BUFFERS

BUFFERS : 

BUFFERS PREPARED BY: MR. JITENDRA PATEL

WHAT ARE BUFFERS? : 

WHAT ARE BUFFERS? A buffered solution is one that resists changing pH when acid or bases is added. A buffered solution contains a weak acid and its salt or a weak base and its salt. The resistance to a change in pH is known as buffer action.

The common ion effect and Buffer equation for a Weak Acid and its Salt : 

The common ion effect and Buffer equation for a Weak Acid and its Salt The pH of buffer and change on pH can be calculated by use of buffer eqn. When Na Ac is added to HAc the dissociation constant for the weak Acid Ka = [H3O+][Ac-] / [HAc]=1.75* 10 -5

The common ion effect and Buffer equation for a Weak Acid and its Salt : 

The common ion effect and Buffer equation for a Weak Acid and its Salt Let’s consider a solution containing a week acid, HA, and its salt, NaAc. Salts are strong electrolytes, so NaAc will completely dissociate in solution:   NaAc(aq) Na+(aq) + Ac-(aq)   The weak acid exists in equilibrium with its ions: HAc(aq) + H2O (l) H3O+(aq) + Ac-(aq)

The ionization constant for the acid is given by : : 

The ionization constant for the acid is given by : Ka = [H3O+][Ac-] / [HAc] Since we are dealing with weak acids, very little conjugate base (Ac-) in solution comes from the acid. The acetate ion supplied by the salt increases the [Ac-]. To reestablish the constant Ka the hydrogen ion term [H3O+] decrease with formation of HAc. Further, the presence of the salt in solution reduces the ability of the acid to ionize (common ion effect). The major source of Ac- is from the salt.

: 

Ka = [H3O+] [salt] / [acid]  log Ka = log [H3O+] + log [salt] – log [acid] -pKa= -pH + log [salt] – log [acid] The Henderson-Hasselbalch equation may be derived from this expression: pH = pKa + log([salt]o/ [acid]o) Ka is dissociation exponent.

Buffer equation for a Weak bases and its Salt : 

Buffer equation for a Weak bases and its Salt Buffer soln are not ordinarily prepared from weak base and their salt bcz of volatility and instability of the base and bcz of the dependence of their pH on pKw. pKw is affected by change in temp. [OH-] = Kb [Base] / [Salt] And using the relation ship [OH-] = Kw/ [H3O+] pH = pkw - pKb + log[Base]/[Salt]

Activity coefficient and Buffer eqn. : 

Activity coefficient and Buffer eqn. In the equilibrium of weak acid we can replace conc with activity. But activity= molar conc * activity coefficient The activity coefficient of the undissociated acid YAc- is one.

Activity coefficient and Buffer eqn. : 

Activity coefficient and Buffer eqn. For an aq solution of univalent ion at 25 oC, having ionic strength not greater than 0.1 or 0.2 we can say The general equation for buffers of polybasic acids is n= stage of ionisation, A= factor that depends on temp and dielectric constant of medium u= ionic strength

Factors influencing pH of buffer : 

Factors influencing pH of buffer Addition of small amt of water cause small +ve or –ve deviation bcz it alters activity coefficient and water itself behave as a weak acid or weak base. Dilution value is the change in pH on diluting the buffer solution to one half of its original strength. +ve value of dilution :pH rises with dilution -ve value :pH falls with dilution. Temp. : pH of acetate buffer increase with rise in temp, pH of boric acid- sodium borate buffer decrease with temp. The pH of basic buffer more markedly change with temp due to the Kw that appears in the equation of the basic buffers and change with T.

Factors influencing pH of buffer : 

Factors influencing pH of buffer SALT EFFECT: Addition of neutral salt to dilute buffer solution lower the pH by lowering the activity coefficint and pH of basic increases.

Buffer capacity : 

Buffer capacity The magnitude of the resistance of a buffer to changes is referred to as a buffer capacity β. Also known as a buffer efficiency, buffer index, buffer value. It is the ratio of the increment of strong acid or base to the small changes in pH brought about by addition. β = Δ B/ ΔpH where Δ B is small increment in gram equivalent / liter of strong base added.

Calculation of Buffer Capacity : 

Calculation of Buffer Capacity Consider acetate buffer containing 0.1 m HAc and 0.1 m NaAc in 1 liter of solution. To this 0.01 m NaOH is added. HAc + NaOH NaAc + H2O pH = pKa + log([salt]+[Base]/ [acid]-[Base]) Before addition pH= 4.76 + log (0.1+0.01/ 0.1- 0.01)=4.77 The buffer capacity changes as log [salt] / [acid] changes. The buffer capacity is also influenced by an increase in total conc of buffer constituents. (0.1-0.01) (0.01) (0.1+0.01)

Calculation of Buffer Capacity : 

Calculation of Buffer Capacity More exact eqn to calculate the buffer capacity (koppel and spiro eqn) β = 2.3 C* Ka* [H3O+]/(Ka +[H3O+])2 Where C = total buffer conc that is sum of the molar conc of the acid and salt.

