ANALYTICAL CHEMISTRY
by Wan Norfazilah Wan Ismail Faculty of Industrial Sciences & Technology [email protected]
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Chapter Description
• Expected Outcomes – Understand and state the principles of titrations. – Define and identify Arrhenius and Brönsted-Lowry acids and bases – Define and identify the conjugate of a given acid or base – Describe and apply the titration curves, calculations and indicators to solve the problem regarding acid base titration.
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Contents
• Acid-Base Theories • Autopyrolysis of Solvents • Acidity of Solution • Acid-Base Titration • End Point Detection • Indicators for titration
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 APPLICATIONS OF NEUTRALIZATION TITRATIONS
General flow: ◦ Preparation of standard solution (acid/base) Standardization of solution with primary standards Titration Results Applications: ◦ Elemental analysis ◦ Determination of inorganic substances ◦ Determination of organic functional groups ◦ Determination of salts
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACID – BASE THEORIES
Arrhenius Theory (Nobel Prize 1894) + • Acid: any species that can produce hydroxonium ions (H3O ) – abbreviated H+ or proton. • Base: produce hydroxyl ions (OH-) in aqueous solution. • Does not include acids or bases that can not produce H+ and OH- ions.
Brönsted – Lowry Theory • Acid = proton (H+) donor • Base = proton acceptor • Amphoteric substance = function as an acid or a base
Lewis Theory • Acid = accept a pair of electrons • Base = donate a pair of electrons
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Strong electrolyte : completely dissociated. Weak electrolyte : partially dissociated.
Strong acid Weak acid Strong base Weak base
HCl CH3COOH NaOH NH3 HBr H2CO3 KOH N2H4 HI HOCN LiOH CH3NH2 HNO3 HCN RbOH H2SO4 HF CsOH HClO4 H2S Sr(OH)2 HOOH Ba(OH)2 HOCl Ca(OH)2 HON=O Mg(OH)2 HOOCCOOH H3PO4 Source: Christian G.D., Dasgupta, P., Schug, K. (2014) CH3CH2COOH Analytical Chemistry. Wiley-VCH
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 AUTOPYROLYSIS OF SOLVENTS
• Autopyrolysis : self-ionization – acts as both an acid and a base.
+ + = 1.0 × 10 + − −14 2 −2 퐻2푂 퐻2푂 ⇌ 퐻3푂 푂� 퐾푤 푚푚푚 퐿 K = + − 퐻 푂� 2 = 퐻 푂 + − 퐾푤 퐻 푂� • Protic solvent : a solvent that involves the transfer of H+ from one
molecule to another. Can undergo self-ionization. i.e. H2O, CH3OH, C6H5OH 2 + + − 3 3 2 Acid3 Base Titration 퐶� 퐶𝐶� ⇌ 퐶� 퐶 푂� 퐶�by퐶 Wan𝐶 Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 AUTOPYROLYSIS OF SOLVENTS
• Polyprotic acids and bases: compounds that can donate or receive more than one proton. i.e. phosphoric acid, phosphate
+ + = 7.11 × 10 − + −3 퐻3푃�4 퐻2푂 ⇌ 퐻2푃�4 퐻3푂 퐾푎� + + = 6.32 × 10 − −2 + −8 퐻2푃�4 퐻2푂 ⇌ 퐻푃�4 퐻3푂 퐾푎� + + = 7.1 × 10 −2 −3 + −13 퐻푃�4 퐻2푂 ⇌ 푃�4 퐻3푂 퐾푎� • Aprotic solvent : a solvent that does not have an acidic proton. i.e.
CH3CN, (C2H5)2O
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACIDITY OF SOLUTIONS
For an aqueous solution of 0.10 M HCl:
+
+ − = 0.01 = 1.0 × 10 퐻𝐻 → 퐻 퐶� + + − −14 ∴0.10퐻 =푀1.0 × 10 퐻[ 푂�] = 1.0 × 10 − −14 − −13 Acidity is related푂 �to pH scale: ∴ 푂� = log = log + 푝� =− 퐻 − 푝𝑝 푂� 푎 푎 Because = 1.0 × 10푝퐾 , pH− is𝑙 related푙퐾 to pOH by + = 14 + − −14 퐻 푂� 푝� 푝𝑝 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF STRONG ACIDS & BASES
100% ionization Example: calculate the pH of a 2.0×10-3M HCl [H+] = 2.0×10-3 pH = -log(2.0×10-3) = 2.70
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 EXAMPLE
What is the pH of a solution containing 0.10 M NaOH?
