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Why pH Value is Measured between 0 and 14?

Value of pH is determined between 0 and 14 that shows the alkalinity and acidity of the solution. pH 7 is considered as neutral.
Curiosity is the first and foremost trait of a science student. Remember when your chemistry teacher in high school taught you about the pH scale? Remember how you were told pH value is measured between 0 and 14? Well, if you are reading this, you're probably a science enthusiast and probably do have the basic traits of a science student. So if you did wonder why pH is measured only on a scale of 0 to 14 and not negative values or in 100s, you can get the answer to that question here.

The answer is, well, pH is not actually measured between 0 and 14. Most commonly found solutions have their pH value lying between this range, but in fact, the pH value of a solution can be lesser or greater than this range.

pH valueThe general definition of the pH value of a solution is the negative base-10 log of the concentration of hydronium ion in the solution.

In the pharmaceutical industry, we're used to not more than 1M in concentration under normal cases. With that being said, the pH value could be on 2 extremes in that case. One far end is not more than 1M of hydrogen ions, which results in a pH value of not more than 0. While on the other end is not more than 1M of hydroxide ions which results in a pH value of not more than 14.
Related: Working Principle of pH Meter

This can be verified by mathematical calculations involving formulae that express equilibrium constant and dissociation constant of the dissociation of water into hydrogen and hydroxide ions which are positive and negative respectively.

Well, with that on one hand, we also must remember that nature ain't a very good fan of following rules that humans lay down. So rest assured, the concentration of solutions in nature need not necessarily be less than 1M.

The pH value goes out of the 0-14 range when the concentration of the solution exceeds 1M. While in basic practice the pH value is the negative log of hydrogen ion concentration, what chemistry graduates would know is that technically the value is a measure of the activity of hydrogen ion. While that is hard to estimate or experimentally find out, the former method suits for most cases with appreciable accuracy.

So if you're wondering about what other factors that could influence the pH value of a solution, one major factor would be the temperature at which the test of the pH of the solution is carried out. Why? I hope you would not ask that question, because we believe that as a science enthusiast you know that temperature is a villain who interferes and alters results in most experiments and tests. So obviously, the temperature would affect the equilibrium and dissociation rates, which would directly result in a change of pH value. For example, water which expresses pH value of 7 in room temperature would express a different pH at a different temperature.

We end this article with a practical implication of this theory. Even the slightest or drop or hike in the pH value of a person's blood is a sign that they are sick because it would have been caused by a hike or a drop in body temperature which had altered the dissociation rates of the blood solution.

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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