Qualitative Analysis : Soda Extracts Preparation

 

PREPARATION OF SOLUTION FOR THE EXAMINATION OF ANIONS

(i)     Preparation of water extract: As all common nitrites, nitrites, thiosulphates and acetates are soluble in water, the confirmatory tests for such anions can be performed with water extract of the mixture.

To prepare water extract, small amount of mixture is boiled with distilled water in boiling tube and filtrate is used as “water extract”.

(ii)    Preparation of sodium carbonate extract: When the mixture under examination is partially or completely insoluble in water, sodium carbonate extract is prepared. To prepare soda extract, mixture/salt is mixed with pure Na2CO3 in 1 : 2 ratio and sufficient amount of water is added. Whole content is boiled for 15-20 minutes and is filtered. Filtrate is used as soda extract.

                   MA2 + Na2CO3 → MCO3 + 2NaA

                   Ca3(PO4)2 + 3Na2CO3 → 3CaCO3 + 2Na3PO4

                   CaC2O4 + Na2CO3 → CaCO3 + Na2C2O4

                                                          Water soluble    

                   PbSO4 + Na2CO3 → PbCO3 ↓ + Na2SO4

                                                          Water soluble   

 

Advantages of Preparing Sodium Carbonate Extract

(a)     Some cations interfere in the confirmatory test of some anions when present in solution. Such cations are eliminated in the form of insoluble carbonates as a result of treatment with sodium carbonate.

(b)     It becomes easier to identify anions in such mixture which are insoluble in water and dil. acids.

 

NOTE:

  1. A few salts such as phosphate of iron and aluminium and sulphides of Zn, Ca, Sb and a few fluorides are not decomposed on boiling with sodium carbonate solution. Hence if fluoride, phosphate and sulphide are not present in the “sodium carbonate extract”, their confirmatory tests must be performed with the residue.         
  2. Some cations such as copper, arsenic and antimony form soluble complexes with sodium carbonate and hence these cations partly go into the sodium carbonate extract. These should be preferably removed by acidifying extract with dil, HCl and passing H2S gas. Excess of H2S can be eliminated by boiling and then the solution can be tested for various anions except Cl– and S2–
  3. Pure sodium carbonate should be sued. “Analar” or A.R. grade sodium carbonate is highly satisfactory. Ordinary sodium carbonate contains chloride and sulphate as impurities and responds to tests for these anions. As a consequence, misleading results are encountered.
  4. If ammonia is given out on boiling the mixture with sodium carbonate solution, the former must be expelled completely because it tends to complex copper, cadmium, nickel etc. into the soluble amine which goes into the filtrate.
  5. Some of the cations like As3+, Sb3+, Sn2+ etc. partly go into the filtrate due to the amphoteric nature of the parent metals. These should be completely removed by acidifying the “sodium carbonate extract” with pure dil. HCl and passing H2S gas, reject the precipitate of their sulphides. Expel H2S from the filtrate by boiling and use it for the detection of anions.

 

Sometimes tartrate and oxalate form soluble complexes with certain cations like copper, iron, chromium and aluminium and these go into the sodium carbonate extract. Remove copper in the above mentioned manner. Boil off H2S from the filtrate and render the same ammonical. A precipitate is formed due to the hydroxides of the iron, chromium or aluminium. Reject the ppt. and expel ammonia from the filtrate by boiling. Use the resulting solution for the detection of anions. 

 

1. Identification of Carbonate and Bicarbonate

Solubility: (i) Carbonate of Na, K and NH4+ are water soluble and all other are insoluble.

          (ii) All bicarbonates are water soluble.

Take small amount of the mixture in a test tube about 4-5 mL of distilled water. Warm and filter and divide the filtrate into two part.

Reactions:

(i)     Na2CO3 + 2HCl → 2NaCl + CO2 ↑­ + H2O

          Ca(OH)2 + CO2 → CaCO3 ↓ + H2O

(ii)    NaHCO3 + HCl → NaCl + H2O + CO2­ ↑

(iii)   CO32– + Mg2+ → MgCO3 ↓

                             White ppt.                            

