42

S:tCTION V

THIAZDLYLAZO DYES AS ANALYTICAL RFAG ENTS

Investigations on the analytical uses of pyr1d1ylazo

dyes induced an interest in the possible analytical

applications of other heterocyclic azo dyes. In the last

2, years, thiazolylazo dyes have been extensively investi­

gated as analytical reagents; however, major part of such

work is devoted to the investigations of these compounds

as chromogenic reagents. Many thiazolylazo dyes have also

proved to be useful indicators in complexometr.1c titrations

of metals. Several workers have reviewed different aspects

ot analytical applications 01' thiazolylazo dyes; of these

two excellent rev laws in Blgl1 sh language are published

in Analyst(2~,2').

In the present section, I have reviewed reported

uSes of thiazolylazodyes as metallochrom1c indicators.

General :

Boni and Hemmler(26,27) synthesised the thiazolylazo

derivatives of pyrocatechml, resorCinol, phlorogluc1nol,

2-naphthol, chromotropic acid, 8-quinolinol, alizarin and

salicylaldoxime and reported that these compounds are

suitable as metal indicators for EDTA titrations.

Kaneniwa(28) prepared the thiazolylazo derivat1ves of

~~resol, resorCinol, 1-naphthol and 2-naphthol and stated

43

that these compounds form coloured metal chelates and th ese

colour react10ns could be appl1ed to the t1 trat10ns of

metals w1th EDTA.

J"ensen(29) repo .... ted thesynthes1s of the following

th1azolylazo compounds:

1. 1- (2-th1azolylazo) -2-naph thol-6-sulphon1c acid (TMf-6-S)

2. 2-(2-thia zolylazo) -p-cresol (TAG)

3. 6-(2-thlazolyla20)-resorcinol (TAR)

4. 6-(~-thlazolylazo)-orcinol (TAO)

,. 6-(2-thlazolylazo)-3-dimethylaminophenol (TAM).

J ensen( 2J) reJX)rted that all these compounds form intens1 vely

coloured metal chelates in neutral or sl1gptly acld solutions

~lth Th(IV), La(II1), U(VI), Hg(II), Pb(II), Cu(II), Co(Il),

Ni(ll), Zn(Il) and Cd(ll) ions.

J" ensen( 29) uS ed 0.1 % methanollc solutions of TAO, TA~,

TAO, TAH· and TAN .68 as ind1ca to rs tor th e EDTA ti tra tions or

several metal 10ns. Acetate-acet1c ac1d, pyr1d1ne-pyridin1um

chloride and ammonla-ammonlumchlorlde butfers were used. Sharp

end points were obtained in d1rect t1tratlons of Cu(II),

Co(Il), and Ni(ll) 10ns in acetate and pyridine buffers but

Co(ll) and Hi(ll) 10ns reacted satisfactor1ly only in hot

solut10ns. The d1rect tltratlons or Zoell) and Pb(ll) 10ns

could 00 performed only 1n pyridine butter but lt could not

be performed in acetate buffer. with all these metal ions, TAN,

1'NA-6S and TAR gave eas11y observable colour changes at the

44

end point 1n E!U'A t1tratlons, but with TAtI, colour change

was d1tticillt to observe because 1t is from red-v10let to

pink.

Subsequently, Jensen(30) reported that stab1lity

constants 01' metal complexes of TAN are large enough to

perm1t their use as indicators for d1rect t1trat10n of Cu(II),

Co(Il), Ni(II) and Zo(II) lons in suitably buftered solutions

giving distlnct cOlour changes.

Chromw and Sommer(31) investlgated 2-(2-thlazolylazo)-,­

methoxy phenol (TAlm) and 2-(2-thlazolylazo)-4-metl'loxy phenol

(TAMH) as indicators for EDTA tltrat1ons. TAMH reacts wlth

metal 10ns gl v1ng green, or blue complexes. TAl.ffi ro:r~ red

or rad-oranb e complexes with metal lons. TAUH is a good

lndicator for the EDTA tltration of Cu(II) (pH 4.,), In(III)

(pH 4.,), Bl(III) (pH 1., to 2.0) and Tl(lII) but TAMR is

not satisfactory for these metals. At pH 6, TAMH gives a

colour change from blue to yellOYl and TAMIl gives a red to

yellow colour change in the EDTA titrations of Hg(ll),

Cd(ll), Zn(II), Pb(II), Ni(Il), (,0-600c) and Co(II)

(60-700 C) ions.

