INTRODUCTION:

Chromium is steel –gray, lustrous, hard metal that takes high polish and has high melting point. Chromium exists in many chemical forms and in nature it is usually encountered in the oxidation states Cr (III) and Cr (VI).Chromium acts as oxidizing agent in many organic and biological reactions as it has free electrons and vacant d-orbital. It is used to prepare many alloys like nichrome, cobalt-alloy steel. It is widely used in coating industries to prepare smooth surfaces as solar collectors.

Chromates, which are chromium (VI) compounds are often used as pigments for photography and in pyrotechnics, dyes, and paints, inks and plastics. Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compouns increases risk of lung cancer^[1] . The reduction of Cr(VI) may serve to activate chromium toxicity if it takes place in or near the cell nucleus of target organs^[2].

More recent studies also disclosed excess risk of lung cancer death resulting from occupational exposure to Cr (VI) compounds. Due to its high toxic nature, the determination of trace amounts of chromium (VI) is of great importance.

A wide variety of techniques are available for the determination of chromium, such as ICP- MS^[3-5] ICP-AES ^[6] HPLC^[7], AAS^[8-9] and spectrophotometry ^[10-12]

The other reagents used for the spectrophotometric determination of chromium include Leuco Xylene cyanol FF^[13], Malachite green^[14], Ferron,^[15], Tetra butyl ammonium iodide^[16] . In the present paper, a simple and sensitive method is developed for the spectrophotometric determination of trace amounts of chromium (VI) by complexing with 5-bromo salicylaldehyde isonicotinoyl hydrazone (5-BrSAINH). The reagent forms light brown coloured complex with chromium (VI) in basic medium in the p^Hrange 5.5-6.5 is stable for more than72 hours. Hence, a systematic study has been made to develop a spectrophotometric method for the determination of chromium.

EXPERIMENTAL:

Apparatus:

The absorbance and pH measurements were made on a Perkin Elmer (LAMDA 25) UV-Visible spectrophotometer (Model UV-160A) controlled by a computer fitted with 1cm path length quartz cells and an ELICO digital P^Hmeter of (Model LI 613), respectively.

Materials and methods:

All chemicals used were of analytical grade. The 5-bromo salicylaldehyde and isonicotinoyl hydrazide were purchased from SD fine chemicals. Potassium chromate (VI) was obtained from Merck laboratories. 0.01M chromium (VI) solutions were prepared by dissolving 0.194gms of potassium chromate (E-Merck) in 100ml distilled water. The stock solutions were diluted appropriately as required. Other metal ion solutions were prepared from their nitrates or chlorides. Buffer solutions were prepared by mixing appropriate volumes of 1M CH3COOH and 1M HCl, (p^H 1.0-3.5) CH₃COOH and CH₃COONa (p^H 4.0-7.0) NH₄OH and NH₄Cl (p^H 8.0-10)

Preparation of 5-BrSAINH:

Equimolar solutions of 5-bromo salicylaldehyde (I) and isonicotinic acid hydrazide (II) were dissolved in sufficient volume of methanol and the mixture was refluxed for 1 hour. The contents were allowed to cool and the product was separated by filtration. A crude sample (yield 80%) was obtained. The resultant product was recrystallized thrice from hot methanol. Pure light yellowish green colored crystals of 5-bromo salicylaldehyde isonicotinoyl hydrazone (III) were obtained (m.p 238-240⁰c)

The light yellow solid was identified and confirmed as NMR Spectral analysis by Infrared and NMR Spectral analysis. 1x10^-2M solution of the reagent was prepared by dissolving 0.320gm of the reagent in 100ml of dimethylformamide (DMF). Working solutions are prepared by diluting the stock solution with DMF.

General procedure:

Direct spectrophotometry:

In each of a set of different 10ml volumetric flasks,4ml of buffer solution( p^H 5.0 ) 0.5ml of 5-BrSAINH (5x10^-5M) and different known volumes of 5x10^-5M .Chromium (VI) solution were taken and the contents were made up to the mark with double distilled water. The absorbance of the metal ion solutions was measured at 430nm against the reagent blank.The calibration plot was prepared by plotting the absorbance against the amount of chromium (VI).

