Detection of anions
prof. Viktor Kanický, Analytická chemie I
1
Detection of anions



no system similar to that for cations
detection reactions are sufficiently selective
reactions:


Neutral solution × acid medium  evolution of gases:
CO32-, SO32-, S2-, HS-, NO3-, NO2Removing of HM:

boiling with 1M Na2CO3  insoluble carbonates, however,
uwanted losses and changes occure :





oxidation SO32-, S2-, NO2adsorption on precipitates of carbonates: SO42-, PO42loss of volatile acids after acidification with HNO3
using ion-exchangers– exchange for Na+
boiling with MgO: can not be used for detection of PO42-,
AsO42-, CO32prof. Viktor Kanický, Analytická chemie I
2
Group reactions of anions
A. precipitation: Ba2+, Ag+ ; solubility = f(conditions)

I. precipitation reactions of Ba2+

Ba salts insoluble in strongly acid medium
2M HCl, 2M HNO3 : SO42-

Ba salts insoluble in weakly acid medium
2M HAc : F-, CrO42-, SO32-, S2O32-

Ba soli insoluble in neutral medium (water)
PO43- , AsO43-, CO32-, BO2prof. Viktor Kanický, Analytická chemie I
3
Group reactions of anions

II. Precipitation reactions of Ag+ (in neutral
medium)

Ag salts insoluble in 2M HNO3
Cl-, Br-, I-, SCN-, CN-, [Fe(CN)6]4-, [Fe(CN)6]3-, S2-

Ag salts stable only in neutral medium, soluble in HNO3
CrO42-, SO32-, S2O32-, PO43-, AsO33-, AsO43-, CO32-, BO2-

Ag salts insoluble in 2M NH3
Br-, SCN-, [Fe(CN)6]4-, I-, S2-
prof. Viktor Kanický, Analytická chemie I
4
Group reactions of anions
B. Redox reactions: MnO4-, I-, I2 ; colour transitions

I. Oxidation reactions of permanganate

Discoloration of MnO4- in acid medium (1M H2SO4) by action of ions
wit reduction capability
SO32-, S2O32-, AsO33-, S2-, [Fe(CN)6]4-, Br-, I-, CN-, SCN-, NO2MnO4  8 H   5e   Mn 2  H 2O
5SO32  2 MnO4  5SO42  2 Mn 2  3H 2O
5S 2O32  8MnO4  14 H   10 SO42  8Mn 2  7 H 2O
5 AsO33  2 MnO4  6 H   5 AsO43  2 Mn 2  3H 2O
5S 2  2 MnO4  16 H   5S  2 Mn 2  8H 2O
5FeCN 6   MnO4  8 H   5FeCN 6   Mn 2  4 H 2O
4
3
5 NO2  2 MnO4  6 H   5 NO3  2 Mn 2  3H 2O
Hal . : X   10 X   2MnO4  16 H   5 X 2  2 Mn 2  8H 2O
prof. Viktor Kanický, Analytická chemie I
5
Group reactions of anions

II. Oxidation reactions of iodine

discoloration of I2 in neutral or weakly acid (pH < 9) medium,
reversible reaction (NaHCO3 solid)
SO32-, S2O32-, AsO33-, S2-, [Fe(CN)6]4-, CN-, SCN-
I 2  2e   2 I 
2S 2O32  I 2  S 4O62  2 I  tetrathionan 
CN   I 2  ICN  I 
SO32 , AsO33 , S 2 , FeCN 6   as with MnO4
4
prof. Viktor Kanický, Analytická chemie I
6
Group reactions of anions

III. Reduction reactions of iodide

Anions with oxidation capability oxidize in acid medium iodide to
iodine→ yellow, red-brown coloration of solution, blue coloration
of starch solution
CrO42-, Cr2O72-, AsO43-, S2-, [Fe(CN)6]3-, NO2-
2CrO42  6I   16H   2Cr 3  3I 2  8H 2O
Cr2O72  6I   14H   2Cr 3   3I 2  7H 2O
AsO43  2I   2H   AsO33   I 2  H 2O
2FeCN 6   2I   2FeCN 6   I 2
3
4
2NO2  2I   4H   2NO  I 2  2H 2O
prof. Viktor Kanický, Analytická chemie I
7
Selective reactions of anions

I. Group of low soluble Ba salts
BaSO4 insoluble X BaS2O3 (decomposition→ SO2, S – turbidity)
separation: SrCl2
sample
solution
precipitate
centrifugation
turbidity
solution
discoloration of solution
Hepar reaction
BaSO4, SrSO4 – detection of SO42- by hepar reaction
prof. Viktor Kanický, Analytická chemie I
8
Group of low soluble Ba salts

