BUFFER SOLUTIONS

BUFFERS Buffers are defined as compound or mixture of compounds that by their presence in solution resist the changes in pH upon addition of small quantities of acid or alkali.

Buffer action. Resistance to change in pH is known as buffer action. When Acid or alkali added to water/Sodium chloride drastic change in pH of solution. Such systems have no buffer action

Buffer capacity Amount of acid or base required to produce a unit change in pH of Solution.

BUFFERS ( contd Characteristic of Buffers

)

a) Definite pH value ( by dissolving known amount compound) b) pH value of buffer does not alter after long time storage or dilution. c) pH value of buffers slightly altered by addition of small quantities of acids and bases.

Applications Enhancing solubility-if pH of solution not adjusted properly , drug may precipitate Improving stability-many drugs hydrolyze in aqueous solution .adjusting pH of solution stabilizes such drugs Improving purity-protein purification Optimizing biological activity-enzymes have max stability at definite pH Comforting the body-if pH of solution greatly differs from normal pH of relevant body fluid cause irritation, hence pH of formulation should match with pH of physiological solution

Buffer Applications1. Should be controlled in formulation of eye drops, ear drops, injections and liquid orals. e. g.Acetified syrup --- 5.0- 7.2 Dextrose injection --- 3.5 6.5 Oxymetzoline nasal drops -- 4.0 -6.5 Chloramphenicol eye drops ---- 7.0 7.5

Buffer Applications Enhancing the solubility : if pH is not maintained Drug may precipitate Sodium salicylate precipitates as salicylic acid when acidified. Acidic drugs more soluble in basic medium and vice versa.

Increasing the stability: Compounds get hydrolyzed in aqueous solutions. Vitamins are stable only at narrow pH range.

Buffer Applications Improving purity : Proteins are least soluble at their isoelectric point. Insulin precipitates from aqueous soln. at pH 5.0-6.0. Used for purification of insulin.

Optimizing Biological activity: At definite pH enzyme have activity For pepsin maximum activity at Ph

Storage of products Glass containers impart alkalinity to products.

BUFFER SYSTEMS -COMPOSITIONS Buffer solutions are aqueous solutions consisting ofa. Mixture of a weak acid and its conjugate base Soln. containing acetic acid and sodium acetate. Soln. containing Ammonium hydroxide and ammonium chloride.

b. Mixture of a weak base and its conjugate acid.

c. Two salts act as acid base pair which can function either as (a) or (b) Solution of Monobasic potassium Phosphate (KH2PO4) and Dibasic potassium Phosphate (K2HPO 4)

BUFFER SYSTEMS COMPOSITIONS (contd .)d. Amphoteric electrolytes Solution of glycine.

e. Strong acids and strong bases High concentration of hydronium and hydroxyl ion Hydrochloric acid buffers 1.2 to 2.2, Strong acid pH less than 3.0, Strong base pH more than 11.0

BUFFER SYSTEMS -COMPOSITIONS (contd .) Mainly borates, citrate, phosphate and phthalate salts used in pharmacy. Ionic strength of buffer is adjusted to definite value by adding sodium chloride and pot. Chloride. Some soln. of drug themselves manifest buffer action Ephedrine weak base in acidic media forms a salt ephedrine Hydrochloride. Salicylic acid stored in soft glass bottle sodium ions in the container react to sodium salicylate.

Buffer Action - Mechanisms In a buffer solution the Components interact with each other and produce dynamic equilibrium. Small quantities of acids or bases added the dynamic equilibrium shifts and nullifies their effects. The mechanism of buffer action for different categories of buffer solution are as follows BUFFER ACTION OF ACID BUFFER Mixture of weak acid and its salt. Mixture of acetic acid and sodium acetate.

BUFFER ACTION OF ACID BUFFER Strong Electrolytes -- Completely ionized and ionization equation are written as H2O CH3COONa CH3COO- + Na+ Weak Electrolytes -- Slightly ionized CH3COOH + H2O CH3COO- + H3O+ Solution contain very less H3O+ ions Excess of Acetate and sodium ions.

When small amount of acid is added H3O+ + CH3COOCH3COOH + H2O The added H3O+ ions not available pH does not change

BUFFER ACTION OF ACID BUFFER (Contd .) When a small amount base is added, hydroxyl ion furnished by base are neutralized by with acetic acid. OH- + CH3COOH CH3COO- + H2O Since added free OH- ions not available pH does not change.

Buffer action maintained when small amount of acid or base added. Process continues until entire acetate or acetic acid consumed. Buffer action is limited.

