was further extended for the extraction and spectrophotometric determination ofvanadium(V)'. successive extraction and photometric determination of iron(III).
/
Annali di Chimica, 86, 1996, by Societa Chimica ltaliana
401
SPECTROPHOTOMETRIC DETERMINATION OF WITH N-PHENYLCINNAMOHYDROXAMIC ACID
Bhagwan Singh CHANDRA VANSHI
C)
VANADIUM(V)
and Berhanu TEMESGEN
Department of Chemistry, Addis Ababa University P.O. Box 1176, Addis Ababa (Ethiopia)
well Scientific Summary - N-PhenylcinnanaJhydroxamic acid (PCHA) reacts with vanadium(V) to form a 1:2 (V:PCHA) bluish-violet coloured complex, quantitatively extractable into chloroform from 3-9 M hydrochloric acid solutions. The spectrum of the complex in the visible region exhibited a broad absorption maximum at 545 nm with a molar absorptivity of 6300 M·I ern I. The coloured system obeyed Beer's law in the concentration range of 0.5-10 ug ml·1 of vanadium. The effects of several experimental variables on the extraction of the complex have been studied to establish the optimum conditions for the determination of vanadium(V). On the basis of these studies a simple, precise, and sensitive method has been developed for the spectrophotometric determination of vanadium(V). The method has been successfully applied to the determination of vanadium in standard steels.
Riassunto - L'acido N-fenilcinnamoidrossamico (PCHA) forma con il vanadio(V) un complesso 1:2 (V:PCHA) colorato in blu-violetto ed estraibile quantitativarrente in cloroformio, da soluzioni 3-9 M di HCL Lo spettro del complesso presenta nella regione del visibile un massimo di assorbimento a 545 nm con un 'assorbivita molare di 6300 M·I crn'. La legge di Beer e seguita in un intervallo di concentrazione di 0.5-10 ug rnl" di vanadio(V), Le condizioni ottirnali per la deterrninazione del vanadio so no state determinate studiando I'effetto di numerose variabili sperimentali sull'estrazione del complesso. II metodo sviluppato, sernplice, preciso e sensibile, stato applicato con successo alia determinazione del vanadio in campioni di acciai standard.
e
n, Author
to whom correspondence
should be addressed.
j 402
CHANDRA
V ANSHI and coworker solution was eva slightly acidifiec acid were filtere volume in a 10( determination 0:
INTRODUCTION
N-PhenyJcinnamohydroxamic acid (PCHA) was first synthesized in 1960 and introduced as an analytical reagent for the gravimetric determination of niobium and tantalum I. Its analytical application was further extended for the extraction and spectrophotometric determination ofvanadium(V)'. successive extraction and photometric determination of iron(III). vanadium(V). and uraniumf Vl ): , spectrophotometric determination of titanium(lV) and niobiumfV)". and gravimetric determination of zirconiurnrl V)'. Recently PCHA has been used for the extraction and spectrophotometric determination of vanadium(V) as mixed ligand complexes with thiocyanate" and p-chlorophenof . extraction and spectrophotometric determination of ironil ll)!", ironrll)". and simultaneous determinations of iron(III) and vanadiumt V)!'. and iron(lIl) and cobalttll)!'. The physical properties of PCHA 13 and its dissociation constant". and stability constants of its complexes with zirconium(lV) and hafniumtlv)", and irorulll)" have also been determined. The physico-chemical studies of the metal complexes ofPCHA need a precise method for its determination. PCHA reacts with vanadiurnt V) to form an intensely coloured complex which could be used for its quantitative determination. The reaction of PCHA with vanadiumrV) has been studied in detail in the present investigation to develop a method for the determination of vanadium and to extend its application to the analysis of real samples. The present investigation has led to the development of a simple. precise. and sensitive method for the spectrophotometric determination ofvanadium(V). Examples are given for the determination of vanadium in steels.
