Solutions

Solutions
They are liquid preparations that contain one or more chemical substances dissolved in a suitable solvent or mixture of mutually miscible solvents.

Classification:
-According to use: oral, otic, ophthalmic, or topical.
-According to composition :
a. Aqueous solutions containing a sugar are classified as syrups (even though some syrups may contain some alcohol).
b. Sweetened hydroalcoholic (combinations of water and ethanol) solutions are termed elixirs
c. Solutions of aromatic materials are termed spirits if the solvent is alcoholic or aromatic waters if the solvent is aqueous.
d. Solutions prepared by extracting active constituents from crude drugs are termed tinctures or fluidextracts, depending on their method of preparation and concentration.

Oral solutions, syrups, elixirs, spirits, and tinctures are prepared and used for the specific effects of the medicinal agents they carry. Additional substances may present, these additional agents are frequently included to provide color, flavor, sweetness, or stability.
In formulating or compounding a pharmaceutical solution, the pharmacist must use information on the solubility and stability of each solute with regard to the solvent or solvent system.








Solubility:
Solubility can be defined in two ways:
• Quantitative term as the concentration of solute in concentrated solution at certain temperature.
• Qualitative term as spontaneous interactions of two or more substances to form a homogenous molecular dispersion.
The solubility of an agent in a particular solvent indicates the maximum concentration to which a solution may be prepared with that agent and that solvent.
When molecules interact, attractive and repulsive forces are in effect. The attractive forces cause the molecules to cohere, whereas the repulsive forces prevent molecular interpenetration and destruction. When the attractive and repulsive forces are equal, the potential energy between two molecules is minimal and the system is most stable.

When a solute dissolves, the substance’s intermolecular forces of attraction must be overcome by forces of attraction between the solute and the solvent molecules. This entails breaking the solute-solute forces and the solvent–solvent forces to achieve the solute–solvent attraction. When a solvent at a given temperature has dissolved all of the solute possible, it is said to be saturated.
The solubility expressed as grams of solute dissolving in milliliters of solvent. These terms are defined in the USP and presented in this table:





Organic compounds may be water soluble if they contain polar groups capable of forming hydrogen bonds with water. In fact, the greater the number of polar groups present, the greater will likely be the organic compound’s solubility in water.
The introduction of halogen atoms into a molecule tends to decrease water solubility because of an increase in the molecular weight of the compound without a proportionate increase in polarity.

Solubility enhancement techniques:
? Using of a different solubilizing agent or different chemical salt form of the medicinal agent.
? Alteration pH of the solution.
? Substitution in part or in whole of the solvent.
? Reduction of particle size of drug.
? Complexation.

Factors affecting solubility:
? Nature of solute and solvent:When two substances are similar they can dissolve each other (like dissolve like). Polar solutes dissolve in polar solvent and vice versa.
? Temperature: Most chemicals absorb heat when they are dissolved and are said to have a positive heat of solution, resulting in increased solubility with an increase in temperature. A few chemicals have a negative heat of solution and exhibit a decrease in solubility with a rise in temperature.
? Particle size of the solute
? Agitation.