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Question

From time to time, I come by to see what people do with my mhchem package. Here, I came across a notation that I did not know yet: $\ce{K2O-nSiO2-xH2O}$ and $\ce{K2O - 3.2SiO2 - 2.7H2O}$

What is this? Is this really a bond? Is this a short form for writing $\ce{K2O-Si_nO_{2n}-H_{2x}O_x}$ and $\ce{K2O - Si_{3.2}O_{6.4} - H_{5.4}O_{2.7}}$? Is this an established notation that mhchem should support? (It doesn't yet with the numbers.) Do you have authoritative References?

I don't think this is standard notation, and the linked question suggests that it is rather a recipe or composition of a mixture of salts rather than an actual compound formula. — Martin - マーチン, Oct 02 '17 at 12:28
They should be dots (i.e. \cdot, $\cdot$) rather than hyphens- they may or may not be bonded. See here for an explanation https://chemistry.stackexchange.com/questions/26699/what-is-the-meaning-of-the-dot-notation-in-chemical-formulas
See the formulas here https://en.wikipedia.org/wiki/Water_of_crystallization — obackhouse, Oct 02 '17 at 16:58
@IvanNeretin Dots make even more sense. I had thought the manufacturer had at least gotten the type of symbols right so my brain was stuck in the distinction between different types of horizontal lines ^^' — Jan, Oct 03 '17 at 05:57

score 1 · Answer 1 · edited May 19 '21 at 02:17

The notation, upon replacing potassium with sodium, can refer to a water glass composition. The unique notation, in my opinion, is warranted based on the following comment from a paper¹:

Despite of years of investigations, there are still questions about the molecular species configuration of these solutions that need to be answered to gain clarity over the best conditions for the various applications

When adding sodium hydroxide ($\ce{NaOH}$) to a water glass, the ratio (molar or weight) $\ce{SiO2/Na2O}$ is decreased. This ratio is called water glass modulus (n) and determines various physical and chemical properties such as the pH and the viscosity of the solution.

[...]

the viscosity is extremely dependent on the modulus and increases as the solution becomes either more siliceous or more alkaline, i.e. at both higher and lower $\ce{SiO2/Na2O}$ molar ratios (n). Since the viscosity of such disperse systems like this is given by the silicate conformation i.e. extended chain conformation and the degree of polymerization

[...]

the degree of polymerization of the predominant silicate species in the alkali-solution [...]

So we have apparently a weakly specified structure characterized by extended chain conformation and degrees of polymerization with its rather unique notation (seems right to me).

Reference

Helén Jansson, Diana Bernin, and Kerstin Ramser , "Silicate species of water glass and insights for alkali-activated green cement", AIP Advances 5, 067167 (2015), DOI: 10.1063/1.4923371

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