Acid-neutralizing capacity change

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... 

Figure 3.10. Time series of predictions with the acidification model PnET-BGC of changes in stream chemistry at Hubbard Brook to changes in past and potential future emissions of sulfur dioxide and nitrogen oxides, including the 1990 Amendments of the Clean Air Act and moderate and aggressive emission control scenarios. Shown are model-predicted stream concentrations of sulfate, nitrate, acid neutralizing capacity, pH and dissolved inorganic aluminum, and soil percent base saturation. Measured values are indicated for comparison...

S deposition (Driscoll et al. 1995 Dillon et al. 1997 Dillon and Evans 2001 Lofgren et al. 2001). Furthermore, other measures of chemical recovery, particularly increases in lake pH and acid neutralizing capacity (ANC), have provided conflicting evidence. In a few cases (Webster and Brezonik 1995 Stoddard et al. 1998), they have indicated limited evidence of chemical recovery, while in many others (Clair et al. 1995 Houle et al. 1996 Dillon et al. 1997), there has been little or no change in these parameters despite declining S deposition. 

The need for this acid neutralization capacity is evident from Fig. 3, which shows the continual decrease in total base number (TEN) and increase in total acid number (TAN) for a high- and low-TAN gasoline engine oil. The TBN/TAN equivalence point occurs at aroimd 3000 to 6000 miles. (The metal content in the drain increases after the equivalence point is reached in field testing.) Typically, at this point, acids build up to unacceptable levels, and it is, therefore, desirable to change the oil before the TEN and TAN cross. 

The amount of acid or base that a buffer is able to neutralize before changing pH, the buffering capacity, is related to the amount of weak acid/conjugate base or weak base/conjugate acid present in the buffer solution. The greater the concentration of the conjugate pairs, the more resistant to a change in pH the buffer will be. In the next section, well look at some quantitative aspects of buffer solutions. 

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. 

The buffer capacity of the pit fluid is equal to the change in alkalinity of the system per unit change of pH. Figure 4-491 shows the buffer intensity (capacity) of a 0.1 M carbonate pit fluid. Calculating the initial buffer capacity of the pit fluid allows for prediction of the pH change upon introduction of live acid and also any addition of buffer, such as sodium bicarbonate, required to neutralize the excess hydrogen ions. 

The slope of the tangent to the curve at the inflection point where oc = is thus inversely proportional to the number of electrons n. The E-oc curves are similar to the titration curves of weak acids or bases (pH-or). For neutralization curves, the slope dpH/doc characterizes the buffering capacity of the solution for redox potential curves, the differential dE/da characterizes the redox capacity of the system. If oc — for a buffer, then changes in pH produced by changes in a are the smallest possible. If a = in a redox system, then the potential changes produced by changes in oc are also minimal (the system is well poised ). 

These equations allow us to calculate the pH or pOH of the buffer solution knowing Kof the weak acid or base and the concentrations of the conjugate weak acid and its conjugate base. Also, if the desired pH is known, along with K, the ratio of base to acid can be calculated. The more concentrated these species are, the more acid or base can be neutralized and the less the change in buffer pH. This is a measure of the buffer capacity, the ability to resist a change in pH. 

Equation (3.70) for the electrical neutrality of the solid, with changes in acidity in the solid related to changes in pH with the soil pH buffer capacity ... 

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