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

Zhu Liya ^1,2, Hu Qiufen ², Yang Guangyu ², Yin Jiayuan ²
(¹Analytical Laboratory, Kunming Institute of Precious Metals. Kunming 650221; ² Department of Chemistry, Yunnan University. Kunming 650091)

Abstract 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 EXPERIMENTAL
1.1 Apparatus
A UV-160 A spectrophotometer (Shimidzu Corporation, Tokyo, Japan) equipped with 1 cm cells was used for all absorbance measurements. The pH values were determined with a Beckman F-200 pH meter (Beckman Instruments, Fullerton, CA, USA).
1.2 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 at 0ºC. 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 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. Elemental analysis: C₁₇H₁₇N₅ found (calculated) C 69.82 (70.08), N 23.83 (24.04), H 6.04 (5.88). IR (KBr) (cm^-1): 3300 (n _N-H), 1600, 1560, 1505, 1426
(n_C=C,N=N), 1380, 1323 (n _C-N), 2850 (n _C-H), 1465 (n _C-H), 3050, 3015 (n _Ar-H), 1175, 1120, 865, 780, 720 (n _Ar-H). ¹HNMR (solvent: d₆-acetone) (d ppm): 2.28 (s, 6H, N-CH₃), 3.69 (s, 2H, -N-H₂); 6.86- 7.85 (m, 9H, Ar-H). MS: 291 (M⁺). All these show that the QADMAA has the structure in Fig.1.
1.3 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 chemicals used were of analytical grade unless otherwise stated.

Fig.1 The structure of QADMAA

1.4 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.
1.5 Application                 
The proposed method has been successfully applied to the determination of palladium in water and catalyst.
    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 were 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 and 4.0 m g of palladium to water samples. A standard method using atomic absorption spectrometry was also used as a reference method. The results are shown in Table1.

Table 1 Determination of palladium in the water sample

2.3 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 of 1.0 % CTMAB solution 1- 4 mL give a constant and maximum absorbance. Accordingly, the use of 2 mL was recommended.
2.4 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.
2.5 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 for at least 20 h.
2.6 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 determinations) was 1.02 %.
2.7 Composition of the complex
The composition of the complex was determined by molar ratio method(Fig.3) and continuous variation method (Fig.4). Both showed that the molar ratio of Pd(II) to QADMAA is 1:2.

Fig.3 Composition of Pd(1I)-QADMAA complex by molar ratio method
The concentration Pd(II) was 0.3×10⁵ mol.L^-1, other conditions as in standard procedure

Fig.4 Composition of Pd(I1)-QADMAA complex by continuous variation method
The concentration of Pd(II)+QADMAA was 1.0×10⁵ mol.L^-1, other conditions as in standard procedure

2.8 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 2.

Table 2 Tolerance limits for the determination of 5 m g of Pd(II) with QADMAA (relative error ± 5% )

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