Interfering radicals in qualitative analysis

This ppt is for BSC chemistry students

1. INTERFERING IONS AND THEIR ELIMINATIONS IN QUALITATIVE ANALYSIS DR P .RAJEESH ASST.PROFESSOR ,DEPT.OF CHEMISTRY NEHRU ARTS AND SCIENCE COLLEGE KANHANGAD,KASARGOD BY 01-Jul-20

2. Interfering ions in qualitative analysis… Oxalate, tartrate, fluoride, borate, silicate and phosphate of the metals are soluble in acidic medium (first and second group) Interfering radicals are oxalate, tartrate, fluoride, borate ,silicate and phosphate and they are anionic radicals. Which interfere the systematic analysis of cations due to the formation of incomplete precipitations in different groups What are interfering radicals? They form complex with 3 rd 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 Why is it necessary to remove them before 3 rd group analysis? 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

3. Why don't they interfere in Ist  or IInd  group analysis?   In basic medium the interfering radicals come into action and disturb the solubility product of cations which causes their premature or incomplete precipitation. See the explanation…(continue….) Remember………… The group reagent First group ( dil : HCl ) Second group ( dil : HCl +H 2 S ) Acidic 3 rd group reagent ( NH 4 Cl +NH 4 OH ) 4 th group reagent ( NH 4 Cl + NH 4 OH + H 2 S ) 5 th group reagent( NH 4 Cl + NH 4 OH+(NH4) 2 CO 3 ) Alkaline or Basic 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

4. 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 Ba (C 2 O 4 ) +2 HCl H 2 C 2 O 4 + BaCl 2 Barium oxalate Oxalic acid Acidic medium Interfering acids(oxalic acid) are weak acids so they do not dissociate completely Which remain in solution in their unionised form. Equilibrium is developed between dissociated and un-dissociated acid             Hydrochloric acid is a strong acid and is ionised completely . HCl H+ + Cl - 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

5. 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(oxalic acid) Therefore, ionic product of C 2 O4 2 - and Ba2+ doesn’t exceed the solubility product of barium oxalate which is why Ba2+ remains in the solution as barium oxalate. So interfering radicals do not interfere as long as the medium remains acidic.   In Acidic medium ….? 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

6. In Alkaline medium…. 3rd  group reagent ammonium hydroxide NH 4 OH, 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 C 2 O 4 2-  and Ba2+ exceeds the solubility product ( Ksp )of barium oxalate Ba2 + gets precipitated in the 3rd group, which actually belongs to the 5th  group. Reaction shift 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

7. Oxalate tartrate borate fluoride silicate (last) phosphate . One or more interfering radicals can be present in the solution. They have to be removed in the following order 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

8. Interfering ions in qualitative analysis ELIMINATION OF INTERFERING ACID RADICALS The anions (Certain acid radicals) such as oxalate, borate, phosphate etc. Present in the give mixture will interfere the analysis of cation. Hence these are to be eliminated before proceeding for the analysis of the cations . Elimination of oxalates and Tartrate Oxalate is eliminated by dry ignition. The substance is heated in a broken china dish or a crucible for about 15 minutes. The residue containing the carbonate or oxides of the metal is extracted with hot dilute HCl and filtered. The filtrate which contains the chlorides of the metal is used for the analysis of the basic radical. 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

9. 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. 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

10. 2. Elimination of fluorides and borates These are eliminated by evaporating nearly to dryness in a china dish about 1 gram of the solid substance with 5 mL of strong HCl . The process is repeated three times by adding fresh quantities of strong HCl acid each time.The fluoride volatilize as HF and the borate as boric acid, H 3 BO 3 in the stream. The residue is extracted with water and solution is filtered off. The filtrate is used for the analysis of the metal . 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

11. Elimination of phosphate To the boiled solution from the group II (free from H 2 S) a drop of conc.HNO 3 is added and boiled gently. One drop of saturated NH 4 Cl solution and three drops of zirconyl nitrate reagent are added. It is digested for two minutes and centrifuged. The process is repeated till all the phosphate has been precipitated. The residue of zirconium phosphate is rejected. Now five drops of saturated NH 4 Cl and slight excess of ammonium hydroxide are added to the centrifuge. Boiled again and centrifuged . 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

12. Chromate. Chromate being a strong oxidising agent, oxidises the hydrogen sulphide in group II to sulphur.   2K 2 Cr 2 O 7 + 10HCl + 3H 2 S → 4KCI + 2CrCl 3 + 8H 2 0 + 3S   Hence the group II metals will not be precipitated by hydrogen sulphide, till all the chromate is reduced to chromic state, causing unnecessary delay and waste of hydrogen sulphide. Besides, much sulphur will be precipitated giving trouble in group II analysis. Therefore, chromate should be eliminated before proceeding to group II . About 300 mg of the mixture is taken in a china dish and boiled with about 2 ml of pure con centrated hydrochloric acid. The solution is evaporated nearly to dryness. The process is repeated thrice, adding hydrochloric acid each time. The residue is extracted with 3 ml of water and centrifuged. 2Ca CrO 4 + 16HCI → 2CaCl 2 + 2CrCl 3 + 8H 2 0 + 3Cl 2 Chromate 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

13. In group II , when hydrogen sulphide is passed through a hydrochloric acid solution of an arsenate, white precipitate of sulphur is first obtained, and only on continued passing of hydrogen sulphide does arsenic sulphide gets precipitated . 2H3AsO4 + 3H2S → 2H3AsO3 + H2O +S 2H 3 AsO 3 + 3H 2 S → As 2 S 3 + 6H 2 O Therefore, arsenates or arsenites , if present, must be completely eliminated as arsenic sulphide by continued passing of hydrogen sulphide in group II till no further precipitation takes place. Otherwise it will get precipitated in other groups subsequently. Arsenate and Arsenite 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

14. Elimination procedure Elimination of Arsenite. The solution after the separation of group I cations is warmed slightly and then saturated with hydrogen sulphide. The precipitate is removed by centrifugation. The centrifugate is transferred into another tube , warmed and again saturated with hydrogen sulphide. The process is repeated till no more precipitate is obtained. The centrifugate is used for analysis in the third and subsequent groups. The combined residue is analyzed for group II cations; arsenic will be an additional cation in group II B . 2H 3 AsO 3 + 3H 2 S As 2 S 3 + 6H 2 Elimination of Arsenate. The centrifugate from group I is boiled with a small quantity of ammonium iodide till the violet vapours of iodine cease to be evolved. Arsenate gets reduced to arsenite. The arsenite thus obtained is eliminated as described under arsenite . H 3 As 2 O 4 + 2H + + 2I - H 2 AsO 3 + H 2 0 + I 2 2H 3 AsO 3 + 3H 2 S As 2 S 3 + 6H 2 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD

15. THANK YOU 01-Jul-20 DEPT OF POLYMER CHEMISTRY NASC KANHANGAD