Raman spectroscopic determination of the degree of dissociation of nitric acid in binary and ternary mixtures with HF and H2SiF6

Abstract

The oxidizing effect of nitric acid in aqueous solutions depends on the concentration of undissociated nitric acid. This makes the concentration of undissociated nitric acid an essential parameter to monitor and control the quality of silicon etching in the industrial manufacturing of solar cells. In the present study, a method known already is extended in such a way that the degree of dissociation of nitric acid can be determined by Raman spectroscopy in HF/HNO3/H2SiF6 acid mixtures over a broad concentration range for the first time and without using an internal or external standard to compensate the typical time‐dependent drift of a Raman spectrometer. The method developed requires the calculation of a peak area ratio from the areas of the unimpeded Raman signals assigned to nitrate (νN − O) at 1,048 cm−1 and to undissociated HNO3 (νN − OH) at 957 cm−1. The correlation between the peak ratio and the degree of dissociation of nitric acid revealed can be described by a simple empirical equation. Using this equation, the degree of dissociation of nitric acid can be determined over a broad concentration range in binary and ternary mixtures of HNO3 with HF and H2SiF6. The impact of the acids HF and H2SiF6 and the total water content in the degree of dissociation of nitric acid is discussed. A method for the determination of the dissociation degree of HNO3 in mixtures with HF and H2SiF6 is presented. The degree of dissociation of nitric acid in any mixture can be calculated by empirically correlating the peak area ratios with the degree of dissociation, provided that the Raman bands required to determine the peak ratio are not interfered. It could be shown that the degree of dissociation of HNO3, even in ternary mixtures, is always a function of the total water content of the mixture.

Full text

RESEARCH ARTICLE
Raman spectroscopic determination of the degree of
dissociation of nitric acid in binary and ternary mixtures
with HF and H
2
SiF
6
Thomas Langner | Anja Rietig | Jörg Acker
Department of Physical Chemistry,
Brandenburg University of Technology
Cottbus‐Senftenberg, Senftenberg,
Germany
Correspondence
Thomas Langner, Department of Physical
Chemistry, Brandenburg University of
Technology Cottbus‐Senftenberg,
Senftenberg, Germany.
Email: thomas.langner@b‐tu.de
Abstract
The oxidizing effect of nitric acid in aqueous solutions depends on the concen-
tration of undissociated nitric acid. This makes the concentration of undissoci-
ated nitric acid an essential parameter to monitor and control the quality of
silicon etching in the industrial manufacturing of solar cells. In the present
study, a method known already is extended in such a way that the degree of
dissociation of nitric acid can be determined by Raman spectroscopy in
HF/HNO
3
/H
2
SiF
6
acid mixtures over a broad concentration range for the first
time and without using an internal or external standard to compensate the typ-
ical time‐dependent drift of a Raman spectrometer. The method developed
requires the calculation of a peak area ratio from the areas of the unimpeded
Raman signals assigned to nitrate (ν
N−O
) at 1,048 cm
−1
and to undissociated
HNO
3
(ν
N−OH
) at 957 cm
−1
. The correlation between the peak ratio and the
degree of dissociation of nitric acid revealed can be described by a simple
empirical equation. Using this equation, the degree of dissociation of nitric acid
can be determined over a broad concentration range in binary and ternary mix-
tures of HNO
3
with HF and H
2
SiF
6
. The impact of the acids HF and H
2
SiF
6
and
the total water content in the degree of dissociation of nitric acid is discussed.
KEYWORDS
dissociation, nitric acid, Raman spectroscopy, silicon etching
1|INTRODUCTION
The HF/HNO
3
/H
2
SiF
6
system is the acid mixture used
most for wet chemical etching of silicon (Si) in microelec-
tronics and photovoltaics. Its major application is to etch
away the so‐called saw damage from the surface of Si
wafers.
[1]
The term “saw damage”designates a disturbed
crystal lattice layer at the wafer surface with a thickness
of several micrometres that is formed during wire sawing
of multicrystalline Si blocks and monocrystals. On the
one hand, the composition of the acid mixture determines
the rate of removal of the disturbed lattice layer, and, on
the other hand, it determines the final topography of the
wafer after the end of the etching process.
[2]
The choice
of etching parameters (vertical/horizontal immersion,
temperature, stirring speed, and additives used) allows
-------------------------------------------------------------------------------------------------------- -------- -------- ------- --
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
© 2019 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd
Received: 2 July 2019 Revised: 30 August 2019 Accepted: 23 September 2019
DOI: 10.1002/jrs.5769
J Raman Spectrosc. 2019;1–7. wileyonlinelibrary.com/journal/jrs 1