SO42
Fe
III
precipitate, at heat with Mg → reduction
resulting S2- - detection: PbAc (lead acetate), black
Na2Fe(CN)5NO (sodium nitroprusside)
CN 5 NO2  S 2  Fe III CN 5 NOS 4
purple in alkaline medium.
prof. Viktor Kanický, Analytická chemie I
9
Group of low soluble Ba salts

SO32
1) with malachite green and fuchsin (Votoček)
malachite green
fuchsin
discoloration of solution (pH 7-12) formed sulfonanes
 interfere: S2- and excess of (OH-) pH > 12
S2- removing by means of CdCO3, ZnCO3
prof. Viktor Kanický, Analytická chemie I
10
Group of low soluble Ba salts

2) with zinc nitroprusside – detection in gaseous phase
SO32  2H   H 2SO3  SO2  H 2O
ZnFeCN 5 NO  Zn2 FeCN 5 NOSO3 
sulphites nitroprusside Zn
(red, low soluble)




H
interferes:
S2O32 
 SO2  S 2
masking: HgCl2
procedure: on filtration paper soaked with reagent – above
crucible in vapours. Unreacted nitroprusside is discoloured
in ammonia vapour → only colour of product
prof. Viktor Kanický, Analytická chemie I
11
Group of low soluble Ba salts

S2O32
1) in acid medium decomposition:
H 2 S 2O3  H 2 SO3  S
colloid sulphur 
H 2 SO3  H 2O  SO2

2) with iron chloride
Fe3  S2O32  FeS2O3 

solution of purple complex
temporary colour because of reduction FeIII → FeII
2Fe 3  2S2O32  2Fe 2  S 4O62


interfere: SCN-, SO32-
3) iodazide reaction – catalysis; in presence of S2(included in S2O32-) fast reaction:

2 N 3 sodium azide   I 2  3N 2  2 I  red solution discolours
interfere: SCN-, S2- (separation as CdS, ZnS)
prof. Viktor Kanický, Analytická chemie I
12
Group of low soluble Ba salts

F
with ZrIV-chelate with xylenol orange


More stable complex of Zr with ligands F- → chelate is
decomposed → red-purple chelate → yellow released
reagent
Generally: use of F- as strongly complexing agent
prof. Viktor Kanický, Analytická chemie I
13
Group of low soluble Ba salts

SiO32
s ammonium molybdate→ molybdate-silicic acid
H4[Si(Mo3O10)4]




originates in acid meduim – yellow solution
before smaple must alkalized, in order to obtain Si(OH)4
by reduction with SnCl2, S2O32- → molybdenum blue – can
be oxidized by HNO3 to yellow H4[Si(Mo3O10)4]
interfere: AsO43-, PO43- → molybdate-phosphoric acid and
molybdate- arsenic acid
prof. Viktor Kanický, Analytická chemie I
14
Group of Ag salts soluble in 2 M HNO3
Soluble in 2 M HNO3
 Coloration of salts: characteristic





Ag2CrO4 – russet, red-brown
Ag3AsO4 – chocolate brown
Ag3PO4, AgAsO2 – yellow
AgBO2 – white
Ag2CO3 – yellowish
2CrO42  2H   Cr2O72  H 2O
neutral, alkal., yellow sol.
acid, orange sol.
Cr2O72  2OH   2CrO42  H 2O
acid  alkaline 
interference: AsO43- x PO43- by molybdate NH4+
AsO2- (HNO3 conc.) → AsO43- the same
prof. Viktor Kanický, Analytická chemie I
15
Group of Ag salts soluble in 2 M HNO3

CrO42-, Cr2O72

1) with hydrogen peroxide (viz Cr3+), acid medium
2) with benzidine → semiquinone of benzidine blue


3) with diphenylcarbazide → red-purple chelate with CrIII,
acid medium; extraction into amylalcohole


interfere: VO2+, MoO42- and generaly oxidizing agents, acid
medium
interfere: Hg2+, Fe3+, MoO42-, VO43-
4) with chromotropic acid→ purple-red solution in acid m.
prof. Viktor Kanický, Analytická chemie I
16
Group of Ag salts soluble in 2 M HNO3

PO43
1) with ammonium molybdate → yellow precipitate




x SiO32- (yellow solution)
NH4 3 AsMo3O10 4 
ammonium tetrakis-trimolybdato phosphate
precipitation at hot , acid medium
interferes: AsO43removing of AsO43- by reduction with Zn powder to AsO33- a
and precipitation by H2S na As2S3
AsO43  Zn  2H   AsO33  Zn 2  H 2O
prof. Viktor Kanický, Analytická chemie I
17
Group of Ag salts soluble in 2 M HNO3

