Flame Test

The flame test is one of the most widely used analytical procedures in Chemistry. It is primarily used to observe and analyze the presence of certain elements in the given compound or salt. Generally, the flame test observes the occurrence of metal ions in a  compound. The flame test for every element is different as ions of each element have a specific feature based on their emission spectrum.

This difference is observed by the color of flames given out when the salt including the metal ion is burnt. It should be observed here that the emission spectrum of each element that discovers the flame color includes atoms rather than ions. The transformation of electrons in the ions has a tendency to produce the visible color lines which are observed in flame tests.

Read the article below to know flame test definition in Chemistry, how to do a flame test, flame test color and limitation of the flame test.

Flame Test Definition

The flame test is a method used by scientists to observe the occurrence of specific metals in a compound by the color they give to a flame. For example, the presence of Sodium turns the flame color to yellow.

How to do a Flame Test?

After knowing what a flame test is? You must be wondering, how to do a flame test? Let us now look at the methods of performing flame tests.

There are two methods to perform the flame test. These are:

• Classic Wire Loop Method.

• Wooden Split or Cotton Swab Method.

Here you will need a clean wire loop. You can use Platinum or nickel-chromium loops. Always clean these loops with hydrochloric acid or nitric acid by dipping in it . After that, rinse with distilled water. Also, test the cleanliness of the loop by inserting it into a gas flame.Remember that if prescribed colors are not produced then, it means loops are not sufficiently cleaned.The loop must be cleaned before the test.

The clean loop is dipped in either a powder or solution of a metallic salt. Place the loop with a sample in the blue part of the flame.After that note the observed color.

Let us now discuss each method in detail:

Classic Wire Loop Method: In Classic Wire Loop Method, you will require a wire loop. The most commonly used loops are platinum or nickel-chromium loops. These can be cleaned by immersing in nitric or hydrochloric acid, followed by washing with distilled or deionized water. Test the cleanliness of the loop by placing it into a gas flame. The loop may not be adequately cleaned if the burst is produced. The loop should be properly cleaned between tests.

The cleaned loop is immersed in either a powder or solution of an ionic (metal) salt. The loop along with a sample is placed in the blue or clear part of the flame and the resulting color is observed.

Wooden Split or Cotton Swab Method: Wooden or Cotton Swab’s method of conducting flame text provides a competitive alternative to wire loops. To use wooden splints, immerse them overnight in distilled water. Discharge the water and wash out the splints with clear water. Be cautious to prevent polluting the water with sodium. Hold a cotton swab or splint that has been soaked in water, immerse it in the sample that has to be tested and flush the splint or swab through the flame. Avoid holding the sample in the flame as this would cause the split or swab to catch fire. Each time use a new splint or swab for a new flame test.

Here you will need a clean wire loop. You can use Platinum or nickel-chromium loops. Always clean these loops with hydrochloric acid or nitric acid by dipping in it . After that, rinse with distilled water. Also, test the cleanliness of the loop by inserting it into a gas flame.Remember that if prescribed colors are not produced then, it means loops are not sufficiently cleaned.The loop must be cleaned before the test.

The clean loop is dipped in either a powder or solution of a metallic salt. Place the loop with a sample in the blue part of the flame.After that note the observed color.

How to Interpret Flame Test Results?

The sample is identified by the distinct observed color with the help of known values from a table or chart.

• Red

Carmine to Magenta: Lithium compounds. Masked by barium or sodium.

Scarlet or Crimson: Strontium compounds. Masked by barium.

Red: Rubidium (unfiltered flame)

Yellow-Red: Calcium compounds. Masked by barium.

• Yellow

Gold: Iron

Intense Yellow: Sodium compounds, even in trace amounts.Here the yellow flame is not the indicator of sodium.It can be indicative of it when 1% of NaCl is added to the dry compound.White

Bright White: Magnesium

White-Green: Zinc

• Green

Emerald: Copper compounds, other than halides. Thallium.

Bright Green: Boron

Blue Green : Moistened phosphate with H2SO4 and B2O3

Faint Green: Antimony and NH4 compounds.

Yellow-Green: Barium, manganese(II), molybdenum.

• Blue

Azure : Lead, selenium, bismuth, caesium, copper(I), CuCl2 and copper compounds. These are moistened with hydrochloric acid, indium and lead . 

Light Blue : Arsenic and its compounds

Greenish Blue: CuBr2, antimony

• Purple

Violet : Generally Potassium compounds except for borates, phosphates and silicates.

Masked by sodium or lithium.

Lilac to Purple Red : Potassium , Rubidium and caesium when seen through blue glass in the presence of sodium.

Flame Test Color

The Flame test color table given below describes the color of each flame as precisely as possible. Generally, most of the metals produce green colors and you can also see various shades of red and blue. The feasible method to observe metal ions is to compare it to the set of standards (known as composition) to determine what color can be expected when using the fuel in a laboratory.

The flame test is not distinctive as multiple variables are included in it. Generally, only a single tool is available to observe the elements present in a compound. While performing a flame test, be cautious of any contamination of the fuel or loop with sodium which has a bright yellow color and hides other colors. Many fuels include sodium contamination. You may observe the flame test color with the help of the blue filter to take off any yellow color.

Here is the list of the common elements that are observed through the flame test. They have different emission spectrums that permit them to show a particular coloured flame in a flame test. However, the color given below of different elements is only guidance as different colors are described differently by different people performing a flame test.

Flame Test Color Table

Limitation of the Flame Test

Some of the limitations of the flame test are given below:

• The ions will not be observed during the flame test as long as the concentration ions are minimum.

• The intensity of the light changes from one sample to another. For example, yellow sodium emissions are much more intense during the flame test in comparison to the red litmus emission.

• The flame test will be affected by the presence of contaminants or impurities. For example, sodium is generally present in most of the compounds and gives the yellow color to the flame. To avoid that, cobalt blue gas is used to filter out the yellow color of sodium.

• The flame test cannot make a distinction between all elements. Most of the metals produce similar colors whereas some of the compounds do not change the color of the flame at all.

Flame Test Colors

This table lists the expected colors for elements in the flame test.

Here the name of colors are subjective. The best way to recognise coloured elements is to test known solutions so that you know what to expect.