A flame test is relatively quick test for the presence of some elements in a sample. The technique is archaic and of questionable reliability, but once was a component of qualitative inorganic analysis. The phenomenon is related to pyrotechnics and atomic emission spectroscopy. [1] The color of the flames is understood through the principles of atomic electron transition and photoemission, where varying elements require distinct energy levels (photons) for electron transitions. [2] [3]
Robert Bunsen invented the now-famous Bunsen burner in 1855, which was useful in flame tests due to its non-luminous flame that did not disrupt the colors emitted by the test materials. [4] [1] The Bunsen burner, combined with a prism (filtering the color interference of contaminants), led to the creation of the spectroscope, capable of emitting the spectral emission of various elements. [1] In 1860, the unexpected appearance of sky-blue and dark red was observed in spectral emissions by Robert Bunsen and Gustav Kirchhoff, leading to the discovery of two alkali metals, caesium (sky-blue) and rubidium (dark red). [4] [1] Today, this low-cost method is used in secondary education to teach students to detect metals in samples qualitatively. [2]
A flame test involves introducing a sample of the element or compound to a hot, non-luminous flame and observing the color of the flame that results. [4] The compound can be made into a paste with concentrated hydrochloric acid, as metal halides, being volatile, give better results. [5] Different flames can be tried to verify the accuracy of the color. Wooden splints, Nichrome wires, platinum wires, magnesia rods, cotton swabs, and melamine foam are suggested for support. [6] [7] [8] Safety precautions are crucial due to the flammability and toxicity of some substances involved. [9] [10] [11] [6] When using a splint, one must be careful to wave the splint through the flame rather than holding it in the flame for extended periods, to avoid setting the splint itself on fire. The use of a cotton swab or melamine foam (used in “eraser” cleaning sponges) as a support has also been suggested. [7] [8] [6] Sodium is a common component or contaminant in many samples, [2] and its spectrum tends to dominate many flame tests others. [5] The test flame is often viewed through cobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.[ citation needed ]
The color of the flames also generally depends on temperature and oxygen fed; see flame colors. [5] The procedure uses different solvents and flames to view the test flame through a cobalt blue glass or didymium glass to filter the interfering light of contaminants such as sodium. [12]
Flame tests are subject of a number of limitations. The range of elements positively detectable under standard conditions is small. Some elements emit weakly and others (Na) very strongly. Gold, silver, platinum, palladium, and a number of other elements do not produce a characteristic flame color, although some may produce sparks (as do metallic titanium and iron); salts of beryllium and gold reportedly deposit pure metal on cooling. [12] The test is highly subjective.
In flame tests, ions are excited thermally. These excited states then relax to the ground state with emission of a photon. The energy of the excited state(s) and associated emitted photon is characteristic of the element. The nature of the excited and ground states depends only on the element. Ordinarily, there are no bonds to be broken, and molecular orbital theory is not applicable. The emission spectrum observed in flame test is also the basis of flame emission spectroscopy, atomic emission spectroscopy, and flame photometry. [4] [13]
Some common elements and their corresponding colors are:
| Symbol | Name | Color [5] | Image |
|---|---|---|---|
| Al | Aluminium | Silver-white, in very high temperatures such as an electric arc, light blue | |
| As | Arsenic | Blue |  |
| B | Boron | Bright green |  |
| Ba | Barium | Light apple green |  |
| Be | Beryllium | White | |
| Bi | Bismuth | Azure blue | |
| Ca | Calcium | Brick/orange red; light green as seen through blue glass. |  |
| Cd | Cadmium | Brick red | |
| Ce | Cerium | Yellow | |
| Co | Cobalt | Silvery white |  |
| Cr | Chromium | Silvery white |  |
| Cs | Caesium | Blue-violet |  |
| Cu(I) | Copper(I) | Blue-green | |
| Cu(II) | Copper(II) (non-halide) | Green |  |
| Cu(II) | Copper(II) (halide) | Blue-green | |
| Fe(II) | Iron(II) | Gold, when very hot such as an electric arc, bright blue, or green turning to orange-brown | |
| Fe(III) | Iron(III) | Orange-brown |  |
| Ge | Germanium | Pale blue | |
| H | Hydrogen | Pale blue | |
| Hf | Hafnium | White | |
| Hg | Mercury | Red | |
| In | Indium | Indigo blue |  |
| K | Potassium | Lilac (pink); invisible through cobalt blue glass (purple) |  |
| Li | Lithium | Carmine red; invisible through green glass |  |
| Mg | Magnesium | Colorless due to Magnesium Oxide layer, but burning Mg metal gives an intense white | |
| Mn(II) | Manganese(II) | Yellowish green |  |
| Mo | Molybdenum | Yellowish green | |
| Na | Sodium | Bright yellow; invisible through cobalt blue glass. See also Sodium-vapor lamp |  |
| Nb | Niobium | Green or blue | |
| Ni | Nickel | Colorless to silver-white |  |
| P | Phosphorus | Pale blue-green | |
| Pb | Lead | Blue-white |  |
| Ra | Radium | Crimson red | |
| Rb | Rubidium | Violet red |  |
| S | Sulfur | Blue |  |
| Sb | Antimony | Pale green |  |
| Sc | Scandium | Orange | |
| Se | Selenium | Azure blue | |
| Sn | Tin | Blue-white | |
| Sr | Strontium | Crimson to scarlet red; yellowish through green glass and violet through blue cobalt glass |  |
| Ta | Tantalum | Blue | |
| Te | Tellurium | Pale green | |
| Ti | Titanium | Silver-white | |
| Tl | Thallium | Pure green | |
| V | Vanadium | Yellowish green | |
| W | Tungsten | Green | |
| Y | Yttrium | Carmine, crimson, or scarlet red | |
| Zn | Zinc | Colorless to blue-green |  |
| Zr | Zirconium | Mild/dull red |
Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.