          Mg2+ + 2HCO3– → Mg(HCO3)2

                             White soluble

          Mg(HCO3)2  \underrightarrow { \quad \Delta \quad } MgCO3 + H2O + CO2 ↑

                                       White ppt.

Test of bicarbonate in presence of a carbonate: Take 0.5 g of the mixture in a test tube containing about 4-5 mL of distilled water. Warm and filter. To this filtrate, add excess of calcium chloride solution when a white ppt. of CaCO3 is formed indicating the presence of carbonate. Calcium bicarbonate formed is soluble in water.

                   Ca2+ + CO32– → CaCO3

                                      White ppt.

                   Ca2+ + 2HCO3– → Ca(HCO3)

                                         Soluble

Filter the white ppt. and add dilute ammonia solution to the clear filtrate when soluble calcium bicarbonate is converted into CaCO3 and white turbidity appears confirming the presence of a bicarbonate in the test solution.

                   Ca(HCO3)2 + 2NH3 → (NH4)2CO3 + CaCO3

                                                              White ppt.

2. Identification of Sulphide

Solubility: (i) Alkali metals sulphides are water soluble while alkali earth metals sulphides are partially soluble in water. All other sulphides are water insoluble.

(ii)    Nitroprusside test: Add 1-2 mL of freshly prepared sodium nitroprusside solution to 2-3 mL of sodium carbonate extract. A purple or violet colour confirms sulphide.

                   Na2S + Na2[Fe(CN)5NO]2 → Na4[Fe(CN)5NOS]

(iii)   Lead acetate test: Acidify 1-2 mL of the sodium carbonate extract with dil. CH3COOH and boil the solution to expel CO2. To this add 1-2 mL of lead acetate solution. If a black ppt. appears, it confirms sulphide.

                   Pb(CH3COO)2 + Na2S →  PbS  +  2CH3COONa

                                                Black ppt.

(iv)   Cadmium carbonate test: Add 2mL of “sodium carbonate extract” to 0.2 of cadmium carbonate in a test tube. Shake the test tube. Appearance of yellow ppt. confirms the presence of sulphide.

                   Na2S + CdCO3 → CdS↓ + Na2CO3

                                                 Yellow ppt.

 

3. Identification of Sulphite

Solubility: (A) Alkali metal and NH4+) sulphites are water soluble while other are practically insoluble in water.

          (B) All the sulphites are soluble in dilute HCl.

(i)     BaCl2 test: In 5 mL soda extract add excess of BaCl2 solution. If a white ppt. appears, it may be due to sulphite, sulphate or excess of sodium carbonate present in the solution.

                   Na2SO3 + BaCl2 → BaSO3↓ + 2NaCl

                   Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl

                   Na2CO3 + BaCl2 → BaCO3↓ + 2NaCl

          Filter the ppt. and divide the ppt. into four parts:

(a) To part I and dil. HCl. If there is evolution of SO2 which turns acidified dichromate paper green. It confirms sulphite.

                   BaSO3 + 2HCl → BaCl2 + H2O + SO2­ ↑        

                   K2Cr2O7 + H2SO4 + 3SO2 → Cr2(SO4)3 + K2SO4 + H2O

                                                        Green

(b) To part II add a few drops of potassium permanganate solution followed by the addition of 3 mL of dil. H2SO4 acid. If the colour of permanganate gets discharged, the presence of sulphite is confirmed.

                   BaSO3 + H2SO4 → BaSO4 + SO2 + H2O

                   2KMnO4 + 2H2O + 5SO2 → K2SO4 + 2MnSO4 + 2H2SO4

(c) To part III add bromine water. On warming this solution, the colour of bromine gets discharged and a white ppt, insoluble in acids is obtained.

                             BaSO3 + Br2 + H2O → BaSO4↓ + 2HBr

                                                         White ppt.

(d) To part IV, and I2 solution. If colour of iodine is decolourised, sulphite is confirmed.

