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2-9. SALTS Previously, it has been stated that one of the properties associated with acids and bases is the neutralization reaction. This reaction involves the production of a salt and water from the reaction of an acid and a base. We will now examine various types of salts produced in neutralization reactions. Salts are the third major classification of inorganic compounds (acids and bases being the first two). They are important in the physiology of the body and are often used as therapeutic agents. Definition. We have already given one definition of a salt in our discussion, that is, the product of a reaction between an acid and a base. A more specific definition, however, would be an ionic compound formed by the replacement of part or all of the acid hydrogen of an acid by a metal or a radical acting like a metal. It is an ionic compound that contains a positive ion other than hydrogen and a negative ion other than hydroxyl (OH-) or "O-2," as in MgO. Types of Salts. There are four types of reactions possible between acids and bases as we classified them (strong or weak) earlier. These are as follows: Relative strength of common acids and bases | STRONG ACIDS - HCl Hydrochloric acid
- H2SO4 Sulfuric acid
- H3PO4 Phosphoric acid
| WEAK ACIDS - HC2H3O2 (HAC) Acetic acid
- H2CO3 Carbonic acid
- H3BO3 Boric acid
| STRONG BASES - KOH Potassium hydroxide
- NaOH Sodium hydroxide
| WEAK BASES - Fe(OH)2 Ferrous hydroxide
- Al(OH)3 Aluminum hydroxide
- NH3 Ammonia
| - * Ca(OH)2 Calcium hydroxide
- Mg(OH)2 Magnesium hydroxide
- MgO Magnesium oxide
Notice that Ca(OH)2, Mg(OH)2, and MgO which forms Mg(OH)2 in water are chemically classified as strong bases because of their high degree of dissociation. Because they are only slightly soluble in water, they produce low concentrations of the hydroxide (OH-) ion in solution. Since calcium hydroxide and magnesium hydroxide do not produce tissue damage, they can be safely used as therapeutic agents (e.g., antacids). | Table 2-1. Relative strength of common acids and bases. - Reaction 1. Strong acid and strong base: HCl+ NaOH ® NaCl + H2O
- Reaction 2. Weak acid and weak base: 2H2CO3 + Fe(OH)2 ® Fe(HCO3)2 + 2H2O
- Reaction 3. Strong acid and weak base: 2HCl + Fe(OH)2 ® FeCl2 + 2H2O
- Reaction 4. Weak acid and strong base. H2CO3 + NaOH ® NaHCO3 + H22O
NOTE: These four reactions result in three types of salts. Reactions 1 and 2 result in neutral salts (that is, in terms of pH), which means a solution of the salt in water will be a neutral solution. Reactions such as 3 result in acidic salts, which produce acidic solutions. Reaction 4 results in basic salts, which produce basic solutions. Determination of Salt Type. To determine the type of salt from a chemical formula, we employ the following steps: - The first element comes from a base. Determine which base and whether it is weak or strong.
- The remainder of the formula comes from the acid. Determine which acid and whether it is weak or strong.
- By knowing the strengths of the acid and base that formed the salt, the salt type can be assigned. Table 2-2 is a summary of salt types resulting from various acid-base combinations.
Example. Al2(SO4)3. - The first element, aluminum, comes from the base Al(OH)3. Since it has a valence of +3, it is a weak base.
- The sulfate radical comes from H2SO4, sulfuric acid, which is a strong acid.
- This compound is an acidic salt since it is the product of a reaction between a strong acid and a weak base.
Example. FeBO3. - The first element, iron, comes from the base Fe(OH)3, and since its valence is +3, it is a weak base.
- The borate radical comes from boric acid, which is a weak acid.
- Thus, this is a neutral salt, since it is the product of a reaction between a weak acid and a weak base.
Importance of Type of Salt. The type of salt is very important when a salt is used medicinally, since the body maintains a specific acidity in the tissues and fluids. The type of salt is also important in the prediction and understanding of incompatibilities. It is important for you to identify the type of salt from its formula. The importance and use of the type will become clear to you as you progress through the course. Reactants ®
¯ | Weak Acid | Strong Acid | | Weak Base | Neutral Salt | Acidic Salt | | Strong Base | Basic Salt | Neutral Salt | Table 2-2. Salt types resulting from various acid-base combinations.
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