Analytical Chemistry - Section 4.pdf

University of Mines and Technology Analytical Chemistry (CH & RP 174) Section 4 – Titrimetric Analysis Dr Benjamin Edem Meteku Department of Chemical & Petrochemical Engineering [email protected] July 2024 ...

University of Mines and Technology Analytical Chemistry (CH & RP 174) Section 4 – Titrimetric Analysis Dr Benjamin Edem Meteku Department of Chemical & Petrochemical Engineering [email protected] July 2024  TITRIMETRIC ANALYSIS ❖ Titration The term titrimetric analysis refers to the quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration (standard solution) which is required to react quantitatively with a measured volume of a solution of a substance (analyte) Titrimetric analysis was formerly termed volumetric analysis BEM 2  TITRIMETRIC ANALYSIS ❖ Although there are 3 main types of titrimetry; - Volumetric titrimetry is used to measure the volume of a solution of known concentration that is needed to react completely with the analyte. - Gravimetric titrimetry is like volumetric titrimetry, but the mass is measured instead of the volume. - Coulometric titrimetry 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 ❖ Titrimetry as used in this course refers to volumetric titrimetry/analysis BEM 3  TITRIMETRIC ANALYSIS ❖ Although there are 3 main types of titrimetry; - Volumetric titrimetry is used to measure the volume of a solution of known concentration that is needed to react completely with the analyte. - Gravimetric titrimetry is like volumetric titrimetry, but the mass is measured instead of the volume. - Coulometric titrimetry 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 ❖ Titrimetry as used in this course refers to volumetric titrimetry/analysis BEM 4  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖In titrimetric analysis the reagent of known concentration is called titrant and the substance being titrated is termed the titrand. ❖The standard solution is usually added from a long- graduated tube called burette. ❖The process of adding the standard solution until the reaction is just complete is termed titration. ❖The point at which this occurs is called equivalence point or the theoretical (or stoichiometric) end point. BEM 5  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖The completion of the titration is detected by some physical change, produced by the standard solution itself or, usually, by the addition of an auxiliary reagent, known as an indicator; alternatively, some other physical measurement may be used. ❖After the reaction between the substance and the standard solution is practically complete, the indicator should give a clear visual change (either a color change or the formation of turbidity) in the liquid being titrated. ❖The point at which this occurs is called the end point of the titration. BEM 6  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration Question 1. Is there a difference between end point and equivalence point? 2. Write a relation for titration error in terms of end point and equivalence point (Define all variables used in the relation) BEM 7  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖The equivalence point of a titration is a theoretical point that can not be determine experimentally. Instead, we can only estimate its position by observing some physical change associated with the condition of equivalence. This change is called the end point for titration. ❖ Titration Error is the difference between the observed end point and the true equivalence point in a titration. BEM 8  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖ Standard Solution is a solution or reagent of known concentration which is used in titrimetric analysis. ❖ Desirable Properties of Standard Solutions used in Titration include; 1. Sufficiently stable 2. React rapidly with analyte 3.React completely with analyte 4.Endure a selective reaction with analyte BEM 9  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖A primary standard is a highly purified compound or chemical reagent (ultrapure)that is used to prepare standard solution for titration. Thus, it serves as a reference material in all volumetric titrimetric analysis. The accuracy depends on the properties of the compound and the important properties are: 1. High Purity 4. Inexpensive 2. High Stability 5. Readily available 3. Low toxicity 6. Not hygroscopic BEM 10 TITRIMETRIC ANALYSIS BEM 11  TITRIMETRIC ANALYSIS ❖Primary Standard ❖ Compounds that meet or even approach this criteria are few and hence only a few Primary standards are available BEM 12  TITRIMETRIC ANALYSIS ❖ Secondary Standard ❖ As a consequence of the lack of commercial availability