The common reactions of IV analytical group cations

Sodium hydroxide (or Potassium hydroxide) precipitate Al³⁺, Cr³⁺, Zn²⁺, Sn²⁺ ions as amphoteric hydroxides which are dissolved in excess of alkali with formation complexes, for example:

Al³⁺ + 3OH^-®Al(OH)₃¯, Al(OH)₃ + 3OH^- ® [Al(OH)₆]^3-;

Cr^{3 +} + 3OH^- ®Cr(OH)₃¯, Cr(OH)₃ + 3OH^- ® [Cr(OH)₆]^3- or Cr(OH)₃ + OH^- ®[Cr(OH)₄]^-;

Zn^{2 +} + 2OH^- ®Zn(OH)₂¯, Zn(OH)₂ + 2OH^-® [Zn(OH)₄]^2-;

Sn²⁺ + 2OH^- ®Sn(OH)₂¯, Sn(OH)₂ + 4OH^- ®[Sn(OH)₆]^4-.

At the presence of Hydrogene peroxide cations Cr³⁺, As³⁺, Sn²⁺ are oxidised accordingly to chromat - arsenat - and hexahydroxostanat (ІV) – ions:

2 [Cr(OH)₄]^- + 3Н₂О₂ + 2OH^- ®2CrO₄^2- + 8Н₂О;

[Sn(OH)₆]^4- + Н₂О₂ ®[Sn(OH)₆]^2- + 2OH^-;

AsО₃^3-+ Н₂О₂ ®AsО₄^3-+ Н₂О.

Reaction performance. Into four test tubes select on 3-4 drops of salts solutions of Aluminium, Chrome (ІІІ), Zinc and Tin (ІІ) and add in each some drops 2 mol/L a solution Sodium hydroxide. At first observe formation of white precipitate of Al(OH)₃, Zn(OH)₂, Sn(OH)₂ and dirty green Cr(OH)₃, and then their dissolution is an exess of alkali solution. Into a test tube with hexahydroxochromate (ІІІ) add 5-6 drops of Hydrogene peroxide solution and heat a test tube on a water-bath, observing change colouring from green to yellow (form chromate ions).

Alkaline metals and ammonium hydrogenphosphate form white crystal precipitates of AlPO₄, Zn₃(PO₄)₂, CrPO₄, Sn₃(PO₄)₂:

Al³⁺ + HPO₄^2- + NH₃ ® AlPO₄ + NH₄⁺

Aluminium, Zinc, Chrome and Tin phosphates are dissolved in solutions of alkalis. Zinc phosphate, also is well dissolved in ammonia, therefore it forms complex:

3Zn²⁺ + 2HPO₄^2- ® Zn₃(PO₄)₂ + 2H⁺;

Zn₃(PO₄)₂ + 12NH₃ ® 3[Zn(NH₃)₄]^{2 +} + 2PO₄^3-.

Reaction performance. Into four test tubes select on 3-4 drops of salts solutions of Aluminium, Zinc, Chrome (ІІІ) and Tin (ІІ), add in each some drops of 0,5 mol/L Sodium hydrogenphosphatesolution and observe occurrence of precipitates. Each of them investigate on solubility in alkalis, and Zinc phosphate - also in 2 mol/L ammonia solution.

Characteristic reactions of ions Al³⁺

The solution of ammonia NH₃ (pharmacopeia’s reaction) with salts of Aluminium in the neutral medium forms white amorphous precipitate Аl(OH)₃:

Al³⁺ + 3NH₃ + 3H₂O ÀAl(OH)₃↓ + 3NH₄⁺,

Al³⁺ + 3OH^- ®Аl(OH)₃↓.

Aluminium hydroxide has amphoteric properties. It is dissolved in acids:

Аl(OH)₃ + 3НNO₃ ®Аl(NO₃)₃ + 3Н₂O,

And also in alkalis:

Аl (OH)₃ + 3NaOH® Na₃[Аl(OH)₆].

After heating of complex it forms metaaluminate:

Na₃[Аl(OH)₆] NNaАlО₂ + 2NaOH + 2H₂O.

