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The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrate studied in reference 1. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrate studied in reference 1. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond studies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrare studied in the reference. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrate studied in reference 1. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrare studied in the reference. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate. Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrare studied in the reference. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.