Trace ions

The foregoing equation reveals that essentially the concentration distribution ratio for trace concentrations of an exchanging ion is independent of the respective solution of that ion and that the uptake of each trace ion by the resin is directly proportional to its solution concentration. However, the... 

Standards for food-grade chemicals in the United States are set by the Committee on Eood Chemicals Codex of the National Academy of Sciences (NAS) which pubHshes them in the Food Chemicals Codex (ECC) (6) (see also Eood additives). Standards for laboratory reagents are set by the American Chemical Society (ACS) Committee on Analytical Reagents and are pubHshed in Feagent Chemicals—A.CS Specifications (7). Standards for electronic-grade chemicals, which have extremely low limits for trace ions, are pubHshed aimually in The Book of SEMI Standards (BOSS) by Semiconductor Equipment and Materials International (SEMI) (8). 

SOLID-PHASE EXTRACTION ON SIMULTANEOUS SPECTROPHOTOMETRIC DETERMINATION TRACE IONS IN WATER WITH l-(2-PYRIDILAZO)-2-NAPHTOL... 

Eor daily limitations = 2 to 3 x average daily levels given above, suggested limits for metals, trace ions ... 

Ion Chromatc aphic Analysis of Trace Ions in Environmental Samples... 

To determine ions at mid pg/1 to mg/1 (ppb to ppm) levels with IC, a sample size of 10 to 50/pi is sufficient. To determine ions at lower levels, then a preconcentration or trace enrichment technique has t3rpically to be utilized [20]. With this method, the analytes of interest are preconcentrated on another column in order to "strip" ions from a measured sample volume. This process concentrates the desired species resulting in lower detection limits. However, preconcentration has several disadvantages, compared with a direct method, additional hardware is required. A concentrator column is used to preconcentrate the ions of interest, a sample pump is needed for loading sample, an additional valve is often required for switching the concentrator column in and out-of line with the analytical column and extra time is required for the preconcentration step. It was of interest to explore the development of a high-volume direct-injection IC method that would facilitate trace ion determinations without a separate preconcentration step. This would represent a significantly simpler and more reliable means of trace analysis. 

Fig. 15.18 Unambiguous identification of the molecules assigned to the trace ions. This identification is only valid for the first 120-s period of Tenax trapping. (Adapted from [199])...

Exchange of trace components The equations for adsorption (diffusion) can be equally applied in the case of isotopic exchange (exchange of isotopes) with minor changes. The same equations can be also be used in the case of the exchange of trace components of different valences (Helfferich, 1962). This is the case where the uptake or release of an ion takes place in the presence of a large amount of another ion in both the solid and liquid phase. In such systems, the amounts removed ate so small that the concentrations in both phases are practically constant, and thus in turn the individual diffusion coefficients also remain unaffected. Moreover, the rate-controlling step is the diffusion of the trace ion. 

Elements present in only minor or trace concentrations are partitioned in quite different ways. These elements are not abundant enough to form their own minerals, so they must be incorporated into minerals composed of the major elements. Because they must fit into crystalline sites whose sizes and coordination are already fixed, the ease with which they can be included depends on ionic size and charge. Trace ions that fit nicely into sites otherwise occupied by major ions having similar sizes and charges are said to be compatible. Conversely, large ions with high charge, referred to as incompatible elements, tend to be excluded from most crystal sites. 

Besides choosing a sample judiciously, we must be careful about storing the sample. The composition may change with time after collection because of chemical changes, reaction with air, or interaction of the sample with its container. Glass is a notorious ion exchanger that alters the concentrations of trace ions in solution. Therefore, plastic (especially Teflon) collection bottles are frequently employed. Even these materials can absorb trace levels of analytes. For example, a 0.2 p.M HgCl2 solution lost 40-95% of its concentration in 4 h in polyethylene bottles. A 2 jlM Ag+ solution in a Teflon bottle lost 2% of its concentration in a day and 28% in a month.3... 

Miro, M. and Frenzel, W., Flow-through sorptive preconcentration with direct optosens-ing at solid surfaces for trace-ion analysis, Trends Anal. Chem., 23, 11-20, 2004. 

Activity effects. The exchange of trace ions in solution with others in the polymer film might, simplistically, be expected to lead to a linear uptake/solution concentration relationship. Unfortunately, this is seldom the case. The thermodynamic restraint is that of electrochemical potential. Thus electroneutrality is not the sole constraint on the ion exchange process. A second thermodynamic requirement is that the activity of mobile species in the polymer and solution phases be equal. (Temporal satisfaction of these two constraints is discussed below, with reference to Figure 4.) The rather unusual, high concentration environment in the polymer film can lead to significant - and unanticipated - activity effects (8). 

The question of trace ionic contaminants is often overlooked. There is no doubt, however, that trace ions initially present in the polymer or subsequently introduced during the manufacturing process can wreak havoc with the device as well as with device-mounted components as the device ages and the ions migrate. Thus, not only can Na ions migrate to and destroy an FET by lowering the threshold resistance in the gate, halide ions can attack some metals, which may lead to a loss of interfacial adhesion and will certainly lead to a loss of conduction. 

