Editing Qualitative inorganic analysis (section)

=== 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