prof. Viktor Kanický, Analytická chemie I
1
Detection of anions

prof. Viktor Kanický, Analytická chemie I
2
Detection of anions
nno system similar to that for cations
ndetection reactions are sufficiently selective
nreactions:
qNeutral solution × acid medium Þ evolution of gases: CO32-, SO32-, S2-, HS-, NO3-, NO2-
qRemoving of HM:
nboiling with 1M Na2CO3 Þ insoluble carbonates, however, uwanted losses and changes occure :
qoxidation SO32-, S2-, NO2-
qadsorption on precipitates of carbonates: SO42-, PO42-
qloss of volatile acids after acidification with HNO3
nusing ion-exchangers– exchange for Na+
nboiling with  MgO: can not be used for detection of PO42-, AsO42-, CO32-

prof. Viktor Kanický, Analytická chemie I
3
Group reactions of anions
nA. precipitation: Ba2+, Ag+ ; solubility = f(conditions)
n
nI. precipitation  reactions of Ba2+
n
nBa salts insoluble in strongly acid medium
q 2M HCl, 2M HNO3 :  SO42-
q
nBa salts insoluble in  weakly acid medium
q 2M HAc : F-, CrO42-, SO32-, S2O32-
q
nBa soli insoluble in neutral medium (water)
q PO43- , AsO43-, CO32-, BO2-

prof. Viktor Kanický, Analytická chemie I
4
Group reactions of anions
nII. Precipitation reactions of Ag+ (in neutral medium)
n
nAg salts insoluble in 2M HNO3
n Cl-, Br-, I-, SCN-, CN-, [Fe(CN)6]4-, [Fe(CN)6]3-, S2-
n
nAg salts stable only in neutral medium, soluble in HNO3 CrO42-, SO32-, S2O32-, PO43-, AsO33-,
AsO43-, CO32-, BO2-
n
nAg salts insoluble in 2M NH3
n Br-, SCN-, [Fe(CN)6]4-, I-, S2-

prof. Viktor Kanický, Analytická chemie I
5
Group reactions of anions
nB. Redox reactions: MnO4-, I-, I2 ;  colour transitions
n
nI. Oxidation reactions of permanganate
qDiscoloration of MnO4- in acid medium (1M H2SO4) by action of ions wit reduction capability
q SO32-, S2O32-, AsO33-, S2-, [Fe(CN)6]4-, Br-, I-, CN-, SCN-, NO2-
q
q

prof. Viktor Kanický, Analytická chemie I
6
Group reactions of anions
nII. Oxidation reactions of iodine
qdiscoloration of I2 in neutral or weakly acid (pH < 9) medium, reversible reaction (NaHCO3 solid)
q SO32-, S2O32-, AsO33-, S2-, [Fe(CN)6]4-, CN-, SCN-
n

prof. Viktor Kanický, Analytická chemie I
7
Group reactions of anions
nIII. Reduction reactions of iodide
qAnions with oxidation capability oxidize in acid medium iodide to iodine→ yellow, red-brown
coloration of solution, blue coloration of starch solution
q CrO42-, Cr2O72-, AsO43-, S2-, [Fe(CN)6]3-, NO2-

anionty
prof. Viktor Kanický, Analytická chemie I
8
Selective reactions of anions
nI. Group of low soluble Ba salts
n BaSO4 insoluble X BaS2O3 (decomposition→ SO2, S – turbidity)
n separation: SrCl2
n
n
n
n
n
n
nBaSO4, SrSO4 – detection of SO42- by hepar reaction
n
sample
solution
precipitate
turbidity
centrifugation
Hepar reaction
solution
discoloration of solution

prof. Viktor Kanický, Analytická chemie I
9
Group of low soluble Ba salts
nSO42-
nprecipitate,  at heat with Mg → reduction
n resulting S2- - detection: PbAc (lead acetate), black
§           Na2Fe(CN)5NO (sodium nitroprusside)

prof. Viktor Kanický, Analytická chemie I
10
Group of low soluble Ba salts
nSO32-
n1) with malachite green and  fuchsin (Votoček)
n malachite green                                         fuchsin
q
q
q
q
q
qdiscoloration of solution (pH 7-12) formed sulfonanes
qinterfere: S2- and excess of (OH-) pH > 12
q S2- removing by means of CdCO3, ZnCO3
§
mala fuchsin

