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Spectrophotometric determination of Cu(II) with sequential injection analysis
Talanta 49 (1999) 1099 – 1108 www.elsevier.com/locate/talanta
Spectrophotometric determination of Cu(II) with sequential injection analysis J.F. van Staden *, A. Botha Department of Chemistry, Uni6ersity of Pretoria, Pretoria 0002, South Africa Received 30 November 1998; received in revised form 2 February 1999; accepted 2 February 1999
Abstract A sequential injection system, based on the reaction of Cu(II) with diethyldithiocarbamate (DDTC), was developed for the determination of Cu(II) in plant food and water samples. The extraction procedure, generally used to extract the Cu(II)–DDTC complex for subsequent analysis was eliminated in this procedure. The complex was detected spectrophotometrically in aqueous solutions at 460 nm. The physical and chemical parameters depicting the system were studied to obtain optimum conditions for sample analysis. The system developed is fully computerized and able to monitor Cu(II) in samples at seven samples per hour with a relative standard deviation of B 4.50%. The calibration curve is linear from 0.5–5.0 mg/l with a detection limit of 0.2 mg/l. Interferences were reduced by introducing multiple flow reversals, to increase mixing between the reagent and sample zones, and subsequently enhance working of the masking agents (EDTA/citrate). © 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper(II); Diethyldithiocarbamate; Sequential injection analysis; Multiple flow reversals; Plant food samples; Water samples
1. Introduction The steady increase in pollution necessitates the analysis and monitoring of toxic species that could become a serious potential hazard if not controlled. Copper fulfils various roles in the agricultural field. Control thereof in terms of nutri Presented at the Ninth International Conference on Flow Injection Analysis (ICFIA’98) held in Seattle, WA, USA, August 23 – 27, 1998. * Corresponding author. Fax: + 27-12-362-5297. E-mail address: [email protected] (J.F. van Staden)
tion of plants and its possible contamination of water, is necessary. Metal dithiocarbamates are widely used for analytical purposes due to their characteristic colours [1]. Diethyldithiocarbamate (DDTC) is often used as reagent for the selective determination of Cu(II) since the Cu(II)–DDTC stability constant is the highest, next to silver, compared with the stability constants of the other DDTC–metal complexes [2]. DDTC coordinates with Cu(II) through the two sulfur atoms to form a four-membered ring complex which can be spectrophotometrically detected. An extensive study has already been conducted on the analysis of copper using its reaction with
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J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
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DDTC and the subsequent extraction of the complex formed into an organic medium [2 –10]. Extraction of the Cu(II) – DDTC complex is preferred since it is slightly soluble in water and dissolves more readily in organic solvents [11]. A disadvantage of solvent-extraction is the use of toxic organic solvents that also have carcinogenic properties. A lot of emphasis is placed on conducting environmentally friendly analysis and eliminating the use of harmful substances. Spectrophotometric analysis of the Cu(II) –DDTC complex, without solvent-extraction, has also successfully been conducted by measurement of the coloured complex in aqueous medium [12–16]. The system developed in this study was based on the spectrophotometric measurement of the Cu(II)–DDTC complex in an aqueous medium.
The development of systems to conduct routine analysis is subjected to stringent requirements. Effective cost control plays a very important role in the efficient management of routine control laboratories and on-line process analysers. Lowcost instrumentation, minimum sample and reagent consumption therefore become important. Detectors like AAS and ICP, although very fast are expensive when compared to UV/Vis spectrophotometers, whereas the reagent consumption in flow injection analysis is relatively high compared to sequential injection analysis (SIA). An additional advantage of SIA is the relative ease of implementation as an on-line analyser. SIA is based on the sequential aspiration of microlitres of reagent and sample, as zones, into a holding
Fig. 1. A schematic diagram of the sequential injection analyser used for the determination of Cu(II) with DDTC. HC, holding coil; RC, reaction coil; SV, selection valve. Table 1 Device sequence for one cycle of the sequential injection system Time (s) 0 5.0 15.7 16.7 17.7 49.8 50.8 51.8 167.8 519.0
Pump
Valve
Description
Off
DDTC/EDTA/citrate
Pump off, select DDTC/EDTA/citrate stream (valve position 1)
Reverse Off Cu/Fe Reverse Off Detector Forward, reverse Forward Off
Home
Draw up DDTC/EDTA/citrate solution Pump stop Select Cu/Fe stream (valve position 2) Draw up Cu/Fe solution Pump stop Select detector line (valve position 3) Twenty-nine flow reversals of 4 s each Pump stack of zones to detector Pump off, return valve to starting position (valve position 1)
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108 Table 2 Influence of different techniques to reduce interference from Fe(II) % Interference Stopped-flow period (s) 0 120 240
31.8 27.9 10.4
Flow reversals ( c ) 1 10 20 30
52.0 32.2 28.6 13.6
Length of flow reversal (s) 0 4 10 20
65.2 59.2 43.9 12.6
coil. After aspiration the flow is reversed and the well-defined zones are then propelled via a reaction coil to the spectrophotometer. The reagent and sample zones mutually penetrate one another to form a product zone which can then be detected. The purpose of this work was to study the possibility of applying DDTC as reagent for the
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spectrophotometric determination of Cu(II) with a SIA system. A sensitive and selective method was developed for the determination of Cu(II) in natural waters and plant food.
2. Experimental
2.1. Reagents and solutions All reagents were prepared from analyticalreagent grade chemicals unless specified otherwise. All aqueous solutions were prepared using deionised water from a Modulab system (Continental Water Systems, San Antonio, TX). The de-ionised water used to prepare the aqueous solutions were degassed, by heating to boiling point and cooling before the solutions were made up.
2.1.1. Standard Cu(II) solution, 1000 mg/l Pure copper metal coarse chips were used in the preparation of the Cu(II) stock solution. The copper metal was cleaned to remove any oxides and dissolved by heating 1.0 g of the copper metal in 10 ml 55% HNO3 and ca. 10 ml of water. The subsequent solution was cooled and then diluted to 1000 ml with water.