Derivative method:

For the solutions as prepared above the first and second derivative spectra were recorded with reference to the reagent blank in the wavelength range 400-550nm. The derivative amplitudes were measured at 442nm for first order and 455nm for second order curves. Calibration graphs were constructed by plotting the derivative amplitudes against the concentration of Cr (VI) ions.

RESULTS AND DISCUSSION:

The absorption spectrum of [Cr (VI)-5-BrSAINH] complex showed maximum absorbance at 430nm where the reagent shows negligible absorbance was shown in Fig 1. The absorbance was found to be maximum and constant in the p^H range 5-5-6.5.Therefore the analytical studies were carried out at p^H6.0. A 10 fold molar excess reagent was found to be necessary to obtained maximum colour intensity for a given metal ion.

The composition of the complex was determined using Job’s continuous variation method. The results indicate a 1:1 stoichiometry between the metal ion and the reagent. The stability constant of the complex was determined as 1.40 X 10⁶. The calibration plot between absorbance and concentration of Cr(VI) shows that Beer’s law is obeyed by the system in the concentration range 0.26-2.6µgmL^-1 of Cr(VI) The straight line obeys the equation A₄₀₀=0.0722C+0.0187. The molar absorptivity and sandell’s sensitivity of the method were found as 2.0 x 10⁴ L.mol^-1cm^-1 and 2.6 X 10^-3 µgcm^-2, respectively.

The effect of diverse ions on the absorbance of the experimental solution showed that all the anions tested possess high tolerance levels (>100fold excess). The metal ions Mo (VI), V (V), Hg (II), and Pt (II) were tolerable up to 50-80 fold excess. U (VI), Ru (III), Fe ( III) was tolerable up to 15-45 fold excess, presented in table 1.

The derivative spectra recorded in the wavelength region 400-550nm for the experimental solutions showed maximum and proportional variable absorbances at 442nm (1^st order) and 455nm (2^nd order) were shown in Fig 2 and Fig 3. The analytical results obtained in direct and first and second derivative methods were tabulated in Table 1. The tolerance limits of some cations in derivative methods were compared with those in direct method and presented in Table 3

Applications:

Zero order method:

The proposed method was employed for the determination of Cr (VI) in soil samples and the results were presented in table 4

Determination of Chromium in soil samples:

A known amount of (1g) air dried homogenized soil samples, spiked with known amounts of chromium (VI) was taken and then fused with 5g anhydrous sodium carbonate in a nickel crucible and evaporated to dryness after the addition of 25ml of water. The dried material was dissolved in water, filtered through Whatman filter paper (No.40) in to 25mL calibrated flask and neutralized with dilute ammonia. It was then diluted to a known volume with water. An aliquot of this sample solution was analyzed for chromium (VI).The obtained results were tabulated in Table 4.

Derivative method:

The second derivative method was applied for the analysis of stanadard steel samples, and the results were presented in Table 5

Table 1: Tolerance limit of foreign ions. Amount of Cr (VI) = 0.25µg ml^-1 ,p^H= 6.0

Foreign Ion	Tolerance limit (Folds)	Foreign Ion	Tolerance limit (Folds)
Thiocyanate	2200	Zr(IV)	955
Tartatate	1800	Zn(II)	910
EDTA	1600	Bi(III)	900
Thiourea	1200	Tl(III)	870
Ascorbic acid	985	Au(III)	855
Iodide	750	W(IV)	790
Sulphate	650	Se(IV)	650
Thiosulphate	640	Sn(IV)	510
Carbonate	570	Pb(II)	480
chloride	490	Te(IV)	450
Fluoride	220	Y(III)	400
Nitrate	130	Ag(I)	330
		Al(III)	250
		Ni(II)	200
		Cu(II)	180
		Co(II)	120
		Cd(II)	100
		Mo(VI)	80
		V(V)	60
		Hg(II)	50
		Pt(II)	50
		U(VI)	45
		Ru(III)	35
		Fe(III)	15

