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I regularly purchase aqueous sodium silicate solutions from commercial providers for my studies and note that a storage recommendation for all different grades (i.e. differing $\ce{SiO2 / Na2O}$ ratios) is to "not store in direct sunlight" with no explanation as to why this is the case. I tried asking the provider why this is a recommendation but didn't get a substantial answer apart from "it turns a weird [brown-like] colour after a while and is not really usable after that".

I believe the issue here is something to do with the polymeric character of sodium silicate - with the formula given as $\ce{Na_{2x}Si_yO_{2y+x}}$ or $\ce{Na2O_x SiO2_y}$ - which would inevitably cause issues with sunlight cleaving parts of the sodium silicate polymer backbone and creating a heterogenous mixture (i.e. with different parts of the aqueous solution having polymers of different chemical formulas) which would be a QAQC (quality assurance and quality control) nightmare for using sodium silicate solutions in commercial developments. Adding to this, I believe the poor usability probably comes from precipitation of something and have found (if useful) that sodium silicate solutions with increased levels of $\ce{Na2O}$ can cause solution precipitation.. however this precipitation is a white gel-like substance (which I believe comes from the solution approaching the saturation concentration of $\ce{NaOH}$) and nothing "brown-like".

My Questions are:

  1. Does anyone have experience/insight here with storing sodium silicate solutions in direct sunlight?
  2. What predictive mathematical and/or empirical models can be used and/or made to assess how much sunlight will cause QAQC issues with sodium silicate?
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    $\begingroup$ These silicate solutions come in plastic bottles, and plastic does degrades with sunlight. Storing away from sunlight is a common warning on reagent bottles, because plastic can also become brittle. Sodium silicate should be UV insensitive. This browning is hard to believe is due to sunlight. $\endgroup$
    – ACR
    Commented Nov 13, 2022 at 15:37
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    $\begingroup$ The brown substance is NOT a compound of silicon and sodium. All sodium silicates are colourless. It must be due to some organic impurity which is sensitive to light. $\endgroup$
    – Maurice
    Commented Nov 13, 2022 at 15:59
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    $\begingroup$ Sodium silicate may attack the paper liner inside some bottle tops, causing the brown coloration, but, as others state, it should be relatively insensitive to light. BTW, never store in a glass container, which it does attack over time. $\endgroup$
    – DrMoishe Pippik
    Commented Nov 13, 2022 at 19:58
  • $\begingroup$ Along with the issues with the container, it seems that photolysis from the sunlight can also degrade the sodium silicate by changing the chemistry of the bonds and thus impacting its QAQC utility in experiments (etc.). $\endgroup$
    – Hendrix13
    Commented Jan 24, 2023 at 12:42

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