Ole1nlk(32) synthesized 4-(2-thiazolylazo)-2-nitroresor­

cinol and 4-(5-sulfo-2-thlazolylaZO)-2-nltroresorcinol and

studied til e1r characteristics as metal indlcators for

complexometrlc t1tratlons.

Sanchez-Pedreno at ale (33) invest1gated 4-(4,5-dlmethYl-2-

th1azolylazo)-2-methyl reso:rcinol as a metallochromic

45

indicator and reported that it gives sharp and points in the

EDTA titration of Cu(ll), Cd(ll), Hg(II), Fe(II), Fe(III),

Zn(II), Pb(II) and Bi(III) ions.

Gonzales Diaz and sanchez-pedreno(3~) reported that

4-(2-benzothiazolylazo)-2-methyl resorcinol can be used as an

indicator for the EDTA t1 t~t1on ot Cu(II) (pH , or 9),

Pb(II) (pH 6., or 9), Zn(II) (pH 6., or 9), Hg(II) (pH 6.,) ions. Ca, Sr, Ba, I-tg did not in tertere in th ese t1trat1ons.

Lopez-Canc10 et ale (35) suggested the use of 2,6-

d1hydroxy-3-(2-th1azolylazo)-benzo1c acid as a metallochromic

ind1cator for EDTA t1trat1ons of Cu(II) (pH 4., or 9.,), Zn(II)

(pH 9.5), Cd(II) (pH 9.5), Ni(II) (pH 9.,) and Pb(II).

Shlbata(36) patented a method for th e preparation of new

heterocyclic azo compounds useful as metallochromic indicators

and described the preparation of 4-(2-thiazolylazO)-3-

dime thylam1no benzoic ac1d.

Kai et ale (37) synthes1zed the following ten thiazolylazo

dyes:

1. 2-(4, 5-d1methyl-2-thiazolylazo) ~-ethylIilenol (rn1TAEP)

2. 2-(4,,-d1methyl-2-thlazolYl~zo)-4-methyl phenol (n1TAC)

3. 2-(4,5-dlmethyl-2-thlazolylazo)-4-methoxy phenol

(PMTAP-OMe)

~. 2-(4,5-dlmethyl-2-thlazolylazo)-4-phenyl phenol (DMTAPP)

,. 2-(4, ,-dimethyl-2-thiazolylazo)-4-chloro Iilenol (Dl1TAC1)

48

6. 2-( 4, ,-dimethyl-2-thiazolylazo) -4-me thYl-6-methoxy phenol

(ll1TAMC )

7. 2-(4, '.dimethyl-2-th1azolylazo) 4,6-dimethylphenol(OOTAX)

8. 2-(4,'-dimethyl.2-th1azolylazo)-resorcinol (DMTAR)

9. 2-(4,,-dimethyl-2-th1azolylazo)-p-dimethylaminopnenol

(Dl1TAM)

10. 2-( 4, '-dimethyl-2.th1azolylazo )-2-naph thol (DHTAN).

They obtained the formation constants of these ligands

with Hn(II), Co(II), IU(II), Cu(Il), Zn(II), Cd(ll), and

Pb(II) ions and suggested their use as metal indicators tor

these ions.

~Je will now see the applications ot thlazolylazo dyes as

inu1cators for EDTA titration of specific metals, namely

copper, calc1 um, Zinc, mercury, rare earths, gallium, lead ,

lodi um, thalllum, thorium, zirconium, bismuth, iron, cobalt

and nickel.

Qopper:

Jensen(29,30) investigated ~r, TAN.6S, TAC, TAP, TAO

and TAM as indicators for the EDTA titration of copper in an

acidic medium and found them satisfactory. TJ~ and TAC give

a ve~y sharp colour change in the EDTA titration of Cu(II)

ions in acetate buffer.

Yanagihara(38) reported th e us e of 2- (4-methyl-2-thiazo-i-

lyazo)-,-methoxy phenol as an indicator for the EDTA "

titration of copper.