Influence of conc on Buffer capacity : 

Influence of conc on Buffer capacity The buffer capacity is also influenced by an increase in total conc of buffer constituents. Consider acetate buffer containing 0.1 m HAc and 0.1 m NaAc in 1 liter of solution. To this 0.01 m NaOH is added. pH= 4.76 + log (0.1+0.01/ 0.1- 0.01)=4.77

Max Buffer Capacity : 

Max Buffer Capacity Koppel and Spiro eqn β = 2.3 C* Ka* [H3O+]/(Ka +[H3O+])2 The max buffer capacity occurs when pH=pKa or when [H3O+] = Ka. β max = 2.3 C* [H3O+]2/(2[H3O+])2 β max = 2.3 C/4 β max = 0.576 C where C is total buffer concentration

Neutralization curves and buffer capacity : 

Neutralization curves and buffer capacity Consider a titration curves of strong acid and weak acids when they are mixed with increasing quantity of alkali. The reaction of an equivalent of acid with an equivalent of base is called neutralization. The neutralization reactions are written as H3O+ (Cl-) + (Na+)OH- =2H20 + Na+ + Cl- HAc + (Na+)OH- =H20 + Na+ Ac- Where H3O+ (Cl-) is hydrated form of HCl in water.

Neutralization curves and buffer capacity : 

Neutralization curves and buffer capacity The neutralization of strong acid by a strong base simply involves a reaction between hydronium and hydroxyl ions H3O+ + OH- = 2 H20 The reaction between strong acid and strong base proceeds to completion. The reaction between weak acid and strong base is incomplete bcz Ac- reacts in part with water to regenerate free acid.

Neutralization curves and buffer capacity : 

Neutralization curves and buffer capacity The neutralization of 10 ml of 0.1 N HCl and 10 ml of 0.1 N HAc by 0.1 N NaOH can be shown by plotting pH versus ml of NaOH added.

Neutralization curves and buffer capacity : 

Neutralization curves and buffer capacity The buffer capacity of a solution of strong acid is shown by Van Slyke to be directly proportional to the hydrogen ion conc. Or β = 2.303 [H3O+] The buffer capacity of a solution of strong base is similarly proportional to the hydroxyl ion conc. β = 2.303 [OH-] The total buffer capacity of water solution of a strong acid or base at any pH is sum of the separate capacities. β = 2.303 ([H3O+] + [OH-])

BUFFERS IN PHARMACEUTICAL AND BIOLOGICN SYSTEM : 

BUFFERS IN PHARMACEUTICAL AND BIOLOGICN SYSTEM

IN VIVO BIOLOGIC BUFFER SYSTEM : 

IN VIVO BIOLOGIC BUFFER SYSTEM Blood is maintained at a pH of about 7.4 by primary buffers in plasma and secondary buffers in the erythrocyte. The plasma contains carbonic acid/ bicarbonate and acid / alkali sodium salts of phosphoric acid as buffers. Plasma proteins, which behave as acids in blood, can combine with base and so act as buffer. In erythrocyte, the two buffer system consist of hemoglobin/oxyhemoglobin & acid/alkali potassium salts of phosphoric acid.

IN VIVO BIOLOGIC BUFFER SYSTEM : 

IN VIVO BIOLOGIC BUFFER SYSTEM The dissociation exponent pK1 for the first ionization stage of carbonic acid in the plasma at body temp. and ionic strength of 0.16 is about 6.1. The buffer eqn for the carbonic acid and bicarbonate buffer of the blood is pH= 6.1 + log ( [HCO3-]/[H2CO3] ) Where [H2CO3] represents the conc of CO2 present as H2CO3 dissolved in blood. The ratio of bicarbonate to carbonic acid in normal blood plasma is log ( [HCO3-]/[H2CO3] ) = 7.4-6.1= 1.3 The lacrimal fluid or tears have a good buffer capacity. The pH of tear is 7.4 with range of 7 to 8.

IN VIVO BIOLOGIC BUFFER SYSTEM : 

IN VIVO BIOLOGIC BUFFER SYSTEM Urine : The urine of a normal adult has a pH of about 6.0 with the range of 4.5 to 7.8 When the pH of the urine is below normal values, hydrogen ions are excreted by the kidneys. When the pH of the urine is above 7.4 , hydrogen ions are retained by the kidneys.

