Elements flame temperatures

Capp, B. 1992. Temperature Rise of a Rigid Element Flame Arrester m Endurance Burning with Propane./. Loss Prev. Process Ind., 5(4), 215-218. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Flame AAS can be used to measure about 70 elements, with detection limits (in solution) ranging from several ppm down to a few ppb (and these can be enhanced for some elements by using a flameless source). Both sensitivity and detection limits (as defined fully in Section 13.4) are a function of flame temperature and alignment, etc. The precision of measurements (precision meaning reproducibility between repeat measurements) is of the order of 1-2% for flame AA, although it can be reduced to <0.5% with care. The accuracy is a complicated function of flame condition, calibration procedure, matching of standards to sample, etc. 

In the same context as the heat of formation, the JANAF tables have tabulated most conveniently the equilibrium constants of formation for practically every substance of concern in combustion systems. The equilibrium constant of formation (KPt[) is based on the equilibrium equation of formation of a species from its elements in their normal states. Thus by algebraic manipulation it is possible to determine the equilibrium constant of any reaction. In flame temperature calculations, by dealing only with equilibrium constants of formation, there is no chance of choosing a redundant set of equilibrium reactions. Of course, the equilibrium constant of formation for elements in their normal state is one. 

The flame temperature calculation is essentially the solution to a chemical equilibrium problem. Reynolds [8] has developed a more versatile approach to the solution. This method uses theory to relate mole fractions of each species to quantities called element potentials ... 

Saita et al. [215] used hydriding combustion synthesis for a direct production of TiFe. In the experiments, an exothermic reaction of Ti with hydrogen (Ti -i- i = TiHj + 144 kJ) was utilized for HCS of TiFe because the adiabatic flame temperature of this reaction was estimated to be 2,000°C, which is sufficiently high for melting both iron and titanium. A 1 1 molar mixture of elemental Ti and Fe pow-... 

Table 5.1 Adiabatic flame temperatures for various elements.

The above reaction is highly exothermic. The stoichiometric proportion of gaseous mixture at equilibrium flame temperature is cooled to 200°C, whereupon the elements combine rapidly to form HCl with over 99% yield. 

To produce this type of atomic emission in a pyrotechnic system, one must produce sufficient heat to generate atomic vapor in the flame, and then excite the atoms from the ground to various possible excited electronic states. Emission intensity will increase as the flame temperature increases, as more and more atoms are vaporized and excited. Return of the atoms to their ground state produces the light emission. A pattern of wavelengths, known as an atomic spectrum, is produced by each element. This pattern - a series of lines - corresponds to the various electronic... 

Such depressions can be encountered when the matrix is refractory (e.g. zirconium, uranium or a rare earth element), and the small amount of analyte can be physically trapped in clotlets of matrix oxide in the flame. Such systems do not show a knee [see type (a)] and can be minimized by higher flame temperature. 

The flames commonly used as atomisers have temperatures in the range 2000-3000 K allowing for the analysis of elements such as Na, K and Cs by OES. The flame temperatures are not high enough to excite many other elements, so other atomisers such as spectroscopic plasmas have to be used. 

The model of the reaction may be hypothesized in the following manner. The total molal flow, m, splits into an infinite number of elemental masses on entering the boundaries of the reactor. A certain fraction, y, of these elemental masses burns to chemical equilibrium composition and attains the adiabatic flame temperature. The remainder (1 — y) remains unburned. It is further assumed that all of the constituents of the burned fraction act only as diluents and can in no way contribute anything further to the reaction. 

In FLAA analysis, the flame temperature may be sufficiently high to ionize neutral atoms and thus reduce the number of ground state atoms. Ionization interference can generally be controlled by the addition to standard and sample solutions of a large excess (1000 mg/1) of an easily ionizable alkaline element. 

There are p + 4 unknowns however, there are 4 flow equations as listed above arid xv element conservation equations. Just as in the solution of the equilibrium flame temperature problem discussed in section II. B. 5., M - a additional equations are required. Except instead of using the equilibrium equations, one must adopt the chemical kinetic rate equations. The form used with the present problem is ... 

While high flame temperature is an essential prerequisite to the sufficient thermal excitation of a wide range of elements to give useful emission intensities (see Chapter 1, equation 7), it should also be realized that some useful atomic emission determinations may be performed using low temperature flames. As discussed in Chapter 1, section 9, only elements with relatively very low excitation potentials, which therefore emit in the visible region of the spectrum, are excited in such flames. Because there are few such elements, those that do may be determined with a low incidence of spectral interferences. The flame used is usually air-propane or air-butane. 

Atoms of some elements are relatively easily ionized at flame temperatures. This is particularly true for the alkali and alkaline earth elements, and other elements to the left of the periodic table. The first ionization potentials also tend to be lower for heavier elements within a particular group. For the group 2 elements, for example, ionization follows the order Ba>Sr>Ca>Mg>Be. This would not matter in flame spectrometric analysis, apart from a slight deterioration in sensitivity, if samples and standards were ionized to exactly the same extent. Suppose barium was to be determined in samples containing potassium, however. The potassium would be ionized ... 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

Rapid and complete solvent evaporation is required for optimum performance. Atomisation occurs in the flame reaction zone, i.e. the conversion of sample molecules into atoms. Three factors affect the number of atoms formed. Firstly, the anion with which the metal atom is combined. Calcium chloride for instance is more easily dissociated than calcium phosphate. The second factor is flame temperature. Higher temperatures cause more rapid decomposition and, indeed, are often specifically required for elements which form refractory oxides. Finally, gas composition may affect the rate of atomisation if the constituents in the gas react with the sample or its derivatives. In the outer zone of the flame the atoms are burned to oxides. In this form they no longer absorb radiation at the wavelength of the uncombined ground state atoms. 

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