SKKU Physical Pharmacy Laboratory 성균관대학교물리약학연구실

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2 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

3 - Solutions which resist changes in ph when small quantities of acid or alkali are added. 3

4 - Solutions which resist changes in ph when small quantities of acid or alkali are added. water Buffered solution

5 Acidic buffers (ph < 7) ; a weak acid + its salt with a strong base Basic buffers (ph > 7) ; a weak base + its salt with a strong acid

6 Combination of a weak acid and its conjugate base HA + OH - A - + H 2 O Combination of a weak base and its conjugate acid A - + H 3 O + HA + OH -

7 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

8 - The ph of a buffered solution and the change in ph upon the addition of an acid or base may be calculated by use of the buffer equation. ph = pk a + log Salt Acid For Acid Buffers : The ph of acid buffer can be calculated from 1. the dissociation constant, Ka of the weak acid and 2. the concentrations of the acid and salt used.

9 A Weak Acid and Its Salt HAc + H 2 O H 3 O + + Ac K 1 HAc H 2 O = K 2 H 3 O + Ac K a = H 3O + HAc Ac log H 3 O + = log K a log Acid + log Salt ph = pk a + log Salt Acid Henderson-Hasselbalch equation 9

10 * when Sod. acetate is added to acetic acid K a = [H 3O + ][Ac - ] [HAc] is momentarily disturbed since the acetate ion supplied by the salt increases the [Ac - ] HAc + H 2 O H 3 O + + Ac - The ionization of HAc is repressed upon the addition of the common ion [Ac - ]

11 A Weak Base and Its Salt B + H 2 O OH + BH + K b = OH BH + OH = K b Base Salt B = K w H 3 O + (b/c K w = OH H 3 O + ) log H 3 O + = log K w log 1 K b log Salt Base ph = pk w pk b + log Base Salt

12 Buffers are not ordinarily prepared from weak bases because of the volatility & instability of the bases and because of the dependence of their ph on pk w, which is often affected by temp. changes.

13 HAc + H 2 O H 3 O + + Ac - K a = [H 3O + ][Ac - ] [HAc] = a H3O+ a Aca HAc activity = (γ H3O+ c H3O+ ) (γ Ac- C Ac- ) (γ HAc C HAc ) Molar conc. Activity coefficients

14 Activity coefficient of the undissociated acid γ HAc is essentially 1 and may be dropped. log a H3 O + = log K a + log a Ac a HAc ph = pk a + log Salt Acid + log γ Ac

15 Ionic strength Activity coefficient Activity μ = 1 2 ΣC iz i 2 log γ z = 0.51z 2 μ 1 + μ α ion = γ z c ion C i : molar concentration of ith ion z i : its charge γ z : activity coefficient for z - charged ions c ion : molar concentration.

16 Ionic strength - Addition of neutral salts - Dilution (alter activity coefficients) Temperature The ph of the most basic buffer was found to change more markedly with temp. than that of acid buffers, owing to Kw.

17 Acid Indicator HIn + H 2 O H 3 O + + In - Acid color Alkaline color K In = [H 3 O + ][ In - ] [HIn] base acid

18 1/10~10/1 ph = pk In + log Base Acid * From experience, one cannot discern a change from the acid color to the salt color the ratio of [base] to [acid] is about 1 to 10 * The effective range of the indicator is ph = pk In ± 1 base 10/1 1/10 acid

19 Characteristics of colorimetric method 1 less accurate 2 less convenient but less expensive than electrometric method 3 difficult to apply for the unbuffered pharmaceutical preparation (change the ph - indicator itself is acids or base) 4 error may be introduced by the presence of salts & proteins

20 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

21 A measure of its magnitude of resistance to change in ph on addition of an acid or a base buffer index = buffer efficiency = buffer coefficient = buffer value β = B ph ΔB : gram equivalent of strong acid/base to change ph of 1 liter of buffer solution [g/l] ΔpH : the ph change caused by the addition of strong acid/base

22 HAc + NaOH ( ) 0.01 NaAc + H 2O ( ) Before the addition of NaOH ph = pk a + log salt acid = After the addition of NaOH salt + base ph = pk a + log acid base = β = B ph = = 0. 11

23 A more exact equation K a H 3 O + β = 2. 3C (K a + H 3 O + ) 2 c : total buffer conc. (sum of the molar conc. of the acid & the salt)

24 β max occurs where ph = pk a ([H 3 O + ] = K a ) β max = C H 3O (2 H 3 O + = ) 2 4 C β max = C at ph = pk a

25 is not a fixed value, but rather depend on the amount of base added depends on the value of the ratio [salt]/[acid] and magnitude of the individual concentrations of the buffer components The greatest capacity(βmax) occurs where [salt]/[acid] = 1 and ph = pka Because of interionic effects, buffer capacities do not in general exceed a value of 0.2

26 - Total buffer capacity of a universal buffer (combination of several buffers) A mixture of weak acids whose pka values are sufficiently alike (differing by no more than about 2 ph units) so that their buffer regions overlap can be used as an universal buffer over a wide range of ph values.

27 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

28 Blood 1 Primary buffers : Plasma ; NaHCO 3 -- H 2 CO 3, NaHPO 4 --NaH 2 PO 4, protein 2 Secondary buffers : Erythrocytes ; hemoglobin-oxyhemoglobin, K 2 Hpo 4- -KH 2 PO 4 Lacriminal fluid - ph: 7.4 (range 7 8 or slightly higher) Urine - ph: 6.0 (range ) - below normal hydrogen ions are excreted by the kidney. - above ph 7.4 hydrogen ions are retained by action of the kidney.