AsIIIO2- (AsIIIO33-)

1) by reduction with SnCl2 – common reaction with AsO432 AsO2  3SnCl 42  8HCl  2 As 0  3SnCl 62  4H 2O  2Cl 


(Bettendorf test)
2) with sulphane → As2S3 yellow, AsV does not react!
AsVO43
1) by reduction with SnCl2 – jako AsIII → brownish As
2 AsO43  5SnCl 42  16HCl  2 As 0  5SnCl 62  8H 2O  6Cl 

2) with ammonium molybdate → yellow precipitate
NH4 3 AsMo3O10 4 

in acid medium HNO3
interferes: PO43prof. Viktor Kanický, Analytická chemie I
18
Group of Ag salts soluble in 2 M HNO3
Differentiation AsIIIO2- x AsVO43- AgNO3
AsIIIO2- : yellow precipitate Ag3AsO3 soluble in HNO3 and NH3
AsVO43- : chocolate brown Ag3AsO4 soluble in HNO3 and NH3
prof. Viktor Kanický, Analytická chemie I
19
As – toxicity, poisons, criminalistics
Common reactions of As– reduction to arsan
(arsin) AsH3

1) Marsh – Liebig test – official evidence
As
V

, As , As  Zn  Zn 2  AsH3 plyn
III
0
H  ,H2SO4
prof. Viktor Kanický, Analytická chemie I
20
As – toxicity, poisons, criminalistics

2) Gutzeit test– AsH3
 AsH3 discolours paper with AgNO3 yellow (arsenide
Ag3As) and than turns black due to decomposition
AsAg3  AgNO3  6 Ag  H 3 AsO3  3HNO3
prof. Viktor Kanický, Analytická chemie I
21
Group of Ag salts soluble in 2 M HNO3

B(OH)4-
borate anion:
BOH 3  H 2O  HBOH 4
Monohydric acid pK = 10

1) flame test – green flame (λ=548,1 a 519,3 nm)


interferes: Cu2+
2) flame test of volatile esters – green flame
BOH 3  3C2 H 5OH  BOC 2 H 5 3  3H 2O

3) with curcumin (curcumin paper)

Red 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
22
Group of Ag salts soluble in 2 M HNO3

CO32
1) with mineral acids
CO32  H   HCO3  H   CO2  H 2O
CO2  BaOH 2  BaCO3  H 2O


interferes: SO32-, S2O32-, HS-, CN- → plyny
removing: decomposition by chromosulfuric acid (K2CrO4 v
H2SO4)
→ oxidation of interfering anions to nonvolatile salts
(also KMnO4)
prof. Viktor Kanický, Analytická chemie I
23
Group of Ag salts insoluble in 2 M HNO3

in 2 M HNO3 precipitate:




AgCl, AgSCN – white
AgBr – yellowish
AgI – yellow
Ag2S – black




Cl-, Br-, SCN- - stable in H+, OHI- v H+oxidation → I2 (yellow)
HS-, S2- - hydrolysis→ H2S, polysulphides
E0 – standard potentials of redox reactions
Cl 2 / 2Cl   1,36 V , Br2 / 2Br   1,07 V , I 2 / 2I   0,58 V
prof. Viktor Kanický, Analytická chemie I
24
Group of Ag salts insoluble in 2 M HNO3

Cl


1) Ag   Cl   AgCl 
AgCl  2NH3  Ag NH3 2 
2) with Deniges reagent (mixture of phenol and aniline)
In conc. H2SO4 Clis oxidized by KMnO4 to Cl2

2MnO4  10Cl   16H   5Cl 2  8H 2O  2Mn 2
Cl 2  2OH   ClO  Cl   H 2Odisproporcion.

+ drop of 1 M NaOH, drop of Deniges reagent
prof. Viktor Kanický, Analytická chemie I
25
Group of Ag salts insoluble in 2 M HNO3




also with pipette: above crucible pipette with NaOH –
absorption of Cl2 → ClOblow out on the drop board with Deniges reagent
dry part of paper colours brown, purple
do not transferred BrO-, IO- !
prof. Viktor Kanický, Analytická chemie I
26
Group of Ag salts insoluble in 2 M HNO3

3) chromylchloride formation

In anhydrous medium (conc. H2SO4)
4Cl   Cr2O72  2CrO2Cl 2  3H 2O (red-brown, volatile)
on paper reaction with OHCrO2Cl 2  4OH   CrO42  2Cl   2H 2O

paper discolurs yellow
by chromate
 
6Br  I   Cr2O72  14H   2Cr 3  3Br2 I 2   7H 2O

interferes: NO2-, NO3- → NOCl
prof. Viktor Kanický, Analytická chemie I
27
Group of Ag salts insoluble in 2 M HNO3

