Why are pH values mostly in a Range of 0-14 pH scale?

Why are pH values mostly in a Range of 0-14 pH Scales?

A pH outside the predictable 0-14 pH scale (pH range) is possible, but various boundaries caused by the instruments and the solution itself limit it from measuring such a substance.

What are the pH scales?

  • The pH scale is used to examine that the substance is acidic or basic in nature, and to estimate how strong a chemical present?
  • A pH value or pH is a number that ranges from 0 to 14 for utmost common chemicals, with pH7 denoted as the middle or neutral point.
  • Less than pH Values 7.0 will be in acidic in nature or pH indicator shows acidity. An increase in pH value 7 indicates alkalinity, which increases as the value increases.
  • The pH scale is not a linear scale. In simple words, an acid with a pH value of 3.0, It is not twice as strong as an acid with a pH value of 6.0.
  • An important difference to know is that the pH scale is a logarithmic scale.
  • pH scale examples 0-14 as shown in below figure,
pH scale
Credit: Freepik

Definition of pH:

By definition, pH = −log10 [aH+]

  • where aH + =is the activity of the hydrogen ion (H+) or hydronium ion (H3O+), and the hydrogen ion activity very closely approximates the hydrogen ion concentration.

The practical pH scale is defined:

pH = pHS + [(E ES)/k]

  • E = recorded potential where the galvanic cell contains the solution under test (pH)
  • ES = recorded potential where the galvanic cell contains the suitable buffer solution for calibration (pHS)
  • k = change in potential/unit change in pH and is derived from the Nernst equation (as follows)

k = loge(10) × (RT/nF)

  • R = 8.314 J/mole/°K
  • T = temperature (°K)
  • n = moles/half-reaction
  • F = Faraday constant, 96485 C/mole

What is the meaning of the p in pH stand for?

  • The idea of pH was 1st introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in the year 1909.
  • In the first papers, the notation had H• as a subscript to the lowercase p, thus: pH•
  • The letter “p” may stand for the French “puissance” word, German word “Potenz”, or Danish word “potens”, which has the meaning of “power”, or “potential”. 
  • Some literature bases state that the “pH” stances for the Latin term pondus hydrogenii (quantity of hydrogen) or potentia hydrogenii (power of hydrogen), although this is not maintained by Sørensen’s writings.

Related: Basic Principle of pH Meter for measuring pH , pH Electrodes

Do we have pH values outside the 0-14 pH scale range?

  • Tentatively speaking, the pH scale should truly range from negative infinity to positive infinity. This assertion is supported by its definition, which indicates that a substance’s pH is determined by the negative log (logarithm) of the concentration of hydrogen ions.
  • However, in truth, most solutions we would find in a standard lab. which have a pH value between 0 to 14 pH value.
  • This is because, in order to reach pH levels below 0 or above 14, one would require extraordinarily acidic or basic solutions, respectively.
  • The pH scale should actually go from negative infinity to positive infinity, according to theory.
  • This statement is supported by its definition, which indicates that a substance’s pH is determined by the negative logarithm of the concentration of hydrogen ions.
  • In actuality, the majority of solutions we would discover in a typical laboratory have a pH value between 0 and 14.
  • This is due to the fact that extremely acidic or basic solutions would be necessary to achieve pH levels below pH 0 or above pH 14.
  • Based on its molarity, a saturated sodium hydroxide solution (NaOH) solution should have a pH of 15.
  • The existence of water, which prevents the solvation (breakdown) of bigger molecules, prevents the chemical from completely solvating.
  • As a result, the amount of hydroxide ions (OH-), which are ions that absorb hydrogen ions and raise the solution’s pH, will start to fall.

In short,

  • The pH value of a solution is often defined as the negative base-10 log of the hydronium ion concentration of the solution. On the other hand, there are no more than 1M hydroxide ions, which leads to a pH of no greater than 14.
  • Calculated pH values typically range from 0 to 14, while negative pH values and values higher than 14 are entirely possible. Since pH is a logarithmic scale, a change of one pH unit corresponds to a tenfold change in hydrogen ion concentration.
  • It is ten times more acidic to have a pH value below 7 than to have one above it. A pH of 4 is ten times more acidic than a pH of 5, while a pH of 6 is 100 times (10 times 10) more acidic.

 Living systems pH value/ pH scale:

Gastric acid1.5–3.5
Human skin4.7
Granules of chromaffin cells5.5
Blood (natural pH)7.34 to 7.45
Cerebrospinal fluid (CSF)7.5
Mitochondrial matrix7.5
Pancreas secretions8.1
ph scale examples 0-14

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