Study on color reaction of 2-(2-quinolylazo)-5-dimethylaminoanline with palladium and its application

Hu Qun ¹, Hu Lin ²，Wang Jian ¹，Wei Yuling ¹ , Chen Yongkuan ^1
(1 Yunnan Academy of Tobacco Science, Kunming 650106; ²Life science school,Yunnan University, Kunming 650091,  China)

Abstrasct A new chromogenic reagent, 2-(2-quinolylazo)-5-dimethylaminoaniline (QADMAA) was synthesized. A sensitive, selective and rapid method for the determination of palladium based on the rapid reaction of palladium(II) with QADMAA was developed. In the presence of 0.5-2.5 mol/L of hydrochloric acid solution and cetyl trimethylammonium bromide (CTMAB) medium, QADMAA reacts with palladium to form a violet complex of a molar ratio 1:2 (palladium to QADMAA). The molar absorptivity of the chelate is 1.35×10⁵ L.mol^-1.cm^-1 at 600 nm. Beer’s law is obeyed in the range of 0.01~ 0.6 m g/ml. The relative standard deviation for eleven replicate sample of 0.2 m g/ml level is 1.02 %. This method had been applied to the determination of palladium with good results.
Keywords 2-(2-quinolylazo)-5-dimethylaminoaniline; palladium; spectrophotometry

1. INTRODUCTION
Palladium is an important element. It is important for industry and biological systems as well ^[1-2]. Many sensitive instruments, such as spectrofluorimetry, X-ray fluorescence spectrometry, neutron activation analysis, atomic absorption spectrometry, chemiluminescence, and the like have widely been applied to the determination of palladium. But the spectrophotometric method still has the advantages of being simple and not requiring expensive or complicated test equipment. For this reason, a wide variety of spectrophotometric methods for the determination of palladium have been developed ^[3-6].
    In our previous work, some 2-quinolylazo-phenol reagents were reported for the determination of metal ions ^[7-9]. This kind reagent has a higher sensitivity than pyridylazo reagents because of its larger conjugated system. However, the 2-quinolylazo-phenol reagent has also a disadvantage of its poor selectivity because both the oxygen atoms and nitrogen atoms donate to the metal ions. To select a more sensitive and selective reagent, we synthesized 2-(2-quinolinylazo)-5-dimethylaminoaniline (QADMAA) and thoroughly studied the color reaction of QADMAA with palladium. This reagent has a higher selectivity than 2-quinolylazo-phenol reagents because of it only donating nitrogen atoms to metal ions. Based on the color reaction of QADMAA with palladium, a highly sensitive, selective and rapid method for the determination of palladium was developed.

2. EXPERIMENTAL
2.1 Apparatus
A UV-160 A spectrophotometer (Shimidzu Corporation, Tokyo, Japanese) equipped with 1 cm cells was used for all absorbance measurements. The pH values were determined with a Beckman Φ-200 pH meter (Beckman Instruments, Fullerton, CA, USA).
2.2 Synthesis of QADMAA                              
2-Aminoquinoline (6.9 g) was dissolved in 500 mL anhydrous ethanol. To which, sodamide (2.0 g) was added and the mixture was refluxed in boiling water bath for 5 h, followed by the addition of isoamyl nitrite (7.4 mL). The solution was refluxed for 30 min with boiling water bath. The solution was cooled and placed over night under 0℃. The diazo salt was obtained by filtering this solution with an isolation yield of 95%. The diazo salt was dissolved in 200 mL anhydrous ethanol, followed by the addition of m-dimethylaminoaniline (5.7g; 0.042 mol). The carbon dioxide was ventilated into the solution with stirring until the pH reaches to about 8.0. The solution stood for two days, and evaporated to dryness. The residue was re-crystallized with 30% ethanol. QADMAA was obtained with 36% yield. The structure of QADMAA was verified by elemental analysis, IR, ¹HNMR, and MS.
2.3 Reagents                            
All of the solutions were prepared with ultra-pure water obtained from a Milli-Q50 SP Reagent Water System (Millipore Corporation, USA). A 5´ 10^-4 mol/L of QADMAA solution was prepared by dissolving QADMAA with 95% of ethanol. A stock standard solution of palladium (1.0 mg/mL) was obtained from Chinese Standard Center, and a work solution of 2.0 m g/mL was prepared by diluting this solution. A 5 mol/L of hydrochloric acid was used. Cetyl trimethylammonium bromide (CTMAB) solution (1.0 % (w/v)) was prepared by dissolving CTMAB with 20% ethanol. All chemical used were of analytical grade unless otherwise stated.
2.4 General procedure                                 
To a standard or sample solution containing no more than 15 m g of Pd(II) in a 25 mL of calibrated flask, 5 mL of 5 mol/L hydrochloric acid, 4.0 mL of 5×10^-4 mol/L QADMAA solution and 2.0 mL of 1.0 % CTMAB solution were added. The mixture was diluted to volume of 25 mL and mixed well. After 10 min, the absorbance was measured in a 1 cm cell at 600 nm against a reagent blank prepared in a similar way without palladium.