Fig. 2. Influence of flow rate on sensitivity and precision.
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Fig. 3. Influence of reagent (DDTC/EDTA/citrate) volume on sensitivity and precision.
Fig. 4. Influence of sample (Cu/Fe) volume on sensitivity and precision.
2.1.2. Standard Fe(II) solution, 100 mg/l Fe(NH4)2(SO4)2.6H2O (0.0702 g) was dissolved in ca. 20 ml of water with 1.1 ml of 18.4 mol/l H2SO4 (98%). The final solution was diluted to 100 ml with water. 2.1.3. Cu(II) /Fe(II) working solution A solution containing 3 mg/l Cu(II) and 15 mg/l Fe(II) was prepared in order to study the effect of Fe(II) as the largest interferent on the
Cu(II)–DDTC reaction [16] in the presence of the analyte. These solutions were made by appropriate dilutions of the 1000 mg/l Cu(II) and 100 mg/l Fe(II) standard solutions respectively.
2.1.4. DDTC/EDTA/citrate reagent solution Sodium diethyldithiocarbamate (DDTC; 0.1% (m/v)) was dissolved in 50 ml water by heating the solution to 60°C. After the DDTC solution was cooled, 1.2 g ethylene-diamine-tetra-acetic acid
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
disodium salt (EDTA) together with 1.5 g triammonium citrate was dissolved in 25 ml water and added to the DDTC solution; 9.5 ml of a 2 mol/l NH4Cl solution was then added to the DDTC/EDTA/citrate solution and the pH adjusted to 8.3 by adding an appropriate volume of 2 mol/l NH3. The final solution was made up to 100 ml with water.
2.2. Apparatus The sequential injection system depicted in Fig. 1 was constructed from the following components: a Gilson Minipuls peristaltic pump (Model M312, Gilson, Villiers-le-Bel, France); a 10-port electrically actuated selection valve (Model ECSD10P; Valco Instruments, Houston, TX); and a UNICAM 8625 UV – visible spectrophotometer equipped with a 10-mm Hellmatype (Hellma, Mu¨lheim/Baden, Germany) flow-through cell (volume 80 ml) for absorbance measurements. Data acquisition and device control was achieved using a PC30-B interface board (Eagle Electric, Cape Town, South Africa) and an assembled distribution board (Mintek, Randburg, South Africa). The FlowTEK [17] software package (Mintek) for computer-aided flow anal-
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ysis was used throughout for device control and data acquisition. The wavelength of maximum absorption was identified by scanning the Cu(II)–DDTC complex solution over the 300–700 nm range with a Spectronic Genesys 5 spectrophotometer (Milton Roy). The optimum wavelength was chosen as 460 nm which corresponds to the wavelength used by previous authors conducting the same determination [13,16]. pH measurements of the DDTC/EDTA/citrate solution were conducted with an Orion pH meter (Model 420A; Orion Research) and an Orion pH Triode™ electrode.
2.3. Sample preparation Cu(II) was determined in plant food (water soluble samples) and water samples. A predetermined mass was weighed, dissolved and finally diluted to 100 ml. The water samples were spiked with a known concentration of Cu(II).
2.4. Procedure The device sequence for the determination of Cu(II) by sequential injection analysis is given in Table 1.
Table 3 Influence of sodium–DDTC concentration on peak height, precision and interference % [Na–DDTC] (m/v) Relative peak height (Cu)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
0.05 0.10 0.20 0.30 0.40
1.41 1.57 1.83 2.06 2.16
1.47 2.86 3.52 4.49 3.08
−19.22 −9.86 0.02 9.47 17.07
1.75 1.74 1.83 1.88 1.85
Table 4 Influence of pH of DDTC/EDTA/citrate solution on peak height, precision and interference pH
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
6.30 7.30 8.30 9.20
1.47 1.67 1.54 1.50
1.89 3.42 1.34 1.84
−10.52 0.95 −2.84 −6.11
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Table 5 Influence of EDTA concentration on peak height, precision and interference % [EDTA] (m/v)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
0.30 0.60 1.20
1.49 1.52 1.50
1.61 1.36 1.18
−9.14 −6.24 −4.35
3. Results and discussion
3.1. Optimization of the sequential injection system It is well-known that changing the physical parameters of an SIA system influenced the sensitivity and precision of measurements [18–20]. These parameters also have a direct influence on the sensitivity and precision obtained when studying a chemical reaction. It was shown that the following chemical parameters affect the Cu(II)– DDTC reaction [16]: DDTC concentration, pH, EDTA and citrate concentration. Optimization of physical and chemical parameters was thus necessary to develop a system with optimum sensitivity and precision.
