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Module 4: Liquid Characteristics - pH
Dissociation Constant of Water
The pH Scale and Air Pollution Control Systems
Practice Problems
Objectives
- Determine the hydrogen and hydroxide ion concentration and the pH
of aqueous liquids.
- Identify the normal operating pH range for air pollution control systems.
Dissociation Constant of Water
Water is a weak electrolyte. It dissociates according to the following reaction:
Since the concentration of water can be treated as a constant, the dissociation
constant (Kw) can be expressed as indicated in Equation
2. This dissociation constant, Kw, has a value of 1.0 
10-14.
- Where:
For pure water, the concentration of H+ ions and OH- ions must be equal.
When acid is added to the water, the H+ ion concentration increases, and the OH- ion concentration decreases due to reactions with the additional H+ ions. However, the product of the H+ ion concentration and the OH- concentration remains constant.
The pH Scale and Air Pollution Control Systems
A convenient way to express the H+ ion concentration is the pH scale. The pH is defined as the negative log to the base 10 of the H+ ion concentration.
Maintaining the proper pH of aqueous liquids used in air pollution control systems is very important. It is often necessary to monitor the pH of these liquids to prevent damage to equipment, plugging of pipes and nozzles, and poor pollutant removal efficiency of control equipment.
Figure 1 shows that the normal pH range for aqueous liquids involved in air pollution control is from 5 to 9, with a pH of 7 being neutral. If the pH is less than 7, the solution is acidic and has an excess of H+ ions. If the pH is above 7, the aqueous liquid is alkaline and has an excess of hydroxide OH- ions.
Very acidic aqueous solutions may cause corrosion damage to components. Damage is caused by the corrosive action of dissolved chloride ions, which increase as the pH decreases. Accordingly, the liquid pH is usually kept above a value of 5.
If the pH exceeds 9, the high concentration of OH- ions react
with dissolved carbon dioxide (CO2) to form carbonate ions
(
).
These carbonate ions can react with dissolved calcium and ma
gnesium ions
to form insoluble solids. These solids can precipitate in piping, nozzles,
and other portions of the control system. (See Figure
2, 
which shows calcium-magnesium buildup on a mist eliminator). Accordingly,
in most systems the liquid pH is kept below 9.
The removal efficiency of acid gas compounds, such as sulfur dioxide,
hydrogen fluoride, and hydrogen chloride, is dependent on the pH of the
scrubbing
liquid. Alkaline slurries (in the pH range of 9 to 11), often composed
of limestone or lime, are added to acid gas systems to enhance the removal
of acid gases from the gas stream. City water by itself is a less effective
medium for the collection of acid gases due to the large quantities of
water that would be necessary for efficient removal. The alkaline slurries
provide alkaline anions, composed mainly of calcium hydroxide, magnesium
hydroxide, and carbonate to react with the H+ formed as a result
of absorption and dissociation of acid gases. For example, carbonate ()
reacts with H+ to form 
and further reacts with another H+ to form H2CO3.
Figure 3 shows which of the three forms of carbonate predominate at different
pH ranges. If too much high-pH alkaline material (e.g. at pH equal to
11) is added to the system, most of the liquid phase carbonate exists
as
with a lower concentration of .
At a pH of 4, virtually all of the carbonate exists as H2CO3,
a form that cannot readily react with more H+ from acid gases.
A low pH indicates that the system requires either a higher flow rate
of alkaline slurry or a slurry of higher pH. In summary, monitoring pH
levels is important for determining the optimal flow rate and concentration
of alkaline slurry into the acid gas control system to prevent equipment
damage as well as to maintain the desired pollutant removal efficiency
levels.
-
#1
- A packed bed wet scrubber is used to remove hydrogen chloride from a gas stream. The pH of the aqueous liquid entering the scrubber is measured at 6 and the aqueous liquid leaving the scrubber is measured at 5. Has the solution become more or less acidic? By what factor has the H+ ion concentration changed?
-
#2
Example Problem 1.
Calculating the pH of a Solution
What is the pH of a solution having a H+ ion concentration
of 1
10-11 moles per liter?
Solution:
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.