The reactions of cobalt compounds with nitroso-naphthols—V[1]

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 4231 4236. Pergamon Press. Printed in Great Britain.

T H E R E A C T I O N S OF C O B A L T C O M P O U N D S N I T R O S O - N A P H T H O L S - - V [ 1]

WITH

THE STOICHEIOMETRY OF THE REACTION BETWEEN COBALTOUS IONS A N D 2-NITROSO-1-NAPHTHOL-4-SULPHONATE S. A. BAJUE and G. C. L A L O R Chemislrv Department. University of the West Indies, Kingston 7, Jamaica (Received29 December 1972) A b s t r a c t - - T h e stoicheiometry of the reaction between Co 2 ÷ and 2-nitroso-l-naphthol-4-sulphonate is reported over a range of concentrations and pH. The product is the Co(III) complex. At low pH values the oxidation is a two electron change with ligand as oxidant. At intermediate p H values even in relatively dilute solution the ligand oxidizes CoII in a four electron change yielding C o L 3 and the unstable 2-amino-lnaphthol-4-sulphonate. Above p H 7 oxidation by oxygen is the d o m i n a n t process.

INTRODUCTION

WI-nLw much has been published on the reactions between cobalt and 1-nitroso-2naphthols the corresponding reactions of 2-nitroso-naphthols have been rather neglected. This paper presents the results of a study on the stoicheiometry of the reaction between the cobaltous and 2-nitroso-l-naphthol-4-sulphonate ions under a variety of conditions of reactant concentrations and of pH. EXPERIMENTAL The purification of 2-nitroso-l-naphthol-4-sulphonic acid and the experimental techniques used were previously reported[l, 2]. Oxygen uptake measurements were carried out on a standard W a r b u r g apparatus at 30°C, at pH values of 5.03 (acetic acid-acetate), 6-86 (phosphate) a n d 9.18 (borax). Concentrations of reactants in the range of 10 -3 M were chosen so that while oxygen was present in excess a significant volume change was observed on reaction. Blank determinations with Co 2 ÷, H L and C o L 3 solutions separately at appropriate concentrations showed no 0 2 uptake for the product, or for either reactant separately, under the present experimental conditions. Spectra were obtained using either a manual Zeiss P M Q I I or a Unicam S.P.700 recording spectrophotometer as appropriate. p H titrations were carried out in a previously described apparatus[3]. Aliquots of Co 2 ÷ were added to a solution of the ligand at a known pH which was maintained by the addition of borax. Slope-ratio measurements[4] were used extensively to examine the spectrophotometry of the reactions. When anaerobic measurements were carried out the nitrogen bubbling technique making use of syringes for transfers was used.

1. 2. 3. 4.

G. C. Lalor and G. A. Taylor, J. inorg, nucl. Chem. 35, 4221 (1973). S. A. Bajue, G. A. Taylor and G. C. Lalor, J. inorg, nucl. Chem. 34, 1353 (1972). H. Cleghorn and G. C. Lalor, Lab. Pract. 19, 394 (1970). J. H. Yoe and A. L. Jones, Ind. Engng Them. Analyt. Edn 16, 111 (1944). 4231

4232

S.A. BAJUE and G. C. LALOR RESULTS

Oxygen uptake The available equipment limited the oxygen u p t a k e studies to a n a r r o w con-. centration range. In this range the rate of oxygen u p t a k e increases with p H but : 1. It is very m u c h lower than the rate of f o r m a t i o n of C o L 3 as observed spectrophotometrically. 2. O x y g e n uptake vs time curves (Fig. 1) show a relatively rapid initial uptake followed by a m u c h slower reaction at p H 7 a n d 9. At p H 5 oxygen u p t a k e is slow. 3. T h e final a m o u n t of oxygen absorbed depends on pH. At p H 5, in the presence of at least sufficient ligand to ensure complete reaction, the [ O 2 ] : [ C o 2+] ratio is only 0"1. At the higher p H values this ratio approaches the value of 0.25 which would be expected from the simplest stoicheiometric equation C o 2+ + 3 H L + 0.2502 -~ C o L a + 0 . 5 H 2 0 + 2 H +.