of primary standards, less pure compounds may be used. ❖ A secondary standard is a compound whose purity has been determined by chemical analysis. ❖ Secondary standard therefore serves as a working standard BEM 13  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖ Indicators are often added to analyte solution in order to give an observable physical change at end point or near the equivalence point. In other words, indicator is a compound having a physical property (usually color) that changes abruptly near the equivalence point of a chemical reaction. BEM 14  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖Indicators Example: Phenolphthalein turns purple in a base. ❖Detection of an end point involves the observation of some property of the solution that change in a characteristic way at or near the equivalent point. BEM 15  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖In the ideal titration the visible end point will coincide with the stoichiometric or theoretical end point. In practice, however, a very small difference usually occurs this represents the titration error. ❖The indicator and experimental conditions should be so selected that the difference between the visible end point and equivalence point is as small as possible. BEM 16  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖ A suitable reaction for titrimetric analysis must have the following conditions; ✓There must be a simple reaction which can be expressed by a chemical equation; the substance to be determined should react completely with the reagent in stoichiometric ratio ✓The reaction should be relatively fast. (Most ionic reaction satisfy this condition.) In some cases the addition of a catalyst may be necessary to increase the speed of a reaction. ✓ There must be an alteration in some physical or chemical property of the solution at the equivalence point BEM 17  TITRIMETRIC ANALYSIS ❖ End Points in Volumetric Analysis ❖Detection of an end point involves the observation of some property of the solution that change in a characteristic way at or near the equivalent point. ❖The properties that have been used for this purpose are numerous and varied; they include: ✓Color due to the reagent, the substance being determined, or an indicator substance. ✓ Turbidity changes resulting from the formation or disappearance of solid phase BEM 18  TITRIMETRIC ANALYSIS ❖ End Points in Volumetric Analysis ✓Electric conductivity of the solution ✓Electric potential between a pair of electrodes immersed in the solution. ✓ Refractive index of the solution. ✓ Temperature of the solution. ✓ Electric current passing through the solution BEM 19  TITRIMETRIC ANALYSIS ❖ Terminologies in Titration ❖ Direct Titration: When a titrant reacts directly with an analyte, the procedure is termed a direct titration ❖ Back Titration is a process in which the excess of standard solution used to react with an analyte is determined by titration with a second standard solution. ❖ Back Titration is often required when the rate of reaction between the analyte and reagent is slow or when the standard solution lacks stability. BEM 20  TITRIMETRIC ANALYSIS ❖ Classification of Reactions in Titrimetric Analysis ❖The reaction employed in titrimetric analysis fall into four main classes. ❖The first three of these involve no change in oxidation state as they are dependent upon the combination of ions. ❖But the fourth class, oxidation-reduction reactions, involves a change of oxidation state or, expressed another, a transfer of electron. BEM 21  TITRIMETRIC ANALYSIS ❖ 1. Neutralization or Acidimetry or Alkalimetry These include the titration of free bases, or those formed from salts of weak acids by hydrolysis with a standard acid (acidimetry), and the titration of free acids, or those formed by the hydrolysis of salts or weak bases, with a standard base (alkalimetry). Example: Milk of magnesia contains Mg(OH)2 to neutralize stomach acid (HCl) The reaction involves the combination of hydrogen and hydroxide ions to form water. Also, under this heading must be included titrations in non-aqueous solvents, most of which involve organic compounds. BEM 22  TITRIMETRIC ANALYSIS ❖ 2. Precipitation Reaction. These depend upon the combination of ions to form a simple precipitate as in the titration of silver ion with solution of chloride. No change in oxidation state occurs. BEM 23  TITRIMETRIC ANALYSIS ❖ 3. Complex Formation (Complexometric) Reaction. These depend upon the combination of ions, other than hydrogen or hydroxide ion, to form a soluble slightly dissociated ion or compound, as in the titration of a solution of a cyanide with silver nitrate. Ethylendiaminetetra-acetic acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titration and has become one of the most important reagents used in titrimetric analysis. BEM 24  TITRIMETRIC ANALYSIS ❖ 4. Oxidation –Reduction (Redox) Reactions. Under this heading are included all reactions involving change in oxidation number or transfer of electrons among the reactive substance. The standard solutions are either oxidizing or reducing agents BEM 25  TITRIMETRIC ANALYSIS ❖ How to Handle Titration Data ❖Example: Molarity of Solutions that have been Standardized A 50 mL volume of HCl solution required 29.71 mL of 0.01963 M Ba(OH)2 to reach an end point with bromocresol green indicator. Calculate the molarity of the HCl (Ans: 0.0233M) BEM 26  TITRIMETRIC ANALYSIS ❖ How to Handle Titration Data ❖Example: Amount of Analyte in Sample from Titration Titration of 0.2121 g of pure Na2C2O4 (134 g/mol) in aqueous sulfuric acid required 43.31 mL of KMnO4. What is the molarity of the KMnO4 solution? (Redox Titration) NB 2KMnO4 + 8H2SO4 + 5Na2C2O4 → 2MnSO4 + 8H2O + 10CO2 + 5Na2SO4 + K2SO4. (Ans: 0.01462 M) BEM 27  TITRIMETRIC ANALYSIS ❖ Titration Curves. ❖To find the end point we monitor some property of the titration reaction that has a well-defined value at the equivalence point. ❖For example, the equivalence point for a titration of HCl with NaOH occurs at a pH of 7.0. ❖We can find the end point, therefore, by monitoring the pH with a pH electrode or by adding an indicator that changes color at a pH of 7.0. BEM 28  TITRIMETRIC ANALYSIS ❖ Titration Curves. BEM 29  TITRIMETRIC ANALYSIS ❖ Titration Curves. ❖Suppose that the only available indicator changes color at a pH of 6.8. Is this end point close enough to the equivalence point that the titration error may be safely ignored? ❖To answer this question, we need to know how the pH changes during the titration. ❖A titration curve provides us with a visual picture of how a property, such as pH, changes as we add titrant. We can measure this titration curve experimentally by suspending a pH electrode in the solution containing the analyte, monitoring the pH as titrant is added. BEM 30  TITRIMETRIC ANALYSIS ❖ Titration Curves. ❖We can also calculate the expected titration curve by considering the reactions responsible for the change in pH. ❖However we arrive at the titration curve, we may use it to evaluate an indicator's likely titration error. For example, the titration curve in the above figure shows us that an end point pH of 6.8 produces a small titration error. ❖Stopping the titration at an end point pH of 11.6, on the other hand, gives an unacceptably large titration error. BEM 31  TITRIMETRIC ANALYSIS ❖ Titration Curves. ❖A titration curve is a plot of reagent volume added versus some function of the analyte concentration. ❖Volume of added reagent is generally plotted on the x axis. The measured parameter that is a function of analyte concentration is plotted on the y axis and its usually pH. BEM 32  TITRIMETRIC ANALYSIS ❖ Types of Titration Curves Two general titration curve types are seen: ✓ Sigmoidal curve - a "z" or "s"-shaped curve where the y axis is a p-function of the analyte (or the reagent reacted with the analyte during titration) or the potential of an ion-specific electrode. ✓ Linear-segment curve - a curve generally consisting of two line segments that intersect at an angle. BEM 33  TITRIMETRIC ANALYSIS ❖ Types of Titration Curves BEM 34  TITRIMETRIC ANALYSIS ❖ Types of Titration Curves BEM 35  TITRIMETRIC ANALYSIS ❖ Types of Titration Curves Linear Titration Curve BEM 36  TITRIMETRIC ANALYSIS ❖Titrations based on Acid-Base Reactions ❖The earliest acid-base titrations involved the determination of the acidity or alkalinity of solutions, and the purity of carbonates and alkaline earth oxides. ❖Various acid-base titration reactions, including a number of scenarios of base in the burette, acid in the reaction flask, and vice versa, as well as various monoprotic and polyprotic acids titrated with strong bases and various weak monobasic and polybasic bases titrated with strong acids. ❖A monoprotic acid is an acid that has only one hydrogen ion (or proton) to donate per formula. Examples are hydrochloric acid, HCl, a strong acid, and acetic acid, HC2H3O2, a weak acid. BEM 37  TITRIMETRIC ANALYSIS ❖Titrations based on Acid-Base Reactions A polyprotic acid is an acid that has two or more hydrogen ions to donate per formula. Examples include sulfuric acid, H2SO4, a diprotic acid, and phosphoric acid, H3PO4, a triprotic acid. A monobasic base is one that will accept just one hydrogen ion per formula. Examples include sodium hydroxide, NaOH, a strong base; ammonium