On pH control

Sulfide ion forms precipitates with heavy metal cations that have solubility products that vary from 10 ° to 10 °° or smaller. In addition, the concentration of S can be varied over a range of about 0.1 M to 10 M by controlling the pH of a saturated solution of hydrogen sulfide. These two properties make possible a number of useful cation separations. To illustrate the use of hydrogen sulfide to separate cations based on pH control, consider the precipitation of the divalent cation M from a solution that is kept saturated with hydrogen sulfide by bubbling the gas continuously through the solution. The important equilibria in this solution are ... 

Enormous differences exist in the solubilities of the hydroxides, hydrous oxides, and acids of various elements. Moreover, the concentration of hydrogen or hydroxide ions in a solution can be varied by a factor of 10 or more and can be readily controlled by the use of buffers. As a consequence, many separations based on pH control are, in theory, available to the chemist. In practice, these separations can be grouped into three categories (1) those made in relatively concentrated solutions of strong acids, (2) those made in buffered solutions at intermediate pH values, and (3) those made in concentrated solutions of sodium or potassium hydroxide. Table 30-2 lists common separations that can be achieved by control of acidity. 

The possible procedures involving titration with SDS or BEC all depend on pH control, and are therefore not applicable to mixtures containing alkali-labile quats. It is better to assume that such mixtures will necessitate separation, and the guidelines given below assume that alkali-labile quats are absent, and that only one quat and one amphoteric are present, although the astute analyst will be able to work out schemes for some mixtures containing more than one surfactant of each class. 

There are multiple interactions between proton pump inhibitors (PPIs) and Helicobacter pylori in vivo. This chapter deals with two aspects First, we discuss the effect of H. pylori on pH control by PPIs and on the clinical effectiveness of these drugs Second, we focus on the effect of PPIs on H. pylori and on gastritis. Of special interest is the question whether long-term treatment with PPIs leads, in H. / y/or/-infected subjects, to the development of atrophic gastritis and associated changes, considered by some authors to represent a precancerous condition. 

Unfortunately, most set points are near the neutral or equivalence points on the titration curve, which coincide with location of the maximum slope. After an upset or start-up, the pH accelerates as the operating point approaches the steepest point on the curve. To the pH controller that knows only what it sees, it appears to be a fast runaway or positive feedback response. Most of the beneficial value of the investment in a well mixed vessel in terms of slowing down a disturbance has been lost, The decrease in the effective process time constant seen by the pH controller will be quantified in Chapter 10 on pH control system selection. Operators will ask whether you can do anything to slow down the response because it tends to zip right past the desired set point. In Chapter 8 on pH control systems, we will see how signal characterization can slow down the response by the translation of the controlled variable from pH to percent reagent demand (X axis of the titration curve) to soothe the operator s nerves and restore the process time constant. 

In addition to the collector, polyvalent ions may show sufficiently strong adsorption on oxide, sulfide, and other minerals to act as potential-determining ions (see Ref. 98). Judicious addition of various salts, then, as well as pH control, can permit a considerable amount of selectivity. 

The success of a reverse osmosis process hinges direcdy on the pretreatment of the feed stream. If typical process streams, without pretreatment to remove partially some of the constituents Hsted, were contacted with membranes, membrane life and performance would be unacceptable. There is no single pretreatment for all types of foulants. Pretreatment methods range from pH control, adsorption (qv), to filtration (qv), depending on the chemistry of the particular foulant. Some of the pretreatment methods for each type of foulant are as foUow (43—45) ... 

Level Dyeing Techniques. It is exceptionally difficult to obtain level dyeings on acryhc, and temperature and pH control depend on fiber type and are not always adequate. Sodium sulfate in limited amounts can be used to some effect. The sulfate ions compete for the dye with the fiber SO3 sites and so retard the rate of dyeing by forming a dye complex with the ions. The effect of sodium sulfate is best with dyes having the lowest... 