Fig. 1. Absorption spectra of

(a) 5-BrSAINH Vs Buffer blank

(b) [Cr(VI) - 5-BrSAINH] Vs reagent blank

[Cr (VI)] = 5 x 10^-5M;[5-BrSAINH] = 5 x 10^-4

Wavelength = 430nm

Fig. 2. First derivative spectra of

Cr (VI) –5-BrSAINH Vs reagent blank

Cr (VI) (µg ml^-1) = (1) 0.42; (2) 0.68; (3) 1.36; (4) 2.72

Table 2: Analytical characteristics of [Cr (VI)-5-BrSAINH]

Parameter	Direct method	First derivative	Second derivative
Analytical wavelength(nm)	430nm	442nm	455nm
Beer’s law range	0.26-2.6	0.16-3.92	0.13-3.92
Molar absorptivity(L mol^-1 cm^-1)	2.0 X10⁴	-	-
Sandell’s sensitivity(µg cm^-2)	2.6 X 10⁴	-	-
Angular Coefficient(m)	0.4876	0.6248	1.0850
Y intercept(b)	0.0843
Correlation coefficient	0.9998	0.9999	0.9999
Relative standard deviation (%)	0.122	0.242	.417
Detection limit(µgmL^-1)	0.046	0.052	0.064
Determinatin limit(µgmL^-1)	0.072	0.085	0.095
Composition(M:L)	1:1	-	-
Stability constant	1.46X10⁶	-	-

Fig. 3. Second derivative spectra of

Cr (VI) –5-BrSAINH Vs reagent blank

Cr (VI) (µg ml^-1) = (1) 0.42; (2) 0.68; (3) 1.36; (4) 2.72

Table.3:Tolerance limits of foreign ions

Foreign ion	Tolerance limit in folds
Foreign ion	Direct method (430nm)	First derivative (442nm)	Second derivative (455nm)
Mo(VI)	80	90	105
V(V)	60	70	90
Hg(II)	50	65	90
Pt(IV)	50	60	85
U(VI)	45	55	75
Ru (III)	20	45	60
Fe(III)	15	25	40

Table 4 : Analysis of Soil samples

Sample	Cr added in µgml^-	Proposed method^*
		Cr found (µgmL^-1)	Relative error	Recovery (%)
Soil sample	4.00	4.02	+0.51	100.5
	5.00	4.98	-0.1	99.6
	6.00	6.04	+0.172	100.6

^*Average of five determinations

Table 5 : Analysis of standard steel samples

Sample

Chromium (VI) certified value (%)

Proposed method *(%)

Relative error

Recovery (%)

SAE304

(Cr18-Ni 9)

17.98

17.94

+0.10

98.86

SAE321

(Cr 18-Ni 10-Ti)

17.58

17.26

+1.23

97.5

^*Average of five determinations

CONCLUSIONS:

Rapid color development, simplicity and selectivity are the advantages of the proposed method. The intensity of the colored species will not be affected by slight variation of the experimental parameters such as concentration of the reagent. The proposed method does not involve extraction, heating or any other stringent reaction conditions and offers the advantage of high color stability (24h). The commonly associated metal ions, especially Zr(IV), Zn(II), Bi(III), T l(III) could be tolerated in considerable excess, which is an advantage over other reported reagents. The proposed method can be used as an alternative method for the determination of trace amounts of chromium in soil and standard steel samples.

ACKNOWLEDGEMENTS:

One of the authors (S. Sobha) thank for financial assistance from UGC and department of Chemistry for providing necessary facilities

REFERENCES:

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Received on 05.05.2013 Modified on 11.06.2013

Asian J. Research Chem. 6(7): July 2013; Page 667-670