How to Do Chemical Analysis

An example of a chemical to be analyzed.

A cornerstone of chemistry is the use of reactions to transform materials into other materials. But often, reactants must be identified before they can be used in a large-scale reaction. And once a product has been made, it must be tested for purity and yield. The techniques used for these tests are in the realm of chemical analysis. Using a host of common chemical analysis strategies, a scientist can get a chemical profile of a substance. This profile can prove invaluable in controlling chemical reactions and obtaining desired products, and is of great use to any chemist.

Things You'll Need

• Unknown chemical (or chemical with unknown properties)
• Merck manual or other chemical identification resource
• Bunsen burner
• Steel loop
• Several 100- and 250-milliliter beakers
• 10- and 100-mL graduated cylinders
• Thermometer
• 50-mL buret
• Universal indicator
• Hydrochloric acid (any precisely known concentration)
• Sodium hydroxide (any precisely known concentration)
• Magnetic stir bar
• Hot plate and electrical outlet
• Various common alkaline earth metal and transition metal solutions, as needed
• Mass balance

Instructions

Determining the Cation: Flame Test

1. Connect the Bunsen burner to a gas source and open the gas flow.

2. Light the Bunsen burner by lighting a match and quickly positioning it near the opening of the burner.

3. Divide the chemical to be analyzed (henceforth referred to as the "analyte") into five or more beakers. Leave enough of the chemical in each container to be reasonably analyzed.

4. Dissolve a portion of the analyte in distilled water (if it is not already in solution) until no solid remains visible.

5. 

   Dip a steel loop in a solution of hydrochloric acid (HCl). Then dip the loop in the solution containing the analyte. Wave the loop through the Bunsen burner flame, and note the new color of the flame. See the References section for a list of flame test colors and their corresponding cations.

Determining the Anion: Precipitation Tests

1. Once the cation is deduced by the flame test, determine the anion of the compound using precipitation techniques, which are largely based on trial and error.

   Begin by obtaining as many solutions as possible from the following list: calcium, magnesium, strontium, barium, copper (II), silver, and cobalt (II) nitrate.

2. Systematically and separately mix each nitrate solution with the analyte solution, noting everything that happens when you do.

3. Consult a Merck index, a Handbook of Chemistry and Physics or an online resource to match the precipitates that you recorded with known compounds. This should allow you to determine the anion of the chemical in question via cross-referencing.

4. Write a general chemical formula for the compound using the cation and anion data collected, using variables x, y, etc. as subscripts. Even if the chemical's identity was already known, sometimes a chemical reaction forms several compounds. The following section will allow you to fill in the variable subscripts.

5. Turn off the Bunsen burner and dispose of the precipitates in a chemical waste container (not a trash can).

Determining the Subscripts of the Analyte: Gravimetric Analysis and Titration Preparation

1. Measure the mass of a sample of solid analyte using a mass balance. Record the value indicated by the balance as precisely as possible.

2. In a fume hood (for safety), heat the solid sample on a hot plate in a crucible. Observe any color changes or effused gases.

3. After about 15 minutes of heating, remeasure the solid sample's mass on the balance. Note whether it has changed. If so, record the new mass of the sample. If there is a change, it is likely caused by waters of hydration attached to the sample, which are important to identify in chemical analysis.

4. Now prepare a separate analyte solution. Transfer a known volume (but unknown concentration) of this solution to a 250-mL beaker. Add a few drops of universal indicator to the beaker, and observe the color.

5. Determine a suitable titration for the analysis based on the anion found in Section 2. For example, if the anion is acidic in solution, titrate with sodium hydroxide (NaOH), and if it is basic in solution, titrate with HCl. Use the link provided in Additional Resources to choose a suitable indicator for the titration.

6. Run the titration by dripping the acid or base in the buret down into the beaker below. Record the volume of titrant you have added when the indicator in the beaker changes color.

7. Once the titration is complete, use the volumes recorded to determine the relative mole ratios of each element in your compound. Use this information to determine the empirical formula of the analyte. See the Additional Resources section for more help in doing this.

Tips & Warnings

• If the unknown material is in solution, you can extract its solid form by slowly boiling it and running it through any common filtration setup using filter paper. Try making a table to organize the findings of the precipitation tests. Include colors and textures.

• Use common sense when handling chemicals. Perform Section 2 in a fume hood because mixing chemicals can sometimes produce volatile, toxic gases. Always wear safety gear and gloves in the laboratory. Be sure to extinguish the Bunsen burner before continuing past Section 1 to keep flammable gases from touching the flame.

References

• Flame Test Identifications
• Determination of Anions--An In-depth Guide

Resources

• Choosing an Appropriate Indicator for Titration
• Determining Empirical Formula of a Compound