A flame test is a procedure used to test qualitatively for the presence of certain metals in chemical compounds. When the compound to be studied is excited by heating it in a flame, the metal ions will emit light. Based on the emission spectrum of the element, the compound will change the color of the flame to a characteristic color. This technique of using certain chemical compounds to color flames is widely used in pyrotechnics to produce the range of colors seen in firework displays.
To perform a flame test,
prepare a solution of the compound to be tested by
dissolving it in deionized
water. Next, clean an inert wire loop
by immersing it in a dilute hydrochloric acid solution, then
rinsing with deionized water. Repeat this process
until no distinct color is seen after placing the loop into
a flame. When the wire loop is clean, dip it into the
solution to be tested and place the loop into the hottest
part of a non-luminous flame. Observe and record the
color of the flame.
A porous wooden splint may be substituted for the metal loop. Pre soak the wood splint in deionized water to wash out any ionic impurities, then soak the splint in the solution to be tested. Place the end of the wood splint into the flame and observe any color changes, but be careful not to leave the splint in too long, causing it to catch fire and burn with a luminous flame, masking the color imparted by the metal ions present.
Certain metal ions will turn the flame distinctive colors; these colors intern can help identify the presence of the particular metal present the compound. However, some colors are produced by several different metals, making it hard to determine the exact ion or concentration of the ion in the compound. Some colors are very weak and are easily overpowered by stronger colors. For instance, the presence of a potassium ions (K+) in a compound will color a flame violet / lilac, on the other hand, even trace amounts of sodium ions (Na+) in a compound produce a very strong yellow flame, often times making the potassium ion very difficult to detect. To counteract the effects of any sodium impurities, one can view the flame through a piece of cobalt blue glass . The cobalt glass absorbs the yellow light given off by sodium ions while letting most other wavelengths of light pass through. More recently, didymium glass has been substituted for cobalt glass due to its superior ability to block undesirable light.
Below is a table listing chemical compounds used to obtain a desired color in a flame.
|Color||Element [common compounds]|
|Red [Pictures]||lithium [Li2CO3], strontium [SrCO3, Sr(NO3)2]|
|Orange [Pictures]||calcium [CaCO3, CaSO4]|
|Yellow [Pictures]||sodium [NaCl, NaNO3, Na2CO3]|
|Yellowish-Green||boron [Borax - Na2B4O7]|
|Green [Pictures]||barium [Ba(NO3)2, BaCl2], copper [CuSO4]|
|Blue [Pictures]||copper [CuO, CuCO3], copper halides [e.g., CuCl2]|
|Purple-Violet [Pictures]||potassium [KClO3, KCl, KNO3, K2SO4]|
|White-Silver [Pictures]||aluminum, magnesium, titanium|
Above: An almost colorless ethanol flame, without any chemical colorants
Left: Lithium chloride and lithium
carbonate color an ethanol flame red.
Right: Strontium chloride and strontium carbonate color an ethanol flame red-orange.
chloride is used to color an ethanol flame orange.
Right (2 pictures): A mixture of potassium chlorate and sugar burns with the coloring agent calcium carbonate (CaCO3) giving it an orange color.
Left: Sodium chloride imparts a bright,
strong, yellow color to an ethanol flame.
Right: Borax (sodium tetraborate) colors the ethanol flame a light yellow-green.
Left: Copper(II) chloride colors a cool
ethanol flame a vivid green.
Right: Trimethyl borate burns with a vivid green flame without the need for the addition of a colorant.
When boric acid is mixed with methanol, trimethyl borate, a volatile (high vapor pressure) and flammable substance, is formed. Trimethyl borate will burn with a green flame without the need to add additional colorants.
A hot, butane flame is colored a bright blue by copper(II)
Right (2 pictures): A mixture of potassium chlorate and sugar burns with a bright blue color due to the copper(II) sulfate (CuSO4) coloring agent.
Above:An ethanol flame is colored purple / lilac by potassium chloride.
Above: Small magnesium metal turnings burn above a butane flame (left) and ethanol flame (right).
|A piece of Titanium metal gives off bright sparks as it is pressed against a fast-spinning sanding pad.|
copper(II) chloride, lithium carbonate, and calcium carbonate.