Anion Interference Control

One of the most important factors involved in control of cation-anion interference effects is flame temperature. If compound formation occurs in the flame, as happens with calcium and the phosphate ion, a severe signal depression is observed. Use of high-temperature flames can minimize this effect. Flames that have been used successfully for this purpose are the nitrous oxide-acetylene flame and the premixed oxygen-acetylene flame. 

Other releasing agents have been used in other cases. Copper has been used as a releasing agent for noble metals and barium has been used for sodium and potassium determinations in the presence of aluminum and iron. 

Two chelating agents, ethylenediaminetetraacetic acid (EDTA) and 8-hydroxyquinoline, also have been used for calcium and magnesium determinations. They are effective in controlling a number of interfering ions, including phosphate, sulfate, aluminum, silicon, boron, and selenium. 

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A method suitable for analysis of sulfur dioxide in ambient air and sensitive to 0.003—5 ppm involves aspirating a measured air sample through a solution of potassium or sodium tetrachloromercurate, with the resultant formation of a dichlorosulfitomercurate. Ethylenediaminetetraacetic acid (EDTA) disodium salt is added to this solution to complex heavy metals which can interfere by oxidation of the sulfur dioxide. The sample is also treated with 0.6 wt % sulfamic acid to destroy any nitrite anions. Then the sample is treated with formaldehyde and specially purified acid-bleached rosaniline containing phosphoric acid to control pH. This reacts with the dichlorosulfitomercurate to form an intensely colored rosaniline—methanesulfonic acid. The pH of the solution is adjusted to 1.6 0.1 with phosphoric acid, and the absorbance is read spectrophotometricaHy at 548 nm (273). 

Anodic limits on mercury. Mercury is readily oxidized, particularly in the presence of anions that precipitate or complex mercury or mercury ) ions, such as the halides, cyanide, thiosulfate, hydroxide, or thiocyanate. For this reason, mercury is seldom used to study anodic processes except for those subtances that are easily oxidized, for example, Cr(II), Cu(I), and Fe(II). Under carefully controlled conditions, mercury can be coated with a thin layer of mercury chloride such that it does not interfere with electron transfer in the oxidation of a number of organic compounds, particularly amines.66... 

Some electron transfer reactions have been studied in supercritical xenon. Two of them have been shown to be diffusion controlled and two are energy controlled. These reactions have been followed by changes in the optical absorption after the pulse. To carry out these studies requires that the rate of electron attachment to the solute be suffidendy fast to compete with ion recombination, which occurs on the picosecond time scale in pulse radiolysis. The solute hexafluo-robenzene satisfies this criterion the rate constant is sufficiently large (see Fig. 6) that millimolar concentrations will allow formation of anions. The rate constant for attachment to 4,4 -bipyridine (bipy) is also sufficiently large to satisfy this need. ° Another requirement for making these studies is to quench the excimers whose optical absorptions are strong and can interfere with detection of ions. As mentioned under Sec. 2, a small concentration of ethane (0.4%) is sufficient for this purpose. 

Where applicable, the use of competitive cations for the control of anionic depressors appears to represent the method of choice. No spectral interference arises in atomic absorption from the addition of another cation, an objection often raised in emission. The concentrations of the added salt required for full anion control usually are less than 1%, a salt level well below that at which light scattering is observed. When working with serum, denaturation or precipitation of proteins may occur from the addition of high concentrations of lanthanum chloride or other salts, and the concomitant changes in solution properties should be taken into... 

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