Question

What is the pH scale, how does it work, why is ocean pH potentially game over for planet Earth?

Answer 1

Ans. pH is a measurement of H⁺ ion or H₃O⁺ (hydronium ion) concentration in a solution.

The standard scale taken is related to equilibrium constant of water, K_w. K_w is exactly equal to 10^-14 at room temperature, as follow-

                        H₂O_(l)   --------> H₃O⁺_(aq) + OH^-_(aq)                  ,

Where, [H₃O⁺] = [OH^-] = 10^-7 M

Or, equilibrium constant, Kw = [H₃O⁺] x [OH^-] = (10^-7) x (10^-7) = 10^-14

Since, pH is taken a measure of [H⁺] = [H₃O]⁺ concentration, we need to focus on [H⁺].

We have, [H⁺] = 10^-7 M. 10^-7is a very small value for creating a scale. Imagine a scale of 0.0000001 measurement units. H⁺ ion concentration can rise several folds more (say, 10000-fold, a pH change of 4 units on normal scale). Depicting a thousand-fold change on the 10^-7 is not feasible. There are similar limitations in adopting the scale.

So, pH is defined as -log [H⁺]. pH = -log [H⁺].

pH of pure water = - log 10^-7 = 7

So, we have a value of normal counting range (7). Similarly, the pH scale ranges between 0 to 14 depending on dissociation of water at either extreme (100% dissociation or no dissociation at all), respectively.

So, a normal pH scale has range of 0 – 14. Where 0 is most acidic, 14 is most basic and 7 is the neutral pH (that of pure water at room temperature).

Ans. 2. Ocean constitute around 70% surface area of the Earth and most of the water on the planet. Ocean water, like other water (that in you glass, or pond or lake) usually dissolves a small amount of CO₂ forming carbonic acid as follow-

H₂O + CO₂ ------> H₂CO₃ ----> H⁺ + HCO₃^-

Carbonic acid is a weak acid and it acts as buffer in aqueous solution.

Change in temperature alters rate of CO₂ dissolution in water, thus can also act to alter CO₂ dissolved in oceanic water. Therefore, ocean is one of the major CO₂ reservoir and buffering system on Earth.