3. RESULTS AND DISCUSSION
3.1 Absorption Spectra
The absorption spectra of QADMAA and its Pd(II) complex are shown in Fig.1. The absorption peaks of QADMAA and its complex are located at 440 nm and 600 nm.

Fig.1 Absorption spectra of QADMAA and its Pd(II) complex
1 QADMAA-CTMAB blank against water; 2 QADMAA-Pd(II)-CTMAB complex against reagent blank

3.2 Effect of Acidity                      
Results showed that the optimal condition for the reaction of Pd(II) with QADMAA is in the acid medium. Therefore, the effect of hydrochloric acid, sulfuric acid, perchioric acid, phosphoric acid and the like, on the color reaction of Pd(II) with QADMAA was studied. Experiment shows that hydrochloric acid has the best effect, and the concentration of hydrochloric acid within 0.5-2.5 mol/L was found to give a maximum and constant absorbance. So 5 ml of 5 mol/L of hydrochloric acid was recommended.
3.3 Effect of surfactants                     
The effect of surfactants on Pd(II)-QADMAA chromogenic system was studied. In the presence of cationic surfactants or nonionic surfactants medium, the absorption of the chromogenic system increases markedly. Experiments show that CTMAB is the best additive. The use 1~ 4 mL of 1.0 % CTMAB solution give a constant and maximum absorbance. Accordingly, the use of 2 mL was recommended.
3.4 Effect of QADMAA concentration                             
For up to 15 m g of Pd(II), the use of about 3.5~ 5 mL of 5×10^-4 mol/L of QADMAA solution has been found to be sufficient for a complete reaction. Accordingly, 4 ml of QADMAA solution was added in all further measurement.
3.5 Stability of the chromogenic system
After mixing the components, the absorbance reaches its maximum within 8 min at room temperature and remains stable for 6 h in aqueous solution. The chelates are stable at least 20 h.
3.6 Calibration Curve and Sensitivity
The calibration curve show that Beer's law is obeyed in the concentration range of 0.01~ 0.6 m g Pd(II) per ml, The linear regression equation obtained was: A = 1.294 C (m g/mL) + 0.0158, (r=0.9996). The molar absorptivity was calculated to be 1.35×10⁵ l.mol^-1.cm^-1 at 600 nm. The relative standard deviation at a concentration level of 0.2 m g Pd(II) per mL (11 repeat determination) was 1.02 %.
3.7 The method precision and recovery
The intra-day precision and inter-day precision were calculated from data obtained during a 7-day validation. The precision of the assay was determined by repeatability (intraday) and intermediate precision (inter-day). To assess intraday variation (repeatability), calibration curve was prepared seven times on the same day. Intermediate precision was assessed by comparing the assays on different days (7 days, n = 7 at each concentration). The results showed that the relative standard derivation of overall intra-day variations were less than 2.5%, and the relative standard derivation of inter-day variations were less than 2.8%. This method is high precision.
    The recovery test of the proposed method was prepared by adding a known amount of standard at three different levels (1.0 m g/mL, 2.0 m g/mL, 5.0 m g/mL) to the pre-analysed sample. The results shown that the recoveries (n=7) were ranged from 94% - 105%. This method is high recovery.
3.8 Composition of the complex
The composition of the complex was determined by molar ratio method (Figure 2) and continuous variation (Figure 3). Both showed that the molar ratio of Pd(II) to QADMAA is 1:2.