3.1.1. Physical parameters 3.1.1.1. Comparison of different techniques to reduce Fe(II) interference. According to previous studies it seems that Fe(II) interfered significantly, even from the smallest interfering ion:Cu(II) ratio when Cu(II) was determined with DDTC [16]. It was therefore decided to optimize the Cu(II)– DDTC complexation reaction in the presence of Fe(II) as interferent. Optimising the chemical parameters to a point where interference from Fe(II) was a minimum possibly also reduced other interferences. Three physical techniques that could contribute to reducing interference were studied. The results are tabulated in Table 2. The techniques were: stopped-flow period, flow reversals and length of the flow reversals. Although the interference for the zero stopped-flow period (‘no stopped flow’) should be equivalent to the interference for no flow reversals, since both just mean
simple sample aspiration and delivery to the detector, the repeated results obtained showed a % interference of 31.8% for ‘no stopped-flow’ and 65.2% for ‘no flow reversal’ respectively (Table 2). The stopped-flow period was implemented for a fixed time directly after the flow was reversed. The length of the stopped-flow period was evaluated. A decrease in Fe(II) interference was observed for the longer periods, due to more time allowed for the masking agents to react with the interferent. A disadvantage of the stopped-flow period was, however, a decrease in sample frequency due to longer analysis times. Flow reversals were obtained by propelling the flow forward and backwards in the flow conduit. By repeating this action more than once, zone penetration was enhanced and a mixing effect was obtained which allowed more effective mixing of the sample and reagent zones. This action contributed to a larger reaction between the masking and interfering components. The results showed that a decrease in Fe(II) interference was obtained with a larger number of flow reversals. Although sensitivity decreased due to increased dispersion of the zones for the larger number of flow reversals, viable analysis times and adequate sensitivity were achieved. The length of a fixed number of flow reversals was changed by changing the time it takes for each reversal (counting one forward and backward motion as one reversal) to be completed. Increasing the length of the flow reversals effectively reduced interference although dispersion of the zones increased dramatically and sensitivity decreased as a result. Thirty flow reversals were chosen as optimum to reduce interference effectively and still gave adequate sensitivity and analysis times.
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
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Table 6 Influence of tri-ammonium citrate concentration on peak height, precision and interference % [Citrate] (m/v)
Relative peak height (Cu)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
1.50 3.00 6.00 10.00
1.54 1.60 1.72 1.82
1.52 1.71 1.86 1.99
2.25 2.01 1.10 1.45
−1.05 7.11 7.98 9.05
3.1.1.2. Flow rate. The effect of flow rate on peak height is shown in Fig. 2. A flow rate of 1.4 ml/min, which gave the best sensitivity and precision compared to other flow rates, was chosen as optimum. A decrease in peak height was observed for the higher flow rates. This was attributed to a low Cu(II) – DDTC reaction rate. The formation of the Cu(II) – DDTC complex can be illustrated by the following equilibrium reaction ((C2H5)2NCSS − =DDTC): Cu(II) (aq)+2(C2H5)2NCSS − (aq) X Cu[(C2H5)2NCSS]2 (s)/(aq) At low flow rates, adequate time was available for the equilibrium to be attained. An adequate amount of Cu(II) – DDTC complex was therefore formed for spectrophotometric detection. With increased flow rates the formed product zone was propelled faster to the detector. The time available for complex formation was therefore shortened, thus less Cu(II) – DDTC complex was formed and a decrease in sensitivity was observed. Although higher flow rates resulted in higher sampling frequencies, it was characterised by a loss of sensitivity and decreased precision (Fig. 2).
3.1.1.3. Reagent and sample 6olume. The reagent volume was studied by aspirating increasing reagent volumes (50 – 750 ml) into the system, while keeping the sample volume constant. In Fig. 3 an increase in sensitivity with increasing reagent volume was initially observed. No further increase for the largest reagent volumes was noticed. An optimum of 250 rather than 500 ml was chosen although the sensitivity for 500 ml was higher. The reason for this was that by using a large reagent volume (e.g. 500 ml) an excess of DDTC reagent would be added to the system,
and subsequently a larger sample volume would be necessary to react with all the reagent to deliver a plateau (no further increase in sensitivity with increasing sample volume) in an increasing sample volume graph. For these systems longer analysis times would be required for flushing the system resulting in lower sampling frequencies. With smaller reagent volumes (e.g. 250 ml), smaller sample volumes would be necessary to deliver a plateau in an increasing sample volume graph. An advantage of SIA is the reduction in reagent and sample consumption. By choosing a smaller reagent volume as optimum (250 ml) a decrease in sensitivity would be inevitable, but the analysis time could be kept as short as possible and reagent and sample consumption would be reduced. Fig. 4 illustrates the change in peak height with changing sample volume (100–1000 ml), whilst the reagent volume is kept constant. The increase in peak height was attributed to the larger quantity of analyte available to react with the excess of reagent. A plateau is reached when the sample
Table 7 Comparison of results obtained by the proposed SIA system and flame-AAS Sample
AAS (mg/l)
SIA (mg/l)
% RSD (SIA)
A B C D E Plant food: A Plant food: B
1.30 1.68 2.87 3.29 4.18 4.88 2.69
1.27 1.82 2.66 3.16 4.28 4.47 2.47
3.98 1.78 1.72 4.54 2.29 2.76 2.02
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Table 8 Interference from foreign ion species studied for a 3.0 mg/l Cu2+ solutiona Tolerance ratio of foreign species (x)
Foreign species
600 300 200 150 40 20 10 5
Mg2+ 2+ SO2− 4 , Mn − NO3 Ca2+ Cl− Zn2+, K+ CO2− 3 Fe2+, Fe3+
a
The ratio of foreign ion (mg/l) to Cu(II) is indicated as x:1.
reacts with most of the reagent in the reagent volume. Optimum sensitivity and precision were obtained for 750 ml of sample.
3.1.2. Chemical parameters 3.1.2.1. Sodium diethyldithiocarbamate (Na– DDTC) concentration. Increasing the % Na– DDTC concentration (m/v) resulted in increased peak heights (Table 3) for the Cu(II)/Fe(II) working solution. Although it was expected that the response would stay constant for higher reagent concentrations due to all the sample being used up by the reagent, the results obtained proved the contrary. The relatively constant peak height obtained for the reaction between Cu(II) and DDTC (in the absence of Fe(II)) suggested that all the Cu(II) had reacted and that by increasing the DDTC concentration no significant increase in complex formation was achieved. An increase in peak height was, however, observed for DDTC and the Cu(II)/Fe(II) sample. This increase was attributed to Fe(II) which was added as interferent to the Cu(II) sample. In this instance Fe(II) was also available to react with the DDTC after an optimum of Cu(II) had reacted. In addition to the detected Cu(II) – DDTC complex, the Fe(II)– DDTC complex might also contribute to the peak height. It is also evident from Table 3 that the % interference changes from a large negative to a large positive value with increasing Na–DDTC concentrations. This is due to the possibility that
Fe(II) might have formed different complexes at various Na–DDTC concentrations. Increased Na–DDTC concentrations could affect the coordination of DDTC towards Fe(II) to form different complexes with differing absorption characteristics. Choosing an optimum was based on a compromise regarding sensitivity and % interference and gaining with respect to precision. The 0.1% Na– DDTC concentration was chosen as optimum. The 0.2% concentration gave better sensitivity with less interference, but the precision was not that satisfactory. The interference experienced for the 0.1% concentration can be reduced by optimization of the masking agent concentrations.
