Chinese Journal of Applied Chemistry ›› 2018, Vol. 35 ›› Issue (6): 729-734.DOI: 10.11944/j.issn.1000-0518.2018.06.170368

• Full Papers • Previous Articles    

Determination of Total Fluoride in Chemical Milling Etching Solution for Titanium Alloys by in-situ Hydrolysis-Lanthanum Nitrate Potentiometric Titration Method

YAN Pengfeia,WU Liangb,YAO Jingfena,LIU Haidonga,GUO Anrub*()   

  1. aAVIC Shenyang Aircraft Corporation,Shenyang 110850,China
    bAerospace Research Institute of Materials & Processing Technology,Beijing 100076,China;
  • Received:2017-10-16 Accepted:2018-02-05 Published:2018-06-01 Online:2018-05-28
  • Contact: GUO Anru

Abstract:

A new method for determination of total fluoride in chemical milling etching solution for titanium alloys based on in-situ hydrolysis of fluoride-titanium complex and on potentiometric titration with La(NO3)3 was developed. In hexamethylenetetramine(HMTA) buffer solution,the fluoride-titanium complex was hydrolyzed to release free fluoride ions. A fluoride ion-selective electrode(F-ISE) was used as the indicator electrode. Various parameters affecting the test(i.e., pH values of the solution, the dosage of HMTA solution and the concentration of titanium ion) were tested and optimized. It is shown that the test results are not interfered by the concentration of Ti(Ⅳ) in the range of 0~20 g/L, the relative standard deviation(RSDs, n=6) of the method ranges from 0.27% to 0.62% and the recovery by standard addition method is in the range of 99.5%~101.1%. The mechanism of hydrolysis of fluoride-titanium complex was discussed. The main form of fluoride presented in etching solution is TiF62-. Appropriate acidity is essential to the hydrolysis reaction. HMTA as a pH buffer provides a constant pH circumstance, while La(NO3)3 as the fluoride scavenger reduces the concentration of free fluoride and promotes the hydrolysis reaction dramatically. Accompanied by the titration reaction, the fluoride-titanium complex is hydrolyzed completely.

Key words: fluoride determination, hexamethylenetetramine, lanthanum nitrate, hydrolysis, potentiometric titration