                             BaSO3 + I2 + H2O → BaSO4↓ + 2HI

                                                       White ppt.

 

(ii)    Sodium nitroprusside test: Take a few drops pf “sodium carbonate extract” in a test tube. To this add 2 drops of 1% sodium nitroprusside solution and one drop of saturated zinc sulphate solution. Appearance of a violet colour confirms the presence of sulphite.

          Na2[Fe(CN)5NO] + Na2SO3  \underrightarrow { \quad ZnS{ O }_{ 4 }\quad }   Na4[Fe(CN)5NO(SO3)]

                                                                        (Rose red colour)

NOTE:

Sodium nitroprusside test must not be performed if S2– is also present in the mixture.

4. Identification of Nitrite

Solubility: All nitrites (except AgNO2) are freely soluble in water.

(i)     Starch-iodide test: Acidify 1-2 mL of the “sodium carbonate extract” with dil. Acetic or dil. Sulphuric acid and boil the solution to expel CO2. Then add a crystal of KI and 1 mL of starcj solution. Also, add a few drops of dil. H2SO4. A deep blue coloration confirms nitrite.

          2NaNO2 + 2KI + 2H2SO4 → Na2SO3 + K2SO4 + I2 + 2NO + 2H2O

                             I2 + Starch → Blue coloured complex

(ii)    Thiourea test: Acidify 2-3 mL of soda extract with dilute acid. Boil off CO2. To this add a pinch of thiourea and a few drops of dil HCl and ferric chloride solution. A characteristic blood red colouration indicates the presence of nitrite.      

                   NaNO2­ + H2NCSNH2 → N2 ↑­ + NaCNS + 2H2O

                   3NaCNS + FeCl3 → Fe(CNS)3 + 3NaCl

                                                    (Blood red colour)

 

NOTE:

As thiocyanates and iodides interfere in this test, these must be removed by precipitation with silver nitrate solution before doing thiourea test for nitrate.

(iii)   Brown ring test: Acidify Na2CO3 extract with dil. acetic acid or dil. sulphuric acid. Add this solution carefully to a concentrated solution of ferrous sulphate acidified with acetic acid or dil. sulphuric acid. If a brown ring due to the compound [Fe, NO] SO4­ is formed at the junction of the two liquids, it confirms the presence of nitrite.

                    3NO2– + 2H+ → NO3– + 2NO + H2O

                   FeSO4 + NO → [FeNO]SO4

                                                (Brown ring)

NOTE:

Nitrate gives a corresponding reaction only when conc. sulphuric acid is added in place of dil. sulphuric acid.

 

(iv)   m-phenylenediamine test: Acidify 1-2 mL of the sodium carbonate extract with dil. acetic and boil to expel CO2. To this add a few drops of m-phenylenediamine hydrochloride solution followed by a few drops of hydrochloric acid, brown ppt. confirms nitrite.               

                             NO2 + H+ → HNO2

   

         

(v)    CoCl2 test: Concentrated CoCl2 solution is added in soda extract acidified with acetic acid. Add KCl and then warm the solution. After sometime formation of yellow ppt. indicates NO2–.

                   CoCl2 + 2NaNO2 → Co(NO2)2 + 2NaCl

                   NaNO2 + CH3COOH → CH3COONa + HNO2

                   2HNO2 → H2O + 2NO + [O]

                   2Co(NO2)2 + 2NaNO2 + H2O + [O] → 2Co(NO2)3 + 2NaOH

                   Co(NO2)3 + 3NaNO2 → Na3[Co(NO2)6]

                                                Colourless and water soluble

                   Na3[Co(NO2)6] + 3KCl →  K3[Co(NO2)6] +3NaCl

                                                   Potassium cobaltinitrate

                                                  (yellow and water insoluble)

(v)    CoCl2 test: Concentrated CoCl2 solution is added in soda extract acidified with acetic acid. Add KCl and then warm the solution. After sometime formation of yellow ppt. indicates