While it is true that, with the exception of the proton, the trace ions found in polymers do not migrate in the absence of water(11,56 58), it is also true that microelectronic devices and components are not hermetically sealed they are always exposed to atmospheric humidity. It is, therefore, necessary to know, both qualitatively and quantitatively, which trace ions are present. This writer has had experience(591 with four techniques which are capable of identifying contaminants in the ppm-ppb (i.e. ug/ml-ng/ml) concentration region. Each has its own advantages and disadvantages, necessitating the simultaneous use of several of the techniques found in Table I. A particular advantage of ion... 

Situations with an excess of ion exchanger occur if,. for example, a given ionic species has to be removed from the solution by a batch operation. An excess of particles with ion exchange properties is usually also present when the sorption of trace ions by soils or soil components (clay minerals, oxides, humic substances) is investigated. Especially in this latter case the particles will be invariably polydisperse. This anomalous kinetic behavior will, of course, only be observed experimentally if the concentration of ions A in the solution and in the various fractions of the ion-ex-changer particles in the mixture are measured continuously. 

Consider a trace ion entering the clinopyroxene M2-site. In general, the ion has a charge which is different from that of the major ion which it is... 

Figure 13 Schematic illustrating two main principles of trace element substitution (1) that the region around the trace defect will have different elastic (E) and electrostatic (e) properties from the matrix and (2) that the electrostatic radius of the defect should be greater than that of the central trace ion.

It would be difficult to find more comprehensive or more detailed studies on the physical chemistry of seawater than those done at the University of Miami (Millero, 2001). Several programs were developed for calculation of activity coefficients and speciation of both major ions and trace elements in seawater. The activity coefficient models have been influenced strongly by the Pitzer method but are best described as hybrid because of the need to use ion-pair formation constants (Millero and Schreiber, 1982). The current model is based on Quick Basic computes activity coefficients for 12 major cations and anions, 7 neutral solutes, and more than 36 minor or trace ions. At 25 °C the ionic strength range is 0-6 m. For major components, the temperature range has been extended to 0-50 °C, and in many cases the temperature dependence is reasonably estimated to 75 °C. Details of the model and the parameters and their sources can be found in Millero and Roy (1997) and Millero and Pierrot (1998). Comparison of some individual-ion activity coefficients and some speciation for seawater computed with the Miami model is shown in Section 5.02.8.6 on model reliability. 

Several hydrous oxides, such as those of aluminum, siTicon and, iron have been used to extract traces ions. Nevertheless, the sorption mechanism is not definitively established. Those oxides probably exhibit some ion exchange capacity among their properties and they can act as anionic or cationic exchangers and sometimes both. The separation of plutonium traces in the presence of HF by sorption onto an alumina column is based on its chemical similarities with thorium and lantanide elements reported by Abrao (2) In this case only thorium and rare earths are sorbed onto alumina from nitric acid-fluoride solutions while uranium remains in the effluent. 

Current surface complexation models were developed with a focus on minor and trace ions and hence do not consider sorption in the diffuse layer. Even the triple-layer model (34), which can include electrolyte sorption as outer-sphere complexes, does not consider sorption in the diffuse layer. To... 

Table 2 Habit modification by addition of trace ions or chemicals...

Many environmentally important chemicals are transported as complexes in natural waters. Complexes may increase or decrease the toxicity and/or bioavailability of elements. Complexation increases the solubility of minerals and may increase or decrease the adsorption of elements. The major monovalent and divalent cations and anions (especially > 10 m) form outer-sphere complexes or ion pairs, in which the bonding is chiefly long-range and electrostatic. Ion pairs are unimportant in dilute fresh waters, but become important in saline waters such as seawater. Minor and trace ions such as Cu, Fe +, Pb +, and Hg are usually complexed, and occur in inner-sphere complexes, which are usually much stronger complexes than the ion pairs. Written in terms of Gibbs free energy. 

The first (-) term on the right is the Debye-Huckel (DH) term for long-range electrostatic effects. The second (+) term defines electrolyte trace-ion (Na-HCOj) interactions. The third term... 

Precipitate flotation is applied in the analysis of natural waters. Trace ions in an aqueous solution are co-precipitated with colloidal metal hydroxide collectors and floated with the aid of a gas stream [94-96]. Tiny gas bubbles are trapped in the interstitial spaces and on the surfaces of the precipitates to give sufficient buoyancy. Surfactant ions having the charge opposite to the precipitate surfaces are used to make the surface hydrophobic. Another important role of the surfactants e.g., sodium oleate, sodium dodecylsulphate) is to form a stable froth layer to support the precipitate on the solution surface, which is important for complete separation of the precipitate. 

The existence of mixed solid phases in which the trace ion lowers its solubility by co-precipitating into another component. 

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