prof. Viktor Kanický, Analytická chemie I
11
Group of low soluble Ba salts
n2) with zinc nitroprusside – detection in gaseous phase
n
n
n            sulphites nitroprusside Zn                             (red, low soluble)
qinterferes:
qmasking: HgCl2
qprocedure: on filtration paper soaked with reagent – above crucible in vapours. Unreacted
nitroprusside is discoloured in ammonia vapour → only colour of product
n

prof. Viktor Kanický, Analytická chemie I
12
Group of low soluble Ba salts
nS2O32-
n1) in acid medium decomposition:
n
n
n2) with iron chloride
n
n temporary colour because of reduction FeIII → FeII
n
qinterfere: SCN-, SO32-
n3) iodazide reaction – catalysis; in presence of S2- (included in S2O32-) fast reaction:
n
qinterfere: SCN-, S2- (separation as CdS, ZnS)

prof. Viktor Kanický, Analytická chemie I
13
Group of low soluble Ba salts
nF-
nwith ZrIV-chelate with xylenol orange
n
qMore stable complex of Zr with ligands F- → chelate is decomposed → red-purple chelate → yellow
released reagent
qGenerally: use of F- as strongly complexing agent
n

prof. Viktor Kanický, Analytická chemie I
14
Group of low soluble Ba salts
nSiO32-
ns ammonium molybdate→ molybdate-silicic acid H4[Si(Mo3O10)4]
n
qoriginates in acid meduim – yellow solution
qbefore smaple must alkalized, in order to obtain Si(OH)4
qby reduction with SnCl2, S2O32- → molybdenum blue – can be oxidized by HNO3 to yellow
H4[Si(Mo3O10)4]
qinterfere: AsO43-, PO43- → molybdate-phosphoric acid and  molybdate- arsenic acid
n

prof. Viktor Kanický, Analytická chemie I
15
Group of Ag salts soluble in 2 M HNO3
nSoluble in 2 M HNO3
nColoration of salts: characteristic
nAg2CrO4 – russet, red-brown
nAg3AsO4 – chocolate brown
nAg3PO4, AgAsO2 – yellow
nAgBO2 – white
nAg2CO3 – yellowish
n
qneutral, alkal., yellow sol. acid, orange sol.
q
qinterference: AsO43- x PO43- by molybdate NH4+
qAsO2- (HNO3 conc.) → AsO43- the same
q

prof. Viktor Kanický, Analytická chemie I
16
Group of Ag salts soluble in 2 M HNO3
nCrO42-, Cr2O72-
n1) with hydrogen peroxide (viz Cr3+), acid medium
n2) with benzidine → semiquinone of benzidine blue
qinterfere: VO2+, MoO42- and generaly oxidizing agents, acid medium
n3) with diphenylcarbazide → red-purple chelate with CrIII, acid medium; extraction into
amylalcohole
n
n
n
qinterfere: Hg2+, Fe3+, MoO42-, VO43-
n4) with chromotropic acid→ purple-red solution in acid m.
difenylkarbazid

prof. Viktor Kanický, Analytická chemie I
17
Group of Ag salts soluble in 2 M HNO3
nPO43-
n1) with ammonium molybdate → yellow precipitate
§x SiO32- (yellow solution)
§
§ ammonium tetrakis-trimolybdato phosphate
qprecipitation at hot , acid medium
qinterferes: AsO43-
qremoving of AsO43- by reduction with Zn powder to AsO33- a and precipitation by H2S na As2S3
n
n

prof. Viktor Kanický, Analytická chemie I
18
Group of Ag salts soluble  in 2 M HNO3
nAsIIIO2- (AsIIIO33-)
n1) by reduction with SnCl2 – common reaction with AsO43-
n
n (Bettendorf test)
n2) with sulphane → As2S3 yellow, AsV does not react!
nAsVO43-
n1) by reduction with SnCl2 – jako AsIII → brownish As
n
n2) with ammonium molybdate → yellow precipitate
§ in acid medium HNO3
qinterferes: PO43-
n