In the presence of NH₄Cl at heating aluminate forms precipitate Аl(OH)₃:

NaАlО₂ + NH₄Cl + H₂O ÀAl(OH)₃¯ + NH₃ + NаCl.

Reaction performance. To 3-4 drops of an investigated solution add 1-2 drops of 1 mol/L chloridic acid and 2-3 drops of tioacetammide solution. After that add some drops of 1 mol/L a Sodium hydroxide solution. If there are ions Аl³⁺, white precipitate forms. It is dissolved in excess of Sodium hydroxide solution. To the received solution add 1 mol/L ammonium chloride solution and observe formation white precipitate.

Alizarin (1, 2-dioksiantrahinon) with ions Al³⁺ in weak acidic medium forms red complex of Aluminium alizarinate Al(OH)₂[C₁₄H₆O₃(OH)] which is not dissolved in acitic acid. It is called as an aluminium varnish.

The ions of Fe^{3 +}, Bi^{3 +}, Cu^{2 +} interfere with the exposure of ions of Al³⁺.

Reaction performance. Reaction are executed in the drop way. On a strip of a filtering paper put a drop of solution K₄[Fe(CN)₆], and in its centre a drop of an investigated solution. After that a strip of a filtering paper are keeped above ammonia, and put on it some drops of alizarin solution. After that a strip of a filtering paper are keeped above ammonia. After that paper are dried, and put on it some drops of 2 mol/L CH₃COOH solution. If there are ions Al^{3 +}, there is a pink ring.

Cobalt nitrate with Aluminium salts (by length heating) forms mixed оxide Aluminium and Cobalt (Cobalt aluminate) Co(AlО₂)₂ dark blue colours:

2Al₂(SO₄)₃ + 2Co(NO₃)₂ ®2Со(AlО₂)₂ + 4NO₂­ + 6SO₃­ + O₂­.

Reaction performance. On a strip of a filtering paper put some drops of 0,1 mol/L investigated solution, and add 2-3 drops of Cobalt nitrate solution. A paper is dried, place into a porcelain crucible and length heating. If there are ions Al^{3 +}, ash will be dark blue colour.

Аluminon with ions Al^{3 +} forms red complex.

The ions of Ca^{2 +}, Cr^{3 +}, Fe^{3 +} interfere with the exposure of ions of Al³⁺.

Reaction performance. To some drops of an investigated solution add 2-3 drops of 2 mol/L CH₃COOH solution, 3-5 drops of aluminon solution and a mix are heated. Then add solution of NH₃ to basic medium (occurrence of a smell of ammonia) and 2-3 drops of 1 mol/L Na₂CO₃ solution. If there are ions Al^{3 +}, the red precipitate forms.

Characteristic reactions of ions Cr^{3 +}

Chrome (ІІІ) in the basic medium is oxidised to CrО₄^2-, and in acidic – to Cr₂O₇^2-. Ions Cr³⁺ to Cr₂O₇^2- are oxidised only strong oxidizers (H₂O₂, Na₂O₂, KMnО₄, Cl₂, Br₂, (NH₄)₂S₂O₈).

Chrome (ІІІ) in the basic medium is oxidised by H₂O₂, chloric or bromic water. In the basic medium chrome (ІІІ) with oxidizing reagents gives complex of tetrahydroxichromit (ІІІ):

2 [Cr(OH)₄]^- + 3Br₂ + 8OH ^- = 2CrO₄^2- + 6Br^- + 8H₂O

Reaction performance. To 2-3 drops of an investigated solution add 3-4 drops of 2 mol/L NaOH solution, 2-3 drops of bromic (chloric) water or 3 % Hydrogene peroxide solution and heat it on a water-bath to change colouring of a solution from green to yellow. Presence of CrО₄^2- ions check by reaction formation peroxichromatic acid H₂CrО₆.

Chrome (ІІІ) in the acidic medium is oxidised by KMnО₄, (NH₄)₂S₂O₈ and many other strong oxidizers:

2Cr^{3 +} + 2MnO₄^- + 5H₂O = Cr₂O₇^2- + 2MnО(OH)₂¯ + 6H⁺.