The final concentrations, = = 0.10
+ = −[ ] 푁푁 푂� 푀 + − = =푤1.0 × 10 0.10 = 1.0 × 10 퐾 퐻 푂� + − −14 −13 퐻 퐾푤⁄ 푂� = 1.0 × 10⁄ = 13 푀
−13 푝� −𝑙푙 @
= log [ ] + = 14
− = log 0.10 = 1 푝𝑝 − 푂� 푝� 푝𝑝 푝𝑝 =−14 1 = 13
푝� − Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID
The dissociation of weak acid
+ (weak acid) + (conjugate− base) 퐻퐻 ⇌ 퐻 � HA (conjugate acid) and A- (conjugate base) are conjugate acid-base pair.
[ ] = [+ ]− 퐻 � 퐾푎 퐻퐻
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 EXAMPLE
A solution of acid HA (0.030 M) was found to have [H+] = 6.5 × 10-4 M.
Calculate the Ka value for the acid.
+ + [ −] 퐻퐻=⇌ 퐻 � [+ ]− 푎 퐻 � 퐾 = = 6퐻퐻.5 × 10 4 + −
= 0.030퐻 �6.5 × 10 4 =− 2푀.9 × 10 + −2 퐻 (6.5 ×푀10−4)(6.5 × 10− 푀4) 푀 = = 1.5 × 10 2.9−× 10 − −5 푎 −2 퐾 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467
WEAK BASE
For every weak acid, there is always an associated weak base.
+ ( ) + − 푎 The anion퐻퐻 A⇌- acts퐻 as �a weak base. This anion퐾 ℎ� 푎can� 푣푣푣푣 undergo𝑠𝑠� hydrolysis푣𝑣푣푣 :
+ + (conjugate− base) (weak acid) (weak acid) (conjugate −base) � 퐻2푂 ⇌ 𝐴 푂� [ ] = = [ ] − 퐻퐻 푂� 퐾ℎ 퐾푏 − �
Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES Partially ionized HA + + A-
Initial C0 0 0 ⇌ H Equilibrium C0-C1 C1 C1
Ka = acidity constant
Assuming 1. Contribution of [H+] from water is negligible
2. Ka<<1, C1< ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 B + + OH- Initial C0 0 0 ⇌ BH Equilibrium C0-C1 C1 C1 Assuming 1. Contribution of [OH-] from water is negligible 2. Kb<<1, C1< ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES Partially ionized HA + + A- Initial C0 0 0 ⇌ H Equilibrium C0-C1 C1 C1 Ka = acidity constant Assuming 1. Contribution of [H+] from water is negligible 2. Ka<<1, C1< ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES - [OH Kw / Ka) * C0) + [H ] = √(Kw / Kb) * C0) ] = √( Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 NEUTRAL IONS • Do not reacts with water to form H+ or OH-. • Are not affected by pH. • Not many, neutral ions are only from strong acid or strong base. Species Neutral Anion − − 3 퐶� − 푁� − 퐵� 퐶𝐶4 Cation − 2− 퐼 푆�4 + 2+ 퐿퐿 + 퐶 � 2 + 푁푁+ 퐵� • When an acid and a base react, they퐾 neutralize each other to form a salt. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID VS STRONG BASE *Completely dissociated Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID VS WEAK BASE *Cation hydrolysis occurs affect the pH value at equivalence point (pH < 7) Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID VS STRONG BASE *Anion hydrolysis occurs affect the pH value at equivalence point (pH > 7) Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID VS WEAK BASE Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STANDARD SOLUTIONS Strong acids or strong bases ◦ Complete reaction with analyte ◦ Sharp end points ◦ Never use weak acids & bases as standard reagents (incomplete reaction) Standard solutions of acids ◦ Dilution of concentrated sulfuric, hydrochloric or perchloric acid. Standard solutions of bases ◦ prepared from solid sodium or potassium and occasionally barium hydroxides. The concentrations of these bases must be established by standardization. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 TITRATION OF STRONG ACIDS AND STRONG BASES A strong acid – strong base titration curve has a large end point break. Figure shows the titration curve for 100 mL of 0.1 M HCl versus 0.1 M NaOH. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 As the concentrations of acid and titrant decrease, the end point break decreases. So the selection of indicator becomes more critical. Dependence of the magnitude of end-point bread on concentration. The concentrations of acid and titrant are the same. 0.001 M 0.01 M 0.1 M Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 This is the mirror image of the HCl titration curve. Titration curve for 100 mL of 0.1 M NaOH versus 0.1 M HCl. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID - STRONG BASE TITRATIONS Approaching the equivalence point, the concentration of [H+] gets very small small Small additions of base large relative change in the concentration of [H+] Thus, near the equivalence point, greater change in pH observed This behavior make it easy by just using an indicator dye to show when we are approaching the equivalence point The indicator may change color at close to pH 7.0 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 TITRATION CURVES OF WEAK ACIDS WITH A STRONG BASE . At the equivalence point, the solution is slightly basic with the present of salt because the salt contains the conjugate base, which is able to recombine with a proton. o After one-half : [HA] = [A-]. o At this point the pH = pKa. o Titration profile is relatively flat around the pH = pKa point. o This means that within this region the pH is not changing much upon the addition of small amounts of base. This is the definition of a "buffered" solution, and explains why the most effective buffering is Source: Dr. Michael Blaber, 2000 at a pH value equal to the pKa. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT 1) using a ruler, draw Weak Acid Titration Curve lines that follow the flat, more horizontal part 12of the curve 10 equivalence point 8 pH 6 4 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT Weak Acid Titration Curve 12 10 equivalence point 8 pH 6 2) draw a line that follows the flat, more 4 vertical part of the curve 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT 3) using a ruler, measureWeak Acid Titration Curve the distance between the top intersection and the bottom12 intersection 10 equivalence point 8 pH 6 4 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT Weak Acid Titration Curve 12 10 equivalence point 8 pH 6 4) the geometric center of this line segment is 4 the equivalence point 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT Weak Acid Titration Curve 5) draw a vertical line 12 from the equivalence point to the x-axis 10 equivalence point 8 pH 6 4 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FINDING EQUIVALENCE POINT Weak Acid Titration Curve 12 10 equivalence point 8 pH 6) where the line crosses the 6 x-axis4) the is the geometric volume centerat the equivalenceof this line point segment is 4 (28.7the mL equivalence in this case) point 2 0 10 20 30 40 50 Buret Volume (mL)Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 FIND THE MID POINT 1) if there is a steep riseWeak in the pHAcid at the Titration Curve beginning of the graph, draw a line that follows12 the steep part of the curve 10 8 pH 6 4 mid point 2 0 10 20 30 Acid Base Titration40 50 by Wan Norfazilah Wan Ismail Buret Volume (mL)http://ocw.ump.edu.my/course/view.php?id=467 FIND THE MID POINT 2) using a ruler, measureWeak the distance Acid Titration Curve between the far left and right intersections12 10 8 pH 6 4 mid point 2 0 10 20 30 Acid Base Titration40 50 by Wan Norfazilah Wan Ismail Buret Volume (mL)http://ocw.ump.edu.my/course/view.php?id=467 FIND THE MID POINT Weak Acid Titration Curve 12 10 8 pH 3) the geometric center 6 between these points is the mid point 4 mid point 2 0 10 20 30 Acid Base Titration40 50 by Wan Norfazilah Wan Ismail Buret Volume (mL)http://ocw.ump.edu.my/course/view.php?id=467 FIND THE MID POINT 4) draw a horizontal lineWeak from theAcid mid Titration Curve point to the y-axis 12 10 8 pH 6 4 mid point 2 0 10 20 30 Acid Base Titration40 50 by Wan Norfazilah Wan Ismail Buret Volume (mL)http://ocw.ump.edu.my/course/view.php?id=467 FIND THE MID POINT Weak Acid Titration Curve 12 10 8 pH 5) where the line crosses the 6 x-axis is the volume at the equivalence point 4 mid point (pH = 7.2 in this case) 2 0 10 20 30 Acid Base Titration40 50 by Wan Norfazilah Wan Ismail Buret Volume (mL)http://ocw.ump.edu.my/course/view.php?id=467 CHOOSING INDICATORS FOR TITRATIONS . Choose an indicator which changes colour as close as possible to that equivalence point. . That varies from titration to titration. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 pH transition range = pKa ± 1. We select an indicator with a pKa near the equivalence point pH. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) pH transition ranges and colors of some common indicators. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACID-BASE INDICATORS An acid/base indicator is a weak organic acid or a weak organic base colourless whose undissociated form differs in color from its conjugate form. red Acid Base Titration phenolphthalein by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACID/BASE INDICATORS acid-type indicator, HIn : - + HIn + H2O In + H3O acid color base color ⇌ base-type indicator, In : + - In + H2O InH + OH base color acid color ⇌ HIn In- + H+ Colour A colour B ⇌ In acidic solution, equilibrium , colour A In basic solution, equilibrium , colour B Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Editor: Wan Norfazilah Wan Ismail Author: Siti Maznah Kabeb Industrial Chemistry Programme Faculty of Industrial Sciences & Technology Universiti Malaysia Pahang Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467