How to Identify the Unknown in a Flame Test

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One of the primary tasks of chemists is the identification of chemicals and the determination of how much of the chemical is present in a sample. Qualitative and quantitative analysis are the terms used for these two tasks. Many samples submitted to chemical laboratories are tested for identification of the metal ions present in the sample. The flame test provides a quick procedure to identify most metal ions in unknowns. The chemist records the color of the flame when a drop of the unknown solution vaporizes in the flame of a Bunsen burner. The emission lines of the metal ions are characteristic of each metal. By matching these lines to the metal that emits them, the metal ions in the unknown are determined.

Things You'll Need

  • Nicrome wire
  • Cork stoppers
  • Bunsen burner
  • 0.1 M hydrochloric acid
  • Standard metal solutions
  • Unknown solution
  • Assemble a test wire for holding the solution in the flame. Push a length of Nicrome wire through a cork stopper from the narrow end. Make a small loop in the end of the wire away from the stopper.

  • Light a Bunsen burner and adjust the air/fuel ratio by moving the vent at the bottom of the burner to provide a colorless or near-colorless flame.

  • Clean the wire before testing each solution. Dip the looped end of the wire into a small beaker containing 0.1 M hydrochloric acid. Place the end of the wire in the flame and observe the color just above the wire. Hold the wire in the flame until the color of the flame returns to normal.

  • Dip the wire in a bottle of the first standard solution of metal ion and then place it in the burner. Record the color of the flame created by the standard solution. Standards are solutions of known metal ion content. The standards will serve as a reference against which you will compare the unknown.

  • Repeat the procedure for each of the standard solutions to which you have access. Clean the wire between each sample.

  • Test the unknown by dipping the wire in a solution of the unknown and record the color observed in the flame.

  • Compare the color of the flame for the unknown with the colors observed with the standards until you match the colors. You will have determined the presence of the metal ion in the standard that the flames from the unknown matched.

References

  • Photo Credit Jupiterimages/BananaStock/Getty Images
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