one to shape the topography of the wafer surface in such
a way that it has a lower reflectivity,
[3]
which leads to a
higher efficiency of the solar cell manufactured. A recent
investigation using a combination of confocal microscopy
and Raman microscopy explains this phenomenon, show-
ing that disturbed lattice areas that are under strong ten-
sile and compressive stress are specifically attacked by
acidic etching solutions.
[4]
Although numerous studies deal with the mechanism
of the acid etching of Si with HF‐HNO
3
mixtures, the
essential step in the initial phase of the reaction is still
unclear. Despite its non‐noble character, Si dissolves only
very slowly (0.03 nm·min
−1
) even in concentrated
hydrofluoric acid (48%).
[5]
Only after the addition of an
oxidizing agent (e.g., HNO
3
) can etch rates of up to sev-
eral thousand nm·s
−1
be achieved.
[6,7]
The characteristic
feature of etching Si in HF/HNO
3
mixtures is the occur-
rence of an induction period, which lasts up to several
seconds. The end of the induction period is usually indi-
cated by a massive etch attack and is accompanied by a
massive formation of gaseous products. Which reactions
occur during the induction period are still unknown,
however, the major outcome of the induction period is a
more or less massive formation of intermediary highly
reactive N (III) species, which determine the etching
process significantly and cause the high etch rate after
the end of the induction period.
[8]
It arises the question
for the reactive species that first initiates a reaction
between Si and nitric acid in freshly prepared HF/HNO
3
mixtures.
[9]
Robbins and Schwartz already assumed that undissoci-
ated nitric acid, that is the HNO
3
species, was the reactive
species for the oxidation of Si in the HF/HNO
3
system.
[7]
First systematic studies on the kinetics of oxidation by
nitric acid were carried out by Berg, who determined
the dissolution rate of copper into nitric acid at different
concentrations.
[10]
Berg attributes the oxidizing effect to
the dissolution of copper solely to undissociated nitric
acid, following the work of Briner.
[11]
In addition, Berg,
for the first time, established an experimentally proven
kinetic relationship in which the dissolution rate of cop-
per is linearly related to the concentration of undissoci-
ated nitric acid in a first order reaction.
In his work, Berg derived the concentrations of undis-
sociated HNO
3
from the data of various authors.
[12–17]
However, these authors assumed that HNO
3
only dissoci-
ates to H
3
O
+
and NO
3
−
(Equation 1).
HNO3þH2O⇄H3OþþNO−
3(1)
Recent Raman spectroscopic investigations on the dis-
sociation of nitric acid by Irish and Puzic showed that the
signal of the nitrate ion at 1,048 cm
−1
is asymmetrically
broadened at a concentration of about 3 mol·L
−1
and
above. The asymmetric widening was attributed to the
presence of an ion pair (H
3
O
+
·NO
3
−
). A further signal
from a concentration of 4 mol·L
−1
is observed at 968
cm
−1
, which is assigned to undissociated nitric acid.
[18]
Based on these findings, they formulated the equilibria
of the individual species in the solution, as shown in
Figure 1.
[18]
Based on the works of Irish and Puzic
[18]
and Potier
et al.
[19]
, Hlushak et al.
[20,21]
deconvoluted the Raman
spectra in the range of 900…1,100 cm
−1
with three
Gaussian‐Lorentz functions. One band at 1,046 cm
−1
±
2cm
−1
was assigned to the nitrate ion, another band at
1,024 cm
−1
±12cm
−1
to the ion pair (H
3
O
+
·NO
3
−
), and
another band at 961 cm
−1
±8cm
−1
to the undissociated
nitric acid. They derived a polynomial from a plot of the
signal area of the nitrate ion band over the total concen-
tration of nitric acid that describes the relationship
between the degree of dissociation and the concentration
of nitric acid.
[20]
Levanov et al. determined the dissocia-
tion constant of nitric acid by a signal deconvolution of
the Raman spectra in the same wave number range into
three Voigt profiles and determined the activity coeffi-
cients in a concentration range of 0–18 mol·L
−1
.
[22]
This results to the following formulation for the degree
of dissociation, α, of nitric acid (Equation 2
α¼cNO−
3þcH3Oþ⋅NO−
3
c0;HNO3
(2)
with cNO−
3as the concentration of nitrate ions, c0;HNO3
as the concentration of the nitric acid submitted, and
cH3Oþ⋅NO−
3as the concentration of the ion pair. The
concentration of undissociated nitric acid, cHNO3, is the
difference between the initial concentration and the dis-
sociated fraction described by the nitrate concentration
and the concentration of the ion pair (Equation 3.
cHNO3¼c0;HNO3
−cH3Oþ⋅NO−
3þcNO−
3(3)
Using the known dissociation degree according to
Equation2, the concentration of undissociated nitric acid
can be calculated from Equation 4.
cHNO3¼c0;HNO3⋅1−α(4)
The papers discussed
[18–22]
provide a sufficient and
reliable database for the degree of dissociation depending
FIGURE 1 Scheme of the equilibrium of HNO
3
in water
[18]
2LANGNER ET AL.

