o-Amino heterocyclic azo-dyes as analytical reagents—II Spectrophotometric determination of cobalt with 3-[(5-chloro-2-pyridyl)azo]-2,6-diaminopyridine

426 Tolonra.

SHORT COMMUNICATIONS

Vol.

20. pp.

426-430.

Pergsmon Press. 1973. Prmled in Great Brrtain

o-AMINO

HETEROCYCLIC AZO-DYES REAGENTS-II

AS ANALYTICAL

SPECTROPHOTOMETRIC DETERMINATION OF COBALT WITH 3-[(5-CHLORO-2-PYRIDYL)AZO]-2,6-DIAMINOPYRIDINE (Received 23 December 1971. Recked 26 October 1972. Accepted 15 Nooember 1972)

In a search for new sensitive and selective reagents, a thorough study of some of the azo compounds containing halogen-substituted pyridine has been made. *A It was found that one of these compounds. 3-[(5-chloro-2-pyridyl)azo]-2,6-diaminopyridine (5-Cl-PADAPy, I), formed a deep blue water-soluble complex with cobalt(H), and a highly selective determination of microgram amounts of cobalt is possible under the conditions established. Recently, Talipov et ~1.~ determined cobalt with 3-(2-pyridylazo)-2.6-diaminopyridine photometrically.

2

EXPERIMENTAL

Reagents S-Cl-PADAPy solution. An ethanolic 0.1% w/v solution was prepared from the pure material (see below). The solution is stable for several months if stored in an amber glass bottle. Cobalt(ll) solution. A stock chloride solution was prepared from 99.99% pure cobalt metal. Bufl^ersolution. Hydrochloric acid (0.2&f)-potassium chloride (0,2M), acetic acid (O.ZM)-sodium acetate (0.2M). boric acid (O.ZM)-potassium chloride (O.ZM)-sodium hydroxide (0.2M) and borax (O.ZM)-sodium hydroxide (O.lM) mixtures. Solutions (1 + 1 dilution) were made from high-purity sulphuric. hydrochloric. nitric. perchloric and phosphoric acid. All other reagents were made from high-purity materials or purified reagents, and were prepared with redistilled water. Preparation of reagent Diazotization. Freshly prepared isopentyl nitrite (5.46 g) was added to a solution containing 6 g of 5-chloro-2-aminopyridine and 2.4 g of sodium amide in 80 ml of absolute alcohol, refluxed for 2 hr and cooled. Coupling. 2,6_Diaminopyridine (5.1 g) was dissolved in IO ml of ethanol and added to the diazonium salt solution at 5’. and carbon dioxide was passed through the solution continuously. The mixture was let cool overnight and the precipitated reddish brown crystals were filtered off. washed with water, dissolved in hot ethanol. recrystallized from aqueous solution. and sublimed at about 200. Purified materials for physicochemical determinations can be obtained by sublimation in vacua. Analysis: calculated C 4X.30”“. N 33.80”,, H 3.65%; found C 48.5%. N 33.7”,,. H 3,8”6. Protonation behaoiow of the reagent

The reagent is sparingly soluble in water, but soluble in various organic solvents including ethanol, acetone and dioxan, as well as in strongly acid solution. Five species, H,L’+. H,L3’. H,L”. HL’. and L are involved in the protonation equilibria. The proton-dissociation constants have values of pK,, = 1.3 for the p-amino group and pK,, = 5.9 for the o-amino group. The pK,, and pK,, values for the pyridine and 2,6diaminopyrtdine heterocyclic nitrogen atoms may exist at about H, = -2.5. but could not be distinguished by the spectrophotometric method. Colour reactiotl with metals The coloured complexes are easily prepared by adding a few drops of a solution of 5-Cl-PADAPy in ethanol to solutions of heavy metals. The ions that give a colour with the reagent are listed in Table I. Ions that do not give a detectable colour at room temperature include silver. aluminium. beryllium.

SHORf

427

COMMUNlCATl0N.S

bismuth, cadmium, eaicium, ~hrornjurn(~~I~, mercury(XI), manga~e(~~)~ nickel (at pH 4). lead, thorium, tungsten, yttrium, zinc, zirconium. titanium. When mineral acid is added to the solution containing metat complexes. only the cobalt complex is stable, and therefore the selectivity for cobalt is excellent.

Table 1. Colour reaction of metals with S-CLPADAPy Colour l0n PH 5

Reagent co2* CU2* Fe3* Ni2*

Orange Green Purple Brown nil.

H&J,

pH 10

added

Orange Green Purple Brown Red purple

Orange red Bluish purple nil* Red brown nil

H,SQ, add& Orange red Blue nil * Red brown nil*

* This means reversion to the c&our of the reagent.

RESULTS

AND

DISCUSSION

Absorption spectra The absorption spectra of the reagent and its cobalt complex in aqueous solution at pH 59 (initial conditions of colour development) are shown in Fig. 1, When hydrochloric acid is added to the sample solution, the cobalt compiex may be changed into a different species, which is suitable for dete~~~ation of cobalt. The spectra of the reagent and its complex in l*2M hydr~h~o~~ acid are shown in Fig. 2.

