The Flame Test

15 SOME SIMPLE TESTS AND OPERATIONS 1.15.1 The flame test 

The vapor of certain chemical elements imparts a characteristic color to the flame of burning gas (e.g., Bunsen burner). This property is used for identifying quahtatively various metallic elements. The flame coloration is caused by electronic transitions occurring between the energy levels of the atoms of the chemical element. For a particular chemical element the flame coloration is always the same, regardless of whether the chemical element is in the free atomic state or chemically in molecules. For example, free sodium metal, sodium chloride, sodium carbonate and sodium sulfate all impart an intense yellow color to the flame (D-line of 589 nm). This yellow color is characteristic of sodium in any form, and hence can be used as a test for sodium. In the making of flame tests, chlorides of the metals are commonly used, since chlorides are more volatile than other salts. 

Flame coloration Wavelength lines (intensity) Chemical element Comments 

Yellow intense 597.3 nm (yellow) 589.2 nm (yellow, s) Sodium (Na) This test for sodium is so delicate that great care must be exercised in using it. Glass blowers Didymium Safety Glasses may be used to block out this emission to observe the less intense colors while the blue cobalt glass masks the color. 

Green bright Boron (B) The addition of 3 parts potassium hydrogenosulfate (KHSO,) and 1 part calcium fluoride (CaF,) impart an emerald green coloration. Boron compounds rarely show an alkaline reaction after ignition. Green color is due to the blue and orange in the spectrum. 

Green (emerald) 535.05 nm (green) Thallium (Tl) Presence of sodium impart a pale green color. Not often observed due to the rarity of thallium-bearing minerals. 

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If an appreciable amount of residue remains, note its colour. Add a few drops of water and test the solution (or suspension) with htmus or with Universal indicator paper. Then add a httle dilute hydrochloric acid and observe whether efiervesceiice occurs and the residue dissolves. Apply a flame test with a platinum wire on the hydrochloric acid solution to determine the metal present. (In rare cases, it may be necessary to subject a solution of the residue to the methods of qualitative inorganic analysis to identify the metal or metals present.) If the flame test indicates sodium, repeat the ignition of the substance on platinum foil. 

It must be remembered that methane can be liberated in a coal mine, not only in the coal itself, but in the nearby stone. It is therefore necessary to use permitted explosives, both in and near a seam of coal. Further, before any shot is fired in a coal mine, tests are made for the presence of methane in the air by means of a safety lamp. This safety lamp is the well-known Davy lamp and an experienced operator can judge the presence of methane in the air from the appearance of the flame. Tests are made... 

When subjected to the flame test, a solution that contains K+ ions produces the color... 

Because its outet valence electrons ate at a gteatet distance from its nuclei, potassium is more reactive than sodium or lithium. Even so, potassium and sodium are very similar in their chemical reactions. Due to potassiums high reactivity, it combines with many elements, particularly nonmetals. Like the other alkali metals in group 1, potassium is highly alkaline (caustic) with a relatively high pH value. When given the flame test, it produces a violet color. 

Mean and turbulent axial velocity was measured in the entire flow field at the same conditions for which the flame tests were conducted. These tests were aimed to elucidate the mixing process at the different forcing conditions. Four cases were compared no forcing and no swirl, no forcing with swirl Ns = 0.3), forcing (forcing level 60%) without swirl, and forcing with swirl. 

Strontium and all its compounds impart crimson red color in the flame test. The metal in trace concentrations can be analyzed by various instrumental methods that include flame-and fumace-AA, ICP-AES, ICP/MS, x-ray fluorescence, and neutron activation analysis. 

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. 

The nature of the flame tests is of the utmost importance in the evaluation of the properties of the fire-resistant mastics. The proper tests are based on careful consideration of the type of combustile or incombustible substrate, the nature of the flame exposure to which these substrates may be subjected in the course of the service, and other requirements which may be needed to meet the desired specifications. 

Test 2. A 1 in 20 solution responds to the oxalate test for calcium, and to the flame test of sodium. 

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. 

Repeat the flame test for each of the other known solutions. Then test each of the unknown solutions. 

Use your observations of the flame tests to confirm or refute the identifications you made in question 5. If you are not sure, check your observations and analysis with other students. If necessary, repeat some of your tests. 

Identification A sample responds to the flame test for Sodium, Appendix IIIA, and gives positive tests for Aluminum and for Sulfate, Appendix IIIA. 

A. A sample responds to the flame test for Potassium, Appendix IIIA. 

Identification A 1 10 solution in 1 2 hydrochloric acid gives positive tests for Aluminum and for Phosphate, Appendix IIIA, and it responds to the flame test for Sodium, Appendix IIIA. Assay Not less than 9.5% and not more than 12.5% of AI2O3, calculated on the ignited basis. 

We can now return to the flame tests. Compounds of certain metals are volatilized in the non-luminous Bunsen flame and impart characteristic colours to the flame. The chlorides are among the most volatile compounds, and these are prepared in situ by mixing the compound with a little concentrated hydrochloric acid before carrying out the tests. The procedure is as follows. A thin... 

Wash well with hot water. Dissolve the ppt. in a little concentrated HC1, evaporate almost to dryness and apply to the flame test. 

Largely SrS04. Wash with a little water. Transfer ppt. and filter paper to a small crucible, heat until ppt. has charred (or burn filter paper and ppt., held in a Pt wire, over a crucible), moisten ash with a few drops concentrated HC1 and apply the flame test (4). 

Calcium is readily identified by precipitation as CaC204, followed by the flame test (best observed through a spectroscope). It may also be converted into CaS04.2H20, and the characteristic crystals examined in a microscope -magnification of about 100 diameters. 

Filter off the ppt. and apply the flame test brick-red flame. 

The mixture of SrC03 and CaC03 is treated cautiously with 83 per cent HN03 the carbonates are thus converted into the nitrates. Sr(N03)2 is insoluble in the medium whilst the Ca(N03)2 dissolves. The Sr(N03)2 is collected by filtration through a sintered glass crucible or funnel the presence of Sr is confirmed by the flame test, preferably with the aid of a hand spectroscope. 

The most satisfactory precipitation for sodium ions is that with uranyl magnesium or zinc acetate (Section III.37, reactions 1 and 3). The flame test, in which an intense persistent yellow colouration is produced, is characteristic. Traces of sodium may be introduced from the reagents during the analysis, and hence it is important to look for a strong persistent yellow colouration a feeble yellow colouration may be ignored. 

Dissolve in a few drops of concentrated HQ, evaporate to dryness in a small crucible and apply the flame test. Green (or yellowish-green) flame. 

Centrifuge, and confirm Ca in the ppt. by the flame test - brick-red flame. 

Separation. In order to separate lithium from the other alkali metals, they are all converted into the chlorides (by evaporation with concentrated hydrochloric acid, if necessary), evaporated to dryness, and the residue extracted with absolute alcohol which dissolves the lithium chloride only. Better solvents are dry dioxan (diethylene dioxide, C4H802) and dry acetone. Upon evaporation of the extract, the residue of lithium chloride is (a) subjected to the flame test, and (b) precipitated as the phosphate after dissolution in water and adding sodium hydroxide solution. 

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