Colormetric determination of total chromium content in water, rice and vegetable samples by 1,5-diphenylcarbazide

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-00080 Chemical and Biological Sci. 2015, Vol. 60, No. 9, pp. 69-76 This paper is available online at Received November 17, 2015. Accepted December 21, 2015. Contact Tran Van Chung, e-mail address: tranchunghhvl@gmail.com 69 COLORMETRIC DETERMINATION OF TOTAL CHROMIUM CONTENT IN WATER, RICE AND VEGETABLE SAMPLES BY 1,5-DIPHENYLCARBAZIDE Khuat Quang Son1, Dao Van Bay1 and Tran Van Chung2 1 Faculty of Chemistry, Hanoi National University of Education 2 Institute of Chemistry and Materials Abstract. A colorimetric determination the total chromium content based on DPC reagent was established in detail. All operations were done under the optimal conditions of pH = 1.65, a ratio of nDPC/nCr(VI) = 2.18 (by mol) and use of a calibration plot for the determination of total chromium established as A = (0.8324 ± 0.00901).CCr + (0. 086 ± 0.00506) that allows determination of chromium in environmental sampl s. Using this method, the total chromium content of water, rice and water spinach samples that were collected along the Nhue River were analyzed. The total chromium content of the water, rice and water spinach samples were analyzed. The obtained results show that the samples were not polluted by chromium. Keywords: Chromate ions, 1,5-diphenylcarbazide, color complex compounds, chromium in rice and other food products. 1. Introduction In general, chromium is an inorganic element present in water, sediments, rocks, soils, plants, animals and volcanic emissions. Chromium has multiple oxidation states. Chromium (III) is present as a trace element that is essential for life, while chromium (VI) is a documented human and animal carcinogen which is quite toxic [1, 2]. The toxicity of chromium depends on its oxidation state [3]. Cr (VI) is much more toxic than Cr (III) [4]. While the amount of Cr (VI) that can safely be ingested by an adult male is 0.57mg/kg/day [5], too much chromium in the diet can cause dise se [6]. For this reason, food products should be carefully examined for chromium content using quantitative determinations. The most widely used method of making a quantitative determination of chromium content is colorimetry. This refers to the color f med when chromium forms a compound in the presence of a diphenylcarbazide reagent [7-9]. In the experiments described in this article, the total chromium content of water, rice and vegetable samples was determined by spectrophotometry using a 1,5-diphenylcarbazide reagent. First, the study focused on establishing optimum Khuat Quang Son, Dao Van Bay and Tran Van Chung 70 experimental conditions for chromium determination and then the total chromium content was determined for the water, rice and water spinach samples that were collected along the Nhue River. This river was chosen because the mass media has printed many stories about its polluted condition. 2. Content 2.1. Experiments 2.1.1. Chemicals All chemicals used were of analytical grade and solutions were prepared using distilled water. - K2Cr2O7 standard stock solution (1 g/L): After an amount of K2Cr2O7 was dried at 150 0C for one hour and left in a desiccator, 2.828 g of the K2Cr2O7 was then weighed out. In a 1000 mL volumetric glass flask, this K2Cr2O7 was dissolved in distilled water and diluted to mark. - 1,5-diphenylcarbazide (DPC) solution (5 mg/mL): 250 mg of 1,5-diphenylcarbazide (CO(NH-NHC6H5)2) was dissolved in 50 ml acetone in a brown glass bottle and kept at 4 0C in a refrigerator. A fresh solution was prepared each week during the course of the experiment. - Other chemical solutions: Other chemical solutions such as NaOH 2M, NaOH 8M, H3PO4 (~ 80%), (NH4)2S2O8 0.01 M, AgNO3 0.1 M, and NaCl 5% of analytical grade were prepared for the experiments when needed. 