We are seldom concerned with just water in the hospital. We are generally more concerned with substances dissolved in water. These are solutions. When we speak of a solution, there are several terms, which are important to understand.
Solute. A solute is the substance, which is dissolved in a solution.
Solvent. The solvent is, the substance, which dissolves the solute. It is usually water in pharmaceutical solutions, but not always.
Solubility. The maximum amount of a compound, which will dissolve in a given amount of solvent at a given temperature is the solubility of that compound.
Dissociation. (Ionization). In general, two things can happen to a solute in a solution. It can dissolve and exist in solution as molecules or it can dissociate and exist entirely or partially as ions. The process of splitting a molecule into ions is known as dissociation.
Electrolyte. When a substance dissociates to a fair extent in water, it will produce enough ions to support an electric current. We can use this property to differentiate between substances that are molecular and substances that are ionic in solution. An electrolyte is a substance, which dissociates sufficiently in solution to carry an electric current. It is therefore ionic in nature (figure 2-1).
Non-Electrolyte. A substance that does not dissociate or carry an electric current in solution is called a non-electrolyte.
Hydrolysis. Some compounds form ions in solution by reacting with water. This reaction with water is called hydrolysis. Hydrolysis is the dissociation of a compound through the splitting and incorporation of water. Hydrolysis occurs when acidic, basic, or neutral (weak acid/weak base) salts are dissolved in water. Consider, for example, the basic salt, sodium bicarbonate.
NaHCO3 + H2O ® Na+ + HCO3 - + H2O ® Na+ + OH - + H2CO3
The hydroxyl ion from water is associated with the sodium ion from the salt. The hydrogen ion from water is associated with the bicarbonate radical, and these two exist primarily as undissociated carbonic acid. The net result of this hydrolysis reaction is a basic solution containing sodium and hydroxyl ions and undissociated carbonic acid.
Figure 2-1. Flow of electric current through electrolyte solution.
Electrolyte Strength. It should be apparent, recalling the discussion of acids and bases that not all electrolytes will dissociate to the same extent in solution.
- Those that dissociate and exist entirely as ions in solution are called strong electrolytes. Strong electrolytes include strong acids, strong bases, and their neutral salts.
- Compounds that dissociate to a small extent and exist only partially as ions in solution are called weak electrolytes. Weak electrolytes include weak acids, weak bases, and salts of weak acids and/or weak bases.
- To identify whether a compound is a strong or weak electrolyte, it is first necessary to identify what type of compound it is. For example, consider NaCl. This compound is a salt formed from a strong base (NaOH) and a strong acid (HCl) and is, therefore, a neutral salt. Since it is a neutral salt of a strong acid and a strong base, it is a strong electrolyte as defined above. Other salts can be determined in a like manner.