Fig.2 Composition of QADMAA-Pd(II) complex by molar ratio method

Fig. 3 Composition of QADMAA-Pd(II) complex by continuous variation method

3.9 Interference
The selectivity of the proposed method was investigated by the determination 5 m g/25ml of Pd(II) in the presence of various ions within a relative error of ± 5% are given in Table-1. The results showed that most foreign ions do not interfere with the determination. This method is high selectivity.

Table 1 Tolerance limits for the determination of 5 m g of Pd(II) with QADEAA (relative error ± 5% )

4. APPLICATION
The proposed method has been successfully applied to the determination of palladium in water and katalyst.
    For water sample, taking an appropriate volume (planting effluents 20 ml, river water 500 ml) of water sample in a 500 mL flask. The samples were concentrated to about 10 mL by heating on a hot plate. 5 mL concentrated nitric acid and 2 mL of 30% hydrogen peroxide was added in this solution. The mixture was heated on a hot plate and evaporated to near dryness. The residue was dissolved with 5 mL of 5% hydrochloric acid and transferred into a calibrated flask, and 2 mL of 10% citric acid was added to mask the nickel and cobalt. The solution was neutralized with sodium hydroxide, and analyzed by general procedure. The recovery of palladium was determined by adding 1.0 m g of palladium to water samples. A standard method using atomic absorption spectrometry was also used as reference method. The results are shown in Table 2.
    For katalyst, 0.1 g of samples was weighted accurately into the Teflon high-pressure microwave acid-digestion bomb (Fei Yue, Analytical Instrument Factory, Shanghai, China). 3.0 ml of concentrated nitric acid, 2 mL of hydrochloric acid and 5.0 ml of 30% hydrogen peroxide were added. The bombs were sealed tightly and then positioned in the carousel of the microwave oven (Model WL 5001, 1000 W, Fei Yue Analytical Instrument Factory, Shanghai, China). The system was operated at full power for 10 min. The digest was evaporated to near dryness. The residue was dissolved with 10 ml of 10% of hydrochloric acid, then transferred into a 50 ml of calibrated flask and diluted to volume with 10% of hydrochloric acid. The palladium content was analyzed by general procedure. The recovery of palladium by added 1.0 m g of palladium in sample was carried out, and a standard method using atomic absorption spectrometry was also used as reference method. The results are shown in Table 2.

Table 2 Determination of palladium in the water sample

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2-(2-喹啉偶氮)-5-二甲基胺苯胺与钯的显色反应及其应用研究
胡群¹，胡琳²，王建¹，魏玉玲¹，陈永宽¹
（¹云南烟草科学研究院 昆明650106；²云南大学生命科学学院,昆明650091）
摘要 研究了新试剂，2-(2-喹啉偶氮)-5-二甲基胺苯胺(QADMAA)与钯的显色反应。基于钯（II）与QADMAA的快速反应，确定了一种具有选择性和快速测定钯的检测方法。在0.5-2.5 mol/L 的盐酸溶液和溴化十六烷基三甲胺(CTMAB)存在下，QADMAA与钯反应生成摩尔比(钯/QADMAA)为1:2的紫色的络合物。在吸收波长600 nm下，摩尔吸光系数 e= 1.35×10⁵L.mol^-1.cm^-1， 比尔法测定钯的浓度范围为0.01~ 0.6 m g/ml。通过11个钯含量为0.2 m g/ml的复杂样品的测定，该方法的相对标准偏差为1.02 %。此方法用于测定样品中的钯，结果令人满意。
关键词 2-(2-喹啉偶氮)-5-二甲基胺苯胺；钯；光度法

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