Two trace chromium determination methods have been developed: Analysis procedure of Cr(VI) and total Cr (Cr(VI+III) in natural water by SqW-AdSV using in situ BiFE;. An[r]
Trang 1HUE UNIVERSITY UNIVERSITY OF SCIENCES
NGUYEN THI HUE
STUDY ON DETERMINATION
OF TRACE CHROMIUM
BY ADSORPTIVE STRIPPING VOLTAMMETRY
SUMMARY OF ANALYTICAL CHEMISTRY DOCTORAL
DISSERTATION
HUE - 2017
Trang 2HUE UNIVERSITY UNIVERSITY OF SCIENCES
NGUYEN THI HUE
STUDY ON DETERMINATION
OF TRACE CHROMIUM
BY ADSORPTIVE STRIPPING VOLTAMMETRY
SPECIALTY: ANALYTICAL CHEMISTRY
CODE: 62 44 01 18
SUMMARY OF ANALYTICAL CHEMISTRY DOCTORAL
DISSERTATION
SCIENTIFIC SUPERVISORS:
1 Assoc Prof Dr NGUYEN VAN HOP
2 Prof Dr LE QUOC HUNG
HUE - 2017
Trang 3INTRODUCTION
1 Rationale
Industrialization, modernization and environmental protection
in the current period require the analytical science to develop highly selective and sensitive analytical methods to accurately determine trace amounts of metals in complex samples To complete this task, many of determination methods have been developed, eg atomic absorption spectrometry, plasma emission spectrometry, capillary gas chromatography, high performance liquid chromatography and modern electrochemical analytical methods, typically differential pulse polarization and stripping voltammetry methods
Before, most studies on adsorptive stripping voltammetry (AdSV) used electrodes either hanging mercury drop electrode (HMDE) or static mercury drop electrode (SMDE), which give stable and sensitive stripping signals However, these electrodes are expensive, difficult to be fabricated Recently, easy-to-make electrodes based on innert rotating disk electrode, such as mercury film electrodes (MFEs), bismuth film electrode (BiFE) have been increasingly used due to their low costs, and being environmentally friendly in the case of BiFE If successful, study using MFE, BiFE for HMDE and SMDE substitution, when applied, will be a feasible solution in our current situation
Chromium is a relatively common element in nature; chromium amount in the environment tends to be increased by industrial activities such as electroplating, tanning, processing and mining In the environment, chromium exists in the form of Cr (III) and Cr (VI), depending on the redox state of the water
The nature and toxicity of chromium forms depend on its oxidation state Chromium is toxic only in Cr (VI) form Cr (III) is necessary for the body Therefore, in environment monitoring, quantifying the total chromium is not enough It is necessary to analyze the different forms of their existence Therefore, the process
of analyzing chromium in environmental samples is a necessary and urgent issue
Starting from these issues, we chose the topic of "Study on determination of trace chromium by adsorptive stripping voltammetry" using the complexing ligand is diethylene triamine
Trang 4pentaacetic acid (DTPA), base component is acetate buffer To implement this topic, it is necessary to address the following:
- Trace chromium determination by AdSV method using HMDE, MFE and BiFE Compare advantages and disadvantages of electrodes and choose the best and most effective electrodes to apply
to the real samples
- Develop a procedure to determine chromium in natural water and in some other water sources in Thua Thien Hue Province In addition, a process is also needed to determine this element in solid samples such as soil, sediment, etc., to quantify chromium in Huong River and Cau Hai Lagoon sediments
3 New scientific and practical contributions
1 This is the first thesis in Vietnam, that systematically study the types of electrodes using for chromium determination by adsorptive stripping voltammetry
2 For the first time in Vietnam, adsorptive stripping voltammetry using bismuth film electrode was used to determine trace chromium
3 Successfully developed a new type of bismuth film
electrode: bismuth film in situ (in situ BiFE) Adsorptive stripping voltammetry using in situ BiFE gives better sensitivity than other electrodes (HMDE, MFE, ex situ BiFE) Limit of detection was
low (0.1 ppb) with short analysis time
With a simple fabrication, this electrode is suitable for not equipped analytical laboratories This could effectively contribute
