Techniques For Analyzing Inorganic Chemical Compounds

Methods for Inorganic Chemical Identification

Flame Test

The flame test is used in the establishment of the identity otherwise possible identity of a metal or metalloid that is found in ionic compounds. The test works by giving off a characteristic color when a compound is placed in the flame.

Test procedure

The use of a clean wire loop that is made of either platinum or nickel-chromium wire is the classic test technique for performing a flame test. The wire is dipped into the solution or the powder that is to be tested and then place in the hottest part of a flame. Observations are made on the resulting color which is then used in predicting the presence of a particular ion. The wire is cleaned by dipping it into hydrochloric acid and then rinsing with water.  The loop is tested to be clean by putting it into a flame of a gas burner which bursts into colors if the wire is not properly cleaned (Schäffer, 2017).

The clean loop is dipped into the solution or powder which is to be tested and then placed on the blue region of a Bunsen burner flame. The color observed is then comparing against the list of the color for various ions. Still, the flame test can be done using a wooden splint. The wooden splint is soaked in distilled water overnight and then rinsed with more clean distilled water. The ends of the splint can be moistened with water using cotton swabs (Toldrá, 2012). The wooden splint is then dipped into the substance that is to be tested and then placed in the flame of a Bunsen burner.

The flame test works by visually determining the identity of an unknown ion or salt using the color characteristics. The ions turn the flame of a Bunsen burner (Kenneth, 2012). The substances tested in this technique include sodium chloride, sodium acetate, sodium carbonates, lithium chloride, calcium nitrate, sodium hydrogen carbonate, potassium chloride, calcium sulfate, glucose potassium chloride among other substances.

 

Table 1: List of colors for various metal ions

Test for Carbonates

Carbonate ions, CO3-, are found in metal carbonates.  A two-step experiment can be used in the establishment of the presence of carbonate ions in a compound

Step 1: Reacting the carbonate with dilute acids

This reaction produces carbon dioxide and water as part of the products. An example could be the reaction between magnesium carbonate and sulphuric acid as shown in the equation below

Flame Test

MgCO₃+ H₂SO₄ – MgSO₄+CO₂+H₂O

Step 2: Collecting the gas produced and bubbling through lime water

Lime water is the commercial name for calcium hydroxide. When carbon dioxide is bubbled through lime water, a white precipitate is formed as shown in the equation below (Rouessac, 2013)

Ca (OH) ₂ + CO₂ – CaCO₃+ H₂O

The presence of a white precipitate is a confirmation of the presence of carbonate ions in the compound being tested.

 

Fig.1: Test showing presence of carbonates

Test for Halide ions

Halide ions are ions formed when elements in Group 7 of the periodic table gain ions. Among the elements in group 7 include chlorine, fluorine, bromine, and iodine. Nitrate solution is used in test the presence of any of these ions in a solution (Williamson, 2016). To achieve this, the procedure below is used:

• Add 2-3 drops of dilute nitric acid into the solution to be tested
• Add 2-3 drops of silver nitrate

Observe any changes with regard to the color of the precipitate

Chloride ions form a white precipitate, bromide ions cream precipitate while iodine ions form a yellow precipitate in the test. These precipitates are silver chloride, silver bromide, and silver iodide respectively.

Test for Sulphates

Barium chloride is used to test for the presence of sulphate ions, SO₄^2- in a solution. To achieve this a few drops of dilute hydrochloric acid are added to the solution to be tested followed by a few drops of a solution of barium chloride (Hansen, 2011). A white precipitate will be formed which is an indication of the presence of sulphate ions in the solution. An example is as illustrated in the equation below for the reaction between barium chloride and sodium sulphate solution

Na₂SO₄+ BaCl₂ – BaSO₄ + 2NaCl

 

Figure 2: Test showing the presence of sulphates

Substances are classified as either acid, basic or neutral depending on their levels of alkalinity. While acids have a sharp taste, bases have a soapy feeling (Rouessac, 2013). An example of an acid is lemon juice which has a sour taste. Toothpaste and detergents are examples of bases. Neutral substances are in between bases and acids. These substances neither burn the body nor have a soapy feeling. An acidic solution is achieved when an acid is dissolved in water. Similarly, a basic solution is achieved when a base is dissolved in water. Indicators are a substance that changes their colors when they are added to a basic or acidic solution.