3.1.2.2. pH of the DDTC/EDTA/citrate solution. The most important feature of the dithiocarbamate ion is its protonation in acidic solutions and the subsequent decomposition into carbon disulphide and the protonated amine [1]. Preliminary experiments therefore showed that complex formation between Cu(II) and DDTC did not occur at a pH below 6, but that complex formation took place above a pH of 6. Due to possible interference from Fe(II) on the spectrophotometric determination of Cu(II), the pH range between 6 and 10 was studied to see whether pH might influence the overall sensitivity as well as interference from Fe(II). Table 4 shows that a change in pH had a minor effect on the sensitivity of the measurements. The pH did however influence the complexation reaction. Since interference was minimised between a pH of 7.3 and 8.3, it can be concluded that the masking agents performed best in the mentioned range. Precision obtained at pH 8.3 was better than at pH 7.3 and was thus chosen as the optimum working condition. This compares well to the optimum pH used in previous studies [13,16]. 3.1.2.3. Masking agents concentration. The selectivity of the complexation reaction between Cu(II) and DDTC is enhanced considerably by using EDTA in combination with citrate as masking agent [1,2,7,13,16]. Increasing the EDTA concen-
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
tration did not affect the sensitivity and had a minor influence on precision (Table 5). As was expected the % interference decreased with higher EDTA concentrations. A 1.2% (m/v) EDTA concentration was chosen for masking interferences. Table 6 shows an increase in sensitivity with increasing citrate concentration, both in the presence and absence of Fe(II). This observation is due to the fact that the citrate might react with either Fe(II) or Cu(II) to form complexes that absorb at the wavelength of interest to contribute to the Cu(II) – DDTC absorption signal. This also explained the increase in % interference with increasing citrate concentration. Least interference was experienced at 1.5% (m/v) citrate and this concentration was therefore chosen as optimum.
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4.2. Interferences Wang and coworkers [16] showed that some ions interfered in the spectrophotometric determination of Cu(II) with DDTC. The interferences of these ions with the proposed SIA system were studied and the results are highlighted in Table 8. It is clear from Table 8 that the level of tolerance is up to at least a 5:1 (foreign ion to Cu(II) ion) ratio for Fe2 + , an interference that has been dealt with in detail. The interference from the carbonate ion is mainly due to a change in pH that was eliminated by selecting the optimum pH working conditions as described.
5. Conclusion 4. Method evaluation The proposed SIA system was critically evaluated with regard to accuracy, precision, detection limit, linear range, sample interaction, sampling frequency and interferences.
4.1. Linearity, accuracy, precision and detection limit The response of the proposed SIA system for the spectrophotometric determination of Cu(II) was evaluated under optimum conditions. The calibration curve was linear from 0.5 to 5.0 mg/l (response =0.4984[Cu(II)] + 0.1367; r = 0.998, n=5). The calculated detection limit was 0.2 mg/l Cu(II). The accuracy was evaluated by analysing two plant food samples and five tap water samples. The results, shown in Table 7, are in good agreement with the results obtained by flame-AAS. The precision determined for the analysed samples (n= 5) is also shown in Table 7. In all cases the RSD was B4.50%. A sampling frequency of seven samples per hour was obtained. The lengthy analysis time (519 s) was ascribed to the time necessary to perform the flow reversals and time needed to rinse the system to minimise possible carry-over between analysis. The sample interaction between samples was smaller than 1%, which is negligible.
The proposed system proved to be successful in analysing Cu(II) in plant food and water samples. Analysis was done without the necessity of introducing an extraction step thus complying with the need for conducting environmentally safe analysis. The lengthy analysis time that contributes to the low sample frequency was due to the flow reversals necessary to reduce interference and to flush the system. The flow reversals successfully reduced interference. The system is fully computerized and can be incorporated on-line if required. The calibration curve is linear from 0.5 to 5.0 mg/l with a detection limit of 0.2 mg/l. The system processes seven samples per hour with a relative standard deviation of B 4.50%.
References [1] A. Hulanicki, Talanta 14 (1967) 1371. [2] J. Szpunar-Lobinska, M. Trojanowicz, Anal. Sci. 6 (3) (1990) 415. [3] T. Blanco, N. Maniasso, M. Fernanda-Gine, A.O. Jacintho, Analyst 123 (2) (1998) 191. [4] X.N. Ge, P.F. Jiang, W.S. Bo, Fenxi Huaxue 22 (8) (1994) 863. [5] Z. Yan, X. Jiang, Y. Yi, Y. Hu, Yaowu Fenxi Zazhi 13 (4) (1993) 275. [6] A.R. Fernandez-Alba, J.L. Martinez-Vidal, P. Aguilera, F. Freniche, A. Aguera, Anal. Lett. 25 (8) (1992) 1581. [7] M. Kan, H. Sakamoto, T. Nasu, M. Taga, Anal. Sci. 7 (6) (1991) 913.
