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 obviously visible changes.

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Detecting cations

According to their properties, cations are usually classified into five groups. Each group has a common reagent which can be used to separate them from the solution. The separation must be done in the sequence specified below, otherwise, for example, some ions of 1st group can also react with 2nd group reagent, so that the solution must not have any ions left from previous groups to obtain meaningful results. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.

1st analytical group of cations

1st analytical group of cations consists of ions that form insoluble chlorides. As such, the group reagent to separate them is hydrochloric acid, usually used at concentration of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion - [PbCl₄]^2- with Pb²⁺. Consequently the Pb²⁺ ion would go undetected

The most important cations in 1st group are Ag⁺, Hg₂²⁺, and Pb²⁺. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl₂ is soluble in hot water, and can therefore be differentiated easily. To distinguish between the other two, ammonia is used as a reagent. While AgCl dissolves in ammonia (due to the formation of the complex ion [Ag(NH₃)₂]⁺ion), Hg₂Cl₂ gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury. Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.

PbCl₂ is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely sedimentize Hg₂²⁺ and Ag⁺, may not be sufficient to do the same to Pb²⁺ and higher concentrations ca not be used for the aforementioned reasons. Thus, a filtrate obtained after first group analysis of Pb²⁺ contains an appreciable concentration of this cation, enough to give the test of the second group, viz. formation of an insoluble sulfide. For this reason, Pb²⁺ is usually also included in the 2nd analytical group.

2nd analytical group of cations

2nd analytical group of cations consists of ions that forms acid-insoluble sulfides. Cations in the 2nd group include: Cd²⁺, Bi³⁺, Cu²⁺, As³⁺, As⁵⁺, Sb³⁺, Sb5+, Sn²⁺, Sn⁴⁺ and Hg²⁺. Pb²⁺ is usually also included here in addition to the first group.

The reagent can be any substance that gives S^2- ions in such solutions; most commonly used are H₂S (at 0.2-0.3 M), AKT (at 0.3-0.6 M). The test with the sulfide ion must be conducted in the presence of dilute HCl. Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone. If dilute acid is not used, the advanced precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results. Acids beside HCl are rarely used. Sulfuric acid may lead to the precipitation of 4th group cations, while nitric acid directly reacts with the sulfide ion (reagent), forming colloidal sulfur.

The precipitates of these cations are almost indistinguishable, except for CdS which is yellow. All the precipitates, except for HgS, are soluble in dilute mineral acids. HgS is soluble only in aqua regia, which can be used to separate it from the rest. The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, while CdS dissolves forming a colourless solution. The sulfides of As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Sn²⁺, Sn⁴⁺ are soluble in yellow ammonium sulfide, where they form polysulfide complexes.

3rd analytical group of cations

3rd analytical group of cations includes ions that form sulfides which are insoluble in basic solution. The reagents are similar to these of the 2nd group, but separation is conducted at pH of 8–9. Occasionally, a buffer solution is used to ensure this pH.

Cations in the 3rd group are, among others: Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Al³⁺, and Cr³⁺.

4th analytical group of cations

Ions in 4th analytical group of cations form carbonates that are insoluble in water. The reagent usually used is (NH₄)₂CO₃ (at around 0.2 M), the pH should be neutral or slightly basic.

Caution should be taken to properly separate all lower analytical groups beforehand, as many of cations in previous groups also form insoluble carbonates.

Most important ions in the 4th group: Ba²⁺, Ca²⁺, and Sr²⁺. After separation, the easiest way to distinguish these ions is by testing flame colour: barium gives a yellow-green flame, calcium orange-red and strontium deep red.

5th analytical group of cations

Cations which are left after carefully separating previous groups are considered to be in the fifth analytical group. The most important ones are Mg²⁺, Li⁺, Na⁺, K⁺ and NH₄⁺.

Detecting anions

Halides are precipitated by silver nitrate; they can be further identified by color. Sulfates can be precipitated by lead. Nitrates can be reduced to ammonia.

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.

See also

• Flame test