Flame testing techniques

One of the most interesting characteristics of matter, and one that drives the study and exploration of chemistry, is the fact that matter changes. By examining a dramatic chemical reaction, such as the reaction of the element copper and the compound silver nitrate in a water solution, you can readily observe chemical change. Drawing on one of the fundamental laboratory techniques introduced in this chapter, you can separate the products. Then, you will use a flame test to confirm the identity of the products. 

Chemists used spectral analysis during the nineteenth century to analyze substances and, sometimes, to discover new elements. Another common technique for analyzing substances, often used in conjunction with spectral analysis is a flame test. One of the foremost practitioners of this technique was a chemist named Robert Bunsen. Find out why he invented his famous burner. Carry on your research to investigate other ways that chemists use spectral analysis to examine the composition of substances. Select an appropriate medium to report your findings. 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Another qualitative analysis technique is a flame test. A dissolved ionic compound is placed in a flame. Table 9.4 lists the flame colours associated with several ions. Notice that all the ions are metallic. The flame test is only useful for identifying metallic ions in aqueous solution. 

Drawing on one of the fundamental laboratory techniques introduced in this chapter, you can separate the products. Then, you will use a flame test to confirm the identity of the products. 

The most common qualitative analytical technique to detect the presence of sodium in aqueous solutions or in solids (which can be dissolved in water) is a flame test. A wire made of an inert material like platinum or nichrome is dipped in a solution and then held in a flame. Sodium ions emit an intense yellow flame, readily verifying the presence of sodium in the sample. 

Over many years, the simple flame test , whereby atoms of, say, sodium are excited in a flame, giving a characteristic yellow colour, has been developed as a sophisticated and sensitive instrumental technique (flame emission spectrophotometry). Sensitivity depends on dissociation of the injected materials into free atoms in order that the characteristic atomic emissions can be given. This in turn demands high flame temperatures. The combination of acetylene fuel with nitrous oxide as oxidant has proved highly successful for this purpose. What temperature is possible in theory The best mixture would correspond to the equation ... 

With the exception of aluminum, which is one of the most abundant elements in Earths crust, most of the boron group elements are rare. None of the elements are found free in nature. Three can be identified by flame tests, as shown in the table. Boron produces a bright green color, while indium produces an indigo blue color. Thallium produces a green color. More precise identification methods involve advanced spectral and imaging techniques. 

While a complete survey of the testing techniques for flame retardants is beyond the scope of this chapter, testing methods such as cone calorimetry, the requirements of the UL 94 testing protocols, and radiant heat panels deserve mention here. 

Analytical techniques used in qualitative analysis include flame tests (Chapter 2) and precipitation reactions (Chapters 3 and 13). Analytical techniques used in quantitative analysis include titrations (Chapter 1), inductively coupled plasma (ICP) spectroscopy (Chapter 22 on the accompanying website), ultraviolet—visible spectroscopy (Chapter 23 on the accompanying website), infrared spectroscopy and various chromatographic techniques (Chapter 23). Analytical techniques used in structural analysis include NMR, IR spectroscopy, mass spectrometry and visible—ultraviolet spectroscopy. Important areas that employ analytical techniques include ... 

Spectroscopic determination of atomic species can only be carried out in the gas phase, where the individual atoms or ions are well separated. Consequently, the first step in the process is atomization, where the sample is volatilized (heated to the gas phase) and decomposed to produce an atomic gas. The differences between the various atomic spectroscopy techniques available, largely lie in the different ways of doing this. The most widely used method is flame atomization, where the sample is decomposed in a flame (a sophisticated version of the common flame test), but other common methods (Table 5.2) are... 

A technique that is somewhat easier to use is the sodium flame test, which produces an aerosol of sodium chloride particles, as with methylene blue, but assesses the dust content of the air upstream and downstream of the filter by the intensity of the yellow colour produced by the salt dust in a hydrogen flame (BS 3928). The size distribution of the salt dust is shown in Table 1.7, with a mean size around 0.6 pm, and no particle larger than 1.7 am. The colour of the flame is observed by a photosensitive cell connected to a meter, which can be calibrated to read the salt concentration directly. 

The flash point of a petroleum liquid is the temperature to which it must be brought so that the vapor evolved burns spontaneously in the presence of a flame. For diesel fuel, the test is conducted according to a closed cup technique (NF T 60-103). The French specifications stipulate that the flash point should be between 55°C and 120°C. That constitutes a safety criterion during storage and distribution operations. Moreover, from an official viewpoint, petroleum products are classified in several groups according to their flash points which should never be exceeded. 

Several coating techniques are now available to overcome the oxidation problems with molybdenum above 300°C. One of these, based on molybdenum disilicide, is finding increased usage in flame breakout shields for aero-engines where tests have shown (unpublished work) that the coated material can withstand a high pressure torching type flame attack at temperatures in excess of 2(X)0°C. 

Separation techniques may have to be applied if the given sample contains substances which act as interferences (Section 21.10), or, as explained above, if the concentration of the element to be determined in the test solution is too low to give satisfactory absorbance readings. As already indicated (Section 21.10), the separation methods most commonly used in conjunction with flame spectrophotometric methods are solvent extraction (see Chapter 6) and ion exchange (Chapter 7). When a solvent extraction method is used, it may happen that the element to be determined is extracted into an organic solvent, and as discussed above it may be possible to use this solution directly for the flame photometric measurement. 

The simultaneous analysis of orthophosphate, glycerol phosphates, and inositol phosphates has been achieved by spectrophotometric analysis of the molybdovanadate complexes. Also, a sensitive and selective chemiluminescent molecular emission method for the estimation of phosphorus and sulphur is described, which is based on passing solutions into a cool, reducing, nitrogen-hydrogen diffusion flame. For organic compounds it was usually necessary to prepare test solutions by an oxygen-flask combustion technique. 

On the other hand, if only specific GC detectors, e.g. the electron capture, nitrogen-phosphorus or flame photometric detectors, are tested, the argument of lack of GC method sensitivity is not acceptable. In most cases mass spectrometric detectors provide the sensitivity and selectivity needed. Unfortunately, tandem mass spectrometry (MS/MS) or MS" detectors for GC are still not widely used in official laboratories, and therefore these techniques are not always accepted for enforcement methods. 

ASTM. 1998a. ASTME1613. Standard test method for analysis of digested samples for lead by inductively coupled plasma atomic emission spectrometry (ICP-AES). Flame Atomic Absorption (FAAS), or Graphite Furnace Atomic Absorption (GFAA) Techniques. American Society for Testing and Materials. 

Flame AA is the oldest of the three techniques and has been widely used for many years. The instruments are relatively inexpensive and methods have been well tested and are well understood. 

The experimental setup for diode-laser sensing of combustion gases using extractive sampling techniques is shown in Fig. 24.8. The measurements were performed in the post-flame region of laminar methane-air flames at atmospheric conditions. A premixed, water-cooled, ducted flat-flame burner with a 6-centimeter diameter served as the combustion test-bed. Methane and air flows were metered with calibrated rotameters, premixed, and injected into the burner. The stoichiometry was varied between equivalence ratios of = 0.67 to... 

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