Editing Qualitative inorganic analysis (section)

== Detecting cations ==
According to their properties, [[cation]]s are usually classified into six groups.<ref name=King />  Each group has a common reagent which can be used to separate them from the [[Solution (chemistry)|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 product]]s 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 ===
The ''1st analytical group of cations'' consists of ions which form insoluble [[chloride]]s. As such, the group reagent to separate them is [[hydrochloric acid]], usually used at a [[concentration]] of 1&ndash;2 [[molar solution|M]]. Concentrated HCl must not be used, because it forms a [[Solubility|soluble]] complex ([PbCl<sub>4</sub>]<sup>2−</sup>) with [[lead|Pb<sup>2+</sup>]]. Consequently, the [[lead|Pb<sup>2+</sup>]] ion would go undetected.

The most important cations in the 1st group are [[silver|Ag<sup>+</sup>]], [[Mercury (element)|Hg{{su|b=2|p=2+}}]], and [[lead|Pb<sup>2+</sup>]].  The chlorides of these [[Chemical element|elements]] cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl<sub>2</sub> 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(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup>), Hg<sub>2</sub>Cl<sub>2</sub> 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.

The silver ammonia complex can react with [[bismuth]] ions and [[iodide]] to generate orange or brown [[Ag2BiI5|Ag<sub>2</sub>BiI<sub>5</sub>]] precipitate.<ref name=":0">{{Cite book |last=李 |first=颖 |title=分析化学实验 |year=2021 |isbn=978-7-04-056004-6 |edition=6th |trans-title=Analytical Chemistry Experiments}}</ref>

PbCl<sub>2</sub> is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg{{su|b=2|p=2+}} and Ag<sup>+</sup> may not be sufficient to do the same to Pb<sup>2+</sup>. Higher concentrations of Cl<sup>&minus;</sup> cannot be used for the before mentioned reasons. Thus, a filtrate obtained after first group analysis of Pb<sup>2+</sup> 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, Pb<sup>2+</sup> is usually also included in the 2nd analytical group.

A signature reaction of lead ions involve the formation of a yellow [[Lead(II) chromate|lead chromate]] precipitate upon treatment with [[Chromate and dichromate|chromate]] ions. This precipitate doesn't dissolve in [[ammonia]] (unlike Cu(II) and Ag(I)) or [[acetic acid]] (unlike Cu(II) and Hg(II)).<ref name=":0" />

This group can be determined by adding the salt in water and then adding dilute hydrochloric acid. A white precipitate is formed, to which ammonia is then added. If the precipitate is insoluble, then Pb<sup>2+</sup> is present; if the precipitate is soluble, then Ag<sup>+</sup> is present, and if the white precipitate turns black, then Hg{{su|b=2|p=2+}} is present.

Hg{{su|b=2|p=2+}} ions, after oxidation in the presence of chloride ions to HgCl<sub>4</sub><sup>2-</sup>, can form a characteristic orange-red precipitate of [[Cu2HgI4|Cu<sub>2</sub>HgI<sub>4</sub>]] with the addition of [[Copper|Cu<sup>2+</sup>]] and I<sup>−</sup>.<ref name=":0" />

Confirmation test for Pb<sup>2+</sup>: 
:Pb<sup>2+</sup> + 2 KI → PbI<sub>2</sub> + 2 K<sup>+</sup>
:Pb<sup>2+</sup> + K<sub>2</sub>CrO<sub>4</sub> → PbCrO<sub>4</sub> + 2 K<sup>+</sup>

Confirmation test for Ag<sup>+</sup>:
:Ag<sup>+</sup> + KI → AgI +  K<sup>+</sup>
:2Ag<sup>+</sup> + K<sub>2</sub>CrO<sub>4</sub> → Ag<sub>2</sub>CrO<sub>4</sub> + 2 K<sup>+</sup>

Confirmation test for Hg{{Su|p=2+|b=2}}:
:Hg{{su|b=2|p=2+}} + 2 KI → Hg<sub>2</sub>I<sub>2</sub> + 2 K<sup>+</sup>
:2 Hg{{su|b=2|p=2+}} + 2 NaOH → 2 Hg{{su|b=2}}O + 2 Na<sup>+</sup> + H<sub>2</sub>O

