Detecting cations

According to their properties, cations are usually classified into six groups.[1] 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; 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 a 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. NH3 is also included in ZERO group of cation (according to NCERT textbooks). The most important cations in 1st group are Ag+, Hg2+2, 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. Ammonia is used as a reagent to distinguish between the other two. While AgCl dissolves in ammonia (due to the formation of the complex ion [Ag(NH3)2]+), 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 precipitate Hg2+2 and Ag+ may not be sufficient to do the same to Pb2+. Higher concentrations of Cl− cannot be used for the before mentioned 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.This group can be determined by adding the salt in water and then adding dilute hydrochloric acid. A white precipitate is formed, to which ammonium hydroxide is then added. If the precipitate is insoluble, then Pb2+ is present; if the precipitate is soluble, then Ag+ is present, and if the white precipitate turns black, then Hg2+

2 is present.

Confirmation test for lead:Pb2+ + 2 KI → PbI2 + 2 K+Pb2+ + K2CrO4 → PbCrO4 + 2 K+Confirmation test for Silver:Ag+ + KI → AgI + K+2Ag+ + K2CrO4 → Ag2CrO4 + 2 K+Confirmation test for dimeric mercury ion:Hg2+2 + 2 KI → Hg2I2 + 2 K+2 Hg2+2 + 2 NaOH → 2 Hg2O + 2 Na+ + H2O3rd analytical group of anionsThe 3rd group of anions consists of SO2−04, PO3−4 and BO3−3. They react neither with concentrated nor diluted H2SO4.Sulfates give a white precipitate with BaCl2 which is insoluble in any acid or base.Phosphates give a yellow crystalline precipitate upon addition of HNO3 and ammonium molybdate.Borates give a green flame characteristic of ethyl borate when ignited with concentrated H2SO4 and ethanol.References ̂ a b E. J. King "Qualitative Analysis and Electrolytic Solutions" 1959, Harcourt, Brace, and World, New York. ̂ C. Parameshwara Murthy (2008). University Chemistry, Volume 1 . New Age International. p. 133. ISBN 8122407420 .

Qualitative inorganic analysisFrom Wikipedia, the free encyclopediaClassical 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.[1]

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

The formation of precipitates with the ions of an unknown salt in aqueous solution and other reagents can be used to help identify and separate the ions of the unknown salt.Chloride Precipitation: Chloride ions, CL, can be added to a solution of an unknown salt. Inspection of the general solubility table (Appendix A) indicates the chloride salts of Ag+, Pb2+, and Hg2

2+ are insoluble in water. Therefore, it is possible to detect the presence of Ag+ and Pb2+ by adding CL ions. To differentiate between Ag+ and Pb2+ additional tests (see complex ions section of Project 6) can be carried out.

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