                   CoCl2 + 2NaNO2 → Co(NO2)2 + 2NaCl

                    NaNO2 + CH3COOH → CH3COONa + HNO2

                   2HNO2 → H2O + 2NO + [O]

                    2Co(NO2)3 + 2NaNO2 + H2O + [O] → 2Co(NO2)3 + 2NaOH

                    Co(NO2)3 +  3NaNO2 → Na3[Co(NO2)6]

                                      Colourless and water soluble

                    Na3[Co(NO2)6] + 3KCl →  K3[Co(NO2)6]    + 3NaCl

                                      Potassium cobaltinitrate

                                      (yellow and water insoluble)

 

5. Identification of Acetate

Solubility: All acetates except silver salts are readily soluble in water.

(i) Ester test: Heat a pinch of dry mixture with a few drops of conc. H2SO4 and 0.5 mL of ethyl alcohol. A pleasant per-like fruity smell shows the presence of acetate.

          2CH3COONa + H2SO4 → Na2SO4 + 2CH3COOH

          CH3COOH + C2H5OH \xrightleftharpoons[\text{}]{\text{}}  CH3COOC2H5 + H2O

                                                       (Fruity odour)

(ii)    Ferric chlorides test: To 1 mL of sodium carbonate extract neutralized with dil. acetic acid, add neutral ferric chloride solution. Appearance of red colour may be due to the presence of acetate. Now dilute and boil the liquid. A dark-red precipitate of basic ferric acetate is formed.

          3CH3COONa + FeCl3 → (CH3COO)­3Fe + 3NaCl

                                      Ferric acetate

                                      (red colour)

          (CH3COO)3Fe + 2H2O \underrightarrow { \quad Boil\quad } Fe(OH)2(O.COCH3) + 2CH3COOH                                           

Basic ferric acetate

                                         (Brown ppt.)

NOTE:

(a)    Thiocyanate if present in the mixture interferes in the test as it also gives red coloration with ferric chloride solution.    

(b)    FeCl3 solution is acidic, due to the hydrolysis which results in the formation of free HCl acid. In order to prepare a neutral solution of ferric chloride, take 5 mL of bench FeCl3 solution. To this add dilute ammonia solution dropwise till a slight ppt, persists. Then boil till a clear reddish brown solution of natural ferric chloride is obtained.

 

(iii)   Cocodyl test: On heating a pinch of the dry mixture with an equal amount of arsenic oxide in a test tube, an extremely unpleasant smell of cocodyl oxide is obtained which confirms acetate.  

4CH3COOK + As2O2 → (CH3)2AsO(CH3)2As + 2K2CO3 + 2CO2

                             Cocodyl oxide

(iv)   Lanthanum nitrated test: This is a very sensitive test for acetate. Take 0.5 mL of 6% lanthanum nitrate solution followed by the addition of 0.5 mL of iodine solution in KI and a few drops of dilute ammonia solution and heat to boiling. A blue colour is obtained. This confirms acetate.

 

6. Identification of Nitrate

          Solubility: All nitrates are water soluble.

          (a) Ring test: See page number 817.

(b) The Lunge test or Diphenylamine test: To the water extract or sodium carbonate extract neutralized with dil. H2SO4 acid, add a few mL of the diphenylamine reagent in a test tube. If a blue ring is formed at the zone of contact of the two liquids, it confirms nitrate.

 

NOTE:

  1. This reaction is also given by many other oxidizing agents such as nitrites chromates, chlorates, permanganate and ferric salts.
  2. Diphenylamine reagent is prepared by dissolving 0.4 g of diphenylamine in 80 mL of conc. Sulphuric acid followed by the addition of 20 mL of water.

 

(c)     Zinc or aluminium and sodium hydroxide test: Take about 2 mL of sodium carbonate extract and add 0.5 g of sodium hydroxide. Boil the solution for few second. Cool and add a pinch of aluminium powder or zinc dust. Heat the contents of the test tube and pass the vapour in the Nessler’s reagent. If a brown ppt. is formed, it confirms nitrate. The formation of brown ppt. is due to the action of ammonia produced during the reaction of Nessler’s reagent.