The effect of initial pN on colour development was studied by developing the colour in a series of solutions varying in pM from 1.5 to 12 and then measuring the absorbance after the acidity had been increased to 1.2M (hydrochloric acid). The absorbance increased steeply in the pH range 2-3. but no change in absorbance was observed over the pH range 3512. In practice the initial pH should be as low as possible bu;ause nickel does not react appreciably with the reagent at pH 3*5-S. Subsequent studies were carried out at an initial pH of 50.

Wavelength, nm 1. Absorbance curves of 5-CI-PADAPy pH 50 with 4 x fCf- ‘M S-Cl-PADAPy,

fig.

and its cobalt complex in aqueous solution at

428

Wovelength. nm

Fig, 2. Absorbance curves of 5-CI-PADAPy and its cobait complex in 1.2&f hydrochloric solution. Con~ntrations of 5-CI-PADAPy and cobalt as for Fig. I.

acid

CobaIt and S-CI-PADAPy in an acid medium did not form any complexes; under these conditions, the Iigand was protonated on the nitrogen atoms and amino groups. In compIex formation the pyridine nitrogen atom plays a very important role: ~nzen~z~~aminopyridine (II) does not react with cobalt under any conditions.

However, the compiex formed at pH M-12 could be changed into another species of higher absorptivity, by addition of mineral acid. The effects of acidity and type of acid were studied. No change in absorbance was observed over the range Z-10 ml of added (1 + 1) hydrochloric, sulphuric. nitric. perchloric or phosphoric acid. Subsequent studies were carried out with addition of 5 ml of hydrochloric acid (1 f 1). The absorbances of a series of solutions containing 20 pg of cobalt and O-l-2.0 ml of 0.17; dye solution were measured. It was found that 0.5 ml of dye solution sufficed to complex up to 20 pg of cobalt. For qualitative detection of cobalt, the excess of reagent can be destroyed by the addition of hydrogen peroxide and heating at about 50-60”’ on a water-bath. Then the colaur of the cobalt complex can be detected more easily by eye. The mjn~rnurn time For complete colour development was found to be l-2 min at room temperature. The absorbance was then stabte for at least a week. The calibration graph proved to be iinear over the range @l-i,2 ppm of cobalt. The effective molar absorptivity for the cobalt complex was 3.6 x lo4 Lmole-” .cm-’ at 620 nm. Efict

of foreign ions Numerous cations and anions were examined by applying the method to fixed amounts of cobalt in the presence of increasing quantities of the ion being studied (Tables 2 and 3). It is evident that many have no effect at the levels studied. Among the cations, interference was caused only by iron(W) and large amounts of copper. Iron(III), about 20 mg, can be masked by the addition of 2 ml of 6% hydrogen peroxide, which interferes appreciably in the formation of the iron complex. Fortunately, hydrogen peroxide does not attack the redgent at room temperature, so 5-Cl-PADAPy can be used for determining microamounts of cobalt in n~any industrial and natural materials without separation.

SHORT COMMUNICATIONS

Table 2. Effect of foreign Metal

Amount added. ?I

Co found. @I

2 4

20.0 19.8 20.0 19.8 19.4 19.7 20.4 20.4 20.9 20.8 20.9 19.6 19.0 20.2 20.3 19.9 20.5 236 19.8 20.0 19.7

Al”’

.

Bi”’

$ 2 4 0.05 2.5 5 0.1

Cd’cu2+ Cr6 + Cr3-

0.2 0.4 5* 10* 15* 208 0.01 2 2 4

Fe” Fe”

Hg” La3+

* 2 ml of 6?’0 H,O,

Amount added. 9

K,SO,

1 1

KNO, KC1 KBr KI

h’nture

1 1 1

qf the cobalt

Error. !Jg

+o.o -0.2 +o.o -0.2 -0.6 -0.3 +0.4 +0.4 +0.9 f0.8 +0.9 -0.4 -1.0 +0.2 +0.3 -0.1 +05 +3.6 -0.2 +o.o -0.3

on determination Metal

of 2OQ 11g of Co Co found, /+I

Amount added. mg

Mg 2’

19.9 19.7 20.2 21.0 20.2 21.2 20.0 20.0 20.0 20.0 20.1 20.3 20.6 24.1 20.1 19.7 20.2 19.8 20.2 20.0

Mn2+ Ni” Pb’+ Th4+ Ti4+ We+ Y’+ Zn2+ Zr4+

Error, !+I

-0.2 -0.3 +0.2 + 1.0 +0.2 +1.2 50.0 50.0 +o.o +o.o +0.1 +0.3 +0.6 +4.1 +0.1 -0.3 +0.2 -0.2 +@2 * 0.0

added.

Table

Salt

cations

429

3. Effect of anions

Co found, !-G? 20.0 20.0 20.0 19.7 16.6

Error.

