Determination of HNO₃, HF and Titanium(Ⅳ) in Etching Solution for Chemical Milling of Titanium Alloys by Potentiometric Titration

A new method of potentiometric titration for determination of HNO₃, HF and Ti(Ⅳ) in etching solution for chemical milling of titanium alloys was proposed. ① For determination of total acidity, an appropriate amount of the etching solution was reacted with 1.000 g of KNO₃ in a solution containing 30 mL of water and 20 mL of ethanol, to restrain the hydrolysis of TiF₆^2- by transforming H₂TiF₆ into precipitate of K₂TiF₆ and HNO₃; and total acidity was titrated potentiometrically with 0.500 mol·L^-1 NaOH solution using HF-resistent pH electrode as indicator. ② For determination of Ti(Ⅳ), an appropriate amount of the sample was reacted with an excess and definite amount of 0.500 mol·L^-1 NaOH solution at 60℃ for 20 min to precipitate Ti(Ⅳ) as Ti(OH)₄, and the excess of NaOH in the solution was titrated with 0.500 mol·L^-1 HCl solution potentiometrically, then the concentration of titanium(Ⅳ) in the sample was calculated. ③ For determination of total fluoride, 5.00 mL of the sample were taken, and after addition of 50 mL of water, its acidity was adjusted to between pH 6.0 to 8.0 with NaOH solution, and its total volume was made up to 100 mL. An aliquot of 5.00 mL was taken, after addition of 10 mL of hexamethylenetetramine buffer solution (pH 6.5) and dilution to 60 mL with water, it was titrated potentiometrically with 0.016 7 mol·L^-1 La(NO₃)₃ solution using F^- selective electrode as indicator. Based on the data obtained, concentration of HNO₃, HF and Ti(Ⅳ) in the sample were calculated by the formulas given. In the analysis of simulated samples with addition of standards of HNO₃, HF and H₂TiF₆, by the proposed method, the results obtained were in consistency with the theoratical values. Tests for recovery were made on the base of substantial samples collected from production front, values of recovery found were in the ranges of 97.8% to 101% for HF, 98.9% to 99.7% for Ti(Ⅳ) and 97.8% to 103% for HNO₃.