Chelate Formation of Tetravalent Vanadium with Pyrocatechol Violet S. P. Mushran, Om Prakash, and J. N. Awasthi Chemical Laboratories, The Unicersity, Allahabad, India denum(V1) and tungsten(V1). In the present communication we have studied the chelate-forming vanadium(1V)-pyrocatechol violet reaction, and the composition and stability of the chelate have been established.
AMONG SULFONPHTHALEIN DYES 3,3 ’,4’-trihydroxyfuchsone2”-sulfonic acid, usually known as pyrocatechol violet (PCV), is of particular interest because of its tendency to form colored chelates in aqueous media. The reagent was obtained in a pure state by Vodak and Leminger ( I ) by condensing o-sulfobenzoic anhydride with pyrocatechol. The reagent has been widely used by Malat and Suk (2) in complexometric titrations. Recently the photometric study of its complexes with tin(1V) and titanium(1V) have been reported by Maltit ( 3 ) ; Majumdar ( 4 ) has investigated its complexes with molyb-
EXPERIMENTAL
A standard solution of vanadyl sulfate (BDH) was prepared by dissolving the salt in double distilled water and was estimated by using ammonium benzoate as precipitant. Pyrocatechol violet solution was prepared by dissolving a weighed quantity (BDH) in double distilled water. Spectrophotometric measurements were made with a Beckman (DU) spectrophotometer. For p H measurements a k e d s & Northrup direct reading pH indicator with a glass-calomel electrode system was used. The instrument was standardized from time t o time with a standard buffer supplied with the instrument. Measurements were made
(1) Z.Vodak and 0. Leminger, Collection Czech. Chem. Cornmuti., 19, 925 (1954). ( 2 ) V. Suk and M. Malat, Chemist-Analyst, 45, 30 (1956). (3) M. Malat, Z . Anal. Chem., 201(4), 262 (1964). (4) A. K. Majumdar and C. P. Saraviar, Naru~~~isset~scl~afteti, 45, 84 (1958).
9
0
400
I
I
I
450
550
500
WaveIength
I
CQO
5
(mp)
Figure 1. Absorption spectra of V(IV)-pyrocatechol violet chelate Total volume, 25 ml; pH, 4.2; Temp, 25’ C Final concentration M X 10-6
voso4 0.00 8.00 4.00 2.00 1.33
PCV 4.00 4.00 4.00 4.00 4.00
Ratio, V(1V) :PCV 0:4.0 2:l.O 1:l.O 1:2.0 1:3.0 VOL. 39, NO. 1 1 , SEPTEMBER 1967
1307
VOSO, (ml)
(Vd?
/
po+y + Pcv]
Figure 2. Determination of chelate composition by method of continuous variations at 600 mp and p H 4.2 Concentration of VOS04and PCV: ( A ) 2.00 X 10-4M (B) 1-33 x 1 0 - 4 ~ (c) 1.00x i o - 4 ~ 4 a t 25" C. All the experiments were performed at pH 4.2, and all the solutions were adjusted to this pH by adding suitable amounts of hydrochloric acid or sodium hydroxide. Total volume in all the cases was 25 ml. RESULTS
The color formation was instantaneous and no fading of color was observed up to 24 hours. The order of addition of reactants had no appreciable effect on the absorbance values. The color intensity of the chelates was stable over a wide range of temperature (5"-60" C). The absorption spectra of the reagent and mixtures of reagent and metal in different proportions a t p H 4.2 are represented graphically in Figure 1. The method of Vosburgh and Cooper ( 5 ) was employed in order to ascertain the formation of only one complex under the conditions of study. At p H 4.2 the, , ,A of the reagent is 450 m p while that of the chelate is 600 mp. Composition of the chelate is 2 :1 (metal :reagent), which is confirmed by the method of continuous variations (6) (Figures 2 and 3) and mole ratio (7) (Figure 4).