For the purposes of this task, we have six solutions namely hydrochloric acid, magnesium hydroxide, sodium nitrate, sulphuric acid, water and calcium oxide solution (Schäffer, 2017). The solutions will be tested for acidity, alkalinity or neutrality using litmus paper and universal indicators

Test Procedure

Litmus indicator

A litmus paper indicator changes red when a solution is red and blue when the solution tested is alkaline. When dipped in neutral solutions, the indicator turns purple. To test for the alkalinity and acidity of the solutions, each of the solutions will be put in a test tube. Both red and blue litmus paper will be dipped into each of the solutions (Dargan, 2013). The color changes of the substances will follow the trend as shown in the table below

	Red Litmus	Blue Litmus
Acidic solution	Stays red	Turns blue
Neutral solution	Stays red	Stay blue
Alkaline solution	Turns blue	Stays blue

While making observations, the term “stays red/blue” as opposed to “no change” or “nothing” is used since it is an illustration of what is actually being seen.

Fig. 3.1: Acids turn blue litmus paper red Fig. 3.2: Bases turn red litmus paper blue

From the solutions used in this experiment, the following are the expected results using the litmus indicator.

Classification
Acids	Neutral	Alkaline
Hydrochloric acid	Water	Magnesium hydroxide
Sulphuric acid	Sodium chloride	Calcium oxide

A universal indicator is used to establish whether a substance is basic or acidic by means of color. It is possible to establish how strong or weak a base or an acid is through the use of universal indicators.  The universal indicator in conjunction with the pH scale can be used in the determination of the pH values of a solution. The pH scale has values ranging from 0-14. Very acidic solutions (having a pH of approximately 0) appear very dark red while neutral ones are green and very alkaline substances dark blue (Skoog, 2017).

 

Figure 4: Universal indicator and the pH scale

To determine whether any of the six solutions are acidic or basic, and the strength of the basicity or the acidity, the following procedures are followed:

• Take a test tube and add 5 mL of sulphuric acid to it.
• Add a drop of the universal indicator solution to the contents of the test tube
• Compare the resulting solution to the universal indicator chart to determine the pH of the solution and hence classify whether it is a strong acid/base or weak acid/base or neutral.
• Repeat the procedures above using the other solutions one after the other.
• Fill the table below for each of the solutions

Solution	Color	pH
Hydrochloric acid
Sulphuric acid
Magnesium hydroxide
Water
Sodium nitrate
Calcium oxide

Chromatography is a method used in the separation of mixtures of colored compounds. Paper chromatography ideal for the analysis of colored substances among them artificial colored and pigments in plants. This method works because some colors have high dissolution ability on the liquid than others hence travel over longer distances (Saukko, 2012).  A spot of the mixture is dropped near the bottom of the chromatography paper and the paper placed in a suitable solvent in an upright position. The solvent carries with it the mixtures as it soaks up in the paper. Various components of the mixture move at a different rate and hence the mixtures are separating out.  Other types of chromatography include gas chromatography which allows the separation of mixtures of compounds and mass spectrometry (Schäffer, 2017). Mass spectrometry can accurately and quickly identify substances and has the capability to detect the smallest amounts of substances in a mixture.

Test for Carbonates

Procedure

A small volume of a solution containing the three amino acids will be placed at the bottom of a rectangular piece of filter paper. Each of the solutions must be applied at the same starting line so as to achieve comparable results. The paper is then rolled into a cylinder and dropped into a beaker containing the liquid mobile phase (Picó, 2012). A solution consisting of ammonia, water and n-propanol are ideal as a mobile phase for separation of amino acids. The solution starts rising up the paper as soon as the paper is dipped into the mobile phase, a concept defined as capillary action.

As the mobile phase continue rising up the paper, it encounters the spots of amino acids. The behavior of each of the amino acids is influenced by its affinity for the stationary and mobile phases. Each of the amino acids will tend to travel as the solvent and not impeded by the filter paper. In cases where the amino acid has a higher affinity for the paper that the solvent, then the amino acid will tend to stick on the paper and hence travel at a very slow pace in relation to the solvent front. It is from these characteristics that the various types of amino acids can be separated using chromatography paper (Picó, 2012). There is a correlation between the affinity of the amino acids for the mobile phase and their solubility in the solvent. An amino acid having a high solubility in the solvent has a relatively higher affinity for the mobile phase in comparison with an amino acid with less solubility in the solvent.

