The influence of alkalinity in the boiler water industry and its solutions

- Sep 04, 2020-

The influence of alkalinity in the boiler water industry and its solutions

1. Definition:


Alkalinity refers to the total amount of substances in water that can neutralize strong acids. Such substances include strong bases, weak bases, strong bases and weak acid salts. The alkalinity in natural water is mainly caused by bicarbonate (bicarbonate, the same below), carbonate and hydroxide, among which bicarbonate is the main form of alkalinity in water. The pollution sources that cause alkalinity are mainly waste water discharged from papermaking, printing and dyeing, chemical, electroplating and other industries, and the loss of detergents, fertilizers and pesticides during use. Alkalinity and acidity are important indicators for judging water quality and wastewater treatment control. Alkalinity is also often used to evaluate the buffering capacity of water bodies and the solubility and toxicity of metals in them. The definition of total alkalinity is more commonly used in engineering, which is generally characterized as the concentration value of calcium carbonate.

alkalinity

2. The harm of alkalinity:


When the alkalinity of the boiler water is too high, it may cause alkaline corrosion and stress corrosion cracking of the water wall tube; it may also cause the boiler water to foam or even produce "steam water" and affect the steam quality. For riveted or expanded boilers, excessive alkalinity can also cause caustic embrittlement. Too high alkalinity will also cause bubbling to cause the steam to carry salt and form salt scale, and it is also easy to cause the co-leakage of steam and water, resulting in superheater water hammer and pipeline water hammer accidents, and serious steam equipment loss.


3. Testing plan:

 

①Titration of acid-base indicator

 

Principle: The water sample is titrated with a standard solution to the specified pH value, and the end point can be judged by the color change of the added acid-base indicator at this pH value.

 

When titrated until the phenolphthalein indicator turns from red to colorless, the pH value of the solution is 8.3, indicating that the hydroxide ions in the water have been neutralized, and the carbonates are all converted to bicarbonates. Reaction formula: OH-+H+ → H2OCO32-+H+

 

When HCO3- is titrated until the methyl is changed from orange to orange, the pH value of the solution is 4.4-4.5, indicating that the bicarbonate in the water has been neutralized. The reaction is as follows; HCO3-+H+-H2O+ CO2 ↑According to the amount of hydrochloric acid standard titration solution consumed when the above two endpoints are reached, the carbonate, bicarbonate and total alkalinity in the water can be calculated.


【Instructions】:

 alkalinity

1. Use a pipette to accurately pipette 10ml of water sample.

 

2. Put the water sample into a clean conical flask.

 

3. Add the 3rd alkalinity (1) (HR) reagent and shake well. At this time, the solution should be red. If it is colorless, it means that the phenolphthalein alkalinity value is 0. Soak in step 5.

 

4. While shaking the cone, add the alkalinity (3) (HR) reagent vertically. Add 1 drop every 3 seconds until the solution just becomes colorless. Record the number of reagent drops consumed (N1).

 

5. Add 3 drops of alkalinity (2) (HR) reagent and shake well. At this time, the solution was green.

 

6. Continue to drip the alkalinity (3) (HR) reagent until the solution turns from green to dark red, and record the number of drops of reagent consumed (N2).

 

7. Result calculation: Phenolphthalein alkalinity: N1×10 (mg/l)

 

②Volume method

 alkalinity

Principle: Alkalinity can be divided into phenolphthalein alkalinity and total alkalinity. Phenolphthalein alkalinity is the amount measured when phenolphthalein is used as an indicator, and the end point pH is 8.3. Total alkalinity is the amount measured when methyl orange is used as an indicator, and the end point pH is 4.2. If the alkalinity is very small, the total alkalinity should be methyl red-methylene blue as the indicator, and the end point pH is 5.0. Total alkalinity: (N1+N2)×10(mg/l)