3.5 - 4
5.0 - 5.5
6.0 - 6.5
2.5 - 3.0
Terpin Hydrate Elixir with Codeine
c. Measurement of pH. There are three common methods for measuring pH,
which you may encounter in medicine.
(1) Litmus paper. Litmus paper is a paper coated with a dye, which is red in
an acid pH or blue in a basic pH. It will only indicate whether a solution is acidic or
basic; it will not give an actual pH value.
(2) pH paper. pH paper works on the same principle as litmus paper but
uses several different dyes. By comparing paper color with a chart, the pH of a solution
can be determined within one pH unit. If a closer measurement is needed, special
narrow-range papers can be used to determine the pH within 0.1-pH unit.
(3) pH meter. The most accurate tool for pH measurement is the pH meter.
This makes use of an electrode dipped into solution and is accurate to about 0.01-pH
unit, depending on the particular machine.
Many drugs are stable in solution only at certain pHs or in narrow pH ranges. If a
solution of one these drugs is desired, the manufacturer must find a way to maintain this
certain pH over a period of time. This is accomplished by the use of buffer systems. A
buffer is a solution of a weak acid and the salt of that weak acid (weak bases could also
be used, but usually are not practical). The function of the buffer is to resist changes in
pH by reacting with any hydrogen or hydroxyl ions that are added to the solution. Two
of the most common buffer systems are:
a. Acetic Acid/Sodium Acetate. This is a common buffer found in many drug
b. Carbonic Acid/Sodium Bicarbonate. This is the buffer system that is most
common in the fluids and tissues of the body and is used to keep the pH of the blood
and body fluids constant.