[gcebrero]

Hello,

I work for a metal finishing company and one of the processes we use is an etch for titanium parts. It is a solution of nitric acid and hydrofluoric acid in a 10:1 ratio. My analysis for the fluoride in solution involves a titration using a fluoride ISE and lanthanum nitrate as the titrant. The concentration of the bath should be around 25 g/L. The problem I'm having is that my plotted titration curve is a little hard to read. I'm not getting a sharp break at the equivalence point but more like a gradual curve. It makes guessing my equivalence point a little difficult. Is there a way to manipulate the solution so I end up getting a titration curve with a better equivalence point break or does Le Chatelier just hate me?

Thanks

[Borek]

I can be completely off, but have you tried to neutralize the solution first? If I understand you correctly, you are titrating highly acidic solution. HF is a weak acid, in low pH it is protonated, which increases the solubility of the LaF₃. At pH 5 it should be no longer a problem.

[gcebrero]

Yes you are correct. The low pH keeps the fluoride as HF but to counter that, the first step in my procedure is that I take a 10 mL sample and dilute it to 100 mL with 15% sodium acetate solution. That gets the pH to around 5 or a little higher which releases the F^- into the solution for analysis.

[Borek]

Hm, have you tested the electrode in other solutions, just to check if it doesn't need replacing?

[gcebrero]

I perform a titration on a 1 g/L standard solution before my analysis and that curve looks fantastic every time and I get the same value for the equivalence point every time as well.