A study of Cr(III) oxidation into Cr(VI) and an experimental processfor determining the total Cr in environmental samples by UV-Vis absorption method

119 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0036 Natural Sciences 2018, Volume 63, Issue 6, pp. 119-126 This paper is available online at A STUDY OF Cr(III) OXIDATION INTO Cr(VI) AND AN EXPERIMENTAL PROCESSFOR DETERMINING THE TOTAL Cr IN ENVIRONMENTAL SAMPLES BY UV-VIS ABSORPTION METHOD Dao Van Bay 1 and Khuat Quang Son 2 1 Fuculty of Chemistry, Hanoi National University of Education 2 University of Fire fighting and Prevention Abstract. The paper focuses on studying the oxidation of Cr(III) onto Cr(VI) in two stages: using mixture (NH4)2S2O8/AgNO3 in acidic medium and following by H2O2/NaOH mixture. This oxidation process was confirmed experimentally by measuring UV-Vis absorption spectra. In the water samples, Cr ions exist mainly in the form of Cr(III) and Cr(VI). In agricultural products (vegetables and rice), Cr exists mainly in organic compounds, mostly in the form of Cr(III). An analytical method based on the oxidation of Cr(III) onto Cr(VI) has been developed for the Cr determination of water, water spinach and rice samples. This method has been evaluated for accuracy through the recovery of experimental data. Keywords: Oxidation mixtures, oxidation method, UV-Vis absorption spectra, experimental procedures. 1. Introduction In water, Cr-ions exist mainly in the form of Cr(III) and Cr(VI). In agricultural products (vegetables and rice) Cr-ions exist mainly in organic compounds, mostly in the Cr (III) form [4-6], so at the high-temperature mineralization, the Cr-ions in organic compounds are almost completely converted to the Cr(III) form. To determine the total Cr content in environmental samples by measuring UV-Vis absorption spectra, before analyzing, Cr(III) must be oxidized onto Cr(VI), and all interfering elements (mainly Fe, Mn) must be removed. In order to simultaneously remove the interfering ions and oxidize Cr(III) onto Cr(VI), it is necessary to proceed with the two-stage method: using oxidation mixtures (NH4)2S2O8/AgNO3 and H2O2/NaOH mixture. 2. Experimental part 2.1. Instruments and chemicals * Instruments - Volumetric flasks including 25; 1000 mL - Pipettes and micropipettes. - TOA DDK HM-25R pH Meter, Japan produced in 2010. Received February 17, 2017. Revised June 29, 2018. Accepted July 6, 2018. Contact Khuat Quang Son, e-mail address: khuatson168@gmail.com Dao Van Bay and Khuat Quang Son 120 - Analytical KERN balance with the accuracy of  10-4 g. - US-made UV-Vis Spectrometer S60 (2013), cuvette 10 mm. * Chemicals All chemicals used in this experiment were analytical purity (PA), including: concentrated HNO3 acid, Cr(III) solutions (5 mg Cr/L); Mn 2 + solution (5 mg Mn/L) and Fe 2 + solution (5 mg Fe/L), 0.01 M (NH4)2S2O8 solution and 0.1 M AgNO3 solution, 30% H2O2 solution and 8 M NaOH solution, 0.0021 M diphenylcarbazide reagent (DPCI). 2.2. Study on oxidation of Cr(III) to Cr(VI) and removal of interering ions 2.2.1. The oxidation process Take 0.25 mL of Cr(III) solution (5 mg Cr/L) into a 50 mL heat-resistant glass beaker, containing 0.5 mL of Mn 2+ solution (5 mg Mn/L) and 0.5 mL of Fe 2+ (5 mg Fe/L) then dilute with distilled water to ~ 10 mL. Ensure that the interfering concentrations of Mn 2 + and Fe 2 + are both double the concentration of Cr(III). - Oxidation in acidic environment using mixed (NH4)2S2O8/AgNO3 Adjust the above solution to pH 1.5 ÷ 2.0 with concentrated HNO3 acid, boil slightly, add 1 mL of 0.1 M AgNO3 solution (to excess), add 2.5 mL of 0.01 M (NH4)2S2O8 solution, continue to boil slightly to maintain the oxidation process. Observing the color of the solution, the metal ions present in the sample will be oxidized to the highest oxidation states of Cr(VI), Mn(VII), Fe(III) indicated in the reactions (1), (2) and (3) [1, 2, 8]. Continue lightly boiling for 5 minutes until the oxidation reaction is complete. 