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It is somewhat surprising to me that there is almost no data about solubility of potassium silicate in water under normal conditions. The only thing I was able to find is the following:

<0.336 g/L @ 25ΕC, increasing to 300 g/L @ 80ΕC. Once dissolved, material will not precipitate out

Technical Document for Potassium silicate also referred to as a BRAD

However, it is not clear what does the last statement mean. Does it mean that the silicate will not precipitate even if the solution cools down to 25°C?

The solubility of sodium silicate can easily be found on Wikipedia. It is mystery in case of potassium silicate.

So let us assume we want to dissovle potassium silicate in water with initial pH of 7. What amount of potassium silicate can be dissolved?

Edit:

Regarding the comment of @M. Farooq about being more specific. The second sentence on Wikipedia says:

The most common potassium silicate has the formula K2SiO3, samples of which contain varying amounts of water.

We can consider the anhydrous form of K2SiO3 and couple of most common hydrates. Also, if the exact amount is not known then at least knowing the order of magnitude is fine as there is huge difference between 0.336g/L and 300g/L.

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    $\begingroup$ The very first line on Wikipedia states that "Potassium silicate is the name for a family of inorganic compounds." The same is true for sodium silicate. You have to be specific about the composition. $\endgroup$
    – AChem
    Oct 13, 2020 at 19:11
  • $\begingroup$ @M.Farooq, thanks for the comment, I edited my post. $\endgroup$
    – n0p
    Oct 14, 2020 at 7:37
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    $\begingroup$ About precipitation of cooled down solutions, it is supposed to be very viscous, up to colloidic - and probably forming easily supersaturated solutions. $\endgroup$
    – Poutnik
    Oct 14, 2020 at 8:11

3 Answers 3

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Potassium silicate is synthesized by melting $\ce{KOH}$ above $400$°C, and adding some solid $\ce{SiO2}$. Its composition depends on the amount of silica used. The dissolution process does not look so. But it is a slow chemical reaction with a rather viscous liquid. The result can be cooled down and gives a solid substance practically insoluble in water at room temperature.

However it is possible to dissolve it with boiling water under pressure. Surprisingly enough, the obtained potassium silicate becomes more easily dissolved at $T>80$°C under pressure. The concentrated solution so obtained can be cooled down. It yields an extremely viscous liquid, more viscous than honey. This liquid is usually not really transparent, but it can be diluted with pure water without producing a precipitate. Such solutions are probably colloidal solutions of $\ce{H2SiO3}$ or $\ce{H4SiO4}$ in a concentrated solution of $\ce{KOH}$. As a matter of fact, if such a solution is heavily diluted, neutralized to destroy KOH, washed and evaporated in a vacuum, hydrated silica is obtained and it is made of $\ce{H2SiO3}$ flakes which are extremely light : the density of the stuff is not much bigger that air, as if it was made of rather long chains $\ce{HO -[- Si(OH)2 - O -]_n-H}$ sometimes directed in all directions around a central Silicium atom.

The chemistry of the silicic acid and its salts is extremely complex. The ternary diagram $\ce{Na2O - SiO2 - H2O}$ contains $11$ different phases.

Ref.: J. G. Vial, Soluble silicates, Reinhold, New York, 1952. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, 1986, § 9.3.4. p. 398

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  • $\begingroup$ Thanks for the answer, @Maurice. It looks really very complex, what I'm looking for is a rough estimate. However, if there will be no better answer in couple of days, I'll accept your answer. Thank you! $\endgroup$
    – n0p
    Oct 14, 2020 at 20:55
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PQ Corporation sells many compositions of sodium and potassium silicates. Concentrations of potassium silicate range up to 40% in water.

The product literature includes pH, viscosity, SDS and just about every measurable property you could ask for. The reason such data are not in academic literature or book form is probably that they are considered lower-class information, used only by industrial engineering minds. I've got lots of brochures that are priceless for indicating what variety and range of commercial products are available: raw materials, not pure compounds.

enter image description here

https://www.pqcorp.com/docs/default-source/typical-properties/pq-corporation/potassium-silicates/17-2d-potassium-silicate.pdf?sfvrsn=a1fd57fb_3

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    $\begingroup$ Thank you James, but I don't see a solubility column in these tables. $\endgroup$
    – n0p
    Oct 18, 2020 at 12:08
  • $\begingroup$ @n0p: For the first 5 potassium silicates, if you add the weight % of K2O to the weight % of SiO2, you get a total weight % which is less than 100. These are liquids (indicated by the viscosity); the remainder is water; the maximum is ~40.9% for Kasil 18. These materials are significantly more viscous than water (~1 cps) and don't drop out crystals at some well-defined low temperature. They just stiffen up like a glass when it cools down from a melt. $\endgroup$ Oct 18, 2020 at 13:45
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The above posts don't seem to answer the question, James Gaidis comes close by introducing you to the different forms though. The Anhydrous form of Potassium silicate does not dissolve in room temp water easely, and if you don't want to have to wait a day or more under stirring for it to dissolve. You have to heat it up under stirring to get it dissolved quickly-ish (Even under heating it is still a slow process).

The Hydrous forms of Potassium Silicate (Spray dried being the most common) do dissolve in room temp water quickly. Not as fast as normal sodium salt, around double the time of normal sodium salt dissolution. Obviously like any soluble salts, if heated while dissolving it speeds up the process, but under room temp the hydrous forms of Potassium silicate dissolve quite rapidly under agitation/stirring. As said above, the molar ratios of potassium too silica also have an effect on the time of dissolution. The higher the potassium ratio the faster it will dissolve.

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