47

svobada(39) suggested the use or TAR, TAN, TAN-6, S,

TAN-3,6S and TANC as indicators for the EDTA titration of

copper.

Havir and Vrestal(40) reported that at pH 3.0 TAN gives

colour only with Cu(ll) and Bi(lll). TAN can be used as

indicator for th e EDTA titration of copper. '!he determ1na tion

is disturbed by the presence of Pb(II), N1(I1), Co(Il), ZneIl),

Bi(II1), La(111), Th(IV).

Wada et ale (41) hQve investigated TAN and TAR as 1nd1 ..

cators for the EDTA titration of copper. In th e pH range 3.0

to 8.0, TAR and TAN show a sharp colour change from red to

ye110\·1. '!be colour change 'With TAR 1s part1cularly sharp,

making 1 t a useful 1ndicator for the titrat10n of eu 'W1th EDTA

at room temperature. Invest1gating 1nfluence of addit10n of

aux1llial'"Y complex forming add1tions on th e rate of metallo­

chromic 1ndicator change, they reported that TAN and TAR give

a sharp colour change for EDTA tit~tion of copper(42).

They reported that 1-(2-th1azolylazo )-2-hydroxy-3-naphthoic

acid (TANK) gives a sharp colour change at the end po1nt in

EDTA titration of eu(II) (43. 44,45). Another good lndicator

for the EDTA t1tration of copper 1s 4-(2-thiazolylazo)-6-

methyl reso rcinol (46).

Gusev et ale (47) investigated isomers of TAN as ind1cators.

For the EDTA titration of copper, 2-{2-th1azolylaZO)-1-naphthol

g1 ves a colour change from blue to orange 1n the pH range 2.7

48

to 3.0. AlCIII), Hg(ll), Ag(!I}Mn(ll), Cr(lll), B1(III), AS(III), I

sn (II), alkali metal sand alkallne earth metals do not

interfere; '-Ihereas 2'll(Il), Cd(IJL), Pb(ll), Co(Il), Ni(II) and

Sb(III) interfere in this determination. 'lbe method was

app11ed to the determination of Cu in duralumin.

subsequently, Gusev et ale (48) investigated the

follol'11ng four til iazolylazo dyes as ind1cators fo r the EDTA

titration of copper:

I. 4-(4-methyl-2-thiazolylazo)-resorcinol (MeTAR)

II. 1-( 4-methyl-2-th1azolylazo) -2-na ph thol (MeTAN)

III. 5-(4-thiazolylazo)-2-ethylamino-p-cresol

IV. 5-(2_thia zoly18zo)-2-ethylam1no-p-cresol.

Th e colour transitions at the endpoint are from blue-violet to

yellow or orange. l1eTAR and l'4eTAU give good results in the pH

range 2 to 3 and compounds I~I and IV give good results at

pH ,.0. Copper '-las estimated in sandstone and in malachite by

EDTA titration uSing HeTAR as 1ndicator.

Gusev et a1.(49) lntroduced 4-(4-methyl-2'-thiazolylazo)­

resorc1nol (}.leTAn) and 3-(2-thiazo1ylazo)-p-cresol (3-TAC)

as indieators for the EDTA titration of copper. 3-TAC was used

in the IiI range 2 to " the colour change at the end po1nt is

green-blue to yellow. l'be process \las apPl1ed to the determi­

nation of coppa'" in sandStone and in malachite.

Kandrac(,o) sugciested the use of 2-(5,?-dimethyl-4,5,6,8-

tetrahydrobenzothiazol-2-ylazo)-4-etqylphenol as an indicator

for the EDTA titration of Cu(lI) at pH 5.0.

49

Garcia Hontelongo et ale (51) suggested that 3-(lt-methyl-

2-thia20lylazo)-pyridine-2,6.diol (MeADHP) is a good indicator

for the EDTA titrat10n of copper. The colour change at the

end point is pink to yellow.

Kal (52) investigated 4-(~,5-d1methyl-2-thiaZDlylazo)

resorcinol as a metallochrom.1c indicator for Cu(ll). ~e,.e is

a sharp colour change from purple to yellow in pH range 3.8

to 7.5.

