PH and Pk

Acidic and basic are two extremes that describe chemicals, just like hot and cold are two extremes that describe temperature. pH is the negative log of the activity of the hydrogen ion in an aqueous solution. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH of 7.

It has also been suggested that the "p" stands for the German Potenz (meaning "power"). Others refer to French puissance (also meaning "power", based on the fact that the Carlsberg Laboratory was French- speaking). Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen.

A pH level of is 7.0 at 25°C is defined as 'neutral' because the concentration of H 3 O + equals the concentration of OH − in pure water. pH is a measure of hydrogen ion concentration; pH is calculated by the formula pH = - log [H + ]

History The concept of p[H] was first introduced by Sørensen It was revised to the modern pH in The exact meaning of the "p" in "pH" is disputed, But according to the Carlsberg Foundation pH stands for "power of hydrogen".power

Knowledge of the dissociation of weak acids and bases is important as it provide base to understand the influence of intracellular pH on structure and biologic activity. Many biochemicals possess functional groups that are weak acids or bases. Carboxyl groups, amino groups, and phosphate esters, whose second dissociation falls within the physiologic range, are present in proteins and nucleic acids, most coenzymes, and most intermediary metabolites.

PH = 1 /[H + ] PH = log 1 – log H + As log 1 =0 so PH= – log H + Example 1: What is the pH of a solution whose hydrogen ion concentration is 3.2x10– 4 mol/L?

PKa

We term the species which donate H + ( R—NH3+) the acid and those species which accept H + (R—NH2) its conjugate base. Similarly, we may refer to a base ( R—NH2) and its conjugate acid ( R—NH 3+).

We express the relative strengths of weak acids and bases in terms of their dissociation constants. Shown below are the expressions for the dissociation constant (K a) for two representative weak acids, R—COOH and R—NH3+.

Definition of Pka pKa is used to express the relative strengths of both acids and bases. For any weak acid, its conjugate is a strong base. Similarly, the conjugate of a strong base is a weak acid. The relative strengths of bases are expressed in terms of the pKa of their conjugate acids. pK a is related to Ka as pH is to [H+]. The stronger the acid, the lower is its pKa value.

The strength of an acid can be determined by considering the extent to which it dissociates in a given solvent. The equilibrium constant in water is given by:

K a is known as the acidity constant of the acid. Generally, the numerical value of K a will be small, and for this reason, K a is usually converted into pK a by the following equation: Thus, the smaller the value of pK a, the stronger the acid.

Similarly, the strength of a base is determined by using pK b. For the example of BOH, a base, in water, K b can be determined by the following formula:

Buffer Solutions

A buffer solution is one in which the pH of the solution is "resistant" to small additions of either a strong acid or strong base.

Types of Buffer Solution Acidic buffer solutions: An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt. A common example would be a mixture of Acetic acid and sodium acetate in solution.

Alkaline buffer solutions: An alkaline buffer solution has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts. A frequently used example is a mixture of ammonia solution and ammonium chloride solution.

How do buffer solutions work? A buffer solution has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it Acidic buffer solutions We'll take a mixture of Acetic acid and sodium acetate as typical. Acetic acid is a weak acid, and the position of this equilibrium will be well to the left: CH3COOH CH3COO- + H +

Adding an acid to this buffer solution The buffer solution must remove most of the new hydrogen ions otherwise the pH would drop markedly. Hydrogen ions combine with the Acetate ions to make Acetic acid. Although the reaction is reversible, since the Acetic acid is a weak acid, most of the new hydrogen ions are removed in this way. CH3COO- + H + CH3COOH

Adding an alkali to this buffer solution Alkaline solutions contain hydroxide ions and the buffer solution removes most of these H +. The most likely acidic substance which a hydroxide ion is going to collide with is an acetic acid molecule. They will react to form acetate ions and water. CH3COOH + OH - CH3COO - + H2O

Adding Acid to alkaline buffer The most likely basic substance which a hydrogen ion is going to collide with is an ammonia molecule. They will react to form ammonium ions.

Adding an alkali to this buffer solution The hydroxide ions from the alkali are removed by a simple reaction with ammonium ions.

References; Harper's Illustrated Biochemistry, Color atlas of biochemistry by J. Koolman and K.H. Roehm Wikipedia.org

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