Concept of Buffer solution GANESH DHAKAL PG MEDICAL

Concept of Buffer solution GANESH DHAKAL PG MEDICAL BIOCHEMISTRY

• BUFFERS. • Definition : Buffers are solutions which can resist changes in p. H when acid or alkali is added.

Composition of a Buffer • Buffers are of two types: • a. Mixtures of weak acids with their salt with a strong base or b. Mixtures of weak bases with their salt with a strong acid. • A few examples are given below: • i. H 2 CO 3/Na. HCO 3 (Bicarbonate buffer) (carbonic acid and sodium bicarbonate)

• ii. CH 3 COOH/CH 3 COO Na (Acetate buffer) (acetic acid and sodium acetate) • iii. Na 2 HPO 4/Na. H 2 PO 4 (Phosphate buffer)

Factors Affecting p. H of a Buffer • The p. H of a buffer solution is determined by two factors: • a. The value of p. K: The lower the value of p. K, the lower is the p. H of the solution. • b. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in a buffer solution may be varying widely, with no change in p. H, so long as the ratio of the concentrations remains the same.

Factors Affecting Buffer Capacity • i. On the other hand, the buffer capacity is determined by the actual concentrations of salt and acid present, as well as by their ratio. • ii. Buffering capacity is the number of grams of strong acid or alkali which is necessary for a change in p. H of one unit of one liter of buffer solution. • iii. The buffering capacity of a buffer is defined as the ability of the buffer to resist changes in p. H when an acid or base is added.

How do Buffers Act? • i. Buffer solutions consist of mixtures of a weak acid or base and its salt. • ii. To take an example, when hydrochloric acid is added to the acetate buffer, the salt reacts with the acid forming the weak acid, acetic acid and its salt. • Similarly when a base is added, the acid reacts with it forming salt and water. Thus changes in the p. H are minimized

• CH 3–COOH + Na. OH → CH 3–COONa + H 2 O CH 3–COONa + HCl → CH 3–COOH + Na. Cl • iii. The buffer capacity is determined by the absolute concentration of the salt and acid. • But the p. H of the buffer is dependent on the relative proportion of the salt and acid

• iv. When the ratio between salt and acid is 10: 1, the p. H will be 1 unit higher than the p. Ka. When the ratio between salt and acid is 1: 10, the p. H will be 1 unit lower than the p. Ka.

Effective Range of a Buffer • A buffer is most effective when the concentrations of salt and acid are equal or when p. H = p. Ka. • The effective range of a buffer is 1 p. H unit higher or lower than p. Ka.

• Since the p. Ka values of most of the acids produced in the body are well below the physiological p. H, they immediately ionize and add H+ to the medium. This would necessitate effective buffering. • Phosphate buffer is effective at a wide range, because it has 3 p. Ka values.

Henderson-Hasselbalch equation • The relationship between p. H, p. Ka, concentration of acid and conjugate base (or salt) is expressed by the Henderson-Hasselbalch equation, • p. H = p. Ka + log [base]/(acid) • When [base] = [acid]; then p. H = p. Ka Therefore, when the concentrations of base and acid are the same, then p. H is equal to p. Ka. Thus, when the acid is half ionized, p. H and p. Ka have the same values.

Application • The Henderson-Hasselbalch's equation, therefore, has great practical application in clinical practice in assessing the acid-base status, and predicting the limits of the compensation of body buffers.

Buffers present in ECF • • Present in plasma, csf, lymph, etc. Bicarbonate buffer. Phosphate buffer Protein buffer

Buffers present in ICF • • Bicarbonate buffer. Phosphate buffer Protein buffer Hemoglobin buffer.

Bicarbonate Buffer System • i. The most important buffer system in the plasma is the bicarbonate-carbonic acid system (Na. HCO 3/H 2 CO 3). • It accounts for 65% of buffering capacity in plasma and 40% of buffering action in the whole body. • ii. The base constituent, bicarbonate (HCO 3–), is regulated by the kidney (metabolic component). iii.

• While the acid part, carbonic acid (H 2 CO 3), is under respiratory regulation (respiratory component). • iv. The normal bicarbonate level of plasma is 24 mmol/liter. • The normal p. CO 2 of arterial blood is 40 mm of Hg. • The normal carbonic acid concentration in blood is 1. 2 mmol/L. The p. Ka for carbonic acid is 6. 1.

The bicarbonate carbonic acid buffer system is the most important for the following reasons: • a. Presence of bicarbonate in relatively high concentrations. • b. The components are under physiological control, CO 2 by lungs and bicarbonate by kidneys.

NOTE: • As compared to plasma bicarbonate buffer the phosphate buffer has less buffering capacity. This is due to the conc. Of bicarbonate buffer is high as compared to that of phosphate buffer.

Protein buffer • Protein buffer plays an imp. role in blood plasma and ICF(other then RBC), but it is of less significance in CSF and lymph. • Hb is the chief protein buffer of RBCs. • All proteins serve as buffer because of there free amino and carboxylic groups, which can accept and donate H+ ions, respectively.

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