Gandhi and Pathak ( 53) have lnvestlgated the following

thiazolylazo dyes as indicators for the direct titration of

co p~er with EDTA, NTA, 00 TA, EGTA and DTPA.

4(2-thiazolylazo) resorcinol (TAR)

5- (2-thiazolylazo) 2,4 dihyd roxy~nzoic acid (TARK)

4-(2-th1azolylazo) Ibloroglucinol (TAPhl)

2- (2-thiazolylazo) p-cresol (TAG)

1-(2-thla zolylazo) 2-naIilthol (TJ.N).

Procedure for th e EDTA titration of copper w-1 th TAN and

TAR indicator'S has be en applied to indirect complexometl"1c

determination of aluminium (54). Althought, the Cu-EDTA complex

has a larger stability constant than Al-EDTA complex, the slow

substitution reaction of Al-EDTA \-Jith Cu makes this method

- P1ltC t icable. The pH range for TAN :Is 3.2 to 7.5 and foY' TAR

it is ~.2 to 7., at room temperature. This method is more

advantageous than that using standard Zn(ll) solution as back­

titrant because the Cu(ll) solutions are stable for a longer

time and becaus e more indicators can be used.

50

Chromy and Vrestal (55) estimated the tin content of

organo-tin compounds by decomposing the organo-tin compound in

acidic medium, adding excess of EDTA to th e solution and

measuring the unreacted EDTA by back titration ~ith standa~d

Cu(II) solution using either 1-(2-th1azolylazo)-2-n~phthol or

4-methox.y 1-(2-th1azolylazo)phenol as indicators.

calcium :

Douglas (56) suggested the follo~ing procedure for the

determination of calcium in blood serum by EDT! titration using

TAG indicator. Place 1 ml of 1 1'4 KOH in a flask, add 0.0"" ml

of 0.1 % solution of 2-{2-th1azolylazo)-p-cresol (TAO) in

metha.nol followed by 0.02 ml. of 0.1 M CUSO".. solution and add

0.1 M disodium El.1rA solution till the colour of the solution

is pale yellow. Add 1 ml of 1 MICOH and 0.2 ml of unhemolyzed

serum, again add 0.02 ml of 0.1 % TAO solution to obtain a

lilac colour and continue titration with 0.1 M disodium EDTA

to a yello\\l end point, carry out a reagent blank. Small amotmts

of added HP04-, N03-, Cl-, CBJcoo- and 50"..-- do not interfere.

lnterference by traces of ferr1c ions 1s masked by addition

of 0.02 ml 1 % KOB.

Zinc:

Iana g1ha ra(57) used 2-(4-methyl-2 th1azolylazo)-4- methoxy

ph enol as an indicato r for th e EDTA ti tra t10n of zinc.

Perez Olmos (58) reported the use of 4-(4-n1tro-2-thiazo­

lylazo)-2-resorcinol and 2-methyl-4-(4-n1tro-2-thiaZOlylazo)-

51

Resorcinol as indicators for the EDTA t1tration of zinc. The

interference of Al(lll) is prevented by NaI whereas that ot

Cu(II) is eliminated by using a mixture of thiosulfate and

ascorbic acid.

Mercury :

Thiazolylazo compounds derived from p-cresol, p-anisole,

p-chlorophenol, 2,4-dichlorophenol and 2-methyl resorcinol

have been investigated as indicators for the EDTA ti tration of

mercury. Kai(59) suggested a method for the determination of

mercur,r with 2-(2.thia zolyla ZO)-p-cresol (TAG) indicator in

the pH ran6e 7.0 to 8.3 (phosphate buffer); there is a sharp

colour change trom blue to yellow (or red). Kai (60) also

suggested the use of 2-(2-thiazoly1a zo)-4-methoxy phenol as an

indicator for mercury at pH 6.3 to 7., (phosphate buffer), the

colour change being from blue to orange. 2-(2-thiazolylazo)-4-

chlorophenol (TACL) was suggested as suitable metallochromic

indicator (61,62) for the direct tit:re.tion of mercury in the

pH range 6.'7 to 8.7; the colour change being from deep blue to

reddish-yellow or violet. Kat (61) stated that thiS optimum

pH range is rather wide compared with that of other metallo­

chromic indicators such as CU-PAN, xylenol orange and methyl

thymol blue. Another indicato!" suggested by Kai (63) tor the

EDTA ti tra tion of Hg(II) 15 6- (2- th 1azolylazo) -2,4- d1ch 10 roph enol

(TAlX!) which is useful. in the pH range 3.8 to ,.0 (citric

acid + NailP04); there is a sharp colour change from deep

violet to orange.