well-to the moniwell-toring and pollution control of chromium for water resources planning and exploitating in Vietnam
4 For the first time, adsorptive stripping voltammetry using
bismuth film electrode in situ was used to determine trace
chromium in fresh, blackish and sea water in Thua Thien Hue area
Trang 54 Composition of the thesis
The thesis consists of 153 pages, 75 tables, 55 pictures, including:
- Contents, List of Abbreviation, Table, Picture (11 pages)
- Introduction (4 pages)
- Chapter 1: Literature Review (31 pages)
- Chapter 2: Research Subjects - Methodology (5 pages)
- Chapter 3: Results and Discussion (86 pages)
- Chapter 4: Conclusion (2 pages)
- References (10 pages)
THESIS CONTENTS Chapter 1 LITERATURE REVIEW
- Introduction to chromium, applications, sources of chromium waste in the environment, the impact of chromium on humans and the environment
- Overview of adsorptive stripping voltammetry: principles, complexing reagents, working electrodes used in adsorptive stripping voltammetry, adsortive stripping voltammogram recording techniques
- Affecting factors needed to be considered when developing a adsorptive stripping voltammetry determination procedure
- Current trends in trace chromium determination: spectrometry methods, modern electrochemical analysis methods
- Noteworthy issues of trace analysis
At present, AdSV method is the most focused research direction to develop new electrochemical methods for chromium determination Of the 44 main studies (listed in Table 1.3) from 1987
to date, 28 have been using the AdSV method, 16 using the HMDE electrode, and 6 on the BIFE electrodes HMDE and BiFE electrodes are commonly used together with DP or SqW techniques In this study, we focus on the determination of chromium with DP/SqW-AdSV using 3 types of electrodes: HMDE, MFE, BiFE to find out the best one which can give analysis results with high sensitivity and repeatability
Trang 6Chapter 2 RESEARCH SUBJECTS AND METHODOLOGY 2.1 Research subjects
Study on the adsorptive stripping voltammetry characteristic of
+ Study on the determination of trace Cr(VI) by differential
pulse stripping voltammetry using in situ BiFE;
+ Study on the determination of trace Cr(VI) by square wave
adsorptive stripping voltammetry using in situ BiFE;
Compare advantages and disadvantages of concerned electrode types to select suitable one and its fabrication for domestic laboratory application
Develop determination procedure:
+ Develop determination procedure;
+ Verify the determination procedure by using certified reference materials;
+ Analyze real samples (sample, preserve, pretreat and analyze)
2.2 Methodology
- Adsorptive stripping voltammetry using HMDE, ex situ MFE, ex
situ and in situ BiFE;
- Evaluate influence factors by monovariant analysis;
- Evaluate method reliability through repeatability, sensitivity, detection limits and linearity;
- Analyse and represent data by using MS Excel 2010
Working electrode preparation:
There are three types of electrode were used in this study:
HMDE used in AdSV/HMDE experiments was prepared according
to the manufacturer's instructions (Metrohm, Swiss)
Trang 7 MFE used in AdSV/ex situ MFE experiments was prepared as
follows:
+ Disk electrode (glassy carbon, GC) cleanning: GC electrode was
cleanned by polishing using fine Al2O3 powder (particle size 0.6 µm), rinsed with distilled water then with 1 M NaOH solution to remove any residual Al2O3 particles on the electrode surface Dip the electrode into 1 M HCl solution to neutralize NaOH Then wash the electrode carefully with distilled water several times, wipe gently with soft filter paper
+ Ex-situ MFE preparing: mount the cleanned GC electrode into the
electrochemical analyzer containing Hg(NO3)2 7.78×104 M solution, apply a constant potential at -1000 mV (vs Ag/AgCl reference electrode) for 120 s, then carefully wash with double distilled water and dry the electrode body (without touching the electrode surface)
Ex situ and in situ BiFE used in AdSV/ BiFE ex situ and AdSV/ BiFE in situ experiments were prepared as follows:
+ GC disk electrode cleanning: same as the preparation of MFE + Ex situ BiFE preparing: mount the cleanned GC electrode into the
