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THE pH SCALE

🎨THE pH SCALE🎨 is something that we often use to describe solutions but do you actually know what the pH scale MEANS?


This post is much more chemistry than it is biology, but we use the pH scale and talk about the effect of pH on reactions so often in biology that it’s really important to know what we are actually talking about. So…


The pH scale is a measurement of the concentration of H+ ions (also referred to as protons) that are present in a solution.


To calculate the pH of any given solution, we use the equilibrium of water ionisation (where water molecules split into ions, the formula is 2H2O(l) <=> H3O+(aq) + OH-(aq)) suggesting that water splits into equal parts hydrogen ions and hydroxide ions = neutral.


This reaction at equilibrium is given a special value called the ionisation constant of water (Kw). The value of this constant at 25°C (room temperature) is 1.0 x 10^-14, where h3O+ concentration is 1.0x10^-7 and OH- concentration is also 1.0x10^-7. The concentrations are equal and therefore this solution(water) is NEUTRAL pH=7.


So, to determine the pH of other solutions, we measure the concentration H+ ions. The relationship between H+ ions and pH value is this equation: -log10[H+].


Therefore, if the value of H+ ion concentration is 1.0x10^-3, the pH value is 3 and the solution is acidic.


If the H+ ion concentration is 1.0x10^-12, the pH value is 12 and the solution is basic (alkaline).


The opposing calculation is to determine concentration of OH- ions. This gives a pOH value instead of pH. The relationship between OH- concentration and pOH value is a similar equation: -log10[OH¯].


pH + pOH = 14.


The higher the pH value, the stronger the base. The higher the pOH value, the stronger the acid.


pH =1, pOH=13 (STRONG ACID)

pH=2, pOH=12 (STRONG ACID)

pH=3, pOH=11 (MEDIUM ACID)

pH=4, pOH=10 (MEDIUM ACID)

pH=5, pOH=9 (WEAK ACID)

pH=6, pOH=8 (WEAK ACID)


pH=7, pOH=7 (NEUTRAL SOLUTION)


pH=8, pOH=6 (WEAK BASE)

pH=9, pOH=5 (WEAK BASE)

pH=10, pOH=4 (MEDIUM BASE)

pH=11, pOH=3 (MEDIUM BASE)

pH=12, pOH=2 (STRONG BASE)

pH=13, pOH=1 (STRONG BASE)




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