Thermo. Titr. Application Note No. H-113
Determination of nitric acid, hydrofluoric acid, and iron in acid
etching baths
Scope:
Determination of nitric acid, hydrofluoric acid, and iron
in acid etch solutions used for cleaning steels.
Principle:
Solutions of HNO3 and HF are employed in the cleaning
of steels, principally stainless steel. As the acid mixtures
become used, dissolved iron accumulates in the solution,
the free HF decreases, and the HNO3 concentration
increases apparently. The proper action of the cleaning
mixture is dependent on an excess of HF remaining in the
liquor.
Two separate titrations are required to determine the
components:
A titration with 2 mol/L aqueous NaOH to determine the
HNO3 and HF contents, where the sample is dissolved in
a mixture of ethanol and acetonitrile.
A titration with 2 mol/L aqueous NaOH to determine the Fe
content, where the sample is dissolved in a mixture
comprising acetonitrile and water.
The compositions of the two solvent solutions have been
optimized to give the sharpest endpoints for the relevant
analytes.
Reagents:
Titrant for both titrations: 2 mol/L NaOH (standardized)
Solvent for HNO3-HF titration: 1:1 by volume denatured
ethanol:acetonitrile
Solvent for Fe titration: 5:95 (v/v)
acetonitrile:distilled water
Method:
Basic Equipment List (for automated titration):
859 Titrotherm
814 USB Sample Processor
800 Dosino with 10 mL burette
120 mL PP beaker
Thermoprobe (fluoride resistant)
Parameters:
HNO3-HF titration
Titrant delivery rate (mL/min)
No. of exothermic EPs, ERC parameter:
No. of endothermic EPs, ERC parameter:
1
4
1, 10
1, -80
Filter factor (data smoothing factor)
Stirring speed
65
9
Fe titration
Titrant delivery rate (mL/min)
4
No. of exothermic EPs, ERC parameters: 2, -40, -20
Data smoothing factor
50
Stirring speed
9
Analysis:
1. HNO3-HF determination:
Dispense accurately 2 mL of etch acid solution into a PP
beaker, and add 35 mL of 1:1 acetonitrile/ethanol mixture.
Titrate to second endpoint.
2. Fe determination:
Dispense accurately 3 mL of etch acid solution into a PP
beaker, and add 35 mL 5:95 acetonitrile/distilled water.
Titrate to the second endpoint.
Results:
Sample
Etching
bath 1
Etching
bath 2
Measured
value
HNO3 (%)
11.7
Exp.
value
HNO3 (%)
1-40%
Measured
value
HF (%)
1.9
Exp.
value
HF (%)
1-10%
Measured
value
Fe3+ (%)
2.1
Exp.
value
Fe
-
17.5
1-40%
3.8
1-10%
<0.4
-
Measured
value
HF (g/L)
Exp.
value
HF (g/L)
10
11
42
10
10
40
Sample
Standard 205
Cal 1
Cal 2
Measured
value
HNO3
(g/L)
135
199
199
Exp.
value
HNO3
(g/L)
126
189
189
Remarks:
All determinations with 3 replicates
Expected values (customer’s information):
HNO3
HF
sample
HNO3
(g/L)
1-40%
1-10%
Etching bath 1
1-40%
1-10%
Etching bath 2
2M
126
0.5 M
Standard 205
3M
189
0.5 M
Cal 1
3M
189
2M
Cal 2
2
HF (g/L)
10
10
40
Calculations:
HNO3-HF titration
HNO3 (%) = ((EP1 mL-HNO3 blank, mL)*NaOH mol/L*Titer
*molar mass HNO3)/(sample vol. mL *density*10)
HF (%) = ((EP2 mL -EP1 mL-HF blank, mL)*NaOH
mol/L*Titer*molar mass HF)/(sample vol. mL*density*10
Fe titration
Fe (%) = ((EP2 mL-EP1 mL)*NaOH mol/L*Titer*molar
mass Fe)/(sample vol. mL*3*density*10)
Density for etching bath 1 = 1.1189 g/mL
Density for etching bath 2 = 1.1204 g/mL
Thermometric Titration Plots etching bath 1:
HNO3-HF titration
Fe titration
Conclusion:
A relatively simple analytical protocol has been developed
for the routine process control analysis of
nitric acid, hydrofluoric acid, and iron. A determination
contains two titrations with a duration of approx. 3
minutes each. Reagents are inexpensive and easily
maintained.
3
Appendix:
Results report for etching bath 1 (“Betbad 1”): Determination of HNO3-HF
4
Results report for etching bath 1 (“Betbad 1”): Determination of Fe
5