Process Description

Neutralization is the restoration of the hydrogen (H+) or hydroxyl (OH-) ion balance in solution so that the ionic concentrations of each are equal. Conventionally, the notation pH (puissance d'hydrogen) describes the hydrogen ion concentration or activity present in a solution as follows:

where:

[H+] = the hydrogen ion concentration, gmol per liter aH+ = hydrogen ion activity

For a dilute solution of strong acids, i.e., acids considered completely dissociate (ionized in solution), the following equation applies:

At neutrality, the concentration of hydrogen and hy-droxyl ions is equal. The product of their ion concentration (Kw) at 25°C is as follows:

(-logic [H+])(-log1o [OH-]) = -log1oKw Kw = 1.008 X 10-14

At neutrality, the following equations apply: —loglnKw pH =-= 7.0 = pOH 7.39(5)

Thus, if a solution has a pH = 2.0 at 25°C, hydrogen ion concentration is 1 X 10—2 moles H+ per liter, pOH =

depends not only on the absolute value of the pH but also on the frequency of pH variation. Thus, for example, the lacrimal fluid of the human eye has a nominal pH of 7.4 and a high buffering capacity, i.e., it resists changes in pH. Variations of the lacrimal fluid as low as 0.1 pH unit can result in eye irritation. The Federal Water Pollution Control Administration has published a report that details not only the pH requirements for water of a designated end use but also the requirements for twenty other ions, as well as organic chemical limitations and physical and microbiological properties. Table 7.39.1 summarizes the preferred or acceptable pH ranges for various water quality categories.

12, and hydroxyl ion concentration is 1 x 10 moles Common Neutralization Reagents

OH— per l. The ion product of water depends highly on temperature, changing approximately two orders of magnitude over a 60°C span (see Figure 7.39.1).

The pH notation as a means of expressing the hydrogen ion concentration is logarithmic. A pH change from 2.0 to 1.0 does not mean that the ion concentration has doubled; a change of one pH unit is an order of magnitude change. Thus, if an acid influent changes by three pH units, the [H+] is changing by a factor of one thousand. This logarithmic nature becomes an important consideration when reagent delivery systems are sized because if the ion load to be neutralized changes by a factor of 1000, the reagent delivery system must have the same turndown (rangeability).

The need to neutralize, or at least place limits on, the pH variation of environmental waters has resulted in the promulgation of water quality standards legislation in virtually every state. The physical well-being of all life forms

FIG. 7.39.1 Ion product of water as a function of temperature.

Wastewater treatment facilities must counter the hydrogen or hydroxyl ion imbalance in a waste effluent by adding a material that restores the ion balance. Thus, if the waste effluent is acidic, i.e., pH < 7.0, they must blend a reagent having basic characteristics with the waste to achieve neutrality. Conversely, if the waste effluent is basic, i.e., pH > 7.0, they must use a reagent having acid characteristics. Table 7.39.2 lists common neutralization reagents.

In addition to the reagents listed in Table 7.39.2, waste acids and bases can also serve as neutralizing reagents. In some cases, particularly in ion-exchange resin regeneration, in which the resin bed is treated first with a caustic solution and then with an acid solution, wastewater treatment facilities can store these solutions and then blend them to achieve a neutral solution rather than discharge them to the sewer immediately after use.

Four widely used reagents are sulfuric acid, caustic soda, hydrated chemical lime, and (to a limited degree) limestone. The main reasons for their popularity are economy and ease of handling.

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