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[8] H. Satoh, Y. Kikuchi, T. Suzuki, K. Sawada, Bunseki Kagaku 40 (10) (1991) 167. [9] K. Fujiwara, Anal. Chim. Acta 212 (1-2) (1988) 245. [10] B. Zuo, G. Chen, J. Han, Huaxue Xuebao 42 (1) (1984) 93. [11] B.J. Mueller, R.J. Lovett, Anal. Chem. 59 (1987) 1405. [12] M.P. San-Andres, M.L. Marina, S. Vera, Analyst 120 (2) (1995) 255. [13] P. Wang, D.L. Jia, Y. Wang, Fenxi Huaxue 23 (1) (1995) 36. [14] T. Kiriyama, Bunseki Kagaku 42 (4) (1993) 223. [15] M.C. Garcia-Alvarez-Coque, M.C. Martinez-Vaya, G.
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Ramis-Ramos, R.M. Villanueva-Camanas, C. MongayFernandez, Quim. Anal. 5 (3) (1986) 329. P. Wang, S.J. Shi, D. Zhou, Microchem. J. 52 (1995) 146. G.D. Marshall, J.F. van Staden, Anal. Instrum. 20 (1992) 79. J.F. van Staden, A. Botha, S.-Afr. Tydskr. Chem. 51 (2) (1998) 100. G.D. Marshall, J.F. van Staden, Proc. Cont. Qual. 3 (1992) 251. T. Gu¨beli, G.D. Christian, J. Ru˚z' ic' ka, Anal. Chem. 63 (1991) 2407.
Spectrophotometric determination of Cu(II) with sequential injection analysis J.F. van Staden *, A. Botha Department of Chemistry, Uni6ersity of Pretoria, Pretoria 0002, South Africa Received 30 November 1998; received in revised form 2 February 1999; accepted 2 February 1999
Abstract A sequential injection system, based on the reaction of Cu(II) with diethyldithiocarbamate (DDTC), was developed for the determination of Cu(II) in plant food and water samples. The extraction procedure, generally used to extract the Cu(II)–DDTC complex for subsequent analysis was eliminated in this procedure. The complex was detected spectrophotometrically in aqueous solutions at 460 nm. The physical and chemical parameters depicting the system were studied to obtain optimum conditions for sample analysis. The system developed is fully computerized and able to monitor Cu(II) in samples at seven samples per hour with a relative standard deviation of B 4.50%. The calibration curve is linear from 0.5–5.0 mg/l with a detection limit of 0.2 mg/l. Interferences were reduced by introducing multiple flow reversals, to increase mixing between the reagent and sample zones, and subsequently enhance working of the masking agents (EDTA/citrate). © 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper(II); Diethyldithiocarbamate; Sequential injection analysis; Multiple flow reversals; Plant food samples; Water samples
1. Introduction The steady increase in pollution necessitates the analysis and monitoring of toxic species that could become a serious potential hazard if not controlled. Copper fulfils various roles in the agricultural field. Control thereof in terms of nutri Presented at the Ninth International Conference on Flow Injection Analysis (ICFIA’98) held in Seattle, WA, USA, August 23 – 27, 1998. * Corresponding author. Fax: + 27-12-362-5297. E-mail address: [email protected] (J.F. van Staden)
tion of plants and its possible contamination of water, is necessary. Metal dithiocarbamates are widely used for analytical purposes due to their characteristic colours [1]. Diethyldithiocarbamate (DDTC) is often used as reagent for the selective determination of Cu(II) since the Cu(II)–DDTC stability constant is the highest, next to silver, compared with the stability constants of the other DDTC–metal complexes [2]. DDTC coordinates with Cu(II) through the two sulfur atoms to form a four-membered ring complex which can be spectrophotometrically detected. An extensive study has already been conducted on the analysis of copper using its reaction with
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 0 6 2 - 4
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
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DDTC and the subsequent extraction of the complex formed into an organic medium [2 –10]. Extraction of the Cu(II) – DDTC complex is preferred since it is slightly soluble in water and dissolves more readily in organic solvents [11]. A disadvantage of solvent-extraction is the use of toxic organic solvents that also have carcinogenic properties. A lot of emphasis is placed on conducting environmentally friendly analysis and eliminating the use of harmful substances. Spectrophotometric analysis of the Cu(II) –DDTC complex, without solvent-extraction, has also successfully been conducted by measurement of the coloured complex in aqueous medium [12–16]. The system developed in this study was based on the spectrophotometric measurement of the Cu(II)–DDTC complex in an aqueous medium.
The development of systems to conduct routine analysis is subjected to stringent requirements. Effective cost control plays a very important role in the efficient management of routine control laboratories and on-line process analysers. Lowcost instrumentation, minimum sample and reagent consumption therefore become important. Detectors like AAS and ICP, although very fast are expensive when compared to UV/Vis spectrophotometers, whereas the reagent consumption in flow injection analysis is relatively high compared to sequential injection analysis (SIA). An additional advantage of SIA is the relative ease of implementation as an on-line analyser. SIA is based on the sequential aspiration of microlitres of reagent and sample, as zones, into a holding
Fig. 1. A schematic diagram of the sequential injection analyser used for the determination of Cu(II) with DDTC. HC, holding coil; RC, reaction coil; SV, selection valve. Table 1 Device sequence for one cycle of the sequential injection system Time (s) 0 5.0 15.7 16.7 17.7 49.8 50.8 51.8 167.8 519.0
Pump
Valve
Description
Off
DDTC/EDTA/citrate
Pump off, select DDTC/EDTA/citrate stream (valve position 1)
Reverse Off Cu/Fe Reverse Off Detector Forward, reverse Forward Off
Home
Draw up DDTC/EDTA/citrate solution Pump stop Select Cu/Fe stream (valve position 2) Draw up Cu/Fe solution Pump stop Select detector line (valve position 3) Twenty-nine flow reversals of 4 s each Pump stack of zones to detector Pump off, return valve to starting position (valve position 1)
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108 Table 2 Influence of different techniques to reduce interference from Fe(II) % Interference Stopped-flow period (s) 0 120 240
31.8 27.9 10.4
Flow reversals ( c ) 1 10 20 30
52.0 32.2 28.6 13.6
Length of flow reversal (s) 0 4 10 20
65.2 59.2 43.9 12.6
coil. After aspiration the flow is reversed and the well-defined zones are then propelled via a reaction coil to the spectrophotometer. The reagent and sample zones mutually penetrate one another to form a product zone which can then be detected. The purpose of this work was to study the possibility of applying DDTC as reagent for the
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spectrophotometric determination of Cu(II) with a SIA system. A sensitive and selective method was developed for the determination of Cu(II) in natural waters and plant food.
