1. Buffered and Isotonic Solutions

2. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

3. Introduction Buffered Solutions ?

4. Buffered Solutions 4.58 3 3 buffer action 0.1N HCl 1ml pH4.7 H2O NaCl
HAc,NaAc
pH7
buffer action
4.58 3 3

5. + Buffered Solutions HA + OH- A- + H2O HA OH- A- + H3O+
• Combination of a weak acid and its conjugate base
A- + H2O
• Combination of a weak base and its conjugate acid HA +
OH-
A- + H3O+

6. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

7. • A Weak Acid and Its Salt
The Buffer Equation
• A Weak Acid and Its Salt
H3O+ + Ac-
HAc + H2O
K1[HAc][H2O] = K2[H3O+][Ac-]
salt
Ka =
[H3O+][Ac-]
[HAc]
acid
-log[H3O+]= - logKa - log[acid] + log[salt]

8. Dissociation exponent
The Buffer Equation
• A Weak Acid and Its Salt
[salt]
pH= pKa+log
[acid]
Buffer equation or
Henderson-Hasselbalch equation
Dissociation exponent

9. Common ion effect [H3O+][Ac-] Ka = [HAc] H3O+ + Ac- HAc + H2O
* when Sod. acetate is added to acetic acid…
Ka =
[H3O+][Ac-]
[HAc]
is momentarily disturbed since the acetate ion supplied by the salt increases the [Ac-]
H3O+ + Ac-
HAc + H2O
The ionization of HAc is repressed upon the addition of the common ion [Ac-]

10. • A Weak Base and Its Salt
The Buffer Equation
• A Weak Base and Its Salt
OH- + BH+
B + H2O
salt
Kb =
[OH-][BH+]
[B]
base

11. -log[H3O+]= - logKw – log1/Kb - log[salt]/[base]
The Buffer Equation
• A weak base and its salt
[OH-] = Kb
[base]
[salt]
[H3O+] • [OH-] = Kw
-log[H3O+]= - logKw – log1/Kb - log[salt]/[base]

12. pH= pKw- pKb + log The Buffer Equation • A Weak Acid and Its Salt
[base]
[salt]
pH= pKw- pKb + log
* 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 pKw, which is often affected by temp. changes.

13. Activity coefficients
H3O+ + Ac-
HAc + H2O
activity
aH3O+ •aAc-
aHAc
Ka =
[H3O+][Ac-]
[HAc] =
Conc activity
(γH3O+•cH3O+)•(γAc-• CAc-)
(γHAc•CHAc) =
Molar conc.
Activity coefficients

14. Activity coefficients
aAc-
- log[aH3O+] = - log Ka + log
aHAc
* activity coefficient of the undissociated acid γHAc is essentially 1 and may be dropped.
[salt]
[acid]
pH = pKa + log log γAc-

15. 1. Altering the ionic strength
pH 에 영향 주는 인자
1. Altering the ionic strength
① Addition of neutral salts
② Dilution (alter activity coefficients)
2. Temperature
The pH of the most basic buffer was found to change more markedly with temp. than that of acid buffers, owing to Kw.

16. KIn = pH indicator HIn + H2O H3O+ + In- • Acid indicator의 경우 [HIn]
Alkaline color
Acid color
base
KIn =
[H3O+ ][ In-]
[HIn]
acid

17. PH indicator pH = pKIn + log pH =pKIn + 1 1/10~10/1 • 10/1 1/10 base
[acid]
1/10~10/1 •
* 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 =pKIn + 1
base
acid
10/1
1/10

18. pH indicator • 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

19. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

20. Buffer capacity
• …the magnitude of the resistance of a buffer to pH changes B pH β=
buffer capacity
= buffer efficiency
= buffer index
= buffer value
ΔB : small increment in gram equivalents/Liter of strong(or acid) added to the buffer soln. to produce a pH change of ΔpH

21. Buffer capacity (근사식 이용)
HAc
( )
NaAC
( )
0.01
+ NaOH
+ H2O
• Before the addition of NaOH
pH=pKa + log
[salt]
[acid]
= 4.76
• After the addition of NaOH
pH=pKa + log
[salt] + [base]
[acid] - [base]
= 4.85
0.01
0.09
= 0.11 = pH β B =

22. • A more exact equation for buffer capacity (1914, 1922)
Ka • [H3O+]
β = 2.3 • C •
(Ka + [H3O+])2
c : total buffer conc.(sum of the molar conc. of the acid & the salt) .
β ---- at any [H3O+]

23. Maximum Buffer capacity
• βmax occurs where pH = pKa ([H3O+] = Ka) 4
2.303
• C
[H3O+]2
βmax = • C •
(2 [H3O+])2 =
βmax = • C
( pH = pKa )

24. Characteristics of Buffer Capacity
• …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

25. Universal Buffer
• Total buffer capacity of a universal buffer (combination of several buffers)

26. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

27. In Vivo biologic buffer systems
• Blood
① Primary buffers : Plasma ;
NaHCO3-- H2CO3, NaHPO4--NaH2PO4, protein
② Secondary buffers : Erythrocytes ;
hemoglobin-oxyhemoglobin, K2Hpo4--KH2PO4
• Lacriminal fluid
- pH: 7.4 (range 7 – 8 or slightly higher)
• Urine
- pH: 6.0 (range 4.5 – 7.8)
- below normal…hydrogen ions are excreted by the kidney.
- above pH 7.4…hydrogen ions are retained by action of the kidney.

