90972

Anions. Anions’ Analytical Classification

Лекция

Химия и фармакология

Except cations, any electrolyte consists of anions. So, the analysis of salts, acids, natural waters and industrial solutions will be incomplete, without identification of anions. Anions are divided into three (3) analytical groups (as it is shown in the table 7.1). First group contains such anions that form insoluble barium salts. Second group contains anions, that form precipitates with

Русский

2015-07-11

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2 чел.

LECTURE 7

ON THE THEME:

Anions

                     

                                                                               

Lecture 7 Anions

Plan

  1.  Anions’ Analytical Classification.
  2.  I Analytical Group of Anions Qualitative Reactions.
  3.  II Analytical Group of Anions Qualitative Reactions.
  4.  III Analytical Group of Anions Qualitative Reactions.
  5.  Colloidal Solutions in Qualitative analysis.
  6.  Analysis of the Mixture with Anions of All Analytical Groups.

  1.    Analytical Classification of Anions.

Except cations, any electrolyte consists of anions. So, the analysis of salts, acids, natural waters and industrial solutions will be incomplete, without identification of anions. Anions are divided into three (3) analytical groups (as it is shown in the table 7.1). First group contains such anions that form insoluble barium salts. Second group contains anions, that form precipitates with Ag+ cation and third group contains others.  

(Table 7/1)

Analytical Groups of Anions

І

II

III

SO42-

Cl-

NO3-

CO32-

Br-

CH3COO-

SiO32-

I-

NO2-

PO43-

S2-

BO2-

Group (general) reactive

BaCl2 or Ba(NO3)2

AgNO3

The amount of anions is larger, than we can see from the table. But in the course we learn the most advanced.

 

  1.   The 1 Analytical Group’s of Anions. Qualitative Reactions.

The characteristic reaction on sulfate anion is also a group reaction. The difference between ВаSО4 and others precipitations of Barium salts of the 1 group is that it is insoluble in all acids, except concentrated sulfuric:

BaSO4↓ + H2SO4 →Ba(HSO4)2

Carbonate ion, both in the solution and in composition of precipitation, evolves carbon dioxide at acids action. CO2 can be identified by its transmission through limewater or baryta water in case of their turbidness:

Ba(OH)2 + CO2 → BaCO3↓ + H2O

Phosphate ion can be determined with magnesia mixture (MgCl2 + NH4Cl + NH4OH). In the result of reaction white crystalline precipitation of MgNH4PO4 is observed. The equations of the reaction are:

Na3PO4 + MgCl2 + NH4Cl → Mg NH4 PO4↓ +  NaCl, or

Na2HPO4 + MgCl2 + NH4OHMg NH4 PO4↓ +  NaCl +  H2O

The determination can be blocked by SiO32-, AsO43- and some other ions. Another reactive on PO43- is molybdic mixture (is also used in arsenate anion determination):

H3PO4 + 12(NH4)2MoO4 + 21HNO3 (NH4)3H4[P(Mo2O7)6] + 21NH4NO3 + 10H2O

Yellow precipitate of hetero poly acid’s ammonia salt is observed.

The characteristic reaction on silicate ion is connected with formation of white jellylike precipitate of metasilicate acid during diluted strong acids addition.

2H+ + SiO32-   H2SiO4

The precipitate superiorly is formed at ammonia chloride action, that not only gives hydrogen ions in a result of hydrolysis, but also promotes coagulation presepitate. Heating promotes hydrolysis and reaction proceeding.

The determination of BO2- is connected with formation fugitive substances (for example BF3), that colour the flame into green:

CaF2 + H2SO4  CaSO4 + H2F2

2 H2F2 + NaBO2  BF3↑ + NaF + 2H2O

The sum equation is:

NaBO2 + 2CaF2 + 2H2SO4cons.)  BF3↑ + NaF + 2CaSO4 + 2H2O

This reaction is toxical and is not accomplished in laboratory.

7.3     II Analytical Group of Anions. Qualitative Reactions. 

During determination the chloride ions can be oxidized it to free chlorine only with strong oxidizing agents (KMnO4, K2Cr2O7 etc). The emitted chlorine gas can be identified by its smell, colour or using wet iodine-starch paper. In the last case, chlorine oxidizes iodide-ion up to free iodine, and that colours the starch of a paper into blue. But this method has the disadvantage: a release of toxic gas. Therefore a silver chloride property to be dissolved in concentrated ammonia solution with  subsequent sedimentation of AgCl by nitric acid is more often used:

 AgCl↓ + 2NH4OH [Ag(NH3)2]+ + Cl- + 2H2O

[Ag(NH3)2]+ + Cl- + 2H+ AgCl↓ + 2NH4+

These reactions are used for determination of silver ions.

These reaction also gives the opportunity to separate AgCl and AgBr or AgI. AgBr is dissolve very weak, and AgI isnt dissolve in ammonia, particularly in special selected buffer solution.

For determination of bromide and iodide ions, their reduction potential is used (comparing with chloride ions). It is easy to use chlorine water (a chlorine dissolved in water) for their oxidation, because the relevant standard electrode potentials Е° have such values (B):

Сl2 + 2е = 2Сl- Е°=1.36

Вr2+-2е = 2Вr- Е°=1.07

І2 + 2е = 21 Е°=1.36

An extraction of free Halogens by benzene is used for better identification of these anions. In case of bromine benzene layer colours in yellow, and iodine - purple.

For determination of iodide ions the reaction of „Golden rain" can be also used.

