molecules Article
Determination of Ultra-trace Rhodium in Water Samples by Graphite Furnace Atomic Absorption Spectrometry after Cloud Point Extraction Using 2-(5-Iodo-2-Pyridylazo)-5-Dimethylaminoaniline as a Chelating Agent Quan Han 1,2, *, Yanyan Huo 1 , Jiangyan Wu 2 , Yaping He 1 , Xiaohui Yang 1 and Longhu Yang 2 1 2
*
School of Chemical Engineering, Xi’an University, Xi’an 710065, China;
[email protected] (Y.H.);
[email protected] (Y.H.);
[email protected] (X.Y.) School of Chemistry and Chemical Engineering, Yan’an Universty, Yan’an 716000, China;
[email protected] (J.W.);
[email protected] (L.Y.) Correspondence:
[email protected]; Tel.: +86-029-8825-8506
Academic Editor: Derek J. McPhee Received: 12 February 2017; Accepted: 15 March 2017; Published: 24 March 2017
Abstract: A highly sensitive method based on cloud point extraction (CPE) separation/ preconcentration and graphite furnace atomic absorption spectrometry (GFAAS) detection has been developed for the determination of ultra-trace amounts of rhodium in water samples. A new reagent, 2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline (5-I-PADMA), was used as the chelating agent and the nonionic surfactant TritonX-114 was chosen as extractant. In a HAc-NaAc buffer solution at pH 5.5, Rh(III) reacts with 5-I-PADMA to form a stable chelate by heating in a boiling water bath for 10 min. Subsequently, the chelate is extracted into the surfactant phase and separated from bulk water. The factors affecting CPE were investigated. Under the optimized conditions, the calibration graph was linear in the range of 0.1–6.0 ng/mL, the detection limit was 0.023 ng/mL for rhodium and relative standard deviation was 3.67% (c = 1.0 ng/mL, n = 11). The method has been applied to the determination of trace rhodium in water samples with satisfactory results Keywords: 2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline; rhodium; TritonX-114; cloud point extraction; graphite furnace atomic absorption spectrometry
1. Introduction As a precious element of the platinum group elements (PGEs), rhodium is widely used in catalysts in the chemical and automotive industries due to its excellent catalytic activity. More than 80% of the world production of rhodium in the last decade was used in the production of automobile catalysts [1]. As a component in the typically used three-way catalysts, rhodium is indeed effective in reducing NOx emissions, but as a result more and more rhodium is being released into the environment during the operation of converters [2]. Hence, a reliable and efficient analytical technique for studying and evaluating its future risk to human health and the ecosystem is required. As rhodium often occurs in environmental samples in complex matrices at low concentrations, its analysis often requires a highly sensitive analytical method with a pre-concentration step. Several sensitive techniques such as graphite furnace atomic absorption spectrometry (GFAAS) [3,4], cathodic stripping voltammetry (CSV) [5–7], neutron activation analysis (NAA) [8,9], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [10,11], and inductively coupled plasma-mass spectrometry (ICP-MS) [12–14] are available for the determination of rhodium in
Molecules 2017, 22, 487; doi:10.3390/molecules22040487
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environmental samples. though it is highly sensitive, it suffersbecause from organic matter interference. NAA couldCSV, hardly be considered a routine approach it is both timeinterference. consuming NAA could hardly be considered a routine approach because it is both time consuming and laborious. and laborious. ICP-MS is a powerful technique for the determination of ultratrace amounts of ICP-MS is but a powerful technique for the determination ultratrace amounts of GFAAS rhodium,isbut is very rhodium, it is very expensive to operate and a of sophisticated method. an itefficient expensive operate and aofsophisticated GFAAS issensitivity an efficientwith method of determination of method of to determination rhodium for method. its appropinquity ICP-MS and lower cost. rhodium for its appropinquity sensitivity with ICP-MS and lower cost. Furthermore, the concentration Furthermore, the concentration of rhodium in environmental samples is very low, so separation and of rhodium in environmental samples is very so separation and pre-concentration steps are often pre-concentration steps are often needed priorlow, to analysis. Widely used techniques for separation and needed prior to analysis. Widely used techniques for separation and pre-concentration of trace amounts pre-concentration of trace amounts of rhodium include liquid-liquid extraction [15,16], solid–liquid