Solutions are homogeneous mixtures of one or more solutes molecularly dispersed in a suitable solvent or a mixture of mutually miscible solvents.
Pharmaceutical
solutions
Introduction
Solutions are homogeneous
mixtures of one or more solutes molecularly
dispersed in a suitable solvent or a mixture of mutually miscible solvents.
A solution composed of only two substances is a binary solution. The
com-ponents making up a binary solution are termed the solute and the solvent
depending on their relative proportions (component in lower proportion is termed
solute).
Pharmaceutical
solutions are used for many routes of administration, including oral, rectal,
vaginal, ophthalmic, parenteral, and otic. The most common solution dosage form
is the oral liquid, which includes aque-ous solutions, syrups, and elixirs. The
physicochemical (e.g., solubility) and stability characteristics of the active
drug determine whether an oral solution dosage form can be prepared. The
required solubility of a drug and its solubility in water and biocompatible
water-miscible solvents help decide the dosage form composition. For example,
if the drug is water sol-uble, a simple aqueous solution can be prepared.
However, if it is soluble in a water–alcohol–glycerin cosolvent system, an
elixir is appropriate.
Drugs
are commonly given in solution in cough/cold remedies and in medi-cations for
the young (pediatric) and elderly (geriatric).
Saturated solutions are solutions which,
at a given temperature and pressure,
contain the maximum amount of solute that can be dissolved in the solvent. Buffer solutions contain a combination
of weak acid and its salt with a strong base or a weak base and its salt with a strong acid. These solutions
resist changes in pH upon the addition of small quantities of acid or base.
Solubility and stability of most ionic drugs change with pH. Therefore, most
pharmaceutical solutions are pH controlled using an appropriate buffer. Isotonic solutions have similar tonicity
as biological fluids. These solutions cause no swelling or contraction of the
tissues with which they come in contact and produce no discomfort when
instilled in the eye, nasal tract, blood, or other body tissues. Parenteral
solutions or solutions for direct administration to mucosal tissues should be
isotonic or hypotonic to avoid local tissue stress and pain upon
administration. Tonicity is usually adjusted using dextrose or sodium chloride.
Isotonic sodium chloride is a 0.9% w/v concentration of NaCl in water and is
also called normal saline. A 5% w/w dextrose, also known as glucose, solution
in water is also isotonic. These are commonly used infusion fluids for
intra-venous administration.
Solutions
intended for oral administration usually contain sweeteners, flavors, and
colors to make the medication more attractive and palatable to the patient. They
may contain stabilizers to maintain the physicochemical stability of the drug
and preservatives to prevent the growth of microorgan-isms in the solution.
A
drug dissolved in an aqueous solution is generally in the most bioavail-able
form. As the drug is already in solution, no dissolution step is neces-sary
before systemic absorption occurs.
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