=== 2nd analytical group of cations ===

The ''2nd analytical group of cations'' consists of ions which form acid-insoluble [[sulfide]]s. Cations in the 2nd group include: Cd<sup>2+</sup>, [[bismuth|Bi<sup>3+</sup>]], [[copper|Cu<sup>2+</sup>]], [[arsenic|As<sup>3+</sup>]], As<sup>5+</sup>, [[antimony|Sb<sup>3+</sup>]], Sb<sup>5+</sup>, Sn<sup>2+</sup>, Sn<sup>4+</sup> and Hg<sup>2+</sup>. Pb<sup>2+</sup> is usually also included here in addition to the first group. Although these methods refer to solutions that contain sulfide (S<sup>2−</sup>), these solutions actually only contain H<sub>2</sub>S and [[bisulfide]] (HS<sup>−</sup>).  Sulfide (S<sup>2−</sup>) does not exist in appreciable concentrations in water.

The reagent used can be any substance that gives S<sup>2&minus;</sup> ions in such solutions; most commonly used are [[hydrogen sulfide]] (at 0.2-0.3 M), [[thioacetamide]] (at 0.3-0.6 M), addition of hydrogen sulfide can often prove to be a lumbersome process and therefore sodium sulfide can also serve the purpose.  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 5th group cations, whereas nitric acid oxidises the sulfide ion in the reagent, forming colloidal sulfur.

The precipitates of these cations are almost indistinguishable, except for [[Cadmium sulfide|CdS]], which is yellow. All the precipitates, except for [[Cinnabar|HgS]], are soluble in dilute nitric acid. 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, whereas CdS dissolves forming a colourless solution. The sulfides of As<sup>3+</sup>, As<sup>5+</sup>, Sb<sup>3+</sup>, Sb<sup>5+</sup>, Sn<sup>2+</sup>, Sn<sup>4+</sup> are soluble in yellow [[ammonium sulfide]], where they form [[polysulfide]] complexes.

This group is determined by adding the salt in water and then adding dilute hydrochloric acid (to make the medium acidic) followed by hydrogen sulfide gas. 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 Bi<sup>3+</sup>, Cu<sup>2+</sup>, Hg<sup>2+</sup> or Pb<sup>2+</sup> is present. Otherwise, if it forms a yellow precipitate, then Cd<sup>2+</sup> or Sn<sup>4+</sup> is present; or if it forms a brown precipitate, then Sn<sup>2+</sup> must be present; or if a red orange precipitate is formed, then Sb<sup>3+</sup> is present.

:Pb<sup>2+</sup> + K<sub>2</sub>CrO<sub>4</sub> → PbCrO<sub>4</sub> + 2 K<sup>+</sup>

Confirmation test for copper:

:2 Cu<sup>2+</sup> + K<sub>4</sub>[Fe(CN)<sub>6</sub>] + CH<sub>3</sub>COOH → Cu<sub>2</sub>[Fe(CN)<sub>6</sub>] + 4 K<sup>+</sup>
:Cu<sup>2+</sup> + 2 NaOH → Cu(OH)<sub>2</sub> + 2 Na<sup>+</sup>
:Cu(OH)<sub>2</sub> → CuO + H<sub>2</sub>O (endothermic)
:(Another very sensitive test for copper utilizes the fact that Cu<sup>2+</sup> can serve as a catalyst for the oxidation of [[thiosulfate]] ions by [[Ferric|Fe<sup>3+</sup>]] ions. In the absence of Cu<sup>2+</sup>, Fe<sup>3+</sup> can form the purple complex Fe(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub><sup>−</sup> without undergoing [[redox]]. If the added sample contains Cu<sup>2+</sup>, the solution will rapidly discolor.)<ref name=":0" />

Confirmation test for bismuth:

:Bi<sup>3+</sup> + 3 KI (in excess) → BiI<sub>3</sub> + 3 K<sup>+</sup>
:BiI<sub>3</sub> + KI → K[BiI<sub>4</sub>]
:Bi<sup>3+</sup> + H<sub>2</sub>O (in excess) → BiO{{su|p=+}} + 2 H<sup>+</sup>
:(Bismuth ions can form th bright yellow complex Bi([[Thiourea|tu]])<sub>3</sub><sup>3+</sup> in the presence of [[thiourea]] under acidic conditions, which can be precipitated as the orange-red Bi(tu)<sub>3</sub>I<sub>3</sub>•Cu(tu)<sub>3</sub>I in the presence of Cu<sup>2+</sup> and I<sup>−</sup>, and this can also act as a test for bismuth.)<ref name=":0" />

Confirmation test for mercury:

