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.

Detecting cations

Because of chemical properties, cations are usually divided into five groups. Each group has a common reagent which can be used to separate them from the solution. The separation has to be done in order, as, 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.

Keep in mind that 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 which chlorides are insoluble. As such, the group reagent to separate them is hydrochloride, usually used at concentration of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion - ^*^2- with Pb²⁺

Most important cations in 1st group are Ag⁺, Hg₂²⁺, Pb²⁺. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. AgCl however is reduced to silver under light and changes its colour to violet. It should be noted that 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 can't be used for the aforementioned reasons. For this reason, Pb²⁺ is usually also included in the 2nd analytical group.

2nd analytical group of cations

2nd analytical group of cations are ions of which sulfides are insoluble in acidic solution. The reagent can be any substance which 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 concentration of the reagent is very important: at higher concentrations Cd²⁺, Pb²⁺, Sn²⁺, who have relatively soluble sulphides, may not sedimentise, while at lower concentrations Zn²⁺, which is more insoluble than other 3rd group elements, may sedimentise. The reaction should be done in pH of about 0.5; if needed, the solution can be made acidic by using HNO₃.

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.

3rd analytical group of cations

3rd analytical group of cations includes ions which sulfides are insoluble in basic solution. The reagents are similar to these of the 2nd group, but separation must be done at pH of 8–9. If needed, a buffer solution can be used to keep it at this level.

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 them is by testing flame colour: barium makes flame yellow-green, 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⁺.