BUFFER ACTION OF ALKALINE BUFFERMixture of weak base and its salt.Ammonium Hydroxide and Ammonium chloride Strong Electrolytes -- Completely ionized NH4Cl + H2O NH4+ + Cl Weak Electrolytes -- Slightly ionized NH4OH + H2O NH4+ + OH Solution contain very less OH- ions Excess of Ammonium and chloride ions.

When small amount of acid is added H3O+ + NH4OH NH4+ + 2H2O The added H3O+ ions not available pH does not change

BUFFER ACTION OF ALKALINE BUFFER When a small amount base is added, hydroxyl ion furnished by base are neutralized OH- + NH4+ NH4OH Since added free OH- ions not available pH does not change. Buffer action is maintained when small amount of acid or base added. Process continues until entire ammonium hydroxide or ammonium ions. Buffer action is limited.

Ampholytic Substances That are capable acting both as acid and base. e.g.. Glycine act as acidNH2CH2COOH + H2O Glycine act as base NH2CH2COO- + H3O+ NH2CH2COO- +H3O+ NH3+CH2COO- +H2O(Zwitter Ions or Dipolar ions)

Ions react with H3O+ ions or OH- and nullify the influence of added substance.

Buffer Equation Henderson-Hasselbalch equation Two separate equations for each type of buffer (acidic and Basic) Derived based on effect of salt on the ionization of weak acid, when salt and weak acid have common ion. Acid buffer : Acetic acid and Sod. Acetate considered for deriving buffer equation.

Buffer Equation of Acid Buffer (Contd .) Acid buffer :acetic acid and sodium acetate Ionization acetic acid -- Slightly ionized CH3COOH + H2O CH3COO- + H3O+ Applying law of mass action, Acid dissociation constant Ka = [CH3COO- ][H3O+] / [CH3COOH]= 1.75x 10-5 When sodium acetate added to acetic acid, the equation disturbed. Since salt supplies acetate ions ,the term in numerator increases (in acetate ion concentration.) In order to reestablish the constant Ka , the concentration of H3O+ instantaneously decreases., the equilibrium is shifted. H3O+ + CH3COOCH3COOH + H2O

Buffer Equation of Acid Buffer (Contd .) Common ion [CH3COO-] repressed ionization of acetic acid. this is e.g. for common ion effect. The pH of final soln. is obtained by rearranging equation[H3O+]= Ka ([CH3COOH]/[CH3COO-]) Since acid is weak ionizes slightly may remain unaltered. hence [CH3COOH]=[acid]) Since salt is completely ionized - directly written as [CH3COO-]=[salt])

Buffer Equation of Acid Buffer [H3O+]= Ka ([acid]/[salt]) Taking log on both sides -log[H3O+]= = -log Ka log ([acid]/[salt]) But pH=-log[H3O+]and pka= -log Ka by substituting in above pka= -log Ka -log([acid]/[salt]) pH= pKa + log ([salt]/[acid]) this equation known as Buffer equation or Henderson-Hasselbalchs equation

Buffer Equation of Acid Buffer For weak Base and corresponding salt [OH-]= Kb ([Base]/[salt]) [OH-]+ [H3O+]= Kw or pH + pOH= pKw pH=pKw- pOH pH = pKw pKb +log ([base]/[Salt])

BUFFER EQUATION APPLICATIONPreparation of buffers 1. To get definite pH of solution-(salt and acid in desired ratio. This ratio is determined by equation). 2. pH of solution can be calculated (Known amount of salt and acid). 3. Percentage of drug in ionized and unionized form in solution Predicting the drug absorption Uniozed drugs more readily absorbed than ionized. Antimicrobial activity of benzoic acid depends on con. of ionized/unionized form. Acidic pH is for maximum activity. Acridines antibacterial activity depends on amount of ionic species. In both the cases the pH is adjusted based on Handerson- Hasselbalch s equations.

BUFFER EQUATION APPLICATION pKa of various drugs can be determined from pH of various solutions. The solubility of substance at any pH can be predicted by provided intrinsic solubility and pKa are known. Suitable salt forming substance can be selected based on Hendersen _hasselbalch s equation.

