Top 16 Properties of Acidity and Alkalinity | Biophysics

The following points highlight the top sixteen properties of acidity and alkalinity.

1. The reaction of the tissues and tissue fluids of the body is nearly natural excepting gastric juice. The slight changes in reaction have a significant physiological effect. In case the reaction of the so­lution is made slightly more acid or more alkaline, the heart stops.

2. Acidity is considered by an excess of hy­drogen ions (H⁺) over hydroxyl ions (OH^–) and al­kalinity by an excess of OH^– over H⁺.

3. The intensity of acidity depends upon the amount of hydrogen ions in excess, and the inten­sity of alkalinity depends upon the excess of hy­droxyl ions.

If the amounts of hydrogen and hy­droxyl ions are the same, the solution is neutral. Pure water is a neutral solution and it contains H⁺ and OH^– in equal proportions. It is never free from H⁺ and OH^–.

The ions are formed by ionisation, thus:

4. The hydrogen ion (H⁺ or proton) with its high ratio of charge cannot exist free in aqueous solution.

It becomes associated with one or more water molecules to form a hydrated hydrogen ion called hydroxonium ion (H₃O⁺).

5. The dissociation of an acid should be

But it is convenient to neglect the hydration and must be written as:

6. The concentration of H⁺ can be increased without increasing OH^– by adding an acid which ionizes to form H⁺.

Example:

Similarly, the concentration of OH^– can be increased without increasing H⁺ by adding an al­kali.

Example:

7. It must be noted that even in the most acid solutions there are always some OH^– and in alka­line solutions there are always some H⁺.

8. Aqueous solutions always contain both H⁺ and OH^– because water always ionizes to some ex­tent.

9. The ionisation of water is a reversible re­action admitting the Law of Mass Action. This clearly explains that the product of the concentra­tions of H⁺ and OH⁻ maintains a constant ratio to the concentration of water in any system.

This is expressed as:

10. If H⁺ is added to this above system the left hand side of the equation will be increased. To re-establish equilibrium OH^– must decrease by com­bining with H⁺ to form water until the equation is balanced.

In other words, if H⁺ are added to a solu­tion, the ionisation of water will be depressed and there will be fewer OH” then in pure water. Simi­larly, addition of OH^– will depress the ionisation of water and reduce the concentration of H⁺.

11. As a matter of fact, water is ionised to such a small extent that even if all the H⁺ and OH⁻ are recombined to form water, the increase in the concentration of water molecules will be negligi­ble. Considering the effect of the addition of H⁺ or OH^– to a system it can be neglected that the addi­tion will cause a minute increase in the concentra­tion of H₂O.

12. In pure water or a perfectly neutral solu­tion, [H⁺] = [OH⁻]. Hence [H⁺]² = Kw.

13. If [H⁺] is greater than 10^-7 (e.g. 10⁻⁴), the solution is acid. If [H⁺] is less than 10⁻⁷ (e.g. 10⁻¹¹), the solution is alkaline.

14. HCl and NaOH are practically 100% ion­ised. Solutions of these substances are almost com­pletely ionised when the solutions are more diluted than N/10. The ionisation is not complete at con­centration greater than this.

15. With weak acids (e.g., acetic acid) 100% dissociation is attained when the solution is weaker than N/10. Therefore, N/10 acetic acid is much less acid than N/10 HCl.

16. The reactions of all biological fluids oc­cur between [H⁺]10⁻¹ and 10⁻¹⁰.