qualitative inorganic analysis
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DESCRIPTIONQualitative Inorganic Analysis. Analytical chemistry could be divided into three main parts; qualitative, quantitative and applied. Qualitative analysis deals with detection and identification of different substances singly or in a mixture. - PowerPoint PPT Presentation
Qualitative Inorganic Analysis
Analytical chemistry could be divided into three main parts; qualitative, quantitative and applied.Qualitative analysis deals with detection and identification of different substances singly or in a mixture. This part deals with the qualitative analysis of anions, which could be defined as the negatively charged fragments of salt or compound. Alternatively anions refer to acid radical.An example is Nacl:
NaCl Na + + Cl- Cation Anion Basic radical Acid radical
Anions are divided into six groups:
1- Carbonates and Bicarbonates group
2- Sulphur-containing anions
4- Cyanogen anions
5- Arsinic and phosphorous containing anions
6- Nitrogen- containing anions
Carbonates and Bicarbonates group CO32- HCO3-I. General characters 1- Parent acid:Carbonic acid (H2CO3) is a very weak volatile acid (stronger than HCN and boric acid) Heating of solution of H2CO3, CO2 will evolve. H2CO3 CO2 + H2O Bicarbonates are considered to be the first step of ionization of carbonic acid, while in the second step carbonates are formed H2 CO3 H+ + HCO3- H+ + CO32-
2-Solubility: All carbonated with the exception of those of the alkali metals (Na+ and K+) and of ammonium are insoluble in water. All bicarbonates are soluble in water. II. General Reactions1- Dry Reactionsa- Action of dilute HClDecomposition with effervescence due to the evolutionof CO2 gas, for both CO3 -- and HCO3-
CO3-- + 2H+ CO2 + H2OThis is a type of displacement reaction in which stronger acid liberates the very weak carbonic acid, which spontaneously decomposes to CO2 & H2O.NaHCO3+ H+ CO2 + H2O + Na+
Test for CO2 gas: The solid substance is placed in a test tube, dilute HCl is added, which immediately displaced the gas, which is evolved(upon warming) and passed into lime water or baryta water contained in another test tube.The production of a turbidity indicates the presence of carbonates or bicarbonates.CO2 + Ca(OH)2 CaCO3 + H2OCO2 + Ba (OH)2 BaCO3 + H2O
With prolonged passage of CO2, the turbidity formed due to the insoluble carbonates, slowly disappears as a result of the formation of a soluble bicarbonate. CaCO3 + CO2 + H2O Ca (HCO3)2 Boiling
2- Wet ReactionsIn order to carry out the wet reactions, a solution of the substance in water must be done.Bicarbonates are mostly decomposed on heating with the liberation of CO2.
2HCO3- CO3-- + H2O + CO2 .a- Reaction with AgNO3A white precipitate of silver carbonate is immediately formed.CO3 -- +2Ag+ Ag2CO3 The precipitate is soluble in mineral acids (nitric acid) and in ammonia.Ag2CO3 + 2H+ 2 Ag+ + CO2 + H2OAg2CO3+4NH3 2[Ag (NH3)2]+ + CO32-
The precipitate becomes yellow or brown if the mixture is boiled.
Ag2CO3 Ag2O +CO2 boiling
b- Reactions with BaCl2, CaCl2 and MgSO4:White precipitates of BaCO3, CaCO3 and MgCO3 will be obtained upon the addition of these reagents to samples of carbonate solution. BaCl2 + NaCO3 BaCO3 + 2 NaCl Ca++ + CO3 -- CaCO3Mg++ + CO3 -- MgCO3The precipitate is soluble in mineral acids
For HCO3- ; No ppt. on cold since all bicarbonates are soluble in water
Ba++ +2HCO3- Ba(HCO3)2 Soluble
H2O + CO2 + BaCO3 Boiling
III. Mixture of CO32- & HCO-3Both anions haves similar reactions, but CO32- form precipitates immediately on cold upon the addition of CaCl2, BaCl2 or MgSO4, while the bicarbonates of these metals are soluble.
Separation:Add excess CaCl2 (BaCl2 or MgSO4) to a solution of the mixture
CO32- /HCO3- a white ppt. indicates CO3-- , centrifuge or filter Contrifugate White ppt. May be HCO3- CaCO32- H+
CO2 + H2OConfirmatory test: 1) Boil 2) Add ammonia solution white ppt.Ca (HCO3)2 + 2 NH3 CaCO3+ (NH4)2 CO3
Sulphur-containing anionsThis group of anions, are;1- Sulphide (S2-) 2- Sulphites (SO32-)3- Thiosulphate (S2O32-)4- Sulphates (SO42-) 5- Perasulphate (S2O82-). I. General characters1- Parent Acids:a- Hydrogren sulphide or Hydrosulphuric acid (H2S) It is a gas with offensive rotten egg odour and poisonous. In solution it gives a weak acid, which ionizes in two steps; H2S H++ HS- (hydrosulphide ion)HS- H++ S-- (sulphide ion)
Both HS- and S-- ions give the same reactions.
b- Sulphurous acid:(H2SO3)This acid is only known in solution (like H2CO3). It has moderate strong acidity. Like H2CO3 in water; present in equilibrium as follows:
H2SO3H++ HSO3-H++ SO3-- H2O + SO2heatAcid sulphitec- Thiosulphuric acid: (H2S2O3)It is not known in the free form, and decomposes to give, H2O, SO2 and S. It's more stronger than sulphurous acid in solutions.It consists of SO32- solution and S, which upon boiling gives S2O32-. d- Sulphuric acid: (H2SO4):It's a colourless oily liquiud (B.P. 3300C). General properties of H2SO41- Acid properties; It is one of the strongest acids, ionize in dilute solutions in two steps, H2SO4 H++ HSO4- (hydrogen sulphate)
HSO4- H++ SO4-- (sulphate)
Metals can liberate hydrogen from H2SO4 solution.
