Pharmaceutical Aids And Necessities - pH,pOH And BUFFERS

The pH Of Solutions

The concept of pH is very convenient for expressing Hydrogen ion concentration.It was introduced by SORENSEN in 1909.Hydrogen ion concentration [H⁺] are typically quite small numbers.Therefore,it can be expressed in terms of pH.

pH is defined as the negative logarithm(log) of the base 10 of the H⁺concentration.

Mathematically,it may be expressed as pH = -log[H⁺],where[H⁺] is the concentration of hydrogen ion in moles per litre.

To calculate Hydrogen ion concentration we go through following step:

Pure water ionises as follows-

H₂O ⇌ H⁺+ OH^-

We can write the equilibrium expression as

K = [H⁺][OH^-] / [H₂O]

Since,water is so little dissociated,the concentration of on-dissociated molecules[H₂O], is presumed to be constant.We can write

K [H₂O] = [H][OH^-]

Kw = [H⁺][OH^-]

where Kw is called product of ionic water and is value is 1 x 10^-14at 25℃,i.e

Kw = [H⁺][OH^-]

log Kw = log[H⁺] + log[OH^-]

-log Kw = -log[H⁺] - log[OH^-]

pKw = pH + pOH

Since,Kw = 1 x 10^-14 and pKw = -log Kw = -(1 x 10^-14) = 14.00

∴ pH + pOH = 14

Pure water has equal concentration of [H⁺] and [OH^-] ions.Thus,

[H⁺][H⁺] = 10^-14

[H⁺] = 10^-7

Thus,for neutral water [H⁺] ion concentration is 10^-7 ion g/litre.The solution becomes acidic if [H⁺] > 1 x 10^-7 ion g/litre and alkaline if  [H⁺] < 1 x 10^-7 ion g/litre.Thus if [H⁺] or [OH^-] is known other can be calculated.

pH SCALE

Hydrogen ion concentration are complex figures to write and use in calculation since they contain negative powers of 10.In 1909,SORENSEN introduced the term pH.The pH expresses [H⁺] of an aqueous solution as a logarithmic function.pH offers a convenient mechanism of expressing a wide range of [H⁺] in small positive numbers.The letter p is used to denote the negative logarithm to the base 10.Measurement of pH can be easily done using a pH metre.

[H+] (mol dm-3)	pH	[OH-](mol dm-3)	pOH	Acidity or Basicity
100	0	10-14	14	Acidity
10-1	1	10-13	13	↑
10-4	4	10-10	10	Increasing
10-7	7	10-7	7	Neutral
10-10	10	10-4	4	Increasing
10-13	13	10-1	1	↓
10-14	14	100	0	Basicity

ACIDOSIS AND ALKALOSIS

Altered carbonic acid bicarbonate levels in the blood,if uncompensated change's the blood pH.By decreasing the amount of breathing (hypoventilation),insufficient carbon dioxide is released.The increased level of carbon dioxide in the blood leads to increased carbonic acid and hydrogen ion levels which results in respiratory acidosis.Fainting can occur from mild acidosis while Coma can be the result of severe acidosis.Alternatively,by hyperventilating (overbreathing) an excessive amount of carbon dioxide is expelled.Excessive loss of carbon dioxide shift the equilibrium to compensate for carbon dioxide loss.Hydrogen ion and bicarbonate ion combine to form carbonic acid,which decomposes to form water and carbon dioxide.The reduction in hydrogen ion concentration leads to respiratory alkalosis.

Acidosis and Alkalosis can occur due to metabolism.Normal carbohydrate metabolism result in the formation of acidic by-product,which can be termed as metabolic by acid.Lactic,pyruvic,and citric acids are examples of metabolic acids. Their concentration is ordmanly such that normal body buffer action prevents continuing acidosis. In diabetes mellitus, abnormal carbohydrate metabolism leads to acidosis. Lack of insulin secretion by the pancreas prevents glucose from being metabolized. In its continuing need for energy, the body instead metabolizes stored fats. Two of the products of fat metabolism are acids.Their dissociation increases the hydrogen ion concentration in extracellular fluids causing metabolic acidosis. If untreated, severe acidosis leads to coma and even death.

