June 22, 2009 A wide range of chemical and microbiological processes influence the acidity of natural waters. Dutch researcher Andreas Hofmann has developed a new method for accurately determining the acidity of water. This can be used to simultaneously view different chemical processes, and the effect of each process on the pH value can also be determined.
Hofmann tested his method at various locations, such as the River Schelde and its estuary, and discovered which processes had the greatest effect on the acidity. The River Schelde rises in North France and flows via Flanders, Belgium, to the North Sea. During its course the river is supplemented with rainwater that mainly enters via the soil. This groundwater contains various chemical substances, including ammonium - a substance that can originate from cow manure. Nitrification, the conversion of ammonium into nitrate by microorganisms, was found to be the most important acidifying process.
Acid and base
In chemistry an acidic substance has a pH lower than 7. For example, lemon juice has a pH value of 2, whereas soapy water, a so-called base, has a pH value of 10.5. Only pure, demineralised water has a pH of 7 and it therefore serves as the calibration point.
During the chemical process of converting ammonium into nitrate - nitrification - acid is formed: the pH value of the river water therefore decreases. However, this acidification leads to higher CO2 concentrations in the water. The water cannot cope with this high CO2 level and the CO2 is therefore released into the air. During this release, the pH value of the river increases (i.e. the water becomes less acidic), as the conversion of carbonates and bicarbonate into CO2 consumes acid. The acidity in the Schelde estuary is therefore mainly determined by the balance between nitrification and the exchange of CO2 between the water and the air.
In general, water has the characteristic of buffering acid production or acid consumption. Consequently the changes in the pH value are far smaller than they otherwise would be. Hofmann expressed ‘buffering' using a buffer factor which indicates how well an aquatic system can cope with acid production or acid consumption without a significant change in the pH. He used this buffer factor to determine the influence of the processes described on the pH value in the Schelde. However, this buffer factor can also be calculated for the global ocean water. Although oceans can buffer quite well, it was found that the buffer factor of oceans will be three to four times lower at the end of this century. Due to the uptake of CO2 produced by humans, oceans in the future will not only be more acidic but also more sensitive for changes in pH.
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