Reaction passes at heating and is accompanied by formation of brown precipitate MnО(OH)₂.

2Cr³⁺ + 3S₂O₈^2- + 7H₂O = Cr₂O₇^2- + 6SO₄^2- + 14H⁺.

Reaction is accelerated in the presence of Silver salts (catalyst).

Reaction performance. To 2-3 drops of an investigated solution add 2-3 drops of 1 mol/L HNO₃ or H₂SO₄ solution, 10-15 drops KMnО₄ and heat this solution. If there are ions Cr^{3 +}, there is an orange solution. A part of the solution is investigated on formation peroxichromatic acid.

Formation of peroxichromatic acid. If to asidic solution of chromate or bichromate add H₂O₂, dark blue solution of peroxichromatic acid forms:

Cr₂O₇^2- + 4H₂O₂ + 2H⁺ = 2H₂CrО₆ + 3H₂O.

In the aqueous solution peroxichromatic acid is very unstable (it is displayed with formation Cr³⁺), therefore to a solution add organic solvent (amyl alcohol or diethyl ether).

Reaction performance. To 2-3 drops of received solution (Cr₂O₇^2-) add 1 mol/L H₂SO₄ to acidic medium, 0,5 mL amyl alcohol and 4-5 drops H₂O₂. If there are Chrome ions the top bed of a solution are painted in dark blue colour. Combining this reaction with any reaction of oxidation Cr³⁺ to Cr₂O₇^2- it is possible to use it for fractional determination of Chrome ions in the presence of all others cations.

Characteristic reactions of ions Zn^{2 +}

Ammonium tetrarhodanomercurate (NH₄)₂[Hg(SCN)₄] with Zn²⁺ ions forms a white crystal precipitate:

Zn²⁺ + [Hg(SCN)₄]^2- = Zn[Hg(SCN)₄]¯.

Reaction passe in the acidic medium. Concentration of acid (it is better H₂SO₄) should not exceed 1 mol/L.

The ions of Fe²⁺, Fe^{3 +} interfere with the exposure of ions of Zn²⁺.

Reaction performance. To 2-3 drops of an investigated solution, add some drops of 1 mol/L sulphatic acid and 2-3 drops of ammonium tetrarhodanomercurate. If there are ions Zn²⁺, the white crystal precipitate forms.

Sodium sulphide Na₂S (pharmacopeia’s reaction) with solutions of Zinc salts forms white precipitate ZnS which is not dissolved in acetic acid, but is dissolved in diluted chloric acid:

Zn²⁺ + S^2- ® ZnS¯

ZnS + 2HCl ZZnCl₂ + H₂S­

The ions of Ag ⁺, Pb^{2 +}, Hg^{2 +} interfere with the exposure of ions of Zn²⁺.

Reaction performance. To 2-3 drops of an investigated solution (рН=5-6) add 1-2 drops of Sodium sulphide solution. If there are ions Zn²⁺, the white precipitate forms.

Potassium hexacyanoferrate (II) K₄[Fe(CN)₆] (pharmacopeia’s reaction) with Zn²⁺ ions forms white precipitate Potassium-Zinc hexacyanoferrate (ІІ) which is not dissolved in diluted chloridic acid:

3Zn²⁺ + 2К₄[Fe (CN)₆] = К₂Zn₃[Fe(CN)₆]₂¯ + 6К⁺.

This reaction may to distinguish Aluminium and Zinc ions.

Reaction performance. To 3-4 drops of an investigated solution add to 1-2 drop Potassium hexacyanoferrate (II). If there are ions Zn²⁺, the white precipitate forms. It is not dissolved in diluted chloric acid.

Dithizon (diphenylthiocarbasol) in solution CCl₄ (or chloroform CHCl₃) with ions Zn²⁺ forms complex bright red colour – Zinc dithizoate, which is extracted by ССl₄ or CHCl₃.

The ions of Ag⁺, Bi³⁺, Pb²⁺, Cu²⁺ interfere with the exposure of ions of Zn²⁺. So before deremination of Zn²⁺ ions their linkage in complexes with 0,5 mol/L Sodium thiosulphate.

