Quick and Green Procedure for the Quantification of Chromium in an Anionic Surfactant System

This study aims to establish a facile, user-friendly, accurate, and repeatable spectrophotometric detection method for Cr(III) ions in trace quantities. The method utilises reagent 1-nitroso-2-naphthol (NNPh) in a sodium dodecyl sulphate (SDS) 1


Introduction
Metals play an important role in environmental and biological systems, 1 many metal ions, such as Zn(II), V(III), Cr(III) and Mn(II) are essential for human and other living organisms.Conversely, metals like arsenic, cadmium, lead, and mercury are toxic to living beings at certain concentration levels. 2,3Even essential metal ions can become toxic at elevated concentrations. 4Chromium, for instance, is essential for humans and other natural systems, 5 being a steel grey, shiny, rigid, and fragile transition metal. 6Its name is derived from the Greek word χρῶμα, chrōma, meaning colour, due to the extensive colouring of many chromium compounds.Chromium is a vital mineral that the body cannot produce on its own, but must be acquired through dietary sources.Chromium is a mineral found in two forms, 7 but only Cr(I-II) is utilised by the body and is present in food sources. 8t is essential for carbohydrate and fat breakdown, 9 supporting brain function and other bodily processes. 10Cr also supports the action of insulin and the breakdown of glucose. 11It is a metal element, which humans require in very small amounts.Brewer's yeast, broccoli, and liver are good sources. 12Supplementation with chromium can increase muscular mass, weight loss, and glucose control. 13Its deficiency may potentially be associated with certain health disorders. 14tallic chromium is highly valued due to its resistance to corrosion and rigidity. 15The addition of chromium metal to steels revolutionised steelmaking, producing stainless steel highly resistant to corrosion and discolouration. 16Cr is not stable in O 2 .It immediately forms a fine layer of oxide that is impervious to O 2 and shields the metal. 17Chromium's chief applications include metal ceramics, alloys, and chrome coating. 18Chromium is used in metallurgy to provide resistance against corrosion and a lustrous finish; 19 in paints and dyes, 20 in the production of artificial rubies; 21 it serves as a catalyst for leather dyeing and tanning; 22 it is used in the production of brick firing moulds; 23 as well as in the manufacture of magnetic tape. 24Cr is extracted from the ore chromite (FeCr 2 O 4 ). 25rious methods exist for the determination of Cr(III) ions, such as UV/Vis spectrophotometry, 26 flame atomic absorption spectroscopy, 27 atomic absorption spectroscopy, 28 inductively coupled plasma atomic emission spectrometry 29 etc.Many of these methods are costly due to the high price of tools, time-consuming, and tedious.New methods are required for the trace-level determination of chromium ions, with the spectrophotometric technique being prevalent due to its simplicity, precision, speed, and inexpensive instruments.In UV-Vis spectrophotometry, metal ions are determined using chelating agents, where the metal reacts with a derivatising agent to form an insoluble metal complex that is solubilised through solvent extraction.Numerous spectrophotometric approaches for analysis of metals are available to replace the previous method of solvent extraction with the use of a micellar system. 30Micellar methods have shown enhancement in the analytical characteristics of metal analysis by solubilising the complex. 31ecently, some spectrophotometrical procedures of lower Quick and Green Procedure for the Quantification of Chromium in an Anionic Surfactant System sensitivity and selectivity have been developed for estimation of Cr(III) ions.In this study, we present a sensitive, rapid, selective, and appropriate procedure for trace analysis of Cr(III) using 1-nitroso-2-naphthol (NNPh) in 1.0 % sodium dodecyl sulphate (SDS) surfactant.This procedure was successfully employed for the determination of Cr(III) ions in various biological and environmental samples.

Experimental
UV-Vis Cecil CE 9500 spectrophotometer having quartz cells with path length of 10 mm, FT-IR spectrophotometer, atomic absorption spectrophotometer, and pH/conductivity meter (model Sension156 HACH Company, USA) were used in this research.