Control of Dyeing Equipment. Over the years, the dyer and machinery manufacturer have appHed any mechanical or electrical equipment that would enable them, day after day, to produce repeatable dyeings of top quaHty. First, thermometers were installed in dye lines these soon evolved into thermocouples with remote recording. Other improvements were soon developed, such as automatic four-way valves with variable-interval controls, flow controls, pressure recorders, hydrauHc and air pressure sets on roUers, pH controls, etc. 

Neutralization. The choice of a reagent for pH adjustment depends on cost ease and safety of storage and handling effectiveness, eg, for removing heavy metals, buffet characteristics of the pH titration curve as they affect pH control and avadabihty. The three principal reagents for neutralization of acid wastes are sodium hydroxide, sodium carbonate, and hydrated calcium hydroxide. 

Three examples of simple multivariable control problems are shown in Fig. 8-40. The in-line blending system blends pure components A and B to produce a product stream with flow rate w and mass fraction of A, x. Adjusting either inlet flow rate or Wg affects both of the controlled variables andi. For the pH neutrahzation process in Figure 8-40(Z ), liquid level h and the pH of the exit stream are to be controlled by adjusting the acid and base flow rates and w>b. Each of the manipulated variables affects both of the controlled variables. Thus, both the blending system and the pH neutralization process are said to exhibit strong process interacHons. In contrast, the process interactions for the gas-liquid separator in Fig. 8-40(c) are not as strong because one manipulated variable, liquid flow rate L, has only a small and indirec t effect on one controlled variable, pressure P. 

Control philosophies for clarifiers are based on the idea that the overflow is the most important performance criterion. Underflow density or suspended sohds content is a consideration, as is optimal use of flocculation and pH control reagents. Automated controls are of three basic types (I) control loops that optimize coagulant, flocculant, and pH control reagent additions (2) those that regulate underflow removal and (3) rake drive controls. Equahzation of the feed is provided in some installations, but the clarifier feed is usually not a controlled variable with respect to the clarifier operation. 

Automated controls for flocciJating reagents can use a feedforward mode based on feed turbidity and feed volumetric rate, or a feed-back mode incorporating a streaming current detector on the flocculated feed. Attempts to control coagulant addition on the basis of overflow turbidity generally have been less successful. Control for pH has been accomplished by feed-forward modes on the feed pH and by feed-back modes on the basis of clarifier feedwell or external reaction tank pH. Control loops based on measurement of feedwell pH are useful for control in apphcations in which flocculated sohds are internaUy recirculated within the clarifier feedwell. 

Uniformity of the rate of feed will be ensured by a constant-weight feeder density control may be automatically obtained through a measuring probe on the media-return line that adjusts delivery of the nec-essai y volume of media from the densifier or media thickener the viscosity can be controlled automatically by continuously testing a predetermined volume of return media and adjusting the divider under the drainage screen for media cleaning as needed pH control can be automated by conventional methods. 

Activities associated with bioreactors include gas/hquid contacting, on-hne sensing of concentrations, mixing, heat transfer, foam control, and feed of nutrients or reagents such as those for pH control. The workhorse of the fermentation industry is the conventional batch fermenter shown in Fig. 24-3. Not shown are ladder rungs inside the vessel, antifoam probe, antifoam system, and sensors (pH, dissolved oxygen, temperature, and the like). Note that coils may lie between baffles and the tank wall or connect to the top to minimize openings... 

Ammonia Injection rate controlled on pH of condensate in accumulator... 

Terminology The International Standards Organization has recently defined a corrosion inhibitor as a chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration, without significantly changing the concentration of any other corrosive agent. This last point is significant since it excludes chemicals employed for deaeration or pH control from the definition of a corrosion inhibitor. On the other hand, it should be noted that the inhibitor is .. . present in the corrosion system. . . , and thus arsenic when added to brasses to prevent dezin-cihcation may be classified as an inhibitor. 

The Payne epoxidation with benzonitrile/ hydrogen peroxide is also an efficient epoxidation process. It is often the method of choice for industrial batch-type applications, but on a small scale the need for continuous pH control is inconvenient. 

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