:Hg<sup>2+</sup> + 2 KI (in excess) → HgI<sub>2</sub> + 2 K<sup>+</sup>
:HgI<sub>2</sub> + 2 KI → K<sub>2</sub>[HgI<sub>4</sub>] (red precipitate dissolves)
:2 Hg<sup>2+</sup> + SnCl<sub>2</sub> →  2 Hg + SnCl<sub>4</sub> (white precipitate turns gray)
:(Hg<sup>2+</sup> may otherwise be detected via Cu<sub>2</sub>HgI<sub>4</sub> formation, see Hg<sub>2</sub><sup>2+</sup> in 1st group cations.)<ref name=":0" />

=== 3rd analytical group of cations ===
The ''3rd analytical group of cations'' includes ions which form hydroxides that are insoluble even at low concentrations.

Cations in the 3rd group are, among others: [[iron|Fe<sup>2+</sup>]], Fe<sup>3+</sup>, [[aluminium|Al<sup>3+</sup>]], and [[Chromium|Cr<sup>3+</sup>]].

The group is determined by making a solution of the salt in water and adding [[ammonium chloride]] and ammonium hydroxide. Ammonium chloride is added to ensure low concentration of hydroxide ions.

The formation of a reddish-brown precipitate indicates Fe<sup>3+</sup>; a gelatinous white precipitate indicates [[aluminium|Al<sup>3+</sup>]]; and a green precipitate indicates Cr<sup>3+</sup> or Fe<sup>2+</sup>. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Cr<sup>3+</sup> is indicated; otherwise, Fe<sup>2+</sup> is present.

=== 4th analytical group of cations ===
The 4th analytical group of cations includes ions that precipitate as sulfides at pH 9. The reagent used is [[ammonium sulfide]] or Na<sub>2</sub>S 0.1 M added to the ammonia/ammonium chloride solution used to detect group 3 cations. 
It includes: [[zinc|Zn<sup>2+</sup>]], [[nickel|Ni<sup>2+</sup>]], [[cobalt|Co<sup>2+</sup>]], and [[manganese|Mn<sup>2+</sup>]]. [[Zinc]] will form a white precipitate, [[nickel]] and [[cobalt]] a black precipitate and [[manganese]] a brick/flesh colored precipitate. [[Dimethylglyoxime]] can be used to confirm nickel presence, while [[ammonium thiocyanate]] in ether will turn blue in the presence of cobalt. This group is sometimes denoted as IIIB since groups III and IV are tested for at the same time, with the addition of sulfide being the only difference.

This includes ions which form sulfides that are insoluble at high concentrations. The reagents used are H<sub>2</sub>S in the presence of NH<sub>4</sub>OH. NH<sub>4</sub>OH is used to increase the concentration of the sulfide ion, by the common ion effect - hydroxide ions from NH<sub>4</sub>OH combine with H<sup>+</sup> ions from H<sub>2</sub>S, which shifts the equilibrium in favor of the ionized form:

:{{chem|H|2|S}} {{eqmR}} 2{{chem|H|+}} + {{chem|S|2-}}

:{{chem|N|H|4|O|H}} {{eqmR}} {{chem|N|H|4|+}} + {{chem|O|H|-}}

:{{chem|O|H-}} + {{chem|H|+}} {{eqmR}} {{chem|H|2|O}}

They contain [[zinc|Zn<sup>2+</sup>]], [[manganese|Mn<sup>2+</sup>]], [[nickel|Ni<sup>2+</sup>]] and [[cobalt|Co<sup>2+</sup>]]

=== 5th analytical group of cations ===
Ions in 5th analytical group of cations form [[carbonate]]s that are insoluble in water. The reagent usually used is [[ammonium carbonate|(NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>]] (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 5th group are [[barium|Ba<sup>2+</sup>]], [[calcium|Ca<sup>2+</sup>]], and [[strontium|Sr<sup>2+</sup>]]. After separation, the easiest way to distinguish between these ions is by testing flame colour: barium gives a yellow-green flame, calcium gives brick red, and strontium, crimson 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 [[magnesium|Mg<sup>2+</sup>]], [[lithium|Li<sup>+</sup>]], [[Sodium|Na<sup>+</sup>]] and [[potassium|K<sup>+</sup>]]. All 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, colorless (although magnesium metal burns with a bright white flame). Magnesium can also be distinguished from other cations in this group by adding sodium hydroxide to drive the pH to 11 or higher, which selectively precipitates Mg(OH)<sub>2</sub>.