The most commonly used neutralization chemicals for acid or base neutralization are 98% Sulfuric acid and 50% Sodium hydroxide. In many cases these are very good choices, however, there are many considerations when selecting chemicals and these may not always be the best selection.
The selection of the chemicals used for the neutralization of an acid or base is almost as important as the design of the neutralization system. There are many considerations ranging from health and safety to cost and convenience of operation. Some of the major points to consider in the selection of chemicals are listed below:
- Health and Safety.
- Cost and Convenience.
- Storage Environment and Location.
To neutralize an acid or base a source of hydroxide ions (OH-) or hydrogen ions (H+) are required, respectively. An acid must be neutralized with a base, which, by definition, is characterized by an excess of OH- ions. Likewise, a base must be neutralized with an acid, which, by definition, is characterized by an excess of H+ ions.
For example: In a simple neutralization process hydrochloric acid (HCl) can be neutralized by using sodium hydroxide (NaOH).
An explanation of chemical selection criteria follows.
Health and Safety: Whenever mixing chemicals extreme caution must be exercised. Hazardous, or noxious reactions may occur. For example: adding any acid to a cyanide bearing solution will result in the release of deadly HCN gas. Due to the complexity of the myriad of processes that are run in industry, most of which are either proprietary or undefined, Digital Analysis usually cannot comment on suitability of an acid or base selection for the neutralization of a wastewater stream. Although adverse reactions are very rare, the possibility exists and must be considered.
Cost and Convenience: Most acids and bases will work in most applications. Therefore the determining criteria are usually cost and convenience. Sulfuric acid (H2SO4), for example, is less costly and more potent than nitric acid, however, if sulfuric is not currently inventoried then nitric may look more favorable because it may already be on hand. Concentrations are also an important consideration in evaluating cost. Sulfuric acid, for example, can be purchased in concentrations ranging from near 0% up to 98%. If a 55 gallon drum of 50% sulfuric is purchased, obviously half of that purchase cost was for water. Higher concentrations are generally less expensive.
Storage Location / Environment: The physical properties of the selected reagent must be considered carefully. 50% Sodium hydroxide (NaOH), for example, begins to freeze at temperatures below 600F. In most places throughout the country, if not all, the possibility of caustic freezing in tanks or in pipelines is a genuine concern. Decreasing the concentration to 25% eliminates this concern altogether. Hydrochloric acid (HCl), for example, out gasses severely. The gas is very highly corrosive and will attack all metallic objects including building structures, sprinkler heads, copper wiring, stainless steel, etc. Therefore, if HCl is used it must be properly vented or used outdoors where the gasses can easily dissipate.
The most commonly used neutralizing chemicals are listed below, Each provides a link to a brief discussion of the selected chemical: