Fluoride and nitric acid in mixed acid etch baths.
In this example, we will explain the process of testing the concentration of fluoride in acidic etch baths. Hydrofluoric acid or ammonium bifluoride is added to nitric acid or sulfuric acid for etching aluminum, pickling stainless, and etching titanium. Volumetric analysis to measure fluoride concentration has historically been done using thorium nitrate; however, thorium nitrate is radioactive and is therefore undesirable in the laboratory. Ion-specific probes can also be used to measure fluoride concentration; however, the procedure requires very dilute samples and probe calibration.
The following table shows concentrations of nitric acid and ammonium bifluoride for a typical titanium etch bath (per BPS 4462 Table III A).
| min | mid | max | |
| HNO3 | 112 g/L | 169 g/L | 225 g/L |
| 15 oz/gal | 22.5 oz/gal | 30 oz/gal | |
| NH4HF2 | 0.75 g/L | 6.0 g/L | 11.25 g/L |
| 0.1 oz/gal | 0.8 oz/gal | 1.5 oz/gal |
Acid-base titrations performed on this bath will measure HF, not NH4HF2: to calculate titration results and additions, we must know how much HF is liberated when NH4HF2 is added to the etch bath. When NH4HF2 is added to water, ~0.33 molecule of HF is liberated from each molecule of NH4HF2 on average. However, when added to an etch bath like the one in the table, almost 2 HF molecules are liberated from each NH4HF2 molecule because the strong acidity of the HNO3 pushes the equilibrium to the right. Textbook analysis for NH4HF2 can be confusing if this is not taken into account. The following method uses acid-base titration to calculate both HNO3 and NH4HF2 concentrations in a strongly acidic bath.
1 Pipette a 5.0 mL bath sample into a 250 mL flask.
2 Add 2-3 drops of phenolphthalein.
3 Titrate with A mL of 1.0N NaOH to a persistent faint pink endpoint.
Approach the endpoint drop-by-drop to avoid overshoot.
The A value can be used to calculate total acid and combined
with the fluoride test to determine nitric acid alone.
4 Add 0.1N H2SO4 drop-by-drop to just barely remove the pink color.
5 Add 40 mL of saturated boric acid (H3BO3).
6 Add 40 mL of reagent alcohol.
7 Add 8 grams of KCl and mix to dissolve.
8 Add 6-8 drops of methyl orange indicator.
9 Titrate with B mL of 0.1N H2SO4 to first sign of faint pink (red-orange).
(The color change can be difficult to see. On the first time,
titrate to pH=3.2 and use that color for future reference.)
[ A mL - B mL x 0.1 ] x 1.68 = oz/gal HNO3
[ A mL - B mL x 0.1 ] x 12.6 = g/L HNO3
B mL x 0.076 = oz/gal NH4HF2
B mL x 0.57 = g/L NH4HF2
In steps 1-3 sodium hydroxide neutralizes the acid in the sample. Since the sample contains both nitric and hydrofluoric acid, we cannot determine the individual constituents with this analysis alone.
In step 4, careful addition of sulfuric acid brings the pH down to 8.5 where the phenolphthalein turns colorless, thereby removing any excess hydroxide. The boric acid, alcohol and potassium chloride reacts with the HF to remove the acid from solution; this increases the pH proportionately to the amount of HF removed. Titrating with 0.1N H2SO4 to the methyl orange color change determines the amount of HF that was removed from the solution (as BF4).
When a 5.0 mL sample of HF is titrated with 1.0N NaOH, as in step 3, the factor is 0.534 (0.534 x mL = oz/gal HF). This factor can be found using the ChemTrak Tools Volumetric Methods. Since we now know the concentration of the HF, we can calculate the the amount of titrant that was needed to neutralize the HF during step 3.
1. HF (oz/gal) = 0.534 x amount of titrant (mL) to neutralize HF in step 3.
2. Amount of titrant used to neutralize HF in step 3 = HF (oz/gal) / 0.534
3.
Amount of titrant used to neutralize HF in step 3 = (B mL x 0.0534) / 0.534
HNO3can be found by subtracting this amount from the total titrant used in Step 3.
4. HNO3 = [ A mL - (B mL x 0.0534) / 0.534 ] x 1.68
5. HNO3 = [ A mL - B mL x 0.1 ] x 1.68
The 1.68 factor for calculating concentration of HNO3 can be found from the ChemTrak Tools Volumetric Methods. Similar analysis can be used to find the concentration of H2SO4 when mixed with HF.
HF molecular weight is 20.01 mmu. NH4HF2 molecular weight is 57.04 mmu. The HF liberation from one molecule of NH4HF2 varies from 0.35 molecules of free HF (if no other acid is present) up to 2 molecules of free HF as more HNO3 (or other acid) is present. So the calculation of NH4HF2 concentration in acid-base titration with NaOH given a 1 mL sample are ...
1. (mL of NaOH) x (Normality of NaOH) x 7.61 / 0.33 = oz/gal NH4HF2 with no other acid.
2. (mL of NaOH) x (Normality of NaOH) x 7.61 / 2 = oz/gal NH4HF2 in other acid.
Similarly, adding 1 oz/gal of NH4HF2 to water will yield 0.123 oz/gal of HF. Adding 1 oz/gal of NH4HF2 to a concentrated strong acid will yield a maximum of 0.7 oz/gal of HF. This varying yield of HF from NH4HF2 shows why it is crucial to mix a small batch of solution with your optimum concentrations, test your method, and dial in your color changes. ChemTrak makes this process easier, ensures repeat performance, and enables continuing statistical process control of your etch bath.
Actane 70 Technical Data Sheet
Enthone Laboratory
July 11, 2000