The paper is lifted from the beaker when the solvent front comes very close to the top during which the amino acids cannot be seen. A compound, nihydrin, is spayed on the paper to make the amino acids visible as it reacts with the amino acids to form a blue-violet compound. The sprayed compound wills thus indicate the number of spots in correspondence to each of the amino acids (Metz, 2012). Amino acids with further spots have a higher affinity for the mobile phase and hence faster in migration.

The R_f values are used in determining the relative extent of movement of the molecules in the chromatography. It is determined by finding the ratio of the component and the solvent distance.

The detection of particular elements or substances is a fundamental aspect of science for chemists. A range of chemical tests are used in determining the identity of the unknown substances. Still, under other circumstances, titration is used to find out the amount or concentration of an acid or an alkali in a solution (Kenneth, 2012). A compound is made up of only cations and anions and thus the chemical tests done in the identification of the unknown substances must be those that are aimed at identifying the cations and anions present in the compound.

Test for Halide Ions

Test for cations (positive ions)

The metal ions can be tested using the flame test. The use of a clean wire loop that is made of either platinum or nickel-chromium wire is the classic test technique for performing a flame test. The wire is dipped into the solution or the powder that is to be tested and then place in the hottest part of a flame. Observations are made on the resulting color which is then used in predicting the presence of a particular ion (Jr., 2016). The wire is cleaned by dipping it into hydrochloric acid and then rinsing with water.  The loop is tested to be clean by putting it into a flame of a gas burner which bursts into colors if the wire is not properly cleaned.

The clean loop is dipped into the solution or powder which is to be tested and then placed on the blue region of a Bunsen burner flame (Hansen, 2011). The color observed is then comparing against the list of the color for various ions. Different metal ions have different colors of flames as shown in the table below.

The procedure for the test of positive ions can be summarized as below:

• Dip a clean loop of the flame test in the solution to be tested
• Hold the flame test loop at the edge of the blue part of a Bunsen burner flame
• Observe the color change of the flame and determine the metal it indicates(Dargan, 2013)
• Clean the lop in an acid and rinse with water
• Repeat the above steps for the other solution to be tested

For the purposes of this experiment, the color of the flame was crimson indicating that the metal was lithium

 

Flame test

Barium nitrate solution would be used in the test of the presence of anions in the solution. The anions tested with barium nitrate are SO42- and SO32-. A few drops of dilute hydrochloric acid are added to the solution to be tested followed by a few drops of a solution of barium chloride. A white precipitate will be formed which is an indication of the presence of sulphate ions in the solution otherwise sulphite ions present (Saukko, 2012). The test tested positive for sulphate ions. This thus means that one of the unknown compounds is composed of Ca²⁺ and SO₄^2- ions. The compound is thus calcium sulphate, CaSO₄²

References

Dargan, P. (2013). Novel Psychoactive Substances: Classification, Pharmacology, and Toxicology. Oxford: Academic Press.

Hansen, S. (2011). Introduction to Pharmaceutical Chemical Analysis. London: John Wiley & Sons.

Jr., S. V. (2016). Analytical Techniques and Methods for Biomass. Cambridge: Springer.

Kenneth, W. (2012). Techniques Labs for Macroscale and Microscale Organic Experiments. London: Cengage Learning.

Metz, C. (2012). Chemistry: Inorganic Qualitative Analysis in the Laboratory. New York: Elsevier Science.

Picó, Y. (2012). Chemical Analysis of Food: Techniques and Applications. New Delhi: Academic Press.

Rouessac, F. (2013). Chemical Analysis: Modern Instrumentation Methods and Techniques. Oxford: John Wiley & Sons.

Saukko, P. J. (2012). Encyclopedia of Forensic Sciences. New York: Academic Press.

Schäffer, A. (2017). Organic Trace Analysis. New York: Walter de Gruyter GmbH & Co KG.

Skoog, D. A. (2017). Principles of Instrumental Analysis. Manchester: Cengage Learning.

Toldrá, F. (2012). Analytical Tools for Assessing the Chemical Safety of Meat and Poultry. Sydney: Springer Science & Business Media.

Williamson, K. L. (2016). Macroscale and Microscale Organic Experiments. London: Cengage Learning.