29 ophthalmic soln. colorimetric determination of ph research studies in which ph must be held constant

30 Clark-Lubs mixtures and ph (a) HCl & KCl, ph (b) HCl & potassium biphthalate, ph (C) NaOH & potassium biphthalate, ph (d) NaOH & KH 2 PO 4, ph (e) H 3 BO 3, NaOH & KCl, ph

31 Steps for development of a new buffer 1 Select a weak acid having a pk a approximately equal to the ph at which the buffer is to be used. 2 Calculate the ratio of salt & weak acid required to obtain the desired ph. 3 Consider the individual conc. Of the buffer salt & acid needed to obtain a suitable buffer capacity * Individual conc. : 0.05 ~ 0.5M * buffer capacity : 0.01 ~ 0.1

32 Steps for development of a new buffer 4 Availability of chemicals, sterility of the final soln, stability of the drug & buffer, cost of materials, freedom from toxicity ex) borate buffer toxic effect not be used for oral or parenteral products. 5 Determine the ph and buffer capacity using a reliable ph meter

33 Influence of buffer capacity and ph on tissue irritation * Tissue irritation will be minimal when (a) Buffer solution β, Volume (b) Physiologic fluid - β, Volume

34 Stability vs. optimum therapeutic response * Undissociated form of a weakly acidic or basic drug has a higher therapeutic activity than the dissociated salt form. * Molecular form is lipid soluble & can penetrate body membranes readily, where the ionic form, not being lipid soluble, can penetrate membranes only with greater difficulty.

35 ph and solubility * Influence of buffering on the solubility of base - At a low ph : base is in the ionic form & usually very soluble in aqueous media - As the ph is raised : more undissociated base is formed when the amount of base exceeds the limited water solubility of this form, free base precipitates from soln. Base soln. should be buffered at a sufficiently low ph for stabilization against precipitation.

36 (Example) GOAL: Compute the mole percent of free base present on 25 and at a ph of 7.4. The pk b of pilocarpine is 7.15 at 25.

37 C 11 H 16 N 2 O 2 + H 2 O C 11 H 16 N 2 O 2 H + + OH - (Pilocarpine base) (Pilocarpine ion) ph = pk w pk b + log Base Salt At ph = log Base Salt Base = 3.56 Salt 1 Mole % of Base = 78% At ph = log Base Salt Base Salt = Mole % of Base = 0. 13%

38 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

39 Red Blood Cell NaCl solution 2.0 % Shrink 0.9 % Isotonic 0.2 % Hemolysis

40 The term Isotonic should be restricted to solutions having equal osmotic pressures which respect to a particular membrane (Husa) Isotonicity value the concentration of an aqueous NaCl soln. having the same colligative properties as soln

41 Hemolytic method apply red blood cells based on the fact that a hypotonic soln. liberates oxyhemoglobin in direct proportion to the number of cells hemolyzed determine colligative properties (chapter 5) modifications of the Hill-Blades Technique based on a measurement of the slight temp. differences arising from differences in the vapor pressure of thermally insulated samples contained in constant-humidity chambers T f = 0.52 ºC (Freezing point lowering of human blood & lacrimal fluid)

42 The van t Hoff expression Tf = L c (Chapter 6) Molal freezing point depression of water L iso = T f / c 0.52 Conc. that is isotonic with body fluids

43 Calculating Tonicity Using L iso Values

44 The Buffer Equation Buffer Capacity Buffers in pharmaceutical and Biologic Systems Buffered Isotonic Solutions Methods of Adjusting Tonicity and ph

45 Class I add Sod. Chloride to lower the freezing point of soln. to Cryoscopic method 2 Sodium chloride equivalent method Class II add Water to form an isotonic soln. 1 White-Vincent method 2 Sprowls method

46 Cryoscopic Method How much NaCl is required to render 100mL of a 1% soln. of apomorphine HCl isotonic with blood serum? Δ T f 0.9% of NaCl soln : 0.52 (Isotonic with blood) Δ T f 1% of apomorphine HCl soln : 0.08 (from table) to reduce the freezing point by an additional 0.44 ( ) Δ T f 1% of NaCl soln : (%)/X = 0.58/0.44 ; X = 0.76 (%) Dissolve 1 g apomorphine HCl g NaCl make 100mL soln. with water

47 Sodium chloride equivalent(e) method by Mellen & Seltzer 1g drug tonicity = Eg NaCl tonicity E : weight of NaCl with the same freezing point depression as 1g of the drug. ΔT f = L iso c ΔT f = L iso 1g/MW 3.4 E E 17 L iso / MW c = 1 g / molecular weight

48 White-Vincent method (Example) GOAL: make 30mL of a 1% soln. of procaine HCl isotonic with body fluid

49 Steps for White-Vincent method 1 Weight in grams of drug(0.3 g) Sod. Chloride equivalent E(0.21..from table) = quantity of sod. Chloride equivalent to w of drug(0.063 g) g/100ml = g / V 3 V = /0.9 4 V = 7.0 ml 5 Add isotonic-buffered diluting soln. to complete V = w E 111.1

50 White-Vincent method water GOAL: make 30mL of 1% soln. of procaine HCl isotonic with body fluid 0.9%NaCl add 0.9%NaCl 0.3g drug (E=0.21) isotonic 7ml or Isotonic buffered sol. 30ml

51 Sprowls method w V E = 0.9 g 100 ml W = 0.3 g (1% solution) TABLE?

Buffered and Isotonic Solutions

Physical Pharmacy Lecture 8 Buffered and Isotonic Solutions Assistant Lecturer in Pharmaceutics Overview Buffered Solutions Definition Buffer Equation Buffer Capacity Buffer in Biological Systems Pharmaceutical