Preparation of reagents
The Cr(III) ion stock solution (1000 µg l −1 ) was prepared in double-distilled water using high-purity salts supplied by Merck.Other solutions of metal ions were also prepared from their chloride and nitrate salts to study the effect of interfering analytes.The NNPh solution was prepared by adding 75 mg into a 50-ml flask, dissolving it in the least amount of ethyl alcohol, and adjusting the volume with 1.0 % SDS up to the mark. 32The 1.0 % SDS solution was prepared by taking 1.00 g of SDS in a 100-ml graduated flask and making up the volume with H 2 O. Buffers with pH values ranging from 1 to 10 were prepared following the methods defined by Perrin, 33 involving the addition of appropriate amounts of hydrochloric acid (HCl) and potassium chloride (KCl) for pH 1.0 to 4.0, acetic acid (CH 3 COOH) and sodium acetate (CH 3 COONa)for pH 5.0 to 6.0, potassium dihydrogen phosphate (KH 2 PO 4 ) and sodium hydroxide (NaOH)for pH 6.5 to 8.0, and hydrochloric acid (HCl) and sodium borate for pH 9.0 to 10.0.

General method for Cr(III) ions determination
Solutions containing concentrations of Cr(III) ions ranging from 0.06 to 10 µg ml −1 , NNPh solution at 185 ppm (185 µg ml −1 ), 2 ml of buffer solution (pH 8), and 2 ml of 1.0 % SDS solution were mixed in a 10-ml calibrated flask.The solutions were shaken well and double-distilled water was added up to the mark.The absorbance of Cr(III)-NNPh complex was recorded at λ max 579 nm against NNPh as the blank.

Cr(III) ions investigation from water
A tap water sample from Ghotki city was collected, filtered using 0.45 μm filter paper, and acidified by adding 2 ml of concentrated HNO 3 to avoid precipitation.Chromium metal was added in the measuring flask, along with 2 ml of 185 ppm NNPh, 2 ml of pH 8 buffer solution, and 2 ml of SDS 1.0 %.The absorbance of the complex was then measured.The results are presented in Table 4.

Chromium determination from alloy
In a 25-ml beaker, 2 g of chromel reference substance was set, followed by the addition of 15 ml of concentrated HCl and 5 ml of concentrated HNO 3 .The sample solution's volume was reduced to 5 ml on a hot plate.Then, 10 ml of concentrated HCl was added, the sample was filtered, and the obtained filtrate was diluted to 25 ml with distilled water.The sample was transferred to a measuring flask.Subsequently, 2 ml of 185 ppm NNPh, 2 ml of pH 8 buffer solution, and 2 ml of 1.0 % SDS were added into the flask for complex formation.The absorbance was measured with a spectrophotometer.The results are presented in Table 5.

Chromium analysis from a reference sample
For each certified reference material (GBW 07605 Tea and GBW 0703 bush branches), 10 g was digested in 10 ml HNO 3 with 2 ml H 2 O 2 in a microwave oven, diluted with deionised water, and filtered.The samples were transferred to a flagon, mixed with 2 ml of 185 ppm NNPh, and 2 ml of SDS was added.The complex absorbance was measured with a spectrophotometer, and the results are presented in Table 6.

Chromium analysis from a real sample
Two grams each of fish, white cheese, beef, black tea, and wheat were digested in a microwave oven with 10 ml of HNO 3 and 2 ml of H 2 O 2 .The obtained solution was diluted to 50 ml with deionised water and filtered.The samples were transferred to a flask, mixed with 2 ml of 185 ppm NNPh, followed by the addition of 2 ml of appropriate buffer and 2 ml of SDS.The complex's absorbance was measured with a spectrophotometer, and the results are given in Table 6.

Chromium determination in pharmaceutical samples
A 25-g multivitamin tablet was ground finely and digested with 10 ml of concentrated nitric acid and 2 ml of H 2 O 2 .The mixture was evaporated to dryness, and the residue was leached with 0.5 M H 2 SO 4 and deionised water was added.The sample was then transferred to a measuring flask, and 2 ml volumes of 185 ppm NNPh, 2 ml of pH 8 buffer solution, and 2 ml of 1.0 % SDS were added for complex formation.The complex absorbance was measured with a spectrophotometer.The results were compared and are given in Table 3.

