This Application Bulletin describes the automated determination of mixtures of HNO3, HF, and H2SiF6 in the range of approximately 200–600 g/L HNO3, 50–160 g/L HF, and 0–185 g/L H2SiF6 using thermometric titration. Etch acid mixtures containing HNO3, HF and H2SiF6 from the etching of silicon substrates can be analyzed in a sequence of two determinations. The first determination involves a direct titration with standard c(NaOH) = 2 mol/L, followed by a back titration with c(HCl) = 2 mol/L. This determination yields the H2SiF6 content plus a value for the combined (HNO3 + HF) contents. The second determination consists of a titration with c(Al3+) = 0.5 mol/L to determine the HF content. The HNO3 content can then be calculated from the results.
For freshly prepared mixtures of HNO3 and HF containing no H2SiF6, a linked two-titration sequence can be used. In the first titration, the combined HNO3 + HF content is determined with c(NaOH) = 2 mol/L. This titration is automatically stopped, and a second titration with c(Al3+) = 0.5 mol/L is started to determine the HF content. The HNO3 content can then be calculated from the results.