                   Zn + 2NaOH → Na2ZnO2 + 2[H]

                    Al + NaOH + H2O → NaAIO2 + 3[H]

                   NaNO3 + 8[H] → NaOH + 2H2O + NH3

                    NH3 + Nessler’s reagent → Brown ppt.

 

NOTE:

This test is interfered by nitrites, cyanides, thiocyanates, ferrocyanides and ferricyanides because these also evolve ammonia under the above conditions. Nitrite is removed by boiling the sodium carbonate extract with ammonia chloride whereas all other nitrogenous anions are removed by warming the sodium carbonate extract with silver sulphate to about 600C and shaking vigorously for about 4-5 minutes when the silver salts of these anions are precipitated which are removed by filtration. The excess of silver is removed from the filtrate by adding NaOH solution and filtering the black ppt. of silver oxide. In the filtrate nitrate is tested as above.

 

7. Identification of Chloride

Solubility: (i) All chlorides are soluble in water, except the silver, Mercurous and cuprous salts. Lead chloride is sparingly soluble in cold water readily dissolves in hot water.

(ii) Chlorides of antimony, bismuth and tin salts are hydrolyzed by water.

(a)    Silver nitrate test: Acidify 2-3 mL of “Soda extract” with dil. HNO3 and add AgNO3 solution, if a white ppt. soluble in ammonia but insoluble in HNO3 is obtained, it confirms chloride.

                   AgNO3 + NaCl → AgCl ↓ + NaNO3

                   AgCl + 2NH3 → [Ag(NH3)2]Cl

                   White ppt.           Soluble

(b)    Sodium arsenite test: Acidify 2-3 mL of “Soda extract” with dil HNO3 and boil the solution to expel CO2. Now add AgNO3 solution. If a white ppt. is obtained then chloride may be present. Filter the ppt. and treat this ppt. with sodium arsenite solution. The formation of yellow ppt. confirms chloride.

                   Na3AsO3 + 3AgCl → Ag3AsO3 + 3NaCl

                                                      Yellow ppt.

 

NOTE;

This test serves as a distinctive test for chloride because silver bromide and silver iodide are not affected by sodium arsenite solution.

 

8. Identification of Bromide

Solubility: (i) All bromides (except those of Ag, Hg22+, Cu22+) are water soluble.         

(ii) Except AgBr, other bromides dissolve in dil. or conc. HCl or HNO3.

(a)    Silver nitrate test: Acidity 2-3 mL of “sodium carbonate extract” with dil HNO3 and add AgNO3 solution. The formation of light yellow ppt. which is sparingly soluble in ammonia solution confirms bromide.

                   NaBr + AgNO3 → AgBr ↓  + NaNO3

                                      (Light yellow ppt.)

(b)    Carbon disulphide test: Acidify 2 mL of “Soda extract” with HCl acid and add 2 mL of CS2 or CCl4 or CHCl3, followed by addition of chlorine water. The brown of organic layer confirms Br

                   2NaBr + Cl2 → 2NaCl + Br2

                                      Brown colour

 

9. Identification of Iodide

Solubility: (i) All iodides (except AgI, Hg2I2, PbI2, HgI2) are water soluble.

(ii) Iodides of Ag, Hg, Cu & Pb are much less soluble than corresponding chlorides or bromides.

(a)    Silver nitrate test: Neutralize 2 mL of “sodium carbonate extract” with dil. HNO3 and boil off CO2. Then add AgNO3 solution. The formation of pale yellow ppt, insoluble in ammonia solution confirms iodide.

                   Ag+ + I– → AgI

                   (Pale yellow ppt.)

(b)    Carbon disulphide test: Same as for bromide and organic layer turns violet which disappears in excess of chlorine water.