on the concentration of nitric acid. However, it is not
advantageous to compensate the time‐dependent signal
drift of a typical Raman spectrometer by adding an inter-
nal standard to concentrated HF/HNO
3
/H
2
SiF
6
acid mix-
tures. A repeating measurement of an external standard
(e.g., CCl
4[21]
) is, in principle, possible, however, less
applicable for continuous measurements, such as online
etch bath monitoring and should, therefore, be avoided.
Consequently, the method of Levanov and Hlushak
[20–
22]
was extended to determine the degree of dissociation
of nitric acid in HF/HNO
3
/H
2
SiF
6
acid mixtures of
unknown nitric acid concentration and the influence of
HF and H
2
SiF
6
on the degree of nitric acid dissociation.
The total nitric acid content in such acid mixtures can
be determined, for example, by ion chromatographic
determination of the nitrate content in highly diluted ali-
quots. Knowing this, the concentration of undissociated
nitric acid can be calculated, which is essential for a
kinetic description of the etching reaction, which is still
pending today.
2|EXPERIMENTAL
2.1 |Chemicals, acids
Etch mixtures were prepared by mass dilution from ana-
lytical grade acids (hydrofluoric acid, HF 48 %[w/w] and
nitric acid, HNO
3
, 69 %[w/w], both from Merck, Darm-
stadt, Germany; hexafluosilicid acid, H
2
SiF
6
, 45 %[w/w]
from Fluorchemie Dohna, Dohna, Germany). All experi-
ments were carried out at room temperature. All concen-
trations are defined as the quotient of the molar amount
of the component i, n
i
, and the total mass of the solution,
m(Equation 5). According to,
[23,24]
this quotient is con-
sidered as specific partial quantity q
i
having the unit
mol·kg
−1
.
qi¼ni
m(5)
2.2 |Raman spectroscopy
Raman spectra from the concentrated etch mixtures were
recorded in the range from 2,500 to 230 cm
−1
using a
DRX SmartRaman (ThermoFisher Scientific) equipped
with a 532‐nm excitation laser at 10‐mW laser energy
and a grating with a resolution of 5 cm
−1
. Single use
UV semi‐micro cuvettes (d= 10 mm, 220–900 nm; Brand
GmbH + Co KG, Wertheim, Germany) were found to be
the most resistant against the etch mixtures. The signals
were assigned as shown in Table 1.
2.3 |Data evaluation
TABLE 1 Wave numbers of the Raman lines found for HNO
3
solution (ν= 600 …1,400 cm
−1
)
Species
Wave
numbers
observed
(cm
–1
)
Wave numbers
literature (cm
–1
)
Vibration
mode
HNO
3
640 640
[25]
δ
rocking
(NO
2
)
688 680,
[26]
688
[25]
δ
bending
(NO
2
)
930 928,
[27]
925
[26,28]
ν
s
(N‐O)
957 955
[25]
ν
s
(N‐OH)
1305 1294,
[27]
1,300,
[26,28]
1303,
[29]
1,304
[25]
ν
s
(NO
2
)
1,558
[25]
2x δ
OOP
1,673
[25]
ν
as
(NO
2
)
NO
3
–
722 720,
[25]
720
[28]
δ
bending
(NO
2
)
1,048 1,050
[27,28]
ν(N‐O)
1,046
[25]
1,049
[30]
H
3
O
+
·NO
3
–
1,036 1,024,
[20]
1,036,
[18]
1,034
[22]
ν(N‐O)
SiF
62‐
656 656
[31]
ν(Si‐F)
N
2
O
3
627 627
[32]
ν
wag
(NO
2
)
784 784
[33]
ν
bending
(NO
2
)
1,288 1,288
[33]
ν
s
(NO
2
)
NO
2
‐
1,328 1,328
[32]
ν
s
(NO
2
)
N
2
O
4
810 810
[27]
δ
bending
(NO
2
)
FIGURE 2 Typical Raman spectrum of a HNO
3
solution (45%
[w/w]), signal deconvolution with peak positions given in Table 1.
Inset: Interferences of signals in an HF/HNO
3
/H
2
SiF
6
etch mixture
in the range of 770–580 cm
−1
LANGNER ET AL.3

