Acid neutralizing capacity natural waters

Acidification the decrease of acid neutralizing capacity in water or base saturation in soil caused by natural or anthropogenic processes. 

One term now used interchangeably with acid neutralizing capacity (ANC) of natural waters is alkalinity (Aik)—defined as the concentration of negative charge that will react with H+. 

Hemond H. F. (1990) Acid neutralizing capacity, alkalinity, and acid-base status of natural waters containing organic acids. Environ. Set Technol. 24, 1486-1489. 

Sullivan T. J., Driscoll C. T., Gherini S. A., Munson R. K., Cook R. B., Charles D. F., and Yatsko C. P. (1989) Influence of aqueous aluminum and organic acids on measurement of acid neutralizing capacity in surface waters. Nature 338, 408-410. 

Alkalinity. The alkalinity of a water sample is its acid-neutralizing capacity. Bicarbonate and carbonate ions are the predominant contributors to alkalinity in most waters, and their chemical equilibria generally maintain the pH of 5—9. The presence of enough hydroxide ion to affect the alkalinity determination in natural waters is rare. Silica, borate, or phosphate do contribute to the overall alkalinity if present in large enough quantities. 

Lake Acidification Lakes acidify when they lose alkalinity (Roth et al. 1985). The total alkalinity or acid-neutralizing capacity is the reservoir of bases in solution. The acidneutralizing capacity of a lake buffers it against large changes in pH. In natural clean waters, most of the acid-neutralizing capacity consists of bicarbonate ion, HCOJ. Carbonate alkalinity is defined by... 

Though water acidification is one of the most important aspects, one would certainly not expect significant changes in water acidity in all exposed areas. The effect is highly dependent on bedrock geology and the nature of the overburden. No acidification of fresh water is to be expected in areas with appreciable amounts of calcareous rocks. The most well known susceptible areas are those with shallow overburden and quartzbearing bedrock. Acidification can occur in catchments with highly weathered sandy soils with low neutralization capacities. 

Hydrogen ion regulation in natural waters is provided by numerous homogeneous and heterogeneous buffer systems. It is important to distinguish in these systems between intensity factors (pH) and capacity factors (e.g., the total acid- or base-neutralizing capacity). The buffer intensity is found to be an implicit function of both these factors. In this chapter, we discuss acid-base equilibria primarily from a general and didactic point of view. In Chapter 4 we address ourselves more specifically to the dissolved carbonate system. 

In this chapter we describe the distribution of CO2, H2CO3, HCOf, and C03 in natural waters, examine the exchange of CO2 between atmosphere and waters, evaluate the buffering mechanisms of fresh waters and seawater, and define their capacities for acid and base neutralization. 

The pH values at the respective equivalence points (around 4.5 and 10.3) for titrations of alkalinity and acidity represent approximate thresholds beyond which most life processes in natural waters are seriously impaired. Thus alkalinity and acidity are convenient measures for estimating the maximum capacity of a natural water to neutralize acidic and caustic wastes without permitting extreme disturbance of biological activities in the water. 

Acidification of natural waters is mainly a problem in areas where the underlying rocks provide poor buffering capacity. Rocks such as granites and gneises offer little buffering protection. Chalk and limestone neutralize added acid, and so soils, lakes and streams in limestone areas are fairly insensitive to acidic precipitation... 

Acidity as applied to natural water systems is the capacity of the water to neutralize OH". Acidic water is not frequently encountered, except in cases of severe pollution. Acidity generally results from the presence of weak acids such as H2P04", CO2, H2S, proteins, fatty acids, and acidic metal ions, particularly Fe +. Acidity is more difficult to determine than is alkalinity. One reason for the dfficulty in determining acidity is that two of the major contributors are CO2 and H2S, both volatile solutes that are readily lost from the sample. The acquisition and preservation of representative samples of water to be analyzed for these gases is difficult. 

The other isolation methods evaluated employed solid adsorbents to isolate the model solutes (6-8). The first of these used XAD-4, a macroreticular, polystyrene-divinylbenzene resin (Rohm and Haas), into which trimethylamine groups had been introduced (9). The purpose of the resulting quaternary ammonium functional groups was to allow more efficient adsorption of acidic compounds without an appreciable loss of capacity for hydrophobic compounds. This feature is important because the vast majority of the organic matter in potable water is neutral or acidic in nature (JO). 

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