H2SO4+ Zno ZnSO4+ H2 Being a strong acid can replace weak acids like, boric acids, hydrocyanic acid and volatile acids or their decomposition products due to its high B.P.
2NaCl + H2SO4 Na2SO4+ 2HCl 2- Dehydrating properties; Conc. H2SO4 has a great tendency to combine with water to from stable hydrates H2SO4.x H2O. So it is used as a dehydrating agent for certain substance, and used mostly in the dissectors. It causes charring for certain organic substances as sugars due to the vigorous abstracting of water from theses substances. 3- Oxidizing properties: It's considered to be as moderately strong oxidizing agent when heated with most reducing agents
H2SO4 H2O + SO2 + [O]
It is reduced to SO2, while with active reducing agents it may be reduced to So or H2S. heat
2-Solubility:All Na+, K+ and NH4+ salts of sulphur containing anions are soluble in water.
Sulphides : Other sulphides are in-soluble except those of Ca++, Ba++, & Sr2+ dissolve due to hydrolysis.
Sulphites: Other sulphites are all in-soluble. Thiosulphates: Most S2O32-are soluble, Ag+, Pb++, Hg2+ & Ba++ salts are slightly soluble. Sulphates: All sulphates are soluble except Pb++, Ba++ and Sr++. Ca++ & Mg++ salts are slightly soluble.
3-Complexing agent: Thiosulphate form complex with Fe3+ Fe3++ 2S2O3-- (Fe(S2O3)2)- purple color4-Reducing agent:Sulphides, sulphites and thiosulphates are reducing agents. They reduce solutions of I2, KMnO4 and K2Cr2O7 with varying activities in acidified solutions.I2+S2- 2I-+So lodine (brown) Colourless H+2KMnO4+ 5S2-+ 16H+ 2Mn+++ 5SO4--+ 8H2O +2K+
I2+SO32-+H2O SO42-+2I-+2H+2 MnO4-+ 5 SO3--+ 6H+ 2Mn+++ 5SO4--+ 3H2OCr2O7--+ 3SO32-+ 8H+ 2Cr3++ 3SO4--+4H2OI2+2S2O3-- H+ S4O62-+2I- Tetrathionate Fe3++2S2O32- S4O62-+Fe2+ H+8MnO4-+ 5 S2O3--+ 14H+ 8Mn+++10SO4--+7H2O4Cr2O72-+ 3S2O32-+ 26H+ 8 Cr3++6SO4--+ 13 H2O
II. General Reactions1- Dry Reactionsa- Action of dilute HClSulphide; S2-H2S gas; evolved upon adding dil. HCl to a solid sample. The gas evolved has its characteristic rotten egg odour, and could be identified by 1- blackening of filter paper moistened with lead acetate sol.
S-- + 2H+ H2S
2- alternatively, a filter paper moistened with cadmium acetate solution, turns yellowH2S + Cd++ CdS Yellow
H2S has reducing character, It reacts with l2 solution, acid KMnO4, acid K2Cr2O7 H2S+Pb++ PbS black
It bleaches the brown color of l2 solution, changes the pink color of acid KMnO4 into colorless and changes the orange color of acid K2Cr2O7 into green.
H2S + l2 2l- + 2H+ +So
5H2S + 6H+ +2 MnO4- 2Mn++ + 8H2O + 5So
3H2S + 8H+ + Cr2O7-- 2Cr3+ + 7H2O + 3So 2- Sulphite: SO32-Upon treatment of SO3-- with dil. HCl, SO2 gas will evolve, due to the decomposition of the liberated unstable H2SO3SO--3 + 2H+ H2SO3 SO2 +H2OThe evolved SO2 gas has a characteristic bunt sulphur odor and turbid lime water (like CO2) due to the formation of the insoluble CaSO3 which is soluble upon prolonged passage of SO2 due to the formation of soluble calcium bisulphite.
Ca (OH)2 +SO2 CaSO3 + H2O
CaSO3 + SO2 + H2O Ca(HSO3)2.SO2 like H2S has reducing character, bleaches the brown color of iodine, reacts with acid KMnO4 and acid K2Cr2O7. l2 + SO2 + H2O SO3 + 2H++ 2l- 2 MnO4- + 5 SO2 + 6H+ 2Mn++ + 5SO3 + 3H2O Cr2O72- +3 SO2 + 8H+ 2Cr3++ 3SO3 + 4H2O 3- Thiosulp