Qualitative Analysis

The value of a material is determined in part by the substances of which it is composed. The operations necessary to determine this composition are known as qualitative analysis. Qualitative analysis is a series of tests; responses to these tests identify the elements and compounds that make up the material.

Every substance is unique. Each has, for example, a certain color, texture, and appearance. These properties are, however, often insufficient to positively identify the substance although they certainly contribute to its identity. One must generally evaluate other physical and chemical characteristics to identify beyond any doubt the exact composition of a material. With 92 naturally occurring elements and an endless variety of possible combinations it is not an easy task to prove with certainty the exact composition of an unknown substance. If, upon testing, an unknown exhibits properties identical in every way to the known properties of a particular substance, then that unknown is identical to the known substance and is identified. Caution is necessary, however, for although some properties may compare within experimental error, all properties must correlate before the known and unknown materials can be termed identical.

Some of the more common physical properties measured for identifying an unknown substance are: melting point, color, boiling point, texture, density, ductility, electrical conductivity, malleability, thermal conductivity, refractive index, and coefficient of linear expansion.

Most of the properties listed exhibit measurable numerical values that can be compared to known values of elements and compounds found tabulated in various reference books. More elaborate physical testing, requiring complex scientific equipment and trained operators, deals with measurements dependent upon the internal structure of a material. Depending upon the arrangement of the particles within a substance, they interact with electromagnetic radiation in different ways. The result of these interactions is an electromagnetic spectrum, a pictorial representation of the absorption and emission of electromagnetic radiations of varying energy as they strike and pass through a substance. X ray, ultraviolet, visible, infrared, and other spectra when compared to similar spectra of known materials produce a match with that of the unknown if they are identical and a mismatch if they are not.

Chemical tests are widely used for qualitative analysis. If an unknown produces the same results when reacted with a certain chemical reagent as does a material of known composition, they may be identical. To be absolutely sure more then one confirmatory test is made, for although reagent A may, when added to both a known and an unknown substance, produce identical responses, reagent B when used for testing might react only with the known and not with the unknown. The analytical chemist who performs these tests must be knowledgeable both in selecting the proper test reagents and in knowing the expected results.

Various schemes for qualitative analysis exist and their study is a part of the training in many college chemistry programs. The most common scheme, the insoluble sulfide scheme, identifies approximately 30 of the more common metallic elements. It uses a single reagent, hydrogen sulfide, to separate solutions of metallic elements into groups of several substances with similar chemical properties. Other, more specific reagents, are then added to further separate within each group. Confirmatory tests are then performed, generating an insoluble colored solid, called a precipitate, or a soluble uniquely-colored product.

The nonmetallic elements, because of the greater number of reactions they can undergo, are more difficult to group. Additional confirmation tests would be necessary to identify single components within each group.

Organic materials, those based primarily on a carbon structure, pose a particular problem for qualitative analysis because of the presence of so many carbon atoms. Distinction between various organic compounds is based upon the arrangement of the carbon atoms and the other non-carbon atoms within a compound. It is possible to divide organic compounds into groups based upon these arrangements and often qualitative analysis for group identification is sufficient rather then identifying a particular compound. Some of the more common organic functional groups, as they are called, and the arrangement of atoms characteristic of the group are listed here. The symbol R represents an underlying arrangement of carbon and hydrogen atoms. R₁ may or may not be the same as R₂: acids R-COOH; alcohols R-OH; aldehydes R-COH; amines R-NH₂; esters R₁-COO-R₂; ethers R₁-O-R₂; hydrocarbons R-H; ketones R₁-CO-R₂.

Organic substances with different functional groups dissolve or remain insoluble in different solvents. They also respond differently to various reagents. It is relatively easy to identify the group into which an organic compound belongs. Once separated, additional tests would be necessary to confirm the presence of a particular functional group.

Identification of a specific organic substance is difficult. Physical tests are often more helpful then chemical tests. As an example, after a tentative identification has been made for an organic compound, a portion of the unknown is mixed with a portion of the pure known substance, and a melting point is measured. The tentative identification was correct if the melting point of the mixture is identical to the literature value melting point for the pure substance but incorrect if a substantially lower melting point is observed.

Spectral identification of organic substances, and this includes complex materials from living species, is probably the best means of qualitative identification. An infrared spectrum of an organic material exhibits numerous peaks and troughs generated by the interaction of the infrared radiation and the atoms within a molecule as the radiation passes through the substance or is reflected from its surface. Each functional group interacts only with infrared rays of specific energy or frequency. A peak observed at the frequency known to be indicative of an alcohol group is evidence that the substance is, indeed, an alcohol. Again, confirmatory tests both physical and chemical, should be made for often the peak generated by one type of functional group overlaps that of another.

Other spectral procedures not related to electromagnetic radiation also evoke specific responses, spectra, from organic compounds based upon the arrangement of the atoms comprising the material. Perhaps best known of these is the technique of nuclear magnetic resonance (NMR) spectroscopy. When applied to living tissue, as a diagnostic tool to observe the misarrangement of molecules within a living organism indicating a certain disease or abnormality, this approach is known as magnetic resonance imaging (MRI) spectroscopy. A sample-placed within a strong magnetic field and subjected simultaneously to a strong electrical signal will, because of the magnetic properties of the protons within its atoms, respond to these outside forces. What results is a nuclear magnetic spectrum. Here, analogous to an infrared spectrum, the location of the peaks which are generated indicates how the atoms within a molecule are arranged. Nuclear magnetic spectra have an advantage over infrared spectra in one respect as they will indicate the presence and position of hydrogen atoms attached to carbon. This is very difficult to determine from an infrared spectrum.

Another spectral technique, mass spectrometry, measures both molecular mass of a material and information relating to how atoms are joined together. By utilizing a combination of electric and magnetic fields coupled with a subatomic bombardment of the material one breaks the substance into fragments. The mass of each fragment is recorded, a mass spectrum, and like pieces of a jigsaw puzzle, this information can be reassembled to identify the structure of the parent substance.

All of these techniques, electromagnetic spectra, nuclear magnetic spectra, and mass spectra are comparative techniques. If the spectrum observed from an unknown matches that from a known material, the two can be assumed identical.

Often substances to be analyzed are composed of complex mixtures requiring a preliminary separation before the individual components can be known. One approach to the separation and simultaneous qualitative identification of complex mixtures uses a variety of related techniques called chromatographic separations. Chromatography is a separation process in which the sample is forced to flow past a stationary adsorbent. Each component in the sample has a different degree of attraction for the stationary adsorbent, those components which are strongly attracted will adhere to the stationary material almost immediately while those with a lesser degree of attraction will be carried farther along before sticking on the stationary material. If the stationary material is an adsorbent paper sheet and the sample in solution is allowed to flow over the paper, the technique is paper chromatography. If the adsorbent is packed in a long vertical tube and the sample solution is poured into the top of the tube, the technique is column chromatography. If the adsorbent is packed into a long narrow pipe and the sample, after being placed at one end of the pipe, is pushed through with a stream of gas, the technique is gas chromatography. With all chromatographic techniques the distance from the starting point traveled on a flat surface by each component or the time necessary for a component to pass through a packed tube from one end to the other is characteristic of that component. Distances or times when matched with the distances or times of known components indicate a qualitative match. It is wise, however, to run additional confirmatory test before a positive match is stated.

One area in which qualitative identification has become very important is the matching of human DNA tissue by law enforcement agencies to prove the presence or absence of a person at a crime scene. The details of how this is done are beyond the scope of this article but make interesting additional reading.