Monday, July 20, 2015

Removal of interfering radicals before IIIrd group analysis


What are the interfering radicals? How do they interfere in systematic separation of cationic radicals? Why is it necessary to remove them before IIIrd gr analysis? Why don't they interfere in Ist or IInd group analysis? Interfering radicals are oxalate, tartrate, fluoride, borate and phosphate and they are anionic radicals. They form complex with IIIrd gr group reagent ammonium chloride and ammonium hydroxide. This leads to incomplete precipitation of IIIrd group cations and causes immature precipitation of IVth and Vth group cations in alkaline medium. Let’s try to understand it.
Oxalate, tartrate, fluoride, borate, silicate and phosphate of the metals are soluble in acidic medium. 

If you remember, for 1stand 2nd analysis medium remain acidic (dilute HCl) that’s why they do not interfere then. But for 3rd group analysis the medium becomes alkaline by group reagents ammonium chloride and ammonium sulphide. Here interfering radicals come into action and disturb the solubility product of cations which causes their premature or incomplete precipitation.

In acidic medium these salts produce their corresponding acids like oxalic acid, phosphoric acid, hydrofluoric acid, boric acid and tartaric acid. For example, barium oxalate reacts with HCl and produces oxalic acid.

            BaC2O4 + 2HCl  BaCl2 + H2C2O4

These interfering acids are weak acids so they do not dissociate completely and remain in solution in their unionised form. Equilibrium is developed between dissociated and un-dissociated acid.

            H2C2O4  2H+ + C2O42-

Hydrochloric acid is a strong acid and is ionised completely.

            HCl  H+ + Cl-

Hydrogen ions acts as common ion among them and higher concentration of H+ suppresses the ionization of interfering acid. Therefore, ionic product of C2O42- and Ba2+ doesn’t exceed the solubility product of barium oxalate which is why Ba2+ remains in the solution as barium oxalate. That’s how interfering radicals do not interfere as long as the medium remains acidic enough. But when we make the medium alkaline by adding 3rd group reagent ammonium hydroxide NH4OH, OH- ions combine with H+and neutralise them. This decreases the concentration of H+ ions which shifts the equilibrium of dissociation of interfering acid forward and increases the concentration of C2O42- . Thus the ionic product of C2O42- and Ba2+ exceeds the solubility product of barium oxalate and Ba2+ gets precipitated in the 3rd group, which actually belongs to the 4th group.

One or more interfering radicals can be present in the solution. They have to be removed in the following order: first we remove oxalate and tartrate, then borate and fluoride, then silicate and in the last phosphate.
Removal of Interfering Radicals
Scheme for the Removal of Interfering Radicals

Procedure for the removal of oxalate and tartrate:  Oxalate and tartrate of metals are soluble in acid and they decompose on heating. Take the filtrate of 2nd group and boil off H2S gas from it. Add 4-5ml concentrated nitric acid HNO3 and heat it till it is almost dry. Repeat this treatment for 2-3 times.

            (COO)22- + H+   (COOH)2
                (COOH)2   HCOOH + CO2
            HCOOH  CO + H2O

Tartrate and tartaric acid decomposes in a complex manner; charring takes place on heating and a smell of burnt sugar develops. Extract the with dilute HCl and filter. Use this filtrate for analysis of 3rd group or use for removal of other interfering radicals.

Procedure for the removal of borate and fluoride: Take the filtrate and evaporate it to dryness. Add concentrated HCl and again evaporate to dryness.

            F- + H+  HF
            CaF2 + 2HCl  CaCl2 + 2HF
  
On heating with HCl fluoride forms hydrofluoric acid and Borate forms orthoboric acid which evaporate on heating.

               BO33- + 3H+  H3BO3
            Na3BO3 + 3HCl  3NaCl + H3BO3

Extract the residue with dilute HCl and filter. Use this filtrate for analysis of 3rd group or use for removal of other interfering radicals.

If fluoride is absent and borate is present then residue use a mixture of 5ml ethyl alcohol and 10ml conc. HCl and evaporate to dryness.

BO33- + 3H+  H3BO3
H3BO3 + 3C2H5OH  (C2H5O)3B + H2O

Procedure for the removal of silicate: Evaporate the filtrate of 2nd group or residue obtained from removal of interfering radicals with concentrated HCl to dryness. Repeat this treatment for 3-4 times.
            SiO32- + 2H+  H2SiO3 
               H2SiO3 ↓  SiO2   + H2O

On heating with HCl silicate converts to metasilicic acid (H2SiO3) which is converted into white insoluble powder silica (SiO2) on repetitive heating with concentrated HCl.

Test for phosphate HPO42-:  test 0.5ml of the filtrate with 1ml ammonium molybdate reagent and a few drops of concentrated HNO3, and warm gently, yellow precipitate indicates the presence of phosphate. Its composition is not known exactly.

Procedure for the removal of phosphate: Ferric chloride is generally used for the removal of phosphate. Fe(III) combines with phosphate and removes all phosphate as insoluble FePO4. Fe(III) is also a member of 3rd group so first we have to test its presence in the filtrate of 2nd group then we can proceed for the removal of phosphate.

            HPO42-  + Fe3+  FePO4  + H+

Test for Fe: To the filtrate of 2nd group add ammonium chloride NH4Cl and a slight excess of ammonia NH3 solution. If precipitate appears, it indicates the presence of 3rd group. It may contain hydroxides Fe(OH)3, Cr(OH)3, Al(OH)3, MnO2.xH2O, traces of CaF2 and phosphates of Mg and IIIA, IIIB and IV group metals. Dissolve the precipitate in minimum volume of 2M HCl. Take 0.5ml solution and add potassium hexacyanoferrate (II) K4[Fe(CN)6] solution. If iron is present, you will get prussian blue coloured precipitate of iron(III) hexacyanoferrate.

4Fe3+ + 3[Fe(CN)6]4- ⟶ Fe4[Fe(CN)6]3

Removal of phosphate: To the main solution add 2M ammonia NH3 solution drop wise, with stirring, until a faint permanent precipitate is just obtained. Then add 2-3ml 9M acetic acid and 5ml 6M ammonium acetate solution. Discard any precipitate if obtained at this stage. If the solution is red or brownish red, sufficient iron Fe(III) is present here to combine with phosphate. If the solution is not red or brownish red in colour then add ferric chloride FeCl3 solution drop wise with stirring, until the solution gets a deep brownish red coloured. Dilute the solution to about 150ml with hot water, boil gently for 1-2min, filter hot and wash the residue with a little boiling water. Residue may contain phosphate of Fe, Al and Cr. Keep the filtrate for test of IIIB group. Rinse the residue in porcelain dish with 10ml cold water, add 1-1.5g sodium peroxoborate and boil gently until the evolution of O2 ceases (2-3min). Filter and wash with hot water. Reject the residue to remove phosphate in the form of FePO4. Keep the filtrate and test for IIIA group.

To test the presence of interfering radicals you need to prepare sodium carbonate extract and then test them separately. Scheme for the test of anionic radicals is not as systematic as cationic radicals. We will study them in coming posts. In the next post we will discuss the analysis of IIIA group cations. 

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