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.

Comprehensive stoichiometric studies on the reaction of silicon in HF/HNO3 and HF/HNO3/H2SiF6 mixtures

“…Considering, on the other hand, that the NO 2 detected in the reaction gas mixture is primarily determined by the content of H 2 O in the etching mixture and that the degree of dissociation of the nitric acid can also be described by the H 2 O content in a first approximation, 45 the data can be described for etching mixtures of any composition (Fig. 4a).…”

“…On the one hand, it is caused by the dependence of the degree of dissociation of HNO 3 on the content of H 2 O in the mixture and, consequently, on the concentration of undissociated HNO 3 . 45 In addition, the concentration of undissociated HNO 3 and the H 2 O content in the etching mixture directly determine the conversion of the reaction product NO to N 2 O 4 via the side reaction eqn (7) ( cf. Fig.…”

“…The concentration of undissociated HNO 3 in the etching mixture was determined by Raman spectroscopy (DXR SmartRaman, Thermo Scientific) according to the method of Langner et al 45 The excitation wavelength of the laser was 532 nm and the laser power was 10 mW.…”

Effect of diluent on the extraction of europium(iii) and americium(iii) with N,N,N′,N′-tetraoctyl diglycolamide (TODGA)

“…54 There is some evidence from experimental spectroscopic data (mostly Raman and Infra-Red), Nuclear Magnetic Resonance data and molecular dynamic simulations for the existence of weak HNO 0 3 ion pairs in relativity concentrated aqueous solutions of nitric acid. [56][57][58][59][60][61] Comprehensive review of the experimental data is available 62 and according to this review values of HNO 0 3 stability (dissociation) constants at innite dilution (K 0 a ) vary signicantly, from 10 to 40. Such a large difference in K 0 a can be explained by the problems associated with experimental or theoretical determination of stability constants of weak ion pairs and strongly indicates that uncertainty associated with this value is large.…”

Inline Raman Spectroscopy and Indirect Hard Modeling for Concentration Monitoring of Dissociated Acid Species

“…29 Some studies approach dissociation of strong mineral acids with Raman spectroscopy to develop process analytical tools for determination of the acid dissociation ratio. 30,31 However, they either rely on available p K a values or use an empirical approach for quantitative evaluation. Besides analysis of mineral acids, inline process monitoring of carboxylic acid species by Raman and IR spectroscopy has been successfully demonstrated during crystallization for liquid and solid phases, for characterization of citric acid and salicylic acid in aqueous solution, and for dissociation of gallic acid in aqueous solution.…”