Buffer Capacity of Water

To calculate the buffer index of an aqueous system we begin by deriving the equation for the acidity titration curve for that system. For pure water we will assume the litrant is NaOH. The charge-balance equation is then simply 

This equation is plotted in Figs. 5.10 and 5.11, which show that strong acids and bases have considerable buffer capacity below pH 4 and above pH 10. /ShjO is negligible in natural waters at intermediate pH s. The units of the buffer capacity, as computed from Eq. (5.101) are equivalents of strong 

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Buffer Capacity of Water.—According to equation (74), the condition for electrical neutrality, when a strong base of concentration b has been... 

It should be noted that the further addition of base does not affect the concentration of A and so its derivative with respect to pH is zero. The buffer capacity of water, as given by equation (79), is negligible between pH values of 2.4 and 11.6, but in more strongly acid, or more strongly alkaline, solutions the buffer capacity of water is evidently quite considerable. This conclusion is in harmony with the fact that the pH-neutralization curve of a strong acid or strong base is relatively flat in its early stages. 

Note that the first two bracketed terms are the buffer capacity of water, and the third term is the buffer capacity due to the weak acid. For intermediate pH s the water terms are negligible and we have... 

Figure 5.10 A linear plot of the buffer capacity of carbonic acid species as a function of pH for Cj= lO M showing that the maximum buffer capacity equals 0.58 Cj, and occurs at pH = pAT,(H2C03) = 6.35. The lower curve is the buffer capacity of water, /Sh,o-...

Conversely, upon addition of a very strong acid, the buffer capacity of water is ... 

The total buffer capacity of water containing a strong acid or strong base is therefore ... 

Equation (53) permits us calculate in simple fashion the buffer capacity of solutions of strong acids and bases at different pH values. Between pH = 2.4 and pOH = 2.4, ir is less than 0.01 and may be neglected. Figure 3 is a graphical representation of the buffer capacity of water plus strong acids and bases. Along the abscissa is plotted pH while w-values constitute the ordinate axis. 

During nitrification hydrogen ions are released which react with hydrogen carbonates commonly present in waters. If the buffering capacity of water is insufficient, the pH value drops significantly. 

We now consider the calcium carbonate equilibrium and aggressive carbon dioxide. Of the various chemical equilibria in natural and service waters, the calcium carbonate equilibrium is of the greatest theoretical and practical importance. It is concerned with the evaluation of water aggressivity, control of deacidiflcation processes, limnology, evaluation of buffering capacity of water, etc. 

Figure 7. Buffering capacity of fractions from the pH-gradient pH 3-10 focused with the anode at the bottom of the column, obtained by titrating 4 id samples on the average containing 10 mg/ml. No corrections have been made for the buffering capacity of water or sucrose.

The buffer capacity of water containing completely dissociated acids or alkahs is... 

Ecologically, accidental releases of solution forms of hydrochloric acid may adversely affect aquatic life by including a transient lowering of the pH (i.e., increasing the acidity) of surface waters. Releases of hydrochloric acid to surface waters and soils will be neutralized to an extent due to the buffering capacities of both systems. The extent of these reactions will depend on the characteristics of the specific environment. 

When calcium carbonate goes into solution, it releases basic carbonate ions (COf ), which react with hydrogen ions to form carbon dioxide (which will normally remain in solution at deep-well-injection pressures) and water. Removal of hydrogen ions raises the pH of the solution. However, aqueous carbon dioxide serves to buffer the solution (i.e., re-forms carbonic acid in reaction with water to add H+ ions to solution). Consequently, the buffering capacity of the solution must be exceeded before complete neutralization will take place. Nitric acid can react with certain alcohols and ketones under increased pressure to increase the pH of the solution, and this reaction was proposed by Goolsby41 to explain the lower-than-expected level of calcium ions in backflowed waste at the Monsanto waste injection facility in Florida. 

Humus/SOM enter into a wide variety of physical and chemical interactions, including sorption, ion exchange, free radical reactions, and solubilization. The water holding capacity and buffering capacity of solid surfaces and the availability of nutrients to plants are controlled to a large extent by the amount of humus in the solids. Humus also interacts with solid minerals to aid in the weathering and decomposition of silicate and aluminosilicate minerals. It is also adsorbed by some minerals. 

The low buffer capacities of the KHPh solution in solvents of aptotic nature is caused by the increase in the (pKa2-pKai) value of phthalic acid. In H20-DMF and H20-DMS0 mixtures, the buffer capacity of 0.05 mol kg-1 KHPh is not enough if the water content is less than 30v/v% [18]. 

Buffer Capacities of Natural Waters. Natural waters are buffered in different ways and to varying degrees with respect to changes in pH, metal ion concentrations, various ligands, and oxidation-reduction potential. The buffer capacity is an intensive variable and is thermodynamic in nature. Hydrogen-ion buffering in natural waters has recently been discussed in detail by Weber and Stumm (38). Sillen (32) has doubted... 

In addition to changing the pH of the water, the uptake and release of CO2 alter the buffer capacity of the water. The effect upon buffer capacity is the result of two factors (1) the dependence of buffer capacity on the hydrogen ion concentration, and (2) the dependence of buffer capacity on the total concentration of weak acid and conjugate base in solution (67, 68). The precipitation of CaCO in natural waters reduces the buffer capacity to a value lower than that predicted on the basis of pH change and respiratory or photosynthetic changes in COL content of the water. 

Acid-Base. The pH of natural waters is determined primarily by the carbonate equilibria. However, organisms may produce amounts of organic matter or ammonia sufficient to influence the pH and buffer capacity of the waters. It would be of interest to determine titration curves of high organic, high color, low alkalinity waters leached from some marshes. It is possible that these waters contain sufficient amounts of organic acids to be significant. 

Many factors control whether a given water body will become acidified as a result of a given deposition regime. In addition to the deposition rate and the lake residence or turnover time, these include the ratio of water surface area to watershed area, the composition of the lake bottom, the residence time of incident precipitation en route through the watershed, and the buffering capacity of the watershed. The presence of organic material can also be important. 

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