2. Experimental
2.1. Reagents and solutions All reagents were prepared from analyticalreagent grade chemicals unless specified otherwise. All aqueous solutions were prepared using deionised water from a Modulab system (Continental Water Systems, San Antonio, TX). The de-ionised water used to prepare the aqueous solutions were degassed, by heating to boiling point and cooling before the solutions were made up.
2.1.1. Standard Cu(II) solution, 1000 mg/l Pure copper metal coarse chips were used in the preparation of the Cu(II) stock solution. The copper metal was cleaned to remove any oxides and dissolved by heating 1.0 g of the copper metal in 10 ml 55% HNO3 and ca. 10 ml of water. The subsequent solution was cooled and then diluted to 1000 ml with water.
Fig. 2. Influence of flow rate on sensitivity and precision.
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Fig. 3. Influence of reagent (DDTC/EDTA/citrate) volume on sensitivity and precision.
Fig. 4. Influence of sample (Cu/Fe) volume on sensitivity and precision.
2.1.2. Standard Fe(II) solution, 100 mg/l Fe(NH4)2(SO4)2.6H2O (0.0702 g) was dissolved in ca. 20 ml of water with 1.1 ml of 18.4 mol/l H2SO4 (98%). The final solution was diluted to 100 ml with water. 2.1.3. Cu(II) /Fe(II) working solution A solution containing 3 mg/l Cu(II) and 15 mg/l Fe(II) was prepared in order to study the effect of Fe(II) as the largest interferent on the
Cu(II)–DDTC reaction [16] in the presence of the analyte. These solutions were made by appropriate dilutions of the 1000 mg/l Cu(II) and 100 mg/l Fe(II) standard solutions respectively.
2.1.4. DDTC/EDTA/citrate reagent solution Sodium diethyldithiocarbamate (DDTC; 0.1% (m/v)) was dissolved in 50 ml water by heating the solution to 60°C. After the DDTC solution was cooled, 1.2 g ethylene-diamine-tetra-acetic acid
J.F. 6an Staden, A. Botha / Talanta 49 (1999) 1099–1108
disodium salt (EDTA) together with 1.5 g triammonium citrate was dissolved in 25 ml water and added to the DDTC solution; 9.5 ml of a 2 mol/l NH4Cl solution was then added to the DDTC/EDTA/citrate solution and the pH adjusted to 8.3 by adding an appropriate volume of 2 mol/l NH3. The final solution was made up to 100 ml with water.
2.2. Apparatus The sequential injection system depicted in Fig. 1 was constructed from the following components: a Gilson Minipuls peristaltic pump (Model M312, Gilson, Villiers-le-Bel, France); a 10-port electrically actuated selection valve (Model ECSD10P; Valco Instruments, Houston, TX); and a UNICAM 8625 UV – visible spectrophotometer equipped with a 10-mm Hellmatype (Hellma, Mu¨lheim/Baden, Germany) flow-through cell (volume 80 ml) for absorbance measurements. Data acquisition and device control was achieved using a PC30-B interface board (Eagle Electric, Cape Town, South Africa) and an assembled distribution board (Mintek, Randburg, South Africa). The FlowTEK [17] software package (Mintek) for computer-aided flow anal-
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ysis was used throughout for device control and data acquisition. The wavelength of maximum absorption was identified by scanning the Cu(II)–DDTC complex solution over the 300–700 nm range with a Spectronic Genesys 5 spectrophotometer (Milton Roy). The optimum wavelength was chosen as 460 nm which corresponds to the wavelength used by previous authors conducting the same determination [13,16]. pH measurements of the DDTC/EDTA/citrate solution were conducted with an Orion pH meter (Model 420A; Orion Research) and an Orion pH Triode™ electrode.
2.3. Sample preparation Cu(II) was determined in plant food (water soluble samples) and water samples. A predetermined mass was weighed, dissolved and finally diluted to 100 ml. The water samples were spiked with a known concentration of Cu(II).
2.4. Procedure The device sequence for the determination of Cu(II) by sequential injection analysis is given in Table 1.
Table 3 Influence of sodium–DDTC concentration on peak height, precision and interference % [Na–DDTC] (m/v) Relative peak height (Cu)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
0.05 0.10 0.20 0.30 0.40
1.41 1.57 1.83 2.06 2.16
1.47 2.86 3.52 4.49 3.08
−19.22 −9.86 0.02 9.47 17.07
1.75 1.74 1.83 1.88 1.85
Table 4 Influence of pH of DDTC/EDTA/citrate solution on peak height, precision and interference pH
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
6.30 7.30 8.30 9.20
1.47 1.67 1.54 1.50
1.89 3.42 1.34 1.84
−10.52 0.95 −2.84 −6.11
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Table 5 Influence of EDTA concentration on peak height, precision and interference % [EDTA] (m/v)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
0.30 0.60 1.20
1.49 1.52 1.50
1.61 1.36 1.18
−9.14 −6.24 −4.35
3. Results and discussion
3.1. Optimization of the sequential injection system It is well-known that changing the physical parameters of an SIA system influenced the sensitivity and precision of measurements [18–20]. These parameters also have a direct influence on the sensitivity and precision obtained when studying a chemical reaction. It was shown that the following chemical parameters affect the Cu(II)– DDTC reaction [16]: DDTC concentration, pH, EDTA and citrate concentration. Optimization of physical and chemical parameters was thus necessary to develop a system with optimum sensitivity and precision.