(1)

Some typical results are summarized in Table 1. Table 1. Amount of oxygen absorbed during the reaction of Co 2+ with 2-nitroso- 1-naphthol-4-sulphonic acid pH

#

moles Co2+

5 6-9 6-9 9-2 9.2

#

moles HL

4-97 4.97 6.63 4-97 6.63

moles 02

[O2]/[Co2+]

0.58 _+ 0.05 1.11 -I- 0.08 1.62 + 0.08 1-3 -I- 0"08 1.38 -t- 0-06

0.12 0.22

#

19.9 19-9 19.9 19.9 19.9

0.26 0.21

Titration studies T h e ratios of [ C o ] / [ H +] and [ C o ] / [ H L ] o b t a i n e d in the titration studies are summarized in Table 2. 1.5--

~fL.---I

/ 0~ :L

o~

....4.....~~

......

~-

I

I

1

50

60

90

rain

Fig. 1. Oxygen uptake by mixtures of Co2+ and HL. [Co2+] = 1-84 x 10.4 M. [HL] = 7.36 x 10-3 M. Solution volume = 2,7 ml. ( x ) pH 5.0; (C)) pH 6.9; (0) pH 9-2.

Reactions of cobalt c o m p o u n d s - - V

4233

Table 2. S u m m a r y of stoicheiometric ratios observed in titration experiments pH

[Co]/[H + ]

5-4 3.2 4.0 5.4

[Co]/[HL]

1 : 1.8 1 : 1.77 1 : 1.77 1 : 1.84

Conditions

1:3.3 1 : 3.44 1:3.1 1 : 3.05

oxygen free air saturated ,,

Spectrophotometric studies In the presence of an excess of C o 2 ÷ the complex Co(II)L is formed. The stability constant of this complex is high enough so that the formation of Co(III)L a is retarded[2]. Generally, however, Co(III)L3 is formed at a rate which is very slow at low pH values, moderately fast about pH 5, and very rapid at pH 9. Above pH 12, CoL a undergoes the usual base substitution reaction of a Co(III) complex. Figure 2 shows some typical slope ratio plots. The results are summarized in Table 3. At [Co 2+] ~< 10-¢M the stoicheiometric ratio is 1:3 over the pH range 4-11. As the concentration increases the [Co] : [HL] ratio approaches 1:3-25. At low pH values, ca.2, the stoicheiometry is 1:3.5; these mixtures require over six weeks for the reaction to go to completion. In 0-05 M NH a solution the 1:3 complex is not formed. The stoicheiometric ratio is now 1:2 indicating the formation of [Co(NHa)2L2]- which has now been prepared[5]. In 0.05 M NaOH there is no reaction.

°I

06

c 04

--

o <

O2

pH = 9 2

I 2

I

I

4

6

2+

rHi ~ / fro ]

Fig. 2. Typical slope ratio plots for the reaction between Co 2 + a n d 2 - n i t r o s o - l - n a p h t h o l - 4 s u l p h o n a t e in air: ( 0 ) 32.500 c m - 1, (O) 40,500 c m - 1, ( × ) 26,800 c m - 1 ; 1 c m p a t h length. [Co 2+] = 8.0 X 10 -6 M. 5. S. Bajue a n d G. C. Lalor, T o be published.

S. A. BAJUE and G. C. LALOR

4234

Table 3. Results of slope ratio measurements on the reaction of cobaltous ions with 2-nitroso-l-naphthol-4sulphonate, at 25°C. Ligand concentrations varied from [HL] = [Co 2+] to [HL] = 6 x [Co 2+] [Co 2 +] 8 8 8 8 8 8

X x

× x x x

10 -6 10 -5 10 -4 10 -4 10- 4 10 4

pH

[Co] : [HL]

5-11 4-9 7-9 4-5 3-0 2.0

1:3 1:3 1:3.1 1:3.2 1 : 3-25 1:3.5

Oxygen free solutions The rate of formation of CoLa is very much reduced in the absence of oxygen as is shown by the comparison in Fig. 3. The observed stoicheiometry is 1:3-25. Spectra of solutions containing stoicheiometric ratios are given in Fig. 4.

