The pH System

we see that varies nearly 100-fold from 0 to 60°C. Note that it is pure chance that at 25°C, near room temperature, we have = 1.0 X 10 The negative log, pK°, is equal to 13.996, or 14.00. There is no magic in the rounded value 7.00 for the pH of ultrapure water at 25°C. Actually, it is 6.998. The pH of pure water is 7.47 at 0 and 6.51 at 60°C. Remember that the definition of neutrality in water is not that the pH is seven, but that the hydronium concentration is equal to the hydroxide concentration. 

When inert salt ions like K Cl are placed in water, the activities of the H30 and OH are affected. If we take as fact that the K° must remain constant, then a change in / values means that molarities must change in such a way as to keep the activity product constant. This has been found experimentally. The molarity product does vary at 25° as KCl is added to water  

we see that the ionic atmosphere affects the degree of ion formation of water. 

These data on the temperature and ionic strength effects on show up the futility of doing calculations with more than one significant digit (or to +10%) when T and ionic strength are unknown. 

When a protonic acid HX is placed in water, it reacts partially (or completely if the acid is strong) to form some ions. The degree of hydronium hydration will not affect the calculations when water activity 1 is assumed for dilute solutions. 

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The maintenance requirements of an ORP-measuring system also mimic the pH system. Electrodes require the same sort of care, never being allowed to dry out and being recalibrated with some frequency. Spare calibrated electrodes should always be available, and the use of dual electrodes is not uncommon. 

A very important fact to remembo whoi using buffers in HPLC mobile phases is that the pH system we commonly refer to is based on an aqueous system. In this system we are dealing with ostensibly pure water with a pH 7. Most HPLC nwbile phases contain significant levels of organic components. Therefore, keep in mind tluit ... 

The pH system, proposed by Sorensen in 1909, is seen from Table 2-3 to be the use of the negative logarithm of H to express the wide range of acidities over about 17 powers of ten. Early workers assumed that they were dealing with the molarity of Equation... 

The resistance of the membrane does not affect the pH response. If the glass is too thick, however, the electrode may depart from normal response. With a high-resistance glass membrane, a noisier reading and a pH error are more likely. As the combined resistance of the pH system, of which the glass electrode component is normally the greatest, approaches the input resistance of the pH meter, the noise and pH error increase. This is further discussed in Section 2.1. 

The interval-halving search method will alw ays converge since there is only one zero crossing of the charge balance if the pH is within the original search interval selected and if the arithmetic precision of the computer is sufficient for the pH system. 

The most often used subphase is water. Mercury and otlier liquids [12], such as glycerol, have also occasionally been used [13,14]. The water has to be of ultrapure quality. The pH value of tire subphase has to be adjusted and must be controlled, as well as tire ion concentration. Different amphiphiles are differently sensitive to tliese parameters. In general it takes some time until tire whole system is in equilibrium and tire final values of pressure and otlier variables are reached. Organic contaminants cannot always be removed completely. Such contaminants, as well as ions, can have a hannful influence on tire film preparation. In general, all chemicals and materials used in tire film preparation have to be extremely pure and clean. 

The data in Tables 4.2 and 4.3 refer to ions in aqueous acid solution for cations, this means effectively [MlHjO), ]" species. However, we have already seen that the hydrated cations of elements such as aluminium or iron undergo hydrolysis when the pH is increased (p. 46). We may then assume (correctly), that the redox potential of the system... 

To illustrate the systematic approach, let us calculate the pH of 1.0 M HF. Two equilbria affect the pH of this system. The first, and most obvious, is the acid dissociation reaction for HF... 

In practice, intermediate, Hquid resins, capable of further reaction are usually prepared. Polymerization is carried to an estabUshed end-point as determined by viscosity or other measurements. When the proper end-point has been reached, the reaction is terminated by adjusting the pH of the system to 5—8. Such hquid resins can be stored for six months or longer, then catalyzed and reacted further to obtain the final, desired product. 

Acid-Base Behavior. The relative acidity-basicity of the filler, generally determined by measuring the pH value of a slurry of a specific mass of filler in 100 mL of deionized water, can influence the behavior of a filler in some systems. For example, the curing behavior of some elastomers is sensitive to the pH value of carbon black. 

In this case, the components are mixed, the pH adjusted to about 6.0 with sodium hydroxide, and the solution appHed to the textile via a pad-dry-cure treatment. The combination of urea and formaldehyde given off from the THPC further strengthens the polymer and causes a limited amount of cross-linking to the fabric. The Na2HP04 not only acts as a catalyst, but also as an additional buffer for the system. Other weak bases also have been found to be effective. The presence of urea in any flame-retardant finish tends to reduce the amount of formaldehyde released during finishing. 

Sodium aluminate [1302-42-7] is another source of soluble aluminum made by leaching bauxite with caustic soda. As with alum, the active species are really its hydrolysis products which depend on the chemistry of the system to which it is added. It tends to raise the pH. It is available both as a soHd and as a solution (see Aluminum compounds, aluminates). 

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

The activity of the hydrogen ion is affected by the properties of the solvent in which it is measured. Scales of pH only apply to the medium, ie, the solvent or mixed solvents, eg, water—alcohol, for which the scales are developed. The comparison of the pH values of a buffer in aqueous solution to one in a nonaqueous solvent has neither direct quantitative nor thermodynamic significance. Consequently, operational pH scales must be developed for the individual solvent systems. In certain cases, correlation to the aqueous pH scale can be made, but in others, pH values are used only as relative indicators of the hydrogen-ion activity. 

Organosulfur Compounds. These compounds, Hsted in Table 8, are used in a variety of appHcations, including cooling water, paint, and metalworking. Methylenebisthiocyanate hydroly2es rapidly at a pH above 8 to cyanate ion which complexes with ferric iron to poison the cytochrome systems (36). 

The viscosity of the latex can also be dependent on pH. In the case of some latices, lowering the pH with a weak acid such as glycine is an effective method for raising the viscosity without destabilising the system. Latices made with poly(vinyl alcohol) as the primary emulsifier can be thickened by increasing the pH with a strong alkaU. 

In recent years, lime treatment has been advocated for corrosion control by removing lead and copper from distribution systems, mainly by raising the pH to around 7.5, which prevents these heavy metals from solubilizing. This type of treatment is appHcable to all water suppHes, and especially for small systems. Itinvolves the use of hydrated lime, generally deHvered in bags (see Water). 

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