52

Bare prths %

Hung(64) suggested the use of ~ as an indicator

for the chelatometric microdetermination of rare earths.

Advantage of TAR over other indicators is that it can be USed

over a wider "range i.e. 5-400jUg of rare~eartho.xide. '!bey

state that TAR is bette~ than Erio chrome black-T, Erio chrome

Blue black-R, Bromopyrogallol red, Chromeazurol-S, and PAR.

Gallium, Indium :

Gallium and Indium have been estimated b,y EDTA titration

uSing 1-(2-thiazolylazo)-2-naphthol-3,6-disulphon1c acid (TAN-3,

68) 1ndicator (65). 111e colour change for indium is violet

to yellO'lJ but that for gallium (orange to yellow) 1s less

satistactory. In the estimation 01' indium, 1 :50 fold excess ot

aluminium does not inter-1'are. with TAN-3. 6S lndicator,

thalllum gives violet to yello¥! colour change at the end point

(66).

5-(2-thiazolylazo)-2~ethylam1no-p-cresol and 5-(4-

methylth1a zolylazo)-2-ethylam1no-p-cresol have been suggested

as ind1cators for the EDTA t1tration of 1ndium; the colour

change be1ng from violet to o~nge (67). 'lhe method is applied

in the determination of indium 1n alloys. 1-(2-Thiazolylazo)-

2-naphthol-3-carboxyllc acid has been suggested as an indicator

tor EDTA titration of ind1um(68) in the presence of a fifty­

fold excess of aluminium. However, it 1s not a satisfactory

indicator for gallium(68).

Thallium :

Busev(69) suggested a pI'ocedu~e for the EDTA titration

of 2 to 11 mg of thal11um(Ill) in the pH range 2.0 to 3.0

using1-(2-thlazolylazo)-2-naphthol-3-carboxylic acid indicator.

~here is a sharp colour change from blu1sh violet to yellow.

Large RlOOunts 01" nitrate, su19hate, oxalate, acetate and

tartarate do not 1nterfere but chloride and bromide do. If the

pH is lowered to O. " lead does not interfere even at a

thousand-fold excess. Bismuth(IlI) interferes.

Chang(70) investigated nine 2-thia zolylazo phenols as

ind1cators for the chelatometr1c titration of thallium(III)

'W i th EDTA. Following four compound s weI'e sa t1sfactory I

I. 4- ( 2- th lazoly la ZO t- reso rc1nol (TAR)

II. 1-(2-th1azolylazo) -2-nalb thol (TAN)

III. 1-(2-th1a zolylazo)-2-na pbthol-3,6-d1sulfon1c ac1d (TAN-3,6S)

IV. 3-a m1no-6-(2-thiazolylazo) -phenol.

Gusev(71) reported the use of 2-(1-thiazolylazo)-1-

naphthol as a metallochrom1c ind1cator for thallium. Large

amounts of Mg(II), Zn(II), Cd(Il), Ag, 1'.1(111), Pb(II), }om(II),

Co(II), SrCII), As(V) do not interfere.

Lead :

Gusev et ale (72) investigated 2-(2-tb1azolylazo)-,­

dlethylamlnophenol as an indicator for ~rA t1tration of lead

and applied the process to the determination of lead in lead

salts, in the presence of other ions and in Sb Cu Sn,Pb-alloys. , ,

Glushkova et al.(73) used 3-(2-thiazolylazo)-p-cresol as an

indicator for the EDTA titration of lead at pH 6.8 to 7.,

(urotropine) in 30 % ethanol. Th e interference of cadmium and

zinc was eliminated by masKing with cyanide. It was poss1ble to

determine lead in the presence of copper or bismuth. '!he

process ltas applied to the determinat10n of lead 1n ceruss1 tee

,Aborium and ZireQnium:

Hemmeier and Scattolar1(7~) evaluated 2-(2.thiazolyla zo)

chromotropic acid (TANC) as an indicator foT' the EDTA ti tration

of thorium and zirconiUIJi. 0 ptimum pH for thorium is 2., to 3.0

and that for Zirconium is 1.5' to 2.,. The colour change at

the end point is from blue-violet to pink-violet. A number of

common cations and T1 do not interfere in the determination

but anions which form stable complexes with tho"'ium and

zirconium interfere even "Jhen present in trace amounts.