2Cr 3+ + 3 + 7H2O → + 6 + 14H + (1) 2Mn 2+ + 5 + 8H2O → 2 + 10 + 16H + (2) 2Fe 2+ + → 2Fe3+ + 2 (3) - Oxidation process in alkaline conditions using H2O2 and NaOH First, add to the reaction mixture ~ 1.5 ÷ 2.0 mL with 30% H2O2 solution (sufficient excess) for the following reactions which will be taken place: + 3H2O2 + 8H + → 2Cr3+ + 7H2O + 3O2↑ (4) 2 + 5H2O2 + 6H + → 2Mn2+ + 8H2O + 5O2↑ (5) 2Fe 3+ + H2O2 → 2Fe 2+ + O2 + 2H + (6) In this case, the metal ions in the sample will be reduced to low oxidation states as Cr 3+ , Mn 2+ , Fe 2+ by the reactions (4), (5) and (6). Then add 2.5 ÷ 5.0 mL of 8 M NaOH solution to alkalize. Observing the effervescent solution releasing the strong gas, a black precipitate will be appeared. Wait until the air bubbles completely, filter to remove the precipitate. Because in the alkaline environment, there should be a reaction with Cr 3+ as follows: Cr 3+ + 3OH  → Cr(OH)3↓ Ks = 6,31.10 -31 (7) Precipitation of Cr(OH)3 is an amorphous hydroxide that will dissolve in excess alkaline and ultimately all components are oxidized as follows [1]. 2Cr(OH)3 + 3H2O2 + 4OH - → 2 + 8H2O (8) Mn 2+ + H2O2 + 2OH  → H2MnO3↓ + H2O (9) 2Fe 2+ + H2O2 + 4OH  → 2Fe(OH)3↓ (10) As a result, Mn 2+ and Fe 3+ ions are separated in the form of H2MnO3 and Fe(OH)3 as reactions (9) and (10). Cr (III) is oxidized to resulting in a solution. A study of Cr(III) oxidation into Cr(VI) and an experimental processfor determining the total Cr... 121 When the air bubbles completely, filter and remove the precipitate. Adjust the obtained solution to pH ~ 1.5 ÷ 2.0 by adding a droplet of concentrated HNO3 acid, where in solution turns to . Transfer the whole solution in beaker to a 25 mL volumetric flask. 2.2.2. Evaluation of oxidation and removal of interfering ions Add 0.25 mL of DPCI reagent solution to the volumetric flask, shake well andset to the mark by distilled water. Leave for 20 minutes for the color to develop, observe the color of the solution and measure the spectrum in wavelengths λ = 400 ÷ 620 nm to find the maximum wavelength. 2.3. Establishment of experimental procedures 2.3.1. Sample treatment * Water sample treatment Water samples taken directly from the Nhue river were opaque, filtered with a 0.45 μm filter to remove the suspended solids, obtain clear water (called the native water sample - denoted Nx). Sample Nx with initial concentration mg /L. When the concentration of Cr in the sample was small, the sample needs tobe enriched before analysis by concentrating with heating. * Vegetable sample treatment - Preliminary treatment of vegetable samples: The samples of vegetables were processed immediately (to ensure freshness sample), picked up, cut and divided into two parts including top and root. The tops of vegetables were about 15 ÷ 25 cm long, and roots of vegetableswere in small pieces. Both parts were thoroughly washed with clean tap water, then rinsed with distilled water, drained and scraped to remove the rest of the surface water. The fresh samples were weighed including for the top, for the root of vegetables. -Vegetable drying by heating: The tops of vegetables and roots were dried at 90 ÷ 100 o C to obtain dry samples. The dried samples’ weight were measured (mass of dried vegetable top is mNR; dried root vegetables denoted as mRR). Diagram of drying process of fresh vegetables is as follow: Fresh samples ( , ) ( ) → Dried samples (mNR, mRR) + H2O↑ (11) Dried vegetables were cooled and stored in a tightly closed glass container, then placed in a desiccator. The percentage of dried root tops and vegetables were calculated according to CT1 and CT2: NR NR o NR m .100 a %driedsamples % m   (CT1) RR RR o RR m .100 a %dried root samples % m   (CT2) In which aNR and aRRwere the percentage of dried matter in the tops and roots vegetable, (%) and and were the fresh weight of the vegetable tops and roots (g). mNRand mRRwere the dried weight of the vegetable tops and roots (g). - Unorganized vegetable samples were transfered to liquid form: Accurately weigh a dried matter (vegetable tops or dried roots) into a baking cup (porcelain or quartz), baking at 300 ÷ 700 o C for 4 hours to fully convert the Cr in sample onto salt or oxide as following: