Solid buffer capacity

R FS. 0.05m Potassium hydrogenphthalate. Dissolve 10.21 g of the solid (dried below 130 °C) in water and dilute to 1 kg. The pH is not affected by atmospheric carbon dioxide the buffer capacity is rather low. The solution should be replaced after 5-6 weeks, or earlier if mould-growth is apparent. 

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. 

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 ... 

The difference in the conductivity of the calibration buffers and sample can cause a very large error on the sample measurement, due to junction potentials in different environments. Solid samples should be dissolved in purified water. It is necessary that the water be carbon dioxide-free. The presence of dissolved carbon dioxide will cause significant bias in the measurement of samples with low buffering capacity. For pH measurements with an accuracy of 0.01 to 0.1 pH unit, the limiting factor is often the electrochemical system (i.e., the characteristics of the electrodes and the solution in which they are immersed). 

The surrounding fluid (Fig. 8-7) serves two purposes 1) it transmits the pressure to stress-load the surface and 2) it allows the surface to equilibrate chemically and thus provides juL in Eqn. (8.61) with physical meaning. Ideally, the Gibbs fluid has a vanishing buffer capacity for the solid so that after a change in an, the fluid becomes resaturated with respect to the solid before a noticeable amount of the solid or its surface dissolves. The key to verify Gibbs relation for solids under non-hydrostatic stress is therefore the existence of such an idealized fluid. 

With electrochemical methods, we determine thermodynamic potentials of components in systems which contain a sufficiently large number of atomic particles. Since the systematic investigation of solid electrolytes in the early 1920 s, it is possible to change the mole number of a component in a crystal via the corresponding flux across an appropriate electrolyte (1 mA times 1 s corresponds to ca. 10 s mol). Simultaneously, the chemical potential of the component can be determined with the same set-tip under open circuit conditions. Provided both the response time and the buffer capacity of the galvanic cells are sufficiently small, we can then also register the time dependence of the component chemical potentials in the reacting solids. ... 

Figure 4 shows a number of computed homogeneous (absence of solid phases) buffer capacity vs. pH relationships for sea water conditions... 

Figure 4. Buffer capacity vs. pH for some chemical systems not involving solid phases...

The equilibrium composition of the solution is independent of the quantities of the three solids as long as all three coexist. To a system with these four phases one may add arbitrary amounts of KOH or HC1. As long as [Cl-] is not changed and none of the three solids disappears, the system would return to the same equilibrium values for [K+] and [H+] as earlier. The system might thus be called a pH-stat rather than a buffer its buffer capacity would be infinite as long as [Cl-] were kept constant (20). 

Elements of /< greater than 3.2 form simple metallate anions in solutions of high pH—i.e., strongly alkaline solutions. As the pH of such solutions is lowered, isopolyanions are produced in many cases, ultimately yielding hydrous oxides, or salts of isopolymetallate ions as heterogeneous solid phases. Again, many of the isopolyanions exhibit buffer capacity. 

Silicates. Both sodium and potassium silicate solids or solutions have valued functionality including emulsification, buffering, deflocculation, and antiredeposition ability. Silicates also provide corrosion protection to metal parts in washing machines, as well as to the surfaces of china patterns and metal utensils in automatic dishwashers. Silicates are manufactured in liquid, crystalline, or powdered forms and with different degrees of alkalinity. The alkalinity of the silicate provides buffering capacity in the presence of acidic soils and enhances the sequestration ability of the builder system in the formulation. The sili-cate/alkali ratios of the silicates are selected by the formulator to meet specific product requirements. Silicate ratios of 1/1 are commonly used in dry blending applications with silicate ratios of 2/1 and higher commonly used in laundry and autodish applications. 

Figure 8A. The relationship between 1/NH3 and 1/NH4 in soil solutions differing in pH and buffering capacity. Experimental data are represented by the solid lines broken lines represent calculated data (from Avnimelech and Laher, 1977, with permission).

The reaction by-products of sulfide oxidation are distinct thus, evidence of sulfide oxidation can be found in the chemical signatures of ground water. In aquifers where sulfide oxidation occurs, ground water chemistry should show a positive correlation of arsenic with sulfate, iron, and trace metals contained in the sulfide minerals. Increases in total dissolved solids and specific conductance also result from sulfide oxidation, due to an increase of dissolved ions in the impacted waters. Ground water impacted by sulfide oxidation may reveal a negative correlation of arsenic with pH, provided that there is minimal buffering capacity provided by the host rocks. 

Process considerations require pH control in a 50,000-gal storage tank used for incoming waste mixtures (including liquid plus solids) at a hazardous waste incinerator. Normally, the tank is kept at neutral pH. However, operation can tolerate pH variations from 6 to 8. Waste arrives in 5000-gal shipments. Assume that the tank is completely mixed, contains 45,000 gal when the shipment arrives, the incoming acidic waste is fully dissociated and that there is negligible buffering capacity in the tank. 

Soil acidity is known to exert adverse effect on crop growth by its effect on nutrient availability and microbial activity. Measurement of soil pH only is not a true representation of soil acidity. This is due to the fact that soil pH which is a measure of active acidity is subject to change due to a number of factors. Substantial amoimts of soil acidity reside in the soil solid phase, in the interlayer spaces, as solid phase minerals and in the fimctional group of soil-organic fraction. All these contribute to the pool of active acidity in response to any shift in the thermodynamic equilibrium and thus helps in maintaining the buffering capacity of soils. 

The most important ion selective electrode using a solid membrane is the glass electrode for pH measurement. Despite their outstanding selectivity for H+ ions glass electrodes are used only seldom in enzyme electrodes because their sensitivity is affected by the buffer capacity of the measuring solution. 

The same principles apply equally to solid matrices, both natural and artificial, and the pH both of the matrix and the solution of the toxicant should be carefully taken into consideration. In general, pH control of the matrices is difficult to maintain and reliance must be made on the natural buffering capacity of soils containing humic material. 

Heterogeneous equilibria are the most efficient buffer systems for natural waters (Stumm and Morgan, 1996). They also represent a potential source of large buffer capacity, because in porous media the solid to water ratio is high. 

Surfactant Chemical Stability. Two approaches were used in assessing surfactant degradation over time. The first consisted of monitoring the pH of surfactant solutions that were in contact with pieces of reservoir rock over several months. Because only commercially available surfactants were tested and almost all of them contained secondary components, the pH data were rather inconclusive. The fact that reservoir solids have some buffering capacity made the interpretation of pH trends even more difficult. 

In addition, the particle size of the solid phase must be sufficiently large to allow the separation of the sohd and aqueous phases and this is, in practice, difficult for small particles. This is probably the main reasons for the widely different solubihty that is reported for Th(IV) hydrous oxide phases. The attainment of solubihty equilibrium is often a slow process, in particular for ciystahine thorium oxides [2001HUB/BAR], [2003NEC/ALT] and care must be taken to ensure that true equihbrium has been attained in the system. The solubility of hydrous thorium oxide is low at pH > 5, but sorption of thorium on the container walls is no problem as the thorium buffer capacity of the system is determined by the solid phase the situation is different in liquid-liquid extraction where the thorium buffer capacity is low. 

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