Hasselbach equation

Henderson-Hasselbach equation A simplified version of the relationships used in calculations on buffer solutions. 

As the acid content within the GI tract is known to change as the body ages, it is also important to evaluate the dissociation constant(s) (Ka) of the drug(s) to be used. From the Henderson-Hasselbach equation of weak acids ... 

Figure 10-3. Theoretical titration curves for the model compounds of Asp and His obtained from REX-CPHMD simulations [41]. Solid curves are the obtained by fitted the computed deprotonated fraction to the generalized Henderson-Hasselbach equation. The dashed lines indicate the computed pKa values...

Acid-base buffers comprise both a weak acid or base and its respective salt. Calculations with buffers employing the Henderson-Hasselbach equation are introduced and evaluated, thereby allowing the calculation of the pH of a buffer. Next, titrations and pH indicators are discussed, and their modes of action placed into context. 

The Henderson-Hasselbach equation, Equation (6.50), relates the pH of a buffer solution to the amounts of conjugate acid and conjugate base it contains ... 

Strategy we first calculate the number of moles of hydrochloric acid added. Second, we calculate the new concentrations of ethanoic acid and ethanoate. And third, we employ the Henderson-Hasselbach equation once more. 

A glass electrode yields a response that is directly linked to the activity of protons. This is not true for optical methods (spectrophotometry and spectrofluorometry) using pH indicators. In fact, these methods are based on the optical determination of the concentrations of the acidic form, [A], and the basic form, [B], of the indicator1 and use of the well known Henderson-Hasselbach equation... 

However, the Henderson-Hasselbach equation results from an oversimplification that deserves special attention. In fact, the equilibrium constant K characterizing the acid-base equilibrium must be written with activities ... 

The Henderson-Hasselbach equation allows the ratio of ionized un-ionized compound to be found if the pH and pKa are known. Consider carbonic acid (H2CO3) bicarbonate (HC03 ) buffer system... 

Drug dissociation constants are experimentally determined by manual or automated potentiometric titration or by spectrophotometric methods.40 Current methods allow determination of pXa values with drug concentrations as low as 10 to 100 pM. For highly insoluble compounds (concentration <1 to 10 pM), the Yesuda-Shedlovsky method41 is commonly used where organic cosolvents (i.e., methanol) are employed to improve solubility. The method takes three or more titrations at different cosolvent concentrations, and the result is then extrapolated to pure aqueous system. The dissociation constant can also be determined with less accuracy from the pH-solubility profile using the following modification of Henderson-Hasselbach equation ... 

We have shown the cysteine thiol group as uncharged. The pAfa for this group in cysteine is about 10.3, and application of the Henderson-Hasselbach equation (see Section 4.9) indicates there will be negligible ionization at pH 7. Nevertheless, under the influence of a suitable basic group, e.g. arginine pATa 12.5, ionization to thiolate may be possible. In such an environment, thiolate may act as the nucleophile in the mechanism. 

By taking logarithms and then substituting the terms pK and pH for the negative logarithms of and [H+], respectively, we arrive at the Henderson-Hasselbach equations ... 

Benzoic acid was found to be more sensitive to acidity changes than was anticipated from the Henderson-Hasselbach equation relating dissociated and non-dissociated fractions at pH 3 and pH 4 [51]. However, the sensitivity of preservative activity to dilution at both pH values remained similar. 

As emphasized earlier, the concentration gradient of the drug in Eq. (1) refers to that of the unbound drug and its ionic distribution, which depends upon its acid-base properties. This can be modified by appropriate choice of excipients to ionize the drug by salt formation, thereby affecting the distribution of ionic versus nonionic species by acid-base equilibrium, using the Henderson-Hasselbach equation. All of the drug will eventually leave the depot and enter the body, but the rate may be reduced if membrane transport is limited by solubility of the neutral species within the membrane. 

The degree of ionization of a compound can be calculated from the Henderson-Hasselbach equation ... 

For example, an acid with a pKa of 4 can be calculated to be mainly nonionized in acidic conditionsat pH 1. Rearranging the Henderson-Hasselbach equation (Eq. 2),... 

These factors ensure that the concentration gradient is maintained, and so weak acids are often absorbed to a significant extent in the small intestine if they have not been fully absorbed in the stomach. Using the Henderson-Hasselbach equation, the degree of ionization can be calculated and the site and likelihood of absorption may be indicated. 

The pH of gastric juice is 1 to 3. The pKa of benzoic acid, a weak acid, is 4. Taking the pH in the stomach as 2, and using the Henderson-Hasselbach equation as described above, it can be calculated that benzoic acid is almost completely nonionized at this pH (Fig. 3.13) ... 

If a gel contains ionizable groups, it becomes a pH sensitive gel since the ionization is determined by pH in terms of ionization equilibrium. The relation of the degree of ionization a and pH is given by the Henderson-Hasselbach equation [122],... 

According to the Henderson-Hasselbach equation (3.6), an accurate approximation of the pK value is obtained from a semilogarithmic plot... 

The Henderson-Hasselbach equation is useful for calculating the molar ratio of base (proton acceptor) to acid (proton donor) for a given pH and pK or for calculating the pK, given the ratio of base (proton acceptor) to acid (proton donor). It can be seen that when the concentration of anion or base is equal to the concentration of undissociated acid (i.e., when the acid is half neutralized), the pH of the solu-tion is equal to the pK of the acid. 

The ratio of base to acid form at any given pH can be calculated from the pK with the help of the Hen-derson-Hasselbach equation. 

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