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 III^rd gr analysis? Why don't they interfere in I^st or II^nd group analysis? Interfering radicals are oxalate, tartrate, fluoride, borate and phosphate and they are anionic radicals. They form complex with III^rd gr group reagent ammonium chloride and ammonium hydroxide. This leads to incomplete precipitation of III^rd group cations and causes immature precipitation of IV^th and V^th 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 1^stand 2^nd analysis medium remain acidic (dilute HCl) that’s why they do not interfere then. But for 3^rd 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⁺ + C₂O₄^2-

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 C₂O₄^2- 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 3^rd 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 C₂O₄^2- . Thus the ionic product of C₂O₄^2- and Ba²⁺ exceeds the solubility product of barium oxalate and Ba²⁺ gets precipitated in the 3^rd group, which actually belongs to the 4^th 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.

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 2^nd 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)₂^2- + 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 3^rd 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.

               BO₃^3- + 3H⁺ ⟶ H₃BO₃

            Na₃BO₃ + 3HCl ⟶ 3NaCl + H₃BO₃

Extract the residue with dilute HCl and filter. Use this filtrate for analysis of 3^rd 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.

BO₃^3- + 3H⁺ ⟶ H₃BO₃

H₃BO₃ + 3C₂H₅OH ⟶ (C₂H₅O)₃B↑ + H₂O

Procedure for the removal of silicate: Evaporate the filtrate of 2^nd group or residue obtained from removal of interfering radicals with concentrated HCl to dryness. Repeat this treatment for 3-4 times.

            SiO₃^2- + 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 HPO₄^2-:  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 3^rd group so first we have to test its presence in the filtrate of 2^nd group then we can proceed for the removal of phosphate.

            HPO₄^2-  + Fe³⁺ ⟶ FePO₄ ↓ + H⁺

Test for Fe: To the filtrate of 2^nd group add ammonium chloride NH₄Cl and a slight excess of ammonia NH₃ solution. If precipitate appears, it indicates the presence of 3^rd 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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