of rhodium[12,17,18], include liquid-liquid extraction extraction [12,17,18], and techniques extraction and techniques based [15,16], on ion solid–liquid exchange [3,6,19]. Cloud point extraction (CPE), based exchange [3,6,19]. Cloud point extraction (CPE),has which an alternative to conventional which on is ion an alternative to conventional solvent extraction, beenis extensively used in analytical solvent extraction, has been extensively used analytical chemistry in the last decade [20,21]. CPE, as a chemistry in the last decade [20,21]. CPE, as ainnew separation and pre-concentration technique, offers new separation and pre-concentration technique,convenience, offers severallow advantages, such as higher simple extraction operation, several advantages, such as simple operation, cost, safety, and convenience, low cost, safety, and[22]. higher extraction pre-concentration factors [22]. CPE also in and pre-concentration factors CPE is also and in agreement with the principles ofis“Green agreement with the principles of “Green Chemistry”, because it does not use toxic organic solvents. Chemistry”, because it does not use toxic organic solvents. The aim of this study was to combine combine CPE, as as aa highly highly efficient efficient separation separation and pre-concentration pre-concentration approach, with withGFAAS, GFAAS,asas a highly sensitive detection technique, and develop a new method simple a highly sensitive detection technique, and develop a new simple method for the determination of trace amounts of rhodium water samples. In the developed for the determination of trace amounts of rhodium in water in samples. In the developed system, a system, a newly synthesized 2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline (5-I-PADMA), newly synthesized reagent, reagent, 2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline (5-I-PADMA), which which forms a hydrophobic complex compound with Rh(III) and is proved to be a highly sensitive forms a hydrophobic complex compound with Rh(III) and is proved to be a highly 613 = 1.86 × 5105 L·mol −1−1·cm−1 ) [23], was used as the chelating chromogenic reagent reagent for for rhodium rhodium(ε (ε613 = 1.86 × 10 L·mol−1·cm ) [23], was used as the chelating agent. agent. The structures of the reagent and its rhodium areinshown in1.Scheme 1. Triton The structures of the reagent and its rhodium complexcomplex are shown Scheme Triton X-114 wasX-114 used was as a nonionic surfactant. The main affecting factors affecting thewere CPE evaluated were evaluated and optimized. as a used nonionic surfactant. The main factors the CPE and optimized. The The method has been successfully applied to determination the determination of trace rhodium in water samples. method has been successfully applied to the of trace rhodium in water samples.
Scheme and its its rhodium rhodium complex. complex. Scheme 1. 1. The The structure structure of of 5-I-PADMA 5-I-PADMA and
2. Result and Discussion 2. Result and Discussion 2.1. Effect 2.1. Effect of of pH pH As shown shown in in Scheme Scheme 1, 1, the the chelating chelating agent agent 5-I-PADMA contains three three nitrogen nitrogen atoms atoms (the (the ring ring As 5-I-PADMA contains nitrogen atom and the two amino group nitrogen atoms) all of which can be protonated. It can nitrogen atom and the two amino group nitrogen atoms) all of which can be protonated. It can therefore + 2+ + +, H R therefore in the in four states: R,2+HR ,H 3R . The concentration distribution exist in theexist solution insolution four states: R, HR , and H23RR+,.and The H concentration distribution of the four 2 of the four species in solution is determined by its acidity. Therefore, pH ofaffects the solution affects species in solution is determined by its acidity. Therefore, the pH of the the solution the formation the formation of the chelate as well as its hydrophobicity, and is a critical parameter. According to of the chelate as well as its hydrophobicity, and is a critical parameter. According to the procedure, the the procedure, influence of evaluated pH on thebyCPE was evaluated changingbuffer the pH of HAc-NaAc influence of pH the on the CPE was changing the pH of by HAc-NaAc solution added in buffer solution added in the range of 3.5–8.0. Figure 1 shows the effect of pH on extraction of rhodium the range of 3.5–8.0. Figure 1 shows the effect of pH on extraction of rhodium chelate. It can be seen chelate. It can be seen that value maximum absorbance value was at pH range of5.5 5.0–6.0. Hence, that maximum absorbance was achieved at pH range ofachieved 5.0–6.0. Hence, a pH of was selected a pH of 5.5 was selected for subsequent experiments. for subsequent experiments.