3.1.1. Physical parameters 3.1.1.1. Comparison of different techniques to reduce Fe(II) interference. According to previous studies it seems that Fe(II) interfered significantly, even from the smallest interfering ion:Cu(II) ratio when Cu(II) was determined with DDTC [16]. It was therefore decided to optimize the Cu(II)– DDTC complexation reaction in the presence of Fe(II) as interferent. Optimising the chemical parameters to a point where interference from Fe(II) was a minimum possibly also reduced other interferences. Three physical techniques that could contribute to reducing interference were studied. The results are tabulated in Table 2. The techniques were: stopped-flow period, flow reversals and length of the flow reversals. Although the interference for the zero stopped-flow period (‘no stopped flow’) should be equivalent to the interference for no flow reversals, since both just mean
simple sample aspiration and delivery to the detector, the repeated results obtained showed a % interference of 31.8% for ‘no stopped-flow’ and 65.2% for ‘no flow reversal’ respectively (Table 2). The stopped-flow period was implemented for a fixed time directly after the flow was reversed. The length of the stopped-flow period was evaluated. A decrease in Fe(II) interference was observed for the longer periods, due to more time allowed for the masking agents to react with the interferent. A disadvantage of the stopped-flow period was, however, a decrease in sample frequency due to longer analysis times. Flow reversals were obtained by propelling the flow forward and backwards in the flow conduit. By repeating this action more than once, zone penetration was enhanced and a mixing effect was obtained which allowed more effective mixing of the sample and reagent zones. This action contributed to a larger reaction between the masking and interfering components. The results showed that a decrease in Fe(II) interference was obtained with a larger number of flow reversals. Although sensitivity decreased due to increased dispersion of the zones for the larger number of flow reversals, viable analysis times and adequate sensitivity were achieved. The length of a fixed number of flow reversals was changed by changing the time it takes for each reversal (counting one forward and backward motion as one reversal) to be completed. Increasing the length of the flow reversals effectively reduced interference although dispersion of the zones increased dramatically and sensitivity decreased as a result. Thirty flow reversals were chosen as optimum to reduce interference effectively and still gave adequate sensitivity and analysis times.
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Table 6 Influence of tri-ammonium citrate concentration on peak height, precision and interference % [Citrate] (m/v)
Relative peak height (Cu)
Relative peak height (Cu/Fe)
% RSD (Cu/Fe)
% Interference
1.50 3.00 6.00 10.00
1.54 1.60 1.72 1.82
1.52 1.71 1.86 1.99
2.25 2.01 1.10 1.45
−1.05 7.11 7.98 9.05
3.1.1.2. Flow rate. The effect of flow rate on peak height is shown in Fig. 2. A flow rate of 1.4 ml/min, which gave the best sensitivity and precision compared to other flow rates, was chosen as optimum. A decrease in peak height was observed for the higher flow rates. This was attributed to a low Cu(II) – DDTC reaction rate. The formation of the Cu(II) – DDTC complex can be illustrated by the following equilibrium reaction ((C2H5)2NCSS − =DDTC): Cu(II) (aq)+2(C2H5)2NCSS − (aq) X Cu[(C2H5)2NCSS]2 (s)/(aq) At low flow rates, adequate time was available for the equilibrium to be attained. An adequate amount of Cu(II) – DDTC complex was therefore formed for spectrophotometric detection. With increased flow rates the formed product zone was propelled faster to the detector. The time available for complex formation was therefore shortened, thus less Cu(II) – DDTC complex was formed and a decrease in sensitivity was observed. Although higher flow rates resulted in higher sampling frequencies, it was characterised by a loss of sensitivity and decreased precision (Fig. 2).
3.1.1.3. Reagent and sample 6olume. The reagent volume was studied by aspirating increasing reagent volumes (50 – 750 ml) into the system, while keeping the sample volume constant. In Fig. 3 an increase in sensitivity with increasing reagent volume was initially observed. No further increase for the largest reagent volumes was noticed. An optimum of 250 rather than 500 ml was chosen although the sensitivity for 500 ml was higher. The reason for this was that by using a large reagent volume (e.g. 500 ml) an excess of DDTC reagent would be added to the system,
and subsequently a larger sample volume would be necessary to react with all the reagent to deliver a plateau (no further increase in sensitivity with increasing sample volume) in an increasing sample volume graph. For these systems longer analysis times would be required for flushing the system resulting in lower sampling frequencies. With smaller reagent volumes (e.g. 250 ml), smaller sample volumes would be necessary to deliver a plateau in an increasing sample volume graph. An advantage of SIA is the reduction in reagent and sample consumption. By choosing a smaller reagent volume as optimum (250 ml) a decrease in sensitivity would be inevitable, but the analysis time could be kept as short as possible and reagent and sample consumption would be reduced. Fig. 4 illustrates the change in peak height with changing sample volume (100–1000 ml), whilst the reagent volume is kept constant. The increase in peak height was attributed to the larger quantity of analyte available to react with the excess of reagent. A plateau is reached when the sample
Table 7 Comparison of results obtained by the proposed SIA system and flame-AAS Sample
AAS (mg/l)
SIA (mg/l)
% RSD (SIA)
A B C D E Plant food: A Plant food: B
1.30 1.68 2.87 3.29 4.18 4.88 2.69
1.27 1.82 2.66 3.16 4.28 4.47 2.47
3.98 1.78 1.72 4.54 2.29 2.76 2.02
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Table 8 Interference from foreign ion species studied for a 3.0 mg/l Cu2+ solutiona Tolerance ratio of foreign species (x)
Foreign species
600 300 200 150 40 20 10 5
Mg2+ 2+ SO2− 4 , Mn − NO3 Ca2+ Cl− Zn2+, K+ CO2− 3 Fe2+, Fe3+
a
The ratio of foreign ion (mg/l) to Cu(II) is indicated as x:1.
reacts with most of the reagent in the reagent volume. Optimum sensitivity and precision were obtained for 750 ml of sample.
