity for solvent (hydrophobic if the solvent is water) ▪ water retained by coagulation is removed easily by heating above 100℃ Gravimetric Analysis 56 TYPES OF PRECIPITATES Curdy ▪ coagulated suspensoids ▪ e.g. AgX Gelatinous ▪ coagulated emulsoids ▪ e.g. FeOH3 Gravimetric Analysis 57 PRECIPITATION From Homogeneous Solution ▪ a precipitating agent is generated in a solution of the analyte by a slow chemical reaction ▪ keeps VWR low by keeping Q low 𝐻2 𝑁 2 𝐶𝑂 + 3𝐻2 𝑂 → 𝐶𝑂2 + 2𝑁𝐻4+ + 2𝑂𝐻− Gravimetric Analysis 58 PURITY OF PRECIPITATES Coprecipitation ▪ the process in which otherwise soluble compounds are removed from solution during precipitate formation Postprecipitation ▪ the process in which an impurity is deposited after precipitation of the desired substance Gravimetric Analysis 59 COPRECIPITATION Surface Adsorption ▪ common in coagulated colloids due to large surface area ▪ may be minimized by washing with volatile electrolyte or by reprecipitation Purity of Precipitates 60 COPRECIPITATION Mixed-Crystal Formation ▪ a contaminant ion (with same charge and almost same size) replaces an ion in the lattice of a crystal Occlusion ▪ foreign ions in the counter-ion layer may become trapped within the rapidly growing crystal Purity of Precipitates 61 COPRECIPITATION Mechanical Entrapment ▪ a portion of the solution is trapped in a tiny pocket Purity of Precipitates 62 DRYING AND IGNITION ▪ conversion of the precipitate into a form suitable for weighing drying in an oven ignition in a furnace Gravimetric Analysis 63 SELECTING A METHOD Purpose of Analysis ▪ Preparation of a databank of figures to establish trends ▪ Acceptance/rejection of a chemical/product before use in a manufacturing operation ▪ Assessment of the value of a consignment of goods before payment ▪ Prosecution of a company for selling a product not up to the stated specification ▪ Criminal charges of a person found to be in possession of drugs Quantitative Chemical Analysis 64 SELECTING A METHOD Sources of Methods ▪ developed by one laboratory for their own special needs ▪ published in the open scientific literature ▪ supplied by trade organizations ▪ books published by professional organizations ▪ standards organizations, e.g. ISO ▪ statutory publications Quantitative Chemical Analysis 65 SELECTING A METHOD Factors to consider ▪ Limit of detection (LOD) ▪ Accuracy ▪ Precision ▪ Speed ▪ Equipment required ▪ Sample size ▪ Cost ▪ Safety ▪ Specificity Quantitative Chemical Analysis 66 SAMPLING ▪ the process by which a portion of material is reduced in size to an amount of homogeneous material that can be conveniently handled in the laboratory and whose composition is representative of the population; ▪ varies according to the physical state and size of the bulk sample. ▪ needs a statistical approach to get a very small fraction of a material to represent it for purposes of laboratory analysis. ▪ the most difficult step in the entire analytical process and the step that limits the accuracy of the analysis Quantitative Chemical Analysis 67 TYPES OF SAMPLES Representative ▪ typical of the parent material for the characteristic under inspection Selective ▪ uses a sampling plan that screens out materials with certain characteristics and/or selects only material with other relevant characteristics Sampling 68 TYPES OF SAMPLES Random ▪ selected by random process to eliminate problems of bias in selection and/or to provide a basis for statistical interpretation of measurement data Composite ▪ consists of two or more portions of material (collected at the same time) selected so as to represent the material being investigated Sampling 69 MOISTURE DETERMINATION Types of Water ▪ Essential Water: forms an integral part of the molecular or crystalline structure of a compound in its solid state Water of crystallization in a stable solid hydrate (e.g. 𝐵𝑎𝐶𝑙2 ∙ 2𝐻2 𝑂) Water of constitution found in compounds that yield stoichiometric amounts of water when heated or otherwise decomposed (e.g. 𝐶𝑎 𝑂𝐻 2(𝑠) → 𝐶𝑎𝑂(𝑠) + 𝐻2 𝑂(𝑔) ) Quantitative Chemical Analysis 70 MOISTURE DETERMINATION Types of Water ▪ Nonessential Water: retained by the solid as a consequence of physical forces adsorbed water resides on the surface of the material sorbed water is contained within the interstices of the molecular structure of a colloidal compound occluded water is trapped in random microscopic pockets of solids, particularly minerals and rocks Quantitative Chemical Analysis 71 MOISTURE DETERMINATION Moisture analysis in solids ▪ the sample is weighed before and after drying the sample ▪ this step is done so that the composition of the sample does not depend on the relative humidity and temperature at the time of analysis Quantitative Chemical Analysis 72 DIRECT METHODS Methods that require application of heat ▪ Oven-drying ▪ Distillation Freeze drying or lyophilization Moisture Determination 73 DIRECT METHODS Chemical methods ▪ Karl-Fischer Titration ▪ Calcium Carbide Method ▪ Cobalt Chloride Paper (blue when dry, pink when moist) Moisture Determination 74 INDIRECT METHODS Those which measure physical property of the sample that is linearly related to its water content ▪ absorbance ▪ refractive index ▪ electrical conductivity ▪ specific gravity Moisture Determination 75 DISSOLVING THE SAMPLES Trial and error method using solvents of progressively increasing reactivity ▪ distilled water at room temperature ▪ hot distilled water ▪ dilute non-oxidizing acid (e.g. HCl) ▪ dilute oxidizing acid (e.g., HNO3, H2SO4, HClO4) ▪ concentrated non-oxidizing acid ▪ concentrated oxidizing acid ▪ acid mixture (e.g., aqua regia = 3 HCl:1 HNO3) Quantitative Chemical Analysis 76 DISSOLVING THE SAMPLES Trial and error method using solvents of progressively increasing reactivity ▪ HF: primarily used for decomposition of silicate rocks and minerals in the determination of species other than silica ▪ use of flux Basic Fluxes: alkali metal carbonates, hydroxides, peroxides and borates Acidic Fluxes: pyrosulfates, acid fluorides, boric oxide Quantitative Chemical Analysis 77 ELIMINATING INTERFERENCES ▪ Interferences are species other than the analyte that affect the final measurement ▪ A scheme must be devised to isolate the analyte from interferences before the final measurement is made Separation by precipitation Separation by extraction Separation by ion-exchange Separation of inorganic species by distillation Quantitative Chemical Analysis 78 CALIBRATION Calibration and Measurement ▪ all analytical results depend on a final measurement of a physical property of the analyte, which varies in a known and reproducible way with the concentration of the analyte Quantitative Chemical Analysis 79 EVALUATION Calibration of results and evaluation of their reliability ▪ Results are calculated and their reliability estimated ▪ The experimenter must provide some measure of the uncertainties associated with computed results if the data are to have any value Quantitative Chemical Analysis 80 REFERENCES Skoog, Douglas A, Donald M West, and Stanley R Crouch. 2014. Fundamentals Of Analytical Chemistry 9E. Australia: Cengage Learning®. Valera, Florenda. n.d. "Intro And Review Of Basic Concepts In Anal Chem". Presentation, University of the Philippines - Diliman. Analytical Chemistry 81