(20 pg of cobalt)

Salt

p9 +OQ * 0.0 kO.0 -0.3 - 3.4

K2CO3

NaCIO, NaCl NH,H2POI Tartaric acid

Amount added,

Co found,

Error,

4

#

IGJ

1 1 1 1 1

18.7 19.2 20.0 19.8 19.7

- 1.3 -0.8 kO.0 -0.2 -0.3

complex

The empirical formulae of the complexes were studied by the continuous-variation and mole-ratio methods. The curves obtained indicate the formation of a complex 1 : 3 metal : ligand ratio at pH 5 and in 1.2M hydrochloric acid. However. it is difficult to see how the three ligands can be co-ordinated to cobalt, on account of the structure of the reagent. A further critical study of the reaction is therefore planned. The present communication discusses only the analytical conditions. Recom~ertdeti

pocedu~.e ,fov the detemirlation

qf cobalt

Into a 25-m) volumetric flask transfer a suitable aliquot of sample solution containing up to 25-30 gg of cobalt. and add 1.0 ml of ethanolic O,l”, reagent solution. Adjust to pH 5 with 5 ml of buffer solution and mix. Then add 5 ml of hydrochloric acid (1 + 1). dilute to volume and mix. Measure the absorbance of the cobalt complex at 620 nm against a reagent blank. Obtain the concentration of cobalt from a standard calibration curve obtamed under identical conditions.

430

SHORTCO~MUNlCATlONS Table 4. Determination

Alloy For electric resistors Konel

For glass fusion EMK

of cobalt in some alloys

Composition, %

Cobalt found.:<

Ni, Co, Ti, si, Al,

73,07 17.16 8.8 0.5 0.26

17.20 17.19 17.21

Ni, Co, Cr, Fe,

30.0 25.0 8.0 37.0

25.3 25.1 253

17.20

25.2

Determination of cobalt in some alloys

A sample (O-1g) of alloy was dissolved in the usual way and cobalt determined on an aliquot of the solution. Some results are shown in Table 4. Government Industrial Research Institute Nagoya, Kita-ku. Nagoya. Japan

S~ozo SHIBATA

MASAMICHIFURUKAWA KAZVO GOTO

REFERENCES 1. S. Shibata, M. Furukawa, Y. Ishiguro and S. Sasaki, Anal. Chim. Acta, 1971, 55, 231. 2. S. Shibata, K. Goto and E. Kamata, ibid., 1%9,4S, 279. 3. S. Shibata, M. Furukawa, E. Kamata and K. Goto, ibid., 1970, Sa, 439. 4. S. Shibata, M. Furukawa and S. Sasaki, ibid., 1970, 51, 271. 5. Sh. T. Talipov, V. S. Podgornova and S. N. Kosolapova, Zh. Analit. Khim.. 1969, 24, 409

Emory-Cobalt(I1) and 3-[(5~hloro-2-pyridyl~o]-2,6~iaminopyridine (5-CI-PADAPy) in slighdy acid, neutral or alkaline media form a blue complex which is very stable even in the presence of mineral acids. The complex has two absorption maxima, at 575 and 620 nm, in I.ZM hydrochlororic acid. The system conforms to Beer’s law; the optimal range for a l-cm ceil is 0.2-1.2 ppm cobalt. Milligram amounts of common anions and cations do not interfere. The molar absorptivity is 3.69 x IO“ l.mole-L.cm-i at 620 nm. Zusammenfaasung-Kobaft(I1) und 3-[(5-Chlor-2-pyridyl)azo]-2,6-diaminopyridin (5X1PADAPy) bilden in schwach sauren, neutralen oder alkalischen Medien einen blauen Komplex, der selbst in Gegenwart von Minerais~uren sehr stabil ist. Der Komplex hat in 1~2M Salzdure zwei Absorptionsmaxima bei 575 und 620 nm. Das System gehorcht dem Beerschen Gesetz. Der beste Konzentrationsbereich fur eine 1 cm-Zelle ist 0.2-1.2 ppm Kobalt. Milligrammengen gangiger Anionen und Kationen stiiren nicht. Der molare Extinktionskoeffizient betragt bei 620 nm 3.69. IO4 I mol-‘cm-‘. R&urn&-Le cobalt (II) et la 3-[(S-chloro 2-pyridyf) azo] 2,6diaminopyridine (5Cl-PADAPy) en milieux I&gerement acide, neutre ou afcalin forment un complexe bleu qui est tres stable meme en la presence d’acides mineraux. Le complexe a deux maximums d’absorption, a 575 et 620 nm, en acide chlorhydrique 1,2M. Le systeme suit la loi de Beer: le domaine optimal pour une cellule de 1 cm est de 0,2-1,2 p.p.m. de cobalt. Des quantites d’anions et de cations communs de l’ordre du mgr n’interferent pas. Le coefficient d’absorption molaire est de 3.69 x lo4 Lmole-’ cm-’ 6 620 nm.