BUFFER SOLUTION

The solutions that are able to resist the changes in pH values are termed as buffer solutions. A buffer solution consists of a mixture of weak acid and its salt or of a weak base and its salt. To such solution when small amount of acid or alkali is added no significant change in the pH takes place.

Buffer solutions can be prepared either by mixing a weak acid with its salt or a weak base with its salt.

Types of Buffer Solutions: These are mainly of following two types:-

• Acidic Buffer Solution: The solution having a mixture of weak acid (e.g.acetic acid) and its salt (e.g.sodium acetate) is known as acidic buffer.
• Basic Buffer Solution: The solution having a mixture of weak base (e.g.ammonium hydroxide) and its salt (e.g.ammonium chloride) is a basic buffer.

Properties of a Buffer solution :

1. The pH of buffer solution remains constant.
2. The pH of buffer solution does not change.
3. The pH of buffer solution does not change even after adding small quantities of acids or base.
4. The pH of buffer solution does not change on dilution.

BUFFER ACTION

Let us consider acidic buffer which is prepared by mixing acetic acid and sodium acetate.Acetic acid is a weak electrolyte and sodium acetate is strong electrolyte and hence dissociation of acetic acid gets suppressed due to common ion(CH₃COO^-).The result is the solution having less H⁺ ions and more Na+ and CH₃COO^- ions.When small amount of acid is added,the H⁺ of it combines with CH₃COO^- to form undissociated CH₃COO^- eq.(ⅰ).The net result is that pH does not change.When small quantity of base is added the OH gets neutralized by acids and pH does not change as given by the reaction below eq(ⅱ).

H⁺ + CH₃COO^- ⇌ CH₃COOH     ------- (ⅰ)

OH^- + CH₃COOH ⇌ H₂O + CH₃COO^-     -------(ⅱ)

Similarly,in basic buffers having NH₄OH and NH₄Cl adding small quantity of base oh reacts with NH₄OH and pH does not alter as given by eq.(ⅲ)

OH^-+ NH₄⁺⇌ NH₄OH  {undissociated}   -------(ⅲ)

When a small quantity of acid is added, NH₄OH neutralises it and pH does not change given by eq. (iv)

H⁺ + NH₄OH ⇌ NH₄⁺ + H₂O --------(ⅳ)

It is possible to calculate the pH of acidic buffer by Henderson-Hasselbalch equation.

pH OF ACIDIC BUFFER SOLUTION AND MAXIMUM BUFFER ACTION : The hydrogen ion concentration,from dissociation of weak acid HA is given by the equation-

HA ⇌ H⁺ + A^-

Ka = [H⁺] + [A^-] / [HA]

OR  [H⁺] = Ka x [HA] / [A^-]

OR -log[H⁺] = -log Ka x [HA] / [A^-]

pH = log Ka x [A^-] / [HA] = pKa + log {[conjugate base] / [Acid]}

With the help of this equation it is possible to calculate the pH of a buffer solution of known concentration.Alternatively, one can make buffer solution of known pH.

The maximum buffer action is obtained when the concentration of the acid and the conjugate base

are equal because under these conditions.

pH = pKa + log 1 = pKa + 0 = pKa {which is constant}

If the concentration of acid is ten times the concentration of base,then

pH = pKa + log 1/10 = pKa - 1,and if

[Conjugate base] / [Acid] = 10 ,then

pH = pKa + log 10 = pKa + 1

It follows by this that a particular acid can employed for making a buffer solution of pH lying within the range of pKa - 1 to pKa + 1.The maximum buffer action can be attained in half neutralized acid i.e. [concentration of Conjugate base = concentration Conjugate acid].The pKa of acetic acid is 4.75 at 25℃ and hence mixtures of acetic acid and sodium acetate can be used to make buffer in the pH range of 3.75 to 5.75.

