PH measurement and control

The use of chemical analysis to monitor the quality of the raw materials or finished products of industrial processes goes back a long way.313,314 Indeed, some techniques owe their development to the need of industry for rapid analytical techniques. However, analytical methods are now often intimately bound up with the production itself, and supply much of the information required for the control and regulation of the process.315 A good example of a continuous monitoring technique that can be used in process control is that of electrodeless conductivity measurement its history has been described.316 A history of early industrial pH measurement and control systems has been given.317... 

John T. Stock, Early industrial pH measurements and control, Bulletin for the History of Chemistry, 10 (1991), 31-34. 

Chapter 1 - The Essentials provides a concise overview of what makes pH applications unique. It covers the total system requirements and Is designed for the widest possible audience. It uses an adventurous style and analogies that should help make this chapter interesting enough to read from start to finish. The other chapters follow up on the seeds planted here and are intended to be used more as a reference for the design of each part of a pH measurement and control system. 

The pH electrode offers by far the greatest sensitivity and rangeability of any measurement. To make the most of this capability requires an incredible precision of mixing, reagent manipulation, and nonlinear control. pH measurement and control can be an extreme sport. 

Key words control algorithms, discontinuous control modes, continuous control modes, composite control modes, analog and digital processing, pH measurement and control, dyeing control. 

In the steaming-out process excess chlorine is used and recycled. The major process conditions that are measured and controlled are temperature, pressure, pH, and oxidation potential. 

Temperature, pH, and feed rate are often measured and controlled. Dissolved oxygen (DO) can be controlled using aeration, agitation, pressure, and/or feed rate. Oxygen consumption and carbon dioxide formation can be measured in the outgoing air to provide insight into the metaboHc status of the microorganism. No rehable on-line measurement exists for biomass, substrate, or products. Most optimization is based on empirical methods simulation of quantitative models may provide more efficient optimization of fermentation. 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

Reactor vessels range in volume from 1.2 to 2.5 liters. The size used may be dependent on the pressure application requirements. A high-pressure reactor can run up to about 60 bar. Porting is provided on the reactor vessel for auxiliary probes (e.g, oxygen sensors, pH measurement, and on-line analysis). These probes may be inserted directly into the reactor contents. Porting is also available for the controlled addition of gas, liquid, or solid materials. 

Ti, or PEEK (polyether ether ketone) to allow measurements under very corrosive conditions. The separated phases pass AMX gadgets for on-line detection (radiometric, spectrophotometric, etc.) or phase sampling for external measurements (atomic absorption, spectrometric, etc.), depending on the system studied. The aqueous phase is also provided with cells for pH measurement, redox control (e.g., by reduction cells using platinum black and hydrogen, metal ion determination, etc.) and temperature control (thermocouples). 

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. 

The sugar industry has always been in the forefront of chemical control. The most important areas of control include measurement of sugar content (done by polarization), pH control (because sugar is unstable at low or high pH, and more so when high temperatures prevail), color measurement, and control of vacuum pans (for good crystallization). 

The dominance of the flow effect suggests that flow information should be used directly to cancel this effect. This can be achieved by adding a flow measurement and controlling the ratio of the effluent and reagent flows based on the pH measurement (Section III.B.4). Despite the measurement lag and minor biases in flow measurement, this makes the concentration effect dominant. As only the two-tank configuration allows the concentration effect to be tolerated, we have two 12-m tanks with ratio control of reagent flow at the inlet to the treatment system as the starting point for the search for an effective plant. 

There has always been some reluctance to assume that the pH in a porous medium is controlled solely by the carbonate buffer system in the porewaters. There are arguments that H+ ions on particle surfaces can affect pH measurements and, even more importantly, comprehensive models of porewater chemistiy are beginning to demonstrate that H+ adsorption on mineral surfaces may play an important role in controlling the pH of porewaters. Conclusions of the pH measurements described here, however, have been confirmed by millimeter-scale measurements of both pcOj and Ca concentration in the porewaters (e.g. Wenzhofer et al, 2001). 

The measurement of analyte concentration is a critical part of successful bioreactor monitoring. Although strategies exist for measuring the majority of relevant analytes, industrial online bioreactor control is carried out primarily by measurement and control of temperature, pH, d02, CO2, and, in some cases, cell density. This is because the available technology cannot be easily and inexpensively adapted to measure the analyte in an aseptic manner and measurement is not often achieved in real time so that online control is challenging. Biosensors for a variety of applications for... 

The set of operations that establish, under specified conditions, the relationship between values indicated by an instrument or system for measuring (for example, weight, temperature and pH), recording, and controlling, or the values represented by a material measure, and the corresponding known values of a reference standard. Limits for acceptance of the results of measuring should be established. 

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