Ionization buffer

FIGURE 16.11 Specific and general acid-base catalysis of simple reactions in solution may be distinguished by determining the dependence of observed reaction rate constants (/sobs) pH and buffer concentration, (a) In specific acid-base catalysis, or OH concentration affects the reaction rate, is pH-dependent, but buffers (which accept or donate H /OH ) have no effect, (b) In general acid-base catalysis, in which an ionizable buffer may donate or accept a proton in the transition state, is dependent on buffer concentration. 

Acids and bases are a big part of organic chemistry, but the emphasis is much different from what you may be familiar with from your general chemistry course. Most of the attention in general chemistry is given to numerical calculations pH, percent ionization, buffer problems, and so on. Some of this returns in organic chemistry, but mostly we are concerned with the roles that acids and bases play as reactants, products, and catalysts in chemical reactions. We ll start by reviewing some general ideas about acids and bases. 

It is relatively easy to overcome ionization interferences. A large excess of an easily ionizable element such as potassium or caesium is added, which maintains the electron concentration constant. The substance added is known as an ionization buffer. Ionization interferences are, as might be expected, substantially worse in the nitrous oxide-acetylene flame than in the air-acetylene flame. It is a common misconception that an ionization buffer totally suppresses determinant ionization, but this is not strictly true. It buffers the degree of ionization at a fixed, reduced level. 

Apart from limitations imposed by aerosol transport inefficiency, conventional analytical flame spectrometry suffers from three other significant limitations. These are the sample requirement (generally 1-2 ml per determination), the time required to add releasing agents or ionization buffers, and the limited linear and useful working ranges of calibration graphs. Sections 4-7 of this chapter consider how the effects of these restrictions may be minimized. 

Generally in analytical flame spectrometry, the pneumatic nebulizer functions as a sample pump, as well as to produce aerosol. Most nebulizers pump sample solution at a rate somewhere between 4 and 8 ml min-1. If the nebulizer is connected, via a suitable capillary T piece, to two pieces of aspiration capillary rather than just one, one capillary may be used to aspirate sample and the other a releasing agent, ionization buffer, or even simply a diluent.20,21 Turbulent mixing occurs at the T ... 

Some analysts prefer to conduct calcium determinations in a nitrous oxide-acetylene flame to minimize the risk of interferences, and this is a sound practice. However, the element has a low ionization potential, so that an ionization buffer such as 5 mg ml-1 potassium must then be added. The AES determination in this flame is very sensitive, and gives a lower detection limit than flame AAS. However flame AAS is sufficiently sensitive to meet the needs of most environmental applications. Flame AFS is really only of academic interest for calcium determination. 

Indium has not proved to be an element of great interest in most environmental samples, in which it is usually present at very low concentrations. The flame AAS determination in a lean air-acetylene flame at 303.9 nm has a detection limit of around 50 ng ml -, and flame AFS is not much better.1 Flame AES in a nitrous oxide-acetylene flame gives a much lower detection limit at 451.1 nm, of around 2 ng ml"1. However the element has a low ionization potential, and addition of potassium at 5 mg ml"1 as an ionization buffer is therefore advised. Sensitivity may be enhanced by solvent extraction pre-concentration using a high extraction ratio.1 Even when pre-concentrated from geological samples by extraction into 4-methylpentan-2-one from 6M hydrochloric acid solution, ICP-AES may be the preferred method of analysis.27... 

The determination of magnesium is so sensitive that there is rarely any reason for attempting to determine the element by AFS or AES. Nevertheless, the element may be determined with good sensitivity at 285.2 nm by AES using a nitrous oxide-acetylene flame, although potassium should then be added as an ionization buffer, especially at very low magnesium concentrations. 

An ionization buffer should also be added when the element is determined by A AS at 766.5 or 769.9 nm (the latter wavelength giving approximately three times poorer sensitivity) in an oxidizing air-acetylene flame. Absence of an ionization buffer results in concave curvature of the calibration graph at low concentrations. 

Sodium is still often determined by flame photometry, measuring the emission intensity of the doublet at around 589 nm, but care is necessary to make sure that excess calcium does not cause spectral interference (from molecular emission). This is unlikely to be a problem if AES is used, with a narrow spectral band-pass, and the intensity of emission at 589.0 nm from an air-acetylene flame is measured. However, at low determinant concentrations it is then advisable to add 2-5 mg ml 1 potassium or caesium as an ionization buffer. This is even more true if a nitrous oxide-acetylene flame is used for FES, although its use is rarely justified in environmental analyses because the additional sensitivity gained is rarely necessary. 

The most sensitive flame spectrometric procedure for the determination of strontium is FES, the emission intensity at 460.7 nm being measured from a nitrous oxide-acetylene flame. A detection limit of 1 ng ml-1 or better is generally readily attainable, although the element has a low ionization potential and addition of potassium or caesium at a final concentration of 2-5 mg ml 1 is essential as an ionization buffer. Chemical interference from phosphate, silicate and aluminium is reduced dramatically in this flame. 

Strontium may also be determined at the same wavelength by AAS, using a nitrous oxide-acetylene flame and ionization buffer to minimize the risk of interference. Although slightly poorer by AAS than by AES on most instruments, the detection limit is still as low as a few ng ml-1. 