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AA
0.3 0.3 0.2 0.2 0.1 0.1 0.0 3.0 0.0 3.0
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pH pH
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Figure 1. Effect of pH on absorbance. 8.0 × 10−6 mol/L 5-I-PADMA; 0.080% (m/v) Triton X-114; Figure 1. of 10 mol/L 5-I-PADMA; 0.080% 0.080% (m/v) (m/v) Triton Triton X-114; X-114; 1. Effect of pH pH on onabsorbance. absorbance. 8.0 10−−66 mol/L temperature: 60 °C; heating time: 15 min;8.0 2.5××ng/mL Rh. 5-I-PADMA; ◦ C; heating time: 15 min; 2.5 ng/mL Rh. temperature: 60 °C; heating time: 15 min; 2.5 ng/mL Rh.
2.2. Effect of the Amount of Chelating Agent 2.2. Effect of the Amount of Chelating Agent 5-I-PADMA is employed to form a hydrophobic complex with rhodium. According to the 5-I-PADMA form hydrophobic complexon with rhodium. According to the −3 mol/L 5-I-PADMA is employed to form hydrophobic complex with rhodium. According procedure, the effect of the amount of 1 ×aa10 absorbance was investigated by −33mol/L 5-I-PADMA on absorbance was investigated by − procedure, the effect of the amount of 1 × 10 procedure, the volumes effect of the of 1 × added mol/L 5-I-PADMA absorbance wasininvestigated changing the of amount the 5-I-PADMA from 20 to 250onµL. As shown Figure 2, the changing the volumesof of 5-I-PADMA added 20 µL. to 250 µL. Asinshown inthe Figure 2, thea the volumes thethe 5-I-PADMA from from 20 to 250 As5-I-PADMA shown Figure 2, absorbance absorbance increased significantly with added the increasing amounts of added and reached absorbance increased significantly with the increasing amounts of 5-I-PADMA added and reached a increased significantly with the increasing amounts of 5-I-PADMA added and reached a constant value constant value within the range of 60–120 µL of 5-I-PADMA. However, the absorbance decreased constant value within the range of 60–120 µL of 5-I-PADMA. However, the absorbance decreased within the range µL of 5-I-PADMA. the 120 absorbance gradually when the gradually when of the60–120 amount 5-I-PADMA However, was beyond µL. As decreased 5-I-PADMA also had strong gradually the amount of 5-I-PADMA beyond 120 As 5-I-PADMA also had strong amount of when 5-I-PADMA was more beyond 120 µL. Aswas 5-I-PADMA also µL. had strong hydrophobicity, there was hydrophobicity, there was 5-I-PADMA and less complex in the surfactant-rich phase with the hydrophobicity, there wascomplex more and less 80 complex in surfactant-rich phase with more 5-I-PADMA and less in the surfactant-rich phase the further increasing of amounts further increasing of amounts of 5-I-PADMA 5-I-PADMA. Hence, µL 1 ×with 10−3the mol/L 5-I-PADMA was used the for −3 − 3 further increasing of amounts Hence, 80 µL 1 ×used 10 mol/L was used for of 5-I-PADMA. Hence, 80 µL 1of × 5-I-PADMA. 10 mol/L 5-I-PADMA was for the5-I-PADMA subsequent experiments. the subsequent experiments. the subsequent experiments.
AA
0.3 0.3 0.2 0.2 0.1 0.1 0 0
100 100 V
(L) V (L)
200 200
Figure 2. Effect Effect of of the theamount amountofof5-I-PADAM 5-I-PADAM cloud point extraction of Rh. 0.08% (m/v) Triton XFigure 2. onon cloud point extraction of Rh. 0.08% (m/v) Triton X-114; Figure 2. Effect of the°C; amount oftime: 5-I-PADAM on cloud point extraction of Rh. 0.08% (m/v) Triton X◦ 114; temperature: 60 heating 15 min; pH = 5.5; 2.5 ng/mL Rh. temperature: 60 C; heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh. 114; temperature: 60 °C; heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh.