Pharmaceutical Buffers : 

Pharmaceutical Buffers Buffer solutions are used in formulations of ophthalmic solutions. As per Gifford when we mix various proportions of boric acid and monohydrated sodium carbonate they yield buffer solutions with pH range 5 to 9. Sorenson proposed mixture of salt of sodium phosphate for buffer of pH 6 to 8. Sodium chloride is added to each buffer system to maintain isotonicity. A buffer system suggested by Palitzsch consist of boric acid, sodium borate and NaCl and used for ophthalmic solution with pH range 7 to 9.

Pharmaceutical Buffers : 

Pharmaceutical Buffers The buffers of clark and Lubs were determined at 20 o C and re-determined at 25 o C. The mix and their ph ranges are: 1. HCl and KCl, 1.2 to 2.2. 2. HCl and KHP, 2.2. to 4.0 3. NaOH and KHP, 4.2 to 5.8 4. NaOH and KH2PO4, 5.8 to 8 5. H3BO3, NaOH, and KCl, 8 to 10. Below pH 2 HCl alone has considerable buffer efficiency and KCl is neutral salt and is added to adjust the ionic strength.

Preparations : 

Preparations Steps to develop a new buffer solution. Select a weak acid having a pKa near to a pH at which the buffer is to be used to ensure a max buffer capacity. Calculate the ratio of salt and weak acid required to obtain the desired pH. The buffer eqn is satisfactory for approximate calculation within the pH range of 4 to 10. Consider the individual concentration of the buffer salt and acid needed to obtain a suitable buffer capacity. A conc of 0.05 to 0.5M is usually sufficient and buffer capacity of 0.01 to 0.1 is generally adequate.

Preparations : 

Preparations Steps to develop a new buffer solution. Availability of chemicals, sterility of the final solution, stabilty of the drug and buffer on aging, cost of materials, and freedom from toxicity should be considered. E.g. a borate buffer, bcz of its toxic effects, certainly can not be used to stabilize a solution to be administered orally or parenterally. Determine the pH and buffer capacity of the completed buffered solution using a reliable pH meter or pH papers.

Preparations : 

Preparations Steps to develop a new buffer solution. When the electrolyte conc is high, the pH calculated by use of the buffer eqn is somewhat different from the experimental value. It is to be expected when activity coefficient are not taken in to account.

pH and Solubility : 

pH and Solubility At low pH the base is in the ionic form, which is usualy 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 the solution. So the solution should be buffered at sufficiently low pH.

Buffered isotonic solutions : 

Buffered isotonic solutions Pharmaceutical solutions that are meant for application to delicate membrane of the body should be adjusted to same osmotic pressure as that of body fluids. Isotonic solutions cause no swelling or contraction. E.g. isotonic NaCl solutions. Mix small quantity of blood with aq. NaCl solutions of varying tonicity. Blood cells + 0.9 % NaCl = cells retain normal size (Isotonic with blood) Blood cells + 2 % NaCl = cells shrink and become wrinkled or crenated. (Hypertonic with blood) Blood cells + 0.2 % NaCl = cells swells and burst liberating hemoglobin (Hypotonic with blood)

Buffered isotonic solutions : 

Buffered isotonic solutions The RBC membrane permit the passage of water molecules, urea, ammonium chloride, alcohol, boric acid. A 2.0 % boric acid solution is isosmotic to blood cell. The molecules of boric acid pass freely through the erythrocyte membrane regardless of conc. As a result boric acid solution is hypotonic and cause hemolysis. So the solution containing quantity of drug calculated to be isosmotic with blood is isotonic only when blood cells are impermeable to solute molecules and permeable to solvent molecules. Mucous lining of the eye is true semi permeable to boric acid solutions and hence 2 % boric acid solution is isotonic ophthalmic preparation.

Measurement of Tonicity : 

Measurement of Tonicity Two approaches 1. Hemolytic method 2. Based on Methods used to determine colligative properties.

Hemolytic method : 

Hemolytic method Suspend the RBC in solutions. Observe the effect of various solution of drug on appearance of RBC. Hypotonic solutions liberate oxyhemoglobin in direct proportion to the number of cells hemolysed. The van’t Hoff i factor (π=iRTC) can be determined and the value compared with that computed from cryoscopic data, osmotic co-efficient, and activity co-efficient.