3.1.2. Chemical parameters 3.1.2.1. Sodium diethyldithiocarbamate (Na– DDTC) concentration. Increasing the % Na– DDTC concentration (m/v) resulted in increased peak heights (Table 3) for the Cu(II)/Fe(II) working solution. Although it was expected that the response would stay constant for higher reagent concentrations due to all the sample being used up by the reagent, the results obtained proved the contrary. The relatively constant peak height obtained for the reaction between Cu(II) and DDTC (in the absence of Fe(II)) suggested that all the Cu(II) had reacted and that by increasing the DDTC concentration no significant increase in complex formation was achieved. An increase in peak height was, however, observed for DDTC and the Cu(II)/Fe(II) sample. This increase was attributed to Fe(II) which was added as interferent to the Cu(II) sample. In this instance Fe(II) was also available to react with the DDTC after an optimum of Cu(II) had reacted. In addition to the detected Cu(II) – DDTC complex, the Fe(II)– DDTC complex might also contribute to the peak height. It is also evident from Table 3 that the % interference changes from a large negative to a large positive value with increasing Na–DDTC concentrations. This is due to the possibility that
Fe(II) might have formed different complexes at various Na–DDTC concentrations. Increased Na–DDTC concentrations could affect the coordination of DDTC towards Fe(II) to form different complexes with differing absorption characteristics. Choosing an optimum was based on a compromise regarding sensitivity and % interference and gaining with respect to precision. The 0.1% Na– DDTC concentration was chosen as optimum. The 0.2% concentration gave better sensitivity with less interference, but the precision was not that satisfactory. The interference experienced for the 0.1% concentration can be reduced by optimization of the masking agent concentrations.
3.1.2.2. pH of the DDTC/EDTA/citrate solution. The most important feature of the dithiocarbamate ion is its protonation in acidic solutions and the subsequent decomposition into carbon disulphide and the protonated amine [1]. Preliminary experiments therefore showed that complex formation between Cu(II) and DDTC did not occur at a pH below 6, but that complex formation took place above a pH of 6. Due to possible interference from Fe(II) on the spectrophotometric determination of Cu(II), the pH range between 6 and 10 was studied to see whether pH might influence the overall sensitivity as well as interference from Fe(II). Table 4 shows that a change in pH had a minor effect on the sensitivity of the measurements. The pH did however influence the complexation reaction. Since interference was minimised between a pH of 7.3 and 8.3, it can be concluded that the masking agents performed best in the mentioned range. Precision obtained at pH 8.3 was better than at pH 7.3 and was thus chosen as the optimum working condition. This compares well to the optimum pH used in previous studies [13,16]. 3.1.2.3. Masking agents concentration. The selectivity of the complexation reaction between Cu(II) and DDTC is enhanced considerably by using EDTA in combination with citrate as masking agent [1,2,7,13,16]. Increasing the EDTA concen-
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tration did not affect the sensitivity and had a minor influence on precision (Table 5). As was expected the % interference decreased with higher EDTA concentrations. A 1.2% (m/v) EDTA concentration was chosen for masking interferences. Table 6 shows an increase in sensitivity with increasing citrate concentration, both in the presence and absence of Fe(II). This observation is due to the fact that the citrate might react with either Fe(II) or Cu(II) to form complexes that absorb at the wavelength of interest to contribute to the Cu(II) – DDTC absorption signal. This also explained the increase in % interference with increasing citrate concentration. Least interference was experienced at 1.5% (m/v) citrate and this concentration was therefore chosen as optimum.
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4.2. Interferences Wang and coworkers [16] showed that some ions interfered in the spectrophotometric determination of Cu(II) with DDTC. The interferences of these ions with the proposed SIA system were studied and the results are highlighted in Table 8. It is clear from Table 8 that the level of tolerance is up to at least a 5:1 (foreign ion to Cu(II) ion) ratio for Fe2 + , an interference that has been dealt with in detail. The interference from the carbonate ion is mainly due to a change in pH that was eliminated by selecting the optimum pH working conditions as described.
5. Conclusion 4. Method evaluation The proposed SIA system was critically evaluated with regard to accuracy, precision, detection limit, linear range, sample interaction, sampling frequency and interferences.
4.1. Linearity, accuracy, precision and detection limit The response of the proposed SIA system for the spectrophotometric determination of Cu(II) was evaluated under optimum conditions. The calibration curve was linear from 0.5 to 5.0 mg/l (response =0.4984[Cu(II)] + 0.1367; r = 0.998, n=5). The calculated detection limit was 0.2 mg/l Cu(II). The accuracy was evaluated by analysing two plant food samples and five tap water samples. The results, shown in Table 7, are in good agreement with the results obtained by flame-AAS. The precision determined for the analysed samples (n= 5) is also shown in Table 7. In all cases the RSD was B4.50%. A sampling frequency of seven samples per hour was obtained. The lengthy analysis time (519 s) was ascribed to the time necessary to perform the flow reversals and time needed to rinse the system to minimise possible carry-over between analysis. The sample interaction between samples was smaller than 1%, which is negligible.
The proposed system proved to be successful in analysing Cu(II) in plant food and water samples. Analysis was done without the necessity of introducing an extraction step thus complying with the need for conducting environmentally safe analysis. The lengthy analysis time that contributes to the low sample frequency was due to the flow reversals necessary to reduce interference and to flush the system. The flow reversals successfully reduced interference. The system is fully computerized and can be incorporated on-line if required. The calibration curve is linear from 0.5 to 5.0 mg/l with a detection limit of 0.2 mg/l. The system processes seven samples per hour with a relative standard deviation of B 4.50%.
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