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3. PH Measurement
Stable chemical compounds, are electrically neutral when mixed with water.
In order to form a water solution, dismember in positive or negative charged particles.
These charged particles are called "i
solution, it is possible to verify that positive ions (H , Na and more) will migrate to the negative terminal, that is
why we call them cations. The reverse happens with negative ions (OH , Cl and more) that will migrate to the
positive terminal and we call them anions. The ions freedom to migrate thru a solution is measured with
the solution
"Electrical Conductivity
"
E
lectrolytes
". Those that totally dissociate (strong acids, strong bases and salt) are called "
Electrolytes
".
The others whose dissociation is not big (acids and weak bases) are called "
hypotetical chemical compound "MA" that dissociates in cations
The arrow indicates that exists a balance between MA not dissociated and the ions M
The extension that this reaction moves to the right or left, varies from one compound to the other and with the solution
Temperature.
For a specific temperature, there is a relation between the ions activity and the non dissociated molecules, that can vary
between 0 up to 100%. This relation is called K Dissociation and it is expressed by the equation:
Where:
K =
CTE dissociation.
aM+ =
ions activity M+.
aA- =
ions activity A-.
aMA =
dissociated molecules activity MA.
For Chloridric acid, the
"K"
On the other hand, Acetic acid has a low
Few hydrogen ions result at the solution, so the acetic acid is an weak acid when the most present ion is the OH-, the
solution is alkaline.
The sodium hydroxide is totally dissociated, this is the reason for being a
Hydroxide
(NH OH)
do not dissociate too much, this the reason for being a
4
As we can see, for both acids and bases the strength of a solution depends on the H+ or OH- ion numbers available that
depends not only on the compound concentration at the water, but also on the " "
Ions H+ and OH-, but it is very weak.
The water molecules number dissociated is low, in comparison with the non dissociated, so we can consider as an
activity of HOH equal to 100%.
ons
". When we apply differential potential in two electrodes immersed in a
". Chemical compounds that produce conductive solutions are called
MA
K = aM * aA- / aMA
is practically infinity, caused by the complete dissociation for ions
HCl
"K"
. Reacts as the following way:
CH COOH
3
NaOH
NH OH
4
HOH
+
+
-
+
M
and anions
++
-
M A
+
(EQ-1)
+ +
-
H Cl
+ +
-
H CH COO
3
+ +
-
Na OH
STRONG BASE.
WEAK BASE
+ +
-
NH OH
4
K dissociation.
+ +
-
H OH
-
Strong
Weak Electrolytes
". For an
-
A
, the reaction can be written as:
+and
-
and A dissociated.
+
-
H
and
Cl
.
So it is a strong acid:
On the other hand the Ammonia
.
Pure water dissociates for
" "
3
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