prof. Viktor Kanický, Analytická chemie I
19
Group of Ag salts soluble  in 2 M HNO3
nDifferentiation AsIIIO2- x AsVO43-  AgNO3
n
nAsIIIO2- : yellow precipitate Ag3AsO3 soluble in HNO3 and NH3
n
nAsVO43- : chocolate brown Ag3AsO4 soluble in HNO3 and NH3
n

prof. Viktor Kanický, Analytická chemie I
20
As – toxicity, poisons, criminalistics
nCommon reactions of As– reduction to arsan (arsin) AsH3
n
n1) Marsh – Liebig test – official evidence
aparatura1

prof. Viktor Kanický, Analytická chemie I
21
As – toxicity, poisons, criminalistics
n2) Gutzeit test– AsH3
qAsH3 discolours paper with AgNO3 yellow (arsenide Ag3As) and than turns black due to decomposition
q
n
n
aparatura2

prof. Viktor Kanický, Analytická chemie I
22
Group of Ag salts soluble  in 2 M HNO3
nB(OH)4-
qborate anion:
qMonohydric acid pK = 10
n1) flame test – green flame (λ=548,1 a 519,3 nm)
qinterferes: Cu2+
n2) flame test of volatile esters  – green flame
n
n3) with curcumin (curcumin  paper)
qRed soluble complex 1:1 in weak acid medium (red-brown discoloration of paper, blank – yellow),
drop of alkaline hydroxide → dark green

prof. Viktor Kanický, Analytická chemie I
23
Group of Ag salts soluble  in 2 M HNO3
nCO32-
n1) with mineral acids
n
n
qinterferes: SO32-, S2O32-, HS-, CN- → plyny
qremoving: decomposition by chromosulfuric acid (K2CrO4 v H2SO4)
q → oxidation of interfering anions to nonvolatile salts
q (also KMnO4)
n

prof. Viktor Kanický, Analytická chemie I
24
Group of Ag salts insoluble  in 2 M HNO3
nin 2 M HNO3 precipitate:
nAgCl, AgSCN – white
nAgBr – yellowish
nAgI – yellow
nAg2S – black
n
qCl-, Br-, SCN- - stable in H+, OH-
qI- v H+oxidation → I2 (yellow)
qHS-, S2- - hydrolysis→ H2S, polysulphides
q
qE0 – standard potentials of redox reactions
n

prof. Viktor Kanický, Analytická chemie I
25
Group of Ag salts insoluble  in 2 M HNO3
nCl-
n1)
n2) with Deniges reagent (mixture of phenol and aniline)
qIn conc. H2SO4 Cl-
qis oxidized by KMnO4 to Cl2
q
q
q
q
q
q
q
q
q
q+ drop of 1 M NaOH, drop of Deniges reagent
q
fenol anilin
reakce

prof. Viktor Kanický, Analytická chemie I
26
Group of Ag salts insoluble  in 2 M HNO3
qalso with pipette: above crucible pipette with NaOH – absorption of Cl2 → ClO-
qblow out on the drop board with Deniges reagent
qdry part of paper colours brown, purple
qdo not transferred BrO-, IO- !
aparatura3

prof. Viktor Kanický, Analytická chemie I
27
Group of Ag salts insoluble  in 2 M HNO3
n3) chromylchloride formation
qIn anhydrous medium (conc. H2SO4)
q     (red-brown, volatile)
qon paper reaction with OH-
q
q
q
q paper discolurs yellow
q by chromate
q
q
qinterferes: NO2-, NO3- → NOCl
n
aparatura4

prof. Viktor Kanický, Analytická chemie I
28
Group of Ag salts insoluble  in 2 M HNO3
nBr-
n1) oxidation to Br2 by chloramine T
n
n
n
qBr2 – yellow, discolorates brown – extraction to CHCl3
qinterfere: SCN-, I- (excess)
qIn presence of I- oxidation to I2 at first (brown discoloration of aequous phase – purple in
CCl3H), then I2 → IO3- (colourless) and then Br2
q SCN- → (CN)2 – dikyane - poisonous
chloramin