Figure 2 shows a typical Raman spectrum of a semi‐
concentrated nitric acid solution. The wave number ranges
of 1,150…900 cm
−1
and 750…600 cm
−1
were deconvoluted
into four individual signals with Voigt profiles.
The baseline‐corrected signal areas for the Raman
band of the symmetric N‐O stretching vibration in the
nitrate ion at 1,048 cm
−1 [25][27][28][30]
with a full width
of half maximum of 6.5…7.5 cm
−1
and the N‐OH
stretching vibration in the molecule of undissociated
nitric acid at 957 cm
−1 [25]
(full width of half maximum
=40…45 cm
−1
) are used to determine the degree of
dissociation. The wave number range of 600…750 cm
−1
,
which also contains the signals of HNO
3
and NO
3
−
,is
interfered by overlapping with the Si‐F stretching vibra-
tion of the reaction product H
2
SiF
6
at 656 cm
−1
.
[29]
The
HNO
3
band at 1,305 cm
−1
is also unsuitable for evalua-
tion, despite its higher intensity, because it interferes
with the signals of N
2
O
3
(1,288 cm
−1
)
[33]
and NO
2
−
(1,328 cm
−1
).
[32]
Signals of other reaction products, such
as N
2
O
4
, at 265 cm
−1
or 810 cm
−1
and (N
4
O
6
)
2+
at 2,246
cm
−1
, do not interfere within the wave number range
selected.
[27]
3|RESULTS
3.1 |Procedure to determine the degree of
dissociation of HNO
3
The degree of dissociation of HNO
3
in the concentration
range 1…69%[w/w] was determined by Raman spectros-
copy, based on the method presented by Levanov and
Hlushak.
[20–22]
This method is based on the analysis of
the baseline‐corrected signal areas of the deconvoluted
signals for the nitrate ion at 1,048 cm
−1
and the ion pair
(H
3
O
+
·NO
3
–
) at 1,034 cm
−1
. The sum of the two signal
areas is plotted over the total concentration of the HNO
3
to determine the degree of dissociation of the nitric acid.
In that case, if the area is linear to the nitrate concentra-
tion (c0;HNO3< 1.8 mol·kg
−1
), the nitric acid is completely
dissociated, and the peak area measured (A
measured
)is
equal to the peak area extrapolated. This concentration
range was used to determine a calibration coefficient k
that is the slope of the linear relationship in Equation 6.
Ameasured ¼cNO3
−þcH3Oþ·NO3
−⋅k(6)
If the nitric acid concentration increases continuously,
A
measured
is smaller than the peak area extrapolated (
c0;HNO3·k). This relationship results from the association
of nitrate ions with protons. The degree of dissociation
can be calculated from Equation 7.
α¼Ameasured
c0;HNO3⋅k(7)
The degree of dissociation thus determined is graphi-
cally shown in Figure 3 in the concentration range up
to 69% [w/w] nitric acid.
Figure 3 compares the results obtained in this study
with the values of Levanov
[22]
, Berg
[10]
and Hlushak
[20]
.
Even without considering the ion pair, the degrees of dis-
sociation used by Berg show only minor deviations. The
linear correlation between the dissolution rate of copper
and the concentration of undissociated HNO
3
by Berg
[10]
is an indication of the validity of the theory that HNO
3
is the species in the rate determining step in the dissolu-
tion of copper.
The method developed in this work is based on the
evaluation of the baseline‐corrected peak areas of the
Raman signals of the undissociated HNO
3
at 957 cm
−1
,
AHNO3957 cm1Þð , and the nitrate ion at 1,048 cm
−1
,
ANO−
31048 cm−1. These were determined anew, because
the peak areas of these signals are not published as a
function of the nitric acid concentrations. The degrees of
dissociation calculated are shown in Figure 3. The ratio
of the signal areas, R, is calculated from the baseline‐
corrected peak areas obtained, according to Equation 8.
R¼AHNO3957 cm−1
ANO−
31;048 cm−1(8)
The peak area ratios determined for different nitric
acid concentrations and the corresponding degrees of dis-
sociation show the correlation plotted in Figure 4. Its
empirical mathematical description is given by
Equation 9.
FIGURE 3 Degree of dissociation; comparison of our own data
with Berg,
[10]
Levanov,
[22]
and Hlushak
[20]
(converted from
mol·L
−1
in mol·kg
−1
to
[34]
). The data from
[10]
are a summary of
calculations without considering the ion pair
[12–17]
4LANGNER ET AL.

