half-titration point 는산또는염기가정확히반만중화되는지점임 Ex. 14-3 의 half-titration point 는 25.0 ml 의염기를가했을때임 half-titration point 에서 buffer capacity 가최대
14C-1 The Effect of Concentration 묽은농도 (0.001000 M) 에서 ph 계산시 2 차방정식풀어야함 ( 근사불가 ) Postequivalence point: excess OH - 만계산 Table 14-3 을 plot 한것임 initial ph 는묽은용액 (curve B) 이높음 equivalence-point 의 ph 는진한용액 (curve A) 이높음 At intermediate titrant volume(25 ml) 농도에상관없이 ph 거의같음 당량점에서 OH- 의변화는묽은용액이작다.
14C-2 The Effect of Reaction Completeness 산의세기가약할수록당량점에서 ph 변화가작아짐 PT BTB BCG 14C-3 Choosing an indicator: The Feasibility of Titration Fig. 14-5) Curve A: BCG 와 BTB 는지시약으로적당하지않음, PT 가적당 Curve B: PT 사용시상당한 titration error 발생, PT 와 BTB 사이에서변색하는지시약으로대체 Fig. 14-6) 산의세기 (Ka 의크기 ) 에따른적절한지시약선택
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. 10
Comparison of Weak Acid/ Base with Strong Base/Acid Titration reaction Initial Weak Acid with Strong Base HA + OH - H 2 O + A - [H + ] = K a F Weak Base with Strong Acid B + H 2 O BH + + OH - [OH-] = K b F =1.4 10 3 Before the equivalence point (0<V a <V e ) ph = pk a + log [A ] /[HA] ph = pk b + log[nh 4+ ]/[NH 3 ] Equivalence point [OH ] = K b F = K w F /K a F =( F V i ) / (V i +V a ) [H + ] = K a F F =( F V i ) / (V i +V a ) After equivalence point (V a >V e ) [OH - ] = F NaOH { (V a V e ) (V i + V a } ) [H + ] = F HCl { (V a V e ) (V i + V a } ) 11
FEATURE 14-5 Determining the pk values for amino acids Amino acids contain both an acidic and a basic group. alanine 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. 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-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). 12
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 적정전 : α 0 거의 1(0.987) HOAc 98.7%, OAc - 1.3% Equivalence point: α 0 =1.1ⅹ10-4, α 1 1 (HOAc=0.011 %) Half-titration point(25 ml): α 0 = α 1 =0.5 From Table 14-3 (column 2)
Experiments. Standardization of 0.1000N NaOH 14.00 5.00 6.00 ph 12.00 10.00 8.00 6.00 4.00 2.00 ph/ V a 4.00 3.00 2.00 1.00 ( ph/ V a )/ V a 4.00 2.00 0.00-2.00-4.00 0.00 0 5 10 15 20 25 30 Volume of 0.09270N NaOH (ml) Fig. Experimental titration curve. 0.06860N KHP 25.00ml vs 0.09270N NaOH 0.00 0 5 10 15 20 25 30 Volume of 0.09270N NaOH (ml) Fig. The 1st derivative experimental titration curve. 0.06860N KHP 25.00ml vs 0.09270N NaOH -6.00 0 5 10 15 20 25 30 Volume of 0.09270N NaOH (ml) Fig. The 2nd derivative 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 Titration of 25.00mL of KHP with NaOH End point=18.50ml 0.06860N 25.00mL= x N 18.50mL x=0.09270 N 18
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When we rearrange this equation slightly, we have the slope-intercept form of a straight line, 23
Figure 14F-5 Gran plot for the titration of 50.00 ml of 0.1000 M weak acid (K a = 1.00 x 10 5 ) with 0.1000 M NaOH. The least-squares equation for the line is given in the figure. 24
Acid base titration in non-aqueous media HOOCCHNH 2 + HClO 4 HOOCCHNH 3+ ClO 4 R CH 3 COOH R (solvent) 0.010M CH 3 COOK in acetic acid HClO 4 + CH 3 COOK CH 3 COOH + K + ClO 4 CH 3 COOH(solvent) Amino acid in acetic acid + 0.020M HClO 4 in acetic acid
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 (pk HS ) 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. HClO 4 + CH 3 COOH = ClO 4 + CH 3 COOH 2 + K = 1.3 10 5 strong acid strong base weak base weak acid HCl + CH 3 COOH = Cl + CH 3 COOH 2 + K = 5.8 10 8 Differentiate acidity or basicity of different acids or bases differentiating solvent for acids differentiating sovent for bases acetic acid, isobutyl ketone 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 pk a, Alpha values (α 0 and α 1 ) Amino acids Indicator, transition range Detection of end-point Conductometry Potentiometry Gran plot Non-aqueous titration