                   2NaI + Cl2 → 2NaCl + I2

                   I2 + CS2 → Violet colour

                    5Cl2 + I2 + 6H2O → 2HIO3 + 10HCl

 

10. Identification of Fluoride

Solubility: Fluorides of alkali metals, Ag, AI and SnF2 are water soluble while other fluorides are insoluble in water.

(a)    Calcium chloride test: Acidify 2-3 mL of “soda extract” with very dil. acetic acid and add CaCl2 solution. A white ppt. is formed, add dil. H2SO4 acid. It this ppt. does not dissolve, then fluoride is confirmed.

                             2NaF + → CaF2↓ + 2NaCl

                                                (White ppt.)

 

11. Identification of Oxalate

Solubility: Alkali metal and magnesium oxalate are water soluble while others are insoluble in water but soluble in dil. HCl.

(a)    Calcium chlorides test: Acidify 2-3 mL “Soda extract” with dil. acetic acid and add CaCl2 solution. A white ppt. is obtained. Dissolve it in minimum quantity of dil. H2SO4 acid and add potassium permanganate solution dropwise. If purple colour gets discharged, oxalate is confirm.

                   Na2C2O4 + CaCl2 → CaC2O4↓ + 2NaCl

                                           (White ppt.)

                   CaC2O4 + H2SO4 → CaSO4 + H2C2O4

2KMnO4 + 3H2SO4 + 5H2C2O4 → 2MnSO4 + 8H2O + 10CO2­ ↑+ K2SO4

(b)    Ferrous sulphate test: Acidify “soda extract” with acetic acid and add ferrous sulphate solution. A yellow precipitate confirms oxalate.

                   2Na2C2O4 + FeSO4 → Na2[Fe(C2O4)2] + Na2SO4

                                      (Yellow)

 

12. Identification of Sulphate

Solubility: (i) Sulphates of alkali metals are soluble in water.

(ii) PbSO4 and BaSO4 are practically insoluble in water. Strontium sulphate is very slightly soluble.

(a)    BaCl2 test: Acidify 2 mL of “soda extract” with dil. HCl acid and add BaCl2 solution. A white ppt. (insoluble in conc. HCl or conc. HNO3) confirms sulphate.

                             Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl

                                                  (white ppt.)

(b)    Sodium rhodizonate test: Take 1-2 mL of barium chloride solution in a test tube. To this add 0.5 mL of freshly prepared 0.1% aqueous rhodizonate solution. A red colour in the test tube is formed. To this add 1-2 mL of “sodium carbonate extract”. If the red colour disappears, it identifies sulphate.

 

13. Identification of Phosphate

Solubility: Phosphates of alkali metals alkaline earths metals are soluble in water. While those of ammonium and lithium are insoluble.

(a)    Ammonium molybdate test: Acidify 2-3 mL of “soda extract” with dil. nitric acid and add 2-3 mL of conc. nitric acid followed by the addition of 5 mL of ammonium molybdate solution. Again, add a mixture of conc. nitric acid, 2 mL tartaric acid (1 gm) and ammonium molybdate solution (3 mL). A slow formation of canary yellow ppt. confirms the presence of phosphate.

                   Ca3(PO4)2 + 6HNO3 → 3Ca(NO3)2 + 2H3PO4

H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)2PO4.12MoO3↓ + 21NH4NO3 + 12H2O

 

NOTE:

The idea of adding tartrate is to make the arsenate and arsenite if they are present in the mixture. Thus, the presence of tartaric acid does not prevent phosphate from responding to this test.

(b)    Magnesia mixture test: To 2-3 mL of “soda extract” and add 2-4 mL of magnesia mixture. A white crystalline ppt. is formed which may be due to arsenate and phosphate. Treat the ppt. with silver nitrate solution containing a few drops of acetic acid. If the ppt. turns yellow, it is due to phosphate whilst the arsenate assumes a brown red colour.

                   Na2HPO4 + Mg(NO3)2 + NH3 → Mg(NH4)PO4 + 2NaNO3

 

Qualitative Analysis : ANALYSIS OF BASIC RADICALS Introduction