Physical Pharmacy Buffers and Buffering Agents PDF

sinaiuniversity.net  Physical Pharmacy Buffers and Buffering Agents 10/31/2015 2 Buffer capacity: It is a measure of the magnitude of the resistance of solution to the change in pH on adding an acid or base Other derivatives: Buffer capac...

sinaiuniversity.net  Physical Pharmacy Buffers and Buffering Agents 10/31/2015 2 Buffer capacity: It is a measure of the magnitude of the resistance of solution to the change in pH on adding an acid or base Other derivatives: Buffer capacity = buffer index = buffer efficiency = buffer coefficient = buffer value. Other definition for buffer capacity: It is the ratio of the increment of strong base or acid to the small change in pH brought by this addition. B = buffer capacity ΔB = the small increment in gm equivalent / liter of strong base or acid added to the buffer. Δ pH = pH change. Van Slyke’s Equation for Buffer Capacity The buffer capacity calculated from the previous equation is only approximate. It gives the average buffer capacity over the increment of base added. Van Slyke developed a more exact equation, Where C is the total buffer concentration (i.e., the sum of the molar concentration of the acid and the salt). Maximum Buffer Capacity: An equation expressing the maximum buffer capacity may be derived from the buffer capacity formula of Van Slyke. The maximum buffer capacity occurs where pH = pKa, or, in pKa equivalent terms, where [H3O+] = Ka. Substituting [H3O] for Ka in both numerator and denominator of equation gives In which C is the total buffer concentration. ( )2  Biological Buffers Blood: pH of blood is maintained at 7.4 by the component of plasma and erythrocytes. The plasma contains: 1- Carbonic acid/carbonate 2- Phosphoric acid / sodium phosphate Erythrocytes (RBSs) contains 1- Oxyhaemoglobin/haemoglobin 2- Phosphoric acid / potassium phosphate The pH range of blood is 7 → 7.8 (above or below this range → life in danger) Lachrymal fluid or tears: pH is about 7.4 ( range 7 → 7.8) Urine: is about 6 (range 4.5 → 7.8) when the pH of urine is altered above or below this range kidney is diseased.  Pharmaceutical Buffers In pharmaceutical industry we adjust the pH of the product for maximum stability or maintain the pH of the product within the optimum physiological range. 1- Buffers in tablet formulation: Buffers employed in tablet or capsule formulations containing acidic drugs to reduce gastric irritation or antacid drugs such as sodium bicarbonate, sodium citrate or magnesium carbonate. 2- Buffers in ophthalmic preparations: Buffers are used here to maintain the pH of the product within the pH range of lachrymal fluids. The lachrymal fluid has good buffering capacity where the solutions within 3.5 – 10.5 can usually be tolerated Outside this range → eye irritation and increase lacrimation. The most commonly used buffers in ophthalmic preparations include borate, phosphate and carbonate buffers. 3- Buffers in parental preparations: In parentral, the consideration of pH is very important, where high pH (above 9) → tissue necrosis while highly acidic pH (below 3) extreme pain in site of injection. The ideal pH for parental is 7.4 similar to the blood. There must be compromised between the blood pH and the solubility and stability of the product. The most widely used buffers in parental are acetate, phosphate, citrate and glutamate buffers. 4- Buffers in creams and Ointments: Topical preparations on storage usually ender go change in pH, this affect the stability of the drugs therefore, buffers must be induced in this formulations. The most widely used buffers here are citrate and phosphate. THANK YOU