This empirical formula is valid for the value range 0.15
≤α≤0.95.
αRðÞ¼K1⋅e−R
k1þK2⋅e−R
k2(9)
with
K1¼0:521 ± 0:020;k1¼0:192 ± 0:009;K2
¼0:425 ± 0:021;k2¼1:85 ± 0:16
Using Equation 9, the degree of dissociation of nitric
acid in HF/HNO
3
/H
2
SiF
6
acid mixtures of unknown
nitric acid concentration can be obtained if the baseline‐
corrected peak area ratio for these mixtures has been
determined previously. However, the determination of
the undissociated nitric acid via the band at 957 cm
−1
is
only possible from a content of 18%[w/w] HNO
3
, because
the intensity of the band clearly differs from the corrected
background only from this content. Therefore, the upper
limit of α= 0.95 was chosen. At this value, the relative
uncertainty of αis about 2%. The determination is carried
out with a relative uncertainty of 10% or lower up to a
peak ratio of about 1.3 and a resulting degree of dissocia-
tion of α= 0.21. At the lower limit of the value range,
which is given by the highest experimentally measured
value (w [HNO
3
] = 69%, R= 1.98, α= 0.15), a result
uncertainty of about 15% must be expected.
The total concentration of nitric acid in the acid mix-
ture must be known in order to obtain the concentration
of undissociated nitric acid in HF/HNO
3
/H
2
SiF
6
acid mix-
tures. Only in the case of freshly prepared acid mixtures is
this value known from the initial concentration of the
nitric acid. In acid mixtures of unknown composition,
that is after a longer etching process, the total nitrate con-
tent has to be determined, for example, by ion chromato-
graphic measurement in high dilution, typically with
factors between 2,000 and 10,000.
[35]
The nitrate concen-
tration determined in such highly diluted solutions corre-
sponds to the total concentration of nitric acid, according
to Equation 10.
cNO−
3¼c0;HNO3(10)
Thus, the concentration of undissociated nitric acid in
any HF/HNO
3
/H
2
SiF
6
acid mixtures can be determined
via Equation 3.
3.2 |Application to etching mixtures of
the HF/H
2
SiF
6
/HNO
3
system
The degree of dissociation of nitric acid in binary and ter-
nary mixtures with HF and H
2
SiF
6
can be determined
using the method described. Figure 5 shows the depen-
dence of the degree of dissociation of HNO
3
on the HF
content (left) and the H
2
SiF
6
content (right) in binary
mixtures.
An increase in the HF or H
2
SiF
6
concentration at a
constant HNO
3
concentration leads to a decrease in
the degree of dissociation of HNO
3
. At a HNO
3
concen-
tration of 5 mol·kg
−1
, the degree of dissociation
decreases almost linearly with increasing HF content.
This is the case in a mixture with H
2
SiF
6
at a HNO
3
concentration of 6 mol·kg
−1
. The graphs are negatively
curved with increasing HF or H
2
SiF
6
content at lower
HNO
3
concentrations. If the HNO
3
concentration is
higher than 5 mol·kg
−1
(HF) or 6 mol·kg
−1
(H
2
SiF
6
),
the curvatures are positive.
Regarding the binary mixtures, both acids HF and
H
2
SiF
6,
each reduce the degree of dissociation of the nitric
acid. This reduction for HF is significantly smaller for the
same HNO
3
concentrations due to the lower acid strength
of HF (pK
a
= 3.16)
[36]
than for H
2
SiF
6
, whose acidity cor-
responds approximately to that of H
2
SO
4
.>
[37]
This tendency also continues in the ternary acid mix-
tures in Figure 6. The composition range of typical acidic
etching mixtures is covered with an HNO
3
concentration
of 4 mol·kg
−1
(25%[w/w]), HF concentrations between 1.5
and 3.5 mol·kg
−1
(3–7%[w/w]) and H
2
SiF
6
concentrations
between 0.2 and 1.3 mol·kg
−1
(3–19%[w/w]).
The increase of the H
2
SiF
6
concentration again shows
a significantly stronger impact on the degree of dissocia-
tion than a change in the HF content. However, the func-
tions do not show exactly the same curvature. It is,
therefore, not straightforward to develop an empirical
FIGURE 4 Degree of dissociation of nitric acid as function of
peak ratio R; the red line is the fitting curve; the red dotted band
indicates the prediction interval
LANGNER ET AL.5

