Wednesday, April 19, 2017

Qualitative Inorganic Analysis

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, so that materials in other forms may need to be brought into this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, solid forming and other visible changes.

Qualitative inorganic analysis is that branch or method of analytical chemistry which seeks to establish elemental composition of inorganic compounds through various reagents.

Detecting Cations

According to their properties, cations are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. This happens because cationic analysis is based on the solubility products of the ions. As the cation gains its optimum concentration needed for precipitation it precipitates and hence allowing us to detect it. The division and precise details of separating into groups vary slightly from one source to another.

[There are also six groups of anions].

Modern Techniques

Qualitative inorganic analysis is now used only as a pedagogical tool. Modern techniques such as atomic absorption spectroscopy and ICP-MS are able to quickly detect the presence and concentrations of elements using a very small amount of sample.

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Flame Test

A flame test is an analytic procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic emission spectrum. The color of flames in general also depends on temperature; see flame color.


The test involves introducing a sample of the element or compound to a hot, non-luminous flame, and observing the color of the flame that results. The idea of the test is that sample atoms evaporate and since they are hot, they emit light when being in flame. Bulk sample emits light too, but its light is not good for analysis. Bulk sample emits light primarily due to motion of the atoms, therefore its spectrum is broad, consisting of a broad range of colors. Separate atoms of sample present in flame can emit only due to electronic transitions between different atomic energy levels. Those transitions emit light of very specific frequencies, characteristic of chemical element itself. Therefore, the flame gets the color, which is primarily determined by properties of the chemical element of the substance being put into flame. The flame test is a relatively easy experiment to set up, and thus is often demonstrated or carried out in science classes in schools.

Samples are usually held on a platinum wire cleaned repeatedly with hydrochloric acid to remove traces of previous analytes. The compound is usually made into a paste with concentrated hydrochloric acid, as metal halides, being volatile, give better results. Different flames should be tried to avoid wrong data due to "contaminated" flames, or occasionally to verify the accuracy of the color. In high-school chemistry courses, wooden splints are sometimes used, mostly because solutions can be dried onto them, and they are inexpensive. Nichrome wire is also sometimes used. When using a splint, one must be careful to wave the splint through the flame rather than holding it in the flame for extended periods, to avoid setting the splint itself on fire. The use of cotton swab or melamine foam (used in "eraser" cleaning sponges) as a support have also been suggested.

Sodium is a common component or contaminant in many compounds and its spectrum tends to dominate over others. The test flame is often viewed through cobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.


The flame test is relatively quick and simple to perform, and can be carried out with the basic equipment found in most chemistry laboratories. However, the range of elements positively detectable under these conditions is small, as the test relies on the subjective experience of the experimenter rather than any objective measurements. The test has difficulty detecting small concentrations of some elements, while too strong a result may be produced for certain others, which tends to cause fainter colors to not appear.

Although the flame test only gives qualitative information, not quantitative data about the proportion of elements in the sample, quantitative data can be obtained by the related techniques of flame photometry or flame emission spectroscopy. Flame Atomic absorption spectroscopy Instruments, made by e.g. PerkinElmer or Shimadzu, can be operated in emission mode according to the instrument manuals.

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The bead test is a traditional part of qualitative inorganic analysis to test for the presence of certain metals. The oldest one is the borax bead test or blister test. It was introduced by Berzelius in 1812. Since then other salts were used as fluxing agents, such as sodium carbonate or sodium fluoride. The most important one after borax is microcosmic salt, which is the basis of the microcosmic salt bead test.

Borax Bead

A small loop is made in the end of a platinum or Nichrome wire (as used in the flame test) and heated in a Bunsen flame until red hot. It is then dipped into powdered borax, and the adhering solid is held in the hottest part of the flame where it swells up as it loses its water of crystallization and then shrinks, forming a colourless, transparent glass-like bead (a mixture of sodium metaborate and boric anhydride)

Allow the bead to cool and the bead is moisturised (traditionally with the tongue) and dipped into the sample to be tested such that only a tiny amount of the substance adheres to the bead. If too much substance is used, the bead will become dark and opaque. The bead and adhering substance is then heated in the lower, reducing, part of the flame, allowed to cool, and the colour observed. It is then heated in the upper, oxidizing, part of the flame, allowed to cool, and the colour observed again.

Characteristic coloured beads are produced with salts of copper, iron, chromium, manganese, cobalt and nickel. After the test, the bead is removed by heating it to fusion point, and plunging it into a vessel of water.

Afterword by the Blog Author

After a full year of high school chemistry, the blog author took a third semester of chemistry entirely dedicated to qualitative analysis.  It was a great opportunity to practice scientific methods at a high level.  It develops a fondness for platinum wire and blue cobalt glass, yes, but it also leads the student to understand the importance of lab bench protocols and clean glassware to getting accurate results.  I went from being afraid of lab work to justifiable pride in accurate qualitative work.  I’m sorry most modern chemists aren’t exposed to this valuable experience.

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