Based on Methods used to determine colligative properties : 

Based on Methods used to determine colligative properties This method is based on the measurement of slight temp differences arising from diff in the vapor pressure of thermally insulated samples contained in constant humidity chambers. The freezing point of blood is -0.56 oC and of tear is -0.80 oC. Now for both it is -0.52 oC. This temp corresponds to the freezing point of 0.9% Nacl solutions, which is therefore considered as a isotonic with both blood and lacrimal fluids

Calculating Tonicity using Liso values : 

Calculating Tonicity using Liso values Freezing point depressions for solutions of electrolytes of both the weak and strong type are greater than those calculated from eqn. ΔTf= Kfc, New factor L=iKf is introduced to overcome difficulty. ΔTf = Lc The L value can be obtained from the freezing point lowering of solutions of representative compounds of a given ionic type at conc c that is isotonic with body fluids. The sp value of L is written as Liso

Calculating Tonicity using Liso values : 

Calculating Tonicity using Liso values The Liso value for a 0.90 % (0.154 M) solutions of NaCl, which has freezing point depression of 0.52 o C is Liso = ΔTf/c = 0.52/0.154 = 3.4 For dilute solutions of non electrolytes, Liso is nearly equal to Kf value.

Methods for adjusting Tonicity & pH : 

Methods for adjusting Tonicity & pH Two type 1. Class I methods : NaCl or another substance is added to the solution of the drug to lower the freezing point of solution to -0.52 oC and thus make isotonic with body fluid. E.g. Cryoscopic Method NaCl equivalent method 2. Class II methods: water is added to the drug in sufficient amount to form isotonic s0lution. The preparation is then brought to its final vol with isotonic or buffered isotonic dilution solution. E.g. White Vincet method and Sprows

Cryoscopic Method : 

Cryoscopic Method The freezing point depression of number of drugs is determined theoretically and experimentally. How much NaCl is required to render 100 ml of 1% solutions of apomorphine HCl isotonic with blood serum. Solutions having freezing point lowering value 0.52 oC is isotonic 1 % solutions of apomorphine HCl have freezing point lowering value 0.08 oC (std) Additional Nacl is added to reduce freezing point lowering value by an additional 0.44 (0.52-0.08)

Cryoscopic Method : 

Cryoscopic Method For 0.58 freezing point lowering 1 % Nacl required (std) So 0.44 freezing point lowering x % Nacl required 0.44* 1% = 0.58 * X X = 0.76 % The solution is prepared by dissolving 1.0 g of drug and 0.76 g of NaCl in sufficient amt of water to make 100 ml of solutions

NaCl / Tonicic equivalent method : 

NaCl / Tonicic equivalent method The NaCl equivalent (E) is amount of NaCl that is equivalent to 1 g of drug. E value can be calculated from Liso value or freezing point depression of the drug. for solutions containing 1g of drug in 1000 ml of solution, the conc c= 1g / M.W And ΔTf = Liso *c = Liso 1 g/ M.W now E is the wt For NaCl with same freezing point depression as 1 g of the drug. ΔTf = 3.4 * E/58.45 Liso / M.W = 3.4 * E/58.45

NaCl / Tonicic equivalent method : 

NaCl / Tonicic equivalent method Multiply the quantity of each drug with its NaCl equivalent and subtract the value from the conc of NaCl that is isotonic with body fluids , 0.9 %

White –Vincet Method : 

White –Vincet Method water is added to the drug in sufficient amount to form isotonic s0lution. The preparation is then brought to its final vol with isotonic or buffered isotonic dilution solution. How to make 30 ml of 1% solution of procaine HCl isotonic with body fluid. The wt of the drug w, is multiplied by the NaCl equivalent, E : 0.3g*0.21( W*E) = 0.063g This is the quantity of NaCl osmotically equivalent to 0.3 g of drug.

White –Vincet Method : 

White –Vincet Method For 0.9 g NaCl 100 ml water required For 0.063 g NaCl V ml water required V = 0.063 * 100/ 9 = 7.0 ml The value of the ratio 100/0.9 = 111.1 So the eqn V = W * E * 111.1 Where V is vol in ml of isotonic solution that may be prepared by mixing the drug with water. For the problem V = 0.3 * 0.21 * 111.1 = 7.0 To complete the isotonic solution, enough isotonic NaCl solution or an isotonic buffered diluting solution is added to make 30 ml of finished pdt.

The Sprowls Method : 

The Sprowls Method The eqn V = 0.3 * 0.21 * 111.1 could be used to construct a table of values of V when the wt of the drug w is fixed. Sprowls chose the wt of drug 0.3 g, the quantity for 1 fluid ounce of 1% solution. Compute the vol V of isotonic solutions of 0.3 g drug with sufficient water for drugs commonly used in ophthalmic and parental preparations.

Slide 46: 

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