28. Pharmaceutical buffers
• ophthalmic soln.
• colormetric determination of pH
• research studies in which pH must be held constant

29. Pharmaceutical buffers
• Clark-Lubs mixtures and pH
(a) HCl & KCl, pH
(b) HCl & potassium biphthalate, pH
(C) NaOH & potassium biphthalate, pH
(d) NaOH & KH2PO4 , pH
(e) H3BO3, NaOH & KCl, pH

30. Preparation of pharmaceutical buffer solutions
• Steps for development of a new buffer
① Select a weak acid having a pKa 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

31. Preparation of pharmaceutical buffer solutions
• 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

32. Buffer in pharmaceutical and biologic systems
• Influence of buffer capacity and pH on tissue irritation
* Tissue irritation will be minimal when…
Buffer solution – β , Volume
(b) Physiologic fluid - β , Volume

33. Buffer in pharmaceutical and biologic systems
Stability vs. optium 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.

34. Buffer in pharmaceutical and biologic systems
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.

35. Buffer in pharmaceutical and biologic systems
(Example)
GOAL: Compute the mole percent of free base present on 25℃ and at a pH of 7.4. The pKb of pilocarpine is 7.15 at 25℃.

36. Buffer in pharmaceutical and biologic systems
Example
C11H16N2O2 + H2O
C11H16N2O2H+ + OH-
(Pilocarpine base)
(Pilocarpine ion)
[base]
[salt]
pH= pKw- pKb + log
At pH 7.4
At pH 4.0
[base]
[salt]
[base]
[salt]
4.0 = 14 – log
7.4 = 14 – log
[base]
[salt]
= / 1
[base]
[salt]
= 3.56 / 1
Mole percent of base = / ( ) • 100 = 0.13%
Mole percent of base = 3.56 / ( ) • 100 = 78%

37. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

38. Buffered isotonic solution
Red blood cell
2.0 %
Hypertonic,
Shrink
0.9 %
Isotonic
0.2 %
Hypotonic,
Hemolysis
NaCl solution

39. Buffered isotonic solution
• 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. (Goyan & Reck)

40. Measurement of tonicity
• 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
Tf = 0.52 ºC (Freezing point lowering of human blood & lacrimal fluid)

41. Calculating Tonicity Using Liso values
• The Van’t Hoff expression
(Chapter 6)
Tf = L · c
Molal freezing point depression of water
Liso = Tf / c
Conc. that is isotonic with body fluids
0.52 °

42. Calculating Tonicity Using Liso values

43. Contents • The Buffer Equation • Buffer Capacity • Buffers in
pharmaceutical and Biologic Systems
• Buffered Isotonic Solutions
• Methods of Adjusting Tonicity and pH

44. Method of adjusting tonicity and pH
Class I …add Sod. Chloride to lower the freezing point of soln. to -0.52°
① Cryoscopic method
② Sodium chloride equivalent method
Class II …add Water to form an isotonic soln.
① White-Vincent method
② Sprowls method

45. Class I methods • Cryoscopic method (빙점강하도법) (Example)
How much NaCl is required to render 100mL of a 1% soln. of apomorphine HCl isotonic with blood serum?
Δ Tf0.9% of NaCl soln : 0.52°(Isotonic with blood)
Δ Tf1% of apomorphine HCl soln : 0.08° (from table)
to reduce the freezing point by an additional 0.44°( )
Δ Tf1% of NaCl soln : 0.58°
1(%)/X = 0.58/0.44 ; X = 0.76 (%)
Dissolve 1 g apomorphine HCl g NaCl make 100mL soln. with water

46. Class I methods
• Sodium chloride equivalent(E) method (염화나트륨당량법) by Mellen & Seltzer
1g drug tonicity = Eg NaCl tonicity
E : weight of NaCl with the same freezing point depression as 1g of the drug.
ΔTf = Liso · c
c = 1 g / molecular weight
ΔTf = Liso · 1g/MW
58.45 E
3.4
E ≈ 17 · Liso / MW

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

48. Steps for White-Vincent method
Class II methods •
Steps for White-Vincent method
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)
0.9 g/100mL = g / V
V = • 100/0.9
V = 7.0 mL
Add isotonic-buffered diluting soln. to complete
V = w • E • 111.1

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

50. Class II methods ? • Sprowls method w E V 0.9 g 100 ml =
TABLE
W = 0.3 g (1% solution) ?

51. Thank you for your attention