4. Physical and
Chemical Properties
When we speak of phenobarbitol we must distinguish
between phenobarbitol itself, a white powder, a weak acid with rather limited solubility
in water, and the sodium salt of phenobarbitol, also a white powder, the salt of the weak
acid, now quite water soluble.
| Concepts:
It is the power of the representations we use in chemistry and allied fields
such as pharmaceutics, that we can understand the differences between these seemingly
identical white powders by looking at their structural representations. |
| Property |
Phenobarbital |
Sodium Phenobarbital |
Comments |
| chemical formula |
C12H12N2O3 |
C12H11N2NaO3 |
Our chemical formula for the two
powders shows that a single hydrogen atom has been replaced by a sodium atom. |
| molecular weight |
232.24 |
254.22 |
As a result, the molecular weight of
the sodium salt is higher than the acid form. This is a critical piece of knowledge
for any experiment or formulation. |
| Chemical structure |
 |
 |
This representation of the
microscopic structure indicates where on the molecule the acidic hydrogen ion is released. |
| ka |
3.9x10-8 |
.3.9x10-8 |
As we will see, the ka
and pKa show a relationship between the acid and base (protonated and sodium
salt) in solution. |
| pKa |
7.41 |
7.41 |
|
| pH of a saturated solution |
5 |
9.2-10.2 |
The pH of the water solutions is a
critical piece of knowledge for pharmaceutic definition of dosages |
| solubility in water |
1g in 1000 mL |
very soluble in water
(1g in 10 mL) |
Solubility in water must be known since the human body
is a water-based system. |
(from the Merck Index and Physical Pharmacy, Alfred Martin, 4th edition)
 to Acid/Base Equilibria
to Acid/Base Determination |
