Direct spectrophotometric determination of cerium sub-group rare earth elements with m-trifluoromethylchlorophosphonazo in the presence of ytrrium sub-group elements in nodular cast iron and steel samples☆

2,7-Bisazo derivatives of chromotropic acid have been used in the spectrophotometric determination of cerium sub-group rare earth elements, the most important being carboxynitrazo [1], due to their metallic complexes with high molar absorptivities. This reagent is applied to the analysis of ores with satisfactory results [2]. Recently, owing to the use of oxalic acid as masking reagent, selective spectrophotometric determination of cerium sub-group rare elements with CPAmN has been developed [3]. In the present work, the reaction of cerium sub-group rare earth elements with a new 2,7-bisazo derivative of chromotropic acid, the m-trifluoromethylchlorophosphonazo, is studied for its higher selectivity compared with the other reagents mentioned: This paper describes the synthesis of the reagent and its use in the direct spectrophotometric determination of cerium sub-group rare earth elements in nodular cast iron and steel samples. The results obtained are favourable. Reagents and Apparatus. Absorbance was measured on a Model UV-300 double-beam spectrophotometer and Model XG-125 spectrophotometer with 1-cm cells. The working standard solutions: 20μg/ml. Synthesis of m-Trifluoromethylchlorophosphonazo. 2-Trifluoromethylaniline was mixed with 20 ml of water and a solution of sodium nitrite (0.3 g in 8 ml of water), and then diazotized by adding 5 ml of 6 M hydrochloric acid by drops at 0–2°C and stirring. The mixture is stirred 1 hr at 0–5 °C. 2 g Chlorophosphonazo I was dissolved in 30 ml of 5% lithium hydroxide solution and cooled to 0–2 °C. the diazotized solution prepared was added while steering, the color of the solution changed from red to blue (the pH of the solution sometimes was adjusted to 9.5–10.5 with dilute lithium hydroxide solution or hydrochloric acid). After continuous stirring for 1 hr, the solution was allowed to stand overnight, acidified to pH 1–2 with hydrochloric acid, filtered and washed with 2 M hydrochloric acid 3 times. The precipitate was dissolved in 80 ml of 5% lithium hydroxide solution and filtered. The solution was acidified to pH 1 with hydrochloric acid then filtered and washed with 2 M hydrochloric acid 2–3 times. The product was dried at 50 °C, yielding 1.5 g of precipitate. Analysis of Nodular Cast Iron and Steel Samples. Weigh a 0.1000–4000 g sample into a 100 ml beaker and dissolve it in 20 ml of 6 N hydrochloric acid and 2 ml of 16 N nitric acid while heating. The solution was evaporated to 2–3 ml, cooled to the room temperature and 20 ml water was added filtering off any residue on fast filter paper and washing the residue and paper several times with dilute hydrochloric acid. Collecting the filtrate and washings in a 100 ml standard flask, diluted to the mark with water, mixed well. An aliquot was tranferred containing not more than 20 μg of cerium sub-group rare earth elements into a 25 ml of 10% oxalic acid and 2.5 ml of 0.05% m-trifluoromethylchlorophosphonazo solution were added and diluted to mark with water. The solution mixed well and absorbance measured at 677 nm in a 1 cm cell against nodular cast iron and steel for the absence of rare earths blank. Absorption Characteristics. The absorption spectra or m-trifluoromethylchlorophosphonazo alone and of the complexes of cerium at 0.012 N hydrochloric acid have been studied. The spectra are shown in Fig. 1. The optimal wavelength for measurement of the cerium (or cerium sub-group elements, ΣCe) complex is found to be 677 nm. A greater amount