754 SHORT PAPERS Analyst June 1983 Spectrophotometric Determination of Exchangeable Calcium in Soils by Chlorophosphonazo-mA Qiu Xing-chu and Zhang Yu-sheng Agricultural Science Research Institute of Ganzhou Prefecture Jiangxi China and Zhu Ying-quan P.O. Box 82 Chengdu China Keywords Chlorophosphonazo-mA ; spectrophotometry ; calcium determina-tion; soils Chlorophosphonazo-mA (CPA-mA) was first proposed by East China Normal University as a spectrophotometric reagent for rare earth elements1 Recently it was used to determine uranium2 and thorium,3 but has previously not been used to determine calcium spectrophoto-metrically. In this work the optimum conditions for the spectrophotometric determination of calcium and the removal of the interference from diverse ions were studied in detail.CPA-mA Experimental Apparatus Analytical Instruments Factory). Instruments Factory). Spectra and absorbances were recorded with a Model 72 spectrophotometer (Shanghai The pH measurements were made with a Model pH S-73 pH meter (Tianjin Analytical Reagents Standard calcizcm solzction 100 pg ml-1. Dissolve 249.8 mg of calcium carbonate (dried at 110 "C) in 40 ml of 2 M hydrochloric acid. Remove the carbon dioxide by boiling and dilute the solution with water to 1 1 and mix well. Dilute an aliquot to give a final calcium con-centration of 20.0 pg ml-l in the working standard solution. CPA-mA aqueous solzction 0.015~0. Triethanolamine (TEA) 1 + 4 solzction. Quinolin-8-ol solzction 0.25% in water. All other reagents were prepared from analytical-reagent grade chemicals.East China Normal University Chem. Indus. Mfg. Procedure Transfer a solution containing not more than 60 pg of calcium into a 25-ml calibrated flask. Add with stirring 2 ml of ammonium acetate (1 M) 1 ml of ammonia (1 + 1) and 3 ml of CPA-mA (0.015%). Dilute to the mark with water. Measure the absorbance of the coloured solution at 630 nm in a 2.0-cm cell against a reagent blank prepared similarly SHORT PAPERS Results and Discussion n z 0.1 a” Q 755 0.2 -n tn 5 0.1 -Absorption Spectra cium. exhibits an absorption maximum at 630 nm. measurements. Fig. 1 shows absorption spectra of aqueous solutions of CPA-mA and its complex with cal-The absorption maximum of the reagent is at 560nm and the calcium complex This wavelength was employed for further 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Effect of Acidity and Salt Medium The optimum pH range in which the absorbance of calcium complex with CPA-mA can be measured is shown in Fig.2. The pH was adjusted with 1 + 1 ammonia by using the pH meter The absorbance is maximal and nearly constant at pH 7.0-9.3. A pH of 9.3 is pre-ferred. In borate medium the sensitivity of this method decreases. 0.6 8 .ff 0 0.4 2 0.2 500 540 580 620 660 Wavelengthlnm Fig. 1. Absorption spectra of CPA - mA and its calcium complex. (1) CPA - mA (against water as reference) 1.0-cm cell. 4.0 0.3 (II +2 n tn a 0.2 7.0 8.0 9.0 10.0 11.0 12.0 PH (2y Ca- CPA-mA complex (against reagent blank as reference) ; Ca taken 60 pg; 2.0-cm cell.Fig. 2. Effect of acidity on absorbance of Ca-CPA-mA complex. Ca taken 40 pg; 3.0-cm cells. (1) Ammoniacal medium; (2) borate medium. Composition of the Complex The composition ratio (calcium to CPA-mA) of the complex was determined by the molar ratio and continuous variation methods. The results obtained showed that the ratio of calcium to reagent in the complex was 1 2 (see Figs. 3 4 and 5). Beer’s Law and Sensitivity of the Reaction 0-60 pg per 25 ml. was 3.9 x lo3 1 mol-l