Chromium investigation in standard alloys
Five millilitres each of NIST 1643 and NIST 1643 were digested in 10 ml of concentrated HNO 3 , 10 ml of 20 % H 2 SO 4 , and 2 ml of H 2 O 2. The solutions were evaporated and reduced; the sample was diluted, neutralised and filtered.The sample was then transferred to a flask; 2 ml of 185 ppm NNPh , 2 ml of buffer, 2 ml of 1 % SDS, and tartrate masking agents were added.The complex's absorbance was measured, and the data are given in Table 7.The UV-Vis spectrum of Cr(III)-NNPh complex indicates a bathochromic shift in the bands of NNPh, shifted by 202 nm to a longer wavelength due to the n to π* electron transition.Ligand to metal charge transfer (L→MCT) occurred from the filled orbital of chelate to the vacant dπ orbital of chromium at λ max 579 nm.These observations indicate that the nitrogen atom of the O=N− and the oxygen atom of the −OH group of NNPh, upon deprotonation, participated in bonding for NNPh-complex formation, as illustrated in Fig. 3.

Metal chelate composition
The metal ligand mole ratio was analysed using Job's method.Stoichiometric and molar ratios were obtained through the technique of continuous variation of metal and ligand ratios. 34Metal complexes of Cr(III) were formed, and absorbance plots against the mole fraction of metal ions are shown Fig. 4. The obtained results indicated a mole ratio of 1 : 3 for Cr(III)-chelate.NNPh demonstrated the capability to form stable chelate complexes with Cr(III).

Effect of 1-nitroso-2-naphthol concentration
The investigation focused on assessing the impact of NNPh concentration on complex-metal absorbance.This was achieved by varying the concentration of the reagent NNPh within the range of 5-80 mM, while maintaining a fixed concentration of metal ions at 1.0 mM.Notably, concentrations of NNPh ranging from 20-50 mM exhibited consistent maximum absorbance, indicating optimal conditions for complex formation.The optimised NNPh concentration, as depicted in Fig. 5, was consequently utilised throughout the entirety of the research endeavour.

Effect of surfactant
For surfactant optimisation, 2 µg ml −1 of Cr(III) metal ion solution, 2 ml of buffer, 2 ml of NNPh reagent, and 2 ml of 1.0 % SDS surfactant were mixed in a 10-ml volumetric flask, and absorbance was measured.It was observed that 2 ml of 1.0 % SDS showed constant maximum absorbance for Cr[NNPh] 3 chelate formation when the concentration of metal ions was kept at 2 µg ml −1 .The 1.0 % SDS was optimised, exceeding the critical micelle concentration value of 8.3 • 10 −3 M, and this concentration was kept constant throughout the entire procedure.

Effect of pH value
To investigate the influence of pH on metal-chelate formation, pH values ranging from 1 to 14 were introduced into the solution during the formation of the metal-NNPh complex.Throughout this process, the metal concentration and NNPh remained constant across a series of volumetric flasks.The pH at which constant absorbance was observed indicated the optimised λ max for the study of metal-chelate formation.The variation in pH altered the spectra of the metal-NNPh complex.The sensitivity and spectra of the metal-NNPh complex absorbance were found to either decrease or increase with changes in pH.Notably, pH 8 was identified as the optimum pH for Cr(III) based on the constant absorbance observed throughout the entire study, as illustrated in Fig. 6.

Effect of time
The stability of metal-complex was studied and demonstrated rapid constant absorbance at room temperature, with the absorbance of the metal-chelate remaining unaltered for 24 h.

Calibration curve
The calibration line of Cr(III) exhibited linearity in the concentration range of 0.5-5.0 µg l −1 with an R 2 value of 0.9993 at λ max 579 nm (Fig. 7).

Molar absorption
The calibration line displayed an average coefficient of molar absorption as 2.05 • 10 4 mol −1 cm −1 for Cr(III) at λ max 579 nm, as provided in Table 1.The molar absorption for metal-chelate showed a better improvement compared to previously reported methods. 26,3610 Sandell's sensitivity The value for Sandell's sensitivity (S.S.) was determined to be 3.49 ng cm −2 for Cr(III), as presented in Table 1.The calculated data calculated represents a more significant improvement than stated earlier.