pH OF ALKALINE BUFFER -The pH of alkaline buffer solutions made from a weak base and its salts with strong acid cn also be calculated in the similar way.The ionisation of weak base BOH is given by,

BOH ⇌ [B⁺] + [OH^-]

∴ Kb = [B⁺][OH^-] / [BOH]

OR [OH^-] = Kb x [BOH] / [B⁺]

 ∴ log [OH^-] = log Kb + log [BOH] / [B⁺]

OR -log [OH^-] = -log Kb + log [BOH] / [B⁺]

OR pOH = pKb + log {[conjucate acid] / [base]}

Now, pH = 14 - pOH =14 - (pKb + log {[conjucate acid] / [base]})

In case  NH₄OH and NH₄Cl buffer, the conjugate acid is the NH₄⁺ and therefore its concentration is equal to the concentration of NH₄Cl i.e.salt.

BUFFER CAPACITY

Often it is necessary to know the effectiveness of a buffer on a quantitative basis. To do so, we employ the term buffer capacity (β), first introduced by van Slyke in 1922. Buffer capacity is defined as the amount of acid or base that must be added to the buffer to produce a unit change of pH. Hence,

β = d[B] /dpH

The buffer capacity always has a positive value, however, since addition of base increases the pH and addition of acid decreases the pH .Thus d[B] and dpH always have the same signs. The value of β depends not only on the nature of the buffer, but also on the pH,which is determined by the relative concentrations of the acid and its conjugate base.The graph shows plots of buffer capacity versus pH for CH₃COOH and CH₃COONa. We can see that buffer functions best around its pKa value of acids.This is not suprising for according to pH = pKa when [HA] = [A-] and there are equal amount of acid and conjucate base to react with added acid or base.

Buffer capacity of the 1M CH3COOH/1 M CH3COONa buffer system.The maximum of the peak occurs at pH = 4.76, which is also equal to pKa.

All this have been said so far applies well equally to buffer system of a weak base and its conjugate acid.

pH = pKa + log[B] / [BH⁺]

The criteria for buffers suitable for research in biological fields may be summarized as follows.They should:-

1. Possess adequate buffer capacity in the required pH range.
2. Be available in a high degree of purity.
3. Be very water soluble and impermeable to biological membranes.
4. Not to be toxic or biological inhibitors.
5. Not alter the solubility of other ingredients.
6. Be safe and should not interfere in the pharmacological actions of active ingredients.

PHYSIOLOGICAL BUFFERS

The pH of blood of a healthy person happens to remain constant. around 7-3-7.4. This occurs because of intricate mechanism of controlling pH by buffering. Normally acidic metabolites are produced in greater quantities than basic metabolites.Carbon dioxide, proteins and amino acids contribute to acidic metabolite formation. This can be counterbalanced by bicarbonate ions. The combined buffering capacity of blood (to neutralize acid) can be designated as 'alkali reserve' when alkali reserve gets depleted metabolic acidosis (acidic pH of blood) develops.

PHARMACEUTICAL BUFFERS

Buffer solutions of different types are needed for different purposes. Standard buffer solutions are solutions of standard pH.They find use for reference purposes in pH measurements and for carrying out specific tests which require adjustments or maintenance of a specified pH The special buffer solutions are also needed for specific microbiological and antibiotic assays. These could be prepared using the reagents specified in pharmacopoeias or standard books. There are many well-known combinations of chemicals to yield standard pH solutions. The some well-known names have been :Feldman's borate buffer (acid and alkaline), Sorenson's modified phosphate buffer etc.

PHARMACISTS THOUGHT

A DOCTOR OPERATES ON YOUR ORGAN BUT ONLY A PHARMACISTS CAN OPERATE ON YOUR CELLS.

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