2.3. X-114 Concentration Concentration 2.3. Effect Effect of of Triton Triton X-114 2.3. Effect of Triton X-114 Concentration The of of itsits advantages such as The nonionic nonionic surfactant surfactantTriton TritonX-114 X-114was wasused usedasasextractant extractantbecause because advantages such The nonionic surfactant Triton X-114 was used as extractant because of its advantages such as commercial availability with high purity, point as commercial availability with high purity,low lowtoxicity toxicityand andcost, cost,and and relatively relatively low low cloud cloud point commercial availability withconcentrition high purity,oflow toxicity and cost, and relatively lowefficiency cloud point temperature (23–26 °C). The Triton X-114 not only affected extraction ◦ temperature (23–26 C). The concentrition of Triton X-114 not only affected extraction efficiency but but temperature (23–26of°C). The concentrition of Triton X-114 not only affected extraction efficiency but also the sensitivity the cloud point extraction. The effect of the amount of 1% (m/v) Triton X-114 on also the sensitivity of the cloud point extraction. The effect of the amount of 1% (m/v) Triton X-114 also the sensitivity of the cloud point extraction. The effect of the amount of 1% (m/v) Triton X-114 on the absorbance was investigated. AsAs shown in in Figure chelate on the absorbance was investigated. shown Figure3,3,the theabsorbance absorbanceof of the the rhodium rhodium chelate the absorbance was investigated. As shown in Figure 3,increases the absorbance of the rhodium chelate significantly increases when the volume of Triton X-114 from 0.1 to 0.6 mL. The optimal significantly increases when the volume of Triton X-114 increases from 0.1 to 0.6 mL. The optimal significantly increases the volumetoof TritonHowever, X-114 increases from 0.1 to 0.6 mL. Theofoptimal amount X-114when ranges from amount of of Triton Triton X-114 ranges from 0.6 0.6 to 1.0 1.0 mL. mL. However, further further increase increase in in the the volume volume of Triton Triton amount of Triton X-114 ranges from 0.6 to 1.0 mL. However, further increase in the volume of Triton X-114 leads to gradual decease in the absorbance signal because of the increment in the X-114 leads to gradual decease in the absorbance signal because of the increment in the volume volume of of the the X-114 leadsphase. to gradual decease inmL theof absorbance signal X-114 because ofemployed. the increment in the volume of the remaining Therefore, 0.8 1% (m/v) Triton was remaining phase. Therefore, 0.8 mL of 1% (m/v) Triton X-114 was employed. remaining phase. Therefore, 0.8 mL of 1% (m/v) Triton X-114 was employed.
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A
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0.0 0.0
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1.2
1.6
V (mL) 0.8
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Figure 3. Effect of the amount of Trition X-114 on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-I0.0 Figure 3. Effect of the amount of Trition cloud1.2 point1.6 extraction of Rh. 8.0 × 10−6 mol/L 0.0 X-114 0.4 on V0.8 (mL) PADMA; temperature: 60 °C; heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh. ◦
5-I-PADMA; temperature: 60 C; heating time: 15 V min; pH = 5.5; 2.5 ng/mL Rh. (mL) Figure 3. Effect of the amount of Trition X-114 on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-I2.4. Effect of 3. Equilibration and Time Figure Effect of theTemperature amount of Trition X-114 on pH cloud point PADMA; temperature: 60 °C; heating time: 15 min; = 5.5; 2.5 extraction ng/mL Rh.of Rh. 8.0 × 10−6 mol/L 5-I-
2.4. Effect PADMA; of Equilibration Temperature and time: Time min; pH = 5.5; 2.5 ng/mL Rh. temperature: 60 °C; and heating Equilibration temperature time are15 two important and highly effective parameters in CPE.