Br
1) oxidation to Br2 by chloramine T



Br2 – yellow, discolorates brown – extraction to CHCl3
interfere: SCN-, I- (excess)
In presence of I- oxidation to I2 at first (brown discoloration of
aequous phase – purple in CCl3H), then I2 → IO3- (colourless)
and then Br2
SCN- → (CN)2 – dikyane - poisonous
prof. Viktor Kanický, Analytická chemie I
28
Group of Ag salts insoluble in 2 M HNO3

I
1) oxidation to I2 by sodium nitrite
2I   NO2  2H   I 2  H 2O  NO



SCN
1) with CeCl3 – in acid medium


DetectionI2: a) extraction to CCl4 or CHCl3 – purple solution
b) starch solution – blue
c) iodide-starch paper- blue
interferences: S2O32-
interferes: F-, H2PO4- (excess) [FeNCS]2+ [Fe(NCS)2]+ red
complex
HS-, S2

1) with Pb2+ →PbS
2) with nitroprusside → [FeII(CN)5NOS]4- purple complex
prof. Viktor Kanický, Analytická chemie I
29
Group NO3-, NO2-, ClO4
NO3-, ClO4

in solutions stable oxidative properties
Do not form: complexes, precipitates



NO3- analogy to Na+ - soluble salts
ClO4- large volume, small charge, formation of ionic
associates with basic organic dyes
NO2

oxidative properties ( 2 I- → I2)
oxidized by atmospheric O2 to NO3-
prof. Viktor Kanický, Analytická chemie I
30
Group NO3-, NO2-, ClO4
NO3
1) with diphenylamine



blue oxidation product (v konc. H2SO4)
interferes: NO2-, CrO42-, MnO4- (oxidation reagents), Fe3+, I-, IO3(formation of I2)
2) formation of azodye after reduction by Zn na NO2in Hac is NO3- reduced by pulver Zn to NO2 detection by diazotation and coupling reaction by formation of
azodye
 Interferes : NO2- → removing by:
a) urea in 1 M H2SO4

2HNO2  CONH2 2  2N 2  CO2  H 2O
b) potassium iodide in Hac (80%)
2NO2  2I   4H   2NO  I 2  2H 2O
prof. Viktor Kanický, Analytická chemie I
31
Group NO3-, NO2-, ClO4c) sodium azide (weak acid media)
HN3  NO2  H   N 2  N 2O  H 2O
d) ammonium salts (acid medium)
NH 4  NO2  N 2  2H 2O
side reaction !!! : 3 HNO2 → HNO3  2NO  H 2O

3) nitration reactions – in acid medium (conc. H2SO4)

yellow to orange comp. character. smell (blank necessary)

NO2- does not interfere – only H2SO4 discolours orange
interferes: I- → removing by precipitation with PbAc2


Nitration 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
prof. Viktor Kanický, Analytická chemie I
32
Group NO3-, NO2-, ClO44) reduction reaction – by Zn or Devard alloy (Cu – Zn - Al)
in alkaline medium → NH3 = reduction by nascent hydrogen
3 NO3− + 8 Al + 5 OH− + 18 H2O → 3 NH3 + 8 [Al(OH)4]−
interfere: NO2-, CN


a)
b)
c)
d)
direct detection: nitration after removing of halogenides by
silver sulfate
Proceedure for reduction reactions:
positive reaction with diphenylamine (proof of oxidation
properties)
reduction by Zn in OH- medium rises NH3 (nitrogen proof)
by means of KI to verify possible presence of NO2- and by
positive reaction to remove
By reducing with Zn in HAc convert NO3- to HNO2 and NO2detect by formation of azodye
prof. Viktor Kanický, Analytická chemie I
33
Group NO3-, NO2-, ClO4
NO2-
3HNO2  HNO3  2 NO  H 2O

unstable acid 
1) with KI – oxidation of I- to iodine
2NO2  2I   4H   2NO  I 2  2H 2O



2) with diphenylamine


it is evidence of NO2- along with the presence of NO3instant reaction unlike with Cr2O72- or ClO3-
oxidation to d. blue
interferes: NO3-, Cr2O72- aj.
3) with KMnO4 – reduction to Mn2+
5NO2  2MnO4  6H   5NO3  2Mn 2  3H 2O
prof. Viktor Kanický, Analytická chemie I
34
Group NO3-, NO2-, ClO4
4) diazotation reaction of HNO2 and coupling to azodyes

diazotation

coupling
in H+ with aromatic amine
prof. Viktor Kanický, Analytická chemie I
35