electrochemical analyzer containing 0.1 M acetate buffer solution,
500 ppb Bi(III) and 4.2×106 M KBr; apply a constant potential at
-1200 mV for 120 s while stirring at 2000 rpm After that, carefully wash with double distilled water and dry the electrode body using soft filter paper
+ In situ BiFE preparing: In situ BiFE electrode was formed during
analyte deposition step as follows: Dip the GC electrode into a electrochemical cell containing reference, auxilỉay electrodes and analytical solution (0.4 M acetate buffer, 0.4 M NaNO3, 0.4 × 103 M DTPA, 600 ppb Bi (III), 5.0×106 M KBr and Cr (VI)); Rotate the electrode at a constant speed and conduct electrodeposition step at -
800 mV (EAd) for a specified time (tad) During this step, Bi(III) is
reduced to metal binding on the GC surface, forming in situ BiFE
Chapter 3 RESULTS AND DISCUSSION
In order to develop analytical procedures for determination of chromium in water, adsorptive stripping voltammetry method using
different electrodes (HMDE, ex situ MFE, ex situ and in situ BiFE)
Trang 8were investigated with DP or SqW scanning techniques in a solution containing acetate buffer, DTPA (complexing agents) and NaNO3
3.1 Adsorptive stripping voltammetry characteristics of chromium
3.1.1 AdSV characteristics of chromium at HDME
To understand stripping voltammetry characteristics of chromium on HMDE, cyclic voltammograms of Cr(VI) solution under various experiment conditions were recorded and investigated Potential was scanned from -800 mV to -1600 mV, at a scan rate of 15 mV/s, and stirring rate = 2000 rpm
(A): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer Without preconcentration step
(B): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103
M DTPA Without preconcentration step
(C): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103
M DTPA, 0.4 M NaNO3 Without preconcentration step
(D): solution contains 0.4 M acetate buffer, 0.4×103 M DTPA, 0.4
M NaNO3 Without preconcentration step
(E): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103
M DTPA, 0.4 M NaNO3
(F): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4 M NaNO3
(G): solution contains 90 ppb Cr(VI), 0.4 M acetate buffer; 0.4×103
M DTPA, 0.4 M NaNO3 Without preconcentration step
(H): solution contains 100 ppb Cr(VI), 0.4 M acetate buffer; 0.4×103 M DTPA, 0.4 M NaNO3 Without preconcentration step Cyclic voltammograms shown in Figure 3.1 (in the thesis) indicated that:
- Cyclic voltammograms in cases A, B, C and D showed that stripping peaks of chromium appeared only when acetate buffer, DTPA and NaNO3 are present together in the analytical solution
- Cyclic voltammograms B and C indicated that stripping peaks of chromium appeared only when analysis solution contained NaNO3 This confirms that only nitrate ions are able to oxidize Cr(II) -DTPA
to Cr(III)-DTPA Chromium(VI) ion in the solution is not able to oxidize Cr(II)-DTPA (Figure 1.4, Section 1.4)
Trang 9- Cyclic voltammograms C and E revealed that only Cr(III) newly generated from the electrochemically reduction of Cr(VI) can react with DTPA
- Cyclic voltammograms C and E indicated that the complex DTPA may have been adsorbed on the surface of electrode
Cr(III)-3.1.2 AdSV characteristics of chromium at ex situ BiFE
From the investigation of cyclic voltammograms recorded using ex
situ BiFE, it is possible to confirm that:
- Chromium(VI) ion only shows electrochemical activity in the analysis solution containing NaNO3
- Zn(II), Co(II) and Cr(III) have no electrochemical activity in this system
- Among Zn(II), Co(II), Cr(III), and Cr(VI) only Cr(VI) own electrochemical activity in this system (containing acetate buffer), complexing agent DTPA, and NaNO3)
3.2 Study on determination of Cr(VI) by stripping voltammetry using HMDE and MFE
- Effect of background composition;
- Effect of mercury concentration on repeatability of the dissolved
signal on the ex situ MFE;
- Effect of rotating speed and differential pulse voltammetry parameters (for DP-AdSV/MFE);
- Effect of adsorption potential, adsorption time, equilibrium time;
- Effect of DTPA concentration and NaNO3 concentration;
- Effect of dissolved oxygen
- Reliability of the method
