Analytical Chemistry II Learning Unit 6 PDF

Analytical Chemistry II Learning Unit 6 AAACA2C www.vut.ac.za 1  CLASSICAL METHODS OF ANALYSIS Titrimetric Methods in Analytical Chemistry (Chapter 13) Titrimetric methods is a group of analytical methods that...

Analytical Chemistry II Learning Unit 6 AAACA2C www.vut.ac.za 1  CLASSICAL METHODS OF ANALYSIS Titrimetric Methods in Analytical Chemistry (Chapter 13) Titrimetric methods is a group of analytical methods that are base on the determining/measuring the amount of reagent of known concentration that is required to react completely with the analyte. There are three main types of titrimetric methods: a) Volumetric titrations is used to measure the volume of a solution of known concentration that is needed to react completely with the analyte. b) Gravimetric titrations is like volumetric titration, but the mass is measured instead of the volume. c) Coulometric titrations is where the reagent is a constant direct electrical current of known magnitude that consumes the analyte; the time required to complete the electrochemical reaction is measured. The benefits of these methods is that that they are rapid, accurate, convenient, and readily available. Vaal University of Technology Department of Chemistry 2 Some terms used in Volumetric Titrimetry Standard Solution: A reagent of a known concentration which is used in the titrimetric analysis. Titration: This is performed by adding a standard solution from a buret or other liquid- dispensing device to a solution of the analyte until the point at which the reaction is believed to be complete. Equivalence point: is the point in a titration when the amount of added standard reagent is exactly equivalent to the amount of analyte. End point: is the point in a titration at which an observable physical (color) change signals the equivalence point. Indicator: is often added to the analyte solution to produce an observable physical change (the end point) at or near the equivalence point. Titration error: is the difference in volume or mass between the equivalence point and the end point. Et = Vep - Veq Where Vep is the actual volume of reagent required to reach the end point Veq is the theoretical volume to reach the equivalence point. Vaal University of Technology Department of Chemistry 3 Back- Titration: This is a process that is sometimes necessary in which an excess of the standard titrant is added, and the amount of the excess is determined by back titration with a second standard titrant. In this instance the equivalence point corresponds with the amount of initial titrant is chemically equivalent to the amount of analyte plus the amount of back- titrant. Example: The amount of phosphate (PO43-) in the sample can be determined by adding a measured excess of standard silver nitrate to a solution of the sample which leads to the formation of silver phosphate. 3Ag+ + PO43- Ag3PO4(s) The excess silver nitrate is then back-titrated with a standard solution of potassium thiocyanate. Ag+ + SCN- AgSCN(s) The amount of silver nitrate is chemically equivalent to the amount of phosphate ion plus the amount of thiocyanate used for the back- titration. Vaal University of Technology Department of Chemistry 4  The titration process Vaal University of Technology Department of Chemistry 5  Primary Standards A Primary Standard is a highly purified compound that serves as a reference material in all volumetric and mass titrimetric properties. E.g. sodium carbonate, potassium hydrogen phthalate (KHP) etc. The accuracy of the method depends on the properties of a compound. The important properties/requirements for a primary standard are: 1) High purity 2) Atmospheric stability 3) Absence of hydrate water 4) Readily available at a modest cost 5) Reasonable solution in the titration medium 6) Reasonably large molar mass Compounds that meet