st analytical group of cations

1st analytical group of cations

The most important cations in 1st group are Ag⁺, Hg²⁺ , 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⁺ and Ag⁺ may not be sufficient to do the same to Pb²⁺. Higher concentrations of Cl⁻ cannot 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.

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 + 2 K⁺

Ag⁺ + K₂CrO₄ → Ag₂CrO₄ + 2 K⁺

Confirmation test for dimeric mercury ion:

Hg2+2 + 2 KI → Hg₂I₂ + 2 K⁺

Hg2+2 + NaOH → HgO + 2 Na + H₂O

2nd analytical group of cations

The reagent can be any substance that gives S²⁻ 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 early 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 the 4th group cations, while nitric acid directly reacts with the sulfide ion in the 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 nitric 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 formpolysulfide complexes.

This group is determined by adding the salt in water and then adding dilute hydrochloric acid followed by hydrogen sulfide. Usually it is done by passing hydrogen sulfide over the test tube for detection of 1st group cations. If it forms a reddish brown or black precipitate then Bi³⁺, Cu²⁺, Hg²⁺ or Pb²⁺ is present. Otherwise, if it forms a yellow precipitate, then Cd²⁺ or Sn⁴⁺ is present; or if it forms a brown precipitate, then Sn²⁺ must be present; or if a red orange precipitate is formed, then Sb³⁺ is present.

To distinguish between ions in the black or reddish brown precipitate, it is first boiled in diluted HNO₃. If it is insoluble, then Hg²⁺ is present. If it is soluble, then Cu²⁺, Hg²⁺ or Pb²⁺ may be present; sulfuric acid is then added to the resulting solution. If a white precipitate forms, then Pb²⁺ may be present; if no precipitate is formed, then a new solution is made by adding an excess of ammonium hydroxide in the original salt solution. A resulting blue color indicates the presence of Cu²⁺, and a white precipitate indicates bismuth. Otherwise, if the precipitate is insoluble then Hg²⁺ is present.

To distinguish between ions in the yellow precipitate, an excess of NaOH is added to the original salt solution to form a white precipitate. The test tube is then shaken, and if the white precipitate dissolves, then Sn⁴⁺ is present; otherwise, Cd²⁺ is present.

Confirmation test for lead:

Pb²⁺ + 2 KI → PbI₂ + 2 K⁺

Pb²⁺ + K₂CrO₄ → PbCrO₄ + 2 K⁺

Confirmation test for copper:

2 Cu²⁺ + K₄[Fe(CN)₆] + CH₃COOH → Cu₂[Fe(CN)₆] + 4 K⁺

Cu²⁺ + 2 NaOH → Cu(OH)₂ + 2 Na⁺

Cu(OH)₂ → CuO + H₂O (endothermic)

Confirmation test for bismuth:

Bi³⁺ + 3 KI (in excess) → BiI₃ + 3 K⁺

BiI₃⁺ + KI → K[BiI₄]

Bi³⁺ + H₂O (in excess) → BiO⁺ + 2 H⁺

Confirmation test for mercury:

Hg²⁺ + 2 KI (in excess) → HgI₂ + 2 K⁺

HgI₂ + 2 KI → K₂[HgI₄] (red precipitate dissolves)

2 Hg²⁺ + SnCl₂ → 2 Hg + SnCl₄ (white precipitate turns gray)

3rd analytical group of cations

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

The group is determined by making a solution of the salt in water and adding ammonium chloride and ammonium hydroxide.

The formation of a reddish brown precipitate indicates Fe³⁺; a gelatinous white precipitate indicates Al³⁺; and a green precipitate indicates Cr³⁺ or Fe²⁺. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Fe²⁺ is indicated; otherwise, Cr³⁺ is present.

Each student is issued with a bottle of the mixtures containing 3 cation of these groups.

Determine which of the cations contained in the mixture (p101-104, application5, circuit10,11). The result of the issue the report, which should reflect the procedure for opening the ion reaction equations the opening, the observed effects, the circuit opening.

4th analytical group of cations

The fourth group of cations include Zn²⁺, Ni²⁺, Co²⁺, and Mn²⁺. This group is determined by the addition of ammonium chloride, ammonium hydroxide, and hydrogen sulfide gas to the solution of the salt. A colored precipitate indicates Mn²⁺; a white precipitate indicates Zn²⁺; and a black precipitate indicates either Ni²⁺, if the color of the original solution is green, or Co²⁺ otherwise.

5th analytical group of cations

Ions in 5th analytical group of cations form carbonates that are insoluble in water. The reagent usually used is (NH₄)₂CO₃ (at around 0.2 M), with a neutral or slightly basic pH. All the cations in the previous groups are separated beforehand, since many of them also form insoluble carbonates.

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

6th analytical group of cations

Cations which are left after carefully separating previous groups are considered to be in the sixth analytical group. The most important ones are Mg²⁺, Li⁺, Na⁺, K⁺ and NH+4. NH+4 gives a brown colored precipitate with Nessler's reagent; the rest of the ions are distinguished by flame color: lithium gives a red flame, sodium gives bright yellow (even in trace amounts), potassium gives violet, and magnesium, bright white.

CONTROL QUESTIONS TO THE REPORT:

1. How to determine the cation group is contained in the investigated salt?

2. The content of any of the cations can assume the colour of the solution?

3. What conclusions can be drawn on the pH value of the studied solution?

4. If adding to the test solution sodium hydroxide precipitate, the cation of which groups may be contained in the solution?

5. As with group reagent to distinguish between cations 4 and 5 analytic groups?

6. If the test solution gave a precipitate with sulphuric acid, which may contain cations in the solution?

7. If the test solution does not give a precipitate with sodium carbonate, which may contain cations in the solution?

8. What is the sequence of actions during the test substances on the solubility?

9. What reagent can determine that the solution contains only cations of the first group?

10. How to distinguish between precipitates of the cations of 5 and 6 groups, resulting from the action of the alkali solution?

11. What ions can be detected in the test solution using silver nitrate?

8-LW Analysis of mixture of cations 1-6 analytical groups (control task).

Each student is issued with a bottle of the mixtures containing 3 cation of these groups.

The task:

Determine which of the cations contained in the mixture (p104-107, application5, circuit12-14). The result of the issue the report, which should reflect the procedure for opening the ion reaction equations the opening, the observed effects, the circuit opening.

A framework for analysis of substances soluble in water

At the beginning of the analysis draw attention to the coloration and the reaction solution. The presence of staining may indicate the presence in solution of ions , , ,

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