2.4. Effect of Equilibration Temperature and Time It has been suggested that CPE processes based the temperature-driven phase separation typical Equilibration temperature and time are twoon important and highly effective parameters in CPE. 2.4. Effect of Equilibration Temperature andbe Time of nonionic micellar solutions should conducted at temperatures well above the cloud point Equilibration and time are twoon important and highly effective parameters in CPE. It has been suggestedtemperature that CPE processes based the temperature-driven phase separation typical of thesolutions system [24].should Thus, the effect of equilibration wasabove investigated within temperature and time two andtemperature highly effective parameters in CPE.point Ittemperature hasEquilibration been suggested that CPE processes based onimportant the temperature-driven phase separation typical of nonionic micellar beare conducted at temperatures well the cloud the range 40–80 °C solutions according to the procedure. The results are shown well in Figure 4.separation Itthe was found that It has been suggested that CPE processes based the phase typical of nonionic micellar should conducted attemperature-driven temperatures above cloud point temperature ofofthe system [24]. Thus, thebeeffect ofon equilibration temperature was investigated within absorbance increased with increase inbe equilibration from toinvestigated 50the °C,cloud andwithin reach of nonionic of micellar solutions should conducted attemperature temperatures well40 above point temperature the system [24]. Thus, the effect of equilibration temperature was ◦ the range of 40–80 C according to the procedure. The results are shown in Figure 4. It was found maximum the range of 50–80 °C. equilibrium time wasin also studied time interval temperature of the system [24]. to Thus, theeffect effectof ofThe equilibration temperature was investigated within the range ofin 40–80 °C according theThe procedure. results are shown Figure 4. It in was found that that absorbance increased with increase in equilibration temperature from 40over to 5010◦ C, and of (Figure 5). It has been observed that when equilibration time is min, thereach the5–30 rangemin of increased 40–80 °C according to the procedure. The results are shown in Figure 4. It was found that absorbance with increase in equilibration temperature from 40 to 50 °C, and reach ◦ C. The effect of equilibrium time was also studied in time interval of maximum in the range of 50–80 quantitative extraction is50–80 achieved. Therefore, 60 °C temperature and 15 min were selected equilibrium absorbancein increased with increase ineffect equilibration from to 50inas °C, and reach maximum the range of °C. The of equilibrium time was also 40 studied time interval 5–30 of min (Figure 5). has been observed that when equilibration over 10over min, themin, quantitative temperature time, maximum the It range ofIt50–80 °C. The effect ofthat equilibrium timetime was is also studied in time interval 5–30 mininand (Figure 5).respectively. has been observed when equilibration time is 10 the extraction is min achieved. 60 ◦Therefore, Cobserved and 15 min were as equilibrium of 5–30 (FigureTherefore, 5).is Itachieved. has been that when equilibration time is over 10 min, the and quantitative extraction 60 °C and selected 15 min were selected as temperature equilibrium 0.35 time,temperature respectively. quantitative extraction is achieved. Therefore, 60 °C and 15 min were selected as equilibrium and time, respectively. temperature and time, respectively. 0.35
A
0.30 0.35
0.30
AA
0.25 0.30
0.25 0.20 0.25
40
50
60
o
70
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T ( C) 0.20 40 50 60 70 80 Figure 4. Effect of equilibration temperature on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-Io 0.20 40 50 60T ( C) 70 80 PADMA; 0.08% (m/v) Triton X-114;heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh. o
T ( C)
Figure 4. Effect of equilibration temperature on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-I-
Figure 4. Effect of equilibration0.35 temperature on cloud point extraction of Rh. 8.0−6× 10−6 mol/L Figure 4. 0.08% Effect(m/v) of equilibration temperature on15cloud point extraction of Rh. 8.0 × 10 mol/L 5-IPADMA; Triton X-114;heating time: min; pH = 5.5; 2.5 ng/mL 5-I-PADMA; TritonX-114;heating X-114;heating time: 15 min; 5.5; 2.5 ng/mL Rh. PADMA;0.08% 0.08% (m/v) (m/v) Triton time: 15 min; pH =pH 5.5;=2.5 ng/mL Rh.
0.30 0.25 0.30
A A
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0.35 0.30 0.35
0.25 0.20 0.25
10
t (min)
20
30
0.20 10 point extraction 20 30 8.0 × 10−6 mol/L 5-I-PADMA; Figure 5. Effect of equilibration time on cloud of Rh, 0.20 t (min) 10 20 0.08% (m/v) Triton X-114; temperature; 60 °C; pH = 5.5; 2.5 ng/mL Rh.30
t (min)
Figure 5. Effect of equilibration time on cloud point extraction of Rh, 8.0 × 10−6 mol/L 5-I-PADMA; Figure(m/v) 5. Effect ofX-114; equilibration time on Rh, 8.0 × 10−6 mol/L 5-I-PADMA; 0.08% Triton temperature; 60 cloud °C; pHpoint = 5.5;extraction 2.5 ng/mLof Rh. Figure 5. Effect of equilibration time on cloud point extraction of Rh, 8.0 × 10−6 mol/L 5-I-PADMA; 0.08% (m/v) Triton X-114; temperature; 60 °C; pH = 5.5; 2.5 ng/mL Rh.
0.08% (m/v) Triton X-114; temperature; 60 ◦ C; pH = 5.5; 2.5 ng/mL Rh.
