Determination of the hydrofluoric acid concentration in the solutions

1. Determination of the hydrofluoric acid concentration in the solutions used for the chemical polishing of crystal. E. BrientCristallerie de Baccarat, 20 rue des Cristalleries - 54120 Baccarat Y. Pillet, M. VilasiLCSM UMR755, Faculté des Sciences et techniques UHP – Nancy I Bd des Aiguillettes BP239 - 54506 Vandoeuvre

2. polishing before after

3.  Savings in consumption of acids  Optimization of the polishing cycles : improvement of the capacity on the existing machines  Control of the « strength » of the bath to avoid quality problems as stuck salts or loss of acceptable dimensional tolerances INDUSTRIAL EXPECTATIONS

4. Solution of HF + H2SO4   HF/H2SO4 ratio : a key factor for the quality of the process Determination of the [F-] in a hyper-acidic medium (H2SO4)  Determination of [HF] to control ageing of polishing bath  Data should be achievable by a simple method CONTEXT + H2SiF6 (leaching of the glass) AIM OF THE STUDY

5. This technique cannot be used directly The classical titration method consists on the following reaction: La3+ + 3 F- => LaF3(solid) acted at pH=5 and the determination of the equivalent volume is realized thanks to a potentiometric measurement In the industrial polishing baths, this reaction is hindered by the presence of H2SiF6 produced by the leaching of the glass

6. The titration of H2SiF6 is feasible according • to the following reaction : • 2 La3+ + SiF62- + 4H2O => 2LaF3(solid) + Si(OH)4 + 4H+ The titration of a mixture of H2SiF6 + HF determines the total amounts of F-I provided by the both acidic species • Laboratory experiments were performed on standard solutions containing : • - pure H2SiF6 • - pure HF • - a mixture of H2SiF6 and HF at pH= 5, H2SiF6 is existing as SiF62-

7. Potentiometric titration of a mixture of H2SiF6 + HF + H2SO4 2 mL of the mixture, stabilized at pH=5 , diluted in 25 mL H2O with La(NO3)3 (9.38x10-3 mole/L) n(La3+) = (1/3) * {n(F -) + 6 * n(SiF62-)} = n/3 [F-I]

8. The determination of [H2SiF6] or [HF] in the same bath needs to use an other technique of titration The titration of standard solutions containing H2SiF6 was studied at the laboratory The easiest titration technique coming in mind is the acid/base method

9. H2SiF6 + 2 OH -SiF62- + 2 H2O SiF62- + 4 OH -Si(OH)4 + 6 F – Titration of 2 mL of H2SiF6 (10-1 mole/L) diluted in 25 mL H2O with NaOH (9.38x10-3 mole/L)

10. DVmL  Titration of a standard mixture of H2SO4 + HF + H2SiF6 gives a similar curve to the one obtained with pure H2SiF6 solution Technique is therefore applicable to an industrial bath [H2SiF6] is determinate by the volume difference DVmL = VEP2 – VEP1

11. [F-I] = [F -] + 6 [SiF62-] Potentiometric Acido/Basic VEP3 DVmL

12. Chemical characterization of fluoride in hyperacidic medium can be done with an accuracy of about 10%.  This method is validated on industrial solutions for crystal polishing Its simplicity allows the application on the glass making site.  It should become a powerful tool to control the ageing of polishing baths CONCLUSIONS

13. Decrease of the ratio HF/H2SO4 (by additional H2SO4 andconstant HF) corresponds to a lower value of H2SiF6 in the bath and lower necessary additions of HF to maintain a constant level of active HF in the bath  • The hypothesis to be checked is the following one: • Ions SiF62- react better with the additional H+ than with K+ to form H2SiF6instead of K2SiF6 • the following reaction permits to re-create active HF: • H2SiF6+2H2O => SiO2 + 6HF  FIRST INDUSTRIAL POSITIVE CONSEQUENCE The “virtuous circle”