Reaction performance. To 2-3 drops of an investigated solution add 1 mL acetic buffer (pН=5) and 1-2 mL of 10 % of dithizon solution in CCl₄ (or CHCl₃). If there are ions Zn²⁺ the organic layer is painted in red colour.

Cobalt nitrate with Zinc salts (by length heating) forms mixed оxide Zinc and Cobalt СоZnО₂ green colour – so-called „Renmarn’s greens”:

Zn(NO₃)₂ + Co(NO₃)₂ ® СоZnО₂ + 4NO₂­ + O₂­.

Reaction performance. On a strip of a filtering paper put some drops of 0,1 mol/L investigated solution, and add 2-3 drops of Cobalt nitrate solution. A paper are dried, place into a porcelain crucible and length heating. If there are ions Zn²⁺, ash will be green colour.

Characteristic reactions of ions Sn^II

Potassium (Sodium) hydroxide with ions Sn^{2 +} forms white precipitate Sn(OH)₂ which is dissolved in excess of alkali with formation tetrahydroxostannate (ІІ):

Sn²⁺ + 2OH^- ®Sn(OH)₂¯;

Sn(OH)₂ + 2OH^-® [Sn(OH)₄]^2-.

Precipitate hydroxide well dissolves in strong mineral acids.

If to solution of Sn (II) salts add some drops of ammonium salt, precipitate Sn(OH)₂ forms.

Reaction performance. To 3-4 drops of an investigated solution add drops of 2 mol/L Sodium hydroxide solution. If there are ions Sn (II), the white precipitate forms, it is dissolved in excess of alkali.

Sodium (Potassium) sulphide and hydrogen sulphide with ions Sn²⁺ forms dark brown precipitate Tin (ІІ) sulphide:

Sn²⁺ + S^2- ®SnS¯.

The precipitate is not dissolved in alkalis, excess of Sodium sulphide.

The ions of Cu^{2 +}, Hg^{2 +} interfere with the exposure of ions of Sn²⁺.

Reaction performance. To 3-4 drops of an investigated solution add 2-3 drops of 0,5 mol/L Sodium sulphide solution. If there are Sn (II) ions, brown precipitate forms.

Salts of Bismuth (ІІІ) with Sn^II ions forms metal Bismuth:

2Bi^{3 +} + 3[Sn(OH)₄]^2- +6ОН^-= 2Bi¯ + 3[Sn(OH)₆]^2-.

Reaction pass in basic medium. Metal Bismuth forms a precipitate of black colour.

The ions which in basic medium form precipitates of hydroxides interfere with the exposure of ions of Sn²⁺.

Reaction performance. To 5-6 drops of an investigated solution add some drops of 2 mol/L a Sodium hydroxide solution to dissolution of a precipitate which can be formed from the first drops. To the received solution add 1-2 drops of 0,5 mol/L Bi(NO₃)₃ solution. If there are Sn (II) ions, black precipitate of metal Bismuth forms.

Mercury (ІІ) chloride HgCl₂. In an acidic solution Sn²⁺ ions reducte HgCl₂ to formation of white precipitate Hg₂Cl₂. If to precipitate add excess of HgCl₂ solution, black precipitate of metal mercury forms:

[SnCl₄]^2- + 2HgCl₂ ®[SnCl₆]^2- + Hg₂Cl₂¯;

[SnCl₄]^2- + Hg₂Cl₂ ®[SnCl₆]^2- + 2Hg¯.

The chloride ions interfere with the exposure of ions of Sn²⁺.

Reaction performance. To 2-3 drops of an investigated solution add 2-3 drops of concentrated HCl and 2-3 drops of solution HgCl₂. If there are Sn (II) ions, white precipitate forms which gradually blackens.

Ammonium tetramolybdatophosphat (NH₄)₃[P(Mo₃O₁₀)₄]×H₂O. Ions Sn²⁺ reducte molybdenum (VI) to dark blue compound (molybdenic blue). In this compound Molybdenum has the lowest oxidation state. The reducer interfere with the exposure of ions of Sn²⁺.