3.3 Study on determination of Cr(VI) by adsorptive stripping voltammetry using BiFE
- Study on determination of Cr(VI) by differential pulse adsorptive
stripping voltammetry using ex situ BiFE (DP-AdSV/ ex situ BiFE)
- Study on determination of Cr(VI) by square wave adsorptive
stripping voltammetry using ex situ BiFE (SqW-AdSV/ex situ BiFE)
- Study on determined Cr(VI) by differential pulse adsorptive
stripping voltammetry using in situ BiFE (DP-AdSV/in situ BiFE)
- Determination of Cr(VI) by square wave adsorptive stripping
voltammetry using in situ BiFE (SqW-AdSV/in situ BiFE)
Linearity, sensitivity, detection limits, and quantitative limits of the AdSV/ex situ BiFE method
Trang 10Figure 3.1 (A) Linear regression line for the SqW-AdSV/in situ
BiFE method; (B) Stripping voltammograms of SqW-AdSV/in situ BiFE: baseline and 9 voltammograms obtained by sequently adding 0.2 ppb Cr(VI) each time into the analysis solution ; (C) I p and C Cr (VI) relationship in DP-AdSV/in situ BiFE method; (D) linear regression line for DP-AdSV/in situ BiFE method
Linearity:
+ SqW-AdSV method: Ip and CCr(VI) had a good linear correlation in
CCr (VI) range from 0.3 to 1.8 ppb with a correlation coefficient R of 0.9994 (Fig 3.1.A, Fig 3.1.B);
Trang 11+ SqW-AdSV method: Ip and CCr(VI) had a good linear correlation in
CCr (VI) range from 0.2 to 1.6 ppb with a correlation coefficient R of 0.9911 (P = 0.95);
+ DP-AdSV method: Ip and CCr(VI) had a good linear correlation in CCr (VI) range from 5.2 to 20.8 ppb with a correlation coefficient R of 0.9995 (P = 0.95);
Sensitivity:
SqW-AdSV method achieved a sensitivity of 21.4 µA/ppb (approximately 15.4 times higher than DP-AdSV, 1.389 µA/ppb)
LOD and LOQ:
+ SqW-AdSV (Ead = -800 mV, tad = 80 s): LOD = 0.2 ppb; LOQ = 0.6 0.8 pbb
+ DP-AdSV (Ead = -800 mV, tad = 120 s): LOD = 0.9 ppb; LOQ = 2.7 – 3.6 ppb
Linearity, sensitivity, detection limits, and quantitative limits of the AdSV/in situ BiFE method
+ DP-AdSV method: Ip and CCr(VI) had a good linear correlation in CCr (VI) range from 2 to 12 ppb with a correlation coefficient R of 0.9989;
Sensitivity:
SqW-AdSV method achieved a sensitivity of 23.0 µA/ppb (approximately 34 times higher than DP-AdSV, 0.682 µA/ppb) (Fig 3.1.C, Fig 3.1.D);
LOD and LOQ:
+ SqW-AdSV (Edep = -800 mV, tdep = 160 s): LOD = 0.1 ppb; LOQ = 0.3 pbb
+ DP-AdSV (Edep = -800 mV, tdep = 50 s): LOD = 0.6 ppb; LOQ = 2 ppb SqW-AdSV method achieved a narrower linear range than the DP-AdSV method, but it got higher sensitivity than the DP-AdSV (due to having lower LOD and higher slope of the standard curve) It can be said that with LOD as above, DP-AdSV and SqW-AdSV
methods can be used with in situ BiFE electrodes to determine Cr
Trang 12different measurement modes were evaluated Some comments were deduced from this evaluation:
pH of analysis solutions were in the range of 5.8 6.2 (except pH of
solutions for DP- AdSV/ex situ MFE experiments were from 5.0 to 5.2)
Appropriate concentration of complexing agent (CDTPA) was of 4×103
M for all experiments (except for DP-AdSV/HMDE experiments of 0.8×103 M)
Appropriate concentration of NaNO3 in DP-AdSV/HMDE experiments were of 1 M, in DP-AdSV/MFE experiments were of 0,5
M, and of 0.4 M for all the others
Suitable adsorption potential (EAd) for DP-AdSV/HMDE experiments was of -1100 mV, for DP-AdSV/MFE of -1000 mV, and of 800 mV for all the others
Suitable adsorption times (tAd) depended on each specific method
With a low detection limit (0.3 ppb), MFE can be used to analyze Cr(VI) in environment samples HMDE had a higher detection limit (1.2 ppb) and some difficulty in using The disadvantage of both electrodes is that they use toxic metal mercury so the use of them is limited
- Two SqW-AdSV/in situ BiFE and SqW-AdSV /ex situ BiFE methods
gained lowest detection limits (0.1 ppb and 0.2 ppb, respetively); in addition, bismuth is a non-toxic element, environmentally friendly Determination procedures are relatively simple, so these two methods are chosen for the real sample analysis However, to be sure about the applicability of these two Cr (VI) methods, it is necessary to control their quality before applying them to reality
3.5 Application and procedure development
Experimental results show that the use of ex situ BiFE and in situ
