The pH of the boiler water decreases after it has been left for some time.
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The phosphate level in the boiler water taken to the laboratory for analysis was 14.95 mg/l, with a pH of 10.5. After being left overnight, the phosphate level remained unchanged, but the pH dropped to 9.24; after another night, the pH further decreased to 7.98. Ammonia is suspected to be the cause of this phenomenon, and the laboratory measured the ammonia nitrogen content in the boiler water at 0.20 mg/l. But could such a small amount of ammonia cause a sudden drop in pH? Or what other factors are at play.
It’s definitely due to the evaporation of ammonia!
Acidic gases in the air dissolve in water, causing the pH level to drop.
Agreed; it may react with carbon dioxide in the air, resulting in a change in pH.
But the drop can’t be that large, right?
But the drop can’t be that large, right?
The ammonia content is too low, which explains why the pH drops so rapidly. Moreover, it’s usually stored in a sealed container – could evaporation really cause such a significant impact?
pH value = -lg, sure! Slight changes in concentration result in significant changes in pH value.
Boiler water is deaerated water (the deoxygenation process is essentially a deaeration process). No matter how well the sample of boiler water is sealed after sampling, the penetration of air into the sample is an inevitable spontaneous process, unless measures are taken to isolate air; even then, air will still get into the sample during the sampling process. Although there are only about 300–400 ppm of carbon dioxide in the air, its solubility in water is high compared to other gases. Moreover, the initial pH of water samples is alkaline, which facilitates the dissolution of carbon dioxide and its neutralization by alkaline substances. To obtain samples of boiler water in a more reliable and representative manner, special samplers should be used, with thorough transfer processes and secure sealing. When transferring the samples to the analysis station, special methods must be employed for their packaging, and such facilities are generally not available in ordinary laboratories.