Buffer capacity Buffer capacity is defined as the ratio of the increment of the strong base (or Acid) to the small change in pH brought about by this addition. F = ( B /(pH B=Concentration of Base or acid added (gram equivalent / lit.) F=1, when 1g equivalent of strong base or strong acid added to one lit. of buffer solution. If the change in pH is 1 unit. Buffer has its greatest capacity when salt and acid concentration are Equal. Therefore H.H. equation; pH = pKa B.C. not a fixed value it depends on the concentration of base added. BC also depends on the Total concentration of buffer constituents. Greater the concentration of salt and acid, the greater is the buffer capacity. (buffer expressed in molar conc.- 0.2M, 0.02M)

buffer capacity (F) Van Slykes s equation can be used for calculating the buffer capacity (F)

C- Total buffer concentration Sum of molar con. Of acid and salt. The maximum buffer capacity occurs when pH = pKa Then [H30+]=Ka hence below equation changes to

= 2.303C( 1/(1+1])2 =2.303C/4 = 0.576 C

Buffers in Biological system Blood : Blood pH 7.4 (Buffer capacity : 7.0-7.8) pH of blood in diabetic coma is 6.8 , Life is in danger. Blood consist of primary and secondary buffer systems

Primary buffer system in plasma Carbonic acid and bicarbonate,(o.o25 M and 0.00125 M) Acid/alkali salts of phosphoric acid as buffers. Plasma proteins, which behave as acids in blood combine with bases and act as buffers.

Secondary buffer systems present in erythrocytes Hemoglobin /Oxyhemoglobin system acid/alkali salts of phosphoric acid system.IMP.

Buffer in Biological system (contd ) Lacrimal fluids (Tears) Tear have greater degree of buffer capacity. Dilution of 1:15 with neutral distilled water (dilution value). pH -7.4 (range 7.0-8.0) Pure conjunctivas fluid acidic than the tear fluid. pH is increase rapidly when fluid is removed for analysis because of loss of Carbon dioxide from tear fluid.

Urine pH :6 (4.5-7.8) pH is low-hydrogen ions excreted by kidneys, high Hydrogen ions retained in kidneys.IMP.

FACTORS INFLUENCING pH OF A BUFFER SOLUTION Temperature studies based on 25 rc As temp. increases Acetate buffer - pH increases Boric acid and Sodium borate - pH decreases

Relatively small change for acidic buffer In alkaline buffer changes are more marked owing to presence of pKw term in buffer equation.

Ionic strength Addition of neutral salts changes the pH of solution due to altered ionic strength. Dilution of buffer - changes the pH of solution due to altered ionic strength. So when ever the pH of buffer soln. is mentioned ionic strength also must be specified.

Pharmaceutical Buffers Formulation of ophthalmic preparation. Boric acid , Sodium borate and sufficient sodium chloride to make the mixture isotonic. pH range is 7.0 to 9.0

Used in colorimetric for determination of pH Preparation of different std. pH solution.

Maintain constant pH in in vitro studies and Biologic research. Dissolution studies, Microbial studies etc.

Pharmacopoeia gives elaborate list of Buffers and their method of preparation. Refer Pharmacopoeia

IMP.

Pharmaceutical Buffers Choice of pharmaceutical buffers depend on the following factors Availability of chemicals Sterility of final solution Stability of drug Stability of buffer on aging Cost of material Freedom from toxicity E.g. Borate buffer cannot be used to stabilize the oral or parenteral solution because of toxic effects.

The experimental pH values slightly differ form calculated pH values using buffer equation

Preparation of Buffer solutions Preparation buffers as follows. 1. A weak acid to be selected which having pKa=desired pH value of solution, this will ensure maximum buffer capacity. 2. 3. From Buffer equation (pH 4-10), the ratio of salt and acid needed for suitable buffer capacity should be calculated. Determine the individual concentrations of buffer salt and acid to obtain suitable buffer capacity. (o.o5 to 0.5 M is sufficient) A con. of 0.5 to 0.05 M is sufficient and buffer capacity of 0.01 to 0.1 is generally adequate.

4. 5.

The ingredients should be dissolved in carbon dioxide free water and allowed some time to establish equilibrium. pH and Buffer capacity of the completed buffer solution to be verified. The procedure remain same for basic buffers.

Buffered Isotonic Solutions

Buffered Isotonic Solutions Pharmaceutical solutions meant for Application of delicate membranes of body should also be adjusted to approximately the same osmotic pressure as that of the body fluids. Isotonic soln. do not cause any swelling or contraction of the tissues with which they come into contact No discomfort when instilled in the eye, nasal tract, blood or other body tissues.