or even approach these criteria are few, and only a few primary standards are available. We use secondary standard solutions. A secondary standard is a compound whose purity has been determined by chemical analysis and it serves as a reference material for a titrimetric method of analysis. Vaal University of Technology Department of Chemistry 6  Standard Solutions Standard solutions play a key role in titrimetric methods. The desirable/ideal Standard Solution for titrimetric method should: 1) Be sufficiently stable 2) React rapidly with analyte 3) React more or less completely with analyte 4) Undergo selective reaction with analyte There are few reagent/solutions that meet all the above requirements. So it is very important to be accurate when one prepares a standard solution. There are two basic methods that are used to establish the concentration of such solutions. Direct method: carefully weighed quantity of a primary standard is dissolved in a suitable solvent and diluted to a known volume in a volume flask. Standardization: in which the titrant to be standardized is used to titrate 1) a weighed quantity of a primary standard 2) a weighed quantity of a secondary standard 3) a measured volume of another standard solution Vaal University of Technology Department of Chemistry 7  A titrant that is standardized against a secondary standard or against another standard solution is sometimes referred to as a secondary standard solution. Standardization is a process in which a concentration of the analyte is determined by titrating it with/against a carefully measured quantity of the primary or secondary standard. e.g. 0.1000 M NaOH (Base) Known concentration Aqueous solution with unknown solute concetration e.g. unknown HCl Soln (Acid) Equation for the reaction HCl + NaOH NaCl + H 2O A Titration setup Vaal University of Technology Department of Chemistry 8  Volumetric Calculations For Standard solutions used in titrimetry, either Molarity (C) or Normality (Cn) is employed. Molarity C ： The number of moles of reagent contained in one liter of solution Normality CN ： The number of equivalents of reagent in the same volume Some useful algebraic Relationships Vaal University of Technology Department of Chemistry 9  Calculating the Molarity of Standard Solutions Example 13-1： Describe the preparation of 2.000L of 0.0500M AgNO3 (169.87g /mol) from the primary standard-grade solid. Solution : n(AgNO3) ＝V × C ＝2.000L X 0.0500 mol/L ＝0.1000 mol mass AgNO3 = 0.1000 mol X 169.87 g/mol = 16.98 g AgNO3 The solution was prepared by dissolving 16.98 g of AgNO3 in water and diluted to 2.000L. Vaal University of Technology Department of Chemistry 10 Treating Titration Data We describe two types of volumetric calculations. 1. The first involves calculating the molarity of solution that have been standardized against either a primary standard or another standard solution. (Calculating Molarities from Standardization Data) 2. The second involves calculating the amount of analyte in a sample from titration data. (Calculating the quantity of the analyte from Titration Data) Vaal University of Technology Department of Chemistry 11  Calculating Molarities from Standardization Data Example 13-4: A 50.00mL portion of HCl solution required 29.71ml of 0.01963M Ba(OH)2 to reach an end point with bromocresol green indicator.Calculate the molarity of the HCl. Solution: Ba(OH)2 + 2HCl BaCl2 + 2H2O n(Ba(OH)2) = 29.71x 10-3 L x 0.01963 mol/L = 0.583 mmol Ba(OH)2 : HCl 1 mol : 2 mol 0.583 x 10-3 mol n(HCl) = X = 0.583 x 10-3 = 1.166 mmol n 1.166 mmol CHCl = = = 0.02332 mol/L v 50 mL Vaal University of Technology Department of Chemistry 12  Calculating the quantity of the analyte from Titration Data Example 13.6: A 0.8040 g sample of an iron ore is dissolved in an acid. The iron is then reduced to Fe3+ and titrated with 47.22 ml of 0.02242 M KMnO4 solution. Calculate the results of this analysis in terms of (a) % Fe (55.847 g/mol) (b) % Fe3O4 (231.54 g/mol). The reaction of the analyte with the reagent is described by the equation MnO4- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O Vaal University of Technology Department of Chemistry 13  Titration Curves in Titrimetric Methods The end point is an observable physical change that occurs near the equivalence point. The two most widely used end points involve : 1) Change in color due to the reagent. 