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Under the optimized conditions identified in Section 2.1 to Section 2.4, the influence of various 2.5. Interference Studies interfering species on the determination of 1.0 ng/mL of rhodium was studied. A foreign species was considered to cause interference whenidentified it caused in a variation lesstothan 5%influence in the absorbance Under the no optimized conditions Sections 2.1 2.4,±the of variousof the sample. The tolerance limits of various species were shown in Table 1. The results indicated that the interfering species on the determination of 1.0 ng/mL of rhodium was studied. A foreign species was common coexisting species did not have a significant effect onless thethan determination of rhodium, and the considered to cause no interference when it caused a variation ±5% in the absorbance of the sample. The tolerance limits of various species were shown in Table 1. The results indicated that the developed method has very good selectivity. common coexisting species did not have a significant effect on the determination of rhodium, and the developed method hasofvery goodspecies selectivity. Table 1. Effect foreign on the pre-concentration/determination of rhodium. Table 1. Effect of foreignForeign speciesSpecies on the pre-concentration/determination of rhodium. Foreign Species to Rh to Rh Species Species Ratio (w/w) Ratio (w/w) Foreign Species to Rh Foreign Species to Rh Species Species + + + 2+ 2+ 2+ Li , Na , K , Mg , Ca , Sr , Ratio (w/w) Ratio (w/w) 2500 500 Cu2+ , Ni2+ , W(VI) 2+ , F− , Cl− , Br− , NO − Zn +, K+, Mg2+, Ca2+,3Sr2+, Li+, Na 2++ 2+, W(VI) 2500 Cu , Ni 500 Ag , Pt(IV), Ir(IV), 2+2+ −, Br2−,−NO3− Zn , F,−I,−Cl 200 2000 Ba , SO 4 Ru(III), Fe3+ Ir(IV), Ag+, Pt(IV), 2+ 2+ 3 − 2+ − 2− 1500 4 2000 200100 Cd Ba , Cr, I ,, SO As(V) Co 3+ Ru(III), Fe 2+ 1000 80 Pb2+3− Pd 2+ 2+ 1500 Co 100 Cd , Cr , As(V) 800 Mn2+ , Al3+ , Mo(IV) 1000 Pd2+ 80 Pb2+ 2+ 3+ 800 Mn , Al , Mo(IV)
2.6. Calibration Curve, Detection Limit and Precision 2.6. Calibration Curve, Detection Limit and Precision
Under the optimized conditions, a calibration curve was obtained by pre-concentrating standard the optimized conditions, a calibration waslinear obtained by pre-concentrating standard solutions Under according to the given procedure (Figurecurve 6). The equation was A = 0.00998 + 0.1304c solutions according to the given procedure (Figure 6). The linear equation was A = 0.00998 + 0.1304c (c: ng/mL), with a correlation coefficient R = 0.9989. The calibration line was linear within the rhodium (c: ng/mL), with a correlation coefficient R = 0.9989. The calibration line was linear within the rhodium concentration range of 0.1–6.0 ng/mL. The limit of detection for rhodium, calculated as three times the concentration range of 0.1–6.0 ng/mL. The limit of detection for rhodium, calculated as three times standard deviation of the blank signals (3s), was determined to be 0.023 ng/mL. The relative standard the standard deviation of the blank signals (3s), was determined to be 0.023 ng/mL. The relative deviation (RSD) was 3.7% = 1 ng/mL, = 11). nThe enrichment factor, defined as the ratio of standard deviation (RSD)(for wasc 3.7% (for c = 1nng/mL, = 11). The enrichment factor, defined as the the aqueous solution volume (10 mL) to that of the surfactant rich phase volume after dilution ratio of the aqueous solution volume (10 mL) to that of the surfactant rich phase volume after dilution with HNOwith solutionsolution (0.05 mL), HNO3-methanol (0.05was mL),200. was 200. 3 -methanol
0.8
A
0.6 0.4 0.2 0.0
0
2
4
6
c (ng/mL) Figure 6. Calibration graph for Rh.
Figure 6. Calibration graph for Rh.
A comparison of the present method with previously reported methods involving rhodium determination thethe literature terms of with detection limits and instruments employed is given in A comparisoninof presentinmethod previously reported methods involving rhodium Table 2. Theindetection limit ofin theterms described method islimits superior to those given in employed the table, with the determination the literature of detection and instruments is given in exception of CPE-ICP/MS. However, ICP/MS, as a multi-elemental analytical technique, is not Table 2. The detection limit of the described method is superior to those given in the table, with available in many laboratories due to the high costs, while GFAAS has the advantages of simplicity, the exception of CPE-ICP/MS. However, ICP/MS, as a multi-elemental analytical technique, is not high sensitivity and low cost. available in many laboratories due to the high costs, while GFAAS has the advantages of simplicity, high sensitivity and low cost.
