Thorium calibration curve

A calibration curve for the range 0.2-10 mg fluoride ion per 100 mL is constructed as follows. Add the appropriate amount of standard sodium fluoride solution, 25 mL of 2-methoxyethanol, and 10 mg of a buffer [0.1 Af in both sodium acetate and acetic (ethanoic) acid] to a 100 mL graduated flask. Dilute to volume with distilled water and add about 0.05 g of thorium chloranilate. Shake the flask intermittently for 30 minutes (the reaction in the presence of 2-methoxyethanol is about 90 per cent complete after 30 minutes and almost complete after 1 hour) and filter about 10 mL of the solution through a dry Whatman No. 42 filter paper. Measure the absorbance of the filtrate in a 1 cm cell at 540 nm (yellow-green filter) against a blank, prepared in the same manner, using a suitable spectrophotometer. Prepare a calibration curve for the concentration range 0.0-0.2 mg fluoride ion per 100 mL in the same way, but add only 10.0 mL of 2-methoxyethanol measure the absorbance of the filtrate in a 1 cm silica cell at 330 nm. 

Figure 6.24 Calibration curves for uranium (a) and thorium (b) determination in brain samples via the standard addition mode using micronebulization in LA-ICP-MS. (/. S. Becker et ai, Anal. Chem. 77, 3208 (2005). Reproduced by permission of American Chemical Society.)...

Fluoride samples of high concentration can be titrated potentiometrically [60] with lanthanum nitrate or thorium nitrate reagent. The potentiometric standard addition technique with NaF standard solution [61] also was found applicable however, direct potentiometry using calibration curves is most often relied on in water analysis. 

Prasad and Dey [1962PRA/DEY] performed potentiometric titrations of 0.005 to 0.05 M ThCl4 solutions with NaOH solutions of different concentrations from pH 3 to pH 11-12 and reverse titrations by adding 0.04 and 0.08 M ThCk to the NaOH test solutions at 27°C. Ionic strength was not kept constant and the calibration of the glass-calomel electrode system used to measure pH is not mentioned. The authors report only the qualitative results that precipitation occurs already at < 4 (which is concluded from the inflection points of the titration curves) and that precipitation is complete near pH 7, independent of the thorium concentration. No thermodynamic data can be derived from this study. 

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