PH stat control

This shows that the number of hydrogen ions used in cathodic reactions is equal to the number of charges transferred in the anodic reaction. The pH value in the solution can then be maintained constant by a pH stat, controlling the addition of acid to the solution at such a rate that the loss of hydrogen ions is compensated. Then the following condition is fulfilled ... 

When an NHg solution is used as a nitrogen source of fermentation, the consumption of NHg decreases the pH of the culture broth. A pH stat control utilizes the pH change as a process variable where the NHg addition is manipulated so as to keep the pH value constant during fermentation. The pH stat is often employed in a fed-batch cultivation of industrial glutamic acid production (high NHg consuming fermentation) using molasses as a feedstock. 

In a pH-stat-controlled reaction system, the uptake of CO2 from the atmosphere is one of the error sources need to be considered. The degree of proteolysis can be calculated from the assumption of base (B) during the reaction. The number of peptide bonds cleaved is equal to the mols of base consumed. That is ... 

Haloferax mediterranei ATCC 33500, a member of the Archaea family, was used to produce PHA using a mixture of extruded rice bran (ERB) and extruded cornstarch (ECS) as carbon source (Huang et al. 2006). This strain caimot use native rice bran or cornstarch as C-sources. By employing pH-stat control strategy in a 5-L jar bioreactor using ERB ECS (1 8 g g ) as the major carbon source, the authors obtained a cell concentration of 140 g L , PHA concentration of 77.8 g L, and PHA content of 55.6 wt% in a repeated fed-batch fermentation. Otherwise when ECS was used as the major carbon source, a 62.6 g L cell concentration, 24.2 g PHA concentration, and 38.7 wt% PHA content were achieved. ... 

To each ml of QD solution, add 50 pi of the EDC/sulfo-NHS stock solution. Maintain the pH at 7.0 by the addition of base, if necessary. Small volume reactions may be controlled using a pH stat. 

Fixed-activity and sliding-activity paths (Sections 14.2-14.3) are analogous to their counterparts in fugacity, except that they apply to aqueous species instead of gases. Fixed-activity paths are useful for simulating, for example, a laboratory experiment controlled by a pH-stat, a device that holds pH constant. Sliding-... 

Various devices can be used to determine the kinetics and rates of chemical weathering. In addition to the batch pH-stats, flow through columns, fluidized bed reactors and recirculating columns have been used (Schnoor, 1990). Fig. 5.15a illustrates the fluidized bed reactor pioneered by Chou and Wollast (1984) and further developed by Mast and Drever (1987). The principle is to achieve a steady state solute concentration in the reactor (unlike the batch pH-stat, where solute concentrations gradually build up). Recycle is necessary to achieve the flow rate to suspend the bed and to allow solute concentrations to build to a steady state. With the fluidized bed apparatus, Chou and Wollast (1984) could control the AI(III) concentration (which can inhibit the dissolution rate) to a low level at steady state by withdrawing sample at a high rate. 

The observed metal phosphate phases agree with thermodynamic models of the ash system described here. These phases control leaching in pH-stat systems and are present after aggressive leaching designed to remove available or leachable fractions. These phases are also similar to ones observed in soil, sediment, smelter dust, industrial wastewater, and slag systems. 

The active membrane separates two compartments and it is possible to get this pH value throughout the system, in presence of the two substrates, by the transient use of a buffer. The pH values outside are controlled and H+ fluxes measured by pH-stat systems. After small asymmetrical perturbations of the pH values at the boundaries (0.05), an inhomogeneous pH distribution arises spontaneously inside the membrane. The initial perturbations are amplified and the pH values in the compartments tend to evolve in opposite directions. The H+ fluxes entering and leaving the membrane can be determined by pH-stat measurements. If the boundary pH values are not maintained constant by a pH stat, the system evolves to a new stationary state characterized by a pH gradient of two pH units across the membrane. 

A hydrolytic reaction that releases acid may be followed by titration with base. This is best done automatically by use of the pH-stat. A glass electrode registers the pH of the solution, which is kept constant by the automatic addition of base from a syringe controlled by an electronic circuit. Reaction volumes as low as 1 mL may be used, and the limit of detectability is about 50 nmol (5 to 10 /tL of base at 5 X 10-3 to 1 X 10-2 M). The usual source of error with this apparatus is the buffering effect of dissolved C02. 

A related form of an automatic potentiometric titrator is instrumentation that permits the maintenance of the acidity or basicity of a solution over a period of time. Such devices are known as pH-stats, and find application in kinetic studies of hydrolysis reactions. The general approach is (by either manual or automatic means) to add either acid or base such that the pH in the solution is maintained constant over a period of time. Normally the amount of acid or base added as a function of time is sought in order that kinetic measurements may be made for the system. In its simplest form the acidity of the solution is monitored with a pH meter and controlled at a preselected value by the addition of acid or base from a burette the quantity delivered as a function of time is recorded in a notebook. Obviously for the fast reactions this becomes difficult and dependent on the dexterity of the individual. 

Another application of pH-stats is to control solution pH without the use of a buffer system. Again, this can be accomplished by either a burette delivery system, a motor-driven syringe, or electrochemical generation of hydroxide ion or hydronium ion. This can be extremely useful for systems where all available buffer solutions interfere with the reaction of interest. 

The present work was carried out with the purpose of relating taste, solubility, emulsifying capacity, foaming capacity, and viscosity of soy protein hydrolysates to the DH of these hydrolysates. This tentative approach to the manufacture of functional soy protein hydrolysates was chosen, because DH is easily controlled during hydrolysis by means of the pH-stat technique (5), and because the properties of the hydrolysate are presumed to be related to the DH-value rather than to hydrolysis parameters such as temperature, substrate concentration, and enzyme-substrate ratio (6). 

We have studied the dissolution kinetics of calcite in stirred CO2 water systems at CO2 partial pressures between 0.0003 and 0.97 atm and between 5° and 60°C, using pH-stat and free drift methods (J ) Our results suggest a mechanistic model for reactions at the calcite-aqueous solution interface that has broad implications to the controls on calcite dissolution and precipitation under diverse chemical and hydrodynamic conditions. 

Figure 1 summarizes our pH-stat results for three CO2 partial pressures at 25°C as a function of pH. In general, the pattern shown in Figure 1 demonstrates three controls on the forward rate. 1) At low pH, dissolution rate shows little dependence on PC02. 

Control of an electrolytic reaction often requires that the proton activity remains within acceptable limits during the electrolysis. For small-scale electrolytic preparations (less than about 10 g/liter of substrate), a sufficiently high initial concentration of buffer, acid, or base is adequate in aqueous solution for large-scale electrolysis a controlled addition of protons during a reduction must be provided. This addition may be controlled by a pH-stat or coupled to the current integrator. In aprotic media a proton donor, electrophile, or nucleophile may play a similar role as buffers in aqueous media. 

Another way to control pH is through the use of a pH-stat, where pH is controlled by titrating with acidic and/or basic solutions (Todd and Winnike, 1994). Ionic equilibrium can be monitored continuously by measuring the solution pH. Equilibrium can be considered to have been reached when the pH no longer changes over a period of time. 

A pH electrode connected to an autotitration instrument (pH meter, controller, and autoburet) is often used to maintain a constant pH (pH-stat) during the reaction. If it is desired to monitor pH changes during the reaction, then the pH meter can be connected to a recorder. It should be remembered I hat autotitration units have finite response times that are longer than many surface-controlled reactions. Thus, pH changes can occur before the autotitra-1 ion system can fully respond, and there will be some delay between the reac-lion and return to pH-stat conditions. 

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