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Table 2. Comparison of the proposed method with reported methods using CPE prior to rhodium determination. Reagent 5-(40 -Nitro-20 ,60 -dichlorophenylazo)-6hydroxypyrimidine-2,4-dione 2-propylpiperidine-1-carbodithioate O,O-Diethydithiophosphate 2-Mercaptobenzothiazole 1-(2-Pyridylazo)-2-naphthol 2-(5-Iodo-2-pyridylazo)-5-dimethylaminoaniline
Detection System
Surfactant
Limit of Detection (ng/mL)
SP
Triton X-114
0.15
[25]
FAAS FAAS ICP/MS ICP/MS TLS GFAAS
Span 80 Triton X-114 Triton X-114 Triton X-100 Triton X-114 Triton X-114
1.2 0.052 0.003 0.001 0.06 0.023
[26] [27] [28] [29] [30] This work
Reference
3. Application of Real Water Samples In order to validate the proposed methodology, the optimized procedure was applied to the determination of rhodium in tap water, well water, spring water and river water samples. Its accuracy was checked by spiking known different concentrations of cobalt into the samples and calculating the recoveries defined as the ratio of measured amount of rhodium to the adding amount of rhodium. The results are shown in Table 3. As shown, the recoveries for the spiked samples were in the range of 96.6%~104.0%, and relative standard deviation was below 5%. Table 3. Determination results of rhodium in the water samples. Sample
Added (ng/mL) -
Tap water
1.0 3.0 -
Reservoir water
2.0 4.0 -
Well water
1.0 3.0 -
River water
2.0 4.0
Found (ng/mL) ND 0.950, 0.960, 0.930 1.02, 0.970, 1.01 3.22, 3.10, 2.94 2.96, 3.16, 3.22 ND 1.94, 2.05, 2.08 1.98, 1.94, 2.08 3.92, 4.06, 3.90 3.92, 3.86, 4.07 ND 0.970, 0.980, 1.00 0.930, 0.960, 1.04 3.02, 2.86, 2.90 2.82, 2.88, 2.92 ND 1.96, 1.95, 2.08 1.92, 2.06, 1.92 3.88, 3.80, 3.88 4.06, 3.92, 3.98
Average (ng/mL)
Recovery (%)
RSD (%)
-
-
-
0.973
97.3
3.6
3.10
103.3
3.9
-
-
-
2.01
101
3.3
3.96
98.9
2.2
-
-
-
0.980
98.0
2.9
2.90
96.7
2.4
-
-
-
1.98
99.0
3.6
3.92
98.0
2.3
ND: Not detected. Tap water: c(Mn2+ ) = 10 ng mL−1 , c(Fe3+ ) = 30 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Cu2+ ) < 50 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . Well water: c(Mn2+ ) = 10 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Cu2+ ) < 50 ng mL−1 , c(Fe3+ ) < 30 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . Reservoir water: c(Fe3+ ) = 30 ng mL−1 , c(Cu2+ ) = 50 ng mL−1 , c(Mn2+ ) = 230 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . River water: c(Fe3+ ) = 30 ng mL−1 , c(Cu2+ ) = 50 ng mL−1 , c(Mn2+ ) = 230 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . All the water samples and their analytical results were provided by Xi’an Hydrographic Bureau, Xi’an, Shaanxi Province, China.
4. Experimental Section 4.1. Apparatus Atomic absorption measurements were performed with a model TAS-990 atomic absorption spectrophotometer (Beijing Purkinje General Instrument Co. Ltd, Beijing, China) equipped with a GFH-990 graphite furnace atomizer system (Beijing Purkinje General Instrument Co. Ltd, Beijing,
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China). A rhodium hollow cathode lamp (Heraeus Noblelight Ltd, Shenyang, Shandong, China) was used as the radiation source. Argon was used as the purge and protective gas. The work condition of graphite furnace atomic absorption spectrophotometer was listed in Table 4. The pH values were measured by a model PB-10 pH meter (Sartorius Scientific Instrument Co. Ltd, Beijing, China) furnished with a combined electrode. A model HH-2 thermostatic bath (Kewei Yongxing Instrument Co. Ltd, Beijing, China), maintained at the desired temperature, was used for cloud point temperature experiments. A model 800-1 centrifuge (Pudong Physical Instruments Factory, Shanghai, China) was utilized to accelerate the phase separation process. Table 4. Operating conditions for GFAAS. Lamp current Wavelength Slit Filter coefficient Pressure (Ar) Injected volume Drying temp. Ashing temp. Atomization temp. Cleaning temp.