The standard personal sampler operates at a flow rate of about 21 min-1 and newer models will sample at up to 41 min-1. Thus, in a two-hour period, 240 to 4801 or about 0.25 to 0.5 m3 can be collected. We have used a 0.25 m3 sample volume to estimate the detection limits for metals in air. The dissolution method in Section II. B. 1 should be modified for analysis of workplace samples use a 10 ml volumetric flask for the final volume, and additions of 0.2 ml of the ionization buffer and releasing agent solutions when required. 

Although ionization of sodium is negligible and potassium small in an air—propane flame, some ionization is experienced in the recommended hotter air—acetylene flame. Ionization should be suppressed by the incorporation of excess potassium or cesium (for sodium determinations) or excess sodium or cesium (for potassium determinations), at concentrations of 1000/igml-1 or greater, in the form of chlorides or nitrates, in both sample and standard solutions. Cesium is the more effective but more expensive ionization suppressant. Extent of ionization is inversely related to analyte concentration with errors due to incomplete suppression thus being greater at low concentrations. As it is difficult to obtain alkali metal salts free from traces of other alkali metals, possible contamination must be considered, especially at low analyte levels. Use of a branched capillary for introduction of ionization buffer has been advocated for flame spectrometry to... 

With alkali metal elements the free atom concentrations in the flame can decrease as a result of ionization, which occurs particularly in hot flames. This leads to a decrease of the absorbances for the alkali metal elements. However, it also may lead to false analysis results, as the ionization equilibrium for the analyte element is changed by changes in the concentration of the easily ionized elements. In order to suppress these effects, ionization buffers can be added. The addition of an excess of Cs because of its low ionization potential is most effective for suppressing changes in the ionization of other elements, as it provides for a high electron number density in the flame. 

This variation of the ionization can occur more or less spontaneously when the matrix contains one or more alkaline elements. To avoid these random errors, an ionization buffer based upon a potassium or sodium salt is added systematically to the solutions. An alternative consists to prepare a series of standards in a medium close to that of sample. 

Three samples of prehistoric ceramics from Greece Fe K Cu Zn Mn Ca Mg Na Comparison of six different decomposition procedures [WDC] [DA] Add SrCl, ionization buffer, A/A flame [FAAS] [WDC-FAAS DA-FAAS] Stratis et al. (1988)... 

They act as ionization buffers, thereby promoting the production of desirabie color species. 

Adding an excess of a more easily ionized element to all standards and samples eliminates ionization interference. This addition creates a large number of free electrons in the flame. The free electrons are captured by the analyte ions, converting them back to atoms. The result is to suppress the ionization of the analyte. Elements often added as ionization suppressants are potassium, rubidium, and cesium. For example, in the AAS determination of sodium, it is common to add a large excess of potassium to all samples and standards. Potassium is more easily ionized than sodium. The potassium ionizes preferentially and the free electrons from the ionization of potassium suppress the ionization of sodium. The detection limit of the sodium determination thereby decreases. The ionization suppression agent, also called an ionization buffer, must be added to all samples, standards, and blanks at the same concentration for accurate results. An example of the use of ionization suppression is shown in Fig. 6.20. Absorbance at a barium resonance line (atomic absorption) and absorbance at a barium ion line (by barium ions in the flame) are plotted as a function of potassium added to the solution. As the potassium concentration increases, barium ionization is suppressed the barium stays as barium atoms. This results in increased atomic absorption at the resonance line and a corresponding decrease in absorbance at the ion line. The trends in absorbance at the atom and ion lines very clearly show that barium ion formation is suppressed by the addition of 1000 ppm of the more easily ionized potassium. 

Halogens are determined by an Orion 901 ion analyzer using 94-09 fluoride, 94-17 chloride, 94-35 bromide, or 94-53 iodide electrodes. These determinations can be carried out sequentially on 1 aliquot of sample in the order chloride, bromide, iodide, and fluoride. Five M KNO3 used as the ionization buffer for the first three ions, while Orion "total ionic strength adjustor buffer" is used for fluoride analysis. All determinations are done by the standard additions technique. 

Ionization buffer. A spectroscopic buffer which is used to minimize or stabilize the ionization of free atoms of the element to be determined. 

Incorrect standard solution Particles greater than 10 pm Incorrect chemistry (no ionization buffer)... 

Ionization effects can be removed or reduced by adding a large excess of an element which ionizes more easily than the analyte. The added element acts as an ionization buffer by reducing the ionization of the analyte. Ionization of potassium can be removed by lithium, although the higher alkali metals (with lower ionization potentials) are even more efficient. The effect of potassium on the determination of calcium, strontium, and barium is shown in Figure 43. 

The potential errors in the use of Eqs. 1 and 2 result from the following facts retention of solutes, especially pro-tonated bases, by processes other than solvophobic interactions, e.g., with exposed silanols or metal contaminants change in values as a function of ionic strength solvophobic effect of ionic strength on solute retention ion-pair interaction of sample ions with ionized buffer species change in the sorption properties of the stationary phase (Cg or Clg) as a result of changing ionization of silanols a... 

---