By the 21st Century, one would think that the chemistry of common inorganic compounds would be firmly established. Not necessarily so! A good example is the existence — or non-existence — of lithium hydrogen carbonate. An online guide to the inorganic component of the British secondary-school chemistry syllabus, in the section on “The thermal stability of the hydrogencarbonates” states in a note: “There is complete disagreement in various sources about lithium hydrogencarbonate. Some say it only exists in solution; some quote it as a solid.”1 The author of the guide concludes that LiHCO3 is a white solid.
The commonly accepted reason for the existence of solid hydrogen carbonates, hydrogen sulfites and so on is that large, low-charge cations are needed to stabilize large, low-charge anions in the crystal lattice. Some sources state that these acid anions form solid compounds with “the alkali metal and ammonium cations.” But the lithium ion is significantly smaller than the other alkali metals. So if the explanation is valid, it would seem unlikely that solid lithium analogs such as lithium hydrogen carbonate exist.
I perused two authoritative sources for their opinion. First, the classic compendium, General and Inorganic Chemistry (1958) by J.R. Partington2 reports that “lithium carbonate … differs from other alkali metal carbonates and resembles the alkaline earth carbonates in being sparingly soluble. … the carbonate dissolves in water containing carbon dioxide, forming lithia water, which may contain LiHCO3.” Second, a more recent encyclopedic work, Handbook of Inorganic Chemicals (2003) by Patnaik3 states: “The [lithium] bicarbonate cannot be separated in solid form. It exists only in solution when carbonate dissolves in water saturated with CO2 under pressure.”
The non-existence of solid lithium hydrogen carbonate makes sense from the lithium cation not being large enough to stabilize a large low-charge anion. It also makes sense from the lithium isodiagonality with the alkaline earth elements.4 Though the lithium-magnesium link is often highlighted, lithium resembles calcium in several ways, and, from the observations of both Partington and Patnaik, this seems to be one of them. That is, similar to calcium carbonate, there is an equilibrium between the aqueous lithium and hydrogen carbonate ions with the sparingly-soluble lithium carbonate, carbon dioxide and water:
2 Li+(aq) + 2 HCO3-(aq) ⇌ Li2CO3(s) + CO2(aq) + H2O(l)
So to test this hypothesis, I mixed saturated aqueous solutions of lithium chloride and sodium hydrogen carbonate. The solution, initially clear, became cloudy, both with a fine precipitate and with a gentle effervescence. The reaction continued for many minutes until there was a thick precipitate. Thus the above reaction does seem to occur. The effervescence is a result of the buildup of carbon dioxide in solution.
So, even in the 21st century, sorting out such simple facts is not always easy. Thus any discussion of the formation of solid compounds of acid oxoanions, such as the hydrogen carbonate ion, should note that they exist only with: “alkali metal (except lithium) and ammonium5 ions.”
- www.chemguide.co.uk/inorganic/group1/compounds.html, page 3, accessed 20 January 2013.
- J.R. Partington, General and Inorganic Chemistry, 3rd edition, Macmillan & Co., London, 1958, page 322.
- P. Patnaik, Handbook of Inorganic Chemicals, McGraw-Hill, New York, 2003, page 498.
- G. Rayner-Canham, “Isodiagonality in the periodic table,” Foundations of Chemistry, 13, 129, 2011.
- The ammonium ion can be considered as a large pseudo-alkali-metal ion. It is a similar size to potassium ion, though chemically, it behaves more like rubidium and cesium. See: G. Rayner-Canham and T. Overton, Descriptive Inorganic Chemistry, 6th edition, W.H. Freeman, New York, page 265.