of oxalic acid slightly depresses the absorption of the cerium complex. In the same condition, ytterbium is almost completely masked. Effect of pH. We have been studying the effect of HCL, HNO3, and H3PO4 on the colour reaction. For the determination of cerium (or ΣCe) in the presence of yttrium sub-group elements (ΣY), the addition of 2–4 ml of 12 N hydrochloric acid was optimal. Hence a 3 ml addition of 12 N hydrochloric acid was chosen for the determination of cerium, which corresponded to 0,12 N hydrochloric acid in the final solution. Effect of Time. The coloured complexes were formed instantly at room temperature. Their absorbances were stable for at least 6 hr. Effect of Reagent Concentration. The chosen final concentrations of the reagent (2–3 ml of 0.05% m-trifluoromethylchlorophosphonazo, 5 ml of 10% oxalic acid) are those giving maximum absorbance. Amounts of 5 ml of 10% oxalic acid solution were added for masking of iron and yttrium sub-group elements. A greater amount of oxalic acid would weaken the absorbance of the cerium complex. Therefore, an addition of 5 ml of 10% oxalic acid is recommended. Beer's Law. In the absence of oxalic acid Beer's Law was obeyed for 0–25 μg/25 ml of cerium. In the presence of 20 mg iron and 5 ml of 10% oxalic acid Beer's Law was obeyed for 0–20 μg/25 ml of cerium or cerium sub-group elements. The apparent molar absoptivities of lanthanum, cerium, praseodynium and neodymium at 677 nm were 9.2 × 104, p.3 × 104 and 9.0 × 104 1-1mol-1cm-1, respectively. Effect of Foreign Ions. An error of 10% in amount is considered tolerable for the determination of 10.0μg of cerium. The following ions when present up to the amount (in mg) shown in brackets, do not interfere in the determination of 10μg of cerium: Fe(III) 20, Al(III) 20, Ca(II) 0.24, Mg(II) 15, Ni(II) 2, Mn(II) 12, Ba(II) 0.23, Cu(II) 1.8 Co(II) 5.3, Pb(II) 3, Cd(II) 25, Zn(II) 12, Bi(III) 2, Cr(III) 6, Mo(VI) 25, W(VI) 22, Zr(IV) 2, Ti(IV) 0.35, V(V) 2, Si(IV) 22, NH+4 22, SO=4 22, Th(VI) 0.002. As shown, most of the ions did not interfere with the determination, 20 mg iron(III) and 20 mg of Al(III) were tolerable. Comparison with Other Reagents. Some results obtained in the study of reagent m-trifluoromethylchlorophosphonazo for the determination of cerium are compared with those obtained with CPAmN. Absorption spectra for both cerium complexes are similar. The optimum pH is different. m-trifluoromethylchlorophosphonazo may react in 0.12 N hydrochloric acid, the change of absorbance with acid is very small. Due to the higher selectivity of m-trifluoromethylchlorophosphonazo, greater interference is found by using CPAmN as reagent in the determination. Analysis of Some Synthetic Samples. Table I gives results for some synthetic samples, showing that the method is satisfactory. t001. Recovery of Cerium Added to Selected Synthetic Samples. Added (Σy) Ce,μg Recovery (%) Added Found 2 7 6.9 98.6 5 10 10.3 103.0 5 12 12.2 101.0 7 15 14.6 97.3 10 10 11.0 110.0 Full-size table Table options View in workspace Download as CSV Applications. The proposed method has been applied to the determination of cerium sub-group elements in nodular cast iron and low alloy steel. (Table II). t002. Determination of Cerium Sub-group Elements in Iron and Steel. Sample Composition Found % % Low alloy steel 1 0.052 0.0508 0.051 2 0.029 0.0293 0.0297 3 0.031 0.0313 0.031 4 0.0205 0.0203 0.0203 5 0.0354 0.0346 0.0342 6 0.082 0.0814 0.0814 Nodular cast iron 1 0.022 0.0236 0.0236 2 0.046 0.0468 0.0468 3 0.085 0.082 0.082 Full-size table Table options View in workspace Download as CSV Table II gives results for some typical samples showing the method is considered satisfactory.