Determination

Fluoride and nitric acid in mixed acid etch baths.

Application

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 hydrofluoric acid for a typical titanium etch bath (per BAC 5753). If you prefer ounces and gallons, convert g/L (grams per liter) to oz/gal (ounces per gallon) by dividing by 7.5. The factor of 7.5 is (28.375 grams/ounce)/(3.7854 liters/gallon).

  min mid max
HNO3 300 g/L 375 g/L 450 g/L 
  40 oz/gal50 oz/gal60 oz/gal
HF 11 g/L 14 g/L 17 g/L 
  1.5 oz/gal1.85 oz/gal2.2 oz/gal

Method

1 Pipette a 10.0 mL bath sample into a 1L volumetric flask.

2 Dilute to mark and take a 100 mL aliquot.

  (A 10mL sample with dilution is an accurate way to

  take a 1.0 mL sample for low fluoride concentrations.)

3 Add 2-3 drops of phenolphthalein.

4 Titrate with A mL of 1.0N NaOH to a persistent faint pink endpoint.

The A value can be used to calculate total acid and combined

with the fluoride test to determine nitric acid alone.

5 Add 0.1N H2SO4 drop-by-drop to just barely remove the pink color.

6 Add 40 mL of saturated boric acid (H3BO3).

7 Add 40 mL of reagent alcohol.

8 Add 8 grams of KCl and mix to dissolve.

9 Add 6-8 drops of methyl orange indicator.

10 Titrate with B mL of 0.1N H2SO4 to first sign of faint pink (red-orange).

Calculations

[ A mL - B mL x 0.1 ] x 8.4 = oz/gal HNO3

[ A mL - B mL x 0.1 ] x 63 = g/L HNO3

B mL x 0.267 = oz/gal HF

B mL x 2.0 = g/L HF

Explanation

In steps 1-4 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 5, 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 in the solution.

When a 1.0 mL sample of HF is titrated with 1.0N NaOH, as in step 4, the factor is 2.67 (2.67 x mL = oz/gal HF). 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 4.

1. HF (oz/gal) = 2.67 x amount of titrant (mL) to neutralize HF in step 4.

2. Amount of titrant used to neutralize HF in step 4 = HF (oz/gal) / 2.67

3. Amount of titrant used to neutralize HF in step 4 = (B mL x 0.267) / 2.67

4. HNO3 = [ A mL - (B mL x 0.267) / 2.67 ] x 8.4

5. HNO3 = [ A mL - B mL x 0.1 ] x 8.4

Similar analysis can be used to find the concentration of H2SO4 when mixed with HF. These calculations are specific to a particular titrant normality and sample size, so we do not show tables of different factors.

Reference

Actane 70 Technical Data Sheet

Enthone Laboratory

July 11, 2000