Reaction performance. To 2-3 drops of Na₂HPO₄ solution add 2-3 drops of molybdenic liquids (a mix of (NH₄)₂MoО₄ and NH₄NO₃) and heat it. The yellow precipitate ammonium tetramolybdatophosphat (NH₄)₃[P(Mo₃O₁₀)₄] forms. Into other test tube add 2-3 drops of an investigated solution, 3-4 drops of concentrated HCl and a strip of iron (some milligrammes of a iron powder). A mix heats for 2-3 minutes. Some drops of receved solution add to the first test tube with a yellow precipitate. If there are Sn (II) ions, blue precipitate.

Characteristic reactions of ions Sn^IV

Sn (IV) ions usually determinate after preliminary are reducted by metal Fe, Mg, Al etc. to Sn (II). Then spend reactions which are characteristic for ions Sn (II).

For determination ions Sn (IV) it is possible to use reaction with hydrogen sulphide.

Hydrogen sulphide with Sn (IV) ions forms yellow precipitate Tin (IV) sulphide:

H₂[SnCl₆] + 2H₂S S SnS₂¯ + 6HCl.

Precipitate Tin (IV) sulphide, unlike Tin (II) sulphide, is dissolved in excess of ammonium or Sodium sulphide with formation tiosalt:

SnS₂ + (NH₄)₂S ((NH₄)₂SnS₃.

Therefore, if to acidic solutions of Sn (IV) salts add a solution of ammonium or Sodium sulphide precipitate SnS₂ will not be formed.

Reaction performance. To 3-4 drops of the investigated solution acidified by 1-2 drops concentrated hydrochloric acid, add some drops hydrosulphuric water. If there are Sn (IV) ions the yellow precipitate is formed. If to this precipitate add excess of Sodium or ammonium sulphide solution, the precipitate is dissolved.

Reduction of ions Sn^IV. The most characteristic reactions of Tin are reactions Sn (ІІ), therefore at first it is necessary to reducte Sn^IV to Sn^II by metal iron:

SnCl₆^2- + Fe S SnCl₄^2- + Fe²⁺ + 2Cl^–.

More active reducers (metal zinc, magnesium) reducte Sn^II and Sn^IV to metal tin.

Reaction performance. To 7-8 drops of an investigated solution add a drop concentrated HCl. 2-3 strips of the iron are dipped into solution; Sn^IV passes at Sn^II.

Characteristic reactions of ions AsО₃^3- or AsО₂^-

Arsene compounds is very toxic! At work with them it is necessary to show extra care!

Silver nitrate with ions AsО₃^3- forms yellow precipitate Ag₃AsО₃ which is dissolved in HNO₃ and NH₄OH.

AsО₂^– + 3Ag⁺ + H₂O A Ag₃AsО₃¯ + 2H⁺;

Ag₃AsО₃ + 6NH₄OH 3 [Ag(NH₃)₂]⁺ + AsО₃^3– + 6H₂O.

The РО₄^3-, J ^- Br ^- ions interfere with the exposure of ions of As^III.

Reaction performance. To 2-3 drops of an investigated solution add 1-2 drop of 0,1 mol/L AgNO₃ solution. If there are Аs (III) yellow precipitate Ag₃AsО₃ forms.

Iodine in a neutral or basic medium becomes colourless by arsenit ions (forms arsenat ions):

AsО₂^– + I₂ + 2H₂O H H₂AsО₄^– + 2I^- + 2H ⁺.

Reaction is carry out in the presence of NaHCO₃.

The other reducers interfere with the exposure of ions of As^III.

Reaction performance. To 2-3 drops of a acidic investigated solution add a some crystals of NaHCO₃, and after its dissolution – one drop of a solution of iodine. If at a solution there are arsenit-ions iodine becomes colourless.

Sodium sulphide Na₂S (pharmacopeia’s reaction) in the acidic medium reacts with arsenits with formation of a yellow precipitate, insoluble in concentrated hydrochloric acid, but soluble in ammonia solution:

AsО₃^3- + 6H⁺ ®As³⁺ + 3H₂O;

2As³⁺ + 3S^2- ®As₂S₃¯.