Atomic absorption spectroscopy (AAS) is a spectroanalytical procedure for the quantitative measurement of chemical elements. AAS is based on the absorption of light by free metallic ions that have been atomized from a sample. An alternative technique is atomic emission spectroscopy (AES).
Rubidium is a chemical element; it has symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher than water. On Earth, natural rubidium comprises two isotopes: 72% is a stable isotope 85Rb, and 28% is slightly radioactive 87Rb, with a half-life of 48.8 billion years – more than three times as long as the estimated age of the universe.
Spectroscopy is the field of study that measures and interprets electromagnetic spectrum. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
X-ray fluorescence (XRF) is the emission of characteristic "secondary" X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects such as paintings.
Chemistry is the physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.
The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to an absorption of energy performed by the chemical.
Chemiluminescence is the emission of light (luminescence) as the result of a chemical reaction, i.e. a chemical reaction results in a flash or glow of light. A standard example of chemiluminescence in the laboratory setting is the luminol test. Here, blood is indicated by luminescence upon contact with iron in hemoglobin. When chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits light by chemiluminescence.
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to electrons making a transition from a high energy state to a lower energy state. The photon energy of the emitted photons is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.
In chemistry and physics, valence electrons are electrons in the outermost shell of an atom, and that can participate in the formation of a chemical bond if the outermost shell is not closed. In a single covalent bond, a shared pair forms with both atoms in the bond each contributing one valence electron.
Didymium is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing and filter lenses for flame testing, especially with a gas (propane)-powered forge, where it provides a filter that selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass without having a detrimental effect on general vision, unlike dark welder's glasses and cobalt glasses. The usefulness of didymium glass for eye protection of this sort was discovered by Sir William Crookes.
A pyrotechnic colorant is a chemical compound which causes a flame to burn with a particular color. These are used to create the colors in pyrotechnic compositions like fireworks and colored fires. The color-producing species are usually created from other chemicals during the reaction. Metal salts are commonly used; elemental metals are used rarely.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), is an analytical technique used for the detection of chemical elements. It is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The plasma is a high temperature source of ionised source gas. The plasma is sustained and maintained by inductive coupling from electrical coils at megahertz frequencies. The source temperature is in the range from 6000 to 10,000 K. The intensity of the emissions from various wavelengths of light are proportional to the concentrations of the elements within the sample.
In spectroscopy, a forbidden mechanism is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that rule is not made. For example, in a situation where, according to usual approximations, the process cannot happen, but at a higher level of approximation the process is allowed but at a low rate.
Colored fire is a common pyrotechnic effect used in stage productions, fireworks and by fire performers the world over. Generally, the color of a flame may be red, orange, blue, yellow, or white, and is dominated by blackbody radiation from soot and steam. When additional chemicals are added to the fuel burning, their atomic emission spectra can affect the frequencies of visible light radiation emitted - in other words, the flame appears in a different color dependent upon the chemical additives. Flame coloring is also a good way to demonstrate how fire changes when subjected to heat and how they also change the matter around them.
Flame photometry is a type of atomic emission spectroscopy. It is also known as flame emission spectroscopy. A photoelectric flame photometer is an instrument used in inorganic chemical analysis to determine the concentration of certain metal ions, among them sodium, potassium, lithium, and calcium. Group 1 and Group 2 are quite sensitive to flame photometry due to their low excitation energies.
Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The sample may be excited by various methods.
Modern spectroscopy in the Western world started in the 17th century. New designs in optics, specifically prisms, enabled systematic observations of the solar spectrum. Isaac Newton first applied the word spectrum to describe the rainbow of colors that combine to form white light. During the early 1800s, Joseph von Fraunhofer conducted experiments with dispersive spectrometers that enabled spectroscopy to become a more precise and quantitative scientific technique. Since then, spectroscopy has played and continues to play a significant role in chemistry, physics and astronomy. Fraunhofer observed and measured dark lines in the Sun's spectrum, which now bear his name although several of them were observed earlier by Wollaston.
Alkali metal nitrates are chemical compounds consisting of an alkali metal and the nitrate ion. Only two are of major commercial value, the sodium and potassium salts. They are white, water-soluble salts with melting points ranging from 255 °C to 414 °C on a relatively narrow span of 159 °C
Resonance ionization is a process in optical physics used to excite a specific atom beyond its ionization potential to form an ion using a beam of photons irradiated from a pulsed laser light. In resonance ionization, the absorption or emission properties of the emitted photons are not considered, rather only the resulting excited ions are mass-selected, detected and measured. Depending on the laser light source used, one electron can be removed from each atom so that resonance ionization produces an efficient selectivity in two ways: elemental selectivity in ionization and isotopic selectivity in measurement.
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