2.1.2. Instruments and experimental procedure * Instruments A spectrophotometer [UV-Vis Biochrom S60 (2013) USA] was used to measure absorbance. All pH measurements were performed using pH- meter TOA, DDK HM-25R, Japan. * Experimental procedure - Determination of absorbance of color compound formed between Cr(VI) - 1,5- diphenylcarbazide (DPC) Three 25 mL volumetric glass flasks were prepared. Each glass flask contained different chemical components as follows: A solution with 5 mL of K2Cr2O7 solution (5 mg Cr/L) + 0.5 mL of H3PO4 (80 %), adding distilled water to mark, denoted DD1. A solution with 0.5 mL of H3PO3 solution (80 %) + 0.5 mL of the DPC solution (5 mg/mL), adding distilled water to mark, denoted DD2. A solution with 5 mL of K2Cr2O7 solution (5 mg Cr/L) + 0.5 mL H3PO4 (80 %) + 0.5 mL DPC solution (5 mg/mL), adding distilled water to mark, denoted DD3. All solutions were allowed to stand for 5 min at room temperature. The absorbance of all samples was taken from 400 to 700 nm with distilled water as the reference. - Factors influencing the absorbance of the color compound + Influence of pH A set of 7 volumetric glass flasks (25 mL) was prepared. Every flask contained a 2 mL solution of K2Cr2O7 (5 mg/L), 0.5 mL of reagent solution DPC (5 mg/mL) and distilled water. The pH of the samples was adjusted to from 0.2 to 3.2 using H3PO4 or NaOH solutions and then adding distilled water to mark. All the samples were allowed to stand for 5 min at the room temperature before absorbance was measured. + Influence of Cr(VI)/(DPC) reag nt ratio at various volumes. Another set of 7 volumetric glass flasks (25 mL) was then prepared. Each flask contained Cr(VI) (1.0 mg/L). The volume of DPC solution (5 mg/mL) added varied from 0.05 to 1.20 mL. Colormetric determination of total chromium content in water, rice and vegetable samples... 71 The pH of the samples was adjusted to optimum value by the addition of an H3PO4 solution, then adding distilled water to mark. All samples were allowed to stand for 5 min before absorbance was measured. + Determination of color compound stability versus the time A 25 mL volumetric glass flask was prepared containing a 2 mL solution of K2Cr2O7 (5 mg Cr/L), 0.5 mL of DPC reagent solution (5 mg/mL), and 0.5 mL H3PO4 (80%), adding distilled water to mark. The sample was allowed to stand for 5 to 60 min before absorbance was measured. - Building the chromium calibration curve A set of 7 volumetric glass flasks (25 mL) was prepared. Each flask contained Cr(VI) at a different concentration, varying from 0.01 to 1.00 mg/L, 0.5 mL H3PO4 (80%), and a 0.5 mL DPC solution (5 mg/mL), and distilled water was added to mark. The samples were allowed to stand for 10 min at room temperature before absorbance was measured. - Determination of total chromium content in the samples + Determination of the total chromium content in the water samples The water samples colle ted along the Nhue River were filtered to remove suspended solids. A 150 mL water sample was heated on a stove to evaporate down to 15 mL. The sample then was mixed with (NH4)2S2O8 in an acidic media to oxidize the Cr(III) to Cr(VI) as suggested in [6]. The presence of Cr(VI) in the samples was determined using spectrophotometry with DPC, using the calibration curve suggested above. + Determination of total chromium content in the water spinach and rice samples The water spinach and rice samples collected along the Nhue River were pre-treat d according to [6]. Before analyzing total chromium content, the water spinach samples (2 g/sample) and the rice samples (10 g of rice/sample) were mineralized using 20 mL of hot concentrated HNO3 solution. They were then heated in an oven at 700 0C to remove all organic compounds. The chromium in the samples were then oxidized using (NH4)2S2O8 in an acidic media to convert Cr(III) to Cr(VI) as suggested in [6, 10].The chromium concentration content of each was analyzed using spetrophotometry with a DPC reagent. 