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• half-titration point는 산 또는 염기가 정확히 반만 중화되는 지점임
• Ex. 14-3의 half-titration point는 25.0 mL의 염기를 가했을 때 임
• half-titration point에서 buffer capacity가 최대
14C-1 The Effect of Concentration
• 묽은농도(0.001000 M)에서 pH계산시→ 2차 방정식 풀어야 함(근사 불가)
• Postequivalence point: excess OH- 만 계산
Table 14-3을 plot 한 것임
• initial pH는 묽은 용액(curve B)이 높음
• equivalence-point의 pH는 진한용액(curve A)이 높음
• At intermediate titrant volume(25 mL)→농도에 상관없이 pH 거의 같음
• 당량점에서 OH-의 변화는 묽은 용액이 작다.
14C-2 The Effect of Reaction Completeness 산의 세기가 약할수록 당량점에서 pH 변화가 작아짐
14C-3 Choosing an indicator: The Feasibility of Titration
Fig. 14-5) Curve A: BCG와 BTB는 지시약으로 적당하지 않음, PT가 적당 Curve B: PT사용시 상당한 titration error 발생, PT와 BTB사이에서 변색하는 지시약으로 대체
Fig. 14-6) 산의 세기(Ka의 크기)에 따른 적절한 지시약 선택
BCG
BTB
PT
14D Titration Curves for Weak Bases
Fig. 14-7. The effect of base strength (dissociation constant) on titration curves. Each curve represents the titration of 50.00 ml of 0.1000 M base with 0.1000 M HCl.
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[H+] =√ KaF’
F’=( F×Vi) / (Vi +Va)
[OH–] =√ KbF’ = √ KwF’ /Ka
F’=( F×Vi) / (Vi +Va)
Equivalence point
After equivalence point (Va>Ve)
pH = pKb + log[NH4+]/[NH3] pH = pKa + log [A–] /[HA]
Before the equivalence point (0<V a<V e)
[OH-] = √ KbF =1.4×10–3 [H+] = √KaF Initial
B + H2O → BH+ + OH- HA + OH- → H2O + A- Titration reaction
Weak Base with Strong Acid Weak Acid with Strong Base
(Vi + Va) } (Va– Ve) { FHCl = [H+]
(Vi + Va) } (Va– Ve) { FNaOH = [OH-]
Comparison of Weak Acid/ Base with Strong Base/Acid
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FEATURE 14-5 Determining the pK values for amino acids Amino acids contain both an acidic and a basic group.
alanine
All naturally occurring amino acids are left-handed (L) form.
The amine group behaves as a base, while the carboxyl group acts as an acid.
Aspartic acid
Figure 14F-1 Structure and molecular model of alanine, an amino acid. Alanine can exist in two mirror image forms, the left-handed (L) form and the right-handed (D) form.
Figure 14F-3 Aspartic acid is an amino acid with two carboxyl groups. It can be combined with phenylalanine to make the artificial sweetener aspartame, which is sweeter and less fattening than ordinary sugar (sucrose).
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Figure 14F-2 Curves for the titration of 20.00 mL of 0.1000 M alanine with 0.1000 M NaOH and 0.1000 M HCl. Note that the zwitterion is present before any acid or base has been added. Adding acid protonates the carboxylate group with a pKa of 2.35. Adding base causes deprotonation of the protonated amine group with a pKa of 9.89. 14
14E The Composition of Solutions During Acid/Base Titrations
From Table 14-3 (column 2)
•적정전: α0 거의 1(0.987)→ HOAc 98.7%, OAc- 1.3%
•Equivalence point: α0 =1.1ⅹ10-4, α1 ≈1 (HOAc=0.011 %)
•Half-titration point(25 mL): α0 = α1 =0.5
Volume of 0.09270N NaOH (ml)0 5 10 15 20 25 30
pH
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Fig. Experimental titration curve.
0.06860N KHP 25.00ml vs 0.09270N NaOH
Primary standard KHP
204.22g/1000mL=1.0000N
0.35g/25mL = x N
x = 0.06860 N
End point=18.50mL
0.06860N×25.00mL= x N×18.50mL
x=0.09270 N
Titration of
25.00mL of
KHP with
NaOH
Volume of 0.09270N NaOH (ml)0 5 10 15 20 25 30
∆pH
/∆V
a
0.00
1.00
2.00
3.00
4.00
5.00
Fig. The 1st derivative experimental titration curve.
0.06860N KHP 25.00ml vs 0.09270N NaOH
Experiments. Standardization of 0.1000N NaOH
Volume of 0.09270N NaOH (ml)0 5 10 15 20 25 30
∆(∆
pH/∆
Va)
/∆V
a
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
Fig. The 2nd derivative experimental titration curve.
0.06860N KHP 25.00ml vs 0.09270N NaOH
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When we rearrange this equation slightly, we have the slope-intercept form of a straight line,
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Figure 14F-5 Gran plot for the titration of 50.00 mL of 0.1000 M weak acid (Ka = 1.00 x 10─5) with 0.1000 M NaOH. The least-squares equation for the line is given in the figure.
Amino acid in acetic acid +
0.020M HClO4 in acetic acid
0.010M CH3COOK in acetic acid
Acid base titration in non-aqueous media
HOOCCHNH2 + HClO4 → HOOCCHNH3+ClO4
–
R CH3COOH R
(solvent)
HClO4 + CH3COOK → CH3COOH + K+ClO4–
CH3COOH(solvent)
Titrants used in non-aqueous titrimetry
Acidic titrants Perchloric acid
p- Toluenesulfonic acid
2,4-Dinitrobenzenesulfonic acid
Basic titrants Tetrabutylammonium hydroxide
Sodium acetate
Potassium methoxide
Sodium aminoethoxide
Selected solvents for non-aqueous titration Solvent Autoprotolysis constant Dielectric
(pKHS) constant
Amphiprotic Glacial acetic acid 14.45 6.1
Ethylenediamine 15.3 12.9
Methanol 16.7 32.6
Aprotic or basic Dimethylformamide 36.7
Benzene 2.3
Methyl isobutylketone 13.1
Pyridine 12.3
Dioxane 2.2
n-Hexane 1.9
Acid and base strengths that are not distinguished in aqueous solution may be distinguishable in non-aqueous solvents.
Ex. Perchloric acid is a stronger acid than hydrochloric acid in acetic acid solvent,
neither acid is completely dissociated. HClO4 + CH3COOH = ClO4
– + CH3COOH2+ K = 1.3×10–5
strong acid strong base weak base weak acid
HCl + CH3COOH = Cl– + CH3COOH2+ K = 5.8×10–8
Differentiate acidity or basicity of different acids or bases
differentiating solvent for acids …… acetic acid, isobutyl ketone
differentiating sovent for bases …… ammonia, pyridine
Summary
Acid-Base titration, Strong acid / Base, Weak acid and base
Standard solution, standardization, Primary standard
Titration curve, First derivative curve, Second derivative curve
pKa , Alpha values (α0 and α1)
Amino acids
Indicator, transition range
Detection of end-point
Conductometry
Potentiometry
Gran plot
Non-aqueous titration