2) Change in potential of an electrode that responds to the concentration of the reagent or the analyte. To understand the theoretical basis of the end point and the sources of titration errors, we use titration curves. Types of Titration Curves Titration Curves are plots of a concentration related variables (e.g. p- function) as a function of reagent volume. Data points are used to construct the titration curves. There are two types of titration curves Vaal University of Technology Department of Chemistry 14  1) Sigmoidal curve:  P-function of analyte (or sometimes the reagent) is plotted as a function of reagent volume. 2) Linear segment curve:  Measurements are made on both sides of, but well away from, the equivalence point Vaal University of Technology Department of Chemistry 15  Titration Curves in Titrimetric Methods Concentration Changes during Titration The equivalence point in a titration is characterized by major change in the relative concentrations of reagent and analyte. The table illustrate this phenomenon. The precipitation reaction for this titration is Ag+ + SCN- AgSCN(s) Vaal University of Technology Department of Chemistry 16  Precipitation Titrimetry (Chapter 17) Precipitation titrimetry, which is based on reactions that yield ionic compounds of limited solubility, It is one of the oldest analytical techniques. Titrimetric methods based on silver nitrate are sometimes called argentometric methods. Argentometric is derived from argentums (Latin word) means silver. Vaal University of Technology Department of Chemistry 17  Precipitation Titration Curves involving silver ion The most common method of determining the halide ion concentration of aqueous solution is titration with a standard solution of silver nitrate. To construct a titration curve three type of calculation are required, each of which corresponds to a distinct stage in the reaction： （1）pre-equivalence, (Before) （2）equivalence, and (At) （3）post-equivalence. (After) The titration curves for this method are the plot of pAg versus volume of AgNO3. Vaal University of Technology Department of Chemistry 18 Example : Calculate the pAg values for the titration of 50.0mL of 0.0500M NaCl with 0.100M AgNO3.(for AgCl, Ksp＝1.82×10-10). Use the following volumes of AgNO3 10.00 ml, 20.00 ml, 25.00 ml and 30.00 ml. Solution: Pre-equivalence point data Equivalence point 0.100 M Post-equivalence point AgNO3 Ag+(aq) + Cl-(aq) AgCl(s) 0.100 M 0.0500 M 10.00 ml 50.0 ml 20.00 ml 25.00 ml 30.00 ml 0.0500 M NaCl 50.00 ml Vaal University of Technology Department of Chemistry 19  Option 1 Vaal University of Technology Department of Chemistry 20  [Excess] (M) pAg Vol(mL) 7.28 x 10-9 8.14 10.00 25.00 30.00 Which one is the Limiting reactant?? Excess Limiting reactant reactant Initial : Change : After rxn : 0 1.5 mmol 1.0 mmol [NaCl] = n/vt = 1.5 mmol/60.00 ml = 0.0250 M ksp = 1.82 x 10-10 [Ag+] [Cl-] = 1.82 x 10-10 ksp = [Ag+] [Cl-] AgCl(s) Ag+(aq) + Cl- (aq) 1.82 x 10-10 -10 [Ag ] = = 1.82 x 10 = 7.28 x 10 M pAg = - log [Ag ] = 8.14 [Cl-] 0.0250 Vaal University of Technology Department of Chemistry 21  [Excess] (M) pAg Vol(mL) (b) Ag+(aq) + Cl- (aq) AgCl (s) 7.28 x 10-9 8.14 10.00 0.100 M 0.0500 M C n 1.35 x 10-5 4.87 25.00 25.00 ml 50.0 ml V C= V 30.00 Which one is the M = mol.L-1 Limiting reactant?? (b) At 25.00 ml : n(AgNO3) = CV = (0.100 