why pH is not greater than 14?

Question

pl.give me details abot why pH=>14.

Answer 1

It is rare but can happen. Materials which are super acids or bases can be outside of the range for pH which most people are familiar with. The reason most people don't see this happen very often is that you need to remember that pH is a logarithmic scale so each unit change represent a 10 fold change in the acidity of a solution so going from 7 for instance to 14 would represent a 10000000 change in the concentration of H+ ions in the solution. This is extreme and not observed in everyday life outside of the laboratory.

Answer 2

It happens all the time. What is the pH of 10% NaOH, 20%, 50%?

It is a matter of definition. The scale is defined between 0 and 14.

You may, however, change the definition and use an extended range based solely on the formula. pH = - log [H+]. A purist chemist will probably through you the book but engineers will be happy to work. You can put on paper anything you want, some will like it other won't.

If you take NaOH for example (MW=40), the highest pH you can use according to the classic definition will be 14 which is equivalent to a concentration of 4%. So 5% NaOH does not have a pH value in the classic sense. But you still can calculate it using the formula using your "extended definition".

Different industries use different "extended definitions" and if the concentration is critical to you as an engineer you need to investigate the definition.

I have a personal story, I was preparing elastomers for compatibility with solutions in the field and I asked the engineer (platinum processing engineer), how about the pH of the solution and he said basic. Well in short, in this industry a "basic pH" is everything above 0. They rarely go above 1. So "0 to 1 is basic", and "less than 0 is acidic". I had to scramble in the field to find, obviously, completely different elastomers.

Answer 3

There are assumptions about the species present when you are using the pH scale. The species are far apart, they don't interact with each other to a large extent, and hydrolysis of water is the dominant equilibrium. There are others such as species "activity". You are not taught most of these when you learn pH, because they can be assumed if you stay within the scale. Outside of these limits you have to go into different scales for measuring and there are several that are used that are fairly complicated to describe.