ADVERTISEMENTS:

In this article we will discuss about:- 1. Definition of pH 2. Variation in pH and Interpretation 3. Determination.

Definition of pH:

pH is defined as the negative of logarithm (base 10) of the hydrogen ion concentration.

Or,

ADVERTISEMENTS:

It is defined as the logarithm of the reciprocal of the hydrogen ion concentration.

All vital activities are affected by H+ concen­tration. Hydrogen ion concentration must be ascer­tained before the pH is calculated. For strong elec­trolytes, [H+] may be substantially the same as the total concentration, if complete ionisation is as­sumed. But for weak electrolytes, [H+] must be ob­tained by the calculation from the ionisation con­stant.

Variation in pH and Interpretation:

A solution of pH 3 contains 10-3 gram H+ per litre.

ADVERTISEMENTS:

A solution of pH 5 contains 10-5 gram H+ per litre.

A solution of pH 8 contains 10″8 gram H+ per litre.

A solution of pH 3 has 10 times the [H+] of one of pH 4 and 100 times that of a solution of pH 5.

As [H+] increases, pH decreases in such a way that for one unit increase in pH the [H+] increases 10 times. It means that the higher the pH the lower will be the acidity.

A neutral solution has pH 7. Pure water (the neutral solution) is ionised as follows:

[H+] x [OH] = Kw

or, [H+] x [OH] = 1 x 10−14

Putting logarithm on both sides,

log [H+] + log [OH] = log1 + log10−14.

ADVERTISEMENTS:

or, −log[H+] – log[OH] = 0 +14 log 10. [Mul­tiplying both sides by – sign]

or, pH + pOH= 14 [Since, pH = 7].

... pH = pOH i.e. [H+] = [OH].

Therefore, a solution with pH less than 7 is acid and higher than 7 is alkaline. The pH range is 0 to 14 only. It should be noted that the pH scale is logarithmic, not numerical.

ADVERTISEMENTS:

pH 6.5 does not represent a [H+] half-way be­tween 6 and 7.

Actually, pH 6.5 = [H+] 3.2 x 10−7

pH 6.0 = [H+] 10 x 10−7 = [H+] 10−7

Determination of pH:

For weak electrolytes with which the physiologic chemistry is concerned, this may be calculated by the law of mass action:

HA = Un-dissociated weak acid, Ka = Dissocia­tion constant for the acid.