mol.L-1)(25.00 mL) = 2.5 mmol n(NaCl) = CV = (0.0500 mol.L-1)(50.00 mL) = 2.5 mmol Ag+ + Cl- AgCl(s) (aq) (aq) Initial : 2.5 mmol 2.5 mmol Change : -2.5 mmol -2.5 mmol +2.5 mmol After rxn : 0 0 2.5 mmol At the equivalence: [Ag+] = [Cl-] ksp = [Ag+] [Cl-] [Ag+] [Cl-] = 1.82 x 10-10 since [Ag+] = [Cl-] AgCl(s) Ag+(aq) + Cl- (aq) + + [Ag ] [Ag ] = 1.82 x 10-10 [Ag+]2 = 1.82 x 10-10 ksp = 1.82 x 10-10 pAg = - log [Ag+] = 4.87 [Ag+] = 1.82 x 10 -10 = 1.35 x 10-5 M Vaal University of Technology Department of Chemistry 22  [Excess] (M) pAg Vol(mL) (c) Ag+ (aq) + Cl-(aq) AgCl (s) 7.28 x 10-9 8.14 10.00 0.100 M 0.0500 M C n 1.35 x 10-5 4.87 25.00 30.00 ml 50.0 ml V C= V 0.00625 2.20 30.00 Which one is the M = mol.L-1 Limiting reactant?? (c) At 30.00 ml : n(AgNO3) = CV = (0.100 mol.L-1)(30.00 mL) = 3.0 mmol n(NaCl) = CV = (0.0500 mol.L-1)(50.00 mL) = 2.5 mmol Excess reactant Ag+ + Cl- AgCl (aq) (aq) (s) Initial : 3.0 mmol Limiting 2.5 mmol reactant Change : -2.5 mmol -2.5 mmol +2.5 mmol After rxn : 0.5 mmol 0 2.5 mmol [AgNO3] = n/vt = 0.5 mmol/80.00 ml = 0.00625 M pAg = - log [Ag+] = 2.20 Vaal University of Technology Department of Chemistry 23  Option 2 Vaal University of Technology Department of Chemistry 24  At 10.00 ml : At 20.00 ml : no. of mol NaCl remaining after the addition of AgNO3 no. of moles remaining CNaCl = C = Total Vol. of solution Total Vol. of solution no. of mol NaCl remaining after the addition of AgNO3 (0.0500 M x 50.00 ml) - (0.100 M x 20 ml) CNaCl = = 70.00 ml Total Vol. of solution (0.0500 M x 50.00 ml) - (0.100 M x 10 ml) = 0.007143 M = 60.00 ml [Cl-] = 0.007143 M = 0.02500 M But NaCl Na+ + Cl- 1 mol 1 mol 1 mol ksp = [Ag+] [Cl-] -10 ksp = 1.82 x 10 [NaCl] = [Cl-] [Ag+] [Cl-] = 1.82 x 10-10 [Cl-] = 0.02500 M -10 1.82 x 10 + - -10 [Ag+] = ksp = [Ag ] [Cl ] ksp = 1.82 x 10 [Cl-] -10 [Ag+] [Cl-] = 1.82 x 10-10 = 1.82 x 10 -10 0.007143 1.82 x 10 [Ag+] = = 2.548 x 10-8 M [Cl-] -10 = 1.82 x 10 0.02500 pAg = - log [Ag+] = 7.59 = 7.0 x 10-9 M pAg = - log [Ag+] = 8.14 Vaal University of Technology Department of Chemistry 25  At 25.00 ml : At 30.00 ml : no. of mol NaCl remaining after the addition of AgNO3 no. of mol AgNO3 in excess CNaCl = CAgNO= Total Vol. of solution 3 Total Vol. of solution (0.0500 M x 50.00 ml) - (0.100 M x 25 ml) (0.100 M x 30.00 ml) - (0.0500 M x 50 ml) = 75.00 ml = 80.00 ml = 0M = 0.00625 M At the equivalence: [Ag+] = [Cl-] But AgNO3 Ag+ + NO3- ksp = [Ag+] [Cl-] ksp = 1.82 x 10 -10 1 mol 1 mol 1 mol [AgNO3] = Ag+ [Ag+] [Cl-] = 1.82 x 10-10 since [Ag+] = [Cl-] [Ag+] = 0.00625 M + + -10 [Ag ] [Ag ] = 1.82 x 10 -10 [Ag+]2 = 1.82 x 10 pAg = - log [Ag+] = 2.20 -10 [Ag+] = 1.82 x 10 = 1.349 x 10-5 M pAg = - log [Ag+] = 4.87 Vaal University of Technology Department of Chemistry 26  Titration Data and Titration curves ✓ ✓ ✓ Equivalent point ✓ Vaal University of Technology Department of Chemistry 27  The effect of Concentration on Titration curves Titration curve for A. 50.00mL of 0.0500M NaCl with 0.100M AgNO3, and B. 50.00mL of 0.00500M NaCl with 0.0100M AgNO3 is shown below The lower the concentration, the smaller the pAg values become (i.e. for more dilute solutions, the change in pAg is small). Vaal University of Technology Department of Chemistry 28  The effect of Reaction Completeness on Titration Curves The change of pAg at the equivalence point becomes greater as the solubility products becomes smaller i.e. the reaction between analyte and AgNO3 becomes more complete. The effect of reaction completeness on precipitation curves is shown below. For each curve, 50.00 mL of a 0.0500M solution of the anion was titrated with 0.1000 M AgNO3. Note that smaller values of Ksp give much sharper breaks at the end point Vaal University of Technology Department of Chemistry 29  Titration Curves for Mixtures of Anion The titration curves for 50.00mL of a solution 0.0800 M in Cl- and 0.0500 M in I- or Br- are shown below. Vaal University of Technology Department of Chemistry 30  Indicators for Argentometric Titrations There are three types of end points that are involved in silver nitrate titrations. 