Reaction performance. To 4-5 drops of an investigated solution add 3-4 drops of 2 mol/L chloric acid solution and a solution of Sodium sulphide. If there are As (ІІІ) ions, the yellow precipitate forms.

Sodium hypophosphite NaH₂PO₂ (pharmacopeia’s reaction) (reaction Bugo and Tille) in the acidic medium reductes compounds of As (III) and As (V) to elementary Arsene which is formed black-brown precipitate:

4H₃AsО₃ + 3H₂PO₂^- ® 4As¯ + 3H₂PO₄^- + 6H₂O.

Reaction performance. To 5-7 drops of an investigated solution add 5-7 drops of Sodium hypophosphite solution. If there are As (III) or As (V) ions, a black-brown precipitate forms.

Characteristic reactions of ions AsО₄^3–

Silver nitrate with ions AsО₄^3– forms a chocolate precipitate:

AsО₄^3– + 3Ag⁺ ®Ag₃AsО₄¯.

All ions which form with Ag⁺ ions precipitate interfere with the exposure of ions of As^V.

Reaction performance. To 2-3 drops of an investigated solution add 4-5 drops of Silver nitrate solution. If there are arsenat-ions, the precipitate of chocolate colour is formed.

Ammonium molybdat (NH₄)₂MoО₄. Arsenitic acid and its salts at presence nitric acid and ammonium nitrate by heating with ammonium molybdat are formed a yellow crystal precipitate (NH₄)₃[As(Mo₃O₁₀)₄]HH₂O

H₃AsО₄ + 12(NH₄)₂MoО₄ + 21HNO₃ ® (NH₄)₃[As(Mo₃O₁₀)₄]×H₂O ¯+ 21NH₄NO₃ + 11H₂O.

The precipitate is not dissolved in nitric acid, considerably dissolved in excess of molybdat and it is easy dissolved – in alkalis and ammonia.

Reaction performance. To 2-3 drops of an investigated solution add 4-5 drops ammonium molybdat, 3-4 drops concentrated nitric acid, some crystals of NH₄NO₃ and heat it to a boiling on the water-bath. At presence in a solution of ions AsО₄^3– the yellow precipitate is formed.

Magnesian mix (MgCl₂+NH₄Cl+NH₄OH) (pharmacopeia’s reaction). Arsenats form with magnesian mix a white crystal precipitate MgNH₄AsO₄:

AsО₄^3– + Mg²⁺ + NH₄⁺ ® MgNH₄AsО₄¯.

This precipitate is similar to Magnesium and ammonium phosphate MgNH₄PO₄. MgNH₄AsO₄ is dissolved in acids and practically insoluble in the diluted ammonia solution.

Reaction performance. To 2-3 drops of an investigated solution add some drops of magnesian mix and wait 5-10 minutes. If the precipitate was not formed, it is necessary rub the wall-side of test tube a glass stick. The white crystal precipitate is formed in the presence of ions AsО₄^3–.

Potassium iodide. Compounds of As (V) in an acidic solution oxidise iodides to free iodine:

AsО₄^3– + 2I^- + 2H⁺ Û AsО₃^3– + I₂ + H₂O.

The other oxidizers interfere with the exposure of As^V ions.

Sensitivity of reaction can be raised, adding to a solution starch or benzene.

Reaction performance. To 2-3 drops of an investigated solution add 2-3 drops acetic acid, same quantity of Potassium iodide and some drops of starch. At presence in solution As (V) it is formed I₂ which paints starch in dark blue colour.

The common reactions of determination As^III and As (V)

High sensitivity reaction of determination As^III and As (V) is reductionthem to AsН₃ and element Arsene.

Reaction of reduction to AsН₃ (it is usedfor determination of small quantities of As^III and As^V). Analytical signal of this reaction is formation black paper moistened by AgNO₃ solution. The paper blackens because AsH₃ reduces ions Ag⁺ to metal silver (in basic or acidic medium).