6.0 mA 343.5 nm 0.2 nm 0.10 0.60 Mpa 10.0 µL 120 ◦ C (Ramp 5 s, hold 10 s) 1100 ◦ C (Ramp 10 s, hold 15 s) 2300 ◦ C (Ramp 0 s, hold 3 s) 2500 ◦ C (Ramp 1 s, hold 3 s)
4.2. Reagents and Solutions A standard stock solution of rhodium (1000 µg/mL) was prepared by dissolving spectroscopically pure (NH4 )2 RhCl5 ·H2 O in 1 mol/L HCl. Working solutions were prepared by appropriate dilution of the stock solution with water. A 1.0 × 10−3 mol/L 5-I-PADMA (Laboratory-synthesized [23]) solution was prepared by dissolving appropriate amounts of this reagent in ethanol. The solution of nonionic surfactant (1% m/v) Triton X-114 (Sigma-Aldrich, Milwaukee, Wisc., USA) solution was prepared by dissolving 1.0 g of surfactant in 100 mL of water. A buffer solution of pH 5.5 was prepared by 0.2 mol/L HAc and 0.2 mol/L NaAc, corrected by using a pH meter. The 0.1 mol/L HNO3 -methanol solution was prepared by mixing of 0.2 mol/L HNO3 and methanol in equal volumes. All the chemicals used were of analytical reagent grade unless otherwise mentioned. All solutions were prepared with ultrapure water (18.2 MΩ cm) obtained from a Simplicity 185 (Millipore Company, Billerica, MA, USA) water purification system. 4.3. Procedure An aliquot of the sample or working standard solution containing rhodium in the range of 0.1–6.0 ng/mL, 2 mL of pH 5.5 HAc-NaAc buffer solution and 80 µL of 1.0 × 10−3 mol/L 5-I-PADMA chelating solution were placed in a 10 mL graduated conical centrifuge tube. For the formation of rhodium complex, the mixture was heated in boiling water bath for 10 min. Afterwords for cloud point extraction, 0.8 mL of 1% (m/v) Triton X-114 solution was added and the mixture was diluted to 10 mL with ultrawater. The resultant solution was shaken and equilibrated at 60 ◦ C for 15 min in a thermostated bath. Separation of the two phases was then accomplished by centrifugation for 5 min at 3500 rpm. The bulk aqueous phase was easily discarded by inverting the tube. The surfactant phase (the remaining phase) in the tube was heated in water bath at 100 ◦ C to remove the remaining water, and then dissolved with 50 µL 0.1 mol/L HNO3 -methanol solution. Then, 10 µL samples were injected into the graphite tube for GFAAS determination of rhodium. 5. Conclusions By combination of CPE as a technique for the separation and pre-concentration, with GFAAS as a detection method, we proposed a novel and sensitive method for the determination of rhodium. GFAAS is a highly sensitive analytical technique. CPE is a powerful technique for pre-concentration
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and separation of metal ions and it has many advantages including low cost, convenience, simplicity, safety, low toxicity and high extraction efficiency. Triton X-114 was selected for the formation of the surfactant-rich phase because of its convenient cloud point temperature and high density of the surfactant-rich phase which facilitates phase separation by centrifugation. 5-I-PADMA was chosen as the chelating reagent because it is a new and good chromogenic reagent for rhodium. The described method has been successfully applied to the selective determination of rhodium at trace levels. Acknowledgments: The present work was supported by National Natural Science Foundation of China (No. 21545014, 21445004), and the Xi’an Science and Technology Plan Project (No. 2016CXML09). Author Contributions: Quan Han conceived and designed the experiments; Yanyan Huo, Jiangyan Wu and Longhu Yang performed the experiments; Yaping He and Xiaohui Yang analyzed the data; Jiangyan Wu, Yaping He and Longhu Yang contributed reagents/materials/analysis tools; Quan Han and Xiaohui Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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Sample Availability: Samples of 5-I-PADMA, are not available from the authors, but can be synthesized according to literature [23]. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