hydroxide, NH4OH, a weak base; and sodium bicarbonate, NaHCO3, a weak base. A polybasic base is one that will accept two or more hydrogen ions per formula. Examples include sodium carbonate, Na2CO3, a dibasic base, and sodium phosphate, Na3PO4, a tribasic base. BEM 38  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves ❖For our first titration curve let’s consider the titration of 50.0 mL of 0.100 M HCl with 0.200 M NaOH. ❖For the reaction of a strong base with a strong acid the only equilibrium reaction of importance is H3O+(aq) + OH-(aq) 2H2O(l) The first task in constructing the titration curve is to calculate the volume of NaOH needed to reach the equivalence point. BEM 39  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves At the Equivalence point, we know from the reaction that Moles HCl = moles NaOH Or CaVa = CbVb where the subscript ‘a’ indicates the acid, HCl, and the subscript ‘b’ indicates the base, NaOH. The volume of NaOH needed to reach the equivalence point, therefore, is 𝐶𝑎𝑉𝑎 0.100𝑀 50.0𝑚𝑙 Veq = Vb = = 𝐶𝑏 (0.200𝑀) BEM 40  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves Before the equivalence point, HCl is present in excess and the pH is determined by the concentration of excess HCl. Initially the solution is 0.100 M in HCl, which, since HCl is a strong acid, means that the pH is BEM 41  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves pH = -log [H3O+] = -log[HCl] = -log (0.100) = 1.00 The equilibrium constant for reaction is (Kw)–1, or 1.00 × 1014. Since this is such a large value we can treat reaction as though it goes to completion. After adding 10.0 mL of NaOH, therefore, the concentration of excess HCl is 𝑚𝑜𝑙𝑒𝑠 𝑒𝑥𝑐𝑒𝑠𝑠 𝐻𝐶𝑙 𝐶𝑎𝑉𝑎 −𝐶𝑏𝑉𝑏 [HCl] = 𝑇𝑜𝑡𝑎𝑙 𝑉𝑜𝑙𝑢𝑚𝑒 = 𝑉𝑎 +𝑉𝑏 0.100𝑀 50.0𝑚𝑙 − 0.200𝑀 (10.0𝑚𝑙 ) = 50.0𝑚𝑙 +10.0𝑚𝑙 = 0.050M giving a pH of 1.30. BEM 42  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves ❖ At the equivalence point the moles of HCl and the moles of NaOH are equal. Since neither the acid nor the base is in excess, the pH is determined by the dissociation of water. BEM 43  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves Kw = 1.00 × 10-14 = [H3O+][OH-]= [H3O+]2 [H3O+] = 1.00 × 10-7 M Thus, the pH at the equivalence point is 7.00. Finally, for volumes of NaOH greater than the equivalence point volume, the pH is determined by the concentration of excess OH–. For example, after adding 30.0 mL of titrant the concentration of OH– is 𝑚𝑜𝑙𝑒𝑠 𝑒𝑥𝑐𝑒𝑠𝑠 𝑁𝑎𝑂𝐻 𝐶𝑏𝑉𝑏 −𝐶𝑎𝑉𝑎 [OH] = 𝑇𝑜𝑡𝑎𝑙 𝑉𝑜𝑙𝑢𝑚𝑒 = 𝑉𝑎 +𝑉𝑏 0.200 𝑀 30.0𝑚𝑙 − 0.100 𝑀 (50.0𝑚𝑙 ) = 50.0𝑚𝑙 +30.0𝑚𝑙 = 0.0125M To find the concentration of H3O+, we use the Kw expression 𝐾𝑤 1.00 × 10−14 [H3O+] = [𝑂𝐻 −] = 0.0125 = 8.00 × 10-13 giving a pH of 12.10. BEM 44  TITRIMETRIC ANALYSIS ❖ Titration of Strong acids and Strong Bases – T. Curves The Table and Figure below shows additional values for the Titration curve BEM 45  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves ❖For this example, let’s consider the titration of 50.0 mL of 0.100 M acetic acid, CH3COOH, with 0.100 M NaOH. ❖Again, we start by calculating the volume of NaOH needed to reach the equivalence point; thus BEM 46  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves Moles CH3COOH = Moles NaOH CaVa = CbVb 𝐶𝑎𝑉𝑎 0.100𝑀 50.0𝑚𝑙 Veq = Vb = = 𝐶𝑏 (0.100𝑀) = 50.0ml Before adding any NaOH the pH is that for a solution of 0.100 M acetic acid. Since acetic acid is a weak acid, we calculate the pH using this method CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO-(aq) BEM 47  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves BEM 48  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves BEM 49  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves BEM 50  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves BEM 51  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curves BEM 52  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curve The pH is then calculated for the weak base BEM 53  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curve After the equivalence point NaOH is present in excess, and the pH is determined in the same manner as in the titration of a strong acid with a strong base. For example, after adding 60.0 mL of NaOH, the concentration of OH– is 𝑚𝑜𝑙𝑒𝑠 𝑒𝑥𝑐𝑒𝑠𝑠 𝑁𝑎𝑂𝐻 𝐶𝑏𝑉𝑏 −𝐶𝑎𝑉𝑎 [OH] = = 𝑇𝑜𝑡𝑎𝑙 𝑉𝑜𝑙𝑢𝑚 𝑒 𝑉𝑎 +𝑉𝑏 0.100𝑀 60.0𝑚𝑙 − 0.100𝑀 (50.0𝑚𝑙 ) = 50.0𝑚𝑙 +60.0𝑚𝑙 = 0.00909M giving a pH of 11.96. The table and figure below show additional results for this titration. BEM 54  TITRIMETRIC ANALYSIS ❖ Titration of weak acid with Strong Bases – T. Curve BEM 55  ACKNOWLEDGEMENT Materials for the lecture presentations were adapted from the works of the following; 1. Dr Gideon Abaidoo Ocran 2. Rev Dr Johannes Ami BEM 56