1) Potentiometric end point 2) Amperometric end point 3) Chemical end point Potentiometric end point They are obtained by measuring the potential between a silver electrode and a reference electrode whose potential is constant and independent of the added reagent. i.e. measuring the potential difference between silver electrode and a reference electrode. Vaal University of Technology Department of Chemistry 31  Potentiometric end point The titration curves for the potentiometric titrations are similar to the one that we had discussed in the previous sections. Burette E vs SCE Vol AgNO3 Silver Electrode Reference Electrode Vaal University of Technology Department of Chemistry 32  Amperometric end point The instrument consists of a pair of silver microelectrodes that are immersed inside the solution of the analyte. The current that is generated by these silver microelectrodes during the addition of the reagent from the burette is measured and plotted as a function of the volume of the reagent. µC Vol AgNO3 Vaal University of Technology Department of Chemistry 33  Chemical end point This end point is produced by the chemical indicators. i.e. the color change of the solution during titration. The requirements for an indicator for precipitation are that I. The color change should occur over a limited range in p-function of the reagent or the analyte. II. The color change should occur within the steep portion of the titration curve for the analyte. There are three types of chemical indicators 1) Chromate Ion indicator (The Mohr Method) 2) Adsorption indicator (The Fajans Method) 3) Iron (III) Ion indicator (The Volhard Method) Vaal University of Technology Department of Chemistry 34  Chemical end point The Mohr Method (Chromate Ion indicator) The sodium chromate (Na2CrO4) is used as an indicator for the argentometric determination of Cl-, Br- and SCN- ions by reacting with Ag+ to form a silver chromate (Ag2CrO4) precipitate which is brick-red. Titration Reaction: Ag+ + Cl- AgCl(s) (White) Indicator Reaction: Ag+ + CrO42- Ag2CrO4(s) (Brick-red) The Mohr titration must be carried out at a pH of 7 to 10 because the chromate ion is a conjugate base of the weak chromic acid (H2CrO4). In more acidic solutions, the chromate ion concentration is too low to produce the precipitate near the end point. Vaal University of Technology Department of Chemistry 35  Chemical end point The Fajans Method (Adsorption indicator) This is an organic compound/substance which tends to be adsorbed on the surface of the solid in a precipitation titration. The adsorption occurs near the equivalent point. After the solution has changed it color during the titration, this color is transferred from the solution to the solid or precipitate at the same time. We use fluorescein as an adsorption indicator for the titration of chlorine ions (Cl-) with silver nitrate. Titration Reaction: Ag+ + Cl- AgCl(s) (White) Indicator Reaction: Ag(ads) + + Fluores- Ag+Fluores-(s) (Pink) Fluorescein Vaal University of Technology Department of Chemistry 36  Chemical end point The Volhard Method (Iron (III) Ion indicator Here, silver ions are titrated with a standard solution of thiocynate ion. Ag+ + Cl- AgCl(s) (White) Ag+ + SCN- AgSCN(s) (White) Iron (III) ion serves as an indicator Fe3+ + SCN- FeSCN2+ (Red) The solution should be acidic, to prevent precipitation of metal hydroxides (e.g. Fe3+ hydroxide) Vaal University of Technology Department of Chemistry 37  The table below represent the lists of some typical applications of precipitation titrations in which silver nitrate is the standard solution. Vaal University of Technology Department of Chemistry 38