大学化学(1)医学基础化学：chapter3 Buffer Solution

1、2022-1-141Chapter ThreeBuffer Solution3-1 Concept of Buffer Solution3-2 The pH of Buffer Solution3-3 Capacity of Buffer Solution3-4 Designing and Preparation of Buffers3-5 Buffer Systems in the Body2022-1-1423-1 Concept of Buffer Solution3-1.1 Definition of buffer solution A solution that can resist

2、 changes in pH when limited amounts of strong acid or base are added to it is called buffer solution (buffer). How do buffers resist changes in H+ or OH- ?3-1.2 Theory of buffer solution How buffer solutions work ? To understand this process, we can analyze the buffer solution of HAc and NaAc. We ca

3、n write such an equilibrium as: 2022-1-143 Adding a small amount of acid. If an amount of H+ ions are added, the above reaction will shift to the left. Adding a small amount of base. If OH- ions are added they will remove H+ ions to form water. Therefore, no matter adding small amount of acid or bas

4、e can not change the pH of buffers significantly.HAclargelargelittleNaAc = Na+ + Ac-H+ + Ac-2022-1-1443-1.3 Composition and Type of buffer solution Type of buffer solution Weak acid and its conjugate base HAc-NaAc, H2CO3-NaHCO3 Weak base and its conjugate acid NH3H2O-NH4Cl Polyprotic acid salt and i

5、ts conjugate base: NaHCO3- Na2CO3 ; anti-base part Composition anti-acid part 2022-1-1453-2 The pH of Buffer Solution3-1.1 Henderson-Hasselbalch equation HA (aq) H+ (aq) + A- (aq) H+A- Ka = - HA HA H+ = Ka - A-We first take the negative log of both sides:2022-1-146 HA -logH+ = -logKa - log- A- HA A-

6、 pH = pKa - log - = pKa + log- A- HA In general: base pH = pKa + log - acid This is known as the Henderson-Hasselbalch equation.It relates pH to pK a and the concentrations of the acid and conjugate base, and can be used to design buffers tomaintain any desired pH.2022-1-147 base pH = pKa + log - ac

7、id From above equation, we can know: The pH of a buffer solution depends on the Ka of conjugate acid and the ratio of conjugate acid and conjugate base. The ratio is called buffer ratio. When buffer ratio =1 , A- = HA, pH = pKa . For certain buffer solution, at a given temperature, Ka is a constant.

8、 pH depends on the buffer ratio. We can design different buffer solutions by changing the buffer ratio.Buffer ratio2022-1-148 When buffer solution is diluted, the buffer ratio is not changed, so pH of buffer solution is not changed. pKa = -logKa H2PO4- - HPO42-, 42POHapkpk4NHapkpkNH4+ - NH3,2022-1-1

9、493-1.2 Calculating the pH of buffer solution Assuming that the volume of buffer is V L, amount of substance of conjugate acid and base is nA, nB respectively. acid=nA / V, base=nB / VSo, nB pH = pKa + log - nA constantvariable2022-1-1410Example 3-1(a) Calculate the pH of buffer system containing 1.

10、0mol/L HAc and 1.0mol/L NaAc. (b) What is the pH of the buffer system after the addition of 0.10 mole of gaseous HCl to one liter of the solution? Assume that the volume of the solution does not change when the HCl is added and pKa HAc= 4.75Solution (a) According to the buffer equation A- 1.0 pH = p

11、Ka + log- = 4.75 + log- HA 1.0 = 4.752022-1-1411(b) After the addition of 0.10mol HCl to one liter of the solution, 0.1mol HCl can consume 0.1mol Ac- so, the amount-of-substance of acetic acid and acetate ions present are nHAc = 1.0 + 0.10 =1.1 mol nAc- = 1.0 0.10 = 0.90 mol The pH of the solution b

12、ecomes nB pH = pKa + log - nA = 4.75 + log(0.90 / 1.1) = 4.662022-1-14123-3 Capacity of Buffer Solution3-3.1 Concept of buffer capacity The buffer capacity () is the amount-of-substance of strong acid or base per liter needed to produce a unit change in pH. b = - |pH| where b: pH:2022-1-14133-3.2 Fa

13、ctors of Influencing Buffer Capacity ()The buffer capacity of a buffer solution depends on the total concentration (ctotal = B +HA) and buffer ratio ( B/HA). When the buffer ratio is fixed, the greater total concentration, the greater the buffer capacity is (See figure 3-1 curve b and c). This can b

14、een seen also from following table (3-1).2022-1-1414-Buffer Ac-(mol/L) HAc ratio ctotal (molL-1pH-1)- 1 0.1 0.1 1:1 0.2 0.115 2 0.02 0.02 1:1 0.04 0.023- Table 3-1 the relationship between capacity and concentration2022-1-1415 When the total concentration is fixed, if the ratio is equal to 1 , the c

15、apacity is the greatest; the pH = pKaat that time. The more ratio deviate 1, the more pHdeviate pKa, the smaller the capacity is.This can been seen also from following table (3-2).2022-1-1416Table 3-2 The relationship between capacity and buffer ratio-Buffer Ac-(mol/L) HAc ratio ctotal (mol L-1pH-1)

16、- 1 0.095 0.005 19:1 0.1 0.0109 2 0.09 0.01 9:1 0.1 0.0207 3 0.05 0.05 1:1 0.1 0.0576 4 0.01 0.09 1:9 0.1 0.0207 5 0.005 0.095 1:19 0.1 0.0109-2022-1-14173-4 Designing and Preparation of Buffers3-4.1 Preparing Principle of Buffer Solution1. Choose the suitable buffer system pH=4.2 buffer, HAc-NaAc,

17、pKa=4.75, buffer range: 3.7-5.62. Choose the best buffer system To select a buffer system, we should first look for an acid with a pKa that is as closes possible to the desired pH. 2022-1-1418 pH= 5.2 buffer, HAc-NaAc, pKa=4.75, KOOCC6H4COOH- KOOCC6H4COONa, pKa=5.43. Choose the suitable total concen

18、tration ctotal = 0.05-0.2mol/L The concentrations of acid and conjugate base canthen be adjusted slightly from the most desirable 1:1ratio to give exactly the desired pH. 2022-1-14193-4.2 Preparing Methods of Buffer Solution Choose the suitable buffer system Choose the suitable total c Calculate the

19、 amount of acid and base Mix the amount of acid and baseExample 3-2 Design a buffer system with pH 4.60.Solution: Step 1: Choose a suitable system, HAc- Ac-. Step 2: Calculation.2022-1-1420 CH3COO- pH = 4.60 = pKa + log - CH3COOH CH3COO- log - = pH pKa CH3COOH = 4.60 - 4.75 = - 0. 15 CH3COO- - = 0.7

20、1 CH3COOH 2022-1-1421Such a ratio could be established by dissolving 0.71mol of sodium acetate and 1.0 mol of acetic acid in oneliter of water. or 0.071 mol NaAc and 0.10 mol HAc inthe same volume. and so on. As long as the ratio ofthe concentrations is 0.71 (and the concentrationsare not too small), the solution will be buffered atclose to pH 4.60.2022-1-14223-5 Buffer Systems in the Body (The application of buffer solution to medical science) The pH of blood: 7.367.44 Intracellular buffers: HHbO2-HbO2- , (氧合血红蛋白氧合血红蛋白) HHb- Hb-, (血红蛋白血红蛋白) H2CO3-HCO3-, H2PO4-HPO42-