BiFE to determine trace chromium by SqW-AdSV method is the most feasible
3.5.1 Determination of chromium in natural water by SqW-AdSV method using BiFE
In natural water samples, chromium usually exists in both Cr(VI) and Cr(III) forms As investigated, the method described in this study is able to determine Cr(VI) and also the total Cr(VI) + Cr(III) if in the sample decomposition step oxidant was added to oxidize Cr(III) to Cr (VI) Thus we can fully determine chromium in individual forms by
Trang 13determining the Cr(VI) (), the total content of Cr(VI) + (III) (), from which Cr (III) is the difference of () and ()
Results from the previous sections show that the square wave
adsorptive voltammetry with ex situ BiFE and in situ BiFE can be
applied to determine the trace amount of Cr(VI) with LOD of 0.2 ppb and 0.1 ppb, respectively With these LOD, SqW-AdSV method using
ex situ BiFE and in situ BiFE can directly determine the trace amount of
Cr(VI) in natural water, without preconcentration step, which is the main advantageous of the SqW-AdSV/BiFE method
To answer the question of whether these methods can be applied
to analyze trace amount of chromium in natural water samples, we have conducted experiments to verify the accuracy and trueness (through analyze CRM sample) and applied to analyze some samples of natural water Based on the experiments mentioned, a determination procedure for Cr(VI) and total chromium (Cr (VI + III)) in water were proposed
using SqW-AdSV/in situ BiFE
3.5.1.1 Quality control through standard sample analysis
In order to confirm the practical applicability of the AdSV method to analyze trace chromium using BiFE, the quality of the analytical procedure needs to be controlled by evaluating the accuracy and repeatability of the result of standard samples analysis
SqW-a Surface water samples
Select Surface water Certified Material Reference (SPS-SW1 Batch 122) to evaluate the accuracy of the method The actual value
of the chromium content of the sample is 2.00 ± 0.02 ppb (95% confidence boundary ε = ± 0.02 ppb) Analysis of standard SPS-SW1 surface water (CRM) samples by the SqW-AdSV using two types of
electrodes ex situ BiFE and in situ BiFE with the appropriate
experimental conditions in Table 3.5.1 Analyse repeatedly 3 times The volume of the solution to be charged to the electrolyser is 2 mL, and the volume of solution in the electrolyser is 10 mL
The results obtained in table 3.5.2 show that:
- The SqW-AdSV method using ex situ BiFE has good
repeatability (RSD = 7% (n = 3) ≤ ½ RSDH (RSDH = 2 (1 - 0.5 lgC), for
concentrations of 2 ppb is 41%), in situ BiFE also has good
repeatability (RSD = 4% (n = 3) ≤ ½ RSDH = 20%)
Trang 14- Both methods have good accuracy due to the determined Cr content within the 95% confidence interval of the CRM sample
- The SqW-AdSV/ in situ BiFE method has better accuracy and repeatability than the SqW-AdSV / ex situ BiFE method
b For sea water samples
Analysis of the standard seawater CRM coded NASS 6 by the
SqW-AdSV method using two types of ex situ BiFE and in situ BiFE
electrodes with the appropriate experimental conditions in Table 3.5.1
Because the concentration of Cr in NASS 6 seawater was too small to be directly analyzed, so only NASS 6 standard specimens were used as the matrix for analysis and validity The actual value of the Cr content in the NASS 6 sample is 0.116 ± 0.008 ppb (the 95% confidence bound ε = ± 0.008 ppb) NAAS 6 standard specimens were added with standard Cr(VI) to attain 3 levels of 2 ppb, 6 ppb and 10 ppb, and then analyzed with standard added samples to determine recovery
The results showed that: When using the SqW-AdSV method
with two types of ex situ BiFE and in situ BiFE for chromium
analysis in seawater samples, both methods gave satisfactory results,
the SqW-AdSV / in situ BiFE gave a (recoverability from 94 to 109%) accuracy better than the SqW-AdSV / ex situ BiFE
(recoverability from 88 to 110%) According to the AOAC (American Association of Analytical Chemists) when analyzing levels of 1.0 to 10 ppb, achieving a recovery rate of 80 to 110% is acceptable Therefore it is possible to use this method to analyze chromium in seawater samples