Products of the reaction The reaction mixtures were examined by TLC on silica using n-propanol/nbutanol/water (4:3:3) as solvent and by column chromatography using Sephadex with 0.01 M HC1 as eluant. In all cases Co(III)L3 is the major product. Where the stoicheiometry is 1:3 no product other than Co(III)L a was detected. In the more concentrated solutions where the stoicheiometry is greater than 1:3 several other products were obtained. These were shown to be oxidation products of 2-amino-1naphthol-4-sulphonic acid.

...//-"I" cf 03 -

/

[Co2~:5.0x 166

, /

[HL] pH

OI

L

:

1 . 8 4 x 1 0 -5

: 9.2

~x ~

x

~

I

I

I

2

m~n Fig. 3. Comparison of the rates of formation of Co(III)L3 in air ( 0 ) and in the absence of air ( x ) at room temperature.

Reactions of cobalt compounds--V

\.X 09

~c

4235

?

''-. \ . I\ ~

--

/

06

o .Q

0 3 - -

J 30 -3 I0 ~ ,

20 cm ~

Fig. 4. Spectra of stoicheiometric mixtures of Co 2÷ and 2-nitroso-l-naphthol-4-sulphonate at pH9.2, at lcm path length. [Co2+] = 1.34 x 10-5 M. [Co2+]:[HL] ratios are 1:3 ( ) ; 1:3'25 in air (--) showing the presence of excess HL, and 1 :3.25 in the absence of air ( .I showing additional absorption due to 2-amino-l-naphthol-4-sulphonate.

DISCUSSION That the complex is Co(IlI)L 3 was confirmed by the titration studies and by spectral comparisons with known pure compounds. The effectiveness of molecular oxygen as an oxidant for Co(II) in the presence of suitable ligands, particularly in alkaline solution, is of course well known. However, the ligand itself is readily reduced by a 4 electron change to 2-amino-l-naphthol-4-sulphonic acid which is readily oxidized to a variety of products. There are therefore two potential oxidants and the reaction path leading to Co(III)L3 may be determined either by the thermodynamics or kinetics of the system. It is not surprising therefore to find that the reaction stoicheiometry depends on reactant concentration and on pH. In the absence of oxygen, the oxidation can only be performed by the ligand with the expected, and observed, stoicheiometry described by equation (2). / Co01) + 3.25 R

NO

/ ~ Co(III)[R

\ OH

NO

/ ]3 + 0.25 R

\ O-

+0.25 H 2 0 + 2H +.

NH2

\ OH (2)

S. A. BAJUE and G. C. LALOR

4236

/

NO

The notation R

is used for the ligand to show the functional groups specifically.

OH At comparable concentrations of ligand and oxygen in alkaline solutions, oxidation by oxygen is much the more rapid reaction and the stoicheiometry is accurately described by Eqn (1). When the concentration of the ligand is considerably greater than that of [O2] in solution, however, the ligand competes with oxygen as oxidant. The ligand also becomes the more effective oxidant relative to oxygen as the pH is lowered and even in dilute solution oxygen apparently plays no primary role in the reaction below pH 4. At pH values in the region of 2 the observed stoicheiometry is 1 : 3.5. This indicates that the ligand is now involved in a two electron redox change according to Eqn (3). / COOI) + 3-5 R

/ ~ Co(III)

\ OH

I

+ 0-5 R

\ O-

/ + 2 H +.

\

(3)

OH

3

The reactions of cobaltous ions with 2-nitroso-1-naphthol-4-sulphonate and with 1-nitroso-2-naphthol-3,6-sulphonate exhibit many similarities but there are some features of substantial difference. Both compounds form red complexes Co(III)L3 and some of the stoicheiometric ratios observed[l] are similar. With the 2-nitrosocompound there is no hint of a one electron redox process involving the ligand ; on the other hand the two electron change observed here is not found for the 1-nitrosoligand. The major difference between the two ligands, particularly in the concentration range of interest for kinetics studies, lies in their relative oxidative ability towards Co(II), compared with that of oxygen. The 1-nitroso-compound is the better oxidant, and is probably the primary oxidant under all conditions since the rate of formation of its cobalt(III) complex is independent of [02] [1]. 02 plays a more important role in the reaction of the 2-nitroso-compound with Co 2 +. No data are available for the E o values of the redox reactions of these nitroso-naphthols, but it is expected that kinetics studies now in progress will provide information on the mechanisms of the reactions.