cm-l. Linear calibration graphs passing through the origin were obtained for calcium in the range From this straight line the apparent molar absorptivity coefficient ( E 756 SHORT PAPERS Analyst VOI?. 108 Effect of Temperature and Stability It can be seen that the absorbance is unaffected by temperature over the range from 10 to 60°C.Therefore normal variations in laboratory temperature will have no effect. Besides the complex was formed instantaneously at room temperature (about 25 "C) and the absorbance of the complex did not change for 6 h and then faded slightly. The effect of temperature on the absorbance of the complex is shown in Fig. 6. $ 0.4 C % 2 0.2 a 1.0 2.0 2.5 3.0 4.0 Ca/m I g 0.2 2 LL 10 30 50 70 90 Tern peratu rePC Fig. 5. Molar ratio plot. Variation of Ca against constant CPA-mA; 0.5-4.5 ml of 2.0 x 1 0 - 4 ~ Ca added to 5.0ml of 2.0 x M CPA-mA per 25 ml. Fig. 6. of Ca2+ - CPA-mA complex. 2.0-cm cell. Effect of temperature on absorbance Ca taken 40 pg; Interferences of Diverse Ions Diverse ions were added to a solution containing 40 pg of calcium per 25 ml and the colour was developed by the usual procedure.In the presence of TEA and quinolin-8-01 as masking agents the following ions when present in the amounts (in micrograms) shown in parentheses, do not interfere Mg(I1) (80) Fe(II1) (50) Al(II1) (20) Mn(I1) (lo) Cu(I1) (5) Zn(I1) (lo), W(V1) (5) Cr(V1) (5) Mo(V1) (5) and Pb(I1) (10). Application The proposed method has been applied to the determination of calcium in soils. Procedure Weigh 1 g of soil sample (air dried) into a 50-ml centrifuge tube and add 30 ml of 1 M ammonium acetate. Centrifuge the tube for 3-5 min. (The procedures were repeated in triplicate.) Combine the clear centri-fugate and dilute to exactly 100 ml with 1 M ammonium acetate.Shake well. Pipette 2 ml of this extract into a 25-ml calibrated flask add 1 ml of TEA (1 + 4) and 1 ml of 0.25% quinolin-8-01 following the above general procedure. Stir the contents for a few minutes with a glass rod. TABLE I RECOVERY OF CALCIUM Calcium/ pg Sample Soil type 1 Violet soil 2 Alluvial soil 3 Red soil 4 Red soil Source of parent material Original Purple sandstone and 15.84 shale weathering products River drift 14.40 Quaternary period red 15.28 Granite weathering 8.86 clayey soil product Added 4 8 12 4 8 12 4 8 12 4 8 12 Found 20.34 24.04 27.89 18.90 22.59 26.44 19.54 23.55 27.09 12.87 16.81 21.15 Error/ pg + 0.50 + 0.20 + 0.05 + 0.50 + 0.19 + 0.04 + 0.26 + 0.27 - 0.19 + 0.01 - 0.05 + 0.29 Recovery, 102.5 100.8 100.2 102.7 100.8 100.2 101.3 101.2 99.3 100.1 99.2 101.0 Jum 1983 SHORT PAPERS 757 Results It can be seen that the recoveries of calcium from standard solutions were >99% and the relative standard deviation (eight determinations) was (3.08%’ The results shown in Table I1 are in reasonable agreement with those obtained by atomic-absorption spectropho tometry.The results obtained by the recommended method are listed in Tables I and 11. TABLE I1 ANALYTICAL RESULTS AND COMPARISON Calcium/mequiv. per 100 g soil f A \ Atomic-absorption Relative standard Sample spectrophotometry Present method* deviation yo 1 10.60 10.65 0.87 2 2.10 2.21 3.08 3 5.06 5.12 1.4 4 7.51 7.48 1.23 * Mean of eight determinations. The authors are grateful to Zhang Lin P.O. Box 82 Chengdu China for her assistance. References 1. 2. 3. East China Normal University Lihua Jianyan 1979 15 1. Xue Guang Huaxue Shiji 1982 4 332. Xue Guang and Zhu Ying-quan unpublished work. Received October 4th 1982 Accepted January 4th 198