2.2. Results and discussion 2.2.1. UV-VIS spectrogram of Cr(VI) with DPC In this experiment, under the experimental conditions presented in (2.3.1.), the maximum absorbance of the color compound of Cr(VI) with DPC appeared at λ = 541 nm, Figure 1. Figure 1. UV-VI spectrogram of Cr(VI) with DPC: spectrogram of Cr(VI) solution (DD1), spectrogram of DPC solution (DD2) and spectrogram of Cr(VI) with DPC (DD3) Khuat Quang Son, Dao Van Bay and Tran Van Chung 72 The appearance of maximum absorbance of Cr(VI) with DPC is due to the formation of the complex compound that proceeded in situ between Cr(III) and diphenylcarbazone (DPCO) [11]. The formation of this color compound in situ is based on the following reactions: Reaction of Cr(VI) with DPC to form Cr(III) and DPCO 2 Cr(VI) + 3 DPC → 2 Cr(III) + 3 DPCO + 6H+ Reaction to form color compound Cr(III) + DPCO → [Cr(III) – DPCO](3-n)+ + nH+ The overall reaction to form the color complex can be written as follows: 2Cr(VI) + 3 DPC → 2 [Cr(III) – DPCO](3-n)+ + DPCO + 2 nH+ Here (n) denotes the unknown number of protons released in the complex. The maximum absorbance of this complex at λ = 541 nm being proportional to concentration of Cr(VI) was used for the analysis. Further experiments focused on finding optimum conditions for analyzing chromium in samples. 2.2.2. Factors influencing absorbance of the color compound * Influence of pH The influence of pH on absorbance of the color compound under the conditions present can be seen in Table 1 and Figure 2. Table.1. Influence of pH on maximum absorbance No. V(Cr2O7) 5mg/L (mL) V (H2O) (mL) V(H3PO4) 80% (mL) VNaOH 1M (mL) VDPC (mL) Vtotal (mL) pH Amax 1 2.00 ~10 12.0 0 0.50 25.0 0.26 0.238 2 2.00 8.0 0 0.50 25.0 0.57 0.299 3 2.00 4.0 0 0.50 25.0 0.90 0.342 4 2.00 0.5 0 0.50 25.0 1.54 0.376 5 2.00 0.1 0 0.50 25.0 1.90 0.368 6 2.00 0.1 0.9 0.50 25.0 2.78 0.356 7 2.00 0.1 1.0 0.50 25.0 3.20 0.349 Figure 2. Influence of pH on absorbance The experimental data obtained shows that the formation of the col r c mpound occurred in the acidic medium. The optimum pH value for color compound formation is 1.54 - 1.90. This value is consistent with the work of Groucho Marx et al. [9]. The pH of 1.65 was selected as an optimum value for further experiments Colormetric determination of total chromium content in water, rice and vegetable samples... 73 * Influence of the reagent ratio (DPC)/Cr(VI) by volume or mol The influence of the reagent ratio (DPC)/Cr(VI) by volume or mol on the absorbance is shown in Table 2 and in Figure 3. Table 2. The influence of the (DPC)/Cr ratio on maximum absorbance Stt V(Cr) 5mg/L (mL) V(H3PO4) 80% (mL) VTT = VDPC 0.021M (mL) Vtotal (mL) Ratio (mL) (DPC)/Cr(VI) Ratio(mol) nDPC/nCr(VI) Amax 1 5.0 0.5 0.05 25.0 0.05/5 0.22 0.577 2 5.0 0.5 0.1 25.0 0.1/5 0.44 0.725 3 5.0 0.5 0.2 25.0 0.2/5 0.87 0.796 4 5.0 0.5 0,5 25.0 0.5/5 2.18 0.853 5 5.0 0.5 0.8 25.0 0.8/5 3.49 0.859 6 5.0 0.5 1.0 25.0 1/5 4.37 0.843 7 5.0 0.5 1.2 25.0 1.2/5 5.24 0.874 Figure 3. The influence of the (DPC)/Cr(VI) ratio on absorbance The obtained results show that a (DPC)/Cr ration that is 0.50/5.0 by volume or 2.18 (by mol) should be selected as the optimum value for the experiments. If the Cr(VI) concentration was higher than the DPC, some Cr(VI) can react with DPCO to form diphenyldicarbazone, which can not form a color complex with Cr(III). Therefore the absorbance would be reduced. This was suggested in the work of Leonard Peltier [11]. * Color compound stability versus time The stability of the compound which resulted