Figure 3. Determination of chelate composition by method of continuous variations employing nonequimolar solutions at 600 mp and p H 4.2 Concentration of VOSO, and PCV: ( A ) 2.00 X 10-4M VOSOd and 1.00 X 10 -4MPCV ( B ) 1.00 x 10-4MVOS04and 2.00 X 10-4MPCV
Table I. Log K and AGc Values of V(IV)-Pyrocatechol Violet Chelate pH 4.2 f 0.1; temperature 25" C Continuous
1308
ANALYTICAL CHEMISTRY
9 - 12
A series of mixtures containing vanadyl sulfate and pyrocatechol violet in the ratio 2 : l were prepared at different p H values, and their absorbances were measured between 380 m p and 625 mp. The, , ,A of the different mixtures was plotted against pH (Figure 5). The range of pH in which the, , ,A of the chelate holds good was taken as the pH range of chelate stability. The stability constant values of the chelate were evaluated b y two different methods: (1) method of continuous variations, employing the following equation p + n - I
,nn-i
( 5 ) W. C. Vosburgh and G. R. Cooper, J . Am. Chem. SOC., 4, 1630
(1942).
Mole ratio method
9 - 12
log K AGC, kcal
K = (6) P. Job, Ann. Chim., (Paris), (X)9, 113 (1928). (7) J. H. Yoe and A. L. Jones, IND. ENG. CHEM., ANAL.ED., 16, 111 (1944).
variation method
where K
=
m
=
n
=
nm-i
p"-1 [ ( p m
(p
-
+ n) x
- ny+n
1)mLn-l [n - (m
+ 4x1
conditional stability constant number of metal ions in the chelate number of reagent molecules in the chelate
(1)
40
Figure 4. Determination of chelate composition by mole ratio method a t 600 mp and p H 4.2 3
Final concentration of PCV: ( A ) 8.00 X 10-5M ( B ) 4.00 x 1 0 - 5 ~
PH Figure 5. Variation of A,
p = ratio of the concentration of the reagent to the x =
concentration of the metal ion mole fraction of metal at absorption maximum
For method (2) the following equation was employed
where a = degree of dissociation A m = maximum absorbance in the mole ratio plot A s = absorbance a t the stoichiometric molar ratio in the mole ratio plot m = number of metal ions in the chelate n = number of reagent molecules in the chelates C = final concentration of the reagent The values of log K and AG' obtained by the above methods have been tabulated in Table I, where K and AGc represent the conditional stability constant and free energy change of formation, respectively. The linearity between the absorbance of vandium(1V)pyrocatechol violet chelate and the vanadium(1V) concentration was tested by varying the metal concentration and measuring the absorbance at 600 m p a t p H 4.2. The range of ad-
of chelate with pH
herence t o Lambert-Bougert-Beer's law is 0.2 to 11.4 ppm of vanadium(1V). transmittance us. log [F4] gave A Ringbom plot (8) of a n optimum range as 1.2 t o 11.4 ppm of vanadium(1V). The relative error per 1% absolute photometric error, as calculated according t o Ayres' (9) equation, is 2.8. The sensitivity of the color reaction, as defined by Sandell, is 0.016 y/cm2. The practical sensitivity, corresponding to an absorbance change of 0.010 unit, is 0.16 y/cm2. Pyrocatechol violet is thus a very sensitive reagent for the detection and microdetermination of vanadium(1V) when present singly, involving a negligible amount of error in photometric analysis. Interference due t o the presence of metal ions from Groups IV, V, and VI were investigated, and it was observed that they interfere at all concentrations. RECEIVED for review November 8, 1966. Accepted May 22, 1967. Financial aid to Om Prakash was furnished by the Council of Scientific and Industrial Research, New Delhi, India. (8) A. Ringbom, 2. Anal. Chem., 115, 332 (1939). (9) G. H. Ayres, ANAL.CHEM., 21,652 (1949).
VOL. 39,
NO.
1 1 , SEPTEMBER 1967
1309