Dried samples (mNR, mRR) ( ) → Rest (salt or oxide) + gas↑ (12) Dao Van Bay and Khuat Quang Son 122 Take the baking cup out of the oven, let cool, add 5 mL of 10% HNO3 solution to dissolve the sample, transfer the whole solution (including rinsingwater) to a 50 mL volumetric flask and set to the mark with distilled water. * Rice sample treatment - Drying rice samples by heating: Accurately weigh the commercial rice, drying at a temperature of 90 ÷ 100 o C to obtain the dried matter, sample weighed for mG. The diagram of rice drying process as follows: Commercial rice sample ( ) ( ) → Dried rice sample (mG) + H2O↑↑ (13) Dried rice was also cooled and stored in a sealed glass container, stored in a desiccator. The percentage of dried matter in rice was calculated according to the formula CT3: G G o G m .100 b %dried rice % m   (CT3) In which bGwas the percent dry matter in rice (%). was the quantity of commercial rice (g), mG was dried rice sample, (g). - Unorganized dried rice samples and transferred to liquid form: Accurately weigh an amount of dried rice, transfer to a baking cup(porcelain or quartz), cook on the stove until the smoke was completely removed. The sample was turned into the dry black stage and then burned (not covered in the oven), keepingat the temperature of 300 ÷ 700 o C within 4 hours to completely transfer black residue into white (white salt and metal oxide) as follows Dried rice sample (mG) ( ) → White residue (salt and metal oxide) + gas ↑ (14) Take the baking cup out of the oven, let cool. Add ~ 5 ÷ 7 mL of 10% HNO3 solution and boil to slightly dissolve the sample completely, obtaining a clear solution Transfer the whole solution (including washing water) into the 50 mL volumetric flask with distilled water to the mark . The dried rice sample taken for analysis is usually mG = 5 ÷ 10 g. 2.3.2. Oxidation procedure of Cr(III) to Cr(VI) and determination of total Cr concentration Using a pipette take accurately v o mL sample (usually v o = 2 ÷ 5 mL) into a 100 mL heatresistant glass beaker, diluted with distilled water to ~ 10 mL. Heat the solution, add 1 ÷ 2 mL of AgNO3 0.1 M solution (sufficient excess), add 2.5 ÷ 4 mL solution (NH4)2S2O8, boil slightly, observe the color of the solution. Add to the beaker about 1 ÷ 3 mL of 30% H2O2 solution (in excess) and 2.5 ÷ 4 mL NaOH solution 8 M to result in enough alkaline, observe the bubbling and black precipitate appearing, continue to boil slightly. At the end of the gas bubbling, the solution must be filteredto remove the precipitate. Transfer all filtered solution into a 25 mL volumetric flask, add a droplet of concentrated HNO3 solution to adjust pH = 1.50 ÷ 2.0, add 0.1 ÷ 0.5 mL of DPCI reagent solution, shake then add distilled water to the mark. Leave the solution for 20 minutes before measuring absorbance A. 2.3.3. Evaluating the correctness of the experimental procedure Prepare 03 standard solutions at 3 diffenent levels with the interfering ions like Mn 2+ and Fe 2+ ions (ensuring the concentration of ions is greater than 2 times the concentration of Cr). At each concentration was analyzed for 4 times. The solutions were prepared as follows: - Solution 1. Take 0.25 mL Cr 3+ solution (5 mg Cr/L) into a 50 mL heat-resistant glass beaker, add 0.5 mL of Mn 2+ solution (5 mg Mn/L) and 0.5 mL Fe 2+ solution (5 mg Fe/L, diluted with distilled water to ~ 10 mL. - Solution 2. Take 1.0 mL of Cr 3+ solution (5 mg Cr/L) into a 50 mL heat-resistant glass beaker, add 2.0 mL of Mn 2+ solution (5 mg Mn/L) and 2.0 mL Fe 2+ solution (5 mg Fe/L), diluted with distilled water to ~ 10 mL. A study of Cr(III) oxidation into Cr(VI) and an experimental processfor determining the total Cr... 123 - Solution 3. Take 4.0 mL of Cr 3+ solution (5 mg Cr/L) into a 50 mL heat-resistant glass beaker, add 8.0 mL of Mn 2+ solution (5 mg Mn/L) and 8.0 mL of Fe 2+ solution (5 mg Fe/L). Heat the solutions in the beakers and add into the beakers: 1 ÷ 2 mL of 0.1 M AgNO3 solution (sufficient excess) and 2.5 ÷ 4.0 mL solution (NH4)2S2O8, boil slightly, observe color of the solution. Add into sample