So, note the difference between strength and
concentration. Strength relates directly to
Ka, the acidity constant for a given dissociation.
Concentration, on the other hand, relates
purely to the amount per... per litre of a
given ion. Strength refers to Ka and concentration
refers to the amount of acid actually in solution.
So, therefore, it's possible to have a concentrated
solution of a weak acid or a dilute solution
of a very strong acid. Concentration is measured
in moles per litre or moles per dm3. And what
you often find is it's referred as mol dm-3
and, sometimes, just as M (molar).
Acid strength can be measured using Ka, as
we've already indicated. The larger the
Ka, the greater the degree of dissociation
of the acid into the conjugate base and, of
course, the important H+. But, these are not
generally convenient ways of measuring it
because you have to use, again, standard form
in this case. 1.8 Ã—10 to the -5 is a little bit
more difficult to work with, especially if
you're looking at it from a biological perspective.
And a lot of the calculations may appear to
be confusing and may even result in some errors
as a consequence.
So, as a consequence of this, pKa is used
with the small letter p denoting negative
log (-log). That's all p means in this context.
So, we take whatever the value of Ka is and
then we carry out the negative log to the
base 10 of that value.
By carrying out the negative log to the base
10, we get a value which is a bit more easy
to deal with. Usually, a value running from
1 through to 14 or, at the very least, a decimal
value which is easy to deal with.
If we look at this equation, what we mean
is, by taking pKa, we are taking the negative
log to the base 10 of the concentrations of
H+ multiplied by a conjugate base A- divided
by the non-dissociated concentration of our
So, if, for example, we were to look at ethanoic
acid, we see that the pKa of this calculation
results in a value of 4.77. The stronger acid,
trichloroethanoic acid, results in a pKa of
0.70. And, as we actually go down in terms
of pKa value, we're increasing the degree
of dissociation and increasing the concentration
of H+ that is actually formed.
When an acid or a base is added to water,
the concentration of H+ will change. And this
is the origins of the term pH. pH, like pKa
in this case, just means that we're taking
the negative log to the base 10 of the concentration
of H+ in solution. Hence, the term pH derived
from the German expression potenz Hydrogen.
Pure water has a pH of 7. And there's a
relatively easy rule of thumb to bear in mind
with this. When the concentration, for example,
is approximately 1 x 10 to the minus 7 in terms of moles
per litre, then you can actually just remove
that -7 and remove the negative value. So,
in other words, if I have 1 x 10 to the minus 7 moles
of H+ in a litre of water, I will have a pH
of 7. So, sometimes, there's a general rule
of thumb which doesn't require you to use
a calculator. Therefore, if we had a concentration
of 1 x 10 to the minus 2, we can just take that power,
remove the negative value and we end up with
a pH of 2, which you should appreciate is
an acidic pH.
Remember, pure water has a pH of 7 and it
also dissociates to a small extent. Acidic
solutions will always give a greater concentration
of H+ that exists in water and have a pH of
less than 7. Basic solutions will have a smaller
concentration of H+ free in solution and have
a pH greater than 7, always.
Typically, physiological conditions result
in a pH of around 7, although obviously,
that varies depending on whether or not you're
talking, for example, about urine samples,
saliva samples or indeed plasma blood samples.