EXPERIMENT
AL
Equipment and reagents A Beckman Model 24 UV- Visible spectrophotometer equipped with l-cm path length silica cells was used for the recording of absorption spectra and absorbance measurements. A Beckman Chern Mate pH meter was used for the measurements of pH. Analytical-grade reagents were used unless otherwise specified. Distilled water was used throughout. Other solvents were redistilled. N-Phenylcinnamohydroxamic acid (PCHA) was prepared by the method reponed ". A 0.005 M solution of PCHA was prepared by dissolving the recrystallized reagent in chloroform (BDH. AnalaR distilled) and was stored in an amber coloured bottle in a dark place. A standard solution of vanadium(V) was prepared by dissolving 1.1700 g of ammonium meta vanadate (BDH. AnalaR) in water and diluted to 1.0 liter with water. The solution was standardized conventionally. Procedures Steel - A weighed quantity of the steel sample (0.1 g) was heated gently in a 400-ml beaker with 10 ml of concentrated nitric acid until the brisk reaction ceased. 5-10 ml of aqua regia was added. and the
Determination a transferred into, to 4-6 M and I 0 M PCHA solutii minutes. and the in a 50-ml beal« twice with a few was transferred were added to th was measured at For calibratic were used throu:
Solvenl for extra Several orga chloroform. and . aqueous phase. C of the vanadium: Absorption speci The absorpt range 400-700 r different coloure The vanadii similar spectra w and strong absorj maximum at 48 wavelengths witl remained consta intensity of the al The intense abso: metal, i.e. with tl J t has been fc phase. Nitric and
'.
Spectrophotometric
Determination
of Vanadium(V)
solution was evaporated to near dryness to expel nitrogen oxides. The residue slightly acidified (hydrochloric acid) water by heating. The undissolved silicic acid were filtered off and washed several times with water. The filtrate and volume in a 100-ml volumetric flask with water. A suitable aliquot of this determination of vanadium.
introduced as an lyrical application .m(V)2, successive j uraniumt Vl)", determination of tric determination If . extraction and ations of iroruf ll) id its dissociation )1;. and ironrl ll)" HA need a precise
403
was dissolved in 50 ml of acid and hydrated tungstic washings were diluted to solution was used for the
Determination of vanadium - An aliquot of the solution containing up to 250 ug of vanadiumtV) was transferred into a 100-ml separatory funnel. The acidity and volume of the aqueous phase were adjusted to 4-6 M and 10 ml with concentrated hydrochloric acid and water, respectively. A 10 ml aliquot of 0.005 M PCHA solution In chloroform was added to the funnel, the mixture was shaken vigorously for two minutes, and the funnel was allowed to stand to separate the two phases. The organic phase was collected in a 50-ml beaker containing about 2 g of anhydrous sodium sulphate. The aqueous phase was washed twice with a few ml of chloroform and the washings were collected in the beaker. The coloured extract was transferred into a 25-ml volumetric flask. The beaker was washed with chloroform. the washings were added to the tlask and diluted to volume with chloroform. The absorbance of the coloured extract was measured at 545 nm against chloroform as reference. For calibration. 0.5. 1.0~ 1.5.2.0, and 2.5 ml of the standard solution ofvanadium(V) (100 flg rnl') were used through the procedure.
lid be used for its
ed in detail in the 'nd its application RESULTS AND DISCUSSION ve method tor the ation of vanadium Solvent for extraction Several organic solvents such as benzene. toluene, xylene, chlorobenzene. 1,2-dichlorobenzene. chloroform. and carbon tetrachloride, were found to extract the vanadium(V)-PCI-IA complex from the aqueous phase. Chloroform was found to be the most suitable solvent because the quantitative extraction of the vanadium(V)-PCHA complex was readily accomplished in it.
th silica cells was n Chern Mate pH unless otherwise ed". A 0.005 .\1 n (BDH, AnalaR 'urn metavanadate -d conventionally.
eaker with 10 ml as added. and the
Absorption spectrum and acidity The absorption spectrum of the vanadium(V)-PCHA complex in chloroform was recorded in the range 400-700 nm. It was found that PCHA reacts with vanadium(V) at different acidities forming different coloured complexes (which are readily extractable into chloroform). The vanadium(V)-PCHA complex formed in the weakly acidic solution. i.e. at pH 6-4. showed similar spectra with increasing absorption in the region 700-490 nm, a plateau or shoulder at 490-.440 nrn, and strong absorption with no maximum below 440 nm. At pH 4-3 an absorption band appeared with a maximum at 480 nm and a minimum at 450-440 nm. The absorption band was shifted to longer wavelengths with increasing acidity of the aqueous phase up to I M with respect to hydrochloric acid and remained constant at higher concentrations of hydrochloric acid up to 10M in aqueous phase. The intensity of the absorption band remained constant in the concentration range 3-9 M hydrochloric acid. The intense absorption band is most probably associated with the charge transfer from the ligand to the metal, i.e. with the electronic transition L - M. It has been found that only hydrochloric acid was suitable for adjusting the acidity of the aqueous phase. Nitric and sulphuric acids were unsuitable because PCHA was unstable at higher concentrations
CHA"iORA
404
V ANSHI and coworker
of these acids. The absorption spectrum of PCHA was also recorded in chloroform in the visible region. The reagent showed negligible absorption in the region 700-500 nm. slightly increasing absorption between 500 and 400 nrn, and strong absorption below