Bismuth:

Havir(~O) sU6bested the determination of bismuth by EDTA

titration using TAN as indicator. However, the presence of

bal1des interferes.

l..I.2n:

Gandhi and pathak(,3) investigated the following thiazo-

lylazo qyes as indicators for the direct t1tration of iron(lll).

?-(2-th1azolylazo) 2,~ dihydroxybenzo1c acid (TA~K)

4-(2-th1azolylazo)ph~oroglucinol (TAPhl)

2-( 2-th iazolylazo )p-cresol (TAC)

55-

1-(2-thia zolylazo) 2-3 dihydroxy naphthalene eTAroN)

1-(2-thiazolylazo) 2-hydroxy 3-naphthoic acid (TAN-2K)

All the five indicators are satisfactory for EDTA and

DTPA titration of iron(lll}. For NTA, TAPhl, TAC, TAN-3K

are good 1ndicators. For E~TA, TARK and TAOON give satisfactory

results. For ICTA, TARK, TAPhl, TAIDN and TAU-3K 1Mere found

to be excellent metallochrom1c indicators.

Cobalt :

Kaw ase(75) reported that TUA-3,6S, TAR, 2-(2-thi&zolylazo}­?

~-methox.yphenol (TM1) and 2-(2-th1azolylazo)·~,6-dimethylphenol

can be used as indicators for th e EDTA titration of cobalt.

Nickel:

Kawase (76 ) reported that TA'F"t, TAN-3,6S, 2-(2.thiazolylazo)­

~methoxy,lb enol (TAM) and 2- (2- th1a zoly lazo) -4, 6-dimethylph enol

are satisfactory 1ndicators for the EDTA t1tration of nickel.

Nakaga~a(77) investigated 2-(2-thiazolylazo}-p-cresol

(TAe) and 2-{2-thlnzolylazo)-4-methoxyphenol (TAU) as

indicators for the EDTA titration of nickel at pH 6.0 to 10.0;

the colour change is from blue to yellow (or orang e).

wada (~3,4,) suggested the use of 1-(2-thiazolylazo)-

2-hydroxy-3-nalil thoic acid (TANK) as an indicator for the

EDTA titration of nickel.

wada (78) reported that the end point of EDTA t1tration

jo'( nickel USing TAG indicator 1s improved by add! tion of 1,10-

phenanthroline.

56

Wada(79) investigated 2-(2-th1azolylazo)-~meth1l phenol

as an indicator tor EDTA titration ot nickel. The optimum pH

range at SOoC is 6.0 to 7.0.

~'iada at al. (do) have also suggested the tOllow:ing

compounds as indicators tor the EI1fA titration ot nickel:

A. 2-(l-thiazolylaZO)-5-(sulfometqylamino)-benzoic acid

B. 2'2-th1azoly~azo)-5-'dimethylam1no)-benzoic acid

c. 3-hydroxy-4-(2-thiazolylazo anllino)-methanesulfonic acid.

Conpound A gave a colour change from purple to yellow in

the pH ~ge 4.0 to 7.0 at 40°C. Coupound B gave a colour

change f~m bluish-violet to orange at ,SOC. The titration

was performed at &Joe ,,11 th compound e.

Ka1 (81, 82) reported the use ot 2-(2-thiazolylazo)-~

chlorophenol (TACt) as indicator for EDTA titration of nickel

in weakly acidic media. Trace amounts of Bi(III), er(lll),

Fe(III), and higher amounts of some bivalent cations

interfere.

KRsuya (83) t1trated nickellJ1th EDTA in ammon1um

chloride-ammonia butter at pH 10.0 and 60°C in the presence

of 2-(2-thiazolylazo)-p-cresol (TAG) lndic&tor.