Protocol Online logo
Top : Forum Archives: : General Biology Discussion

question about pH - (Nov/05/2005 )

hi >>>>>>>>>>

PH makes me so confused
when we say high PH, do we mean that OH ions are in high concentration, i mean above 7


it means that H ions are are high in concentration (below 7)?

help, unsure.gif


The "p" in pH is actually a mathematical operator that means "take the negative logarithm of the quantity following". Square brackets ([]) dentote that the quantity contained within them is a concentration. Thus, the pH of a solution is the negative logarithm of the molar concentration of hydrogen ions. In other words, pH is the solution to this equation:

pH = -log[H+]

As you can see pH is a logarithmic function; this explains why a change of one pH unit represents a ten-fold change in hydrogen ion concentration (for example, pH 2 is not twice as acidic as pH 4, it is one hundred times as acidic).

Since it is a negative (opposite) mathematical operation, there is an inverse relationship between the hydrogen ion concentration and pH. Increasing the H+ concentration solves to a lower number, thus more H+ (more acidic) equals a lower pH (also more acidic).

Because of this inverse relationship, when one speaks of a "high pH", one means a solution with a low concentration of H+, and the pH is thus a high number (greater than 7) and the solution is basic. The opposite is also true -- when speaking of a "low pH", one is refering to an acidic pH (less than 7), thus the solution is high in H+ concentration.

In summary, then:

High pH = low H+ concentration = high number on scale = basic.
Low pH = high H+ concentration = low number on scale = acidic.


BTW, just in the interest of a full explanation, you should also note that at room temperature (25ºC), about 1 molecule in every 10 million (10^7) of pure water is dissolved into H+ and OH-. Another way of saying this is that there are 10^-7 moles of H+ in a quantity of pure water at 25ºC, and, doing the math, -log[10^-7 moles of H+] = 7.

This is why neutral pH is defined as 7, and why pH measurements are temperature dependent...


thans a lot >>>>>>>i alredy know some of the information you mentioned

but, this was really useful and clear

thanx again dear