Limit of detection
The limit of detection (D.L.) was determined to be 3.49 for Cr(III), as presented in Table 1.The data value demonstrated better enhancement than reported.

Interference from foreign ions
In metal-chelate formation, the interference of various cations and anions was investigated.Potassium thiocyanate (KSCN), sodium tartrate, and potassium chlorate (KClO 3 ) interfered less, and showed beyond 800 μg l −1 , confirming interference in complex absorbance for Cr(III), Mn(II), V(III), and Cd(II) at their minute level.Masking agents such as ascorbic acid, ethylenediaminetetraacetic acid (EDTA), Fig. 6 -pH effect on Cr(III)-NNPh complex and dimethyl glyoxime were employed to reduce interference from foreign ions. 35The diverse effect of analytes on Cr(III) complex formation was mitigated by the masking agent ammonia buffer, as shown in Table 2.

Validation of the method
The developed technique was validated through standard addition test % recovery, comparison with AAS, and with reference materials at a confidence interval of 95 %.The results obtained demonstrated good agreement with all aforementioned official methods, indicating good reproducibility, accuracy, and precision, as shown in Tables 3-6.
The developed method exhibited greater sensitivity and selectivity than previously reported spectrophotometric techniques for metal ion determination.The method presented advantages over the earlier extraction procedure, being selective, straightforward, sensitive, rapid, and versatile.A simple, rapid, and inexpensive technique was oped for the investigation of chromium at minute levels using NNPh in a micellar 1.0 % SDS solution.The proposed procedure was applied for the investigation of Cr(III) ions in biological, pharmaceutical, environmental, industrial, certified material, and water samples.

Conclusion
This developed procedure for the quantification of Cr(III) ions at the trace level is quick, green, novel, non-extractive, sensitive, and versatile.It is environmentally friendly, demonstrates higher selectivity and sensitivity, and replaces previous solvent extraction methods that are slow, expensive, and hazardous.The results show significant improvement in sensitivity and molar absorption compared to reported methods, as shown in Table 8.The proposed method was successfully employed to determine Cr(III) ions in biological, environmental, real, and industrial samples.

3. 1
Spectrophotometric analysis of chromium with NNPh reagent Chromium(III) ions give coloured complexes in reaction with NNPh.NNPh exhibited n to π* transition, and due to this transformation of electronic charge, NNPh displayed a sharp absorption peak.The oxygen atom of the −OH and nitrogen of the −N=O group in NNPh donate electron pairs to the chromium ion.The structure of the bonding sites of NNPh with the metal ion is shown in Fig. 1.

3. 13
FTIR spectra FTIR of NNPh revealed a stretching band at 2998 cm −1 for ν(O=N) ν(O−O).The band of ν(C−N), ν(N=O), and δ(C−OH) were measured at 1718, 1545, 1221, 1099, and 898 cm −1 wave number.Absorption bands in the bending region at 1095, 900, and 800 cm −1 of nitroso and naphthyl are shown in Fig. 8(a).The band of metal-chelate bonding is changed from NNPh reagent.The ν(N=O) stretching band showed an increase of 20-30 cm −1 due to bonding with N of the nitroso group.The ν(C−O−H…N=O) stretching band increased to a high band by 20-30 cm −1 next to 1750, and 1250 cm −1 ν(O=N) bond between metal and NNPh through ν(M−O).The change in absorption bands showed bonding between metal and NNPh through chelation via the M−O−H and M−N sites.The newest peaks were observed at 1440 cm −1 for the ν(M−O) in the Cr(III)-chelate, as presented in Fig. 8(b).

Table 2 -
Interference of foreign ions/salts in absorbance of Cr(III)-NNPh complex

Table 5 -
Investigation of Cr(III) in alloy samples

Table 6 -
Analysis of chromium in certified materials and food samples

Table 7 -
Analysis of Cr in standard reference materials

Table 3 -
Investigation of Cr(III) in reference materials * Glucose Tolerance Factor (GFT)