EDTA Titrimetric Determination of Fluorion in Chemical Milling Solution and Acid Pickling Solution for Titanium Alloys

For the determination of content of F- in samples of chemical milling and acid pickling solution used for titanium alloys, an aliquot of the sample solution was adjusted to pH 2.0 and buffered with 10 mL of HOAc-NaOAc buffer solution; an excess of standard La3+ solution was added to the sample solution and the mixture was heated to boiling for 2 min to precipitate completely the F- as LaF3. Excess of La3+ in the solution was then titrated with standard EDTA solution, and the content of F- was determined indirectly by calculation on the base of the amount of titrant consumed. To test for the accuracy and precision of the proposed method, 10 successive determinations were made with a known HF solution of 45.00 g·L-1 F- by the proposed method, giving values of recovery very close to 100% and of RSD′s of 0.080%. Substantial samples of chemical milling solution and acid pickling solution were analyzed by the present method and checked by the Th(NO3)4 titrimetric method, consistent results were obtained.