equation to estimate the degree of dissociation of HNO
3
out of the concentrations of the acids used.
A very interesting relationship is obtained if the
degrees of dissociation of the differently concentrated
HNO
3
of the binary mixtures (HF/HNO
3
;H
2
SiF
6
/HNO
3
)
and the ternary mixtures (HF/H
2
SiF
6
/HNO
3
) are plotted
over the total water content of the respective mixture
(Figure 7). The resulting plot shows that the degree of dis-
sociation of the HNO
3
is essentially dependent on the
concentration of the water in the acid mixture in a first
approximation.
The resulting empirical relationship between the water
concentration and the degree of dissociation is given by
Equation 11.
αqH2O

¼a⋅q4
H2Oþb⋅q3
H2Oþc⋅q2
H2Oþd⋅qH2Oþe
with
a¼3:54 ± 2:01⋅10−6kg4⋅mol−4;
b¼−5:28 ± 2:83⋅10−4kg3⋅mol−3;
c¼0:0275 ± 0:0149 kg2⋅mol−2;
d¼−0:551 ± 0:343 kg⋅mol−1;e¼3:65 ± 2:95
(11)
This dependence provides a useful approach for a sim-
plified kinetic description of the etching reaction. The
influence of the concentrations of HF and H
2
SiF
6
on the
concentration of undissociated HNO
3
Equation 4) in a
formalized kinetic model can now be simplified,
described by the water content of the acid mixture.
FIGURE 5 Degree of dissociation of different HNO
3
concentrations as a function of the HF concentration (a) and H
2
SiF
6
concentrations,
(b) the grey areas represent values outside the detection limit
FIGURE 6 Degree of dissociation of HNO
3
(4 mol‐kg
‐1
)
depending on the HF and H
2
SiF
6
concentration
FIGURE 7 Degree of dissociation of different HF/HNO
3
/H
2
SiF
6
mixtures as a function of water concentration; the red line is the
fitting curve; the red dotted band indicates the prediction interval
6LANGNER ET AL.

4|CONCLUSION
A method for the Raman spectroscopic determination of
the degree of dissociation of HNO
3
in HF/HNO
3
/H
2
SiF
6
acid mixtures was developed. This method is based on
the determination of the peak area ratio of the Raman
signals of the undissociated HNO
3
at 957 cm
−1
and
the nitrate ion at 1,048 cm
−1
. The degree of dissociation
of nitric acid in each acidic mixture can be calculated by
means of an empirical correlation between the peak
area ratios and the degree of dissociation. The only pre-
conditions are that the Raman bands required for the
determination of the peak ratio (a) are not interfered
with by other Raman bands and (b) have sufficient
intensities. The concentration of undissociated nitric
acid is obtained either by knowing the initial content
of nitric acid or after the determination of the total
nitrate concentration by ion chromatography, as in
highly diluted etch solutions.
The impact of HF or H
2
SiF
6
on the dissociation of
nitric acid is basically correlated to the dissociation
strength of the two acids. However, it could be shown
that the degree of dissociation of HNO
3
in the ternary
etch mixtures is a function of the total water content in
the respective mixtures (Figure 7), providing a simplified
approach for a later formalized kinetic description of the
etch reaction.
The method presented can be applied independently
on the Raman device used and without the use of internal
or external standards to compensate for the typical device
drift. This provides a further important parameter for
characterizing acid etching mixtures, which can be con-
tinuously determined and used as an instrument for con-
trolling etching baths in industrial production lines for
the manufacture of solar cells.
ORCID
Thomas Langner https://orcid.org/0000-0002-5979-6977
Anja Rietig https://orcid.org/0000-0002-2448-6856
Jörg Acker https://orcid.org/0000-0002-1325-1111
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How to cite this article: Langner T, Rietig A,
Acker J. Raman spectroscopic determination of the
degree of dissociation of nitric acid in binary and
ternary mixtures with HF and H
2
SiF
6
.J Raman
Spectrosc. 2019;1–7. https://doi.org/10.1002/jrs.5769
LANGNER ET AL.7