آزمایش شعله (Flame Test)

Flame Test

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 begin to emit light.   Based on the emission spectrum of the element, the compound will turn the flame 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 a firework display.

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 submerging 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 change to 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 as it may catch fire.

Top: Parking lot bathed in Yellow, Sodium vapor, light. Bottom: The parking lot as seen through a, blue, Cobalt glass plate.

Certain metal ions will turn the flame very distinctive colors, these colors intern can help identify the presence of a particular metal in a 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 Ion in a compound will color a flame violet / lilac, on the other hand, even trace amounts of Sodium ions 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 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.

Coloring Fire

Below is a table listing Chemical compounds used to obtain a desired color in a flame.

Plain

Above: An almost colorless ethanol flame, without any chemical colorants

Red

Left: Lithium Chloride and Lithium Carbonate color an ethanol flame.
Right: Strontium Chloride and Strontium Carbonate used in ethanol flame.

Orange

Left: Calcium Chloride used to in an ethanol flame.
Right (2 pictures):  A mixture of Potassium Chlorate and sugar burns with the coloring agent Calcium Carbonate (CaCO₃) giving it an orange color.

Yellow / Yellowish-Green

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.

Green

Left: Copper(II) Chloride colors a cool ethanol flame a vivid green.
Right: Trimethyl Borate burns with a strong 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.

Blue

Left: A very hot, butane flame is colored a bright blue by Copper(II) Chloride.
Right (2 pictures):  A mixture of Potassium Chlorate and sugar burns with a very bright blue color due to the Copper(II) Sulfate (CuSO₄) coloring agent.

Purple-Violet

Above: An ethanol flame is colored a light purple / lilac by Potassium Chloride.

White-Silver

Additional Pictures

Above: Potassium Chlorate and sugar react along with various
flame coloring agents to produce flames of every color in the spectrum.