Buffered Isotonic Solutions Definition-solution which maintains the isotonicity and pH as that of body fluids

Isotonic dilute fluids

Iso-osmotic solutions Are those solutions which produce the same osmotic pressure as that of cell content in question -but solvent permeable through the cell membrane their by altering tone of the cell Example-1.8% solution of urea Iso osmotic means It does not necessarily isotonic 1.8% urea soln. has same Iso-osmotic pressure that of 0.9% Nacl solution, but not isotonic solution 1.8% urea solution produces haemolysis due to permeability of water

Isotonic solutions Are those solutions which produce the same osmotic pressure as that of cell contents in question without net gain or loss of water by both the solution-provided cell membrane impermeable to solutes Isotonic solutions are Iso-osmotic as well as isotonic with cells and membrane

Buffered Isotonic Solutions Small qty. of blood mixed with aqueous concentration of sodium chloride solutions for verifying toxicity. When Blood cells mixed with 0.9% NaCl Solution (isotonic) Retain their normal size. Cells to shrink. Cells to Swell and burst.

2.0% NaCl Solution (hypertonic) 0.2% NaCl Solution (hypotonic) Isotonic Eq. Equal Osmotic Pressure

0.9% w/v Sodium chloride solution 5.0% w/v Dextrose solution 2.0% w/v Boric acid solution

Hypertonic solutions Defined as those solutions containing the solute in higher conc. than that is required for isotonic solutions Which causes red blood cell -shrinkage

Hypotonic solutions Defined as those solutions containing the solute in lower concentration than that is required for isotonic solutions Cause red blood cell to burst- hemolysis

Measurement of tonicity Isotonicity value: Con. Of an aqueous NaCl solution having the same colligative property as the solution in question. Apart form Nacl no of chemicals and drugs used in the formulation These ingredients contribute to tonicity of solution Methods are needed for verifying the tonicity and adjusting the isotonicity of the solution Determination of tonicity Hemolytic method Cryoscopic method or Depression of freezing point.

Measurement of tonicity (contd ) Hemolytic method (used to determine isotonic value) Red blood cells are suspended in various solutions and the appearance of red blood cells is observed for swelling, bursting and wrinkling. In hypotonic solution oxyhemoglobin released-which is directly proportional to no of cells hemolysed In hypertonic solution, the cells shrink and become wrinkled or crenated In isotonic solution ,the cells do not change their morphology

Colligative properties Lowering of volatile point Elevation of boiling point Depression of freezing point Osmotic pressure of solution These properties depend on no of particles and not on the nature of solvent

Colligative property of Molecule Depression of freezing point.( Tf)

Used to determine isotonicity values Freezing point of water 0r C, when Sodium chloride added-freezing point decreases. The depression of FP of blood ( o.52 rC), the drug solution should have FP similar to blood ( o.52 rC). Same osmotic pressure as blood.

Depression of freezing point.( Tf) Given by equation Tf = K f C Tf = Where freezing point of depression K f = Molar depression constant C = Molar concentration Actually FD is always grater than value obtained To compensate for this deviation from Vant Hoff factor i Tf = i K f C new factor L= i K f

Tf = L C

OR

L =

Tf / C

The L value is obtained from the freezing point lowering of a solution of a compound that is isotonic with blood or lacrimal fluid The specific value for L is represented by L iso which is defined as the specific value of L which is equal to i K f at a con. of drug which is isotonic with blood and lacrimal fluid L iso value for 0.9% Nacl whose FD is O.52 -0.5/ 0.154= 3.4 L = Tf / C 0.154 is molar conc. of 0.9% w/v Nacl

Methods of adjusting the tonicity Osmotic pressure is not readily measurable quantity But freezing point of depression is easily measurablemethods types

Class- IClass-II

1) Cryoscopic method 2) Sodium chloride equivalent method

1) White - Vincent method 2) Sprowls method

Class I method of adjusting isotonicity In this type ,sodium chloride or other substances are added to the solution in sufficient quantity to make it isotonic Then the preparation is brought to its final volume with isotonic or buffered isotonic diluting solutionmethods types

Class- I

1) Cryoscopic method 2) Sodium chloride equivalent method

Class-II method of adjusting isotonicity In this type ,water is added in sufficient quantity to make solution isotonic Then the preparation is brought to its final volume with isotonic or buffered isotonic diluting solutionmethods types

Class-II

1) White - Vincent method 2) Sprowls method

Cryoscopic method of adjusting isotonicity Water freezing point is 0 0C Blood contains number of substances-carbonic acid , carbonates, salts of phosphoric acid Results in DOFP -0.52 0C When substances added such of Nacl to water freezing point of water decreases-which depend on con. Of Nacl added 1% w/v of Nacl decreases freezing point of water -0.58 0C In order to make drug solution isotonic FPD should be maintained at -0.52 0C