under conditions (VCr(VI) 5mg/L + V H2O + V H3PO4 80% + V DPC (mL) in a 25 mL volumetric flask)is shown in Table 3. Table 3. Stability of compound versus time Amax 0.338 0.338 0.338 0.338 0.338 0.338 0.338 0.338 0.338 t mn 5 5 5 5 25 25 25 25 25 The experimental data shows that complex Cr(III)-DPCO is very stable. Khuat Quang Son, Dao Van Bay and Tran Van Chung 74 * Determination of calibration curve The dependence of absorbance on Cr(VI) concentration is shown in Table 4. Table 4. Dependence of absorbance on Cr(VI) concentration No. V(Cr2O7 5mg/L) (mL) V(H3PO4 80%) (mL) VDPC (mL) Vtotal (mL) CCr mg/L A(541nm) 1 0.05 0.50 0.50 25 0.01 0.015 2 0.50 0.50 0.50 25 0.10 0.091 3 1.00 0.50 0.50 25 0.20 0.174 4 2.00 0.50 0.50 25 0.40 0.339 5 3.00 0.50 0.50 25 0.60 0.515 6 4.00 0.50 0.50 25 0.80 0.686 7 5.00 0.50 0.50 25 1.00 0.829 The calibration curve is presented in Figure 4. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 A C(mg/l) Equation y = a + b*x Adj. R-Square 0.9993 Value Standard Error B Intercept 0.0086 0.00506 B Slope 0.8324 0.00901 Figure 4. The calibration curve of Cr(VI) The calibration equation is as follows: A = (0.8324 ± 0.00901).Cr + (0.0086 ± 0.00506) This calibration curve was used to determine th pres nce of chromium in samples. * Determination of total chromium content in samples - Total chromium content in the water samples Our results showing the presence of chromium in the surface water samples that were collected along the Nhue River are presented in Table 5. Table 5. Total chromium concentration in the water samples Sample Signal Places of samples CCr (mgCr /L) N1 Lang Cao 0.0067 N2 Trat Cau 0.0083 N3 Tram Bom 0.0107 N4 Vinh Bo 0.0075 Colormetric determination of total chromium content in water, rice and vegetable samples... 75 Referring to Vietnamese Standard QCVN08:2008/BTNMT, the concentration of chromium in the surface water collected from the Nhue River is at a level below that which would be called chromium pollution. - Determination of the total chromium content in water spinach samples Our results showing the presence of chromium in the water spinach samples that were collected along the Nhue River are presented in Table 6. Table 6. Total chromium concentration in the water spinach samples Sample signal Places of samples Cr concentrations (mg Cr/2g in dried samples) Cr concentrations (mg Cr/kg fresh samples) R1 Lang Cao 0.002 0.086 R2 Trat Cau 0.014 0.594 R3 Tien Phong 0.014 0.606 Referring to Vietnamese Standard 02/2011/TT-BYT, the concentration of chromium in the water spinach samples that were taken from long the Nhue River is at a level below that which would be called chromium pollution. - Determination of the total chromium content in rice samples Our results showing the presence of chromium in the rice samples that were collected along the Nhue River are presented in Table 7. Table 7. Total chromium concentration in the rice samples Sample signal Places of samples Cr concentrations (mg Cr/10g rice) Cr concentration (mg Cr/kg rice) G1 Trat Cau 0.0095 0.9475 G2 Vinh Bo 0.0068 0.6832 Referring to Vietnamese Standard 02/2011/TT-BYT, the concentration of chromium in the rice samples that were collected along the Nhue River is at a level below that which could be called chromium pollution. 3. Conclusion A colorimetric measurement method was used to determine chromium content, based on the color compound with DPC reagent, was systematically studied in detail and compared to other published works. Optimal experimental conditions in terms of pH, Cr(VI) concentration and its ratio with DPC reagent were established and then applied. Total chromium concentration in the water, rice and water spinach samples that were collected along the Nhue River was analyzed. 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