prof. Viktor Kanický, Analytická chemie I
29
Group of Ag salts insoluble  in 2 M HNO3
nI-
n1) oxidation to I2 by sodium nitrite
n
qDetectionI2: a) extraction to CCl4 or CHCl3 – purple solution
§ b) starch solution – blue
§ c) iodide-starch paper- blue
qinterferences: S2O32-
nSCN-
n1) with CeCl3 – in acid medium
qinterferes: F-, H2PO4- (excess) [FeNCS]2+ [Fe(NCS)2]+ red complex
nHS-, S2-
n1) with Pb2+ →PbS
n2) with nitroprusside → [FeII(CN)5NOS]4- purple complex
q

prof. Viktor Kanický, Analytická chemie I
30
Group NO3-, NO2-, ClO4-
nNO3-, ClO4-
nin solutions stable oxidative properties
nDo not form: complexes, precipitates
qNO3- analogy to Na+ - soluble salts
qClO4- large volume, small charge, formation of ionic associates with basic organic dyes
nNO2-
noxidative properties ( 2 I- → I2)
noxidized by atmospheric O2 to NO3-
n

prof. Viktor Kanický, Analytická chemie I
31
Group NO3-, NO2-, ClO4-
nNO3-
n1) with diphenylamine
qblue oxidation product (v konc. H2SO4)
qinterferes: NO2-, CrO42-, MnO4- (oxidation reagents), Fe3+, I-, IO3-      (formation of I2)
n2) formation of azodye after reduction by Zn na NO2-
qin Hac is NO3- reduced by pulver Zn to NO2-
qdetection by diazotation and coupling reaction by formation of azodye
qInterferes : NO2- → removing by:
n a) urea in 1 M H2SO4
n
n b) potassium iodide in Hac (80%)
a)
difenylamin

prof. Viktor Kanický, Analytická chemie I
32
Group NO3-, NO2-, ClO4-
qc)  sodium azide (weak acid media)
q
qd) ammonium salts (acid medium)
n
n      side reaction !!! : 3 HNO2 →
n3) nitration reactions – in acid medium (conc. H2SO4)
qyellow to orange comp. character. smell (blank necessary)
q
q
qNO2- does not interfere – only H2SO4 discolours orange
qinterferes: I- → removing by precipitation with PbAc2
qNitration of other componds: phenolsulfonic acid, phenol-2,4-disulfonic acid, α-naphtolsulfonic
acid, α-naftylamin (red-purple) pyrocatechol (green), m-phenylendiamien (žl.,červený), chromotropic
acid
nitrace

prof. Viktor Kanický, Analytická chemie I
33
Group NO3-, NO2-, ClO4-
n4) reduction reaction – by Zn or Devard alloy (Cu – Zn - Al)   in alkaline medium → NH3 =
reduction by nascent hydrogen
n3 NO3− + 8 Al + 5 OH− + 18 H2O → 3 NH3 + 8 [Al(OH)4]−
interfere: NO2-, CN-
qdirect detection: nitration after removing of halogenides by silver sulfate
qProceedure for reduction reactions:
a)positive reaction with diphenylamine (proof of oxidation properties)
b)reduction by Zn in OH- medium rises NH3 (nitrogen proof)
c)by means of KI to verify possible presence of NO2- and by  positive reaction to remove
d)By reducing with Zn in HAc convert NO3- to HNO2 and NO2- detect by formation of azodye

prof. Viktor Kanický, Analytická chemie I
34
Group NO3-, NO2-, ClO4-
nNO2-
n
n1) with KI – oxidation of I- to iodine
n
qit is evidence of  NO2- along with the presence of NO3-
qinstant reaction unlike with Cr2O72- or ClO3-
n2) with diphenylamine                             oxidation to d. blue
qinterferes: NO3-, Cr2O72- aj.
n3) with KMnO4 – reduction to Mn2+
q
difenylamin

prof. Viktor Kanický, Analytická chemie I
35
Group NO3-, NO2-, ClO4-
n4) diazotation reaction of HNO2 and coupling to azodyes
qdiazotation
q
q
q
qcoupling
q in H+ with aromatic amine
diazotace kopulace