beaker about 1 ÷ 3 mL of 30% H2O2 solution (sufficient excess) and still 2.5 ÷ 4.0 mL of 8M NaOH solution to adjust sufficiently alkaline medium. Observe the bubbling and the black precipitate appearing. Continue to boil gently until the air bubbles and then filter to remove precipitate. Transfer all filtered sample into 25 mL volumetric flasks, add droplets of concentrated HNO3 solution to adjust pH = 1.50 ÷ 2.0. Add 0.1 ÷ 0.5 mL of DPCI reagent solution, shake and then adddistilled water to the mark. Allow the solution to stand for 20 minutes before measuring the optical absorbance A. The sample was repeated four times to calculate the mean concentration and evaluate the degree of accuracy by determining the recovery [3]. 2.4. Results and discussion 2.4.1. Oxidation of Cr(III) onto Cr(VI) and removal of interfering ions From the experiments, the obtained spectrum of color complexes in the wavelength range λ = 400 ÷ 620 nm was presented in the spectrum (Figure 1). Figure 1. The spectrum of the solution after the oxidation phase The spectral results in Figure 1 show that when the DPCI reagent present, the reddish violet color appears and the maximum absorption at wavelength λmax = 541 nm, which is the characteristic maximum absorption wavelength complex formed by Cr(VI) with DPCI reagent. This result is consistent with the study [9]. Experimental results show that after oxidation of the first stage with the mixture (NH4)2S2O8/AgNO3, beside to the reaction to form , also formed which are purple existing in the solution so might not be separated, thus affecting measuring data. As oxidation continues in stage 2 with a mixture of H2O2 + NaOH, here only Cr ions are converted to form, while the ions of Mn and Fe are separated as precipitated H2MnO3↓ and Fe (OH)3↓. Dao Van Bay and Khuat Quang Son 124 This proves that the two-stage oxidation method is feasible. This oxidation method has been selected for use in this study. 2.4.2. Results of experimental design and its evaluation * Results of experimental process On the basis of the experiments, an experimental procedure of analysis of Cr(III) in the water samples is present in Figure 2 and the Cr(III) analysis in the rice samples is shown in Figure 3. Figure 2. Procedure of analysis of water samples In Figure 3: mNR, mG: is the dried sample of vegetables and rice for analysis (g), aNR, bG is percentage of dried vegetables and rice (%), v o is the color-generating volume (mL). ci is the concentration determined in (mg/L), is the concentration of Cr in the initial water sample (mg/L), , are the volumetric flask ( , = 50 mL). xNR: is the content of Cr in the tops of vegetables (mg/kg of fresh vegetables), yG: is the Cr content in rice (mg/kg of commercial rice). A study of Cr(III) oxidation into Cr(VI) and an experimental processfor determining the total Cr... 125 Figure 3. Procedure of analysis of vegetables and rice * Evaluation results of experimental process From the experiments conducted as in Section 2.3.3, the results of Cr concentrations at different concentration levels are presented in Table 1. Table 1. Results of determination of Cr concentration and recovery level Sol. Cc (mg/L) repeatability Ai ci (mg/L) iC (mg/L) R (Recovery rate) (%) 1 0.05 1 0.041 0.049 0.049  0.006 98.0 2 0.039 0.046 3 0.042 0.050 4 0.041 0.049 2 0.20 1 0.165 0.196 0.195  0.006 97.5 2 0.163 0.194 3 0.165 0.196 4 0.161 0.192 3 0.80 1 0.674 0.802 0.792  0.019 99.1 2 0.668 0.795 3 0.661 0.787 4 0.662 0.788 + 10% HNO3 solution Dao Van Bay and Khuat Quang Son 126 The results in Table 1 show that: for a range of Cr values determined from: 0.049 ÷ 0.792 mg/L (or 0.049 ÷ 0.792 ppm), the recovery rate is: 97.5 ÷ 99.1%. The calculated recoverable values meet the requirements of the AOAC organization, allowed at the corresponding concentration range of: 80 ÷ 110%. Thus the analytical method has guaranteed accuracy, so the obatined processes of experiments shown in Figures 2 and 3 are reliable. These experimental procedures are well used to analyze environmental samples. 3. 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