The sulfide ion can form poorly soluble salts with most of cations. Soluble are only sulfides of alkaline and alkaline-earth metals and ammonium. It is used in the qualitative analysis. So, if to 1-2 drops of sulphide solution add  2-3 drops of  РЬ(СН3СОО)2 solution, a black precipitate of lead sulfide is observed. A reduction property of use S2- is also used, applying as an oxidant potassium permanganate, potassium dichromate or chlorine water. Usually the solution decolorizing occurs and fine sediment precipitates of sulfure during this process.

7.4. III Analytical Group of Anions. Qualitative Reactions.

Diphenylamine (reacts in the sulfuric medium) is one of the most wide-spread reactive on nitrate ions. In this reaction solution changes the colour to dark blue, caused by products of oxidizing diphenylamine). Other oxidizers must be absent. This reaction is also useful for determination of nitrite ions, if they are apart.

 Acetic acid and its salts form ethyl acetate in the reaction with ethanol in presence of sulfuric acid. Ethyl acetate can be detected by the flavour:

Beside this, in analytical practice the most spread is the reaction of acetate ion with salts of Iron (III). During the boiling the solution changes its colour to tea-red and then the formation of brown-red precipitation of the basic iron salt occurs:

7.5. Colloidal Solutions in Qualitative analysis.

During the precipitation of many cations and anions in a form of hydroxides, sulfides, and during conduction of other analytical operations colloidal systems (colloidal solutions or sols) are often formed.

For example, it is often observed the weak turbidness during the addition of AgNO3 in the solution that contains chloride ions, and precipitation isn’t observed despite the exceed of WSAgCl. It is explained by formation of colloidal solution.

Colloidal solutions are not homogeneous systems, but superfine micro heterogenic systems despite the real (molecular) solutions. Though colloidal particles have sizes from 1 to 100 nm and go beyond sizes of molecules and ions in hundred times (< 1 nm), they are visible neither by naked eye nor often by microscopes. But these particles disperse the light and become visible with ultra microscope due to Tyndall effect. Colloidal particles pass through the most dense paper filters. This fact perplexes procedure of analysis or even does it impossible.

The particles of colloids carry the positive charge (Al(OH)3, Fe(OH)3, Cr(OH)3) or negative (H2SiO3, H2Sb(OH)6, CdS, As2S3, AgCl etc.), that it triggers by adsorption of anions solution in first case and cations in second on their surface. These particles attract ions of the opposite charge from the solution and form micelle. The facilitation variant of micelle oxide is:

     

The presence of colloidal particles of charges, that force them repulse each other, perplexes the formation of massive particles that have the ability to precipitate under the gravitation action.

The charged colloids can be connected (rolled) by special methods in more massive aggregates. This process is named as coagulation. Stability of sol can be decreased and transfer it into jelly (precipitations) by addition of electrolytes (coagulators) into solutions. Fugitive acids or ammonia salts are usually used for this purpose in qualitative analysis (because  such coagulators can be deleted during heating, so the don’t cause difficulties in analysis process). The increasing of the temperature promotes the process of coagulation.

During washing and long-term presenting of precipitation in contact with a solution, and also at the action of some chemicals the phenomenon of reverse coagulation – peptizing is observed. Peptizing, that is seen most often during the washing precipitation with clean water, is explained due to a reduction of the ion concentration - coagulants, surrounding sediment particles. For preventing of this negative as for qualitative, and especially the quantitative analysis, precipitations should not be washed with water, but with solutions of appropriate electrolytes. So, precipitation AgCl is washed with diluted nitric acid, NiS and Fe(OH)3 - solution of ammonium chloride, etc.

  1.  Analysis of the Solution with Anions of All Analytical Groups.

Unlike cations, anions, in most cases, do not interfere the determination of each other. Therefore, they, with a few exceptions, are determined by partial method. Firstly, the presence or absence of anions of I and II groups is determined, using group reactives.

Among anions of the I group only SiO32-  interfere the determination of PO43-, so the precipitate of polysilicate acids, that precipitates after the action of NH4Cl, must be separated. Analysis of anions of II group is the characteristic of a systematic analysis and is conducted in the following sequence: the precipitation of silver salts, dissolving in ammonia AgCl and determination Cl- in the filtrate. The determination of  I- and Br- is conducted by the one test using chlorinated water and benzene in the following sequence. Add a bit of benzene to water solution, that is analysed, and dropwise freshly prepared chlorine water. The mixture must be constantly shaken. Under the condition of iodide-ions presence, they will be oxidized firstly:

2I + Сl22 + 2Сl

Iodine is extracted by benzene and coloures it into lilac. In case of continuing of chlorine water addition, iodide is further oxidized:

 

2I- + 5С12 + 6Н2О = 2103 + 12H+ + 10Cl-

  and benzene layer is decolorized. 

Finally, bromide ions become oxidized (as they have less reduction ability).

r- + Сl2 = Вr2 + 2Cl-

 and benzene layer changes its colour to yellow

          So, for determination of vanadium (V) such metod is used: firstly it is reduced: 

VО3- + Fе2+ = VО32-' + Fе3+

Then in excess of the Iron (II) it is oxidized by potassium persulfate (in such conditions doesnt act on Vanadium (IV):

2Fе2+ + S2О82- = 2Fе3+ + 2SO42- 

Finally, vanadium is titrated by potassium permanganate:

5VО32-   + МnО4- + 8е- = 5VО3+ Мn2+ + 4Н2О.

The example of reducers using (as working solution) is potassium thiosulfate in iodometry. 


 

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